Role of Sb in the epitaxial growth of Au-nucleated III-V nanowires

Abstract

III-Sb NWs and related heterostructures have many excellent properties, such as the largest electron and hole mobilities, the narrowest bandgap, large g-factor, strong spin-orbit interaction and possibility of forming different types of band alignment. As a consequence, III-Sb NWs are highly suitable for infrared photodetection, ultra-fast and low power optoelectronic devices and quantum physics applications, such as Majorana fermion pseudo-particles. Therefore, it is essential to gain understanding of III-Sb NW growth and push the development of III-Sb NWs for device applications. This thesis aims to understand the fundamental growth mechanisms of III-V NWs together with growth optimization of III-Sb NWs and related NW heterostructures for future optoelectronic applications. Growth of high-quality ternary GaAs1-xSbx NWs is comprehensively studied. Sb content is distributed uniformly along the NWs (even in tapered NWs) and can be tuned between 0.09~0.61 by changing the AsH3 flow. In contrast, the morphology and crystal structure of the NWs are sensitive to TMSb flow, resulting in nearly taper-free but kinked NWs under high TMSb flow. A reduction in surface energy of the liquid-vapour interface under high TMSb flow can dislodge the Au seed from the growth front to wet the NW sidewall. Using obtained high quality GaAs1-xSbx NWs, near infrared single NW photodetectors are fabricated and characterized. GaAs1-xSbx/InP core/shell NWs show a triangular shape with unexpected A-polar {112} side facets instead of {112}B facets usually found in other III-V NWs. The occurrence of A-polar {112} facet is a result of the surfactant role of Sb, which strongly reduces the surface energy and driving force for InP shell to grow along the <112>A direction. The core/shell NWs show strong photoemission intensity between 1.3 to 1.5 micron, high internal quantum efficiency (up to 56%) and long photo excited carrier lifetime (~800 ps) thanks to the efficient passivation effect of the InP shell. The surfactant effect of Sb can be exploited to control the polarity of GaAs NWs. By preloading the Au seeds with either Sb or Ga to change its surface energy balance, perfect vertical yield of <111>A-polar GaAs NWs is demonstrated for the first time. Larger (111)A interface energy leads to higher Au wetting angle. This causes the Au droplet move to the side facets via an interface energy minimization process, leading to NW kinking. Moreover, large wetting angle thermodynamically favours centre nucleation, which is energetically unfavourable for the formation of twin defects. Furthermore, (111)A polar GaAs1-xSbx NW array is obtained with high crystal quality. GaAs1-xSbx QW shell is grown on the GaAs core NWs. Surprising, twins are found in these QW NWs, which is ascribed to the lattice mismatched induced strain. It is noticed that lattice mismatched shell can result in twin formation in the core. The optimized GaAs1-xSbx QW NWs show efficient PL emission. Strong periodic emission is observed even at room temperature thanks to the Fabry-Perot cavity mode of the NWs. These high optical quality GaAs1-xSbx QW NWs are potential candidates for NW lasing applications. Show more