Growth and Characterisation of InP Nanowires and Nanowire-Based Heterostructures for Future Optoelectronic Device Applications

Abstract

Indium Phosphide (InP) forms a cornerstone amongst direct band-gap III-V compound semiconductors with the possibility for a wide range of other III-V alloys to be lattice matched with it. It is commonly used in optical communications related device applications, high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs). The very low surface recombination velocity of InP has made its nanowire counterpart a standout amongst nanowires of other III-V materials with successful demonstrations in nanowire solar cells, lasers and single photon sources. Considerable progress has been made in terms of InP nanowire growth in the past decade. Defect-free wurtzite (WZ) phase nanowires with good optical quality have been achieved on InP (111)B substrates. However, there are unexplored areas related to nanowire heterostructures that may hold promise for future device applications. Furthermore, InP nanowires aimed for future integrated devices need to be grown on the Si (111) substrates, and preferably on Si (100) substrates, in order to be integrated with microelectronics and other planar devices on a single chip. This dissertation presents a progressive advancement of Au seeded InP nanowire growth by MOVPE, from heterostructures grown on InP (111)B substrates to nanowire growth on Si (111) substrates and [100] oriented InP substrates. A number of diverse techniques have been employed to understand the growth process and characterise the samples. Scanning and transmission electron microscopy, atomic force microscopy, X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) have been used for structural and compositional analysis, while room and low temperature photoluminescence (PL) and PL mapping have been used for optical characterisation. InP-InxGa1-xAs nanowire quantum wells (QWs) emitting in the 1.3 μm optical communications wavelength region are grown on InP (111)B substrates. Detailed structural and optical analysis carried out using cross-sectional TEM (X-TEM) and PL mapping reveal asymmetric diffusion at the two interfaces of the QW, and broad, yet bright and homogenous PL emission along the complete length of the nanowire, with no emission visible from the InP nanowire core or outer barrier. The emission wavelength of the QW is tuned in the 1.3 μm range by varying the QW thickness as well as composition. The WZ phase QWs are optically modelled using the kp method. Multiple QWs comprised of three QWs and showing strong emission is also demonstrated. InP nanowire growth on Si (111) substrates has been carried out using an intermediate buffer layer. A two-step approach is used for the growth of the buffer layer and the growth parameters are optimised for both steps in order to achieve a smooth layer that covers the underlying Si substrate. It is seen that the layer fully relaxes by forming dislocations at the interface and is of (111)B polarity. Over 97% vertical nanowire yield is achieved on the buffer layers, and these nanowires are found to be similar in morphology and optical properties to those grown homoepitaxially on InP (111)B substrates under the same growth conditions. InP nanowires grown on the industry standard [100] orientated substrates are examined by studying the growth directions, facets and crystal structure of the different types, namely, vertical, non-vertical and planar nanowires grown on InP (100) substrates. The seemingly random growth directions of the non-vertical nanowires are actually found to be <111> and <100> directions that acquire complex orientations with respect to the substrate due to the consecutive three dimensional twinning that takes place at the initial stages of growth. These directions are mathematically calculated and verified by the measurements carried out on individual nanowires. It is shown that 99% of the nanowires grown on InP (100) substrate are either <100>, <111> or <110> oriented with growth facets of either {100} or {111}. The relative yields of each type of nanowire grown on InP (100) substrates are controlled by optimising the pre-growth annealing and growth conditions. A maximum of 87%, 100% and 67% yield is achieved for vertical, planar and non-vertical nanowires, respectively. The novel families of side facets of <100> nanowires are engineered to obtain cross-sectional shapes ranging from square to octagonal while maintaining a high vertical yield. Growth parameters and post-growth in-situ annealing conditions are tuned in order to achieve this. Finally, InGaAs QWs are grown on a novel and asymmetric facet combination of [100] nanowires, demonstrating the intended non-uniform complex growth that results in different thicknesses and compositions on the different types of nanowire facets. Overall, this work explores new avenues of InP nanowire and heterostructure growth aimed for future optoelectronic devices that are directly integrable with planar devices and Si technology. The findings presented, especially those on growth on [100] oriented substrates, bring many unforeseen opportunities for nanowire device development to light.