Nitride laser semiconductor device

Blu-ray y HD DVD
Llega el láser azul

Después de varios años oyendo hablar de ellos ya están aqui. Los primeros dispositivos -y discos- de láser azul se han presentado en sociedad y amenazan con derRocar al DVD. Veamos qué nos ofrecen.

En febrero de 2002 se anunció el primer formato de disco que utilizaba láser azul. Se llamaba Blu-ray y, aunque estaba promovido, entre otras, por nueve de las compañías que forman parte del DVD Forum (Hitachi, LG, Matsushita, Philips, Pioneer, Samsung, Sharp, Sony y Thomson) no se propuso a esta organización para su aceptación, pues sus características lo hacían incompatible con el DVD. Seis meses más tarde, Toshiba y NEC (también miembros del DVD Forum) desarrollaron su propio sistema, al que denominaron AOD (Advanced Optical Disc, Disco Óptico Avanzado), hoy HD DVD, y que tenía la ventaja de ser compatible con los actuales DVD. Toshiba y NEC sí propusieron su formato al DVD Forum, que es quien debe aprobar el formato que mejor sustituya al DVD. La guerra estaba servida.

El problema de la existencia de dos formatos de soporte de vídeo incompatibles entre sí no es nuevo, los más veteranos recordarán lo que sucedió con los sistemas de cinta Beta y VHS (y el fugaz Vídeo 2000), y cómo al lanzarse el DVD se impidió que se repitiera la historia unificando las dos propuestas existentes. Sin embargo, ahora no parece que vaya a haber acuerdo y será la industria, o los consumidores, los que inclinen finalmente la balanza.

La diferencia fundamental entre un DVD-ROM y un CD-ROM es la densidad con que se almacenan los datos y, puesto que el tamaño del soporte es idéntico, el tamaño de las marcas utilizadas para registrar dichos datos. Los nuevos discos de láser azul emplean la misma técnica, puesto que siguen reduciendo el tamaño de las marcas para que quepan más datos en la misma superficie. Para conseguirlo es necesario que el láser sea de color azul (consulta el cuadro ¿Por qué azul?). Sin embargo, el mismo tipo de láser se puede usar de varias formas, lo que ha dado lugar a dos formatos incompatibles entre sí.

Ambos formatos están destinados tanto al almacenamiento de datos, como a aplicaciones de vídeo de alta definición, pensando en poder grabar las emisiones de HDTV (televisión de alta definición), pues este estándar, que ya se utiliza en otros países y pronto entrará en España, requiere de mucha más capacidad que la que ofrece el DVD. En un disco de 25 GB se podrán grabar en torno a dos horas de vídeo de alta definición en formato MPEG-2 o hasta 13 horas de vídeo en calidad VHS. Por supuesto, los discos contarán con un sistema de protección para evitar las copias ilegales de discos originales, lo que supone uno de los aspectos más controvertidos, pues los estudios de cine no están dispuestos a apoyar un formato que permita que se vulneren de ninguna manera los derechos sobre sus obras. También en este sentido se repite la historia, pues ya en los DVD se exigieron sistemas de protección, pero no tardaron en ser quebrantados. Y es que no existe ningún sistema invulnerable, sólo es cuestión de tiempo que alguien encuentre la manera de hacerlo.

Blu-ray: grande y rápido

Blu-ray fue el primero en llegar, con la Blu-ray Disc Association (BDA), y los esfuerzos se centraron en ganar velocidad y capacidad (25 GB por capa frente a los 15 del HD DVD) pero... olvidando la compatibilidad. Sin embargo, esto no implica que tengamos que olvidarnos del DVD en los nuevos dispositivos Blu-ray ya que, aunque no sea obligatorio, la BDA sí recomienda que hay

a esa compatibilidad. Por otra parte, es también lógico que los fabricantes den soporte al DVD en los nuevos dispositivos, de lo contrario sería una apuesta excesivamente arriesgada, teniendo en cuenta lo que ofrece el rival. Tiene también otro gran inconveniente: para los fabricantes supone la construcción de nuevas plantas de fabricación, lo que encarecerá los dispositivos y los discos.

En el mercado de videojuegos cuenta con el apoyo de Sony, por lo que la nueva PlayStation 3 utilizará Blu-ray. Es posible, por cierto, que pienses que si Blu-ray viene por lo del láser azul lo correcto sería denominarlo Blue-ray (con la e) y que llevamos todo el artículo confundiendo el nombre. No es así, ya que para poder registrarlo como marca tuvo que perder la e final, quedando en el actual Blu-ray. La BDA, además, informa que ésa es su escritura correcta y no Blue-Ray (con la R mayúscula). De la misma forma, su correcta abreviatura es BD, no BR o BRD.

