domingo, 27 de junio de 2010

GaN-based high-frequency detectors for telecom applications, Gerald Soto, CRF 2010-1, (2do Parcial).

For fiber-based optical data transmission, opto-electronic components are reaching their intrinsic speed limits.The next generation of telecommunications components will require ultra-high frequency detectors that do not suffer from parasitic capacitance due to space-charge effects. Given the physical constraints in terms of operation wavelength, a technology taking advantage of both a resonant absorption process and optical non-linearities would be ideal.

Such a process occurs between the bound levels of a quantum well (QW) fabricated from ultra-thin semiconductor layers. Intersubband (ISB) absorption, as it is called, is not only highly efficient but has the additional advantage of ultra-fast recovery time. In the past, it has been used in compound semiconductor systems (e.g., GaAs/AlGaAs and InGaAs/InAlAs) for fabricating photoconductive quantum-well infrared photo-detectors (QWIPs) in the mid-infrared (mid-IR) range. However, in order to access the shorter near-IR wavelengths, a different class of materials, with considerably larger conduction-band discontinuity, must be employed.

In our work, we stack and interleave thin gallium nitride quantum well (GaN QW) and aluminum nitride (AlN) barrier layers. This combination offers a conduction band discontinuity of nearly 2eV, which is sufficiently large to accommodate ISB transitions in the near-IR range. In order to avoid dark current noise, a photovoltaic detection mechanism is used for this work. As Figure 1 shows, electrons undergo a small lateral displacement (along the growth direction) when being excited into the upper-bound level of a QW. This displacement results in the formation of an electrical dipole that polarizes the surrounding material. Added up across the entire quantum well and repeated several times, such polarization results in an appreciable photoinduced voltage. This mechanism, first described in 1989, is also called resonant optical rectification.

Figure 1. Asymmetric quantum well in gallium nitride/aluminum nitride (GaN/AlN) superlattice.

Our photodetector structures are epitaxially grown on c-face sapphire substrates using plasma enhanced molecular beam epitaxy. The active region consists of a 40-period superlattice with 1.5nm thick Si-doped GaN QW layers and 1.5nm undoped AlN barriers. This superlattice is deposited on a 1μm AlN buffer and covered with a 100nm AlN cap layer. Processed photodetectors are entirely planar and consist of a pair of evaporated metal contacts: a dark reference contact and an illuminated signal contact with a size of 100μm^2. The photovoltage produced by illumination is then measured between the reference and the signal contact.

Due to the ISB nature of the involved optical transition, a spectrally narrow response results with a center wavelength of 1.55μm and a relative linewidth on the order of 10%. Although maximum performance occurs at a temperature of 200K, room temperature operation with a maximum response of 10V/W could also be observed.

For high frequency testing, we illuminated the detector with a directly modulated 1.55μm laser diode coupled to an optical fiber. The detector signal was amplified with a single low-noise amplifier and measured in a spectrum analyzer. As Figure 2 shows, the maximum frequency for which a signal could be seen was 2.94GHz. Because no special care was taken to reduce parasitic capacitance and impedance mismatch, this value does not constitute the intrinsic speed limit of this novel optoelectronic component. In addition, it shows that the wide bandgap semiconductor GaN is not only suitable for applications in the ultraviolet wavelength region, but also has considerable potential as a material for future optical data transmission systems.

Figure 2. Frequency response of the detector. The inset shows the signal at the maximum frequency for which a signal could be seen.

In sum, the use of an optically non-linear effect in ultra-thin layers of a strongly piezo- and pyro-electric semiconductor has resulted in the demonstration of a high-speed photo-detector for future multi-gbit/s data rate telecom applications. We have demonstrated the high-speed operation of such a device up to frequencies in the lower GHz range. In future experiments, we will optimize these detectors and reduce the parasitic effects in the amplifier electronic circuit. This should eventually result in components operating beyond 40 GHz.

GaN-based waveguide devices for long-wavelength optical communications, Gerald Soto, CRF 2010-1, (2do Parcial).

In order to make full use of the wide bandwidth provided by optical fibers, more and more wavelength channels have been used in wavelength division multiplexed ~WDM! optical networks. Among others, WDM optical demultiplexers, optical switches, and wavelength routers are fundamental devices in multiwavelength, dynamic optical networks. With the rapid advancement of photonic integrated circuits (PIC), silica-based array waveguide grating (AWG)has become a popular approach for WDM multiplexing and demultiplexing.1 Since silica is not a semiconductor material and hence purely passive, the transfer function of a silicabased AWG is usually not tunable, or can only be slowly tuned by thermal effect.2,3 InP has been another popular material to make planar waveguide PICs. As a semiconductor material, InP-based PIC can potentially be made fast tunable with carrier injection,4 however, due to its high refractive index, high temperature sensitivity, and high insertion loss,5 InP-based AWG devices have so far not become commercially competitive.

