Nitruro de galio ( Ga N )

Nitruro de galio ( Ga N )

nitruro de galio ( Ga N ) es un binario III / V bandgap directo semiconductores de uso común en brillantes diodos emisores de luz desde la década de 1990. The compound is a very hard material that has a Wurtzite crystal structure . El compuesto es un material muy duro que tiene una estructura cristalina Wurtzita . Its wide band gap of 3.4 eV affords it special properties for applications in optoelectronic , high-power and high-frequency devices. Su amplia banda prohibida de 3,4 eV ofrece sus propiedades especiales para aplicaciones en optoelectrónica , de alta potencia y dispositivos de alta frecuencia. For example, GaN is the substrate which makes violet (405 nm) laser diodes possible, without use of nonlinear optical frequency-doubling . Por ejemplo, GaN es el sustrato que hace violeta (405 nm) láser de diodos de lo posible, sin el uso de óptica no lineal de frecuencia doble . Its sensitivity to ionizing radiation is low (like other group III nitrides ), making it a suitable material for solar cell arrays for satellites . Su sensibilidad a la radiación ionizante es baja (al igual que otros del grupo III nitruros ), por lo que es un material adecuado para la célula solar arrays para satélites . Because GaN transistors can operate at much hotter temperatures and work at much higher voltages than gallium arsenide (GaAs) transistors, they make ideal power amplifiers at microwave frequencies. Debido a que los transistores de GaN pueden funcionar a temperaturas mucho más caliente y el trabajo a mayor tensión mucho más arseniuro de galio (GaAs) transistores, amplificadores de potencia hacen ideal a frecuencias de microondas.

 

Propiedades físicas GaN is a very hard, mechanically stable material with large heat capacity . In its pure form it resists cracking and can be deposited in thin film on sapphire or silicon carbide , despite the mismatch in their lattice constants . GaN can be doped with silicon (Si) or with oxygen to N-type and with magnesium (Mg) to p-type ; however, the Si and Mg atoms change the way the GaN crystals grow, introducing tensile stresses and making them brittle. Gallium nitride compounds also tend to have a high spatial defect frequency, on the order of a hundred million to ten billion defects per square centimeter. GaN es muy dura y estable mecánicamente con material de gran capacida calorífica . En su forma pura que resiste el agrietamiento y pueden ser depositados en capa fina sobre zafiro o de carburo de silicio , a pesar de la falta de coincidencia en sus constantes de red . puede GaN ser dopado con silicio (Si) o con oxígeno de tipo N y con el magnesio (Mg) a tipo p ; Sin embargo, el "Sí" y los átomos de Mg cambiar la forma de los cristales de GaN crecer, la introducción de tensiones de tracción y haciéndolos quebradizos. galio nitruro de compuestos también tienden a tener una frecuencia espacial defecto alta, del orden de un cien millones hasta diez mil millones defectos por centímetro cuadrado. GaN-based parts are very sensitive to electrostatic discharge . Basados en GaN partes son muy sensibles a las descargas electrostáticas.

Evolucion
GaN cristalina de alta calidad se puede obtener a baja temperatura depositado capa de tecnología de amortiguación. Esta calidad cristalina de alta GaN llevó al descubrimiento de GaN tipo p, de unión pn blue/UV- LED y la temperatura ambiente- emisión estimulada (indispensables para la acción láser). Esto ha llevado a la comercialización de los LED de alto rendimiento azul y violeta-vida-láser de diodos de largo, y al desarrollo de la base de nitruro de dispositivos tales como detectores de UV y de alta velocidad -transistores de efecto de campo .
High-brightness GaN light-emitting diodes (LEDs) completed the range of primary colors, and made applications such as daylight visible full-color LED displays, white LEDs and blue laser devices possible. De alto brillo de GaN diodos emisores de luz (LED) completaban la gama de colores primarios,hizo aplicaciones tales como la luz del día visible color del LED muestra llena, los LEDs blancos y azules con láser los dispositivos posibles. The first GaN-based high-brightness LEDs were using a thin film of GaN deposited via MOCVD on sapphire . El brillo de los LED GaN--base de alta primero estaban usando una fina capa de GaN depositado a través de MOCVD sobre zafiro . Other substrates used are zinc oxide , with lattice constant mismatch only 2%, and silicon carbide (SiC). Group III nitride semiconductors are in general recognized as one of the most promising semiconductor family for fabricating optical devices in the visible short-wavelength and UV region. Otros sustratos utilizados son el óxido de zinc , con una constante de red no coincide sólo el 2%, y el carburo de silicio (SiC). Grupo de semiconductores de nitruro III, en general, reconocido como uno de los semiconductores de la familia más prometedora para la fabricación de dispositivos ópticos en el espectro visible a corto longitud de onda y de la región UV.
The very high breakdown voltages, high electron mobility and saturation velocity of GaN has also made it an ideal candidate for high-power and high-temperature microwave applications, as evidenced by its high Johnson's Figure of Merit . El reparto de tensiones de muy alta, alta movilidad de electrones y la velocidad de saturación de GaN también ha hecho un candidato ideal para la alta potencia de microondas y la temperatura de las aplicaciones de alto, como lo demuestra su alta figura de Johnson al Mérito . Potential markets for high-power/high-frequency devices based on GaN include microwave radio-frequency power amplifiers (such as used in high-speed wireless data transmission) and high-voltage switching devices for power grids. Los mercados potenciales para los dispositivos basados en GaN high-power/high-frequency incluyen microondas , radiofrecuencia amplificadores de potencia (tales como los utilizados en la velocidad de transmisión de datos inalámbrica de alta) y de alta tensión de dispositivos de conmutación de redes eléctricas. A potential mass-market application for GaN-based RF transistors is as the microwave source for microwave ovens, replacing the magnetrons currently used. Un mercado masivo potencial de aplicación basado en RF GaN transistores es como la fuente de microondas para hornos de microondas , para sustituir a las magnetrones se utilizan actualmente. The large band gap means that the performance of GaN transistors is maintained up to higher temperatures than silicon transistors. El boquete de la venda grande significa que el rendimiento de los transistores de GaN se mantiene hasta temperaturas más altas que los transistores de silicio. First gallium nitride metal/oxide semiconductor field-effect transistors (GaN MOSFET ) were experimentally demonstrated in 1993 and they are being actively developed. En primer lugar nitruro de galio metal / óxido de semiconductores transistores de efecto de campo (GaN MOSFET ) se demostró experimentalmente en 1993 y que están siendo desarrollados.

Bárbara Scarlett Betancourt Morales

CAF

CARACTERIZACIÓN Y CRECIMIENTO DE PELÍCULAS DELGADAS DE GaN DOPADAS

CARACTERIZACIÓN Y CRECIMIENTO DE PELÍCULAS DELGADAS DE GaN DOPADAS

Introducción

El estudio de películas delgadas de semiconductores de tipo III-V dopados con tierras raras (TR) es de gran interés por sus propiedades ópticas y electrónicas. En esta investigación se crecieron películas delgadas de GaN usando la técnica de deposición por láser pulsado (DLP) y fueron dopadas in situ con TR. Las tierras raras son elementos pertenecientes a la serie de los lantánidos y, de esta serie, los elementos que se encuentran desde Ce hasta Yb tienen un nivel 4f parcialmente lleno que se encuentra protegido por los niveles externos 5s² 5p⁶. Si son introducidos como impurezas en un material anfitrión entonces al ser excitados emiten luz en el espectro visible como también en el infrarrojo.¹ En GaN las TR usualmente se ubican en la sub-red del catión (Ga); por eso se encuentran en un estado iónico +3 (TR ⁺³). El GaN es un anfitrión de interés para las TR⁺³ debido a que este material es un semiconductor de brecha ancha, lo cual lo hace transparente a la emisión visible de las TR⁺³, y a su estabilidad química y térmica.² Además, por la ubicación de la mayoría de las TR⁺³ en la sub-red del catión (Ga) a diferencia del caso cuando se usan materiales II-VI como anfitrión, los cuales sufren defectos debido a la falta de neutralidad de carga.