HD DVD: compatible

Al desarrollar HD DVD se centraron los esfuerzos en conseguir compatibilidad, lo que permitirá reproducir los actuales discos DVD en los nuevos reproductores. Sin embargo consigue menores capacidades y velocidades que su competidor y esto obligará, ineludiblemente, a una actualización más temprana de la tecnología. Otra ventaja es que permite aprovechar los actuales sistemas de fabricación de DVD, por lo que las inversiones serán menores y los productos más baratos. Además, cuenta con el apoyo de la mayor parte de los estudios de cine, que pueden inclinar la balanza rápidamente si inundan el mercado con sus películas en este formato; si bien tampoco existe una alianza exclusiva, por lo que podrían cambiar de bando si el mercado lo requiere.

En el caso de los juegos su mentor es Microsoft, de modo que la nueva Xbox podrá utilizar los nuevos HD DVD.

La clave de la compatibilidad y economía del HD DVD es que la capa de datos se sigue colocando en el centro de la estructura del disco, igual que en los DVD, mientras que en Blu-ray se sitúa en un extremo con una capa protectora de sólo 0,1 mm, de modo que, además de ser mas complejos de fabricar, también son mucho más sensibles a los arañazos, lo que obliga a usar un cartucho protector o un material extremadamente resistente para esa capa. Por ejemplo TDK ha desarrollado un material de nombre Durabis 2, que ofrece una capa protectora de mayor resistencia que las actualmente usadas en los discos convencionales CD y DVD, sobre los que también puede ser usado, ya que no está limitado a Blu-ray sino a soportes ópticos en general.

El último, a la cola

O quien no corre, vuela. De hecho, los primeros dispositivos están listos a la espera de un acuerdo final para la implementación de los sistemas de protección. Toshiba, Samsung, Pioneer y Sony tienen ya sus sistemas preparados, tanto Blu-ray como HD DVD, tanto dispositivos independientes (lectores y grabadores) como montados en equipos.

Los nuevos Qosmio, buque-insignia de los portátiles de Toshiba, están ya en la rampa de lanzamiento con su reproductor HD DVD en las entrañas. Sony hará lo propio con sus Vaio, incluyendo, lógicamente, unidades Blu-ray. Samsung también prepara sus reproductores y grabadores, así como Pioneer, uno de los modelos de los que sabemos ya nombre y especificaciones. Se llamará "BDR-101A" (Blu-ray, acostúmbrate a las nuevas nomenclaturas) y ofrecerá una velocidad de grabación de 2x tanto para BD-R (disco grabable) como para BD-RE (disco regrabable), con lo que grabar un disco de una capa nos supondrá alrededor de 45 minutos. Además, y como ya comentábamos antes, ofrecerá soporte para discos DVD como si de una unidad tradicional se tratase: 8x para grabación DVD ±R, 2x/2,4x para doble capa y 4x para regrabables. Junto a ello, búferes de 8 MB para BD y 2 MB para DVD, con interfaz IDE. De lo que no hay cifras fiables es de los precios, aunque se barajan cifras entre 500 $ y 1.000 $ (de 400 a 800 euros, aproximadamente) para los primeros reproductores (en los casos del BDP-S1 de Sony y los modelos HD-A1 y HD-XA1 de Toshiba). En cuanto a consumibles, las cifras estarían entre 20 y 30 euros para los discos Blu-ray de una sola capa y entre 30 y 50 ¤ para los de doble. Claro que habrá que esperar a tenerlos finalmente en el mercado de consumo para ver la tendencia.

Está claro que la guerra está servida pero no tanto quién será el ganador. La industria de Hollywood y los fabricantes informáticos quieren llevarse el trozo de pastel más grande, repartiendo sus alianzas y apoyos a uno y otro formato, no sin temor a verse relegados al grupo perdedor, que lo tendrá que haber tarde o temprano porque a esto de la coexistencia parece que se niega la historia. Quizá sea cuestión de rescribirla.

¿Por qué azul?