III-nitride wide-band-gap semiconductor materials have attracted much attention in recent years.6,7 In addition to emitting in UV/blue wavelength region, III-nitride optoelectronic devices are able to operate at high temperatures and high power levels due to their mechanical hardness and larger band offsets. Research in III nitrides has so far been focused on their applications in blue/UV wavelength regions. Their optical characteristics and potential applications in infrared for optical communications remain largely unknown. In this letter, we propose to make functional optical waveguide devices using GaN/AlGaN semiconductor materials and explore their potential applications in infrared wavelength regions for fiber-optic communications.

In order to design guided-wave optical devices, the knowledge of material refractive indices in the operating wavelength region is essential. Due to the unavailability of experimental data in infrared, we have conducted the refractive index measurements for AlxGa1-xN with different Al molar fractions. In order to perform this measurement, a number of samples of AlxGa1-xN films were grown by metmetalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The films thickness range from 1.1 to 1.5 mm and Al molar fractions range from x50.1 to x50.7.8 To evaluate the refractive index of each film, optical transmission spectra were measured. Due to the Fabry–Perot ~FP! interference caused by the two facets of the film ~one facet is between AlxGa1-xN and the air and the other facet is formed between AlxGa1-xN and sapphire!, optical transmission efficiency is wavelength dependent. With the knowledge of the film thickness, the film refractive index can be obtained by best fitting the measured optical transmission spectrum to a well-known FP transmission equation.

Figure 1(a) shows the measured refractive indices of AlxGa1-xN versus wavelength for several different Al molar fractions. The continuous curves in the same figure were numerical fittings by using the first order Sellmeier dispersion formula:

The coefficients for best fitting are displayed in Fig. 1~a! and their variations versus Al molar fraction x are shown in Fig. 1(b). Since we are mostly interested in the refractive indices in 1550 nm wavelength window, this information can be collected from Fig. 1 and the following polynomial expression is obtained for the Al molar fraction ~x! dependence of the refractive index at 1550 nm wavelength:

The monotonic decrease of AlxGa12xN refractive index with the increase of Al molar fraction x makes the design of single-mode optical waveguide devices straightforward.

We used beam-propagation method (BPM) simulation tools to design single-mode optical waveguide devices. Figure 2 shows schematically the cross section of the designed single-mode waveguide based on GaN core and AlxGa1-xN cladding. Several different waveguide configurations have been designed, including straight waveguides, and 232 waveguide couplers. To verify the design, a number of waveguide samples were prepared. In the fabrication process, a 4-mm-thick epitaxial film of AlxGa1-xN was grown on a sapphire substrate and a 3-mm-thick GaN film was deposited on top of the AlxGa12xN layer and then the optical waveguide structures were formed by photolithographic patterning and inductively coupled plasma (ICP) dry etching.9 According to the design, the etching depth is controlled at ;2.8 mm and the Al molar fraction x is about 3%. As an example,Fig. 3 shows a typical 232 waveguide coupler fabricated with this process. The power splitting ratio of this particular coupler was designed to be 3 dB.

Terahertz detection by GaN/AlGaN transistors, Gerald Soto, CRF 2010-1, (2do Parcial).

More than ten years ago, Dyakonov and Shur proposed using submicron field effect transistors (FETs) as sources and detectors of terahertz electromagnetic radiation [1, 2]. Since that time both detection and generation in the sub-THz and THz frequency range have been experimentally demonstrated using compound semiconductor and silicon transistors [3–10]. For detection, the regimes of the resonant and non-resonant operation are distinguished. When wt<< 1,the FET operates in the non-resonant regime, where o is the radian frequency and t the plasmon decay time; the transistor response to the electromagnetic radiation is a decreasing function of the gate voltage. When wt>> 1,the transistor has a resonant response at resonant plasma frequencies Wn=Wo(1+2N), where N= 0, 1, 2, . . . , Wo= (pi=2Lg).SQR(e^2.ns/Cm) is the fundamental radian plasma frequency, Lg is the gate length, ns is the channel concentration, C is the gate capacitance per unit area, m is the effective mass, and e is the electron charge. The decay time is equal to or shorter than the scattering rate. It might be affected by the ballistic transport [11], the viscosity of the electron fluid owing to electron-electron collisions [1], and by a possible effect of oblique plasma modes [12].

