Understanding the growth of III-V nanowires incorporated with In by molecular beam epitaxy

Abstract

III-V semiconductor nanowires have attracted a great attention due to their peculiar electronic and optical properties. Among them, GaAs semiconductor nanowires, with direct band gap, high carrier mobilities (than Si) and the heterojunction formation with other III-V semiconductor nanowires, such as InAs and AlGaAs, have potential for application in future nanoscale devices. To realize such optoelectronic devices fabrication, the III-V nanowires growth must be under control to enable their effective integration into devices, which can be achieved by Au catalyzed epitaxial nanowires growth under the vapor-liquid-solid (VLS) mechanism. Compared with binary III-V nanowires, ternary nanowires have been attracting an increasing interest as they allow the tunability of desired band gap for different applications by modulating the composition fraction of their alloys. In particular, InGaAs nanowires have been demonstrated to be technologically important for a wide range of applications, attributed to their tunable band gap ranging from near-infrared to infrared regions by tuning the InGaAs alloy composition, which makes them constitute the ideal material system for numerous optoelectronic devices, such as light-emitting diodes and nanolasers. Accordingly, the motivation to investigate ternary InGaAs nanowires system and its related binary GaAs nanowires system is triggered for their optoelectronic applications in the future. In this thesis, binary GaAs nanowires system was first studied. With the help of Au nanoparticles, GaAs nanowires were grown on GaAs {111}B substrates. By modulating the growth parameters, such as growth temperature, V/III ratio and growth duration, the morphological and structural characteristics of grown GaAs nanowires were investigated. Through extensive electron microscopy investigations, we found that by lowering the group-V flux to the low V/III ratio, nanowire growth is As-limited and thermodynamically controlled, leading to the slow nanowire growth and defect-free wurtzite structured grown nanowires. On the other hand, we found that by prolonging the growth duration, the structural quality of GaAs nanowires was significantly enhanced from defected to defect-free wurtzite structure. Moreover, the growth behavior and compositional characteristics of ternary InxGa1-xAs nanowires were studied by tuning the In concentration from 50 to 85 at.%. With extensive cross-sectional study of the grown nanowires by advanced electron microscopy, we found that when x = 0.5, core-multishell structure spontaneously formed at the nanowire bottoms and Ga concentration in nanowire cores increases towards the nanowire tops; and when x = 0.85, InGaAs nanowires formed the core-shell structure, with the In-rich core and the Ga-enriched shell, in which the composition of the cores and shells were varied at varied nanowire regions. The fundamental reasons behind these new phenomena were investigated and the corresponding growth mechanism was unveiled.