---------------------

Utilizar un rayo láser para leer las marcas del disco implica un tamaño mínimo para estas marcas relacionado con la longitud de onda de la luz empleada. Si esta longitud de onda es muy grande sólo se podrán leer marcas grandes, pues en las más pequeñas el haz de luz abarcaría varias de ellas simultáneamente. Por ejemplo, en los CD-R se utilizaba un láser infrarrojo de 780 nm y en los DVD se pasó al láser rojo de 640 nm. En el espectro de luz visible las longitudes de onda menores se sitúan en el violeta, llegando al rojo con las mayores. Esto significa que se ha pasado de un extremo a otro del espectro visible, cambiando el láser rojo de 640 nm por otro azul/violeta de sólo 405 nm.

La fabricación de un láser azul de semiconductor ha llevado cierto tiempo de desarrollo, pues es el color más difícil de conseguir. De hecho, incluso los LED normales (no láser) de color azul son relativamente recientes, mientras que los rojos, verdes y amarillos ya hace años que se desarrollaron y se utilizan ampliamente. El color de la luz emitida por un LED depende del material con el que esté construido. Los láseres azules están basados en Nitruro de Galio (GaN), un material que, ya en su presentación en círculos científicos, se mostró como una revolución para la electrónica del futuro.

Blu-Ray HD DVD

http://www.idg.es/pcworld/blu-ray-y-hd-dvd_llega-el-laser-azul/art175582.htm

Marcos Pinto D`derlee

C.I. 17862728

EES

---

Connect to the next generation of MSN Messenger  Get it now!

Nitride semiconductor laser device

Semiconductor sources advance deeper into the ultraviolet

The cutting edge in semiconductor light sources has moved into the ultraviolet. Developers are pushing the wide-bandgap III-V gallium nitride compounds used in blue and violet lasers to shorter wavelengths by adding aluminum to increase the bandgap. In theory, that family can emit at wavelengths as short as 205 nm if the active layer is aluminum nitride, and electroluminescence has been observed at wavelengths shorter than 240 nm. However, practical gallium aluminum nitride (GaAlN) devices now are limited to about 280 nm for LEDs and 370 nm for laser diodes, says John Carrano, manager of the semiconductor UV optical sources program at the Defense Advanced Research Projects Agency (DARPA; Washington, D.C.).

Shorter wavelengths have been reported in laboratory devices. "But who cares who puts out a nanowatt?" says Carrano. "I only consider it to be a real device when it's milliwatt class and can last for at least hundreds of hours." Those powers and lifetimes bring devices close to the levels DARPA wants for applications such as biosensors, water purification, sterilization, and non-line-of-sight atmospheric communications. Carrano thinks those requirements are achievable, and that both LEDs and lasers can be pushed to somewhat shorter wavelengths.

UV semiconductor materials

The raw material for short-wavelength sources is GaxAl1-xN. Increasing the aluminum concentration decreases the wavelength, because the bandgap increases from 3.4 to 6.2 eV when moving from pure GaN to pure AlN (see Fig. 1). However, as bandgaps and aluminum concentrations increase, several problems arise in making LEDs and lasers.

As the bandgap increases, nonradiative mechanisms can dissipate more of the energy released by recombining current carriers, leading to lower power conversion and contributing to heat-management problems. The refractive-index difference that confines light within quantum wells becomes smaller at higher aluminum levels, increasing the light leakage. Nonradiative mechanisms also increase the fraction of light emitted at long wavelengths, which can interfere with fluorescence-based sensors.

Gallium nitride is prone to internal defects that speed device degradation and failure. Although developers have made steady progress, defects degrade electrical characteristics and contribute to device failure. As aluminum concentration increases, it becomes harder to make good electrical contacts, and p-type AlGaN becomes increasingly resistive. As contact and bulk resistance increase, the voltage drop across the diode increases, reducing power conversion efficiency and increasing internal heating-contributing to shorter lifetimes.

Adding indium to create the quaternary compound AlGaInN gives an additional degree of freedom, so bandgap and lattice constant can be adjusted independently. Indium also helps in the growth process, and causes some inhomo genieties that actually increase radiative efficiency. Indium was important in developing blue diode lasers, but it reduces bandgap energy, so its use is increasingly limited at shorter wavelengths.

The allure of UV LEDs

Light-emitting diodes have important practical advantages over diode lasers. One is their simplicity-they do not require elaborate internal microstructures to confine light and form a laser resonator. Another is higher reliability, because their lower drive currents lead to lower internal drive-current densities, and less power dissipation and internal heat. Short-wavelength operation is inherently limited by device degradation, so the slower degradation and greater reliability of LEDs allows them to operate at much shorter wavelengths than laser diodes.

