lunes, 22 de marzo de 2010
Dowa Succeeds in Practical Application for Mass Production of a Deep Ultraviolet LED Chip Having the World`s Highest Output
Mar 17, 2010
Dowa Electronics Materials Co., Ltd., a subsidiary of Dowa Holdings Co., Ltd., has successfully developed practical applications for a deep ultraviolet LED that generates shorter wavelengths than the ultraviolet LEDs currently available on the market. Dowa Electronics Materials has begun to offer samples to develop a market.
The deep ultraviolet LED chip is expected to be used in a wide range of fields, including resin cure, adhesion, drying, medical treatment, analyses, photo catalysts, water purification, and sterilization, and is attracting the attention of companies worldwide. Compared with the existing mercury light source, the LED chip is expected to diversify wavelengths, create mercury-free light sources, and have a longer life. Because of a lack of suppliers able to mass-produce deep ultraviolet LEDs, the market is immature. However, if a supply system is established, we expect a market worth tens of billions of yen will emerge.
The deep ultraviolet LED consists of a nitride semiconductor and has been very difficult to manufacture. No manufacturers have therefore succeeded in mass-production. Dowa Electronics Materials has succeeded in developing a practical application for a deep ultraviolet LED chip that emits light with wavelengths of 300 nm to 350 nm. The company created an LED with the world's highest output power in wavelengths by combining its own AIN template (high-quality AIN film growing on the sapphire substrate) technology, and the newly obtained ultraviolet LED epi growth technology from Palo Alto Research Center (PARC) and RIKEN. The currently available sample has achieved an optical output power of 1.4mW with 20mA in wavelengths of 320nm to 350nm.
Dowa Semiconductor Akita Co., Ltd., a subsidiary manufacturer of Dowa Electronics Materials, is creating prototypes and is striving to start mass production. It will seek to increase the output power while at the same time developing deep ultraviolet LED chips that will emit light with shorter wavelengths.
Dowa Electronics Materials has the ability to manufacture many types of GaAs products and has more than 20 years of experience in the red and infrared LED business. The company has also in recent years been rapidly enhancing the Iineup of nitride semiconductors. In the first stage, it has launched a nitride electronic device
(HEMT) epi for high-frequency waves used at next-generation mobile phone base stations, etc. and for power semiconductors. The deep ultraviolet LED chip is a nitride product in the second stage. With the introduction of this product Dowa aims to bolster the base of its semiconductor business.
ALONZO DAVID LINARES MORA
COMUNICACIONES DE RADIO FRECUENCIA
FUENTE ORIGINAL:
http://www.compoundsemiconductor.net/csc/news-details.php?id=19674925&name=Dowa%20Succeeds%20in%20Practical%20Application%20for%20Mass%20Production%20of%20a%20Deep%20Ultraviolet%20LED%20Chip%20Having%20the%20World%60s%20Highest%20Output
BreconRidge Expands Monolithic Microwave Integrated Circuit (MMIC) Assembly Capabilities to Address Future Gallium-Nitride Based Programs and Technologies.
BreconRidge Expands Monolithic Microwave Integrated Circuit (MMIC) Assembly Capabilities to Address Future Gallium-Nitride Based Programs and Technologies.
(PRWEB) March 18, 2010 -- BreconRidge Corporation packaged and shipped over 90 Gallium-Nitride (GaN) modules to the Canadian Space Agency (CSA) as part of its corporate strategy to extend its core capabilities in next generation Micro Electronic Modules. This delivery is the latest step in BreconRidge's goal to become a key partner with the Aerospace and Defense industries. Prior milestones include collaborative design and manufacturing contributions to programs involving defense and aerospace radars, radio-astronomy systems and defense communication systems.
"Emerging technologies like Gallium-Nitride require an innovative approach in all aspects of product design and manufacturing." commented John Pokinko, VP Engineering at BreconRidge. "We are aggressively pursuing all opportunities to further our expertise in applying these new technologies in advanced RF and microelectronic solutions. Successful completion of the CSA GaN packaging contract represents a key stepping stone in this strategy."
Gallium-Nitride leads an emerging class of semiconductor technologies designed to tackle the RF challenges of next-generation cellular network base stations and satellite communications systems. Compact packaging and stringent linearity requirements challenge today's designers to meet heat dissipation and bandwidth allocation objectives. GaN based electronics offers the potential to cost-effectively address these challenges. The National Research Council's Canadian Photonics Fabrication Centre (NRC-CPFC) was responsible for fabricating the GaN die used in this project at their world-class industrial grade facility. "Few companies in the Electronic Manufacturing Services sector have the capabilities to assemble and package Gallium-Nitride electronics.", observed Cyril McKelvie, President at BreconRidge. "Being able to deliver these first modules is a reflection of our desire, skills and capabilities to address the emerging needs of the Aerospace and Defense sectors. Ultimately, our customers will benefit from our accumulated experience with Gallium-Nitride electronics."
About BreconRidge
BreconRidge is a world class provider of innovative design and manufacturing services for electronic products. With facilities located in North America and Asia, BreconRidge collaborates on voice, video and data applications with the world's leading communications, industrial, medical, aerospace and defence OEMs (original equipment manufacturers). Specializing in RF, microwave and optical technologies and applications, BreconRidge delivers a complete range of life cycle services from collaborative design, new product introduction, supply chain management, process and test engineering, DFx services, manufacturing, repair, distribution and reverse logistics. BreconRidge's unique portfolio of core competencies provides customers with significant competitive advantages in their respective markets.
Read more: http://www.earthtimes.org/articles/show/breconridge-delivers-first-gallium-nitride-micro-electronic-modules-to-csa,1210454.shtml#ixzz0isLIvAO6
ALONZO DAVID LINARES MORA
COMUNICACINES DE RADIO FRECUENCIA
FUENTE DE ORIGEN:
http://www.earthtimes.org/articles/show/breconridge-delivers-first-gallium-nitride-micro-electronic-modules-to-csa,1210454.shtml
(PRWEB) March 18, 2010 -- BreconRidge Corporation packaged and shipped over 90 Gallium-Nitride (GaN) modules to the Canadian Space Agency (CSA) as part of its corporate strategy to extend its core capabilities in next generation Micro Electronic Modules. This delivery is the latest step in BreconRidge's goal to become a key partner with the Aerospace and Defense industries. Prior milestones include collaborative design and manufacturing contributions to programs involving defense and aerospace radars, radio-astronomy systems and defense communication systems.
"Emerging technologies like Gallium-Nitride require an innovative approach in all aspects of product design and manufacturing." commented John Pokinko, VP Engineering at BreconRidge. "We are aggressively pursuing all opportunities to further our expertise in applying these new technologies in advanced RF and microelectronic solutions. Successful completion of the CSA GaN packaging contract represents a key stepping stone in this strategy."
Gallium-Nitride leads an emerging class of semiconductor technologies designed to tackle the RF challenges of next-generation cellular network base stations and satellite communications systems. Compact packaging and stringent linearity requirements challenge today's designers to meet heat dissipation and bandwidth allocation objectives. GaN based electronics offers the potential to cost-effectively address these challenges. The National Research Council's Canadian Photonics Fabrication Centre (NRC-CPFC) was responsible for fabricating the GaN die used in this project at their world-class industrial grade facility. "Few companies in the Electronic Manufacturing Services sector have the capabilities to assemble and package Gallium-Nitride electronics.", observed Cyril McKelvie, President at BreconRidge. "Being able to deliver these first modules is a reflection of our desire, skills and capabilities to address the emerging needs of the Aerospace and Defense sectors. Ultimately, our customers will benefit from our accumulated experience with Gallium-Nitride electronics."
About BreconRidge
BreconRidge is a world class provider of innovative design and manufacturing services for electronic products. With facilities located in North America and Asia, BreconRidge collaborates on voice, video and data applications with the world's leading communications, industrial, medical, aerospace and defence OEMs (original equipment manufacturers). Specializing in RF, microwave and optical technologies and applications, BreconRidge delivers a complete range of life cycle services from collaborative design, new product introduction, supply chain management, process and test engineering, DFx services, manufacturing, repair, distribution and reverse logistics. BreconRidge's unique portfolio of core competencies provides customers with significant competitive advantages in their respective markets.
Read more: http://www.earthtimes.org/articles/show/breconridge-delivers-first-gallium-nitride-micro-electronic-modules-to-csa,1210454.shtml#ixzz0isLIvAO6
ALONZO DAVID LINARES MORA
COMUNICACINES DE RADIO FRECUENCIA
FUENTE DE ORIGEN:
http://www.earthtimes.org/articles/show/breconridge-delivers-first-gallium-nitride-micro-electronic-modules-to-csa,1210454.shtml
Nitruración de Dientes de Engranajes en un Plasma de N2-H2-CH4
1. INTRODUCCIÓN
Existen varios tratamientos termo-químicos que, mediante la incorporación de ciertos elementos químicos, permiten mejorar el comportamiento funcional de muchos componentes mecánicos de acero. Con esta incorporación, normalmente se genera una capa superficial de dureza mucho mayor que la del material original, lo que permite mejorar la resistencia tanto a la fatiga como al desgaste del componente. Entre los tratamientos más conocidos utilizados con este fin están: cementación, que incorpora carbono; nitruración, que incorpora nitrógeno; y carbonitruración, que incorpora carbono y nitrógeno [1].
En el caso de la nitruración, el tratamiento generalmente se realiza ya sea en atmósferas gaseosas a base de amoníaco disociado, o en sales fundidas a base de cianuro/cianato [2]. El uso de plasma como atmósfera nitrurante es una tecnología desarrollada en Alemania a comienzos de la década del 30 [3], pero cuya utilización industrial comenzó sólo en la década del 70. En Chile, esta tecnología fue implementada experimentalmente en la Universidad de Chile, a comienzo de la década del 80 [4], y a la fecha existe en el país cierta capacidad instalada para efectuar tratamientos de nitruración iónica a escala semi-industrial [5].
Para realizar la nitruración por plasma se necesita, básicamente, el siguiente equipamiento: un recipiente de vacío con un sistema de bombeo que permita obtener una presión entre
1-10 mm Hg, un suministro de gases, una unidad de potencia eléctrica para producir una descarga luminosa (glow-discharge), y un sistema de control del proceso. El proceso consiste, fundamentalmente, en ionizar una atmósfera gaseosa a baja presión que contenga nitrógeno, mediante la aplicación de un alto voltaje entre un ánodo, que puede ser el cuerpo del recipiente de vacío, y la pieza a nitrurar, conectada como cátodo. En el caso de piezas relativamente pequeñas, sólo con el calentamiento provocado por el bombardeo iónico, la pieza alcanza la temperatura adecuada para el proceso, la que normalmente es inferior a los 600 ºC. En caso de piezas grandes, el calentamiento debe ser asistido por algún sistema de potencia adicional.
En general, las atmósferas que más se han utilizado en nitruración iónica han sido las mezclas de nitrógeno e hidrógeno [4-11]. Según Edenhofer [13], el proceso se basa en la formación en la superficie de la pieza, del nitruro FeN, el que por ser inestable a la temperatura del proceso, se transforma en nitruros de menor contenido de nitrógeno (Fe2-3N, Fe4N). De esta forma se genera nitrógeno monoatómico, el que en parte difunde hacia el interior de la pieza, para formar nitruros con algunos elementos de aleación del acero (Cr, Al, V, W, Mo), y en parte difunde hacia el plasma.
Algunos autores han observado que la composición de la atmósfera del plasma tiene influencia en los resultados del tratamiento de nitruración. Por ejemplo, al utilizar mezclas de 25%N2+75%H2 existe formación de capa compuesta, en tanto que con mezclas 2%N2+98%H2 esta capa no se forma, [4]. Por otro lado, con estas atmósferas de sólo nitrógeno más hidrógeno, se produce algún grado de descarburi-zación [10] por la formación de CH1-3 [15], lo que hace disminuir la microdureza en la superficie de las piezas nitruradas [4 -7].
En este trabajo se utilizó un plasma de nitrógeno, hidrógeno y metano, a fin de disminuir el efecto descarburizante de los plasmas de sólo nitrógeno más hidrógeno. Se estudió la influencia que dicho plasma tiene tanto en el espesor de la capa compuesta como en la dureza de dientes de engranajes.
2. DESARROLLO EXPERIMENTAL
Probetas de acero SAE 4140 (0.4%C-1%Cr-0.65%Mn-0.2%Mo), bonificado por temple y con una microdureza entre 320 y 340 HV, fueron nitruradas iónicamente en una atmósfera de 80%N2+19%H2+1%CH4. Las probetas se construyeron de barras de 14 mm de diámetro y 40-45 mm de largo, a cada una de las cuales se les mecanizó tres dientes de cremallera (Fig.1) de distinto tamaño: de módulos (M) 1, 2 y 3 mm. La nitruración iónica se realizó con corriente continua, a las temperaturas (T) de 480, 525 y 570 °C, por tiempos (t) de 2, 3 y 5 h, y a presiones (P) de 2, 5 y 12 mm Hg.
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image002.jpg
Figura1
A todas las probetas nitruradas se les midió el espesor de la capa compuesta en tres zonas típicas de los dientes: en la zona externa (ZE); en la zona media (ZM); y en la zona de fondo (ZF). Además, a los dientes se les midió la microdureza superficial de la capa compuesta y se les determinó su perfil de microdureza.
3. RESULTADOS Y DISCUSIÓN
Espesor de la Capa compuesta
En las Figs. 2 y 3 se muestran los valores del espesor de la capa compuesta para la zona externa, la zona media y la zona de fondo, para nitruraciones realizadas a 570 ºC, 12 mm Hg y con tiempos de 2 y 5 h, respectivamente. En la Fig.4 se muestra la influencia de la temperatura en el espesor de esta capa (medido en la zona media ZM), en tanto que en la Fig.5 se muestra la influencia de la presión.
En las Figs.2 y 3 se puede observar que el espesor de la capa compuesta no fue uniforme a lo largo del diente: el espesor aumentó desde el fondo hacia la punta. Esta no-uniformidad del espesor se explica por la no-uniformidad del campo eléctrico a lo largo del diente, como consecuencia del efecto de cavidad que se produce entre los dientes [12]. Debido a esto, el tratamiento es menos efectivo en la zona interna de los dientes, debido a una menor densidad de corriente y, por lo tanto, de un bombardeo iónico menos efectivo. Además, al comparar las mismas Figs.2 y 3 se constata que el espesor de la capa compuesta aumentó con la duración del tratamiento de nitruración. En la Fig.5 se observa que el espesor de esta capa también aumentó con la presión del tratamiento. Así, los mayores espesores de la capa compuesta se obtuvieron en la zona exterior de los dientes, para los tratamientos con mayor duración y mayor presión; en tanto que los menores espesores se obtuvieron en la zona interior, para los tratamientos de menor duración y menor presión. Por otro lado, no se encontró que el tamaño de los dientes (Figs.2-5) ni la temperatura del proceso (Fig.4) tuvieran una influencia significativa en el espesor de la capa.
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image003.gif
Figura 2:Espesor de la capa compuesta en las distintas. T=570 ºC, P=12 mm Hg, t=2 h
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image004.gif
Figura 3:Espesor de la capa compuesta en las distintas.
T=570 ºC, P=12 mm Hg, t=5 h
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image005.gif
Figura 4. Influencia de la temperatura en el espesor de la capa compuesta: P=12 mm Hg, t=5 h, ZM
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image006.gif
Figura 5. Influencia de la presión en el espesor de la capa compuesta: T=570 ºC, t=5 h, ZM
Composición de la capa compuesta
En relación con la composición de la capa compuesta, en la Fig.6 se muestra el espectro de rayos X obtenidos de una muestra nitrurada a 525 ºC, durante 3 h a 12 mm Hg. En la misma figura se han indicado los distintos nitruros que normalmente aparecen en este tipo de capa. Claramente, en el caso de este estudio, en la capa compuesta predominó el nitruro Fe3N. Este resultado sería una consecuencia de la composición del plasma utilizado, puesto que, por ejemplo, al nitrurar este mismo acero (SAE 4140) en plasmas de sólo nitrógeno más hidrógeno, también se forma el nitruro Fe4N [5 -11-14]. Por otra parte, en la misma Fig.6 se observa que en la capa compuesta apareció cromita (FeCr2O4). La formación de este compuesto podría haber sido evitada, si las probetas hubieran sido sometidas, previo a la nitruración, a una limpieza superficial, por ejemplo mediante bombardeo iónico con argón. La importancia de llegar a obtener capas compuestas monofásica radica en que este tipo de capas son menos frágiles que las típicas capas polifásicas [13].
4. CONCLUSIONES
Al nitrurar dientes de engranajes de acero SAE 4140 con una mezcla de 80%N2+19%H2+1%CH4, a temperaturas entre 480 y 570 ºC, por tiempos entre 2 y 5 h, y a presiones entre 2 y 12 mm Hg, se deducen las siguientes conclusiones:
- El espesor de la capa compuesta no es uniforme a lo largo del diente. Las capas más gruesas tienen un espesor de aproximadamente 30 mm en la zona del fondo y de 45 mm en la zona exterior. Los mayores espesores se obtienen para las mayores duraciones y las mayores presiones del tratamiento.
- La máxima microdureza de la capa compuesta es de 1000-1100 HV, y se obtiene a las mayores temperaturas, a los mayores tiempos y a mayores presiones.
- En la capa compuesta se detectó predominan-temente la presencia del nitruro Fe3N, lo cual abre la posibilidad de obtener capas monofásicas, con la consiguiente reducción de su fragilidad.
- La microdureza siempre aumenta con el tiempo y con la temperatura (hasta un cierto valor máximo).