Since the channel concentration is a function of the gate voltage, the FET in the resonant mode of operation is a tunable detector of THz radiation. In spite of the experimental demonstration of resonant sub- THz detection by GaAs-based transistors (see [7] and references therein) and THz detection by InGaAs-based transistors [13], this mode of operation remains a challenge to achieve. Since GaN/AlGaN heterostructure FETs (HFETs) have extremely high electron concentration in the channel (exceeding 1013cm 2), these transistors with gate length Lg <>

One of the most important parameters of the detectors is noise equivalent power (NEP), which can be found as N/RV, where N is the noise of the transistor in V/Hz^0.5 and RV is the responsivity in V/W. Fig. 2 shows the NEP for transistors with Lg=250 nm at 300 K for frequencies of radiation f=0.2 and 0.7 THz. The responsivity, which is the ratio of the output in volts to the radiant input in watts, was estimated in two different ways. For f=0.7 THz, the responsivity was estimated as RV=DV/(PtSa/St), where Pt is the total power of the source, St is radiation beam spot area, and Sa is the transistor area, which includes contact pads. In the experiment with 0.2 THz, the radiation was focused to the diameter approximately of the same size as the transistors, including the contact pads. Therefore, the total power of the source was taken for the NEP estimate. Since detection was studied at zero bias, the noise was taken equal to the thermal noise N=SQR(4kTRfet), where RFET is the gate voltage dependent drain to source resistance, which can be extracted from the transfer current voltage characteristic of the field effect transistor. Fig. 2 shows the NEP at 300 K against gate voltage. The inset in Fig. 2 shows the responsivity for the 0.2 THz experiment as a reference. As seen, in spite of a relatively low responsivity, the minimum NEP is of the order of 5 10 9 W/Hz^0.5. The minimum NEP corresponds approximately to the voltage, where the responsivity is the largest. This value of NEP is slightly higher than for such commercial detectors as Golay cells, pyroelectric detectors and Schottky diodes, having, meanwhile, the potential advantage of operation at very high sampling frequency of several tens of gigahertz.

Conclusions: We have investigated different GaN/AlGaN devices and demonstrated an efficient detection of electromagnetic radiation at THz frequencies. While varying the temperature in the range from 4 K to 300 K and the excitation frequency within 0.2–2.5 THz, we have shown the resonant detection due to excitation of the plasmon modes. These detectors demonstrate reasonably low noise equivalent power suitable for practical applications.

Dispositivos de potencia de GaN basado en mejorar la conversión de energía, Gerald Soto, CRF 2010-1,(2do Parcial).

With radical improvements in R(ON) x Qsw figure of merit (FOM), leading to an application value proposition - efficiency x density/cost - that is an order of magnitude better than state-of-the-art silicon, gallium nitride (GaN)-based power devices promise a revolution in high efficiency, high density, cost effective power conversion solutions.

By enabling rapid adoption of switch mode power supplies (SMPS), silicon power MOSFETs have been at the forefront of power conversion for last three decades.

From planar HEXFETs, introduced in 1978 by IR, to TrenchFETs and superjunction (SJ) FETs, power MOSFETs have given bipolars a run for their money for nearly 30 years. But now this silicon power device has approached a performance plateau. That means, going forward it does not have the juice to deliver performance/cost ratio demanded by next -generation applications. Consequently, any performance increment will result in unwarranted excessive expenses.

New materials and transistor structures are therefore needed to fill this power conversion performance gap. Even though silicon carbide (SiC) developers have been tackling these issues in the past ten years, it has not made any dent in this market because of cost. Besides the intrinsic cost structure of SiC, the limited supply of quality material also makes this technology very expensive, adding to the non-scalability of the substrate size and the expitaxial deposition throughput shortcomings.

Anticipating a need to look for solutions beyond silicon, scientists and engineers have been researching new technologies. One of these is the proprietary GaN-based power device technology platform developed by IR, which delivers a FOM performance at least 10x better than existing silicon MOSFETs.

GaN-on-Si Benefits

Since bulk GaN substrates are uneconomical, developers have taken the hetero-epitaxial route for building GaN-based power devices. However, until now, major substrates used for GaN epitaxy have been SiC or sapphire. But, both are relatively expensive propositions.

Although silicon was an attractive low-cost alternative, it remained difficult because of defects and deformations due to intrinsic mismatch in lattice constants and thermal expansion coefficients. Leveraging the extensive industry experience in GaN epitaxy and devices, significant engineering efforts have been made to resolve these issues. As a result, GaN-on-silicon technology platform has been developed to offer high epitaxial film uniformity, lower defect levels and higher device reliability. In addition, the device manufacturing process is CMOS compatible, thus allowing high volume deposition of GaN-based material on low-cost silicon wafers with larger diameter substrates. This novel GaN-on-Si design and fabrication technology platform is referred to as GaNpowIR.

As shown in the Fig, the basic GaN-on-Si-based power device is a high electron mobility transistor (HEMT), based on the presence of a two dimensional electron gas (2DEG) spontaneously formed by the intimacy of a thin layer of AlGaN on a high-quality GaN surface. As this device structure is a HFET with a high electron mobility channel that conducts in the absence of applied voltage (normally on), several techniques have been developed to provide a built-in modification of the 2DEG under the gated region that permits normally off behavior.

In essence, this novel power device delivers dramatic improvements in three basic FOMs, namely specific ON resistance, device switching and power conversion application. Fundamental physics indicates that GaN-based HEMTs can achieve a factor of 10 improvement in RDS(ON) over silicon MOSFETs in the 100 to 300V application range.