Fortunately, the key advantages of laser emission-narrow wavelength range and high directionality-are not essential in most applications of semiconductor UV sources. DARPA is stressing the need for powers in the milliwatt-range, continuous-wave (CW) operation at room temperature, and reasonable lifetimes. So far, milliwatt LEDs emitting near 280 nm have room-temperature lifetimes of hundreds of hours. That wavelength is a critical target, because it marks the edge of the "solar-blind" spectrum. Atmospheric absorption blocks solar UV radiation at shorter wavelengths from reaching the ground, eliminating the effects of background sunlight.

Carrano is more concerned with improving LED efficiency than with pushing their wavelengths much deeper into the UV. So far, the best power-conversion efficiency in the 280-nm range is 1% to 2%. That might be acceptable in the laboratory, but not in field devices that require battery power or are hard to replace if they burn out. So DARPA's target is 10% power-­conversion efficiency. Higher efficiency will also help extend operating lifetime of the LED itself, another major concern for systems in the field. DARPA wants lifetimes of at least 10,000 hours so they can operate for an entire year without service.

One goal is a family of inexpensive battery-powered biosensors that could be distributed in the field to give early warning of possible biological agents. Some compounds found in living cells fluoresce strongly when excited by UV light. Although cheap LED-based biosensors would not be able to identify specific biological agents, they could serve as an early warning system so troops could take protective measures.

So far, milliwatt 280-nm LEDs last for hundreds of hours. That improves at longer wavelengths, reaching thousands of hours near 340 nm, but DARPA wants shorter wavelengths because they promise new applications. Wavelengths shorter than 280 nm could be used for short- distance covert communications that don't require a line of sight. Strong Rayleigh scattering by the atmosphere would spread the signal over a range of angles, while atmospheric absorption would limit transmission range and block the solar background (see Fig 2).

"There really aren't any huge challenges to get to 260 to 265 nm," Carrano says. Those wavelengths affect DNA in living cells, forming bonds between the two strands that block their replication, killing cells. DARPA envisions applications in UV sterilization of water or surfaces, which now require bulky mercury-vapor lamps with peak emission at 254 nm.

The UV-laser problem

A UV laser diode is "just a tougher device to make" than an LED, Carrano says. The more complex heterostructure and high current density in laser diodes impose far more stringent requirements on semiconductor materials than the lower currents and simpler structure of LEDs.

The 400-nm violet diodes available commercially are based on the ternary compound indium gallium arsenide, but they can't be pushed beyond about 360 nm without adding aluminum, which creates new problems that affect laser lifetime. Moving to shorter wavelengths also increases problems with optical confinement, nonradiative processes, and sustaining population inversions.

All these factors limit CW room-temperature operation to much longer wavelengths than LEDs. For milliwatt lasers, the cutting edge is 370 nm, where lifetimes are hundreds of hours. At CLEO 2004 (San Francisco, CA), Cree (Durham, NC) reported diodes with CW operation to 348 nm and pulsed operation to 343 nm, but powers and operating lifetimes have been very low. DARPA wants lasers that operate for thousands of hours at room temperature.

The Palo Alto Research Center (PARC; Palo Alto, CA) is working to push diode lasers to 320 nm and have demonstrated optically pumped laser hetero structures at wavelengths to 308 nm. They are now working to improve the electrical properties. Noble Johnson of PARC thinks his group is close to a 320‑nm current-driven diode laser, but can't predict when they will reach laser threshold. "Right now, the hurdle is to get the threshold voltages and currents down to reasonable values and we are making steady progress," he adds.

The main interest in semiconductor lasers is for high-end biosensors that could give more-precise information than early-warning LED sensors. The lasers could be tuned to match peak absorption wavelengths, with sensors monitoring wavelengths at which specific agents fluoresce most strongly. These sensors would protect high-value targets such as important buildings or military facilities. Definitive identification of specific pathogens to diagnose people who may have been exposed to bioagents might still require biological techniques that take several hours and laboratory culture.

Outlook

Development of semiconductor ultraviolet sources builds upon Shuji Nakamura's breakthrough fabrication of blue and violet diode lasers in InGaN. Commercial developers have found a sweet spot at 400 nm, which will be the base of an emerging generation of optical-storage devices.