ALONZO DAVID LINARES MORA
COMUNICACINES DE RADIO FRECUENCIA
FUENTE DE ORIGEN:
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/
Existen varios tratamientos termo-químicos que, mediante la incorporación de ciertos elementos químicos, permiten mejorar el comportamiento funcional de muchos componentes mecánicos de acero. Con esta incorporación, normalmente se genera una capa superficial de dureza mucho mayor que la del material original, lo que permite mejorar la resistencia tanto a la fatiga como al desgaste del componente. Entre los tratamientos más conocidos utilizados con este fin están: cementación, que incorpora carbono; nitruración, que incorpora nitrógeno; y carbonitruración, que incorpora carbono y nitrógeno [1].
En el caso de la nitruración, el tratamiento generalmente se realiza ya sea en atmósferas gaseosas a base de amoníaco disociado, o en sales fundidas a base de cianuro/cianato [2]. El uso de plasma como atmósfera nitrurante es una tecnología desarrollada en Alemania a comienzos de la década del 30 [3], pero cuya utilización industrial comenzó sólo en la década del 70. En Chile, esta tecnología fue implementada experimentalmente en la Universidad de Chile, a comienzo de la década del 80 [4], y a la fecha existe en el país cierta capacidad instalada para efectuar tratamientos de nitruración iónica a escala semi-industrial [5].
Para realizar la nitruración por plasma se necesita, básicamente, el siguiente equipamiento: un recipiente de vacío con un sistema de bombeo que permita obtener una presión entre
1-10 mm Hg, un suministro de gases, una unidad de potencia eléctrica para producir una descarga luminosa (glow-discharge), y un sistema de control del proceso. El proceso consiste, fundamentalmente, en ionizar una atmósfera gaseosa a baja presión que contenga nitrógeno, mediante la aplicación de un alto voltaje entre un ánodo, que puede ser el cuerpo del recipiente de vacío, y la pieza a nitrurar, conectada como cátodo. En el caso de piezas relativamente pequeñas, sólo con el calentamiento provocado por el bombardeo iónico, la pieza alcanza la temperatura adecuada para el proceso, la que normalmente es inferior a los 600 ºC. En caso de piezas grandes, el calentamiento debe ser asistido por algún sistema de potencia adicional.
En general, las atmósferas que más se han utilizado en nitruración iónica han sido las mezclas de nitrógeno e hidrógeno [4-11]. Según Edenhofer [13], el proceso se basa en la formación en la superficie de la pieza, del nitruro FeN, el que por ser inestable a la temperatura del proceso, se transforma en nitruros de menor contenido de nitrógeno (Fe2-3N, Fe4N). De esta forma se genera nitrógeno monoatómico, el que en parte difunde hacia el interior de la pieza, para formar nitruros con algunos elementos de aleación del acero (Cr, Al, V, W, Mo), y en parte difunde hacia el plasma.
Algunos autores han observado que la composición de la atmósfera del plasma tiene influencia en los resultados del tratamiento de nitruración. Por ejemplo, al utilizar mezclas de 25%N2+75%H2 existe formación de capa compuesta, en tanto que con mezclas 2%N2+98%H2 esta capa no se forma, [4]. Por otro lado, con estas atmósferas de sólo nitrógeno más hidrógeno, se produce algún grado de descarburi-zación [10] por la formación de CH1-3 [15], lo que hace disminuir la microdureza en la superficie de las piezas nitruradas [4 -7].
En este trabajo se utilizó un plasma de nitrógeno, hidrógeno y metano, a fin de disminuir el efecto descarburizante de los plasmas de sólo nitrógeno más hidrógeno. Se estudió la influencia que dicho plasma tiene tanto en el espesor de la capa compuesta como en la dureza de dientes de engranajes.
2. DESARROLLO EXPERIMENTAL
Probetas de acero SAE 4140 (0.4%C-1%Cr-0.65%Mn-0.2%Mo), bonificado por temple y con una microdureza entre 320 y 340 HV, fueron nitruradas iónicamente en una atmósfera de 80%N2+19%H2+1%CH4. Las probetas se construyeron de barras de 14 mm de diámetro y 40-45 mm de largo, a cada una de las cuales se les mecanizó tres dientes de cremallera (Fig.1) de distinto tamaño: de módulos (M) 1, 2 y 3 mm. La nitruración iónica se realizó con corriente continua, a las temperaturas (T) de 480, 525 y 570 °C, por tiempos (t) de 2, 3 y 5 h, y a presiones (P) de 2, 5 y 12 mm Hg.
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image002.jpg
Figura1
A todas las probetas nitruradas se les midió el espesor de la capa compuesta en tres zonas típicas de los dientes: en la zona externa (ZE); en la zona media (ZM); y en la zona de fondo (ZF). Además, a los dientes se les midió la microdureza superficial de la capa compuesta y se les determinó su perfil de microdureza.
3. RESULTADOS Y DISCUSIÓN
Espesor de la Capa compuesta
En las Figs. 2 y 3 se muestran los valores del espesor de la capa compuesta para la zona externa, la zona media y la zona de fondo, para nitruraciones realizadas a 570 ºC, 12 mm Hg y con tiempos de 2 y 5 h, respectivamente. En la Fig.4 se muestra la influencia de la temperatura en el espesor de esta capa (medido en la zona media ZM), en tanto que en la Fig.5 se muestra la influencia de la presión.
En las Figs.2 y 3 se puede observar que el espesor de la capa compuesta no fue uniforme a lo largo del diente: el espesor aumentó desde el fondo hacia la punta. Esta no-uniformidad del espesor se explica por la no-uniformidad del campo eléctrico a lo largo del diente, como consecuencia del efecto de cavidad que se produce entre los dientes [12]. Debido a esto, el tratamiento es menos efectivo en la zona interna de los dientes, debido a una menor densidad de corriente y, por lo tanto, de un bombardeo iónico menos efectivo. Además, al comparar las mismas Figs.2 y 3 se constata que el espesor de la capa compuesta aumentó con la duración del tratamiento de nitruración. En la Fig.5 se observa que el espesor de esta capa también aumentó con la presión del tratamiento. Así, los mayores espesores de la capa compuesta se obtuvieron en la zona exterior de los dientes, para los tratamientos con mayor duración y mayor presión; en tanto que los menores espesores se obtuvieron en la zona interior, para los tratamientos de menor duración y menor presión. Por otro lado, no se encontró que el tamaño de los dientes (Figs.2-5) ni la temperatura del proceso (Fig.4) tuvieran una influencia significativa en el espesor de la capa.
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image003.gif
Figura 2:Espesor de la capa compuesta en las distintas. T=570 ºC, P=12 mm Hg, t=2 h
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image004.gif
Figura 3:Espesor de la capa compuesta en las distintas.
T=570 ºC, P=12 mm Hg, t=5 h
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image005.gif
Figura 4. Influencia de la temperatura en el espesor de la capa compuesta: P=12 mm Hg, t=5 h, ZM
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/10168_arquivos/image006.gif
Figura 5. Influencia de la presión en el espesor de la capa compuesta: T=570 ºC, t=5 h, ZM
Composición de la capa compuesta
En relación con la composición de la capa compuesta, en la Fig.6 se muestra el espectro de rayos X obtenidos de una muestra nitrurada a 525 ºC, durante 3 h a 12 mm Hg. En la misma figura se han indicado los distintos nitruros que normalmente aparecen en este tipo de capa. Claramente, en el caso de este estudio, en la capa compuesta predominó el nitruro Fe3N. Este resultado sería una consecuencia de la composición del plasma utilizado, puesto que, por ejemplo, al nitrurar este mismo acero (SAE 4140) en plasmas de sólo nitrógeno más hidrógeno, también se forma el nitruro Fe4N [5 -11-14]. Por otra parte, en la misma Fig.6 se observa que en la capa compuesta apareció cromita (FeCr2O4). La formación de este compuesto podría haber sido evitada, si las probetas hubieran sido sometidas, previo a la nitruración, a una limpieza superficial, por ejemplo mediante bombardeo iónico con argón. La importancia de llegar a obtener capas compuestas monofásica radica en que este tipo de capas son menos frágiles que las típicas capas polifásicas [13].
4. CONCLUSIONES
Al nitrurar dientes de engranajes de acero SAE 4140 con una mezcla de 80%N2+19%H2+1%CH4, a temperaturas entre 480 y 570 ºC, por tiempos entre 2 y 5 h, y a presiones entre 2 y 12 mm Hg, se deducen las siguientes conclusiones:
- El espesor de la capa compuesta no es uniforme a lo largo del diente. Las capas más gruesas tienen un espesor de aproximadamente 30 mm en la zona del fondo y de 45 mm en la zona exterior. Los mayores espesores se obtienen para las mayores duraciones y las mayores presiones del tratamiento.
- La máxima microdureza de la capa compuesta es de 1000-1100 HV, y se obtiene a las mayores temperaturas, a los mayores tiempos y a mayores presiones.
- En la capa compuesta se detectó predominan-temente la presencia del nitruro Fe3N, lo cual abre la posibilidad de obtener capas monofásicas, con la consiguiente reducción de su fragilidad.
- La microdureza siempre aumenta con el tiempo y con la temperatura (hasta un cierto valor máximo).
ALONZO DAVID LINARES MORA
COMUNICACINES DE RADIO FRECUENCIA
FUENTE DE ORIGEN:
http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10168/
Effects of N Incorporation on the Electronic Properties of GaAsN-Based Modulation-Doped Heterostructures
Introduction:
Dilute nitride (In)GaAsN alloys are useful for applications in infrared laser diodes, high-efficiency solar cells, and high-performance heterojunction bipolar transistors. The addition of N to GaAs-based semiconductors decreases the bandgap energy without drastically affecting the lattice parameter. To date, literature reports have presented substantially lower electron mobilities for dilute nitride semiconductor alloys in comparison with those of (In)GaAs. Furthermore, for (In)GaAsN alloys, the electron mobility has been reported to decrease as the N incorporation increases. At present, the precise role of N in reducing the electron mobility is not well understood. In order to study nitrogen-related electron scattering effects in GaAsN, with minimal contributions from ionized impurity scattering, we are examining the transport properties of modulation-doped AlGaAs/GaAs(N) heterostructures, with Si dopants in the AlGaAs barrier layer spatially separated from the nominally undoped GaAs(N) channel layer. We will discuss the dependence of the mobility on carrier density, with a focus on the insulating phenomena associated with multiple scattering effects past a critical carrier density.
Experimental Procedure:
The heterostructures consisted of modulation-doped AlGaAs/GaAs(N) heterostructures grown via molecular beam epitaxy (MBE), on GaAs (001) substrates. An initial 500 nm thick GaAs buffer layer was grown at 580°C. After buffer layer growth, a 50 nm thick GaAs(N) channel was grown at 400°C. Next, a 5 minute pause was used to ramp the substrate temperature to 580°C, and layers of 1 nm GaAs, 20 nm undoped Al0.3Ga0.7As, 60 nm Si-doped Al0.3Ga0.7As, and 10 nm GaAs were then grown in succession. Carriers from the Si-doped AlGaAs layer migrated into the GaAs(N) channel and were confined in a triangular well, producing a two dimensional electron
The undoped AlGaAs layer enabled spatial-separation of the 2DEG and the ionized impurities, thereby minimizing the Coulombic long-range scattering effects on the carriers [2]. The density of free carriers in the channel was further controlled by front gating, or by illumination at low temperature.
Electron transport measurements were implemented with eight-arm gated-Hall bars (1050 x 150 µm), fabricated by standard contact photolithographic processes, with e-beam evaporated Ni/Ge/Au/Ti/Au (200/325/650/200/2000 Å) contacts and Ti/Au (100/
1000 Å) gates. The mesa and contacts/gate were defined using positive and negative photolithography and a phosphoric acid etch [6]. Prior to gate deposition, the contacts were annealed at 410ºC for 2 minutes in argon gas.
Figure 1: Magnetic field dependence of rxy and rxx,
at T = 4.2K for a gate-controlled 2DEG.
http://photos-a.ak.fbcdn.net/hphotos-ak-snc3/hs082.snc3/15024_1359735027977_1069579604_1067442_3896747_n.jpg
Figura1
Magnetoresistance measurements were performed at 4.2K, with the magnetic field swept from 0 to 7 Tesla in a superconducting NbTi magnet. To modulate the free carrier density in the 2DEG, the gate voltage was swept from -60 mV to 150 mV, and data were collected every 10 mV. A near-infrared light emitting diode was also used to illuminate the sample surface, to increase the free carrier density in the 2DEG, through the persistent photoconductivity effect [3].
Results and Conclusions:
In Figure 1(a), the minima of the Shubnikov-de Haas oscillations in the magnetoresistance data correspond to the Quantum Hall plateaus in Figure 1(b). Both of these quantum phenomena result from the increasing magnetic field altering the spacing between the Landau levels, thus sweeping the Landau levels with respect to the Fermi level. As the gate voltage decreases from 150 mV to -30 mV, the carrier density (resistivity) decreases (increases), and the Shubnikov de Haas oscillations and quantum Hall plateaus become less apparent, as shown in Figure 1.
From the gated resistivity and Hall effect measurements, we calculated the carrier density and carrier mobility at each gate voltage, shown in Figure 2. One characteristic of the relation between carrier density and mobility was the metal-insulator transition behavior due to multiple scattering effects at carrier densities below a critical carrier density (Nc). Based on the structure and doping level of our 2DEG, we predicted our critical carrier density to be around 7 x 1010cm-2 [4]. The gated 2DEG data exhibited a deviation from power law dependence, recognized as the critical carrier density, at about 9 x 1010cm-2 as shown in Figure 2.
http://photos-g.ak.fbcdn.net/hphotos-ak-snc3/hs082.snc3/15024_1359739308084_1069579604_1067448_6530222_s.jpg
Figure2
The relationship between carrier density and carrier mobility for densities above 9 x 1010cm-2 reveals information about the mechanisms of electron scattering by N atoms. The dependence of mobility on carrier density can be expressed as µ~na [5]. The a values correspond to the slope of a linear-least squares fit to log (µ) vs. log (n). In the control 2DEG, a~1, suggesting remote ionized impurity scattering due to Coulombic interactions between the free carriers and the ionized impurities, is the dominant scattering mechanism. The mobility increases rapidly with n, due to increased screening of the ionized impurity potential. For the 0.08%N 2DEG, a~0.1, indicating ionized long-range scattering is likely not the dominant mechanism. The increased carrier density does not have the same screening effect on neutral scatterers, and therefore does not increase the mobility. For n
> Nc, µ is independent of n, similar to calculations which assume N acts as a neutral independent local scatterer [1], as shown by the line in Figure 2. Thus, we tentatively conclude that N acts as a neutral, short-range scatterer.
Acknowledgements:
We gratefully acknowledge the National Science Foundation through grant #DMR-0606406 and the NNIN Grant ECS-0335765, as well as the Goldman and Kurdak Groups and the MNF at the University of Michigan.
ALONZO DAVID LINARES MORA
COMUNICACIONES DE RADIO FRECUENCIA
FUENTE DE ORIGEN:
http://www.nnin.org/doc/NNINreu06Mangan.pdf
Dilute nitride (In)GaAsN alloys are useful for applications in infrared laser diodes, high-efficiency solar cells, and high-performance heterojunction bipolar transistors. The addition of N to GaAs-based semiconductors decreases the bandgap energy without drastically affecting the lattice parameter. To date, literature reports have presented substantially lower electron mobilities for dilute nitride semiconductor alloys in comparison with those of (In)GaAs. Furthermore, for (In)GaAsN alloys, the electron mobility has been reported to decrease as the N incorporation increases. At present, the precise role of N in reducing the electron mobility is not well understood. In order to study nitrogen-related electron scattering effects in GaAsN, with minimal contributions from ionized impurity scattering, we are examining the transport properties of modulation-doped AlGaAs/GaAs(N) heterostructures, with Si dopants in the AlGaAs barrier layer spatially separated from the nominally undoped GaAs(N) channel layer. We will discuss the dependence of the mobility on carrier density, with a focus on the insulating phenomena associated with multiple scattering effects past a critical carrier density.
Experimental Procedure:
The heterostructures consisted of modulation-doped AlGaAs/GaAs(N) heterostructures grown via molecular beam epitaxy (MBE), on GaAs (001) substrates. An initial 500 nm thick GaAs buffer layer was grown at 580°C. After buffer layer growth, a 50 nm thick GaAs(N) channel was grown at 400°C. Next, a 5 minute pause was used to ramp the substrate temperature to 580°C, and layers of 1 nm GaAs, 20 nm undoped Al0.3Ga0.7As, 60 nm Si-doped Al0.3Ga0.7As, and 10 nm GaAs were then grown in succession. Carriers from the Si-doped AlGaAs layer migrated into the GaAs(N) channel and were confined in a triangular well, producing a two dimensional electron
The undoped AlGaAs layer enabled spatial-separation of the 2DEG and the ionized impurities, thereby minimizing the Coulombic long-range scattering effects on the carriers [2]. The density of free carriers in the channel was further controlled by front gating, or by illumination at low temperature.
Electron transport measurements were implemented with eight-arm gated-Hall bars (1050 x 150 µm), fabricated by standard contact photolithographic processes, with e-beam evaporated Ni/Ge/Au/Ti/Au (200/325/650/200/2000 Å) contacts and Ti/Au (100/
1000 Å) gates. The mesa and contacts/gate were defined using positive and negative photolithography and a phosphoric acid etch [6]. Prior to gate deposition, the contacts were annealed at 410ºC for 2 minutes in argon gas.
Figure 1: Magnetic field dependence of rxy and rxx,
at T = 4.2K for a gate-controlled 2DEG.
http://photos-a.ak.fbcdn.net/hphotos-ak-snc3/hs082.snc3/15024_1359735027977_1069579604_1067442_3896747_n.jpg
Figura1
Magnetoresistance measurements were performed at 4.2K, with the magnetic field swept from 0 to 7 Tesla in a superconducting NbTi magnet. To modulate the free carrier density in the 2DEG, the gate voltage was swept from -60 mV to 150 mV, and data were collected every 10 mV. A near-infrared light emitting diode was also used to illuminate the sample surface, to increase the free carrier density in the 2DEG, through the persistent photoconductivity effect [3].