With on-going improvements in the GaN power devices, it will offer a 10x improvement in RDS(ON) over current state-of-the-art silicon MOSFETs within five years.

Likewise, in the 600 to 1,200V application range, the calculated material limit curves for unipolar devices show that GaN-based power devices have the potential of further reducing RDS(ON) by a factor of 100 over silicon MOSFETs. Results from early stage development of GaNpowIR devices are compared with silicon and SiC devices in this figure.

Besides enhancing specific ON resistance, GaN-based power devices also offer a significant boost in the device switching FOM RDS(ON) x Qg(RQ). Simulated results from early prototypes fabricated indicate that the first generation of GaN-on-Si-based power HEMTs are expected to realize about a 33% improvement over the state-of-the-art silicon MOSFETs. As this momentum continues, it is estimated that GaN-based power devices will further cut RQ FOM by an order of magnitude within five years of introduction of GaNpowIR technology platform in 2009.

Paradigm Shift

The combination of low gate capacitance and low ON resistance permits switching at much higher frequencies. Internal simulations show that GaNpowIR devices have the potential to switch at frequencies above 50MHz, far beyond the capabilities of silicon MOSFETs. Consequently, DC-DC converters using GaN-on-Si-based HEMTs will achieve much higher power density without compromising conversion efficiency. Current state-of-the-art multi-phase silicon-based solutions perform 12V to 1.2V conversion efficiently up to about 2MHz per phase. By comparison, GaN-on-Si-based power devices will enable efficient power conversion to greater than 50MHz per phase. That translates into fewer external components and lower parasitic-related power losses. The end result is the achievement of a high density, high efficiency and low cost system.

Analog vs Digital Regulation

In order to take advantage of this fundamental improvement in basic value proposition of power conversion, the regulation scheme must provide the required precision at a much higher bandwidth than that which is currently deployed. This presents a fundamental challenge to the adoption of digital circuits to provide the regulation function, as the precision frequency cost FOM is strained. Switching at frequencies above 20MHz presents a real challenge to digital regulation.

In order to achieve the required regulation performance, including absolute accuracy of <0.5% of VOUT and switching at >20MHz, an effective resolution of some 12 bits is needed for analog-to-digital converters (ADC) clocking at hundreds of MHz. This results in larger die area, coupled with high speed designs using deep sub-micron CMOS technologies. Such solutions substantially increase the cost of the regulator and reduce the dynamic headroom of the regulator, making it much more susceptible to noise-induced error.

Consequently, the benefits of digital regulation (non-linear control, self compensation, etc) are second order to the fundamental value proposition presented by the ability to switch at high frequency. Hence, it is likely that the proposition of digital regulation will not be compelling for the most demanding applications, which will require cost effective high density and high efficiency power conversion in the near future.

By achieving dramatic improvements in specific ON resistance, device switching FOM R(ON) x Qsw and power conversion application FOM efficiency x density/cost, a commercially viable GaN-on-Si-based power technology platform is all set to stimulate a new revolution in high frequency, high density, highly efficient cost effective power conversion solutions. As a result, new conversion architectures and control schemes will be developed to take full advantage of the capabilities of GaN-on-Si-based power HEMTs.

Nueva célula fotovoltaica mediante la adición de cobalto a una película delgada tipo Nitruro de Galio, Gerald Soto, CRF 2010-1, (2do Parcial).

The PV cell was realized by adding "3d transition metals" including manganese (Mn) to transparent composite semiconductors with a wide bandgap such as gallium nitride (GaN). It could enable to develop a highly-efficient PV cell by using a simply-joined cell without making a multi-junction cell.

Currently, the conversion efficiency of the new PV cell is low, but its open voltage (Voc) is as high as 2V.

The research group delivered a 90-minute lecture on the cell under the title "Nitride Semiconductor Added With Transition Metals as a Photoelectric Conversion Material for Ultraviolet, Visible and Infrared Lights ~ In the Aim of Realizing the Next-generation Super-efficient PV Cell With a Simple Element Structure."

Sonoda found that when Mn is added to GaN, which is transparent because its bandgap is as large as 3.4eV, until its component ratio reaches several to 20%, the absorbing coefficient of the GaN becomes continuously high for a wide wavelength band of light including ultraviolet, visible and infrared lights. In fact, a PV cell made by adding Mn to p-type GaN is black and transparent unlike an element that does not contain Mn.

Sonoda explained the "impurity band" model, which is mainly composed of Mn's energy levels in the 3d orbit. There has been a technology to set a ladder to a forbidden band, to which electrons with small energy levels cannot climb, by adding impurities to a semiconductor material with a large bandgap so that light with a longer wavelength can be absorbed. And such a band-gap structure is commonly called "intermediate band." However, it is not clear whether the new mechanism is the same as that of the intermediate band, Sonoda said.