Pushing to shorter wavelengths is an uphill battle, and at least in the near term LEDs will be more practical than lasers. But most applications don't require lasers, and real progress is being made in areas from fundamental materials issues to device structure. Semiconductor sources are marching deeper into the ultraviolet. 

http://www.optoiq.com/index/photonics-technologies-applications/lfw-display/lfw-article-display/221443/articles/laser-focus-world/volume-41/issue-2/features/semiconductor-sources-advance-deeper-into-the-ultraviolet.html

Marcos Pinto D´derlee

C.I. 17862728

EES

---

Explore the seven wonders of the world Learn more!

nitride semiconductor laser device

"El reto del láser azul"

 

El nitruro de galio (GaN) es un material semiconductor de banda ancha que permite obtener –combinado con aluminio– luz láser, corazón de los sistemas lectores de DVD de alta definición como: HD-DVD y Blu-Ray Disc.

Al respecto, el doctor Godofredo García Salgado, del Centro de Investigaciones en Dispositivos Semiconductores de la BUAP, en colaboración con el doctor Víctor Sánchez Reséndiz del Cinvestav-IPN, trabaja en un proyecto para obtener fases puras del GaN, mediante un sistema de MOCVD (Metal Organic Chemical Vapor Deposition por sus siglas en inglés) aplicando un modelo termodinámico fuera de equilibrio, con el cual se adquiere el valor de la fuerza impulsora del proceso denominado afinidad termodinámica.

Para ello, el doctor García Salgado, como parte de su tesis doctoral, realizó un estudio del proceso de crecimiento y desarrolló ecuaciones que relacionan la afinidad termodinámica con todos los parámetros de crecimiento de un sistema de MOCVD: "si se conoce el flujo de los gases, la temperatura y la presión del sistema, entonces se puede determinar la afinidad termodinámica del proceso".

El reto es obtener una fase pura durante el proceso, en particular la cúbica, ya que permite hacer cortes más adecuados para fabricar un láser.

"El nitruro de galio, posee dos fases: una estable Wurtzita, de estructura hexagonal y otra metaestable Zincblenda con estructura cúbica. Normalmente el crecimiento del material, llevado a cabo por distintos métodos, presenta una mezcla de fases, que va en detrimento de la calidad del mismo y la de los dispositivos fabricados con él, como láseres o transistores de potencia", explica. Esta investigación permitirá fabricar láseres azules con mejor calidad.

http://www.ecienciaytecnologia.gob.mx/wb2/eMex/eMex_El_reto_del_laser_azul

Marcos Pinto D`derlee

C.I. 17862728

EES

---

Discover the new Windows Vista Learn more!

Nitride semiconductor laser device

Pure gallium nitride wafers likely to change blue-violet laser market

The short-wavelength blue-violet laser diode, with its potential for increased data density, higher operating speed and smaller system size, is expected to become the main light source in such applications as data storage, communications, measurement and medical inspection.

Sony Corp. will unveil the first blue-violet laser-based product, the Blu-Ray Disc recorder, on April 10. With a suggested retail price tag of about $3,800, however, the product will be out of the reach of most buyers in the consumer market. But the potential demand for short-wave lasers is expected to grow significantly.

Although shortwave laser diodes were originally dubbed "blue" lasers, they are really blue-violet ones that emit light at 405 nanometers. Gallium nitride (GaN) is used for the lasers since they oscillate most efficiently around this wavelength. (Blue is defined as light in the range of 455 to 485 nm, according to a Japanese industrial standard).

In the still-small blue-violet laser market, Nichia Corp. has been the sole supplier. But the company's strength may be undermined by the emergence of GaN wafers. Once GaN wafers become available in volume, most of the technical obstacles currently hindering the production of blue-violet lasers in volume will be solved, said industry sources.

GaN wafers may solve patent issues as well. Nichia is fabricating lasers by growing a GaN layer epitaxially on a sapphire substrate and owns patents covering a wide range of related technologies. GaN wafers have the potential to help other laser developers avoid infringing on Nichia's patents, since those companies may be able to design GaN lasers in different structures.

GaN wafers

After researchers succeeded in getting GaN to emit light in the 1980s, GaN devices have been grown on a sapphire substrate. But sapphire has not been an ideal substrate for GaN device fabrication because of the big gap — over 16 percent — between the lattice constant for GaN and sapphire.

That difference in the lattice constants causes a lot of dislocations, or defects, in the GaN crystal layer that is grown. Those defects have a negative impact on the laser's quality and make it difficult to develop lasers that last long. The difference also makes it difficult to cleave the crystal and obtain a laser reflective surface.

