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

在过去的十年中，III-V族材料的半导体纳米线（NWs）作为纳米级器件和互连的有前途的成分出现。NWs为纳米级光电子器件，包括场效应晶体管、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，NWs是下一代太阳能电池的理想成分，因为它们通常是直径约5nm，长度约几微米的单晶六边形棒，因此为光产生的电荷提供了良好的传导途径。III-V型半导体目前保持着传统平面太阳能电池光电转换效率的记录，但由于生产成本高，它们通常只用于空间任务和太阳能集中器阵列等专门应用。制造更便宜、更高效的太阳能电池板的能力，将改变有利于光伏发电的经济状况，并使太阳能电池在发电中所占的比例大大提高。纳米线的生产成本相对较低，因为它们的生长基质不需要是单晶，而且可以回收利用。此外，纳米线的纳米级几何结构可以很容易地控制，以尽量减少入射阳光的反射损失。然而，虽然NW光伏的早期结果非常有希望，但这些也强调了NW在太阳能电池中的应用至关重要地依赖于准确和可复制地电掺杂它们。因此，无法可靠地涂覆纳米线已成为开发和利用任何新型纳米线器件的主要障碍。获得这样的控制是至关重要的，因为它允许定向电荷沿预定的设备路线流动。在本研究计划中，我们将采用两种方法来解决这一主要障碍。(1)开发核壳纳米线调制掺杂新技术，实现可靠的纳米线掺杂和表面阱钝化；(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通道电荷流方法来探索掺杂的替代方案。我们将与纳米线生长技术和先进光谱分析方面的广泛专家团队一起解决这些目标。由于纳米线的几何结构，适合评估其载流子浓度的技术相对较少。我们将利用基于光泵太赫兹探针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，探索通过一系列途径开发的纳米线。这种光谱方法得益于非接触方法，即从测量中得到的物理观测值不会被接触的变化所掩盖，而是反映了纳米线系综的内在特性。通过这些尖端的分析技术，我们将推进目前领先的自下而上生长单晶半导体纳米线的方法，即分子束外延（MBE）和金属有机化学气相沉积（MOCVD）。MBE（澳大利亚国立大学）和MOCVD（洛桑联邦理工学院）的领先研究小组作为这个项目的合作伙伴，将使他们第一次直接比较纳米线掺杂的不同方法。通过这种联合方法，我们将建立通用的纳米线设计参数，这对半导体纳米线在纳米级光电子器件和下一代太阳能电池中的生长和实施具有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.