Debido a que el radio atómico covalente de las TR⁺³ ( 0.185 nm - 0.157nm ) es más grande que el de Ga ( 0.126 nm) su electronegatividad de Pauli es menor en las TR⁺³ (1.1 – 1.25) que en el catión de Ga (1.81). Estas diferencias pueden crear trampas isoelectrónicas.⁴ Son llamadas isoelectrónicas porque las TR son isovalentes con el catión de Ga, o sea su configuración electrónica externa es la misma en Ga (3s 3p) y en las TR ( 5s 5p). Se ha demostrado que las trampas isoelectrónicas juegan un papel importante en el mecanismo de excitación de las TR⁺³ .⁴

El GaN:TR⁺³ tiene aplicaciones en el desarrollo de dispositivos electroluminiscentes en los rangos visible e infrarrojo, en dispositivos de almacenamiento óptico de información y para la creación de pantallas planas. En la actualidad existen diversos métodos de crecimiento para películas de GaN y distintos métodos para dopar dichas películas delgadas de GaN con TR⁺³ . Las técnicas de crecimiento utilizadas comúnmente son las de "Metal Organic Molecular Beam Epitaxy" (MOMBE) , "Chemical Vapor Deposition" (CVD) , "Molecular Beam Epitaxy" (MBE) y las películas son dopadas in situ por evaporación del elemento dopante o ex situ por implantación de iones . Hasta donde el autor conoce, no se ha informado sobre el crecimiento de películas delgadas de GaN:TR⁺³ por el método de deposición por láser pulsado (DLP). Esto nos motiva a implementar una técnica para dopar con TR⁺³ películas de GaN crecidas por deposición por láser pulsado (DPL).

En esta investigación se crecieron películas delgadas de GaN por DLP asistido por un haz de nitrógeno atómico y una serie de estas películas fueron dopadas in situ con TR⁺³. Esto se hizo con el propósito de obtener películas de GaN de alta calidad para así al ser dopadas con TR⁺³ emitan luz en el rango visible como también en el infrarrojo. La estructura, morfología y propiedades ópticas de estas películas fueron evaluadas con las siguientes técnicas de caracterización. Para estudiar la estructura cristalina de estas películas se utilizó un difractómetro de rayos x. Para determinar su luminiscencia las películas fueron fotoestimuladas a temperatura ambiente con un láser de argón Coherent modelo Innova 300. La emisión fue analizada por un espectrofotómetro doble Spex modelo 1430. La morfología de la superficie de las películas fue estudiada con un microscopio de fuerza atómica (AFM) y un microscopio óptico.

Mecanismos de excitación de las TR⁺³

• Niveles de Energía

En las películas de GaN:TR⁺³ la transición intraconfiguracional 4f-4f de las TR⁺³ son responsables por la emisión de luz. Estos niveles de energía 4f son afectados por las fuerzas electrostáticas producidas por los átomos de las capas externas y levemente por el cristal anfitrión .⁵ Como los iones de las TR⁺³ crean trampas isoelectrónicas (Figura II-1) los excitones acoplados crean mecanismos de excitación y recombinación diferentes a los casos cuando se tienen excitones acoplados a donantes neutrales o aceptantes.⁴ En el caso de un excitón acoplado a un donante neutral o aceptante el tiempo característico de luminiscencia es del orden de nanosegundos, mientras que en el caso de un excitón acoplado a una trampa isoelectrónica son del orden de miles de nanosegundos. Otra característica de importancia es que las trampas isoelectrónicas de las TR⁺³ dependen poco de la temperatura. Esto se debe al tipo de mecanismo dominante en la excitación de las TR⁺³.

• Excitación directa

Cuando ocurre fotoexcitación directa los electrones de los niveles 4f en las TR⁺³ absorben fotones con las energías adecuadas para ser excitados (Figura II-2). Lo mismo ocurre en el caso de excitación directa por catodoluminiscencia (CL) y electroluminiscencia (EL) en donde los electrones de los niveles 4f de los núcleos isoelectrónicos de las TR⁺³ son excitados por choques con electrones calientes.

• Excitación indirecta

Cuando las TR⁺³ son excitadas indirectamente estas son estimuladas por un par de electrón y hueco generado por un fotón con energía mayor que la de brecha de banda, o por un electrón caliente en CL , o inyectado como en el caso cuando tenemos una unión p-n por "forward bias". Tomaremos de ejemplo el caso cuando las trampas isoelectrónicas son estimuladas por un fotón .⁴

Primero el material es estimulado por un fotón con una energía mayor que la de brecha de banda. La absorción del fotón hace que un electrón en la banda de valencia suba a la banda de conducción, dejando así un hueco en la banda de valencia.

Características y Descripción del GaN

El GaN es un semiconductor de brecha ancha (3.4 eV) perteneciente a los nitruros III-V, con enlace químico sp³ y coordinación tetrahedral. Este enlace del GaN no es covalente puro y presenta una ionicidad de 35% debido al fuerte enlace con el anión de N. ⁶ Su estructura cristalina es la de la wurzita, que consiste de dos subredes hexagonales interpenetradas (hcp). Una de las subredes tiene átomos de Ga y la otra átomos de nitrógeno, con una secuencia de apiñamiento ABABAB entre las bicapas en la dirección <0001>.

El GaN es un semiconductor de transición directa, o sea que no necesita de un fonón para recombinarse a través de la brecha entre la banda de conducción y la banda de valencia. A consecuencia de ser un semiconductor de transición directa el número estados que logran recombinarse es mayor. Además, el tiempo de recombinación es menor, ya que no se necesita de un fonón en el proceso de recombinación, a diferencia de los semiconductores de transición indirecta como Si. Otra propiedad de importancia para su uso en dispositivos optoelectrónicos es que este material exhibe altos niveles de actividad óptica aún con una alta densidad de defectos (~10 ¹⁰ cm²) .⁸

Algunas de sus aplicaciones en la industria lo son en diodos emisores de luz azul, verde y blanca, en láser de emisión azul y ultravioleta (que pueden ser usados en tecnología de almacenaje y lectura de información) y en dispositivos electrónicos con capacidad de resistir altas temperaturas y distintos tipos de radiación

Fabricación de GaN:TR⁺³

El GaN:TR⁺³ ha demostrado gran promesa para el desarrollo de dispositivos emisores de luz. Estos podrían ser utilizados en el desarrollo de pantallas planas, gracias a que se pueden desarrollar dispositivos electroluminiscentes para obtener emisión en los 3 colores primarios; rojo (Eu, Pr), verde (Er) y azul (Tm). ¹ Wilson et al. en el 1996 observaron una fuerte emisión en el infrarrojo centrada en 1.54 μm para una película delgada de GaN/zafiro dopada con Er. ¹⁶ Esta fue depositada por MOMBE y dopada con Er por el método de implantación de iones y se encontró que su activación óptica mejoraba al codoparla con oxígeno. Steckl y Birkhahn fueron los primeros en reportar emisión en el rango visible para películas de GaN/zafiro. Estas fueron crecidas por la técnica de MBE y dopadas in-situ por una fuente sólida de Er.¹⁷ Estos reportaron la emisión de luz verde al fotoestimular las películas. Poco después Birkhahn y Steckl reportaron sobre la emisión de un verde visible a simple vista en películas de GaN:Er crecidas sobre Si(111) al ser fotoestimulada. Con esto demostraron la posibilidad del uso de estas películas en tecnología de silicio. ¹⁸ A.J. Steckl y su grupo encontraron que las películas de GaN:Er / Si(111) ¹⁹ pueden ser estimuladas eléctricamente utilizando contactos Schottky de aluminio. Se observó una emisión en el verde sin la ayuda de sensores. Luego se encontró que utilizando contactos de óxido-indio-estaño (ITO) en vez de contactos de metal la intensidad de luz emitida aumentaba. ²⁰

