Crecimiento de nitruros del grupo III por epitaxia de haces moleculares para la fabricación de diodos electroluminiscentes en el rango visible-ultravioleta

En esta memoria se presenta el crecimiento por epitaxia de haces moleculares asistida por plasma (PA-MBE) de compuestos (0001) III-N. Su estudio viene motivado por la necesidad de mejorar las prestaciones de los diodos electroluminiscentes (LED) con emisión en el rango visible-ultravioleta para su aplicación en sistemas de iluminación con luz blanca, sistemas de señalización, dispositivos de almacenamiento óptico de alta densidad y sistemas de esterilización. Los modos de crecimiento y la morfología del (0001) GaN se analizaron en función de la temperatura del sustrato y de la relación de flujos Ga/N. Los resultados evidencian tres modos de crecimiento: tridimensional (3D), capa a capa (2D) y por corrimiento de escalones (1D). Las condiciones de crecimiento que dan lugar a cada uno de estos modos se resumieron en un diagrama universal para el crecimiento de (0001) GaN por PA-MBE. La morfología y composición del ternario (0001) InAlN fueron analizadas en función de la relación de flujos y de la temperatura del sustrato. De manera análoga al GaN, los resultados obtenidos se emplearon para construir un diagrama universal de crecimiento. La obtención de capas de InxAl1-xN con x ~ 0.17 abrió las puertas a la fabricación de espejos Bragg (DBR) de InAlN/GaN con ajuste en red centrados a ~ 400 nm y con reflectividades de hasta un 60 % para un total de 10 períodos. El análisis presentado en esta memoria acerca de la incorporación de In en (0001) InAlGaN en función de la temperatura del sustrato y la relación de flujos Al/Ga muestra que la incorporación de In disminuye con ambos parámetros. La calidad cristalina de las capas de InAlGaN resultó ser comparable a la de los sustratos de GaN/zafiro empleados para su crecimiento. El estudio de sus propiedades ópticas, mediante fotololuminiscencia (PL) y espectroscopía de absorción, reveló la presencia de fenómenos de localización excitónica causados por fluctuaciones de composición. Para un contenido de In fijo, estos fenómenos resultaron ser tanto más notables cuanto mayor fue el contenido de Al. Debido a que la localización excitónica reduce la probabilidad de recombinación no radiativa, los resultados sugieren que el cuaternario InAlGaN puede ser empleado para mejorar la eficiencia de los LED de emisión ultravioleta. La optimización del crecimiento de los distintos compuestos III-N permitió la fabricación de LED con pozos cuánticos (QW). Variando la composición y el espesor de los QW se consiguieron LED con emisiones a 355 y 400 nm. Con objeto de fabricar en un futuro matrices de nano-LED se analizaron el crecimiento y las propiedades de las nanocolumnas (NC) de (0001) (Al)GaN. La optimización de su crecimiento auto-ensamblado, sobre sustratos de Si (111), hizo posible la fabricación de discos cuánticos (QDisk) de GaN embebidos en NC de AlGaN. Las propiedades de estos sistemas fueron estudiadas mediante PL en función del espesor de los QDisk y del contenido de Al de las NC y barreras. Para finalizar, se propone un modelo teórico para explicar el crecimiento auto-ensamblado de NC de III-N sobre Si(111). Abstract This thesis addresses the research on the growth by plasma-assisted molecular beam epitaxy (PA-MBE) of (0001) III-N compounds. The studies carried out along this thesis were motivated by the need to improve the performance of the visible and ultraviolet light emitting diodes (LEDs) to be used in white light generation, traffic signals, high density optical storage devices and ultraviolet germicide sterilization. Growth mode and surface morphology of (0001) GaN were analysed as a function of growth temperature and impinging Ga/N flux ratio. The results reveal three different growth modes: three-dimensional (3D), layer-by-layer (2D), and step-flow (1D). A universal growth diagram was established, where the growth modes and the properties of the layers are depicted as a function of the growth parameters. Surface morphology and alloy composition of (0001) InAlN were investigated as a function on impinging fluxes and growth temperature. As in the case of GaN, the results were used to build up a universal growth diagram. The achievement of InxAl1-xN with x ~ 0.17 paved the way to the fabrication of ten periods lattice-matched InAlN/GaN distributed Bragg reflectors (DBRs) with peak reflectivity values up to 60 % at 400 nm. The studies presented in this thesis about In incorporation into (0001) InAlN show that it decreases with both growth temperature and impinging Al/Ga flux ratio. The crystal quality of InAlGaN layers was comparable to that of the GaN/sapphire templates used as substrate. The analysis of the optical properties by photoluminescence (PL) and optical absorption revealed an Al-enhanced exciton localization at potential fluctuations caused by alloy inhomogeneities. Since a stronger localization makes carriers less sensitive to defects and non radiative recombination, InAlGaN seems a promising choice for the active region in efficient ultraviolet LEDs. A proper optimization of the growth conditions for the different III-N compounds allowed the fabrication of LEDs with quantum wells (QWs) as active region. The LED emission wavelength was tuned at 355 and 400 nm by changing the QWs composition and thickness. To fabricate nano-LED arrays in the near future, the growth and properties of (0001) (Al)GaN nanocolumns (NCs) were studied. The optimization of the self-assembled PA-MBE growth of (Al)GaN NCs on Si(111) substrates enabled the subsequent growth of GaN quantum disks (QDisks) embedded in AlGaN NCs. The properties of these low dimensional systems were studied by PL as a function of QDisk thickness and the NC Al content. Finally, a theoretical model is proposed to explain the self-assembled growth of III-N NCs on Si(111)