The research group added a variety of 3d transition metals other than Mn and obtained similar results in many cases. A 3d transition metal is an element whose number of electrons increases in the 3d orbit, which is inside the outermost orbit, as its atomic number (the number of protons in the atomic nucleus) increases. Specifically, scandium (Sc), titanium (Ti), vanadium (V), chrome (Cr), Mn, iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) and zinc (Zn) are 3d transition metals.

By appropriately choosing those additive elements, even aluminum nitride (AlN), which has a very large bandgap, can possibly have an absorbing region in the visible light range, Sonoda said.

This time, the PV cell was prototyped by adding cobalt to p-type GaN. Its Voc is 2V or more at 1 sun. In general, when a unijunction cell has a Voc of 2V or more, its bandgap is large, and only the short-wavelength part of visible light (blue, green, etc) can be converted into electricity. However, it does not apply to the new PV cell.

On the other hand, the short-circuit current density of the PV cell is about 10μA/cm2, which is about 1/1,000 that of a normal crystalline silicon PV cell. Because the cell and electrodes are separated, the electric resistance of the p-type GaN connecting them is very large, Sonoda said.

This time, it was not possible to accurately measure the output current because photolithography machines could not be used for designing the cell. As a result, the current cell conversion efficiency is only slightly higher than 0.01%.

Recently, many researchers are adding indium (In) to GaN-based PV cells in the aim of narrowing the bandgap and enabling to absorb visible lights. However, in such cases, multi-junction cells using materials with, for example, different ratios of indium are necessary for converting a wide wavelength band of light into electricity. The findings of the research group are expected to pave the way to a GaN-based PV cell with a totally different mechanism.


Las capas antirreflectantes son comúnmente usadas en las componentes de los sistemas ópticos y en dispositivos optoelectrónicos como los detectores de radiación. En este trabajo presentamos la obtención de Nitruro de Silicio a partir de depósito químico en fase vapor asistido por plasma (PECVD), y su caracterización. La obtención de este material tiene la ventaja de llevarse a cabo dentro del mismo proceso de fabricación de dispositivos optoelectrónicos, por lo que se evita cualquier contaminación. La reflectividad alcanzada con esta capa es del 99 % para el valor de longitud de onda donde se optimiza el grosor de la capa, y de 96 %, al menos, para longitudes de onda diferentes. Se muestra la variación de reflectividad para el caso en que la incidencia de la radiación no sea perpendicular a la superficie.

El presente trabajo se generó durante la fabricación de un detector de radiación infrarroja, basado en la generación de una barrera Schottky entre Silicio cristalino y una aleación amorfa de Silicio Germanio (a-SiGe) [1] obtenida por el método de depósito químico asistido por plasma (PECVD) [2]. El esquema general de este dispositivo es mostrado en la figura 1. El detector es iluminado por su cara trasera, donde tenemos a la oblea de silicio, con un índice de refracción de 3.5, y una reflectividad de más del 50 % en el intervalo entre 0.5 y 5 μm; por lo que es necesario el uso de capas antirreflectantes para lograr un mejor acoplamiento de la radiación incidente dentro del detector, o dicho de otra manera, disminuir la reflectividad de la interfaz Silicio-aire.

Una gran variedad de materiales pueden ser usados como capas antirreflectantes, sin embargo, nuestro problema fundamental radica en el hecho de que la capa de SiGe es altamente oxidante una vez que entra en contacto con el medio ambiente. Por eso necesitábamos un material que pudiese ser depositado durante el proceso de fabricación del dispositivo, de preferencia, depositado por el mismo método que la capa amorfa, para así evitar la oxidación de la capa por contacto con el medio ambiente; por esta razón se decidió trabajar con Nitruro de Silicio.

Si usamos una capa antirreflectante, la reflectividad viene dada por el coeficiente de Fresnel:

Donde n0 es el índice de refracción del medio (aire, n0 = 1), ns es el índice de refracción del substrato (Silicio), narc es el índice de refracción de la capa antirreflectante y δ1 es la diferencia de fase que introduce la capa en la onda incidente,


Donde λ es la longitud de onda de la radiación y tarc es el espesor de la capa antirreflectante.
Si seleccionamos el espesor de la capa, de modo que,

Obtenemos un valor mínimo para la reflectividad dado por:


De esta última expresión se concluye que la reflectividad será cero cuando:


Y por supuesto se halla elegido correctamente el espesor de la capa, según ecuación (4).

Si la incidencia de la radiación no es perpendicular a la superficie entre los dos medios, entonces el índice de refracción debe ser sustituido por el índice de refracción efectivo:

(7) y (8)

Si la incidencia de la radiación no es perpendicular a la superficie entre los dos medios, entonces el índice de refracción debe ser sustituido por el índice de La capa de Nitruro de Silicio fue obtenida por PECVD usando el sistema AMP 3300 de Applied Materials. SiH4 y Amoníaco fueron usados como gases reaccionantes. Las películas fueron depositadas sobre Corning 1737. Las condiciones de depósito fueron las siguientes: Pressión de 0.6 Torr, Potencia de 350 W, frecuencia de 110 kHz y temperatura de depósito de 300 0C.