Otros colores como el rojo y el azul se han obtenido con películas de GaN con impurezas de Pr, Eu y Tm como lo indican las investigaciones citadas a continuación. Se ha encontrado que las películas de GaN/Si al ser dopadas con Pr o Eu pueden emitir en el rojo.^21,22 El método de crecimiento de estas películas fue por MBE y fueron estimuladas por electroluminiscencia y fotoluminiscencia. Para obtener el color azul el GaN fue dopado con Tm. ²³

Otro método de excitación utilizado fue el de catodoluminiscencia (CL) para películas de GaN implantadas con Dy, Er y Tm .²⁴ El mecanismo utilizado para dopar las películas fue el de implantación de iones. Esta fue la primera ocasión en que se reporta emisión en el rango visible para las películas de GaN dopadas por el método de implantación de iones.

Colores como azul verdoso y amarillo se han obtenido al codopar Tm y Er ²⁵, y Er y Eu ²⁶ respectivamente. Esto demuestra que es posible obtener cualquier color en el espectro visible con la combinación adecuada de emisores basados en GaN:TR⁺³.

D. S. Lee y A.J. Steckl demostraron que era posible la integración lateral de GaN:TR⁺³ para dispositivos electroluminiscentes.²⁷ Estos autores reportaron la integración lateral de GaN:Er y GaN:Eu sobre un substrato de Si (111). Con este dispositivo obtuvieron independientemente el color rojo por emisión del Eu y verde por emisión del Er. En este dispositivo resultó posible obtener el color rojo o el verde en la emisión aplicando voltajes de polarización opuestos.

Luego Y.Q. Wang y A.J. Steckl reportaron sobre la integración lateral para los tres colores primarios, rojo Eu (621nm), verde Er (537nm,558 nm) y azul Tm (477nm) en películas electroluminiscentes de GaN depositadas por MBE sobre Si(111).²⁸ Comparando los largos de onda obtenidos con la gráfica de cromaticidad y tomando en cuenta su relativamente sencilla fabricación se puede apreciar la capacidad de estos dispositivos para implementación en pantallas planas.

Algunos trabajos recientes sobre GaN:TR⁺³ se concentran en la optimización de estos materiales para su utilización en dispositivos prácticos. Ejemplo de esto es el trabajo de D.S. Lee and A.J. Steckl ²⁹ sobre el crecimiento de películas de GaN dopadas con Eu, Er, Tm sobre silicio, crecidas a temperatura ambiente. La intención al crecer películas a temperatura ambiente y sobre silicio es que resulta ser un proceso de bajo costo y simple, con posible utilidad en circuitos basados en silicio.

También se ha estudiado la relación entre el flujo de Ga, N y el dopante para así mejorar la calidad de las películas. ³⁰ Se encontró que al ser crecidas bajo un alto flujo de N con una concentración de aproximadamente 1% atómico de Er se obtenían películas con una mejor calidad.

Otro trabajo de importancia fue sobre dispositivos electroluminiscentes de GaN:Er que pueden ser excitados usando voltajes bajos (5V-6V) lo cual es ventajoso. ³¹ Se ha encontrado que al codopar Er con Mg o Er con C y O su actividad óptica incrementa y mejora su estabilidad térmica. ^32,33

Uno de los problemas que enfrenta el proceso de implantación de iones son los daños en la estructura cristalina. Debido a esto, las películas tienen que ser calentadas a altas temperaturas para rearreglar su estructura cristalina. Con esta técnica se calienta toda la muestra, inclusive las áreas no dopadas, causando difusión de átomos entre las capas. Para evitar este efecto indeseado se ha desarrollado una técnica en donde se usa un láser para calentar la muestra que luego es enfriada por conducción. Los resultados fueron películas con mejores cualidades cristalinas. ³⁴ Sin embargo, es deseable el dopaje in situ por su simplicidad y porque se espera una cantidad menor de defectos que los ocasionados por la implantación.

Películas de nitruro de Galio El propósito de esta investigación era desarrollar una técnica para fabricar películas delgadas de GaN dopadas in situ con TR mediante deposición por láser pulsado. Se intentó mejorar además la estructura cristalina de las películas y sus cualidades ópticas.

Para esto se tiene que tomar en consideración los siguientes factores:

• La tasa de deposición

• El flujo de nitrógeno atómico

• La fluencia del láser

• La temperatura y estructura cristalina del substrato

Para fabricar las películas GaN se tomaron como punto de partida los parámetros utilizados por M.E. Pumarol ³⁵. Como se quería crecer el GaN sin rotar el blanco con el fin de disminuir el particulado se busco la fluencia del láser para la cual la pluma de Ga era parecida al caso cuando el blanco era rotado. Esto se hizo variando la distancia lente-ventana y la energía de los pulsos.

Las características de las películas que resultan dependen fuertemente de los substratos usados. El caso ideal se daría al crecer directamente sobre substratos de GaN, pero hasta el momento no ha sido posible la comercialización de cristales de GaN. Algunos investigadores han tenido éxito en la fabricación de cristales de GaN pero sus procesos son muy complejos y costosos. En parte por esta razón se utilizan otros materiales cristalinos como substratos para la fabricación de GaN, algunos de los cuales tienen una estructura bastante distinta a la de GaN. Entre los substratos más utilizados están los de Si(111) para la estructura wurzita. Con estos substratos no se obtienen los mejores resultados pero son de bajo costo y pueden ser utilizados en tecnologías basadas en silicio

