• Energy Research

We report on the dynamics of in situ etching of nanowires using an etching agent which allows for parameter optimization for nanowire synthesis without concerns of tapering issues. Upon etching of InP nanowires using HCl it is found that HCl mainly reacts with the precursor TMI, its decomposition species, and physisorbed In. The reaction with solid InP is less rapid and diffusion limited. We find a gas-phase etch process which is metal assisted and has a high aspect ratio of 1:100. (C) 2011 Elsevier B.V. All rights reserved.

Improving Nanowire Photovoltaics In principle, solar cells based on arrays of nanowires made from compound inorganic semiconductors, such as indium phosphide (InP), should decrease materials and fabrication costs compared with planar junctions. In practice, device efficiencies tend to be low because of poor light absorption and increased rates of unproductive charge recombination in the surface region. Wallentin et al. (p. 1057 , published online 17 January) now report that arrays of p-i-n InP nanowires (that switch from positive to negative doping), grown to millimeter lengths, can be optimized by varying the nanowire diameter and length of the n-doped segment. Efficiencies as high as 13.8% were achieved, which are comparable to the best planar InP photovoltaics.

Highly controlled particle-assisted growth of semiconductor nanowires has been performed for many years, and a number of novel nanowire-based devices have been demonstrated. Full control of the epitaxial growth is required to optimize the performance of devices, and gold seed particles are known to provide the most controlled growth. Successful nanowire growth from gold particles generated and deposited by various different methods has been reported, but no investigation has yet been performed to compare the effects of gold particle generation and deposition methods on nanowire growth. In this article we present a direct comparative study of the effect of the gold particle creation and deposition methods on nanowire growth characteristics and nanowire crystal structure, and investigate the limitations of the different generation and deposition methods used.

Nanowires grown with the vapor-liquid-solid method commonly exhibit polytypism, showing both zincblende and wurtzite crystal structure. We have grown p-type InP nanowires using DEZn as a dopant precursor and studied the wetting of the seed particle and the nanowire crystal structure. The nanowires grown with high DEZn molar fractions exhibit deformed seed particles after growth. We observe 20% smaller nanowire diameter at the highest DEZn molar fraction, indicating a significant increase in contact angle of the seed particle during growth. The decrease in diameter correlates with an increase in zincblende segment length as measured by TEM. We explain the results with a modified nucleation model.

Solar energy harvesting for electricity production is regarded as a fully credible future energy source: plentiful and without serious environmental concerns. The breakthrough for solar energy technology implementation has, however, been hampered by two issues: the conversion efficiency of light into electricity and the solar panel production cost. The use of III-V nanowires (NWs) in photovoltaics allows to respond to both these demands. They offer efficient light absorption and significant cost reduction. These low-dimensional structures can be grown epitaxially in dense NW arrays directly on silicon wafers, which are abundant and cheaper than the germanium substrates used for triple-junction solar cells today. For planar structures, lattice matching poses a strong restriction on growth. III-V NWs offer to create highly efficient multijunction devices, since multiple materials can be combined to match the solar spectrum without the need of tightly controlled lattice matching. At the same time, less material is required for NW-based solar cells than for planar-based architecture. This approach has potential to reach more than 50% in efficiency. Here, we describe our work on NW tandem solar cells, aiming toward two junctions absorbing different parts of the solar spectrum, connected in series via a tunnel diode.

Aerosol technology provides efficient methods for producing nanoparticles with well-controlled composition and size distribution. This review provides an overview of methods and results obtained by using aerosol technology for producing nanostructures for a variety of applications in semiconductor physics and device technology. Examples are given from: production of metal and metal alloy particles; semiconductor nanoparticles; semiconductor nanowires, grown both in the aerosol phase and on substrates; physics studies based on individual aerosol-generated devices; and large area devices based on aerosol particles.

AbstractWe present x-ray diffraction based methods to quantitatively determine the wurtzite content of nanowire ensembles and to investigate the effect of twinning. An increased lattice constant in growth direction is found for all investigated InAs and InP nanowire samples. This increase is independent of the wurtzite content. Using x-ray pole figures we find that twinning is present in GaAs/Si branched nanowires, which leads to 60° rotations of the lattice.

Axially defined GaInP single nanowire (NW) p-i-n junctions are demonstrated, with photocurrent response and yellow-green electroluminescence near the indirect bandgap crossover point at 2.18 eV (569 nm). We use DEZn and H2S as p- and n-type dopants, and find that they both affect the material composition and the crystal structure. The photovoltaic efficiency is comparable to single NW devices from binary III-V materials. These results demonstrate the potential of GaInP nanowires as a high-bandgap material for multijunction solar cells and light-emitting devices in the visible regime.