Sapphire substrates have other problems, like low thermal conductivity, which may cause the laser diode grown on the substrate to heat up. And because sapphire is electrically nonconductive, the laser diodes grown on the sapphire substrate have to have both electrodes on the surface of the GaN layer. That constraint makes the size of the chip larger than a device that can have an electrode on the bottom as well as on the top.

GaN devices that are expitaxially grown on GaN substrates, on the other hand, do not suffer from those problems. But the GaN crystal has only been available in millimeter-sized pieces, which have been produced in high-temperature, high-pressure conditions. As GaN does not exist in a liquid state, the pulling method that is widely used to produce single-crystal ingots, which are then sliced into wafers, cannot be applied to separate single-crystal GaN. Thus, sapphire and silicon carbide are mainly used as substrates for GaN device fabrication.

Wafer developers have been trying to find a solution to the problems associated with the epitaxial growth process. Three companies, Sumitomo Electric Industries Ltd. (SEI) (Osaka, Japan), Hitachi Cable Ltd. (Tokyo) and Crystal Photonics Inc. (Sanford, Fla.) have developed freestanding GaN wafers. Sumitomo and CPI have already started offering samples to potential laser suppliers and Hitachi Cable will start sampling soon.

These wafer developers grew a GaN crystal layer on a substrate — usually sapphire — that was several hundred microns thick, and removed the substrate from the GaN layer, leaving the layer as a freestanding wafer. But this process causes the same problem — many dislocations in the crystal — as GaN devices grown on a sapphire substrate have. Each company had a different approach to lowering the number of dislocations in the crystals.

Sumitomo Electric announced in February 2000 that it had developed the world's first freestanding single-crystal GaN wafer, which measured 2 inches in diameter. It grew a GaN layer on a sapphire substrate and reduced the number of dislocations using the company's proprietary Deep (Dislocation elimination by epitaxial growth with inverse-pyramidal pits) method.

In the Deep method, many microscopic pits are generated in the GaN crystal layer while the crystal layer grows. Each side of a pit is matched with a facet of the GaN crystal. Along with the epitaxial growth of the crystal layer, dislocations move and gather to the center of the pits. The pits, which are several 100 microns in diameter, remain on the surface of the crystal layer, leaving the rest of the crystal with fewer dislocations.

Sumitomo Electric engineers said that in the first generation, it was impossible to control the position of the pits. In the second generation, announced last June, they added a control technology to arrange the pits in a more orderly way. As the position of the pits is controlled, the areas that have fewer dislocations appear in a more orderly fashion. Users can grow laser diodes efficiently on those areas, each of which is about 500 microns in diameter.

The dislocation density per square centimeter of these usable areas is between 10,000 and 100,000, which is about 1/100,000 of the density of conventional GaN epitaxial layers on sapphire substrates, according to the company.

SEI started shipping samples with a thickness of 0.25 to 0.6 mm last June and is presently preparing volume production. The company plans to begin operation sometime in April with a capacity of 300 wafers a month.

"Multiple potential users are evaluating our wafers and are giving us feedback about the result of the evaluation. Lower dislocation is mandatory because it has a direct effect on the quality of lasers, on which we have confidence. The next several months will be the most important period for us because major users will select which wafer to use for their laser production," said a spokesman of Sumitomo Electric.

Hitachi Cable, a Hitachi group company, was the second company in Japan to announce the development of a freestanding 2-inch GaN wafer. The company announced the accomplishment in February, and will begin sampling this spring.

Hitachi Cable expitaxially grows a single GaN crystal wafer on a sapphire substrate and the sapphire substrate is removed after the crystal layer has been fully grown. Hitachi Cable said that it becomes difficult to remove the sapphire base completely without damaging the GaN wafer when the wafer diameter becomes as large as 2 inches. Hitachi Cable's solution is to use an approach they call the Void Assistance Separation (VAS) method.

In the VAS method, a nitride titanium film is inserted between the sapphire substrate and the GaN growth layer. The film is actually meshwork consisting of 20- to 30-micron stripes of nitride titanium. During epitaxial growth, large numbers of microscopic voids are formed in the openings of the nitride titanium meshwork. After the GaN crystal layer has been fully grown, it can be removed from the substrate without being damaged at the void layer. This method will allow the company to make wafers larger than 2 inches in diameter in the future, according to a Hitachi Cable spokesman.