Top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation

We demonstrate the top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation of pre-patterned GaN(0001) layers grown by hydride vapor phase epitaxy on Al$_{2}$O$_{3}$. Arrays with nanowire diameters and spacings ranging from 50 to 90 nm and 0.1 to 0.7 $\mu$m, respectively, are simultaneously produced under identical conditions. The sublimation process, carried out under high vacuum conditions, is analyzed \emph{in situ} by reflection high-energy electron diffraction and line-of-sight quadrupole mass spectromety. During the sublimation process, the GaN(0001) surface vanishes, giving way to the formation of semi-polar $\lbrace1\bar{1}03\rbrace$ facets which decompose congruently following an Arrhenius temperature dependence with an activation energy of ($3.54 \pm 0.07$) eV and an exponential prefactor of $1.58\times10^{31}$ atoms cm$^{-2}$ s$^{-1}$. The analysis of the samples by low-temperature cathodoluminescence spectroscopy reveals that, in contrast to dry etching, the sublimation process does not introduce nonradiative recombination centers at the nanowire sidewalls. This technique is suitable for the top-down fabrication of a variety of ordered nanostructures, and could possibly be extended to other material systems with similar crystallographic properties such as ZnO.Comment: This is the accepted manuscript version of an article that appeared in Nanoscale Advances. The CC BY-NC 3.0 license applies, see http://creativecommons.org/licenses/by-nc/3.0

Small-angle X-ray scattering from GaN nanowires on Si(111): facet truncation rods, facet roughness and Porod's law

Small-angle X-ray scattering from GaN nanowires grown on Si(111) is measured in the grazing-incidence geometry and modelled by means of a Monte Carlo simulation that takes into account the orientational distribution of the faceted nanowires and the roughness of their side facets. It is found that the scattering intensity at large wavevectors does not follow Porod's law I(q) ∝ q-4. The intensity depends on the orientation of the side facets with respect to the incident X-ray beam. It is maximum when the scattering vector is directed along a facet normal, reminiscent of surface truncation rod scattering. At large wavevectors q, the scattering intensity is reduced by surface roughness. A root-mean-square roughness of 0.9 nm, which is the height of just 3-4 atomic steps per micrometre-long facet, already gives rise to a strong intensity reduction. open access

A growth diagram for chemical beam epitaxy of GaP1-xNx alloys on nominally (001)-oriented GaP-on-Si substrates

The compound GaP1-xNx is highly attractive to pseudomorphically integrate red-light emitting devices and photovoltaic cells with the standard Si technology because it is lattice matched to Si with a direct bandgap energy of ≈1.96 eV for x = 0.021. Here, we report on the chemical beam epitaxy of GaP1-xNx alloys on nominally (001)-oriented GaP-on-Si substrates. The incorporation of N into GaP1-xNx was systematically investigated as a function of growth temperature and the fluxes of the N and P precursors, 1,1-dimethylhydrazine (DMHy) and tertiarybutylphosphine (TBP), respectively. We found that the N mole fraction exhibits an Arrhenius behavior characterized by an activation energy of (0.79 ± 0.05) eV. With respect to the fluxes, we determined that the N mole fraction is linearly proportional to the flux of DMHy and inversely proportional to the one of TBP. All results are summarized in a universal equation that describes the dependence of x on the growth temperature and the fluxes of the group-V precursors. The results are further illustrated in a growth diagram that visualizes the variation of x as the growth temperature and the flux of DMHy are varied. This diagram also shows how to obtain single-phase and flat GaP1-xNx layers, as certain growth conditions result in chemically phase-separated layers with rough surface morphologies. Finally, our results demonstrate the feasibility of chemical beam epitaxy to obtain single-phase and flat GaP1-xNx layers with x up to about 0.04, a value well above the one required for the lattice-matched integration of GaP1-xNx-based devices on S

A growth diagram for chemical beam epitaxy of GaP1-xNx alloys on nominally (001)-oriented GaP-on-Si substrates

The compound GaP1-xNx is highly attractive to pseudomorphically integrate red-light emitting devices and photovoltaic cells with the standard Si technology because it is lattice matched to Si with a direct bandgap energy of ≈1.96 eV for x = 0.021. Here, we report on the chemical beam epitaxy of GaP1-xNx alloys on nominally (001)-oriented GaP-on-Si substrates. The incorporation of N into GaP1-xNx was systematically investigated as a function of growth temperature and the fluxes of the N and P precursors, 1,1-dimethylhydrazine (DMHy) and tertiarybutylphosphine (TBP), respectively. We found that the N mole fraction exhibits an Arrhenius behavior characterized by an activation energy of (0.79 ± 0.05) eV. With respect to the fluxes, we determined that the N mole fraction is linearly proportional to the flux of DMHy and inversely proportional to the one of TBP. All results are summarized in a universal equation that describes the dependence of x on the growth temperature and the fluxes of the group-V precursors. The results are further illustrated in a growth diagram that visualizes the variation of x as the growth temperature and the flux of DMHy are varied. This diagram also shows how to obtain single-phase and flat GaP1-xNx layers, as certain growth conditions result in chemically phase-separated layers with rough surface morphologies. Finally, our results demonstrate the feasibility of chemical beam epitaxy to obtain single-phase and flat GaP1-xNx layers with x up to about 0.04, a value well above the one required for the lattice-matched integration of GaP1-xNx-based devices on S