Results and Conclusions:
In Figure 1(a), the minima of the Shubnikov-de Haas oscillations in the magnetoresistance data correspond to the Quantum Hall plateaus in Figure 1(b). Both of these quantum phenomena result from the increasing magnetic field altering the spacing between the Landau levels, thus sweeping the Landau levels with respect to the Fermi level. As the gate voltage decreases from 150 mV to -30 mV, the carrier density (resistivity) decreases (increases), and the Shubnikov de Haas oscillations and quantum Hall plateaus become less apparent, as shown in Figure 1.
From the gated resistivity and Hall effect measurements, we calculated the carrier density and carrier mobility at each gate voltage, shown in Figure 2. One characteristic of the relation between carrier density and mobility was the metal-insulator transition behavior due to multiple scattering effects at carrier densities below a critical carrier density (Nc). Based on the structure and doping level of our 2DEG, we predicted our critical carrier density to be around 7 x 1010cm-2 [4]. The gated 2DEG data exhibited a deviation from power law dependence, recognized as the critical carrier density, at about 9 x 1010cm-2 as shown in Figure 2.
http://photos-g.ak.fbcdn.net/hphotos-ak-snc3/hs082.snc3/15024_1359739308084_1069579604_1067448_6530222_s.jpg
Figure2
The relationship between carrier density and carrier mobility for densities above 9 x 1010cm-2 reveals information about the mechanisms of electron scattering by N atoms. The dependence of mobility on carrier density can be expressed as µ~na [5]. The a values correspond to the slope of a linear-least squares fit to log (µ) vs. log (n). In the control 2DEG, a~1, suggesting remote ionized impurity scattering due to Coulombic interactions between the free carriers and the ionized impurities, is the dominant scattering mechanism. The mobility increases rapidly with n, due to increased screening of the ionized impurity potential. For the 0.08%N 2DEG, a~0.1, indicating ionized long-range scattering is likely not the dominant mechanism. The increased carrier density does not have the same screening effect on neutral scatterers, and therefore does not increase the mobility. For n
> Nc, µ is independent of n, similar to calculations which assume N acts as a neutral independent local scatterer [1], as shown by the line in Figure 2. Thus, we tentatively conclude that N acts as a neutral, short-range scatterer.
Acknowledgements:
We gratefully acknowledge the National Science Foundation through grant #DMR-0606406 and the NNIN Grant ECS-0335765, as well as the Goldman and Kurdak Groups and the MNF at the University of Michigan.
ALONZO DAVID LINARES MORA
COMUNICACIONES DE RADIO FRECUENCIA
FUENTE DE ORIGEN:
http://www.nnin.org/doc/NNINreu06Mangan.pdf
Introduction to Dilute Nitrides
Arsenide and phosphide based III-V semiconductors have been extensively researched for many years, resulting in mature technologies that are used for many of today's electronics and optoelectronics. These materials are however limited to medium and narrow bandgap applications, preventing the fabrication of wide gap devices such as high power electronics and short wavelength optoelectronics. The development of wide-gap material has been slow and problematic, nevertheless, recent advances in nitrides, such as the development of blue LEDs and lasers, have seen these alloys catapulted into the limelight. This has lead to an explosion of research interest, and the natural progression to bridge the gap between the wide-gap nitrides and the medium-gap arsenides. However, instead of the addition of small amounts of nitrogen to GaAs resulting in the expected increase of bandgap, it was found to have the opposite effect, resulting in a rapid reduction of the bandgap.[1] This unusual behaviour has sparked considerable interest both from a fundamental physics point of view as well as for potential narrow-gap applications. These new hybrid alloys, which have become known as "dilute nitrides", are the topic of this current work.
In the late eighties interest in wide-gap materials for short wavelength optoelectronics (and electronics) began to grow, most research concentrated on zinc selenide (ZnSe) although some work was also carried out on nitrides and silicon carbide (SiC). Despite the indirect bandgap of SiC, it was used in the late eighties to produce the first commercial blue LEDs; unfortunately, the indirect gap meant low efficiencies (~0.03 % [2]) and no scope for producing lasers. Instead, ZnSe-based materials were pursued, leading to the first pulsed blue laser diode in 1991 [3]. Despite subsequent advances that lead to continuous wave room temperature lasing, ZnSe was problematic. Unlike SiC and nitrides that have wide-gaps resulting from strong chemical bonds, ZnSe's gap is due to its highly ionic bonds that are weak. As a result, defect growth and propagation is common, leading to device degradation. In addition, ZnSe suffers from poor electrical/thermal properties and Fermi level pinning.
Nitride research dates back to the early seventies [4] when many physical properties such as refractive index,[5] bandgap [6] and lattice constant [7] were measured. However, interest soon diminished, as it was discover that the strong background n-type doping prevented the growth of p-type material and no suitable substrate materials were available. Interest in nitrides was rekindled in the early nineties when a small research group led by Nakamura at a virtually unheard of chemical company in Japan reported blue emission from InGaN devices. The application of innovative growth techniques to the nitride system led to rapid advances in material quality and commercially viable devices. The problems associated with substrate mismatch were overcome using buffer layers,[8, 9] and background doping was reduced via optimised growth. The key breakthrough of p-doping was finally achieved using magnesium dopant activated by electron beam irradiation [10] or thermal annealing.[11] This allowed control over both n and p-type doping, hence nitride based p-n junctions could be produced and subsequently advance heterostructures LEDs and lasers.
http://photos-c.ak.fbcdn.net/hphotos-ak-snc3/hs082.snc3/15024_1359727747795_1069579604_1067434_526700_n.jpg
Figura 1
The first steps towards bridging the gap between arsenides and nitrides was made in 1992 by Weyers et.al.,[1] resulting in the unexpected discovery of a rapid reduction in bandgap energy with increasing nitrogen. In most, if not all other, III-V alloys, when a semiconductor element is replaced by one of smaller ionic radius, the band gap energy increases. When small amounts of nitrogen were added to GaAs, instead of the expected blue shift in emission, a considerable red shift was observed.[1] Furthermore, the decrease in energy per atomic percent of nitrogen was more than ten times greater than the typical increase in other semiconductor alloys. While this behaviour meant that the dilute nitrides could not be used for visible light emitters, fundamental physics as well as potential applications were clear incentives to pursue these materials. Surprisingly, the next few years saw very little published work on dilute nitrides.
Interest in dilute nitrides really began in the mid nineties after Kondow et.al. published results on the quaternary alloy, GaInNAs.[13, 14] This new alloy allowed independent control over the In:Ga and N:As ratios. Increasing the In:Ga ratio causes a reduction in bandgap and an increase in lattice parameter, while increasing the N:As ratio also causes bandgap reduction, but a decrease in the lattice parameter. GaInNAs, therefore, gives the flexibility of tailoring both bandgap and lattice parameter. Such tailoring potential opens up a wide range of possible applications, however, the 1.3 mm lasers based on GaAs for optical communications were identified as a key application. At this wavelength silica fibre has zero dispersion and relatively low attenuation, making it an attractive communications window. Many of today's medium haul systems operate in the 1.3 mm window, and if transceiver systems could be made economical then they could be used in local area networks. The first laser to operate in the 1.3 mm window was produced in 1976 by J. Hsieh, using InGaAsP/InP,[15] since then most, if not all, 1.3 mm systems have used InP based lasers. Unfortunately, this material system is not ideal for producing cheap lasers as a number of problems ultimately increase costs. While some innovative attempts have been made to solve these problems, many are far from ideal, often causing additional problems or using processing techniques that are not easily integrated into commercial production. Instead of solving the problems, considerable research efforts are currently being put into finding alternative materials that avoid the problems altogether. InGaAs has been extensively researched and investigations have been done to see how far the material can be pushed towards long wavelength emissions. While highly strained InGaAs QWs lasers operating out to around 1.2 mm have been produced with good characteristics using strain-compensated QWs,[16, 17] increasing the wavelength further becomes very difficult. Lasers operating close to 1.3 mm based on GaAsSb QWs have been demonstrated,[18] however, the conduction band discontinuity between GaAsSb and GaAs is believed to be small [19] resulting in high leakage currents and hence poor performance.
At present, the main research drive for alternative 1.3 mm lasers is split between self-organised quantum dots (QD) and dilute nitrides. QD lasers have attracted substantial interest, as the physics of QDs could potentially improve laser performance considerably. They are expected to have reduced temperature dependence,[20] reduced thresholds and higher efficiencies.[21] However, this can only be realised if methods are found to control dot density, distribution and, most importantly, size. Even small size fluctuations can 'smear' the density of states, producing bulk-like behaviour. While considerable progress has been made towards dot uniformity, there appears to be some trade off with dot density, hence gain.[22] In addition, the expected temperature performance of QD lasers has not yet been demonstrated, so far devices have exhibited poor characteristic temperatures no better than InP lasers.[23]
Considerable interest in dilute nitrides for long wavelength lasers was stimulated by the first reported GaInNAs lasers.[13] The materials were expected to have very good temperature characteristics resulting from the predicted large electron confinement. In addition, being based on GaAs, dilute nitrides would be easily integrated with the established GaAs technology, including AlGaAs based Bragg mirrors. However, producing high quality dilute nitride has proved difficult, with increasing nitrogen fraction resulted in reduced optical quality.[13, 24-28] Post-growth annealing was soon found to improve the optical quality, however, the increase in optical quality was usually accompanied by a blue shift,[29] which partly offset the red shift induced by nitrogen incorporation. Despite these problems, early research lead to the demonstration of a range of devices and a gradual improvement in material quality.[30] Progress in dilute nitride devices has been rapid, 1.3 mm laser emission was realised in 1998,[31] and good characteristic temperatures have been reported.[32, 33] The emission wavelength has even been pushed out to 1.515 mm using GaInNAs with high indium and nitrogen fractions.[34, 35] Unfortunately, many early lasers had high thresholds and low slope efficiencies compared to nitrogen-free lasers. These problems have largely been attributed to poor crystal quality, which is hardly surprising in a new material system. However, considerable improvements have been reported recently, with some 1.3 mm single and triple quantum well lasers having thresholds as low as 400 and 680 A/cm2 respectively.[36] A number of dilute nitride VCSELs have also been successfully fabricated, the first in 1997 operated at 1.22 mm under optical pumping,[37] however, it was not long before an electrically pumped version was demonstrated.[38] Subsequently, VCSELs operating close to 1.3 mm were demonstrated, first optically pumped [39, 40] and then electrically pumped.[41] In addition to lasers, dilute nitrides have been used in a number of other devices including:
• Resonant cavity enhanced (RCE) photodetectors,[42, 43]
• Electro-absorption (e/a) modulators,[44]
• Solar cells,[45-47]
• Heterojunction bipolar transistors (HBTs).[41, 48-52]
ALONZO DAVID LINARES MORA
COMUNICACIONES DE RADIO FRECUENCIA
In the late eighties interest in wide-gap materials for short wavelength optoelectronics (and electronics) began to grow, most research concentrated on zinc selenide (ZnSe) although some work was also carried out on nitrides and silicon carbide (SiC). Despite the indirect bandgap of SiC, it was used in the late eighties to produce the first commercial blue LEDs; unfortunately, the indirect gap meant low efficiencies (~0.03 % [2]) and no scope for producing lasers. Instead, ZnSe-based materials were pursued, leading to the first pulsed blue laser diode in 1991 [3]. Despite subsequent advances that lead to continuous wave room temperature lasing, ZnSe was problematic. Unlike SiC and nitrides that have wide-gaps resulting from strong chemical bonds, ZnSe's gap is due to its highly ionic bonds that are weak. As a result, defect growth and propagation is common, leading to device degradation. In addition, ZnSe suffers from poor electrical/thermal properties and Fermi level pinning.
Nitride research dates back to the early seventies [4] when many physical properties such as refractive index,[5] bandgap [6] and lattice constant [7] were measured. However, interest soon diminished, as it was discover that the strong background n-type doping prevented the growth of p-type material and no suitable substrate materials were available. Interest in nitrides was rekindled in the early nineties when a small research group led by Nakamura at a virtually unheard of chemical company in Japan reported blue emission from InGaN devices. The application of innovative growth techniques to the nitride system led to rapid advances in material quality and commercially viable devices. The problems associated with substrate mismatch were overcome using buffer layers,[8, 9] and background doping was reduced via optimised growth. The key breakthrough of p-doping was finally achieved using magnesium dopant activated by electron beam irradiation [10] or thermal annealing.[11] This allowed control over both n and p-type doping, hence nitride based p-n junctions could be produced and subsequently advance heterostructures LEDs and lasers.
http://photos-c.ak.fbcdn.net/hphotos-ak-snc3/hs082.snc3/15024_1359727747795_1069579604_1067434_526700_n.jpg
Figura 1
The first steps towards bridging the gap between arsenides and nitrides was made in 1992 by Weyers et.al.,[1] resulting in the unexpected discovery of a rapid reduction in bandgap energy with increasing nitrogen. In most, if not all other, III-V alloys, when a semiconductor element is replaced by one of smaller ionic radius, the band gap energy increases. When small amounts of nitrogen were added to GaAs, instead of the expected blue shift in emission, a considerable red shift was observed.[1] Furthermore, the decrease in energy per atomic percent of nitrogen was more than ten times greater than the typical increase in other semiconductor alloys. While this behaviour meant that the dilute nitrides could not be used for visible light emitters, fundamental physics as well as potential applications were clear incentives to pursue these materials. Surprisingly, the next few years saw very little published work on dilute nitrides.
Interest in dilute nitrides really began in the mid nineties after Kondow et.al. published results on the quaternary alloy, GaInNAs.[13, 14] This new alloy allowed independent control over the In:Ga and N:As ratios. Increasing the In:Ga ratio causes a reduction in bandgap and an increase in lattice parameter, while increasing the N:As ratio also causes bandgap reduction, but a decrease in the lattice parameter. GaInNAs, therefore, gives the flexibility of tailoring both bandgap and lattice parameter. Such tailoring potential opens up a wide range of possible applications, however, the 1.3 mm lasers based on GaAs for optical communications were identified as a key application. At this wavelength silica fibre has zero dispersion and relatively low attenuation, making it an attractive communications window. Many of today's medium haul systems operate in the 1.3 mm window, and if transceiver systems could be made economical then they could be used in local area networks. The first laser to operate in the 1.3 mm window was produced in 1976 by J. Hsieh, using InGaAsP/InP,[15] since then most, if not all, 1.3 mm systems have used InP based lasers. Unfortunately, this material system is not ideal for producing cheap lasers as a number of problems ultimately increase costs. While some innovative attempts have been made to solve these problems, many are far from ideal, often causing additional problems or using processing techniques that are not easily integrated into commercial production. Instead of solving the problems, considerable research efforts are currently being put into finding alternative materials that avoid the problems altogether. InGaAs has been extensively researched and investigations have been done to see how far the material can be pushed towards long wavelength emissions. While highly strained InGaAs QWs lasers operating out to around 1.2 mm have been produced with good characteristics using strain-compensated QWs,[16, 17] increasing the wavelength further becomes very difficult. Lasers operating close to 1.3 mm based on GaAsSb QWs have been demonstrated,[18] however, the conduction band discontinuity between GaAsSb and GaAs is believed to be small [19] resulting in high leakage currents and hence poor performance.
At present, the main research drive for alternative 1.3 mm lasers is split between self-organised quantum dots (QD) and dilute nitrides. QD lasers have attracted substantial interest, as the physics of QDs could potentially improve laser performance considerably. They are expected to have reduced temperature dependence,[20] reduced thresholds and higher efficiencies.[21] However, this can only be realised if methods are found to control dot density, distribution and, most importantly, size. Even small size fluctuations can 'smear' the density of states, producing bulk-like behaviour. While considerable progress has been made towards dot uniformity, there appears to be some trade off with dot density, hence gain.[22] In addition, the expected temperature performance of QD lasers has not yet been demonstrated, so far devices have exhibited poor characteristic temperatures no better than InP lasers.[23]
Considerable interest in dilute nitrides for long wavelength lasers was stimulated by the first reported GaInNAs lasers.[13] The materials were expected to have very good temperature characteristics resulting from the predicted large electron confinement. In addition, being based on GaAs, dilute nitrides would be easily integrated with the established GaAs technology, including AlGaAs based Bragg mirrors. However, producing high quality dilute nitride has proved difficult, with increasing nitrogen fraction resulted in reduced optical quality.[13, 24-28] Post-growth annealing was soon found to improve the optical quality, however, the increase in optical quality was usually accompanied by a blue shift,[29] which partly offset the red shift induced by nitrogen incorporation. Despite these problems, early research lead to the demonstration of a range of devices and a gradual improvement in material quality.[30] Progress in dilute nitride devices has been rapid, 1.3 mm laser emission was realised in 1998,[31] and good characteristic temperatures have been reported.[32, 33] The emission wavelength has even been pushed out to 1.515 mm using GaInNAs with high indium and nitrogen fractions.[34, 35] Unfortunately, many early lasers had high thresholds and low slope efficiencies compared to nitrogen-free lasers. These problems have largely been attributed to poor crystal quality, which is hardly surprising in a new material system. However, considerable improvements have been reported recently, with some 1.3 mm single and triple quantum well lasers having thresholds as low as 400 and 680 A/cm2 respectively.[36] A number of dilute nitride VCSELs have also been successfully fabricated, the first in 1997 operated at 1.22 mm under optical pumping,[37] however, it was not long before an electrically pumped version was demonstrated.[38] Subsequently, VCSELs operating close to 1.3 mm were demonstrated, first optically pumped [39, 40] and then electrically pumped.[41] In addition to lasers, dilute nitrides have been used in a number of other devices including:
• Resonant cavity enhanced (RCE) photodetectors,[42, 43]
• Electro-absorption (e/a) modulators,[44]
• Solar cells,[45-47]
• Heterojunction bipolar transistors (HBTs).[41, 48-52]
ALONZO DAVID LINARES MORA
COMUNICACIONES DE RADIO FRECUENCIA
domingo, 21 de marzo de 2010
Dilute nitrides tailor the wavelength of semiconductor disk lasers
Mircea Guina, Jussi Rautiainen, Pietari Tuomisto, Ville-Markus Korpijärvi, Antti Härkönen, and Oleg Okhotnikov
A newly developed gain region enables high-brightness orange-red laser radiation.
