Epitaxial growth of semiconductor nanowires

Sammanfattning: This thesis describes the results obtained from investigations carried out on epitaxially grown III-V semiconductor nanowires aimed at improving our understanding of and knowledge on the growth mechanism of nanowires. This is important to be able to control their growth, in order to make future applications possible. Nanowire growth was carried out using chemical beam epitaxy (CBE). This is a growth technique with the advantage of a fast response in the flow at the substrate surface and a slow growth rate, which makes it possible to realize size-controlled heterostructures with atomically abrupt changes in materials within a single nanowire. The growth mechanism of epitaxially nucleated wires is usually described by the vapour-liquid-solid (VLS) mechanism, where metallic seed particles, often gold (Au), are used to form a eutectic system with the growth material. These particles act as seeds for nanowire growth by creating supersaturation in the particle, which is the driving force for anisotropic growth. It was shown that the seed particle, often described as being in the liquid phase, is probably in the solid phase in this growth system. Despite the solid phase of the seed particle, it is believed that it maintains its catalytic property by locally changing the surface below the seed particle, at the wire-particle interface. The growth rate at this interface will increase in comparison with the growth rate on surrounding surfaces, resulting in anisotropic growth, i.e., wire growth. The effect of different growth conditions was studied and the results indicate that Au-seeded growth of both GaAs and InAs nanowires on the (111)B surface is strongly dependent on the diffusion of group-III material (Ga and In precursors) from the surrounding surfaces to the growth point. Based on these results, a model was proposed for epitaxial growth of nanowires with CBE. More complex nanowires were also grown and studied, such as nanowires of ternary compounds, InAs(1-x)P(x), as well as InAs nanowires containing InP and InAs(1-x)P(x) segments. The crystal structure, direction and composition were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray energy dispersive spectrometry (XEDS), showing that growth is almost always in the <111>B direction, a result of the low interfacial free energy of this plane. The crystal structure is zincblende or wurtzite, sometimes intermixed through stacking faults in the growth direction. The electronic properties of the homogeneous InAs and InAs(1-x)P(x) nanowires were characterized as a function of the applied source-drain voltage, gate voltage and temperature. The InAs nanowires are n-type and degenerated, with a resistance that decreases with decreasing temperature. The InAs(1-x)P(x) nanowires are also n-type but are not degenerated and therefore show a resistance that increases with decreasing temperature. Finally, measurements on InAs nanowires containing InAs(1-x)P(x) barriers with x varying from 0.22-1 were carried out to map the conduction band offset of InAs(1-x)P(x) relative to InAs. These measurements were carried out by thermal excitation of the carriers across the barrier. With this information, band gap engineering in nanowires based on the InAs(1-x)P(x) system could be performed, resulting in applications such as resonant-tunnelling-diodes, infrared detectors and improved transistor behaviour.