InP Nanowires grown by Selective-Area Metalorganic Vapour Phase Epitaxy

Abstract

Semiconductor nanowires have attracted significant attention over the past decade. InP nanowires, with a direct bandgap and high electron mobility, are suitable materials for many electronic and optoelectronic devices. Several techniques have been used to grow InP nanowires, and among them, selective-area metalorganic vapour phase epitaxy (SA-MOVPE) is a versatile and powerful method owing to its advantages of being catalyst free, and its ability to produce position controlled nanowire arrays with high uniformity and excellent reproducibility, which are highly desirable for nano-scale device applications. This dissertation presents the growth of stacking fault-free and taper-free wurtzite (WZ) InP nanowires with a wide range of diameters using SA-MOVPE. This involves a systematic investigation of the growth conditions such as growth temperature and precursor flow rates. Furthermore, a detailed study revealing the fundamental growth mechanisms is presented, which reveals that selective area growth of nanowires does not necessarily lead to the pure selective-area-epitaxy (SAE) regime. There is a competing process between a pure SAE and a self-seeded vapour-liquid-solid mechanism, depending on the growth conditions and the mask opening. A comprehensive model is developed to combine the major features of these two mechanisms to understand the complex growth process. The optical properties of InP nanowires are studied by micro-photoluminescence (µ-PL) and time-resolved PL measurements. The internal quantum efficiency (IQE) of stacking fault-free WZ InP nanowires is quantified, with results equivalent to the best quality 2D layers. As a result of the excellent structural and optical quality of the nanowires, low threshold room temperature lasing is achieved from conventional guided modes. These nanowires can be transferred to different targeted substrates with high spatial accuracy using a nanoscale transfer printing technique. Lasing emission from the nanowires is maintained after the transfer, which highlights the robustness of our InP nanowires and the gentle nature of the transfer process. Doping in semiconductors is important for device applications and is thus investigated in this thesis. Using our newly developed optical method, doping concentration and IQE of the doped InP nanowires are quantified. This contact-free method can spatially resolve the doping profile along the length of the nanowire. The effect of doping on the morphology and crystal structure of Si-doped and Zn-doped InP nanowires are studied. It is found that doping favours radial growth and the nanowires remain pure WZ crystal phase after doping. Finally, this dissertation presents the design, fabrication and characterisation of axial p-i-n InP nanowire array solar cells. Optical modelling is performed to optimise the nanowire array geometry to achieve good light absorption. Electron beam induced current measurement is used to visualize and quantify the width and position of the p-n junction for device optimisation. It is shown that by varying the doping profile of the solar cell structures, the junction position and width can be adjusted and placed towards the top of the nanowires, where light absorption is most favourable. Solar cell devices with good efficiency (up to 6%) are demonstrated with future improvement achievable after further optimisation in nanowire structure design and fabrication procedures.