Competitors Sanyo Electric Co. Ltd. and Sharp Corp. also have aggressive plans for blue-violet lasers in place.

Sanyo characterizes its blue-violet laser business as one of the new mainstays of the company, along with several other areas such as organic light-emitting diodes and a charge-coupled device module for mobile gears.

Sanyo developed a blue-violet laser with 35-mW output early this year using GaN substrates. The laser has the highest output power thus far announced. Sanyo is the first company to claim the use of a GaN substrate to fabricate a laser diode, though the company has not disclosed the name of the supplier. As the diode does not have an insulating sapphire substrate, it can have one electrode on the top and anther on the bottom, which reduces the chip size to about half that of other devices, according to Sanyo.

Sanyo plans to start operating a blue-violet laser production line at a Tottori Sanyo fab sometime this spring and will ramp volume production this autumn. The first sample will be available in May for about $1,680 each.

Sanyo said it plans to take its aggressively named Blueimpulse blue-violet laser family to about $127 million in sales, including blue LEDs, by 2006.

Sanyo's laser diode does not infringe on Nichia's patents because its structure is different from Nichia's sapphire-based lasers, said Yukinori Kuwano, president of Sanyo.

Sharp Corp., another major player in the red-laser market, is also positioning a blue-violet laser as the next key device but has been keeping silent about its laser diode, whose development it apparently completed at the end of last year. Last May, Sharp opened a new fab in Mihara, Hiroshima Prefecture, for compound semiconductor production. The fab will be used as the production base of blue-violet lasers and will begin volume production by the end of this year.

Toyoda Gosei Co. Ltd., which is fiercely competing with Nichia in the blue-LED market, also completed development of a 410-nm laser in April 2001, as the result of a research project funded by Japan Science and Technology Corp.. The company has continued the development of shortwave laser diodes in cooperation with an undisclosed system manufacturer. Last autumn Toyoda Gosei developed a 405-nm laser with an output power of 30 mW using a sapphire substrate, and it continues to evaluate the device.

Rohm Ltd. is working with Cree Inc. for silicon carbide-based lasers and with Pioneer Corp. for sapphire-based lasers.

With practical applications in sight, blue-violet lasers have already entered into a volume-production phase, and several companies will come out with their own products this year.

http://www.eetimes.com/story/OEG20030325S0038

Marcos Pinto D`derlee

C.I 17862728

EES

---

Explore the seven wonders of the world Learn more!

nitride semiconductor laser device

High Quality Superlattices for Effective P-Type Doping

Effective p-type doping of wide bandgap III-nitride semiconductors is difficult due to the high activation energy of most acceptors. This problem is more pronounced for AlGaN compounds with increasing Al content.

 Through theoretical modeling and material growth optimization, researchers at CQD have successfully demonstrated the highest p-type conductivity conductivity ever reported for AlGaN. The technique made use of AlGaN based superlattices, based on a patent awarded in 1997 to Northwestern University ("III-Nitride superlattice structure, The method of increasing acceptor level and decreasing contact resistance," US patent, 5831277, March 19, 1997). These superlattices exhibited a very high free hole concentration of 4.2x10¹⁸ cm^-3 and a very low resistivity of 0.19 Ω.cm.

N-type doping also poses a significant challenge for high Al content AlGaN layers. By careful optimization of the SiH4 flow, and the use of a high-quality AlGaN/AlN superlattice template on AlN buffer we were able to realize Al0.5Ga0.5N:Si with n ~1x1018 cm-3 and μ~40 cm²/V·s. The addition of indium yields a carrier concentration of n~5x1018 cm-3 and mobility of μ~80 cm²/V·s.

World's First Lateral Epitaxial Overgrowth of GaN on Silicon

Because of the lack of commercially available native substrates, obtaining low defect III-Nitride semiconductors has remained one of the most challenging issues in this area for numerous years. The dislocations that are formed when growing III-Nitrides heteroepitaxially, i.e. on non-native substrates, have proved detrimental not only to the material's structural, optical, and electrical quality, but also to the device performance.

Researchers at CQD have successfully and extensively explored innovative approaches to reduce the dislocation density in GaN by several orders of magnitude through the use of lateral epitaxial overgrowth (LEO). In particular, researchers at CQD were the first to demonstrate the LEO of GaN on silicon substrates.

aa f p' p flow, and the use of a high-quality AlGaN/AlN superlattice template on AlN buffer we were able to realize Al0.5Ga0.5N:Si with n ~1x1018 cm-3 and μ~40 cm²/V·s. The addition of indium yields a carrier concentration of n~5x1018 cm-3 and mobility of μ~80 cm²/V·s.