Bárbara Scarlett Betancourt Morales

Caf

Growth and applications of Group III- nitrides

Growth and applications of
Group III-nitrides

Introduction
Group III-nitrides have been considered a promising system for semiconductor devices applications since 1970,especially for the development of blue- and UV-lightemitting diodes. The III–V nitrides, aluminium nitride (AlN), gallium nitride (GaN) and indium nitride (InN), are candidate materials for optoelectrical applications at such photon energies, because they form a continuous alloy system (InGaN, InAlN, and AlGaN) whose direct optical bandgaps for the hexagonal wurtzite phase range from 1.9 eV for _-InN and 3.4 eV for _-GaN to 6.2 eV for _-AlN. The cubic modifications have bandgaps in the range from 1.7 eV for _-InN and 3.2 eV for _-GaN to 4.9 eV for _-AlN (figures 1 and 2) [1–6]. Other advantageous properties include high mechanical and thermal stability, large piezoelectric constants and the possibility of passivation by forming thin layers of Ga2O3 or Al2O3 with bandgaps of approximately 4.2 eV and 9 eV. The spontaneous and piezoelectric polarization in the wurtzite materials) and the high electron drift velocities (2 _ 105 m s−1 [7]) of GaN can be used to fabricate highpower transistors based on AlGaN/GaN heterostructures. In addition, AlN is an important material with a variety of applications such as passive barrier layers, high-frequency acoustic wave devices, high-temperature windows, and dielectric optical enhancement layers in magneto-optic multilayer structures Very informative reviews of the growth techniques and structural, optical and electrical properties of Group IIInitrides and their alloys have been presented by Strite et al .A good overview of applications of Group III-nitride based heterostructures for UV emitters and high-temperature, high-power electronic devices is provided in [12] and [13]. This review focuses on the development of the different growth techniques successfully applied to the deposition of Group III-nitride epitaxial films and heterostructures, such as chemical transport and metalorganic chemical vapour deposition (MOCVD), sputtering and molecular beam epitaxy (MBE). The quality of state-of-the-art material and its application for optical and electronic devices are discussed in detail in order to point out possible limitations, promising developments and future trends. The first systematic effort to grow InN, GaN and AlN by chemical vapour deposition or sputtering processes took place in the 1970s in order to characterize the optical and structural properties of thin films. At that time, neither metalorganic precursors containing In or Al with electronic grade purity, plasma sources for nitrogen radicals compatible with MBE systems, nor substrate material with reasonably good thermal and lattice matches to the nitrides were available. The InN and GaN material had large concentrations of free electrons, presumed to result from oxygen impurities and intrinsic defects, and the structural quality of the AlN films was not good enough for optical or electronic applications. Primarily, the development of MOCVD and plasma-induced molecular beam epitaxy (PIMBE) over the last eight years has led to a number of recent advances and important improvements in structural properties.

Crystal structure, polarity and polarization of InN, GaN and AlN
In contrast to cubic III–V semiconductors like GaAs and InP with the zincblende structure, the thermodynamically stable phase of InN, GaN and AlN, is the hexagonal wurtzite structure (_-phase). Beside the _-phase, a metastable _-phase with zincblende structure exists and a cubic high-pressure modification with NiAs structure was observed for pressures above 25 kbar in the case of AlN. Because the _- and _-phases of Group III-nitrides only differ in the stacking sequence of nitrogen and metal atoms (polytypes), the coexistence of hexagonal and cubic phases is possible in epitaxial layers, for example due to stacking faults. The hexagonal crystal structure of Group III-nitrides can be described by the edge length a0 of the basal hexagon, the height c0 of the hexagonal prism and an internal parameter u defined as the anion–cation bond length along the (0001) axis. Because of the different cations and ionic radii (Al3C: 0.39 A° , Ga3C: 0.47 A° In3C: 0.79 °A[15]), InN, GaN and AlN have different lattice constants, bandgaps and binding energies as shown in table Both wurtzite and zincblende structures have polar axes (lack of inversion symmetry). In particular, the bonds in the h0001i direction for wurtzite and h111i direction for zincblende are all faced by nitrogen in the same direction and by the cation in the opposite direction. Both bulk and surface properties can depend significantly on whether the surface is faced by nitrogen or metal atoms. The most common growth direction of hexagonal GaN is normal to the f0001g basal plane, where the atoms are arranged in bilayers consisting of two closely spaced hexagonal layers, one with cations and the other with anions, so that the bilayers have polar faces. Thus, in the case of GaN a basal surface should be either Ga- or N-faced. By Gafaced we mean Ga on the top position of the f0001g bilayer, corresponding to polarity. Ga-faced does not mean Ga-terminated; termination should only be used.

Growth of Group III-nitride films and crystals
The history of the production of Group III-nitrides covers more than a century. AlN powder was first made in 1862 from liquid Al and N2 gas. The major difficulty with this direct reaction method is that the surface film of AlN on Al is very adherent and impedes further reaction. In another technique, AlN powder can be formed from Al electrodes in a direct current arc. This produces only small amounts of AlN per day and the powder usually has several per cent excess aluminium because even here the AlN skin is protective. A more useful method for making AlN is to react AlF3 powder with NH3 gas at high temperatures. The overall chemical reaction at 1000 _C is: AlF3.s/ C NH3.g/ ! AlN.s/ C 3HF.g/: (12) In order to promote the formation of AlN, it is necessary to keep the NH3 partial pressure above 1 bar and the HF gas must be continually removed. At 1 atm pressure, a minimum of 25 molecules of NH3 are needed for each AlN molecule produced. The earliest investigations of GaN powder were reported by Johnson and co-workers, who described the conversion of metallic Ga in a NH3 stream by the chemical reaction: 2Ga.l/ C 2NH3.g/ ! 2GaN.s/ C 3H2.g/: (13) They obtained a black powder by flowing ammonia over metallic gallium at 1000 _C. Ejder contained Ga in an open sintered alumina boat. The boat was placed in a silica glass tube in a tube furnace, and a mixture of nitrogen and ammonia was passed over the boat. The reaction between Ga and NH3 started below 1000 _C and the thin crust ofGaN which formed on the surface of the molten Ga then decreased the rate of further GaN formation by lowering the evaporation rate of Ga from the metal surface. By this method whiskers, needles and prisms (sizes up to 500 _m) were formed on the edges of the boat and on the walls of the surrounding silica glass tube. These early growth processes and investigations resulted in AlN and GaN powders and very small crystals which were used to determine basic physical properties like crystal structure, lattice constants and optical properties. These results enabled the identification of substrate materials suitable for the heteroepitaxy of Group III-nitrides.

Substrates for heteroepitaxy
One particular difficulty in the growth of thin films is the unavailability of sufficiently large (>1 cm) single crystals for use as substrates for homoepitaxial growth. Thus up to now, heteroepitaxial growth is a practical necessity and the choice of substrate is critical. This problem is well recognized and there have been a number of studies on the effects of the substrate on the structural, electrical and morphological properties of thin films of these compound semiconductors [76–88]. Possible substrate materials with low thermal expansion and lattice mismatch for vapour phase epitaxy (VPE) and MOCVD are limited to those unaffected by high concentrations of ammonia and hydrogen at temperatures in excess of 1000 _C. This limits the use of Si, GaAs and GaP, if no low-temperature buffer layer can be grown as a first step of device fabrication. Even for PIMBE, in which the growth temperatures are about 250 _C lower than for VPE and MOCVD, the substrate surfaces have to be stable under the influence of nitrogen radicals at 800 _C. For device production processes, the substrate of choice has to be available in a minimum size of two inches, with atomically flat surfaces, and in large quantities at acceptable prices. Under the presuppositions mentioned above, sapphire and silicon carbide are the most popular substrate materials. Although sapphire (_-Al2O3) has a rhombohedral structure, it can be described by a hexagonal cell that is larger than the basic rhombohedral unit cell. To date, four orientations of sapphire have been used as substrates: (10N10), (0001), (2N1 N10) and (11N20). The lattice mismatch between GaN and (0001) sapphire is 13.9%. This table also includes the thermal expansion mismatch which is just as important for epitaxial growth. For various commercially available sapphire orientations, the relationship of orientation, lattice mismatch and crystallographic symmetry with a GaN epitaxial film are given. From the viewpoints of lattice mismatch and crystal symmetry, (10N10) sapphire (m-plane) seems the most suitable for GaN growth. However, the c-axis of a GaN film grown on a (10N10) sapphire substrate has a nonzero inclination with respect to the c-axis of the sapphire substrate. This means that twins may be generated. This is a major disadvantage of a (10N10) plane compared to a (0001) plane. Because of the high in-plane lattice mismatch (up to−29%) between the (0001) oriented films of InN, GaN and AlN and the (0001) sapphire, it seems surprising that epitaxial growth is possible. Transmission electron microscopy (TEM) was used to investigate the epitaxial growth of hexagonal GaN by MBE on the (0001) basal plane of Al2O3. The in-plane orientation obtained between layer and substrate gives a high lattice misfit of −13:9%, which induces a stress relaxation process directly in the interface region. High resolution TEM (HRTEM) images reveal f11N20gSapphire lattice fringes terminating at the interface between GaN and Al2O3(0001) [79]. Because the Burgers vectors of misfit dislocations are parallel to the (0001) interface plane, gliding is limited to the (0001) planes and therefore these misfit dislocations are confined at the interface. This interfacial relaxation process is very effective and the extension of dislocations into the GaN layer is suppressed. In the case of pseudomorphic growth the number of f10N10gGaN lattice fringes is expected to be the same as the number of f11N20gSapphire lattice fringes. The plastic relaxation of GaN by the formation of misfit dislocations leads to a termination of f11N20gSapphire lattice fringes at the interface. The quantitative evaluation of Fourier-filtered HRTEM images reveals an average of 8:3 _ 0:7 Al2O3 lattice spacings between two terminating f11N20gSapphire fringes. By forming a coincidence lattice between GaN and Al2O3, a large portion (−11:8%) of the high mismatch is compensated by dislocations confined at the interface. The relaxation process allows an epitaxial growth of GaN on (0001)Al2O3 with a density of non-confined dislocations of about 1010 cm−2 in the GaN. The non-confined dislocations are caused by the residual strain reduction from −2:1% measured near the interface to −0:2% obtained at the surface for a 1 _m thick GaN film. The lattice constant a.GaN/ D 3:1892 °A in the surface region reveals a residual strain of −0:2% in the GaN film after growth. With thermal expansion coefficients perpendicular to the c-axis of 5:6_10−6 K−1 for GaN and 7:3_10−6 K−1 for Al2O3 a lattice misfit of −13:75% can be calculated for a growth temperature of 800 _C. Cooling of the sample from growth temperature to room temperature causes a film strain of −0:12%, which is similar to the measured surface strain. Consequently it can be concluded that at the growth temperature, the film is almost completely relaxed and the residual strain close to the surface is only a thermal effect.