18 June 2008, SPIE Newsroom. DOI: 10.1117/2.1200806.1177
Interest is growing rapidly in efficient and compact optically pumped semiconductor disk lasers (OP-SDLs) since they provide a practical solution for generating high-power radiation at visible wavelengths. In particular, red-green-blue SDLs are thought to satisfy the demanding requirements of laser displays and projectors. Other applications, such as spectroscopy and biomedicine, are also expected to benefit greatly from the wavelength versatility and excellent beam quality offered by OP-SDLs. Also known as vertical external cavity surface-emitting lasers,1,2 OP-SDLs combine many of the advantages of traditional solid-state lasers with the versatility offered by semiconductor gain materials. Such laser sources can deliver Watt-level diffraction-limited output beams in a broad spectral range determined by the gain material.
The key element of an OP-SDL is the semiconductor gain mirror, which is placed in an external laser cavity configuration, as shown in Figure 1. This arrangement allows a nonlinear crystal to be placed into the cavity for frequency conversion. The most notable results demonstrated with visible SDLs have been obtained in the blue-green region by frequency doubling 940nm and 1060nm laser radiation.3 However, the development of frequency-doubled OP-SDLs at red-orange wavelengths has been hindered by the lack of suitable semiconductor materials for fabricating high-quality gain mirrors operating at around 1200–1250nm. To tackle this problem, we have developed gallium indium nitride arsenide / gallium arsenide (GaInNAs/GaAs) gain regions that can be integrated with high-quality gallium arsenide / aluminum arsenide (GaAs/AlAs) distributed Bragg reflectors (DBRs). DBRs act as mirrors for a specific wavelength range related to the optical thickness of its constituent layers, so the GaInNAs/GaAs gain regions and GaAs/AlAs DBRs together act as a gain mirror.
http://photos-b.ak.fbcdn.net/hphotos-ak-snc3/hs102.snc3/15024_1359715387486_1069579604_1067427_7430112_n.jpg
Figure 1:Schematic of an OP-SDL with an intra-cavity heat-spreader.
GaInNAs/GaAs quantum-wells (QWs) have traditionally been used as gain regions in edge-emitting semiconductor lasers operating at around 1.3µm. As a downside for the use of dilute nitride materials, it should be noted that incorporation of nitrogen (N) within the GaInAs lattice leads to the creation of non-radiative recombination centers that have a detrimental effect on the laser efficiency, ultimately limiting the maximum power. The power scaling depends largely on the efficiency of heat removal from the gain region. This is commonly achieved by using a transparent high thermal conductance material, such as diamond, silicon carbide (SiC), or sapphire as a heat spreader, which is placed between the semiconductor sample and a metallic heat-sink. Our research was aimed primarily at developing high quality GaInNAs/GaAs gain structures incorporating a large number of QWs that would allow high power operation. The first trial resulted in the demonstration of 1230nm OP-SDL emitting ~1.2W of optical power at room temperature4 and >300mW of red radiation by frequency doubling.
The gain chip also has been used in a Z-shaped cavity to demonstrate efficient intracavity frequency doubling.7 The total output power emitted at 612nm reached a value of ~2.7W, yielding a conversion efficiency of 7.4% from pump to red radiation. We have also demonstrated an 8nm tuning range for the output spectrum.
http://spie.org/Images/Graphics/Newsroom/Imported/1177/1177_fig3.jpg
Figure 2. A frequency doubled orange-red OP-SDL in operation.
To conclude, our results demonstrate that GaInNAs technology has good potential for the development of high-power disk lasers operating at 1200–1300nm that are capable of efficiently generating orange-red radiation by frequency conversion. Future work will focus on improving the gain structure and the laser cavity to demonstrate higher output power and improved efficiencies.
This work was supported by EU FP6 project NATAL (IST-NMP- 016769) and the "Nanoscience and Nanophotonics" program financed by the Finnish Ministry of Education.
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A newly developed gain region enables high-brightness orange-red laser radiation.
18 June 2008, SPIE Newsroom. DOI: 10.1117/2.1200806.1177
Interest is growing rapidly in efficient and compact optically pumped semiconductor disk lasers (OP-SDLs) since they provide a practical solution for generating high-power radiation at visible wavelengths. In particular, red-green-blue SDLs are thought to satisfy the demanding requirements of laser displays and projectors. Other applications, such as spectroscopy and biomedicine, are also expected to benefit greatly from the wavelength versatility and excellent beam quality offered by OP-SDLs. Also known as vertical external cavity surface-emitting lasers,1,2 OP-SDLs combine many of the advantages of traditional solid-state lasers with the versatility offered by semiconductor gain materials. Such laser sources can deliver Watt-level diffraction-limited output beams in a broad spectral range determined by the gain material.
The key element of an OP-SDL is the semiconductor gain mirror, which is placed in an external laser cavity configuration, as shown in Figure 1. This arrangement allows a nonlinear crystal to be placed into the cavity for frequency conversion. The most notable results demonstrated with visible SDLs have been obtained in the blue-green region by frequency doubling 940nm and 1060nm laser radiation.3 However, the development of frequency-doubled OP-SDLs at red-orange wavelengths has been hindered by the lack of suitable semiconductor materials for fabricating high-quality gain mirrors operating at around 1200–1250nm. To tackle this problem, we have developed gallium indium nitride arsenide / gallium arsenide (GaInNAs/GaAs) gain regions that can be integrated with high-quality gallium arsenide / aluminum arsenide (GaAs/AlAs) distributed Bragg reflectors (DBRs). DBRs act as mirrors for a specific wavelength range related to the optical thickness of its constituent layers, so the GaInNAs/GaAs gain regions and GaAs/AlAs DBRs together act as a gain mirror.
http://photos-b.ak.fbcdn.net/hphotos-ak-snc3/hs102.snc3/15024_1359715387486_1069579604_1067427_7430112_n.jpg
Figure 1:Schematic of an OP-SDL with an intra-cavity heat-spreader.
GaInNAs/GaAs quantum-wells (QWs) have traditionally been used as gain regions in edge-emitting semiconductor lasers operating at around 1.3µm. As a downside for the use of dilute nitride materials, it should be noted that incorporation of nitrogen (N) within the GaInAs lattice leads to the creation of non-radiative recombination centers that have a detrimental effect on the laser efficiency, ultimately limiting the maximum power. The power scaling depends largely on the efficiency of heat removal from the gain region. This is commonly achieved by using a transparent high thermal conductance material, such as diamond, silicon carbide (SiC), or sapphire as a heat spreader, which is placed between the semiconductor sample and a metallic heat-sink. Our research was aimed primarily at developing high quality GaInNAs/GaAs gain structures incorporating a large number of QWs that would allow high power operation. The first trial resulted in the demonstration of 1230nm OP-SDL emitting ~1.2W of optical power at room temperature4 and >300mW of red radiation by frequency doubling.
The gain chip also has been used in a Z-shaped cavity to demonstrate efficient intracavity frequency doubling.7 The total output power emitted at 612nm reached a value of ~2.7W, yielding a conversion efficiency of 7.4% from pump to red radiation. We have also demonstrated an 8nm tuning range for the output spectrum.
http://spie.org/Images/Graphics/Newsroom/Imported/1177/1177_fig3.jpg
Figure 2. A frequency doubled orange-red OP-SDL in operation.
To conclude, our results demonstrate that GaInNAs technology has good potential for the development of high-power disk lasers operating at 1200–1300nm that are capable of efficiently generating orange-red radiation by frequency conversion. Future work will focus on improving the gain structure and the laser cavity to demonstrate higher output power and improved efficiencies.
This work was supported by EU FP6 project NATAL (IST-NMP- 016769) and the "Nanoscience and Nanophotonics" program financed by the Finnish Ministry of Education.
ALONZO DAVID LINARES MORA
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High responsivity GaNAsSb p-i-n photodetectors at 1.3μm grown by radio-frequency nitrogen plasma-assisted molecular beam epitaxy
1. Introduction
The GaNAsSb material system has attracted great interest for potential photodetector applications in the near infrared region (0.9-1.6µm) [1]. By keeping the ratio of nitrogen (N) content to antimony (Sb) content at 1.0 to 2.6, the GaNAsSb material can be tailored to lattice-match GaAs at wavelengths from 0.9ìm to 1.3µm. Compared with the incumbent InPbased technology, GaNAsSb offers significant advantage due to the use of lower cost GaAs substrate and availability of larger GaAs substrate. Achieving high photoresponsivity is one of the key challenges in GaAs-based dilute nitride photodetector research. So far, reported results of GaAs-based dilute nitride photodetectors can be categorized into two groups: one based on quantum well (QW) absorption layers [2-4] and the other one based upon bulk absorption layers [5-10]. For devices based on a quantum well absorption layer, a thin dilute nitride layer (<10nm) is used. While such QW devices enable the utilization of highly strained dilute nitride layers and thus offer a photo-response up to 1.6ìm, their photoresponsivity is generally low [2, 3] (typically less than 0.03A/W) due to the thin QW photoabsorption layer. To overcome this limitation [4], a resonant cavity has been incorporated into the device structure. On the other hand, photodetectors based on bulk dilute nitride absorption layers (>0.4ìm thick) suffer from reduced photo-response at long wavelengths. So far, the highest reported cut-off wavelength is ~1.4 ìm [10]. This is due to the difficulty in incorporating more than 3.5% of nitrogen into the material. Nevertheless, photodetectors based on bulk dilute nitride absorption layers exhibit a higher photoresponsivity compared to QW-based devices. Recently, photoresponsivities of up to about 0.1A/W have been reported for bulk GaNAsSb/GaAs devices [5, 6, 11]. These photoresponsivity values are still much lower as compared to those of commercial InGaAs photodetectors, with a typical photoresponsivity of up to ~0.9A/W at 1.3ìm.
In this paper, we report on a significant improvement in the photoresponsivity of GaNAsSb/GaAs photodetectors with a GaNAsSb bulk photoabsorption layer at 1.3ìm wavelength. The devices exhibit characteristics which strongly suggest the presence of photogenerated carrier multiplication due to the avalanche effect.
2. Technology
The device structure shown in Fig. 1 was grown using a molecular beam epitaxy (MBE) system in conjunction with a radio frequency (RF) N plasma-assisted source and a valved Sb cracker source. The i-GaNAsSb (bulk) photoabsorption layer was 0.5ìm-thick for i-GaNAsSb layer grown at 350oC and 400oC, and 2ìm-thick for the i-GaNAsSb layer grown at 440oC and 480oC. The RF nitrogen plasma power was 180W and the beam equivalent pressure (BEP) of the Sb flux was ~1×10-7 torr. Under these conditions ~3.3% of N and 8% of Sb were incorporated into the i-GaNAsSb layer, which was confirmed by x-ray diffraction (XRD). Using the band anti-crossing (BAC) model [12], the optical bandgap of the i-GaNAsSb layer was estimated to be ~0.9eV. The doping concentrations of the p-type (C-doped) and n-type (Si-doped) GaAs contact layers were approximately 2×1019cm-3 and 5×1018cm-3, respectively, and the growth temperature of these layers was 600oC. The use of carbon as p-type dopant minimizes the out-diffusion, compared to beryllium The devices have a diameter of 80 ìm. The devices were fabricated using standard photolithography and wet etch process. After defining the mesa patterns by photolithography, an acid-based solution, NH4OH: H2O: H2O2 (5: 250: 2) was used to etch away GaAs and GaNAsSb layers, which were not protected by the photoresist, at room temperature. The etch depth was measured using a surface profiler. The Ohmic p- and n-contacts were formed by Ti (50nm) / Au (200nm) and Ni (5nm) / Ge (25nm) / Au (100nm) / Ni (20nm) / Au (100nm), respectively. In addition, the n-contact was annealed at 380oC for 60s. These contacts were connected to metal banding pads using air bridge metallization technology. There is no passivation for the devices.
http://photos-a.ak.fbcdn.net/hphotos-ak-snc3/hs082.snc3/15024_1359699267083_1069579604_1067297_2766786_n.jpg
Figure 1
3. Results and discussion
The photoresponsivity measurement was carried out using a quartz tungsten halogen lamp as the light source, in conjunction with a monochromator. Furthermore, the light source was calibrated using a commercial InGaAs photodetector to measure the power of light arriving at surface of the devices. The photocurrent was measured using combination of a low noise preamplifier and a lock-in amplifier. Figure 2(a) shows the plot of photoresponsivity at a reverse bias of 3V vs. wavelength for the devices whose i-GaNAsSb layers were grown at 350oC to 480oC. The photodetectors show a photo-response up to wavelength of 1350nm. Figure 2(b) shows the photoresponsivity at different reverse biases measured at the wavelength of
1300nm.
From Fig. 2(b), it is interesting to note that the photodetector with GaNAsSb layer grown at 350oC shows an extremely high photoresponsivity value of ~12A/W under 4.8V reverse bias at 1300nm. This is more than 2 orders higher than previously reported results. Assuming a unity quantum efficiency, a photodetector at 1300nm exhibits a maximum responsivity of ~0.75A/W, taking into account 29% incident power reflection due to refractive index difference at the air/GaAs interface. Thus, a photoresponsivity value of 12A/W implies a quantum efficiency value significantly larger than 1, possibly due to the presence of an
avalanche carrier multiplication effect. From Figs. 2(a) and 2(b), it can also be seen that the photoresponsivity of the devices increases as the growth temperature of the i-GaNAsSb photoabsorption layer decreases, except for the device with the i-GaNAsSb layer grown at 480oC. As can be seen from Fig. 2(b), the photoresponsivity of the device grown at 480oC is the lowest at reverse biases below 1V, confirming that the responsivity generally decreases with increasing growth temperature. As the reverse bias is increased, the responsivity of the 480°C rises much stronger as compared to the other devices. This behavior will be further explained below.
http://photos-b.ak.fbcdn.net/hphotos-ak-snc3/hs102.snc3/15024_1359701387136_1069579604_1067302_1104171_n.jpg
Figure2
The photodetectors with GaNAsSb layer grown at different temperatures have different depletion widths under the same reverse bias due to different unintentional doping concentration in the i-GaNAsSb layer. From capacitance-voltage (C-V) measurement, the unintentional doping concentrations in the i-GaNAsSb layer grown at 350oC, 400oC, 440oC and 480oC were experimentally determined to be 2?1016cm-3, 6?1016cm-3, 3?1017cm-3 and 1.5?1018cm-3, respectively. These unintentional doping is p-type and is induced by defects states, especially nitrogen related defects. Based on these unintentional doping concentrations, the depletion region width in the i-GaNAsSb layer at different reverse biases can be calculated. From our previous report [13], the absorption coefficient ??was measured using a spectroscopic ellipsometer and has a value of 1.3?104cm-1 at the wavelength of 1300nm.
Using the measured photoresponsivity values, calculated depletion region widths and values of ?, the photocurrent multiplication factor M for all devices at different reverse biases are calculated and shown in Fig. 3. From Fig. 3, it can be seen that the photodetector with i- GaNAsSb layer grown at 350oC has a M value of ~30 under 4.5V of reverse bias. This high value of M confirms our earlier suggestion of the presence of a photogenerated carrier multiplication due to the avalanche effect. As the carrier avalanche effect is directly dependent on the electric field strength at the depletion region, the values of M in Fig. 3 are re-plotted against the average electric field strength at the depletion region and shown in Fig.4. As mentioned earlier, the photodetector whose i-GaNAsSb layer was grown at 480oC showed a different characteristic in that its photoresponsivity rises much stronger with increasing reverse bias as compared to the other devices. At high reverse voltages (>1.5V) it thus exhibits a higher responsivity compared to the device with i-GaNAsSb layer grown at 440oC. This can be explained by the high electric field in the depletion region of this device as shown in Fig. 4. Due to the high unintentional doping concentration of 1.5?1018cm-3 in the i- GaNAsSb layer grown at 480oC, the depletion region is comparably thin thus resulting in a high electric field strength of about 200-400KV/cm in the depletion region. This is in contrast to the other devices which exhibit an average electric field strength of <200kV/cm in their depletion regions.
http://photos-e.ak.fbcdn.net/hphotos-ak-snc3/hs082.snc3/15024_1359705827247_1069579604_1067338_860612_n.jpg
Figura3
It is interesting to note that as the growth temperature of the i-GaNAsSb layer deceases from 440oC to 350oC, the devices showed a higher value of M, even at much lower electric fields. The photodetector with i-GaNAsSb layer grown at 350oC exhibits a high carrier multiplication factor at average electric field strengths of <100kV/cm. Even when considering a non-uniformly distributed electric field in the depletion region, the maximum electric field strength is ~100kV/cm and 180kV/cm at reverse bias of 1V and 5V, respectively. This electric field strength is unexpectedly low, considering the fact that GaAs or InGaAs based avalanche photodetectors only show carrier multiplication at electric field strength higher than ~200kV/cm [14, 15]. These results suggest that the decrease in growth temperature of the i- GaNAsSb layer leads to a higher impact ionization coefficient in the material, resulting in initiation of the carrier avalanche process at low electric field. The high ionization coefficient and initiation of the carrier avalanche process at low electric field in photodetector with low temperature grown i-GaNAsSb layer could be explained by the existence of mid-gap As antisite defects (AsGa) in the material. It is known that dilute-nitride materials contain AsGa defects [7, 16] as they are grown at non-equilibrium low temperature (<500oC) growth conditions. We expect that the i–GaNAsSb layer grown at 350oC has the highest amount of AsGa defects as content of these defects increases proportionally in response to the decrease in the growth temperature of dilute-nitride material [16]. Generally, carriers in a p-n junction require energy of (3/2)Eg to start an impact ionization and thus avalanche process [17]. E g is the bandgap of the material. Mid-gap defects, such as AsGa are reported [18] to enhance the impact ionization process by lowering the energy required in the impact ionization process. Instead of energy of (3/2)Eg , the impact ionization process through the mid-gap defects states requires only energy of Eg /2 [18]. By lowering the required energy, the existence of mid-gap defects enables a more efficient impact ionization and carrier multiplication process at a lower electric field. This explains our observation that photodetectors, which have i-GaNAsSb layer with more AsGa defects, have higher carrier multiplication and initialize the impact ionization process at a lower electric field.
http://photos-h.ak.fbcdn.net/hphotos-ak-snc3/hs102.snc3/15024_1359709067328_1069579604_1067341_6330720_n.jpg
Figura4
The detectivity, D* of the devices at 1300nm were estimated byD* =Ri( (Ro . A)/(KT))^(1/2)
Where Ri?is the responsivity of devices at zero bias at 1300nm, Ro is the dark impedance at zero bias and A is the detector area. To calculate the Ro, the dark current-voltage (I-V) data was fitted using I=a.( e) ) 1 ( ) 1 ( ????????dv bv e c e a I [19]. By taking the derivative (dV/dI) of the fitted curve equation at zero bias, the value of Ro can be obtained. The value of Ro for device with i-GaNAsSb layer grown at 350oC, 400oC, 440oC and 480oC are 3?106, 5?106, 5.5?106 and 9?105?, respectively. The low value of Ro could be due to the defect states in the GaNAsSb layer. Using the fitted value of Ro, the detectivity at 1.3?m for device with i- GaNAsSb layer grown at 350oC, 400oC, 440oC and 480oC are estimated to be 2.6?109, 8.5?109, 1.7?109 and 9.5?107 cm??Hz/W, respectively.