El índice de refracción fue medido en el intervalo de 0.5 a 5 μm, usando un espectrofotómetro de doble haz; una muestra de vidrio Corning sin película depositada fue empleada como referencia para realizar las mediciones.refracción efectivo:

En la figura 2 se muestra la gráfica de índice de refracción del Nitruro de Silicio, y a continuación en la figura 3 aparece la reflectividad que se obtiene al usar una capa de 150 μm, lo que significa optimizar el espesor para la longitud de onda λ0 = 1.1 μm.

La televisión 3D con espejos deformables de Nitruro de Silicio, Gerald Soto, CRF 2010-1, (2do Parcial).

La televisión 3D (o 3D TV) es un sistema de televisión que permite ver las imágenes en 3 dimensiones dando sensación de profundidad. Basado en la esterovisión, en la que también se basa el cine en 3D, se captan, procesan, emiten, reciben y muestran dos imágenes similares, que son captadas por dos cámaras de televisión situadas una cerca de la otra.

Existen al menos dos tecnologías diferentes para la realización práctica de esta técnica, la más extendida utiliza gafas cpn cristales polarizados y la otra se basa en pantallasde LCD o plasmaque incorporan unas microlentes o espejos colocados en cada píxel, que permiten enviar una imagen ligeramente diferente a cada ojo (el mismo principio que la holografía) y con ello nuestro cerebro puede utilizar la diferencia entre imágenes para componer el espacio. Algunos aspectos de esta nueva tendencia aún se encuentran en desarrollo.

Tipos de televisores 3D

  • Televisión stereo
La necesidad de variar el ángulo del haz de luz de cada píxel hizo que se utilizasen en un principio espejos deformables. Cada uno de estos espejos consiste en una membrana fina y circular de nitruro de silicio recubiertos de aluminio y suspendidos sobre unos electrodos. Cuando se le aplica un voltaje al electrodo la membrana del espejo se deforma de manera parabólica. El frente de onda del haz de luz incidente en el espejo puede ser cambiado variando el voltaje aplicado sobre el electrodo. Si no se aplica ningún voltaje, la membrana de espejos se mantendrá plana. De esta manera se pueden realizar imágenes en tres dimensiones manteniendo también el modo de dos dimensiones habitual. Esta metodología es la que se denomina televisión stereo y sólo permite que haya un observador. También tiene la problemática de que el sistema necesita saber desde dónde mira el observador. Para tener esta información se requiere tener una cámara con un algoritmo que localice los ojos del espectador y sea capaz de enfocar los espejos en aquella dirección. La imagen captada por el observador será igual en cualquier punto del ángulo de visión, es decir, aunque nos movamos no percibiremos un cambio de posición relativa a la imagen. El vídeo reproducido en este sistema tiene que tener dos imágenes por frame.

  • Televisión autoestereoscópica
La televisión autostereoscópica se considera una mejora respecto al sistema anterior. Además de representar la información de profundidad permite la selección arbitraria del punto de vista y dirección dentro de la escena. De esta manera, un cambio de posición del espectador afecta a la imagen que éste observa. La sensación es que la escena gira con el movimiento del observador. Este fenómeno se conoce cómo Free viewpoint (punto de vista libre) y estos están limitados a 8 actualmente por cuestiones tecnológicas. Cada Free Viewpoint son dos imágenes (una por cada ojo) lo que hace que podamos mostrar en la actualidad 9 imágenes a la vez, diferentes en el plano horizontal, lo que quiere decir que la pantalla tendrá que tener una resolución mucho mayor que la HDTV. Se resuelve también el problema con la capacidad de espectadores, puede haber más de uno, ya que no es necesario localizarlos. El principal cambio es la utilización de microlentes que permiten controlar la difracción de los haces de luz. También permiten mantener el modo de dos dimensiones.

Tener diferentes puntos de vista significa incrementar el número de imágenes mostradas a la vez. Esto quiere decir que el monitor debe tener una resolución 4 veces mayor que la resolución estándar (SDTV) y soportar corrientes de vídeo de millones de bytes por segundo. Además, la utilización de lentes delante de la pantalla puede suponer una pérdida de brillo, contraste y color si no se aplica un sistema de control riguroso sobre las microlentes.

