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

在过去的十年里，III-V族材料的半导体纳米线(NW)已经成为纳米级器件和互连的有希望的成分。纳米网络为纳米级光电子器件，包括场效应管、激光器、光电探测器和单电子存储器件提供了巨大的机会。此外，NW是下一代太阳能电池的理想原料，因为它们通常是直径约5纳米、长度几微米的单晶六角棒，因此为光生电荷提供了良好的传导路径。III-V半导体目前保持着传统平面太阳能电池光电转换效率的效率纪录，但由于生产成本较高，它们通常只用于空间飞行任务和太阳能集中器阵列等专门应用中。制造更便宜、更高效的太阳能电池板的能力将改变经济状况，有利于光伏发电，并看到太阳能电池发电的比例要大得多。纳米线的生产成本相对较低，因为它们的生长衬底不需要是单晶，而且可以回收利用。此外，纳米线的纳米级几何形状可以很容易地进行操作，以最大限度地减少入射太阳光的反射损失。然而，虽然关于NW光伏的早期结果非常有希望，但这些结果也突显了NW在太阳能电池中的应用关键依赖于对它们进行准确和可重复的电子掺杂。因此，无法可靠地掺杂纳米线已经成为开发和利用任何基于纳米线的新器件的主要障碍。获得这样的控制是至关重要的，因为它允许沿预定的器件路线定向电荷流动。在这个研究方案中，我们将用两种方法来解决这一主要障碍.(1)我们将开发新的核壳纳米线调制掺杂技术，以实现可靠的纳米线掺杂和表面陷阱钝化；(2)我们将通过开发基于内置半导体异质结界面电荷转移的沟道电荷流动方法，探索替代掺杂的方法。我们将在纳米线生长技术和先进的光谱分析方面拥有一个广泛的专家团队来实现这些目标。由于纳米线的几何形状，适用于评估纳米线中载流子浓度的技术相对较少。我们将探索通过一系列路线开发的纳米线，使用基于光泵太赫兹探测光谱和时间和空间分辨光致发光光谱的光谱方法的强大组合。这种光谱方法的好处在于它是一种非接触式方法，即测量得到的物理可观测值不会因接触的变化而变得模糊，而是反映了纳米线系综的内在性质。通过这些前沿的分析技术，我们将推动目前单晶半导体纳米线自下而上生长的两种领先方法，即分子束外延(MBE)和金属有机化学气相沉积(MOCVD)。让MBE(澳大利亚国立大学)和MOCVD(洛桑联邦理工学院)生长方面的领先研究小组作为该项目的合作伙伴，将首次使人们能够直接比较他们对纳米线掺杂的不同方法。通过这种联合方法，我们将建立一般的纳米线设计参数，这些参数将对纳米级光电子器件和下一代太阳能电池中半导体纳米线的生长和实施起到至关重要的推动作用。

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.