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

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.

The proposed project "III-V Semiconductor Nanowires: Attaining Control over Doping and Heterointerfaces" has two strands of high importance for the UK. Firstly, there is high potential for long-term impact since it addresses two of the UK's key societal challenges of energy security and global warming. The UK has set itself a target to reduce carbon emissions by 80% (from 1990 levels) by 2050. Future development in pushing photovoltaic technology forward, as outlined in this proposal, can make crucial contributions towards the UK being able to fulfilling such aims. Secondly, the UK has a long history of manufacturing semiconductors and semiconductor technology, and is host to a number of key semiconductor growth and processing companies. This project will help put the UK at the forefront of a rapidly developing field that has the potential to be a disruptive technology.

The project will overcome the major hurdles in the commercialisation of semiconductor nanowire based optoelectronic devices. Semiconductor nanowires offer huge potential as the building blocks for the next generations of miniaturised electronics components including high-efficiency solar cells, nanolasers, nanosensors, single photon detectors, and three-dimensional nano-transistor arrays. However for nanowires it has proved extremely difficult to form reliable and reproducible p-n junctions - the essential component of conventional electronic and optoelectronic devices. The difficultly in implementing and understanding p and n doping in nanowires is thus currently limiting the commercial exploitation of a plethora of novel nanowires devices. This project will tackle this obstacle by developing methods to dope nanowires and characterise doping in nanowires. Furthermore it will also explore novel device architectures that avoid the need to dope. The advancement of knowledge gained in the project will foster the development of nanowire solar cells, and seed the development of a wide range of optoelectronic and electronic nanowire devices.

The highest efficiency solar cells currently available are all based on group III-V semiconductors in planar geometry. Solar cells based on similar III-V semiconductor materials, but in a single crystal nanowire morphology, should offer efficiencies exceeding these owing to enhanced light coupling. Furthermore the long term stability and low energy cost of producing nanowires (owing to substrate recycling and low growth temperatures) should lead to cheap, highly efficient solar cells. Reducing the cost of energy production in a country has an immediate benefit to energy intensive industries in a country, and hence promotes economic growth. A reduction in the cost of a renewable energy source also accelerates its replacement of fossil fuel based energy generation, leading to an important environmental and health benefits through the reduction in the emission of carbon oxides, particulates and noxious gases.
The pathway to achieving such economic and societal impact involves the generation and dissipation of scientific knowledge and technology to academia, industry, government and the general public. In addition, training researchers and inspiring the next generation of scientists will be key outcomes of the project. The UK is strong in the area of next generation PV, an area where it hosts a number of established and start-up companies. Training needs are high in this technology area; hence this project will be important in maintaining the UK's competitiveness in industry and academia through its training of early-stage researchers.