sábado, 26 de junio de 2010


Los dispositivos de ondas acústicas basados en materiales piezoeléctricos se han estado utilizando desde hace más de sesenta años. Debido a su versatilidad, dichos dispositivos se utilizan en gran cantidad de aplicaciones, como líneas de retardo, osciladores, resonadores, sensores, actuadores, etc.; su principal usuario es la industria de las telecomunicaciones, que consume aproximadamente tres mil millones de filtros de ondas acústicas anualmente. Normalmente estos dispositivos son filtros de onda acústica en superficie (SAW) que actúan como filtros paso banda tanto en las secciones de radiofrecuencia como en las de frecuencia intermedia de la electrónica de los equipos de transmisión y recepción. Sin embargo, en los últimos años han aparecido nuevas aplicaciones de los dispositivos de ondas acústicas que pueden llegar a igualar la demanda del mercado de las telecomunicaciones, como por ejemplo la industria de automoción (sensores del par motor o de la presión de los neumáticos), las aplicaciones médicas (biosensores) o aplicaciones comerciales o industriales (sensores de gases, de humedad, de temperatura o de masa). Los sensores de ondas acústicas tienen precios competitivos, son resistentes, muy sensibles e inherentemente fiables. Durante mucho tiempo los materiales más comúnmente utilizados en la tecnología de fabricación estándar de dispositivos de ondas acústicas han sido monocristales de cuarzo (SiO2), tantalato de litio (LiTaO3) o niobato de litio (LiNbO3). Cada uno de estos materiales tiene distintas ventajas y desventajas dependiendo de factores como por ejemplo el coste, la dependencia con la temperatura, la atenuación o la velocidad de propagación.

Actualmente hay nuevos materiales que están cobrando interés en este tipo de aplicaciones, como el nitruro de galio (GaN), el arseniuro de galio (GaAs), la langasita (LGS), el óxido de zinc (ZnO), el titanato circonato de plomo (PZT) o el nitruro de aluminio (AlN).

Las propiedades piezoeléctricas del nitruro de aluminio lo convierten en un material muy adecuado para la fabricación de dispositivos electroacústicos como filtros de onda acústica en superficie (SAW) y en volumen (BAW), líneas de retardo, osciladores, resonadores, etc. La velocidad de propagación de las ondas SAW en las películas epitaxiales de nitruro de aluminio es muy elevada (casi el doble que en el caso del cuarzo); este tipo de material tiene un coeficiente de variación de la velocidad de propagación de las ondas SAW con la temperatura muy bajo y un valor del coeficiente de acoplo electromecánico razonablemente alto.

Hasta la fecha no ha sido posible crecer capas puramente epitaxiales (monocristalinas) de nitruro de aluminio. Existen diferentes técnicas de depósito que permiten obtener películas policristalinas de nitruro de aluminio con características epitaxiales, como por ejemplo el depósito químico en fase de vapor asistido por plasma (PECVD), de metalorgánicos (MOCVD), CVD por jet de plasma en continua, o la epitaxia por haces moleculares (MBE). Sin embargo, estas técnicas, además de requerir equipos muy caros en algunos casos, suponen el uso de costosos sustratos cristalinos y altas temperaturas que no son compatibles con las tecnologías del silicio que se utilizan actualmente en la fabricación de dispositivos ni con el crecimiento del material sobre electrodos metálicos. No obstante, existen otras técnicas que permiten obtener material en forma de película delgada policristalina (con una cierta dispersión en la alineación de sus cristales) con buenas propiedades piezoeléctricas a pesar de no ser epitaxial.

La pulverización reactiva o sputtering, en la que los iones generados en una descarga arrancan átomos de la superficie de un blanco, es una técnica de depósito que permite obtener un material policristalino no epitaxial con propiedades piezoeléctricas muy similares a las de las películas epitaxiales trabajando a bajas temperaturas. Por este motivo en los últimos tiempos se han dedicado numerosos estudios a esta técnica de depósito en sus distintas modalidades:

- Tensión continua (DC).
- Tensión continua pulsada.
- Resonancia electrón ciclotrón (ECR).
- Radiofrecuencia (RF).

La técnica de pulverización reactiva de radiofrecuencia, es una técnica sencilla, versátil y adecuada para depositar nitruro de aluminio policristalino con buenas propiedades cristalinas y piezoeléctricas que lo hacen apropiado para su uso en dispositivos de ondas acústicas.

Tesis Doctoral
Lucía Vergara Herrero
Ingeniero de Telecomunicación

Gerald Soto, CRF 2010-1.

Iluminacion Con Diodos Led de Nitruro de Galio y Nitruro de Indio, Gerald Soto, 2010-1 (2do Parcial)

La introducción al mercado de dispositivos de iluminación con diodos LED es reciente. Esto fue debido a que los diodos rojos y verdes eran muy fáciles y baratos de producir, pero no ocurría lo mismo con los de color azul.

Todo cambió en 1993 cuando el investigador Shuji Nakamura descubrió un proceso más barato y revolucionario de fabricación en LED, estos compuestos son: Nitruro de Galio y Nitruro de Indio, componentes que reducen costos y que su nivel de consumo energético es bajo, actualmente estos componentes se usan para la producción de LEDs.

