The vapor-liquid-solid (VLS) process of semiconductor nanowire growth is an attractive approach to low-dimensional materials and heterostructures because it provides a mechanism to modulate, in situ, nanowire composition and doping, but the ultimate limits on doping control are ultimately dictated by the growth process itself. Under widely used conditions for the chemical vapor deposition growth of Si and Ge nanowires from a Au catalyst droplet, we find that dopants incorporated from the liquid are not uniformly distributed. Specifically, atom probe tomographic analysis revealed up to 100-fold enhancements in dopant concentration near the VLS trijunction in both B-doped Si and P-doped Ge nanowires. We hypothesize that radial and azimuthal inhomogeneities arise from a faceted liquid-solid interface present during nanowire growth, and we present a simple model to account for the distribution. As the same segregation behavior was observed in two distinct semiconductors with different dopants, the observed inhomogeneity is likely to be present in other VLS grown nanowires.

GaN-InGaN core-shell nanowire array devices are characterized by spectrally resolved scanning photocurrent microscopy (SPCM). The spatially resolved external quantum efficiency is correlated with structure and composition inferred from atomic force microscope (AFM) topography, scanning transmission electron microscope (STEM) imaging, Raman microspectroscopy, and scanning photocurrent microscopy (SPCM) maps of the effective absorption edge. The experimental analyses are coupled with finite difference time domain simulations to provide mechanistic understanding of spatial variations in carrier generation and collection, which is essential to the development of heterogeneous novel architecture solar cell devices.

Barrier heights between metal contacts and silicon nanowires were measured using spectrally resolved scanning photocurrent microscopy (SPCM). Illumination of the metal-semiconductor junction with sub-bandgap photons generates a photocurrent dominated by internal photoemission of hot electrons. Analysis of the dependence of photocurrent yield on photon energy enables quantitative extraction of the barrier height. Enhanced doping near the nanowire surface, mapped quantitatively with atom probe tomography, results in a lowering of the effective barrier height. Occupied interface states produce an additional lowering that depends strongly on diameter. The doping and diameter dependencies are explained quantitatively with finite element modeling. The combined tomography, electrical characterization, and numerical modeling approach represents a significant advance in the quantitative analysis of transport mechanisms at nanoscale interfaces that can be extended to other nanoscale devices and heterostructures.

The decreasing cost of silicon-based photovoltaics has enabled significant increases in solar electricity generation worldwide. Silicon photoanodes could also play an important role in the cost-effective generation of solar fuels, but the most successful methods of photoelectrode passivation and performance enhancement rely on a combination of precious metals and sophisticated processing methods that offset the economic arguments for silicon. Here we show that metal-free carbon-based nanomaterial coatings deposited from solution can protect silicon photoanodes carrying out the oxygen evolution reaction in a range of working environments. Purified semiconducting carbon nanotubes (CNTs) act as a hole extraction layer, and a graphene (Gr) capping layer both protects the CNT film and acts as a hole exchange layer with the electrolyte. The performance of semiconducting CNTs is found to be superior to that of metallic or unsorted CNTs in this context. Furthermore, the insertion of graphene oxide (GO) between the n-Si and CNTs reduces the overpotential relative to photoanodes with CNTs deposited on hydrogen-passivated silicon. The composite photoanode structure of n-Si/GO/CNT/Gr shows promising performance for oxygen evolution and excellent potential for improvement by optimizing the catalytic properties and stability of the graphene protective layer.

Two-dimensional scattering rates are formulated using a two-subband triangular well approximation for GaInAs/InP and GaInAs/AlInAs single-well heterostructures, and two-dimensional electron gas (2-DEG) velocity characteristics are calculated using one-particle Monte Carlo simulations. Although the alloy scattering in GaInAs limits the low field mobility at low temperatures, a significant increase in the velocity characteristics is obtained for the 2-DEG at both 77 and 300 K. This result clearly illustrates that GaInAs is a promising material for high speed devices at 300 K and even at 77 K.