n-GaAs films were grown epitaxially on n(+)-GaAs substrates by a close-spaced vapor transport method and their photoelectrochemical energy conversion properties studied. Under 100 mW cm(-2) of ELH solar simulation, conversion efficiencies up to 9.3% for CSVT n-GaAs photoanodes were measured in an unoptimized ferrocene/ferrocenium test cell. This value was significantly higher than the 5.7% measured for similarly doped commercial n-GaAs wafers. Spectral response experiments showed that the higher performance of CSVT n-GaAs films relative to the commercial wafers was due to longer minority carrier diffusion lengths (L(D)), up to 1,020 nm in the CSVT films compared to 260 nm in the commercial n-GaAs wafers. Routes to improve the performance of CSVT GaAs and the implications of these results for the development of scalable GaAs-based solar energy conversion devices are discussed.

Laser assisted atom probe tomography is used to characterize the alloy distribution in BGaN. The effect of the evaporation conditions applied on the atom probe specimens on the mass spectrum and the quantification of the III site atoms is first evaluated. The evolution of the Ga++/Ga+ charge state ratio is used to monitor the strength of the applied field. Experiments revealed that applying high electric fields on the specimen results in the loss of gallium atoms, leading to the over-estimation of boron concentration. Moreover, spatial analysis of the surface field revealed a significant loss of atoms at the center of the specimen where high fields are applied. A good agreement between X-ray diffraction and atom probe tomography concentration measurements is obtained when low fields are applied on the tip. A random distribution of boron in the BGaN layer grown by molecular beam epitaxy is obtained by performing accurate and site specific statistical distribution analysis.

Methods to simultaneously optimize carrier collection and light in-coupling in semiconductors are important for developing low-cost, high-efficiency photovoltaics and photoelectrodes. We anodically etched nanostructures into planar (100) n-GaAs wafers with different bulk minority carrier diffusion lengths L(D). The structures were varied by changing the anodization parameters. A ferrocene/ferrocenium electrolyte provided a conformal rectifying contact to the anodized n-GaAs and enabled the measurement of carrier generation and collection as a function of nanostructure geometry and L(D). Internal quantum efficiency Φ(int) of photoelectrodes varied with nanostructure geometry and L(D). External quantum efficiency Φ(ext) also depended on the reflectance of the nanostructured GaAs-electrolyte interface. Reflectance was minimized using anodization current densities of 100-150 mA cm(-2), which etched subwavelength trigonal prismatic nanostructures ~400 nm in width at their base. For Si-doped n-GaAs with L(D) = 170 nm, peak Φ(ext) of ~75% and Φ(int) of ~85% was achieved using J(anod) = 150 mA cm(-2). The control of both surface nanostructure (to minimize reflection) and pore depth and spacing (to optimize 3D carrier collection) via two-step anodization yielded photoelectrodes with peak Φ(ext) of ~85% and peak Φ(int) of ~95% for Te-doped n-GaAs with a bulk L(D) of only 420 nm. The measured short-circuit current densities for the nanostructured photoelectrodes were up to 2.5 times that of planar controls, demonstrating that appropriate nanostructuring significantly improves carrier collection even for direct bandgap materials with large absorption coefficients like GaAs.

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Incorporating boron into gallium nitride to make BxGa1-xN solid solutions would create an avenue for extreme alloys due to the fact that wurtzite phase BN has a larger band gap and smaller lattice parameters compared to GaN. In this paper, the authors report the growth of high crystal quality, random alloy BxGa1-xN thin films with x up to 3.04% grown on (0001) Ga-face GaN on sapphire substrates using plasma assisted molecular beam epitaxy and BBr3 gas as a B source. High resolution x-ray diffraction was used to measure both the c plane spacing and the strain state of the films. It was determined that the films were fully coherent to the GaN substrate. Elastic stress-strain relations and Vegard's law were used to calculate the composition. Atom probe tomography was used to confirm that the BxGa1-xN films were random alloys. In addition to demonstrating a growth technique for high crystal quality BxGa1-xN thin films, this paper demonstrated the use of BBr3 as a novel B source in plasma assisted molecular beam epitaxy.