Bárbara Scarlett Betancourt Morales
CAF

Growth and optical properties of III-nitride

Growth and optical properties of III-nitride
semiconductors for deep UV (230–350 nm)
light-emitting diodes (LEDs) and laser diodes (LDs)

Introduction
GaN and III-nitride compound semiconductors are attracting considerable attention for the realization of visible or ultraviolet (UV) laser diodes (LDs) and light-emitting diodes (LEDs). Blue LDs and violet, blue and green LEDs are already commercially available.1) In particular, AlGaN is expected to be applied in deep UV LDs and bright LEDs, because the direct transition energy can be adjusted between 6.2 eV (AlN) and 3.4 eV (GaN). Shows the relationship between the direct transition energy and the lattice constant of wurtzite (Al,Ga,In)N alloy.The emission wavelength range of GaN and related materials covers the red to deep UV region, which can be realized by excimer lasers, He-Cd laser or solid-state SHG lasers. Deep UV LDs or LEDs are useful for realizing large-capacity optical memories or longlifetime fluorescent light.Moreo ver, they are important in the biochemical and medical fields.Ho wever, there are some severe technical problems that prevent realization of UV optical devices.The most serious problems are difficulty in obtaining efficient UV emission from AlGaN quantum wells (QWs), in contrast to InGaN QWs,1) as well as the difficulty in achieving p-type doping in high-Al-content AlGaN. First, we studied the growth and optical properties of high- Al-content AlGaN.W e obtained single-peak emission of AlxGa1−xN over the entire compositional range, i.e., from GaN to AlN, from near the band edge.2) We obtained the shortest wavelength (208 nm) photoluminescence (PL) of a semiconductor from AlN grown on SiC.We also demonstrated the shortest wavelength efficient UV emission at 230–250nm from AlN(AlGaN) /AlGaN multi- (M-)QWs.3) The 200 nmband emissions from AlGaN QWs were as strong as those from commercially available InGaN QWs at 77 K,3) however, they were weak at room temperature (R.T.). For the purpose of obtaining R.T. bright UV emission and high hole conductivity of wide-bandgap AlGaN, we propose the use of the emission from a localized electron-hole pair in the In segregation region in InAlGaN quaternary.It was reported that the quantum-dot-like region formed by In segregation in InGaN QWs is very effective for the suppression of nonradiative recombination and that an InGaN QW emits well at room temperature.4,5) It was also reported that the In content segregation of more than 5% in InGaN is necessary for high-current injection devices such as LDs.W e need a high Al content ranging from 40 to 60% in order to achieve 300 nm-band-emitting InAlGaN quaternary with 5% In incorporation, because of the very large band bowing of InAlGaN.W e report on the growth and optical properties of InAlGaN quaternary and demonstrate R.T. intense deep-UV emission at 300–340nm from InxAlyGa1−x−yN quaternary QWs for the first time.6,7) As for the current injection of deep-UV-emitting QWs, we have already realized 333nm current injection emission using Mg-doped GaN/AlGaN superlattice (SL) hole conducting layers.8) We report on the possibility of p-type doping for wider bandgap AlGaN by introducing In.

Experimental details and discussions
Structures were grown at 76 Torr on the Si-face of an on-axis 6H-SiC(0001) substrate by low-pressure (76Torr) metalorganic vapor phase epitaxy (MOVPE).The layer structures consisting of GaN, AlxGa1−xN, InxGa1−xN or InxAlyGa1−x−yN were grown on a several-hundred-nm-thick AlxGa1−xN (x = 0.1–0.5) buffer layer which was grown on SiC under optimized growth conditions in order to achieve a flat surface suitable for the WQ layer growth and to reduce the threading dislocation density (TDD).As precursors, ammonia (NH3), trimethylaluminum (TMAl), trimethylgallium (TMGa), trimethlindium di-i-propyalamine adduct (TMI-adduct), tetraethylsilane (TESi), and bisethylcyclopentadienylmagnesium (BECp2Mg) were used with H2 and N2 carrier gas.The typical growth temperatures of GaN, AlxGa1−xN, InxGa1−xN, and InxAlyGa1−x−yN are 1000–1100 ◦C, 1050–1250 ◦C, 650–800 ◦C, and 800–870 ◦C, respectively. The typical growth rates of GaN, AlxGa1−xN, InxGa1−xN, and InxAlyGa1−x−yN are 2.4 μm/hour, 0.5– 2.4 μm/hour, 0.1 μm/hour and 0.12 μm/hour, respectively. First, we show the optical properties of AlGaN.Figure 2 shows the PL spectra of AlxGa1−xN films over the entire Al compositional range, i.e., from GaN to AlN, emitting from near the band edge measured at 77 K.The AlGaN alloy was grown directly on a very thin (∼5 nm) AlN layer deposited on SiC.The thickness of AlGaN film was approximately 250 and 400nm for AlN and Al0.11Ga0.89N, respectively.As seen, single-peak spectra were obtained for the entire Al compositional range emitting from near the band edge.2) We obtained the first PL emission of AlN and record the shortest wavelength (208 nm) PL of a semiconductor.The deep level yellow emission at approximately 500–550nm was negligible even for high-Al-content AlGaN, indicating the good crystal quality of AlGaN. Then, we fabricated several series of AlGaN MQW samples, consisting of various Al-content AlGaN barriers.Figure 3 shows the schematic layer structure and the PL spectra for various QW thicknesses of fabricated AlN/Al0.18Ga0.82N MQW samples.In order to achieve a flat surface suitable for the growth of AlGaN quantum wells, an approximately 250– 400 nm-thick AlN buffer layer was deposited.W e confirmed a step-flow grown surface by atomic force microscopy (AFM). The PL measurement was performed with excitation by a Xelamp light source (215 nm) measured at 77K.W e obtained single-peak intense PL emission from each MQW.The most efficient emission was obtained at a wavelength of 234 nm. This is the shortest intense UV emission of a semiconductor QW.The optimum value of well thickness was approximately 1.6 nm. The PL intensity of the MQWs was several tens of times higher than that of bulk AlGaN.The quantized level shift was clearly observed.A rapid reduction in the PL intensity with an increase in the well thickness was caused by a reduction in the radiative recombination probability due to a large piezoelectric field in the well.
The PL intensities were compared among AlN/AlGaN, Al- GaN/GaN and InGaN/InGaN MQWs under the same measurement conditions, as shown in Fig.4. We found that the PL intensity of 230 nm-band emission from AlGaNbased QWs was as strong as that of 420 nm-band emission from commercially available InGaN-based QWs, and much stronger than that from GaN-based QWs at 77K.Ho wever, at room temperature, the emissions from AlGaN and GaN QWs were much weaker than that from InGaN QWs.Th us, the next purpose was to obtain efficient UV emission at room temperature.

