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In this letter, the photoluminescence spectra of n-type doped GaAs nanowires, grown by the metal organic chemical vapor deposition method, are measured at 4K and 77 K. Our measurements indicate that an increase in carrier concentration leads to an increase in the complexity of the doping mechanism, which we attribute to the formation of different recombination centers. At high carrier concentrations, we observe a blueshift of the effective band gap energies by up to 25 meV due to the Burstein-Moss shift. Based on the full width at half maximum (FWHM) of the photoluminescence peaks, we estimate the carrier concentrations for these nanowires, which varies from 6 × 1017 cm–3 (lightly doped), to 1.5 × 1018 cm–3 (moderately doped), to 3.5 × 1018 cm–3 (heavily doped) as the partial pressure of the disilane is varied from 0.01 sccm to 1 sccm during the growth process. We find that the growth temperature variation does not affect the radiative recombination mechanism; however, it does lead to a slight enhancement in the optical emission intensities. For GaAs nanowire arrays measured at room temperature, we observe the same general dependence of band gap, FWHM, and carrier concentration on doping. [ABSTRACT FROM AUTHOR]

A stack design for carbon-doped GaAs single junction solar microcells grown in triple-layer epitaxial assemblies is presented. As-grown materials exhibit improved uniformity of photovoltaic performance compared to zinc-doped systems due to the lack of mobile dopants while a slight degradation exists in middle and bottom devices. Detailed electrical and optical characterizations of devices together with systematic studies of acceptor reactivation reveal carbon-related defects accompanied by carrier compensation, and associated scattering and recombination centers are primarily responsible for the degraded contact properties and photovoltaic performance, resulting from prolonged thermal treatments of early-grown materials during the multilayer epitaxial growth. [ABSTRACT FROM AUTHOR]

Recently, we reported enhanced H 2 evolution on TiO 2 -passivated GaAs. Based on density functional theory (DFT) calculations, this enhancement was attributed to Ti 3+ states, which bind reactant species and increase charge transfer across the semiconductor–liquid interface. Here, we provide a quantitative correlation between Ti 3+ density, as measured by X-ray photoemission spectroscopy (XPS) and photoluminescence (PL) spectroscopy, and photocatalytic performance, which substantiates the hypothesis put forth previously. In the photo- I – V characteristics reported here, passivating GaAs with TiO 2 produces a shift in the onset potential of +0.35 V at 1 mA/cm 2 and enhances the photocurrent by 32-fold over bare GaAs (at 0 V vs. RHE), resulting in a peak photoconversion efficiency of 1.5% under AM1.5 G illumination. We find that just 1 nm of TiO 2 produces the best conditions for photocatalysis. XPS spectra show that thinner TiO 2 films (1 nm) have a higher density of Ti 3+ states than thicker films (5 nm), which have lower photocatalytic performance. PL spectroscopy provides further evidence for these Ti 3+ surface states, which cause increased surface recombination. While it is apparent that the TiO 2 films cause strong electron–hole recombination, the benefit that they provide by providing catalytically active sites outweighs their detriment associated with charge recombination. No enhancement is observed for TiO 2 thicknesses above 10 nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO 2 films. [ABSTRACT FROM AUTHOR]