4. Conclusion
In conclusion, this paper reports on the RF nitrogen plasma-assisted MBE growth of four P-i-
N photodetectors whose the i-GaNAsSb layer were grown at 350oC, 400oC, 440oC, and
480oC. The device with the i-GaNAsSb photoabsorption layer grown at 350oC exhibited
extremely high photoresponsivity of 12A/W corresponding to a photogenerated carrier
multiplication factor of 30. This high photoresponsivity and multiplication factor in this
photodetector is considered to be due to the high impact ionization coefficient in the i-
GaNAsSb layer grown at low substrate temperature.
Acknowledgments
This work was supported by the European Commission within the European Network of
Excellence ISIS, www.ist-isis.org under grant no. 26592. University Duisburg-Essen further
acknowledges support by the European IPHOBAC project, www.ist-iphobac.org under grant
no. 35317. Support from the MERLION Program (France Embassy) project no. 09.01.06 is
acknowledged.
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Negative Differential velocity in dilute nitride alloyz for terahertz electronics
From mobile telecommunications to crime prevention, modern technologies use a wide electromagnetic frequency spectrum. However, for the frequency range 0.3-10 terahertz (THz), referred to as the THz gap, there remains an urgent need for convenient compact solid state devices that emit/detect THz radiation in a selective and tuneable way. Such devices are relevant for many emerging applications, such as THz imaging in biomedicine, and for fundamental studies of excitations in several material systems, such as semiconductors, gaseous plasmas and proteins. The THz gap lies above the frequency range of conventional electronics (transistors) and below the frequency range of traditional optical sources (lasers). To fill this gap, we propose a novel approach to the generation/detection of THz radiation, which exploits the unique electron dynamics occurring in the semiconductor dilute nitride Ga(AsN) alloy. A small concentration of nitrogen (N) atoms has a remarkable effect on the electronic properties of GaAs. The admixing of the N-impurity levels with the extended conduction band states of GaAs leads to the formation of a fully developed energy gap in the conduction band and to subbands with highly non-parabolic energy-wavevector å(k) dispersions. Of particular interest is the form of the lower energy subband in which an inflection point, k*, occurs in the å(k) curve at wavevectors much smaller than the size of the Brillouin zone of GaAs. The electron group velocity in this subband has a maximum at k* and falls off rapidly at higher k-values, Figure a. At large electric fields, electrons can gain sufficient energy to approach k* and the energy level of the N-atoms, at which they become spatially localised, thus causing a negative differential velocity (NDV) effect in which the current flow through the device decreases with increasing applied voltage, Figure b [5-6]. This is an entirely new physical concept of great technological potential.
We discovered a strong dependence of the NDV on magnetic field (Figure b), which reveals that the acceleration of electrons towards the Nlevel is a fast (<10-12 s) process of relevance for THz electronics [5]. This is confirmed by model calculations of electron transport, which indicate that the NDV can lead to harmonic generation of ac current and detection/stimulated emission of THz radiation. More importantly, the maximum response frequency associated with the NDV can be tuned by the applied electric field into the THz frequency range of interest for tuneable sources of THz radiation [7]. The development and investigation of this unusual NDV effect in Ga(AsN) will enhance not only our understanding of the fundamental electronic properties of this novel alloy, but also lead to new discoveries in a much wider variety of dilute nitride semiconductors, such as In(PN), In(SbN), In(AsN) …. Novel regimes of electron dynamics could be revealed in these material systems with exciting prospects for applications in innovative technologies.
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We discovered a strong dependence of the NDV on magnetic field (Figure b), which reveals that the acceleration of electrons towards the Nlevel is a fast (<10-12 s) process of relevance for THz electronics [5]. This is confirmed by model calculations of electron transport, which indicate that the NDV can lead to harmonic generation of ac current and detection/stimulated emission of THz radiation. More importantly, the maximum response frequency associated with the NDV can be tuned by the applied electric field into the THz frequency range of interest for tuneable sources of THz radiation [7]. The development and investigation of this unusual NDV effect in Ga(AsN) will enhance not only our understanding of the fundamental electronic properties of this novel alloy, but also lead to new discoveries in a much wider variety of dilute nitride semiconductors, such as In(PN), In(SbN), In(AsN) …. Novel regimes of electron dynamics could be revealed in these material systems with exciting prospects for applications in innovative technologies.
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QuantaSol Adopts Dilute Nitrides to Boost Absorption and Solar Efficiency
QuantaSol Ltd, an independent designer and manufacturer of strain-balanced quantum-well solar cells has exclusively licensed advanced materials growth technology from the University of Houston to make its manufacturing process simpler and cheaper, while further improving solar cell efficiency.
"We've already tested the benefits of using Houston's dilute nitride materials in the way we engineer quantum wells in our cells," said Keith Barnham, CSO and co-founder of QuantaSol. "The exclusive worldwide licence is a strategic move to ensure we maintain our performance advantage, and we will work with our colleagues in Houston to develop the techniques further in commercial production in 2010."
QuantaSol combines nanostructures, 'quantum wells', of two or more different alloys, in order to obtain synthetic crystals. The crystalline structure can be tuned during manufacture to overcome the absorption problems associated with current concentrator photo-voltaic (CPV) cell designs. The quantum well effect also greatly enhances the photovoltaic conversion efficiency, as already proven by its recent world record efficiency single junction device. Ultimately QuantaSol will produce highly efficient triple junction CPV devices in 2010.
The use of dilute nitrides will allow QuantaSol to reduce the number of quantum well layers it needs to introduce into each junction, while maintaining or increasing solar efficiency. This further reduces the thickness and manufacturing cost of its production devices.
"This is the first major collaboration QuantaSol has announced," said Chris Shannon, QuantaSol's new CEO. "It indicates just how close the company is getting to being able to produce very efficient devices in production quantities. I'm really looking forward to the progress we will deliver over the next 12 months."
Chris Shannon is an experienced semiconductor, optics and photonics leader. He joined QuantaSol in September in a planned move to structure the company for volume manufacture.
"We are excited to cooperate with QuantaSol in its application of the basic patents of Prof. Alex Freundlich on quantum well solar cells. These joint efforts will advance solar cell technology and help increase our use of renewable resources, " said Alex Ignatiev, director, Center for Advanced Materials, and distinguished university professor of physics, chemistry, and electrical and computer engineering for the University of Houston.
QuantaSol was established in June 2007 as a spin-out of Imperial College London to commercialise the University's solar cell IP and offer devices to concentrator Photovoltaic (PV) systems developers. The company has a product development and test laboratory in Kingston-upon-Thames in the UK.
QuantaSol is funded and backed by the Low Carbon Accelerator and Imperial Innovations, and its strain-balanced quantum-well solar cell (SB-QWSC) is believed to be the highest performing single-junction concentrator cell in the world with the potential to produce very durable multi-junction cells with record operating efficiencies.
QuantaSol is ranked 85th in The Guardian's 2009 'Global Cleantech 100' ranking of the world's most promising clean technology companies. About the Center for Advance Materials at the University of Houston: The Center for Advanced Materials (CAM) addresses research and development of advanced materials and their fundamental science while maintaining a strong applications focus. CAM develops new materials leading to technologies of importance to the nation's future energy needs within its industry-academia-government partnerships, and moves these and other nano-engineered advances into the commercial sector for economic and social benefit, while training the next generation of scientists and engineers.
The University of Houston, Texas' premier metropolitan research and teaching institution, is home to more than 40 research centers and institutes and sponsors more than 300 partnerships with corporate, civic and governmental entities. UH, the most diverse research university in the country, stands at the forefront of education, research and service with more than 37,000 students.
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"We've already tested the benefits of using Houston's dilute nitride materials in the way we engineer quantum wells in our cells," said Keith Barnham, CSO and co-founder of QuantaSol. "The exclusive worldwide licence is a strategic move to ensure we maintain our performance advantage, and we will work with our colleagues in Houston to develop the techniques further in commercial production in 2010."
QuantaSol combines nanostructures, 'quantum wells', of two or more different alloys, in order to obtain synthetic crystals. The crystalline structure can be tuned during manufacture to overcome the absorption problems associated with current concentrator photo-voltaic (CPV) cell designs. The quantum well effect also greatly enhances the photovoltaic conversion efficiency, as already proven by its recent world record efficiency single junction device. Ultimately QuantaSol will produce highly efficient triple junction CPV devices in 2010.
The use of dilute nitrides will allow QuantaSol to reduce the number of quantum well layers it needs to introduce into each junction, while maintaining or increasing solar efficiency. This further reduces the thickness and manufacturing cost of its production devices.
"This is the first major collaboration QuantaSol has announced," said Chris Shannon, QuantaSol's new CEO. "It indicates just how close the company is getting to being able to produce very efficient devices in production quantities. I'm really looking forward to the progress we will deliver over the next 12 months."
Chris Shannon is an experienced semiconductor, optics and photonics leader. He joined QuantaSol in September in a planned move to structure the company for volume manufacture.
"We are excited to cooperate with QuantaSol in its application of the basic patents of Prof. Alex Freundlich on quantum well solar cells. These joint efforts will advance solar cell technology and help increase our use of renewable resources, " said Alex Ignatiev, director, Center for Advanced Materials, and distinguished university professor of physics, chemistry, and electrical and computer engineering for the University of Houston.
QuantaSol was established in June 2007 as a spin-out of Imperial College London to commercialise the University's solar cell IP and offer devices to concentrator Photovoltaic (PV) systems developers. The company has a product development and test laboratory in Kingston-upon-Thames in the UK.
QuantaSol is funded and backed by the Low Carbon Accelerator and Imperial Innovations, and its strain-balanced quantum-well solar cell (SB-QWSC) is believed to be the highest performing single-junction concentrator cell in the world with the potential to produce very durable multi-junction cells with record operating efficiencies.
QuantaSol is ranked 85th in The Guardian's 2009 'Global Cleantech 100' ranking of the world's most promising clean technology companies. About the Center for Advance Materials at the University of Houston: The Center for Advanced Materials (CAM) addresses research and development of advanced materials and their fundamental science while maintaining a strong applications focus. CAM develops new materials leading to technologies of importance to the nation's future energy needs within its industry-academia-government partnerships, and moves these and other nano-engineered advances into the commercial sector for economic and social benefit, while training the next generation of scientists and engineers.
The University of Houston, Texas' premier metropolitan research and teaching institution, is home to more than 40 research centers and institutes and sponsors more than 300 partnerships with corporate, civic and governmental entities. UH, the most diverse research university in the country, stands at the forefront of education, research and service with more than 37,000 students.
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Physics and technology of dilude nitrides
Dilute nitrides have emerged from conventional III–V semiconductors such as GaAs or InP by the insertion of nitrogen into the group V sub-lattice, which has a profound influence on the electronic properties of these materials and allows widely extended band structure engineering. This is expected to lead to novel devices, e.g. for optical data transmission, solar cells, biophotonics or gas sensing, some of which are already making their way into the market. Unlike in all other cases, where a reduction in bandgap energy is achieved by inserting an element that increases the lattice constant, N accomplishes this and at the same time reduces the lattice constant. Thus smaller bandgaps can be achieved and the unusual role of N in the lattice also allows a tailoring of band alignments. Both of these effects have opened up a new dimension of bandgap engineering and the rapid progress in the field led to the demonstration of high quality 1300 nm lasers on GaAs and eventually to the realization of the first VCSELs that can be mass produced at low cost and emit at 1300 nm. This in turn will allow extending inexpensive data transmission through optical fibers from the present range of about 300 m to a distance of 10 to 20 km and at the same time increasing the data rate by about a factor of four. Thus it will enable metro-area data links, which are presently considered to be the bottleneck for large-scale optical communications. Furthermore, the fact that GaNP and related alloys can be grown lattice-matched on Si substrates has offered intriguing new possibilities of OEIC and integration of efficient III–V optoelectronic devices with the mainstream microelectronics based on Si.
Despite their promising applications and the first encouraging experimental results, very little is known about the physical properties of such alloys. For instance the difficulty of incorporating nitrogen into GaInAs while maintaining good optical quality has provoked much work to establish an understanding of the underlying factors determining the optical quality of GaInNAs, such as composition, growth and annealing conditions. We are still far from establishing an understanding of the band structure and its dependence on composition. Fundamental electronic interactions such as electron–electron and electron–phonon scattering, dependence of effective mass on composition, strain and orientation, quantum confinement effects, effects of localized nitrogen states on high field transport and on galvanometric properties, and mechanisms for light emission in these materials, are yet to be fully understood. Nature and formation mechanisms of grown-in and processing-induced defects that are important for material quality and device performance are still unknown. Such knowledge is required in order to design strategies to efficiently control and eliminate harmful defects. For many potential applications (such as solar cells, HBTs) it is essential to get more information on the transport properties of dilute nitride materials. The mobility of minority carriers is known to be low in GaInNAs and related material. The experimental values are far from reaching the theoretical ones, due to defects and impurities introduced in the material during the growth. The role of the material inhomogeneities on the lateral carrier transport also needs further investigation.
From the device's point of view most attention to date has been focused on the GaInNAs/GaAs system, mainly because of its potential for optoelectronic devices covering the 1.3–1.55 µm data and telecommunications wavelength bands. As is now widely appreciated, these GaAs-compatible structures allow monolithic integration of AlGaAs-based distributed Bragg reflector mirrors (DBRs) for vertical cavity surface-emitting lasers with low temperature sensitivity and compatibility with AlOx-based confinement techniques. In terms of conventional edge-emitting lasers (EELs), the next step is to extend the wavelength range for cw room-temperature operation, as well as improving the spectral purity, modulation speed and peak power output. Many applications in medicine, environmental sensing and communications can be addressed with the achievement of significant improvements in these parameters. Semiconductor optical amplifiers (SOAs) are also important devices of interest, since it is widely predicted that the market for SOAs in photonic access networks will increase dramatically in the next few years. In addition to EELs and SOAs, vertical cavity surface-emitting lasers (VCSELs), vertical external cavity surface-emitting lasers (VECSELs), vertical cavity semiconductor optical amplifiers (VCSOAs), and semiconductor saturable absorber mirrors (SESAMs) are of increasing importance.
The VECSELs can potentially incorporate saturable absorbers for very high repetition rate (~100 GHz) pulsed and potentially MEMS-tuneable sources. VECSEL devices in the 2–3 µm range for applications in e.g. free-space optical (FSO) communications, are possible using InAsN/InGaAs/InP with AlGaAs metamorphic mirror growth. Semiconductor saturable-absorber mirror structures (SESAMs) have demonstrated widespread applicability for self-starting passive mode locking of (diode-pumped) solid-state lasers, to produce high-performance picosecond and femtosecond laser sources for scientific, instrumentation and industrial use. Very recently, these devices have also shown applicability for ultra short pulse generation at >GHz repetition rates, both in DPSS lasers and surface-emitting semiconductor lasers. These devices are undoped monolithic DBR structures incorporating one or more quantum wells for saturable absorption. Low-loss and high-damage threshold requirements demand pseudomorphic growth, and have, until very recently, essentially limited these devices to the 800–1100 nm range, but extension beyond this range is urgently required by a host of mode locking applications. In addition to these devices modulators and photodiodes, including quantum well infrared photodetectors (QWIPs) and resonant cavity-enhanced photodiodes (RCEPDs) based on dilute nitrides need to be investigated extensively.