Para conseguir luz blanca hay que mezclar en partes iguales luz roja, verde y azul. Se puede hacer el experimento de mirar de cerca una parte blanca de la pantalla del ordenador, y se comprobará que está compuesta de diminutos puntos de estos colores. Al alejarse, se ve el color blanco.

Principales ventajas de la iluminación LED

Tamaño: A igual luminosidad, un diodo LED ocupa menos espacio que una bombilla incandescente.

Luminosidad: Los diodos LED son más brillantes que una bombilla, y además, la luz no se concentra en un punto (como el filamento de la bombilla) sino que el todo el diodo brilla por igual.

Duración: Un diodo LED puede durar 50.000 horas, o lo que es lo mismo, seis años encendidos constantemente. Eso es 50 veces más que una bombilla incandescente.

Medio ambiente: Los diodos LED son amigables con el medio ambiente, debido a que reducen emisiones de CO2 gracias a su larga duración y bajo consumo de energía. Además los LED no contienen mercurio, lo que si ocurre con muchas de las bombillas utilizadas actualmente.

Hoy la iluminación LED es utilizada en innumerables ambientes y dispositivos. Entre los que están: pantallas de tv y computadores, dispositivos móviles, linternas, iluminación exterior, publicidad exterior, señalización y semaforización, etc.

Growth and In Situ Characterisation of Dilute Nitride Quantum Well Structures

In this work epitaxial growth and in situ characterisation of dilute nitride quantum well (QW) structures are studied. Dilute nitrides are III-V compound semiconductors with low (typically < 5%) composition of nitrogen in the lattice. Metal organic vapor phase epitaxial (MOVPE) system was used to grow the structures and in situ reflectance monitoring in normal incidence at a wavelength of 635 nm was utilised to study the growth process of the structures. Ex situ characterisation was performed by x-ray diffractometry and photoluminescence measurements.
The nitrogen content of (In)GaAsN was found to depend on many MOVPE growth parameters. Increased nitrogen content was obtained with decreasing growth temperature, increasing DMHy/V and TBAs/III ratios and when nitrogen (N2) was used as a carrier gas instead of hydrogen (H2).
In situ reflectance data was measured during growth of GaAsN/GaAs, InGaAs/GaAs and InGaAsN/GaAs multi quantum well (MQW) structures. The reflectance curve was observed to be different when MQW structures with different compositions were grown. The reflectance data was analysed by an experimental method utilising the dependence of the reflectance change observed during growth of the QW on the QW composition. Additionally, reflectance curves for multi layer stacks were calculated and the measured curves were compared to the calculated ones. With both methods the sample composition can be determined in situ if the growth rates of the layers are known.
In the process of comparing calculated and measured reflectance curves, the high temperature complex refractive indices of the various QW materials were obtained. The imaginary part of the complex refractive index of (In)GaAsN was found to be linearly dependent on the nitrogen content, as well as both real and imaginary parts of the complex refractive index of InGaAs depended linearly on the indium content. However, changing the nitrogen content did not change the real part of the complex refractive index of (In)GaAsN.


Liquid Phase Epitaxial Growth of Dilute Nitrides for the Mid-infrared

We are interested in the incorporation of nitrogen into semiconductors such as GaAs, InAs and GaSb. This is important because the band gap of the parent III/V semiconductor is substantially reduced by the incorporation of very small amounts of nitrogen. These so-called "dilute nitrides" show promise for use in tailoring the wavelength and efficiency of novel semiconductor lasers and other optoelectronic devices. Although GaAsN and InGaAsN are currently being studied mainly for their applications in photodetectors and lasers in the 1.3 to 1.55 um telecomms wavelength range there is far less research into dilute nitride compounds for the mid-infrared (2-5 um) spectral range which is rich in applications. However, there are problems associated with incorporation of N and degradation of the crystalline quality and especially as nitrogen content in the material is increased beyond 1%.
This project seeks to investigate the growth of dilute nitrides for the mid-infrared spectral range using growth from the liquid phase rather than from the gas phase.One key advantage of this approach is that we do not need any N plasma to introduce the nitrogen atoms and so we can avoid all the damage from the energetic N ion species generated as a by-product from the plasma source normally used in vapour phase growth. Liquid phase epitaxy (LPE) is well known to produce material of excellent crystalline perfection. The proposed project seeks to build on our existing expertise in LPE growth and mid-infrared optoelectronics at Lancaster and study the resulting material properties of GaAsN, InAsN, GaSbN with a view towards evaluating their potential for use in mid-infrared optoelectronic devices. We aim to investigate both bulk materials and also corresponding dilute N nanostructures.
The preparation of dilute N III-V alloys with high quantum efficiency would be a real breakthrough, particularly for use within mid-infrared light sources and detectors for which there are many practical applications. Moreover, if the approach proves successful it can be readily extended to other technologically important alloys such as InGaAsN and GaAsPN.