Conclusions
We demonstrated intense UV emission at 230–350nm from QW structures consisting of III-nitride semiconductors grown by MOVPE.W e obtained 230 nm-band intense UV emission from AlN/AlxGa1−xN QWs at 77 K.The emission efficiency of AlGaN-based QW was as high as that of blue LEDs at a low temperature, however, at R.T., it was not significantly high.Then, we introduced In into AlGaN to improve R.T. emission efficiency by the radiative recombination of the localized electron-hole pair in In segregation regions.The efficiency of R.T. UV emission was dramatically enhanced by introducing several percent of In into AlGaN.W e obtained 320 nm-band R.T. intense emission from InAlGaN/InAlGaN
QWs.The emission intensity of the InAlGaN-based QW was as strong as that of commercially available InGaN QWs at R.T. We also achieved p-type doping into wide-bandgap Al- GaN using several methods and accomplished the first successful operation of a 330 nm-band LED.F rom these results, it was shown that the InAlGaN quaternary was very promising for use as active layers of 300–350 nm-band LDs or LE.

Bárbara Scarlett Betancourt Morales
Caf

Progress on Gallium Nitride Semiconductor Growth by Plasma Sputtering

Progress on Gallium Nitride Semiconductor Growth by Plasma Sputtering

Introduction

The growth of III-V nitride semiconductors is traditionally achieved by epitaxial techniques such as Molecular Beam Epitaxy (MBE) or Metal-Organic Chemical Vapor Deposition (MOCVD).These growth techniques produce films and heterostructures of high crystalline quality, which is necessary for short-wavelength optoelectronic devices, and high-power high-frequency electronic devices. The device costs and costof- ownership from using these epitaxial growth techniques are typically high, owing to the slow growth rates and infrastructure investment, and alternate techniques to grow large area GaN layers on various substrates are being considered. One such technique is Hydride Vapor Phase Epitaxy (HVPE), which has recently been used to grow GaN layers on sapphire, as well as InGaN/GaN heterostructures. In this work, preliminary results from an attempt to grow GaN using a novel technique – Biased Target Deposition (BTD) are reported. This growth technique uses a remote plasma source and a widely variable bias on the sputter target to give unprecedented control of process particle energetics and allows the growth of crystalline films. Sputtering has been applied to the growth of GaN, but none of these techniques proved suitable, scalable or cost effective enough for widespread deployment. BTD promises to meet these criteria with future development.

Experimental

Gallium nitride (GaN) was grown on sapphire wafers and characterized by x-ray diffraction (XRD) and photoluminescence. The growth reactor consisted of a commercially-available 4Wave Laboratory Alloy Nanolayer Sputtering (LANS) biased target sputter deposition (BTD) vacuum chamber with a base pressure of 2E-8 Torr. A modified Kaufman & Robinson Inc. (KRI) EH-1000 end-Hall plasma source coupled with a KRI LHC thermionic hollow-cathode electron source were used to provide low energy plasma to fill the interior of the chamber. The plasma forming gas was a mixture consisting of 30 sccm of argon (Ar) and 3 to 30 sccm of nitrogen (N2). The anode of the end-Hall source was biased at ~65 volts positive with respect to ground (the walls of the vacuum chamber were grounded) and the cathode of the hollow cathode was biased 13 volts negative with respect to ground to sustain a discharge in the anode region of 8 amperes. The emission current of the cathode was set to 8.5 amperes. Under these conditions, the plasma consisted of mainly Ar+ and N2 + ions totaling ~2 amperes of continuous ion current flow with a kinetic energy distribution ranging from 65 electron volts (eV) to 0 eV (median energy 35-40 eV) and a slight over supply of electron flux. Pumping via a Helix Technology CT-10 cryo-pump resulted in an ambient pressure of 5 – 7x10-4 Torr, depending upon N2 flow. The sputtering geometry was a right-angled geometry. The plasma source directly faced the growth substrate at a distance of 240 mm, with the axis between them horizontal. A gallium metal sputter target was located 114 mm gravitationally below this axis, centered between the plasma source and the substrate. The gallium target was 105 mm diameter and faced gravitationally upward to contain the gallium metal as a liquid. In this geometry, it is essential to spin the growth substrate on the source-substrate axis to obtain lateral thickness uniformity of the growth across the substrate. The spin rate was 10 rpm.
The gallium sputter target was contained in a water-cooled, copper-topped heat exchanger 112 mm diameter with an 8 mm tall ring of boron nitride ceramic glued to it. The container thus formed was filled with 99.999% pure gallium pellets. The pellets were then melted with a hot air gun and allowed to cool to form a solid mass. The sputtering process itself remelts the gallium (m.p. = 30°C) even at low sputtering power of ~100 watts and with 15°C cooling water flowing through the heat exchanger. The melted gallium initially had dross on the surface, probably due to native oxide on the pellets. Three cycles of sputtering the target for about ½ hour after it melted, then cooling, venting the vacuum chamber and lightly sanding the target with 400 grit carbide sandpaper eliminated the dross and gave a mirror-like surface. Sputtering of the gallium metal was accomplished by bi-polar pulsed DC bias of the target heat exchanger. The negative pulses were 11 microseconds ( μs) long and -850 volts with respect to ground while the positive pulses were 3 μs long and +10 volts. Average currents drawn by the two pulse trains were 165 mA for the negative, of which 33 mA was capacitive, and 100 mA for the positive. The substrate holder had a removable titanium carrier plate, 140 mm diameter, to hold and center the 2-inch (50.8 mm) sapphire wafer. The wafers were 0.432 mm thick, epi polished on one side and C-plane oriented. Various methods were used to clamp the wafer to the Ti plate, since it was gravitationally vertical during growth. As mentioned, the Ti plate was spun at 10 rpm on the plasma source-substrate axis. Behind the Ti plate was a 1500 watt infrared emissive heater from Heraeus Noblelight which was not spun. A bare-junction thermocouple of 0.010" (0.25 mm) dia. wire was attached to the heater.
The usual growth process consisted of mounting a sapphire wafer on the Ti carrier plate and masking a small portion (1 mm dia. spot) with graphite colloid in isopropyl alcohol, so that it later be dissolved away and growth thickness measured with a Tencor P12 profilometer, load-locking the sapphire wafer into place, starting wafer spin and heating the substrate holder to the desired temperature behind a closed shutter. The plasma source was started and allowed to stabilize for 10 min. The shutter was opened for two minutes to pre-clean the sapphire surface with gentle ion bombardment, as specified above. After that, the shutter was closed and the gallium target was sputtered, to melt it and condition it. When ready to do growth, the shutter was opened. Growth was stopped by closing the shutter. In some cases, a bare-junction thermocouple of 0.005" (0.12 mm) dia. wire was fastened to the edge of the wafer using a screw and washer, so substrate spin could not be used for those runs. An attempt was made to correlate the wafer temperature so measured with the temperature of the heater. High resolution x-ray diffraction (HRXRD) was used to characterize the epitaxial film quality/crystallinity. The HRXRD measurements were taken with a Panalytical X'pert Pro MRD system. The characterization was done with X-rays of 30 keV. Photoluminescence (PL) spectra emitted by the GaN sample were measured when the sample was pumped by a 3 ps pulsed coherent radiation at the wavelength of 208 nm. The pump beam was the output of quadrupling the frequency of the laser pulses at a central wavelength of 832 nm using two BaB2O4 crystals.The average power of the pump beam focused on the samples surface was set to 1.0 mW. The collected PL signal was sent through a double monochromator and then detected by a photomultiplier tube. A lock-in amplifier was used to reduce the measurement noise.

