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

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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

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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/

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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

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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]

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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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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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http://www.shef.ac.uk/eee/nc35t/bulletin/Patane_NDR_GaInNAs.pdf

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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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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
COMUNICACIONES DE RADIO FRECUENCIA
FUENTE DE ORIGEN:
http://iopscience.iop.org/0953-8984/16/31/E01?ejredirect=migration

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