To date, most theoretical attention has been focused on understanding the band structure of the GaInAsN/GaAs system and on evaluating gain spectra and threshold conditions for 1.3 µm lasers. However, as our understanding of band structure and the effects of strain, defects, etc in dilute nitrides improves we can calculate the electrical and optical properties, including radiative and non-radiative recombination for the materials and structures of interest. The spontaneous and stimulated emission rates have already been calculated for GaInNAs at 1.3 µm by many authors, but extension to other dilute nitrides and other wavelength ranges still represents a major challenge. Many-body effects, including exchange-correlation effects, are essential for accurate models of gain spectra in lasers and optical amplifiers. The differential gain is a key parameter for laser modulation and remains an important subject of study as new materials and structures are explored. Similarly the differential refractive index and linewidth enhancement factor have strong influences on laser spectrum (chirp, linewidth), dynamics and noise, and these must also be studied theoretically. As regards to non-radiative recombination, in addition to recombination through defects, the Auger effect is of especial significance for wavelengths beyond 1 µm and is a worthy subject for theoretical study. The converse effect, impact ionization, is of key importance for avalanche photodiodes (APDs) and has yet to be evaluated for the dilute nitride materials. Inter-valence band absorption (IVBA) is of significance, as a possible cause of temperature sensitivity in lasers and this must be investigated theoretically in the dilute nitrides. Third-order non-linear optical coefficients should be calculated in order to assess the scope for all-optical signal processing components within the dilute nitrides. Electro-absorption and electro-refractive effects—Franz-Keldysh (FK) and quantum-confined Stark effect (QCSE) need to be studied theoretically in view of their importance for optical modulators.
The aim of this special issue is to review the recent progress in theory, growth, characterization and device applications of dilute nitrides, and to collate what is known and what is not known in the field and address important fundamental physical properties and key material and device issues. The issue brings together a wide selection of papers from over 27 prominent research groups that have made key contributions to the field in the areas of research including growth, characterization and physical properties, devices and device integration, and theory and modelling. The editor is very grateful to all the invited authors for their contribution to this issue of Journal of Physics: Condensed Matter.
ALONZO DAVID LINARES MORA
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Despite their promising applications and the first encouraging experimental results, very little is known about the physical properties of such alloys. For instance the difficulty of incorporating nitrogen into GaInAs while maintaining good optical quality has provoked much work to establish an understanding of the underlying factors determining the optical quality of GaInNAs, such as composition, growth and annealing conditions. We are still far from establishing an understanding of the band structure and its dependence on composition. Fundamental electronic interactions such as electron–electron and electron–phonon scattering, dependence of effective mass on composition, strain and orientation, quantum confinement effects, effects of localized nitrogen states on high field transport and on galvanometric properties, and mechanisms for light emission in these materials, are yet to be fully understood. Nature and formation mechanisms of grown-in and processing-induced defects that are important for material quality and device performance are still unknown. Such knowledge is required in order to design strategies to efficiently control and eliminate harmful defects. For many potential applications (such as solar cells, HBTs) it is essential to get more information on the transport properties of dilute nitride materials. The mobility of minority carriers is known to be low in GaInNAs and related material. The experimental values are far from reaching the theoretical ones, due to defects and impurities introduced in the material during the growth. The role of the material inhomogeneities on the lateral carrier transport also needs further investigation.
From the device's point of view most attention to date has been focused on the GaInNAs/GaAs system, mainly because of its potential for optoelectronic devices covering the 1.3–1.55 µm data and telecommunications wavelength bands. As is now widely appreciated, these GaAs-compatible structures allow monolithic integration of AlGaAs-based distributed Bragg reflector mirrors (DBRs) for vertical cavity surface-emitting lasers with low temperature sensitivity and compatibility with AlOx-based confinement techniques. In terms of conventional edge-emitting lasers (EELs), the next step is to extend the wavelength range for cw room-temperature operation, as well as improving the spectral purity, modulation speed and peak power output. Many applications in medicine, environmental sensing and communications can be addressed with the achievement of significant improvements in these parameters. Semiconductor optical amplifiers (SOAs) are also important devices of interest, since it is widely predicted that the market for SOAs in photonic access networks will increase dramatically in the next few years. In addition to EELs and SOAs, vertical cavity surface-emitting lasers (VCSELs), vertical external cavity surface-emitting lasers (VECSELs), vertical cavity semiconductor optical amplifiers (VCSOAs), and semiconductor saturable absorber mirrors (SESAMs) are of increasing importance.
The VECSELs can potentially incorporate saturable absorbers for very high repetition rate (~100 GHz) pulsed and potentially MEMS-tuneable sources. VECSEL devices in the 2–3 µm range for applications in e.g. free-space optical (FSO) communications, are possible using InAsN/InGaAs/InP with AlGaAs metamorphic mirror growth. Semiconductor saturable-absorber mirror structures (SESAMs) have demonstrated widespread applicability for self-starting passive mode locking of (diode-pumped) solid-state lasers, to produce high-performance picosecond and femtosecond laser sources for scientific, instrumentation and industrial use. Very recently, these devices have also shown applicability for ultra short pulse generation at >GHz repetition rates, both in DPSS lasers and surface-emitting semiconductor lasers. These devices are undoped monolithic DBR structures incorporating one or more quantum wells for saturable absorption. Low-loss and high-damage threshold requirements demand pseudomorphic growth, and have, until very recently, essentially limited these devices to the 800–1100 nm range, but extension beyond this range is urgently required by a host of mode locking applications. In addition to these devices modulators and photodiodes, including quantum well infrared photodetectors (QWIPs) and resonant cavity-enhanced photodiodes (RCEPDs) based on dilute nitrides need to be investigated extensively.
To date, most theoretical attention has been focused on understanding the band structure of the GaInAsN/GaAs system and on evaluating gain spectra and threshold conditions for 1.3 µm lasers. However, as our understanding of band structure and the effects of strain, defects, etc in dilute nitrides improves we can calculate the electrical and optical properties, including radiative and non-radiative recombination for the materials and structures of interest. The spontaneous and stimulated emission rates have already been calculated for GaInNAs at 1.3 µm by many authors, but extension to other dilute nitrides and other wavelength ranges still represents a major challenge. Many-body effects, including exchange-correlation effects, are essential for accurate models of gain spectra in lasers and optical amplifiers. The differential gain is a key parameter for laser modulation and remains an important subject of study as new materials and structures are explored. Similarly the differential refractive index and linewidth enhancement factor have strong influences on laser spectrum (chirp, linewidth), dynamics and noise, and these must also be studied theoretically. As regards to non-radiative recombination, in addition to recombination through defects, the Auger effect is of especial significance for wavelengths beyond 1 µm and is a worthy subject for theoretical study. The converse effect, impact ionization, is of key importance for avalanche photodiodes (APDs) and has yet to be evaluated for the dilute nitride materials. Inter-valence band absorption (IVBA) is of significance, as a possible cause of temperature sensitivity in lasers and this must be investigated theoretically in the dilute nitrides. Third-order non-linear optical coefficients should be calculated in order to assess the scope for all-optical signal processing components within the dilute nitrides. Electro-absorption and electro-refractive effects—Franz-Keldysh (FK) and quantum-confined Stark effect (QCSE) need to be studied theoretically in view of their importance for optical modulators.
The aim of this special issue is to review the recent progress in theory, growth, characterization and device applications of dilute nitrides, and to collate what is known and what is not known in the field and address important fundamental physical properties and key material and device issues. The issue brings together a wide selection of papers from over 27 prominent research groups that have made key contributions to the field in the areas of research including growth, characterization and physical properties, devices and device integration, and theory and modelling. The editor is very grateful to all the invited authors for their contribution to this issue of Journal of Physics: Condensed Matter.
ALONZO DAVID LINARES MORA
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El nitruro de galio podría revolucionar la iluminación a corto plazo
El nitruro de galio podría revolucionar la iluminación a corto plazo
Los últimos avances logrados con el nitruro de galio, una sustancia que emite luz, podrían revolucionar la iluminación de los hogares y las oficinas en un plazo de unos cinco años, según afirma un experto en ciencia de los materiales, Colin Humphreys, catedrático de la Universidad de Cambridge (Reino Unido). El uso de esta fuente de luz brillante podría contribuir a reducir hasta un 75% el consumo eléctrico.
El nitruro de galio (GaN) es un compuesto emisor de luz que ya se usa en los flashes de las cámaras, los faros de las bicicletas, los teléfonos móviles y en la iluminac
ión del interior de autobuses, trenes y aviones, pero un equipo de investigadores británicos prevé que sus posibilidades van mucho más allá.Los científicos consideran que cuando se pueda usar el GaN para iluminar las casas y las oficinas será como haber encontrado el Santo Grial. Si se consigue, se podría reducir en un 75% el consumo habitual de luz eléctrica en los países desarrollados y, a la vez, se contribuiría a disminuir enormemente las emisiones de dióxido de carbono de las centrales eléctricas y a preservar las reservas de combustibles fósiles.
"Los diodos emisores de luz (LED, por sus siglas en inglés) de nitruro de galio (GaN) tienen un futuro muy prometedor", asegura Humphreys, que coordina las investigaciones como catedrático del Centro para el Nitruro de Galio en la Universidad de Cambridge (Reino Unido). Estos LED son "asombrosamente duraderos, ya que pueden proporcionar 100.000 horas de luz (100 veces más que una bombilla convencional), y en la práctica eso significa que con el uso normal de una familia habría que cambiarlos a los 60 años".
"Además, a diferencia de las luces fluorescentes compactas de bajo consumo que se usan ahora, los LED de nitruro de galio no contienen mercurio, por lo que desecharlos no es un problema medioambiental tan complicado", añade el catedrático.
Pero para aprovechar estas ventajas los científicos deben superar algunos obstáculos importantes, como que los LED de GaN son demasiado caros para fabricarlos para un uso a gran escala en hogares y centros de trabajo. Además, otro de los factores limitantes es la luz fuerte que emiten. Los investigadores han desvelado recientemente porque se produce ese fenómeno gracias a una nueva y completa teoría, desarrollada en colaboración con Phil Dawson, catedrático de la Universidad de Manchester (Reino Unido).
"Comprender esto es vital para mejorar la calidad y la eficiencia de las luces de GaN", afirma Humphreys, que añade: "Nuestro centro también está trabajando en una técnica innovadora para depositar el GaN sobre discos de silicio de 15 centímetros, en lugar de los discos de zafiro que se usaban hasta ahora. Esto podría reducir los costes de fabricación a la décima parte, y así contribuiría a que las luces de nitruro de galio se introdujesen en nuevos mercados". Otro de los proyectos del centro se centra en lograr que la iluminación con GaN imite a la luz del sol, lo que podría tener importantes beneficios para quienes padecen trastornos afectivos estacionales.
Humphreys predice que las luces de GaN "deberían empezar a dejar notar su presencia en las casas y oficinas en un plazo de unos cinco años", lo que no sólo será bueno para el medio ambiente, sino que también beneficiará a los consumidores en cuanto a comodidad, ahorro de electricidad y calidad de vida".
Las posibilidades futuras de las luces de nitruro de galio son muy diversas. Actualmente los LED de este compuesto se recubren con fósforo para transformar la luz azul en luz blanca. Pero existe la posibilidad de retirar la cubierta e incluir varios LED en miniatura, cada uno de los cuales emitiría luz en un color diferente dentro de la "bombilla" general.
Los LED en miniatura emitirían juntos la luz blanca, pero la gente en casa o en la oficina podría modificar el equilibrio exacto (para conseguir, por ejemplo, una luz azulada) según su estado de ánimo. "Ésta y otras aplicaciones, como en medicina para detectar tumores o para el tratamiento de aguas en países en vías de desarrollo, podrían estar disponible en 10 años", vaticina Humphreys
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SÍNTESIS Y CARACTERIZACIÓN DE NITRUROS Y CARBUROS DE VANADIO
SÍNTESIS Y CARACTERIZACIÓN DE NITRUROS Y CARBUROS DE VANADIO
Synthesis and characterization of vanadium nitrides and carbides José Alberto LUMBRERAS1,2, Gabriel ALONSO NÚÑEZ2, Guillermo SANDOVAL1, José Aarón MELO1, Sergio ROBLES1 y Ricardo GARCÍA ALAMILLA1 1 Instituto Tecnológico de Ciudad Madero, J. Rosas y J. Urueta s/n, Col. Los Mangos, C.P. 89440. Ciudad Madero, Tam. México. Correo electrónico: rgalamilla@yahoo.com.mx 2 Centro de Investigación en Materiales Avanzados, S. C., Av. Miguel de Cervantes 120, Complejo Industrial Chihuahua. C.P. 31109. Chihuahua, Chih. México(Recibido febrero 2007, aceptado noviembre 2007)
RESUMENSe prepararon 2 carburos de vanadio (CV–a y CV–A) usando dos flujos de gas de carburización (660 mL/min y 8 mL/min, respectivamente) y un nitruro de vanadio (NV) se preparó usando 660 mL/min de NH3. Los catalizadores se caracterizaron por difracción de rayos X, adsorción de nitrógeno, titulación potenciométrica con n–butilamina, deshidratación de 2–propanol y se midió su actividad en la hidrodesulfuración de tiofeno y de un gasóleo pesado de vacío. La hidrodesulfuración del gasóleo pesado de vacío se llevó a cabo con el carburo de vanadio preparado con 660 mL/min de la mezcla del gas carburante y con el nitruro de vanadio. La deshidratación de 2 propanol indica que ambos carburos de vanadio tienen propiedades ácidas predominando los sitios débiles, mientras que el nitruro de vanadio tiene propiedades anfóteras. En la hidrodesulfuración de tiofeno, el carburo de vanadio preparado con 8 mL/min de CH4/ H2 fue el catalizador más activo, mientras que en la hidrodesulfuración de gasóleo, el carburo de vanadio preparado usando 660 mL/min de la mezcla de carburización fue más activo que el nitruro de vanadio, alcanzando un 40 % de eliminación de azufre y un 51 % de eliminación de nitrógeno.Palabras clave: carburo de vanadio, nitruro de vanadio, deshidratación de 2–propanol, HDS de tiofeno, HDS de gasóleo pesado de vacío ABSTRACTTwo vanadium carbides catalysts (CV–a and CV–A) were prepared using two carburation gas mixture flows (660 and 8 mL/min, respectively) and a vanadium nitride (NV) catalyst was also prepared with a 660 mL/min of NH3 flow. The catalysts were characterized by X–ray diffraction, nitrogen adsorption, n–butylamine potentiometric titration techniques and their acid and catalytic properties were tested in 2–propanol dehydration, hydrodesulphurization of thiophene and a heavy gas oil. Heavy gas oil hydrodesulphurization was carried out with the vanadium carbide prepared at 660 mL/min of carburation gas mixture and with the vanadium nitride catalysts. Results obtained in 2–propanol dehydration indicate that both vanadium carbides have acid properties with the prevalence of weak acid sites, while the vanadium nitrides have acid–base properties. In the thiophene hydrodesulphurization, the vanadium carbide prepared with 8 mL/min of CH4/H2 gas mixture was the most active catalysts, this catalyst shows a crystalline phase mixture. In the gas oil hydrodesulphurization the vanadium carbide "CV–a" was more active than the vanadium nitride, with a 40 wt % of sulphur remotion and a 51 wt % of nitrogen remotion.Key words: vanadium carbides, vanadium nitrides, 2 propanol dehydration, thiophene HDS, heavy gas oil HDS
INTRODUCCIÓNEn los últimos años, se ha generado una mayor conciencia ambientalista, de la cual ha emergido una nueva generación de científicos, tecnólogos e inclusive políticos, que enfatizan la necesidad de un cambio tecnológico en referencia a la decisión de los modelos de crecimiento de los países en vías de desarrollo, intentando con esto disminuir las emisiones de contaminantes por el uso indiscriminado de combustibles fósiles, las cuales afectan el ambiente y son las causantes de los cambios climáticos recientes. La relación entre la contaminación y el desarrollo económico sugiere que existe una tendencia para las emisiones per cápita y por unidad de producto interno bruto (PIB) que convergen con el tiempo, relación que se debe reducir en países en vías de desarrollo mediante aplicación de nuevas tecnologías y políticas (Stern 2005, Brock y Taylor 2004). La primera valoración del impacto ambiental en México enfocado al sector industrial después de ingresar al TLC, indicó que el crecimiento de la economía durante este periodo se llevó a efecto con un aumento de la degradación del ambiente; las emisiones de SO2, NOX y dióxido de carbono aumentaron significativamente. Sin embargo, la contaminación ambiental no es un problema exclusivo de países en vías de desarrollo sino que también afecta a los países altamente desarrollados, que finalmente son los que generan la mayor cantidad de contaminantes. Es, por tanto, que en los últimos años se han realizado esfuerzos para desarrollar tecnologías que reduzcan el contenido de este tipo de compuestos en los combustibles. El proceso de hidrotratamiento es de gran relevancia ya que mediante éste se reduce el contenido de O, N y S, familias de hidrocarburos insaturados y metales como Ni y V, principales contaminantes que contiene el crudo. En los últimos años se han realizado diversos estudios para desarrollar catalizadores que posean características particulares, que favorezcan la eliminación de los contaminantes antes mencionados, resistan las condiciones de operación requeridas para el procesamiento de cortes cada vez más pesados y que logren disminuir a los valores previstos la concentración de las fracciones más refractarias de los compuestos de azufre contenidos en el petróleo.Los carburos y nitruros de metales de transición han demostrado tener buena actividad catalítica en reacciones de hidrodesulfuración (HDS), hidrodesnitrogenación (HDN) e hidrogenación (HID) en especial de moléculas modelo como dibenzotiofeno o mezclas de dibenzotiofeno, quinoleina y tetralina (Dhandapani et al. 1998, Markel et al. 1990). Los carburos de W y Mo soportados en Al2O3, se han evaluado en la hidrogenación de tetralina a presión elevada, en ausencia y presencia de H2S, obteniendo resultados promisorios debido a estabilidad de la fase activa y resistencia al efecto de envenenamiento por azufre. Sin embargo, este tipo de catalizadores no se han evaluado con cargas reales de hidrocarburos como gasóleos o cargas similares, por lo cual en este trabajo se reporta la preparación de dos carburos y un nitruro de vanadio evaluando sus propiedades anfóteras y su capacidad de eliminación de azufre mediante la hidrodesulfuración de tiofeno y de un gasóleo pesado de vacío.Parte experimentalSíntesis de catalizadores. Se prepararon dos carburos de vanadio CV–a y CV–A, empleando dos flujos de carburación, 660 y 8 mL/min respectivamente, para analizar su efecto sobre el área específica y las propiedades estructurales y ácidas de los materiales resultantes. La síntesis de los carburos y nitruro de vanadio se llevó a cabo por el método de temperatura programada de reducción, usando un reactor de cuarzo de 10 mm de diámetro interno similar a lo reportado en la literatura (Neylon et al. 1999).Síntesis de carburo de vanadio (CV–a)Los carburos de vanadio se prepararon a partir de pentóxido de vanadio (Aldrich), el cual se carburizó con un flujo volumétrico de 660 mL/min por gramo de precursor de una mezcla constituida por CH4/H2 (Praxair 66.9 % en volumen de metano); la preparación se inició con un calentamiento lento desde 25 °C hasta 900 °C con un incremento lineal de temperatura de 1 °C/min, permaneciendo a 900 °C por 60 minutos. Finalizada la etapa de carburización, el sólido se enfrió hasta temperatura ambiente y su pasivación se hizo con una mezcla de Ar/O2 (Praxair 1 % en volumen de O2) por 60 minutos, el sólido obtenido se denominó CV–a.Síntesis de carburo de vanadio (CV–A )El carburo de vanadio (CV–A) se preparó con un flujo de carburización de 8 mL/min por gramo de precursor, manteniendo los demás parámetros de síntesis similares a los usados para el catalizador CV–a.Síntesis de nitruro de vanadio (NV)El pentóxido de vanadio se nitruró con un flujo volumétrico de 660 mL/min de amoniaco (Praxair 99.9 %) por gramo de precursor, calentando lentamente a una velocidad lineal de 1 °C/min desde temperatura ambiente hasta 530 °C, permaneciendo a esta temperatura por 60 minutos; finalizada la etapa de nitruración, la pasivación del catalizador obtenido se hizo con una mezcla de Ar/O2 por 60 minutos. El sólido obtenido se denominó NV.Caracterización de los catalizadoresLos catalizadores se caracterizaron por medio de difracción de rayos X, utilizando un difractómetro de rayos X Brukker, modelo Advance D8 con radiación CuKa. La medida del área específica se realizó en un equipo RIG 100–19 por adsorción física de nitrógeno, a temperatura de nitrógeno líquido. Previo a la determinación del área específica, las muestras fueron desgasificadas a 200 °C por 4 horas con un flujo de helio 40 mL/min (Praxair 99.9 %). La titulación potenciométrica con n–butilamina permitió la evaluación del número total de sitios ácidos así como la fuerza ácida de estos sitios. El potencial (E) del electrodo indica la máxima fuerza de los sitios ácidos superficiales y el rango donde una meseta es observada (meq/g de sólido) indica el número total de sitios ácidos. La fuerza de los sitios ácidos se asigna de acuerdo con los siguientes rangos: E > 100 mV sitio muy fuerte, 0 <>
RESULTADOS Y DISCUSIÓN
Estudio de difracción de rayos XLa figura 1 muestra el diagrama de difracción de rayos X del polvo del CV–a, preparado con un flujo volumétrico de 660 mL/min de la mezcla de CH4/H2. El material presenta la estructura cúbica característica del carburo de vanadio, la cual se identificó por sus principales líneas de difracción a 37.5, 43.5, 63.2 y 75.8° de la escala 2 theta a través de la tarjeta 73–476 del Joint Committee on Powder Diffraction Standards (JCPDS), esta fase cristalina es similar a la reportada en la literatura (Neylon et al. 1999) y es considerada la fase activa en este tipo de materiales. Por otra parte, el catalizador CV–A sintetizado con un flujo bajo del gas carburante (8 mL/min) presenta una mezcla de carburo y trióxido de vanadio, este último se identificó por las líneas de difracción que caracterizan al trióxido de vanadio a través de la tarjeta 71–343 del JCPDS; la presencia de esta fase probablemente fue causada por el bajo contenido de H2 de la mezcla de carburación, el cual no alcanzó a desplazar completamente el oxígeno de la estructura del pentóxido e impidió la completa carburación del vanadio. Un comportamiento similar se observó en la síntesis de un carburo de molibdeno obteniendo una mezcla de carburo y trióxido de molibdeno (Li et al. 1996).