Results
A first growth of GaN was done with the sapphire substrate at room temperature and using 30 sccm N2 flow through the plasma source. The resultant 260 nm thick film was transparent with a slightly grey color. The XRD confirmed that the film showed no long-range order. The attempt to correlate a direct wafer temperature measurement with the temperature of the heater failed, within the time allotted, for two reasons. The direct measurement gave temperature readings that drifted during a run and varied from run to run for the same heater power. Also, the Ti wafer carrier plate warped, after heating, in a cylindrical fashion by about 2-3 mm total bending over its 140 mm diameter. Crystalline GaN growth was attempted at nitrogen flow rates of 3, 7, 10, 12, 15 and 30 sccm with two different heater power settings. The resultant growths for 3 sccm were black and powdery, for 7 sccm were transparent brown and dense, for 10 sccm were transparent grey and dense and for 12, 15 and 30 sccm were transparent light grey and dense. For the grey, dense films, the growth rate was ~600 nm/hour. Figure 1 shows the X-ray diffraction scan of the GaN sample grown with 15 sccm of N2 and at 896 watts of heater power. The large peak at 20.709° confirms the crystallinity of the sapphire substrate, and a small GaN (002) peak is observed. Figure 2 shows the photoluminescence response of the GaN sample grown with 7 sccm of N2 and at 896 watts of heater power. A small emission at 365 nm is consistent with the GaN bandgap.

Discussion
The first growth of GaN at room temperature having no long range order in the XRD is entirely expected, and is consistent with much experience that sputtering normally produces "amorphous" dielectric films at low substrate temperature. The failed attempt to correlate a direct wafer temperature measurement with the temperature of the heater probably has several combined causes. The washer used to clamp the thermocouple junction to the wafer may have partially electrically short-circuited the junction. Thermal shifting of the washer probably exacerbated the situation. The warped substrate holder meant that the wafer could not lay flat on the holder, and there probably were temperature variations across the wafer. GaN growth with N2 flows > 10 sccm are probably providing fully nitrided GaN, as evidenced by the color changes up to that flow and the lack of color changes above it. The process, at the settings used, is energetic enough (in the sense of ion assist by N2 + and bombardment by fast N atoms) that the degree of nitridation will be largely independent of substrate temperature, though some small effect would be expected. Both the XRD and PL results are consistent with poor crystalline quality of the GaN. The many emitting states observed in the bandgap (wavelengths longer than 365 nm) by PL are tentatively attributed to defects. Attributing them to metallic or high-Z impurities is extremely unlikely given past studies on other materials systems and the inherent cleanliness of the BTD technique itself. It is possible that oxygen and hydrogen from residual gases in the LANS could get incorporated in the growing GaN, thus explaining some of the crystalline disorder and emitting states. But it is considered much more likely that the lack of control of wafer temperature, probably giving too low a temperature, combined with the energetic sputtering wafer environment explains the results. Future work will focus on getting the wafer temperature under control first, then starting to vary the energetics of the process to achieve acceptable GaN crystallinity. A reflection high-energy electron diffraction (RHEED) instrument will be added to the LANS to assist in this work. Lastly, if contaminants from residual gases turn out to be a problem, growth rates and vacuum pumping can be increased dramatically, thereby increasing the ratio of Ga and N arrival relative to the impurity arrival at the growing GaN. The LANS is a small, economical research system, not optimized for semiconductor growth, but newer systems could be so optimized.

Conclusions
GaN has been grown by sputtering from a liquid gallium sputter target in an Ar+/N2 + plasma environment. In these preliminary results, crystalline quality was poor. Future work to improve this will proceed along the lines discussed.

Bárbara Scarlett Betancourt Morales

Caf

Gallium nitride (GaN)

Gallium nitride (GaN)

Gallium nitride (GaN) is a binary III/V direct bandgap semiconductor commonly used in bright light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crysta structure. Its wide band gap of 3.4 eV affords it special properties for applications in optoelectronic, high-power and high-frequency devices. Its sensitivity to ionizing radiation is low (like other group III nitrides), making it a suitable material for solar cell arrays for satellites. Because GaN transistors can operate at much hotter temperatures and work at much higher voltages than gallium arsenide (GaAs) transistors, they make ideal power amplifiers at microwave frequencies.

Physical properties
GaN is a very hard, mechanically stable material with large heat capacity. In its pure form it resists cracking and can be deposited in thin film on sapphire or silicon carbide, despite the mismatch in their lattice constants. GaN can be doped with silicon (Si) or with oxygen to N-type and with magnesium (Mg) to p-type; however, the Si and Mg atoms change the way the GaN crystals grow, introducing tensile stresses and making them brittle. Gallium nitride compounds also tend to have a high spatial defect frequency, on the order of a hundred million to ten billion defects per square centimeter.

GaN-based parts are very sensitive to electrostatic discharge.

Developments
High crystalline quality GaN can be obtained by low temperature deposited buffer layer technologyThis high crystalline quality GaN led to the discovery of p-type GaN,[ p-n junction blue/UV-LEDs and room-temperature stimulated emission[9] (indispensable for laser action).This has led to the commercialization of high-performance blue LEDs and long-lifetime violet-laser diodes, and to the development of nitride-based devices such as UV detectors and high-speed field-effect transistors.
High-brightness GaN light-emitting diodes (LEDs) completed the range of primary colors, and made applications such as daylight visible full-color LED displays, white LEDs and blue laser devices possible. The first GaN-based high-brightness LEDs were using a thin film of GaN deposited via MOCVD on sapphire. Other substrates used are zinc oxide, with lattice constant mismatch only 2%, and silicon carbide (SiC). Group III nitride semiconductors are in general recognized as one of the most promising semiconductor family for fabricating optical devices in the visible short-wavelength and UV region.
The very high breakdown voltages, high electron mobility and saturation velocity of GaN has also made it an ideal candidate for high-power and high-temperature microwave applications, as evidenced by its high Johnson's Figure of Merit. Potential markets for high-power/high-frequency devices based on GaN include microwave radio-frequency power amplifiers (such as used in high-speed wireless data transmission) and high-voltage switching devices for power grids. A potential mass-market application for GaN-based RF transistors is as the microwave source for microwave ovens, replacing the magnetrons currently used. The large band gap means that the performance of GaN transistors is maintained up to higher temperatures than silicon transistors. First gallium nitride metal/oxide semiconductor field-effect transistors (GaN MOSFET) were experimentally demonstrated in 1993 and they are being actively developed.

