The behavior of the quasi-Fermi levels of electrons and holes at various semiconductor/liquid interfaces has been probed through the use of thin, high purity, low dopant density single crystal Si photoelectrodes. Since standard Air Mass 1.5 illumination is sufficient to produce high level injection conditions in such samples, minimal electric fields can be present near the solid/liquid interface. Under these conditions, efficient charge separation relies on establishment of kinetic asymmetries at the back contacts while effectively sustaining photogenerated carrier concentration gradients in the photoelectrode. These conditions were achieved for Si/CH3OH interfaces in contact with the 1,1‘-dimethylferrocene+/0, cobaltocene+/0, methyl viologen2+/+, and decamethylferrocene+/0 redox couples. For redox couples having energies near the top of the Si valence band, such as 1,1‘-dimethylferrocene+/0, the sample acted like an n-type photoelectrode, yielding large photovoltages for collection of electrons at the back contact and small photovoltages for collection of holes. For redox couples having energies near the bottom of the Si conduction band, such as cobaltocene+/0, the sample acted like a p-type photoelectrode, yielding large photovoltages for collection of holes at the back contact and small photovoltages for collection of electrons. The Si sample exhibited both photoanodic and photocathodic currents in contact with redox couples having electrochemical potentials in the middle of the Si band gap. A simple explanation, based on the fundamental carrier statistics of semiconductor/liquid contacts under illumination relative to the situation at equilibrium, is advanced to describe this behavior. This explanation is also applicable to a description of the photovoltage behavior of semiconductor particles and to undoped photoconductive semiconductor electrodes that are operated under high level injection conditions. In additional experiments, measurement of the apparent electrochemical potentials of electrons and holes in contact with various redox couples has allowed quantification of the amount of recombination and experimental determination of the separation of the quasi-Fermi levels for various redox couples at the semiconductor/liquid contact. These measurements are important to verification of key elements of the Shockley−Read−Hall and Marcus−Gerischer theories for semiconductor/liquid junctions.

We present a novel two-step approach for the selective area growth (SAG) of GaAs nanowires (NWs) by molecular beam epitaxy which has enabled a detailed exploration of the NW diameter evolution. In the first step, the growth parameters are optimized for the nucleation of vertically-oriented NWs. In the second step, the growth parameters are chosen to optimize the NW shape, allowing NWs with a thin diameter (45 nm) and an untapered morphology to be realized. This result is in contrast to the commonly observed thick, inversely tapered shape of SAG NWs. We quantify the flux dependence of radial vapour-solid (VS) growth and build a model that takes into account diffusion on the NW sidewalls to explain the observed VS growth rates. Combining this model for the radial VS growth with an existing model for the droplet dynamics at the NW top, we achieve full understanding of the diameter of NWs over their entire length and the evolution of the diameter and tapering during growth. We conclude that only the combination of droplet dynamics and VS growth results in an untapered morphology. This result enables NW shape engineering and has important implications for doping of NWs.

This report describes the successful realization of high‐quality semi‐polar (112¯2) ‐GaN templates grown on 100 mm diameter r‐plane patterned sapphire. Trench patterning is accomplished by plasma etching using a slanted SiNx mask that is formed by a resist‐reflow process and subsequent dry etching. Epitaxial overgrowth by MOVPE is optimized with the aid of in situ monitoring to trace GaN coalescence behavior and surface morphology. Wafer curvature at growth temperature exceeds typical values of c‐oriented GaN, whereas room‐temperature bow is spherical and comparable to polar material. Morphological and structural properties compare well with published data on 2 in. substrates. Threading dislocation densities of about 2 × 108 cm−2 and basal stacking fault densities in the order of 1 × 103 cm−1 are deduced from cathodoluminescence studies. Low residual impurity concentrations ([O, Si] < 1 × 1017 cm−3) have been verified.

Use of thin, nearly intrinsically doped Si electrodes having implanted, interdigitated n+ and p+ back contact points has allowed electrical control over the potential of either electrons or holes in the solid. During potential control at the n+ point contacts, the open-circuit potential of holes could be monitored, while during potential control of the p+ point contacts, the open-circuit potential of electrons was measured. In combination with current density−voltage measurements of either electrons or holes passing through the back contact points, these data allowed a comparison of the behavior of a given carrier type when generated by an applied bias (i.e., as majority carriers) relative to their behavior when generated with band gap illumination of the solid (as minority carriers). Data have been collected for Si/CH3OH junctions having 1,1‘-dimethylferrocene+/0, decamethylferrocene+/0, methyl viologen2+/+, and cobaltocene+/0 as redox couples. These data have been used to validate certain key predictions of the quasi-Fermi level concept in photoelectrochemistry. In addition, digital simulations that include two-dimensional representations of the charge density distribution and of the current fluxes in the solid have been utilized to provide a quantitative understanding of the observed experimental behavior.

The selective area growth of Ga-assisted GaAs nanowires (NWs) with a high vertical yield on Si(111) substrates is still challenging. Here, we explore different surface preparations and their impact on NW growth by molecular beam epitaxy. We show that boiling the substrate in ultrapure water leads to a significant improvement in the vertical yield of NWs (realizing 80%) grown on substrates patterned by electron-beam lithography (EBL). Tentatively, we attribute this improvement to a reduction in atomic roughness of the substrate in the mask opening. On this basis, we transfer our growth results to substrates processed by a technique that enables the efficient patterning of large arrays, nano imprint lithography (NIL). In order to obtain hole sizes below 50 nm, we combine the conventional NIL process with an indirect pattern transfer (NIL-IPT) technique. Thereby, we achieve smaller hole sizes than previously reported for conventional NIL and growth results that are comparable to those achieved on EBL patterned substrates.arXiv:1708.02454v1 [cond-mat.mtrl-sci]