El patrón de difracción del NV, preparado a partir de V2O5 se muestra en la parte superior de la figura 1, sus principales ángulos de difracción localizados en 37.7, 43.8, 63.6 y 74.8° de la escala 2 theta identifican completamente un nitruro de vanadio con estructura cúbica, la cual se corroboró por medio de la tarjeta 78–1315 del JCPDS, fase cristalina también reportada como la fase activa para este tipo de catalizador (Neylon et al. 1999).Área específicaEn la preparación del carburo de vanadio, el V2O5 se transforma lentamente en el correspondiente carburo o nitruro dependiendo de la atmósfera de síntesis empleada en la preparación del material, durante este proceso se genera agua in situ continuamente. Las reacciones que prevalecen durante el proceso de carburación o nitruración en los catalizadores preparados se muestran a continuación:Para el carburo de vanadio CV–aV2O5 + 2CH4 + H2 –> 2CV + 5H2OPara el carburo de vanadio CV–AV2O5 + 4CH4 + 4H2 –>4CV + V2O3 + 12H2OPara el nitruro de vanadio NVV2O5 + 2NH3 –>2NV + 3H2O + O2El agua producida durante la reducción debe ser evacuada del medio de reacción para disminuir su contacto con el carburo en formación y evitar la posible sinterización del mismo, la cual puede ocasionar una reducción en el área específica. En el cuadro I se muestran los resultados de área específica obtenida con los tres catalizadores. El área obtenida en los tres materiales es baja, el nitruro de vanadio tiene únicamente 6 m2/g mientras que el catalizador CV–a preparado con un flujo volumétrico de 660 mL/min de la mezcla de CH4/H2 tiene un área de 9 m2/g mayor que la del catalizador CV–A; la mayor área obtenida con el material CV–a es debido probablemente a la mejor remoción del agua generada durante la reducción del vanadio.Determinación de acidezEn el cuadro I también se muestran los resultados de acidez total y la máxima fuerza ácida de los catalizadores determinado de acuerdo a la técnica de titulación potenciométrica para lo cual 0.15 gramos de la muestra se diluyen en acetonitrilo puro (J.T. Baker), siguiendo la neutralización de los sitios ácidos mediante su titulación con una solución de n–butilamina (Aldrich) en acetonitrilo al 0.05N (Cid y Pecchi 1985). Los resultados de acidez obtenidos por esta técnica en el catalizador CV–a indican la presencia de sitios ácidos muy débiles, (mV < –100), mientras que en los catalizadores CV–A y NV se observan la coexistencia de sitios ácidos fuertes y débiles (0 < mV < 100) (Vázquez et al. 2002). El flujo de carburización tiene un marcado efecto sobre la fuerza ácida de los sitios que se generan en los materiales y pueden ser analizados a partir de los resultados del cuadro I, en el cual se observa que el catalizador CV–a, preparado con un flujo elevado de carburización tiene sitios ácidos muy débiles, mientras que el carburo sintetizado con un flujo de 8 mL/min de la misma mezcla de carburización desarrolló sitios ácidos fuertes [12 E(mV)]; por otra parte, el nitruro de vanadio preparado con 660 mL/min de NH3 también desarrolló sitios ácidos fuertes.En la figura 2 se muestran los perfiles de neutralización de los sitios ácidos obtenidos mediante la titulación potenciométrica de n–butilamina con los tres catalizadores preparados. El nitruro de vanadio tiene sitios ácidos fuertes (+50 mV), sin embargo la concentración de estos es baja predominando los sitios ácidos débiles a muy débiles (–50 a –150 mV. El mismo comportamiento se observa con el material CV–A, el cual al inicio de la neutralización presenta sitios ácidos fuertes (+15 mV) pero estos se neutralizan rápidamente, observándose una concentración más homogénea de sitios ácidos débiles entre –50 y –100 mV; por otro lado, el perfil de neutralización del CV–a muestra la existencia de sitios ácidos de débiles a muy débiles (–100 a –170mV).
Deshidratación de 2–propanolLa deshidratación del 2–propanol se estudió para determinar la existencia o ausencia de sitios anfóteros (ácido–base) en los carburos y nitruro de vanadio, con base en el análisis de las reacciones de eliminación que toman lugar en la transformación de este tipo de moléculas. Se ha reportado que si en la deshidratación de un alcohol se obtienen olefinas, se establece la existencia de sitios duales ácido–base, la presencia de una cetona indica la existencia de sitios básicos.En el caso de la deshidratación del 2 propanol, la formación de propileno indica la existencia de sitios anfóteros y la presencia de la acetona indica la presencia de sitios básicos. Por otra parte, la relación entre la selectividad de las reacciones de deshidrogenación y deshidratación es un índice indirecto del carácter acido o básico del catalizador, si el cociente es igual o mayor a 1 el sólido tiene un fuerte carácter básico (Domínguez et al. 2002).En el cuadro II se reportan los resultados de la deshidratación de 2–propanol de los tres materiales evaluados. Con el catalizador CV–a la deshidratación del 2–propanol se lleva a cabo con ambas temperaturas de reacción evaluadas, 200 y 150 °C, observándose que la temperatura tiene un efecto importante sobre la distribución de productos; mientras que a 200 °C, el propileno es el único producto de reacción con ambos carburos, a 150 °C se obtiene propileno y éter diisopropílico con el material CV–a. El catalizador CV–A deshidrata el 2–propanol obteniéndose propileno como único producto de reacción a las condiciones de operación evaluadas, lo cual establece la presencia de sitios ácidos. Por otra parte, con el nitruro de vanadio a ambas temperaturas de reacción se obtiene acetona y propileno, a 200 °C se obtiene éter diisopropílico. En este caso, la distribución de productos revela la coexistencia de sitios ácidos y básicos, mientras que la fuerza de los sitios ácidos es de intensidad variable y la existencia de sitios ácidos débiles puede ser la responsable de la formación del éter diisopropílico. La mayor actividad catalítica observada con carburos de vanadio CV–a, podría ser consecuencia de una mayor área especifica, además este material posee la estructura cristalina del carburo en forma pura, mientras que CV–A muestra menor área específica y la mezcla de dos fases cristalinas. A pesar de obtener acetona como subproducto de reacción, la relación entre las propiedades de deshi–drogenación–deshidratación le confieren al nitruro de vanadio un fuerte carácter ácido.De acuerdo con los resultados mostrados en el cuadro II y a los mecanismos de reacción E1cb y E2 para la deshidratación de alcoholes, se deduce que la deshidratación del 2–propanol sobre los carburos y nitruro de vanadio proceden mediante ambos mecanismos de reacción. Por una parte, la transformación del 2–propanol sobre el material NV ocurre por un mecanismo E1cb, en el cual la velocidad de formación de propileno es lenta y requiere una alta proporción de sitios básicos, lo que conduce a una elevada formación de acetona; mientras que sobre los materiales CV–a y CV–A, la deshidratación del 2–propanol ocurre mediante el mecanismo E2, en donde la velocidad de formación de propileno es rápida; este mecanismo difícilmente permite la formación de acetona por la falta de un intermediario propoxi, lo cual explica la baj a proporción de acetona obtenida con e stos materiales (Di Cosimo et al. 1998).
Hidrodesulfuración de tiofenoLa figura 3 muestra los resultados obtenidos en la hidrodesulfuración de tiofeno con los 3 catalizadores preparados. La reacción se llevó a cabo a 350 °C y presión atmosférica. Los carburos y el nitruro de vanadio fueron pretratados a 400 °C en hidrógeno, para remover la capa de oxígeno superficial causada por el tratamiento de pasivación.
El NV fue el menos activo de la serie, al inicio de la reacción se obtiene hasta un 40 % de conversión de tiofeno, pero su actividad catalítica decrece hasta un 25 % hacia los 180 minutos de reacción. Ambos carburos de vanadio, al inicio de la reacción fueron menos activos que el nitruro de vanadio; sin embargo, con ambos materiales se presenta una ligera activación en función del tiempo de reacción. La progresiva activación de los catalizadores con el tiempo de reacción puede estar relacionada con la evacuación de oxígeno superficial remanente depositado durante la etapa de pasivación o bien, por la sulfuración de la superficie del carburo de vanadio durante la reacción debido al azufre contenido en la molécula de tiofeno. El catalizador CV–A alcanzó una conversión del 40 % hacia los 180 minutos de reacción y 30 % con el catalizador CV–a. Los productos de la HDS de tiofeno fueron principalmente C4 predominando el butano.Hidrotratamiento de gasóleo pesado de vacíoEn el cuadro III se muestran los resultados de eliminación de azufre y nitrógeno, así como la densidad del gasóleo obtenidos antes y después del hidrotratamiento de un GPV. El VC–a fue el más activo en la reacción de hidrodesulfuración e hidrodesnitrogenación, eliminando 40 y 51 % de azufre y nitrógeno, respectivamente contra un 37 y 44 % de eliminación de azufre y nitrógeno obtenido con el NV. La relación N/S superior a la unidad en cada caso pone de manifiesto la mayor capacidad desnitrogenante de estos materiales; por otra parte, la densidad API del gasóleo tratado con el carburo de vanadio se reduce de 22 hasta 28° API, y el tratado con el nitruro de vanadio a 27° API. La diferencia existente en la densidad entre el gasóleo sin tratar y el tratado es consecuencia de la transformación de las moléculas presentes en el gasóleo, las cuales sufren la ruptura de enlaces C–S y C–N, lo cual puede ser promovido por la acidez del catalizador. CONCLUSIONESLos carburos y nitruros de vanadio son activos en la hidrodesulfuración de tiofeno. Sin embargo, el nitruro se desactiva progresivamente con el tiempo de reacción, mientras los carburos por el contrario se activan superando la actividad del nitruro de vanadio. En la hidrodesulfuración e hidrodesnitrogenación del gasóleo pesado de vacío, la fase pura del carburo de vanadio fue más activa que la fase del nitruro de vanadio. La densidad API del gasóleo después de la reacción se incrementó indicando la modificación de las moléculas que componen el gasóleo, haciendo más ligero el combustible como consecuencia de la ruptura de enlaces C–S y C–N que llevan a moléculas de menor peso molecular; esta ruptura debe ser promovida por la acidez del catalizador, mientras el vanadio debe ser responsable de la saturación de las moléculas insaturadas.Las propiedades ácidas de los carburos son fuertemente dependientes de las condiciones de síntesis; un flujo alto de la mezcla carburante lleva a la obtención de un carburo con más alta área específica y sitios ácidos débiles. El nitruro de vanadio es un sólido con propiedades ácido–base con sitios ácidos de intensidad variable.