Applications
GaN, when doped with a suitable transition metal such as manganese, is a promising spintronics material (magnetic semiconductors). A GaN-based violet laser diode is used in the Blu-ray disc technologies, and in devices such as the Sony PlayStation 3. The mixture of GaN with In (InGaN) or Al (AlGaN) with a band gap dependent on ratio of In or Al to GaN allows the manufacture of light-emitting diodes (LEDs) with colors that can go from red to blue.

Nanotubes of GaN are proposed for applications in nanoscale electronics, optoelectronics and biochemical-sensing applications

GaN HEMTs have been offered commercially since 2006, and have found immediate home in various wireless infrastructure applications due to their high efficiency and high voltage operation. Second generation technology with shorter gate lengths will be addressing higher frequency telecom and aerospace applications.

Synthesis
GaN crystals can be grown from a molten Na/Ga melt held under 100 atm pressure of N2 at 750 °C. As Ga will not react with N2 below 1000 °C, the powder must be made from something more reactive, usually in one of the following ways:
2 Ga + 2 NH3 → 2 GaN + 3 H2
Ga2O3 + 2 NH3 → 2 GaN + 3 H2O

Safety
The toxicology of GaN has not been fully investigated. The dust is an irritant to skin, eyes and lungs. The environment, health and safety aspects of gallium nitride sources (such as trimethylgallium and ammonia) and industrial hygiene monitoring studies of MOVPE sources have been reported recently in a revi.

Bárbara Scarlett Betancourt Morales

CAF

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Tecnologías epitaxiales de crecimiento de cristales semiconductores

Tecnologías epitaxiales de crecimiento de cristales semiconductores

El término epitaxia (del griego epi, sobre, taxis, orden) apareció por primera vez hace aproximadamente 50 años. Este término se refiere a un proceso de crecimiento orientado de una película sobre un substrato, que puede ser del mismo material que la película (homoepitaxia) o bien de un material diferente (heteroepitaxia). A los procesos de cristalización de películas sobre la superficie de un substrato se les llama, en forma genérica, métodos de crecimiento epitaxial de cristales. En la tecnología actual de crecimiento de heteroestructuras láser las técnicas de crecimiento epitaxial son utilizadas de manera extensiva. Estas técnicas ocupan un lugar muy importante en la tecnología de semiconductores relacionada con el diseño de circuitos integrados,así como de dispositivos semiconductores entre los que se cuentan fotodetectores, fotodiodos y transistores de alta frecuencia. Dependiendo de la forma de transportar el material a crecer desde la fuente hasta el substrato, todos los procesos epitaxiales se dividen en tres tipos: (a)epitaxia por fase líquida, (b) epitaxia por fase gaseosa, (c) epitaxia por haces moleculares.
Hoy en día todos estos procesos son utilizados en el crecimiento de los más diversos dispositivos y estructuras semiconductoras. Por otro lado, también son útiles en la búsqueda de nuevos materiales semiconductores. Debido a que cada uno de estos procesos tiene una serie de particularidades muy específicas, su análisis detallado se realiza en este artículo..

La epitaxia o crecimiento epitaxial es uno de los procesos en la fabricación de circuitos integrado.

A partir de una cara de un cristal de material semiconductor, o sustrato, se hace crecer una capa uniforme y de poco espesor con la misma estructura cristalina que este. Mediante esta técnica se puede controlar de forma muy precisa el nivel de impurezas en el semiconductor, que son los que definen su carácter (N o P). Para hacer esto se calienta el semiconductor hasta casi su punto de fusión y se pone en contacto con el material de base para que, al enfriarse, recristalice con la estructura adecuada.

Hay varios métodos:

• Crecimiento epitaxial en fase vapor (VPE).
• Crecimiento epitaxial en fase líquida (LPE).
• Crecimiento epitaxial por haces moleculares (MBE).

La tecnología de sensores basada en materiales semiconductores que puedan funcionar a temperatura ambiente es un área de interés relevante en el campo de la investigación, la medicina [y la industria. Se pueden considerar un gran número de aplicaciones donde estos materiales pueden ser utilizados, en particular en el campo de detección de rayos X y radiación gamma para medicina nuclear, seguridad y no proliferación de materiales dañinos, aplicaciones medioambientales, etc.

Entre los materiales semiconductores que potencialmente tienen alto número Z, gap de banda ancho y propiedades eléctricas aceptables, los que proporcionan mejores resultados tecnológicos son los que pertenecen a la familia II-VI basados en Cd(Zn)Te [4],los únicos que han demostrado estar lo suficientemente desarrollados como para poder ser aplicadso en disciplinas que requieren un alto nivel de fiabilidad es el compuesto CdxZn1-xTe [5-8]. Esta familia de materiales específicos II-VI combinan propiedades excelentes como para ser los mejores candidatos a sensores por:

- su alta sensibilidad como consecuencia de los valores altos en los productos de

vida media y movilidad, (τ·μ),

- su alta resolución en energías como consecuencia de la energía de par electrónhueco

de 4.41 eV,

- su razonable madurez en la tecnología microelectrónica requerida para la tecnología de sensores, etc... que hacen en resumen que esta familia de materiales sean los mejores candidatos para la tecnología de sensores. En este contexto, la familia de CdTe puede contribuir a mejorar aspectos críticos como son la reducción de la dosis en el paciente y la visualización de alto contraste.

Nanoparticulas de SiO2

Otro método muy prometedor para fabricar substratos nanoperfilados, que empezamos a estudiar muy recientemente, es la deposición de nanoparticulas de SiO2 sobre obleas de Si mediante un tratamientos químicos. Con microscopia de fuerzas atómicas AFM determinamos el tamaño de grano y la forma de una distribución superficial de un substrato de Si al que hemos depositado nanoparticulas de Si02, mediante un tratamiento químico. También determinamos el tamaño de grano y la forma cuando la distribución de nanocristales sea superficial.

Crecimiento epitaxial de columnas de Cd(Zn)Te en alúmina porosa

La epitaxia en fase de vapor (VPE) de CdTe y Cd1-xZnxTe también se lleva a cabo para investigar el crecimiento de columnas nanoestructuradas de Cd(Zn)Te, las cuales pueden servir como base para micro-pixels utilizables en detectores de rayos X –y gamma, con alta resolución en imágenes. Se ha demostrado la posibilidad de crecer nanoestructuras de "Cd(Zn)Te en alúmina porosa" por crecimiento VPE, en un rango de temperaturas entre (600-850)ºC [13]. Los cristales de Cd(Zn)Te integrados en alúmina porosa tienen propiedades PL compatibles con materiales en volumen y capas epitaxiales. SEM ampliadas de la sección transversal de "Cd(Zn)Te en alúmina

porosa". En la micro-escala (FIGURA 5A), observamos como algunos de los poros están rellenos

monolíticamente con Cd(Zn)Te (flecha A) mientras que otros están rellenos de dendritas de Cd(Zn)Te, como cristales. Esta peculiaridad, se ve mejor a nano-escala (FIGURA 5B), y sugiere la necesidad de utilizar algunos procesos post-growth para mejorar la calidad cristalina de estas estructuras.

Bárbara Scarlett Betancourt Morales 

CAF

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