III-V Semiconductor Nanowires: Attaining Control over Doping and Heterointerfaces

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Semiconductor nanowires (NWs) of group III-V materials have emerged over the past decade as promising ingredients for nanoscale devices and interconnects. NWs offer great opportunities for nanoscale optoelectonic devices, including field-effect transistors, lasers, photodetectors and single-electron memory devices. In addition, NWs are ideal ingredients for next-generation solar cells as they are typically single crystal hexagonal rods of around 5nm in diameter and a few microns length, thus offering excellent conduction pathways to photo-generated charges. III-V semiconductors currently hold the efficiency records of light to electrical power conversion efficiency for conventional planar solar cells, yet they are generally only used in specialised applications such space missions and in solar concentrator arrays owing to their high production cost. The ability to make cheaper, and more efficient solar panels will change the economics in favour of photovoltaics and see a much larger proportion of electricity generation from solar cells. Nanowires are relatively cheap to produce as their growth substrates need not be single crystals and can be recycled. Furthermore the nanoscale geometry of nanowires can be easily manipulated to minimise reflective loss of incident sunlight. However, while early results on NW photovoltaics have been highly promising, these also highlighted that the application of NWs in solar cells crucially relies on electrically doping them accurately and reproducibly. Thus the inability to reliably dope nanowires has become the major obstacle to developing and exploiting any new nanowire based devices. Attaining such control is crucial as it allows directional charge flow along intended device routes. In this research programme we will attack this major obstacle using two a two-fold approach. (1) We will exploit novel techniques of modulation doping in core-shell nanowires to achieve reliable nanowire doping and surface trap passivation; and (2) We will explore alternatives to doping by developing methods to channel charge flow based on interfacial charge transfer at built-in semiconductor heterojunctions. We will tackle these aims with a broad team of experts on both nanowire growth technology and advanced spectroscopic analysis. Relatively few techniques are suitable for assessing the carrier concentration in nanowires, owing to their geometry. We will explore nanowires developed through a range of routes, using a powerful combination of spectroscopic methods based on Optical Pump Terahertz Probe spectroscopy and time- and spatially-resolved photoluminescence spectroscopy. This spectroscopic methodology benefit from being a non-contact method, i.e. the physical observables derived from the measurement are not obscured by variations in the contacts, but reflect the intrinsic properties of the nanowire ensemble. Through these cutting-edge analytical techniques we will advance both of the current leading approches to bottom-up growth of single crystal semiconductor nanowires, which are molecular beam epitaxy (MBE) and metal organic chemical vapour deposition (MOCVD). Having leading research groups on both MBE (Australian National University) and MOCVD (Ecole Polytechnique Federale de Lausanne) growth as partners on this project will allow for the first time a direct comparison of their different approaches to nanowire doping. Through this joint-up approach, we will establish general nanowire design parameters that give a crucial boost to the growth and implementation of semiconductor nanowires in nanoscale optoelectronics devices and next-generation solar cells.