IBAÑEZ JESUS
CRF
FUENTE: http://www.uaemex.mx/Red_Ambientales/docs/congresos/TLAXCALA%202009/REVISTA/contaminacion/acervo/vol_24_1/2.pdf
RESUMENSe prepararon 2 carburos de vanadio (CV–a y CV–A) usando dos flujos de gas de carburización (660 mL/min y 8 mL/min, respectivamente) y un nitruro de vanadio (NV) se preparó usando 660 mL/min de NH3. Los catalizadores se caracterizaron por difracción de rayos X, adsorción de nitrógeno, titulación potenciométrica con n–butilamina, deshidratación de 2–propanol y se midió su actividad en la hidrodesulfuración de tiofeno y de un gasóleo pesado de vacío. La hidrodesulfuración del gasóleo pesado de vacío se llevó a cabo con el carburo de vanadio preparado con 660 mL/min de la mezcla del gas carburante y con el nitruro de vanadio. La deshidratación de 2 propanol indica que ambos carburos de vanadio tienen propiedades ácidas predominando los sitios débiles, mientras que el nitruro de vanadio tiene propiedades anfóteras. En la hidrodesulfuración de tiofeno, el carburo de vanadio preparado con 8 mL/min de CH4/ H2 fue el catalizador más activo, mientras que en la hidrodesulfuración de gasóleo, el carburo de vanadio preparado usando 660 mL/min de la mezcla de carburización fue más activo que el nitruro de vanadio, alcanzando un 40 % de eliminación de azufre y un 51 % de eliminación de nitrógeno.Palabras clave: carburo de vanadio, nitruro de vanadio, deshidratación de 2–propanol, HDS de tiofeno, HDS de gasóleo pesado de vacío ABSTRACTTwo vanadium carbides catalysts (CV–a and CV–A) were prepared using two carburation gas mixture flows (660 and 8 mL/min, respectively) and a vanadium nitride (NV) catalyst was also prepared with a 660 mL/min of NH3 flow. The catalysts were characterized by X–ray diffraction, nitrogen adsorption, n–butylamine potentiometric titration techniques and their acid and catalytic properties were tested in 2–propanol dehydration, hydrodesulphurization of thiophene and a heavy gas oil. Heavy gas oil hydrodesulphurization was carried out with the vanadium carbide prepared at 660 mL/min of carburation gas mixture and with the vanadium nitride catalysts. Results obtained in 2–propanol dehydration indicate that both vanadium carbides have acid properties with the prevalence of weak acid sites, while the vanadium nitrides have acid–base properties. In the thiophene hydrodesulphurization, the vanadium carbide prepared with 8 mL/min of CH4/H2 gas mixture was the most active catalysts, this catalyst shows a crystalline phase mixture. In the gas oil hydrodesulphurization the vanadium carbide "CV–a" was more active than the vanadium nitride, with a 40 wt % of sulphur remotion and a 51 wt % of nitrogen remotion.Key words: vanadium carbides, vanadium nitrides, 2 propanol dehydration, thiophene HDS, heavy gas oil HDS
INTRODUCCIÓNEn los últimos años, se ha generado una mayor conciencia ambientalista, de la cual ha emergido una nueva generación de científicos, tecnólogos e inclusive políticos, que enfatizan la necesidad de un cambio tecnológico en referencia a la decisión de los modelos de crecimiento de los países en vías de desarrollo, intentando con esto disminuir las emisiones de contaminantes por el uso indiscriminado de combustibles fósiles, las cuales afectan el ambiente y son las causantes de los cambios climáticos recientes. La relación entre la contaminación y el desarrollo económico sugiere que existe una tendencia para las emisiones per cápita y por unidad de producto interno bruto (PIB) que convergen con el tiempo, relación que se debe reducir en países en vías de desarrollo mediante aplicación de nuevas tecnologías y políticas (Stern 2005, Brock y Taylor 2004). La primera valoración del impacto ambiental en México enfocado al sector industrial después de ingresar al TLC, indicó que el crecimiento de la economía durante este periodo se llevó a efecto con un aumento de la degradación del ambiente; las emisiones de SO2, NOX y dióxido de carbono aumentaron significativamente. Sin embargo, la contaminación ambiental no es un problema exclusivo de países en vías de desarrollo sino que también afecta a los países altamente desarrollados, que finalmente son los que generan la mayor cantidad de contaminantes. Es, por tanto, que en los últimos años se han realizado esfuerzos para desarrollar tecnologías que reduzcan el contenido de este tipo de compuestos en los combustibles. El proceso de hidrotratamiento es de gran relevancia ya que mediante éste se reduce el contenido de O, N y S, familias de hidrocarburos insaturados y metales como Ni y V, principales contaminantes que contiene el crudo. En los últimos años se han realizado diversos estudios para desarrollar catalizadores que posean características particulares, que favorezcan la eliminación de los contaminantes antes mencionados, resistan las condiciones de operación requeridas para el procesamiento de cortes cada vez más pesados y que logren disminuir a los valores previstos la concentración de las fracciones más refractarias de los compuestos de azufre contenidos en el petróleo.Los carburos y nitruros de metales de transición han demostrado tener buena actividad catalítica en reacciones de hidrodesulfuración (HDS), hidrodesnitrogenación (HDN) e hidrogenación (HID) en especial de moléculas modelo como dibenzotiofeno o mezclas de dibenzotiofeno, quinoleina y tetralina (Dhandapani et al. 1998, Markel et al. 1990). Los carburos de W y Mo soportados en Al2O3, se han evaluado en la hidrogenación de tetralina a presión elevada, en ausencia y presencia de H2S, obteniendo resultados promisorios debido a estabilidad de la fase activa y resistencia al efecto de envenenamiento por azufre. Sin embargo, este tipo de catalizadores no se han evaluado con cargas reales de hidrocarburos como gasóleos o cargas similares, por lo cual en este trabajo se reporta la preparación de dos carburos y un nitruro de vanadio evaluando sus propiedades anfóteras y su capacidad de eliminación de azufre mediante la hidrodesulfuración de tiofeno y de un gasóleo pesado de vacío.Parte experimentalSíntesis de catalizadores. Se prepararon dos carburos de vanadio CV–a y CV–A, empleando dos flujos de carburación, 660 y 8 mL/min respectivamente, para analizar su efecto sobre el área específica y las propiedades estructurales y ácidas de los materiales resultantes. La síntesis de los carburos y nitruro de vanadio se llevó a cabo por el método de temperatura programada de reducción, usando un reactor de cuarzo de 10 mm de diámetro interno similar a lo reportado en la literatura (Neylon et al. 1999).Síntesis de carburo de vanadio (CV–a)Los carburos de vanadio se prepararon a partir de pentóxido de vanadio (Aldrich), el cual se carburizó con un flujo volumétrico de 660 mL/min por gramo de precursor de una mezcla constituida por CH4/H2 (Praxair 66.9 % en volumen de metano); la preparación se inició con un calentamiento lento desde 25 °C hasta 900 °C con un incremento lineal de temperatura de 1 °C/min, permaneciendo a 900 °C por 60 minutos. Finalizada la etapa de carburización, el sólido se enfrió hasta temperatura ambiente y su pasivación se hizo con una mezcla de Ar/O2 (Praxair 1 % en volumen de O2) por 60 minutos, el sólido obtenido se denominó CV–a.Síntesis de carburo de vanadio (CV–A )El carburo de vanadio (CV–A) se preparó con un flujo de carburización de 8 mL/min por gramo de precursor, manteniendo los demás parámetros de síntesis similares a los usados para el catalizador CV–a.Síntesis de nitruro de vanadio (NV)El pentóxido de vanadio se nitruró con un flujo volumétrico de 660 mL/min de amoniaco (Praxair 99.9 %) por gramo de precursor, calentando lentamente a una velocidad lineal de 1 °C/min desde temperatura ambiente hasta 530 °C, permaneciendo a esta temperatura por 60 minutos; finalizada la etapa de nitruración, la pasivación del catalizador obtenido se hizo con una mezcla de Ar/O2 por 60 minutos. El sólido obtenido se denominó NV.Caracterización de los catalizadoresLos catalizadores se caracterizaron por medio de difracción de rayos X, utilizando un difractómetro de rayos X Brukker, modelo Advance D8 con radiación CuKa. La medida del área específica se realizó en un equipo RIG 100–19 por adsorción física de nitrógeno, a temperatura de nitrógeno líquido. Previo a la determinación del área específica, las muestras fueron desgasificadas a 200 °C por 4 horas con un flujo de helio 40 mL/min (Praxair 99.9 %). La titulación potenciométrica con n–butilamina permitió la evaluación del número total de sitios ácidos así como la fuerza ácida de estos sitios. El potencial (E) del electrodo indica la máxima fuerza de los sitios ácidos superficiales y el rango donde una meseta es observada (meq/g de sólido) indica el número total de sitios ácidos. La fuerza de los sitios ácidos se asigna de acuerdo con los siguientes rangos: E > 100 mV sitio muy fuerte, 0 <>
RESULTADOS Y DISCUSIÓN
Estudio de difracción de rayos XLa figura 1 muestra el diagrama de difracción de rayos X del polvo del CV–a, preparado con un flujo volumétrico de 660 mL/min de la mezcla de CH4/H2. El material presenta la estructura cúbica característica del carburo de vanadio, la cual se identificó por sus principales líneas de difracción a 37.5, 43.5, 63.2 y 75.8° de la escala 2 theta a través de la tarjeta 73–476 del Joint Committee on Powder Diffraction Standards (JCPDS), esta fase cristalina es similar a la reportada en la literatura (Neylon et al. 1999) y es considerada la fase activa en este tipo de materiales. Por otra parte, el catalizador CV–A sintetizado con un flujo bajo del gas carburante (8 mL/min) presenta una mezcla de carburo y trióxido de vanadio, este último se identificó por las líneas de difracción que caracterizan al trióxido de vanadio a través de la tarjeta 71–343 del JCPDS; la presencia de esta fase probablemente fue causada por el bajo contenido de H2 de la mezcla de carburación, el cual no alcanzó a desplazar completamente el oxígeno de la estructura del pentóxido e impidió la completa carburación del vanadio. Un comportamiento similar se observó en la síntesis de un carburo de molibdeno obteniendo una mezcla de carburo y trióxido de molibdeno (Li et al. 1996).
El patrón de difracción del NV, preparado a partir de V2O5 se muestra en la parte superior de la figura 1, sus principales ángulos de difracción localizados en 37.7, 43.8, 63.6 y 74.8° de la escala 2 theta identifican completamente un nitruro de vanadio con estructura cúbica, la cual se corroboró por medio de la tarjeta 78–1315 del JCPDS, fase cristalina también reportada como la fase activa para este tipo de catalizador (Neylon et al. 1999).Área específicaEn la preparación del carburo de vanadio, el V2O5 se transforma lentamente en el correspondiente carburo o nitruro dependiendo de la atmósfera de síntesis empleada en la preparación del material, durante este proceso se genera agua in situ continuamente. Las reacciones que prevalecen durante el proceso de carburación o nitruración en los catalizadores preparados se muestran a continuación:Para el carburo de vanadio CV–aV2O5 + 2CH4 + H2 –> 2CV + 5H2OPara el carburo de vanadio CV–AV2O5 + 4CH4 + 4H2 –>4CV + V2O3 + 12H2OPara el nitruro de vanadio NVV2O5 + 2NH3 –>2NV + 3H2O + O2El agua producida durante la reducción debe ser evacuada del medio de reacción para disminuir su contacto con el carburo en formación y evitar la posible sinterización del mismo, la cual puede ocasionar una reducción en el área específica. En el cuadro I se muestran los resultados de área específica obtenida con los tres catalizadores. El área obtenida en los tres materiales es baja, el nitruro de vanadio tiene únicamente 6 m2/g mientras que el catalizador CV–a preparado con un flujo volumétrico de 660 mL/min de la mezcla de CH4/H2 tiene un área de 9 m2/g mayor que la del catalizador CV–A; la mayor área obtenida con el material CV–a es debido probablemente a la mejor remoción del agua generada durante la reducción del vanadio.Determinación de acidezEn el cuadro I también se muestran los resultados de acidez total y la máxima fuerza ácida de los catalizadores determinado de acuerdo a la técnica de titulación potenciométrica para lo cual 0.15 gramos de la muestra se diluyen en acetonitrilo puro (J.T. Baker), siguiendo la neutralización de los sitios ácidos mediante su titulación con una solución de n–butilamina (Aldrich) en acetonitrilo al 0.05N (Cid y Pecchi 1985). Los resultados de acidez obtenidos por esta técnica en el catalizador CV–a indican la presencia de sitios ácidos muy débiles, (mV < –100), mientras que en los catalizadores CV–A y NV se observan la coexistencia de sitios ácidos fuertes y débiles (0 < mV < 100) (Vázquez et al. 2002). El flujo de carburización tiene un marcado efecto sobre la fuerza ácida de los sitios que se generan en los materiales y pueden ser analizados a partir de los resultados del cuadro I, en el cual se observa que el catalizador CV–a, preparado con un flujo elevado de carburización tiene sitios ácidos muy débiles, mientras que el carburo sintetizado con un flujo de 8 mL/min de la misma mezcla de carburización desarrolló sitios ácidos fuertes [12 E(mV)]; por otra parte, el nitruro de vanadio preparado con 660 mL/min de NH3 también desarrolló sitios ácidos fuertes.En la figura 2 se muestran los perfiles de neutralización de los sitios ácidos obtenidos mediante la titulación potenciométrica de n–butilamina con los tres catalizadores preparados. El nitruro de vanadio tiene sitios ácidos fuertes (+50 mV), sin embargo la concentración de estos es baja predominando los sitios ácidos débiles a muy débiles (–50 a –150 mV. El mismo comportamiento se observa con el material CV–A, el cual al inicio de la neutralización presenta sitios ácidos fuertes (+15 mV) pero estos se neutralizan rápidamente, observándose una concentración más homogénea de sitios ácidos débiles entre –50 y –100 mV; por otro lado, el perfil de neutralización del CV–a muestra la existencia de sitios ácidos de débiles a muy débiles (–100 a –170mV). Deshidratación de 2–propanolLa deshidratación del 2–propanol se estudió para determinar la existencia o ausencia de sitios anfóteros (ácido–base) en los carburos y nitruro de vanadio, con base en el análisis de las reacciones de eliminación que toman lugar en la transformación de este tipo de moléculas. Se ha reportado que si en la deshidratación de un alcohol se obtienen olefinas, se establece la existencia de sitios duales ácido–base, la presencia de una cetona indica la existencia de sitios básicos.En el caso de la deshidratación del 2 propanol, la formación de propileno indica la existencia de sitios anfóteros y la presencia de la acetona indica la presencia de sitios básicos. Por otra parte, la relación entre la selectividad de las reacciones de deshidrogenación y deshidratación es un índice indirecto del carácter acido o básico del catalizador, si el cociente es igual o mayor a 1 el sólido tiene un fuerte carácter básico (Domínguez et al. 2002).En el cuadro II se reportan los resultados de la deshidratación de 2–propanol de los tres materiales evaluados. Con el catalizador CV–a la deshidratación del 2–propanol se lleva a cabo con ambas temperaturas de reacción evaluadas, 200 y 150 °C, observándose que la temperatura tiene un efecto importante sobre la distribución de productos; mientras que a 200 °C, el propileno es el único producto de reacción con ambos carburos, a 150 °C se obtiene propileno y éter diisopropílico con el material CV–a. El catalizador CV–A deshidrata el 2–propanol obteniéndose propileno como único producto de reacción a las condiciones de operación evaluadas, lo cual establece la presencia de sitios ácidos. Por otra parte, con el nitruro de vanadio a ambas temperaturas de reacción se obtiene acetona y propileno, a 200 °C se obtiene éter diisopropílico. En este caso, la distribución de productos revela la coexistencia de sitios ácidos y básicos, mientras que la fuerza de los sitios ácidos es de intensidad variable y la existencia de sitios ácidos débiles puede ser la responsable de la formación del éter diisopropílico. La mayor actividad catalítica observada con carburos de vanadio CV–a, podría ser consecuencia de una mayor área especifica, además este material posee la estructura cristalina del carburo en forma pura, mientras que CV–A muestra menor área específica y la mezcla de dos fases cristalinas. A pesar de obtener acetona como subproducto de reacción, la relación entre las propiedades de deshi–drogenación–deshidratación le confieren al nitruro de vanadio un fuerte carácter ácido.De acuerdo con los resultados mostrados en el cuadro II y a los mecanismos de reacción E1cb y E2 para la deshidratación de alcoholes, se deduce que la deshidratación del 2–propanol sobre los carburos y nitruro de vanadio proceden mediante ambos mecanismos de reacción. Por una parte, la transformación del 2–propanol sobre el material NV ocurre por un mecanismo E1cb, en el cual la velocidad de formación de propileno es lenta y requiere una alta proporción de sitios básicos, lo que conduce a una elevada formación de acetona; mientras que sobre los materiales CV–a y CV–A, la deshidratación del 2–propanol ocurre mediante el mecanismo E2, en donde la velocidad de formación de propileno es rápida; este mecanismo difícilmente permite la formación de acetona por la falta de un intermediario propoxi, lo cual explica la baj a proporción de acetona obtenida con e stos materiales (Di Cosimo et al. 1998).
Hidrodesulfuración de tiofenoLa figura 3 muestra los resultados obtenidos en la hidrodesulfuración de tiofeno con los 3 catalizadores preparados. La reacción se llevó a cabo a 350 °C y presión atmosférica. Los carburos y el nitruro de vanadio fueron pretratados a 400 °C en hidrógeno, para remover la capa de oxígeno superficial causada por el tratamiento de pasivación.
El NV fue el menos activo de la serie, al inicio de la reacción se obtiene hasta un 40 % de conversión de tiofeno, pero su actividad catalítica decrece hasta un 25 % hacia los 180 minutos de reacción. Ambos carburos de vanadio, al inicio de la reacción fueron menos activos que el nitruro de vanadio; sin embargo, con ambos materiales se presenta una ligera activación en función del tiempo de reacción. La progresiva activación de los catalizadores con el tiempo de reacción puede estar relacionada con la evacuación de oxígeno superficial remanente depositado durante la etapa de pasivación o bien, por la sulfuración de la superficie del carburo de vanadio durante la reacción debido al azufre contenido en la molécula de tiofeno. El catalizador CV–A alcanzó una conversión del 40 % hacia los 180 minutos de reacción y 30 % con el catalizador CV–a. Los productos de la HDS de tiofeno fueron principalmente C4 predominando el butano.Hidrotratamiento de gasóleo pesado de vacíoEn el cuadro III se muestran los resultados de eliminación de azufre y nitrógeno, así como la densidad del gasóleo obtenidos antes y después del hidrotratamiento de un GPV. El VC–a fue el más activo en la reacción de hidrodesulfuración e hidrodesnitrogenación, eliminando 40 y 51 % de azufre y nitrógeno, respectivamente contra un 37 y 44 % de eliminación de azufre y nitrógeno obtenido con el NV. La relación N/S superior a la unidad en cada caso pone de manifiesto la mayor capacidad desnitrogenante de estos materiales; por otra parte, la densidad API del gasóleo tratado con el carburo de vanadio se reduce de 22 hasta 28° API, y el tratado con el nitruro de vanadio a 27° API. La diferencia existente en la densidad entre el gasóleo sin tratar y el tratado es consecuencia de la transformación de las moléculas presentes en el gasóleo, las cuales sufren la ruptura de enlaces C–S y C–N, lo cual puede ser promovido por la acidez del catalizador. CONCLUSIONESLos carburos y nitruros de vanadio son activos en la hidrodesulfuración de tiofeno. Sin embargo, el nitruro se desactiva progresivamente con el tiempo de reacción, mientras los carburos por el contrario se activan superando la actividad del nitruro de vanadio. En la hidrodesulfuración e hidrodesnitrogenación del gasóleo pesado de vacío, la fase pura del carburo de vanadio fue más activa que la fase del nitruro de vanadio. La densidad API del gasóleo después de la reacción se incrementó indicando la modificación de las moléculas que componen el gasóleo, haciendo más ligero el combustible como consecuencia de la ruptura de enlaces C–S y C–N que llevan a moléculas de menor peso molecular; esta ruptura debe ser promovida por la acidez del catalizador, mientras el vanadio debe ser responsable de la saturación de las moléculas insaturadas.Las propiedades ácidas de los carburos son fuertemente dependientes de las condiciones de síntesis; un flujo alto de la mezcla carburante lleva a la obtención de un carburo con más alta área específica y sitios ácidos débiles. El nitruro de vanadio es un sólido con propiedades ácido–base con sitios ácidos de intensidad variable. IBAÑEZ JESUS
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FUENTE: http://www.uaemex.mx/Red_Ambientales/docs/congresos/TLAXCALA%202009/REVISTA/contaminacion/acervo/vol_24_1/2.pdf
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