Influence of growth temperature on structural and optical properties of laser MBE grown epitaxial thin GaN films on a-plane sapphire

Epitaxial thin GaN films (similar to 60 nm) have been grown on a-plane sapphire substrates at different growth temperatures (500-700 degrees C) using laser molecular beam epitaxy (LMBE). The effect of growth temperatures on the structural and optical properties of GaN layers grown on low temperature (LT) GaN buffer on prenitridated a-sapphire have been studied systematically. The in situ reflection high energy electron diffraction pattern revealed the three-dimensional epitaxial growth of GaN films on a-sapphire under the adopted growth conditions. The full width at half maximum (FWHM) value of x-ray rocking curves (XRCs) along GaN (0002) and (10-12) planes decreases with increasing growth temperature. The FWHM values of (0002) and (10-12) XRC for the 700 degrees C grown GaN film are 1.09 degrees and 1.08 degrees, respectively. Atomic force microscopy characterization showed that the grain size of GaN increases from 30-60 to 70-125 nm with the increase in growth temperature as GaN coalescence time is shorter at high temperature. The refractive index value for the dense GaN film grown at 600 degrees C is obtained to be similar to 2.19 at the wavelength of 632 nm as deduced by spectroscopic ellipsometry. Photoluminescence spectroscopy confirmed that the epitaxial GaN layers grown on a-sapphire at 600-700 degrees C possess near band edge emission at similar to 3.39 eV, close to bulk GaN. The GaN growth at 700 degrees C without a buffer still produced films with better crystalline and optical properties, but their surface morphology and coverage were inferior to those of the films grown with LT buffer. The results show that the growth temperature strongly influences the structural and optical quality of LMBE grown epitaxial GaN thin films on a-plane sapphire, and a growth temperature of >600 degrees C is necessary to achieve good quality GaN films. Published by the AVS

Influence of growth temperature on structural and optical properties of laser MBE grown epitaxial thin GaN films on a-plane sapphire

Epitaxial thin GaN films (similar to 60 nm) have been grown on a-plane sapphire substrates at different growth temperatures (500-700 degrees C) using laser molecular beam epitaxy (LMBE). The effect of growth temperatures on the structural and optical properties of GaN layers grown on low temperature (LT) GaN buffer on prenitridated a-sapphire have been studied systematically. The in situ reflection high energy electron diffraction pattern revealed the three-dimensional epitaxial growth of GaN films on a-sapphire under the adopted growth conditions. The full width at half maximum (FWHM) value of x-ray rocking curves (XRCs) along GaN (0002) and (10-12) planes decreases with increasing growth temperature. The FWHM values of (0002) and (10-12) XRC for the 700 degrees C grown GaN film are 1.09 degrees and 1.08 degrees, respectively. Atomic force microscopy characterization showed that the grain size of GaN increases from 30-60 to 70-125 nm with the increase in growth temperature as GaN coalescence time is shorter at high temperature. The refractive index value for the dense GaN film grown at 600 degrees C is obtained to be similar to 2.19 at the wavelength of 632 nm as deduced by spectroscopic ellipsometry. Photoluminescence spectroscopy confirmed that the epitaxial GaN layers grown on a-sapphire at 600-700 degrees C possess near band edge emission at similar to 3.39 eV, close to bulk GaN. The GaN growth at 700 degrees C without a buffer still produced films with better crystalline and optical properties, but their surface morphology and coverage were inferior to those of the films grown with LT buffer. The results show that the growth temperature strongly influences the structural and optical quality of LMBE grown epitaxial GaN thin films on a-plane sapphire, and a growth temperature of >600 degrees C is necessary to achieve good quality GaN films. Published by the AVS

Hybrid Reduced Graphene Oxide/GaN Nanocolumns on Flexible Niobium Foils for Efficient Photoelectrochemical Water Splitting

We present the photoelectrochemical (PEC) water-splitting properties of pristine and reduced graphene oxide (rGO)-coated GaN nanocolumns (NCs) on flexible niobium (Nb) metal foils. The structural, optical, and electronic structure analyses of rGO-coated GaN-NCs on Nb foils revealed the formation of a rGO/GaN-NCs hybrid structure. Further, the valence-band studies of pure GaN-NCs shows valence-band maxima at similar to 3.0 eV below the Fermi level, which decreased to similar to 2.8 eV for rGO/GaN-NCs. The PEC measurement performed on pristine GaN-NCs under standard (1 Sun) conditions shows an effective photocatalytic nature with a photocurrent density of similar to 60 mu A/cm2 at 0.8 V (vs Ag/AgCl) in a 1 M oxalic acid electrolyte, which increases to similar to 110 mu A/cm2 for rGO/ GaN-NCs. The efficient PEC characteristics of rGO/GaN-NCs are attributed to the effective charge separation/transport of photogenerated carriers. Furthermore, transient photocurrent measurement reveals that hybrid and pristine GaN-NCs on flexible metal foil have a fast and stable photoresponse in an aqueous solution. The development of a hybrid nitride nanostructure-based PEC device on flexible metal foils paves the way toward developing scalable PEC devices for hydrogen production applications