World's First Lateral Epitaxial Overgrowth of GaN on Silicon

Because of the lack of commercially available native substrates, obtaining low defect III-Nitride semiconductors has remained one of the most challenging issues in this area for numerous years. The dislocations that are formed when growing III-Nitrides heteroepitaxially, i.e. on non-native substrates, have proved detrimental not only to the material's structural, optical, and electrical quality, but also to the device performance.

World's First Solar Blind Ultraviolet Photodetectors and Focal Plane Arrays

The Center for Quantum Devices (CQD) has demonstrated many record-breaking achievements in the development of UV, visible -blind and solar-blind photodetectors.

Photodetectors are devices that transform electromagnetic radiation (light) into an electrical signal (current or voltage). Several applications demand that these devices detect specific energies of light while ignoring others, e.g. sensitive to UV and not visible or infrared light. Using III-Nitride technology, inexpensive, efficient, highly sensitive and robust devices can be achieved.

These high efficiency Al-GaN based UV photodetectors can be used in a number of military and civilian applications including early missile threat warning, secure space-to-space communications, chemical and biological agent detection, engine/flame detection, furnace monitoring, UV dosimetry, ozone/pollution monitoring, and UV astronomy.

Numerous types of photodetectors have been investigated at CQD, including photoconductors, Metal-Schottky-Metal detectors, Schottky photodiodes, pin photodiodes, and most recently avalanche photodiodes.

World's First Back Illuminated Avalanche Photodiodes and Geiger-Mode Single photon Detectors

Research into avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet detectors in numerous civilian and military applications. By utilizing low-noise impact ionization based gain, GaN APDs can deliver gains of more than 10000 in linear mode; and operating in Geiger mode, the gain can exceed 1×107 and thus single photon detection becomes possible.

We have experimentally studied impact ionization in GaN and found that the hole ionization coefficient is significantly larger than that for electrons. This means that back-illuminated devices exhibit larger gains; we have also observed excess multiplication noise factors that are more than three orders of magnitude lower.

By using high-quality delta-doped p-GaN a consistently lower breakdown voltage can be realized, without significantly affecting the dark current. This has allowed a more than 50x improvement in the maximum gain realized, as compared to devices grown with traditional bulk p-GaN.

In addition to controlling the material and doping quality it is also possible to carefully design the structure to minimize the external bias necessary to reach the critical electric field necessary for breakdown (typically of 3 ×106 V×cm-1). This can be accomplished by reducing the width of the combination absorption/multiplication layer in a traditional p-i-n device structure; however this also increases the leakage current of the diodes, and thus, the dark count rate in Geiger mode. To help overcome this, we have developed a separate absorption and multiplication APD structure (SAM-APD). By separating the absorption and multiplication regions using a p-i-n-i-n structure it becomes possible to absorb more than 99% of the light in the bottom layers resulting in nearly pure hole-injection into the multiplication region. This maximizes the advantage of the higher hole-ionization coefficient resulting in a device with a maximum linear mode gain of more than 40,000.

Violet, Blue, Green Light Emitting Diodes and Lasers

The Center for Quantum Devices has developed the III-Nitride material growth and processing of visible Light Emitting Diodes LED) and lasers.

Light-Emitting Diodes

Using InxGa1-xN/GaN double hterostructures and multiquantum well structures, researchers at the Center have demonstrated high-brightness blue and green LEDs.

Lasers

High-performance laser diodes fabricated from III-nitride materials are complex, yet highly attractive devices due to their potential use in a number of applic ations. The Center for Quantum Devices has been pursuing the MOCVD growth and development of these devices, which incorporate In_xGa_1-xN/GaN multi-quantum wells. To achieve high efficiency lasers, the CQD has been investigating lateral epitaxial overgrowth (LEO) as a means to improve III-Nitride materials by reducing defects during the growth on both sapphire and silicon substrates.

Semiconductor lasers are highly efficient, compact devices that emit an intense, coherent, monochromatic beam of light.

Blue laser diodes stand to satisfy a number of application needs including high-density data storage, high capacity DVD, high resolution color printing, and laser displays.

http://cqd.eecs.northwestern.edu/research/nitride.php

Marcos Pinto D`derlee

C.I. 17862728

EES

---

Discover the new Windows Vista Learn more!