GROWTH OF SELF-CATALYZED GaAs NANOWIRES BY MOLECULAR BEAM EPITAXY DIRECTLY ON GaAs (111)B SUBSTRATES

Abstract

This dissertation is a systematic investigation to improve the growth method of GaAs nanowires by molecular beam epitaxy via self-catalytic VLS mode. The nanowire can be grown directly on GaAs (111)B substrates instead of SiO2-covered substrates. The merit of this technique is to eliminate the common problems that occur with nanowires grown by conventional VLS mode such as tilted nanowire, and parasitic structure. Growth parameters namely Ga flux pressure, and growth temperature were varied to search the optimal conditions to grow nanowires. After obtaining the optimal conditions, the nanowire growth was performed on both GaAs (111)B and SiO2 coated Si (111) for a comparison. Moreover, the nanowire samples were characterized by in-situ RHEED under varied growth time to study the growth evolution. Results show that GaAs nanowires are able to form directly on GaAs (111)B substrates without the existence of tilted wire, and parasitic structure. Density of nanowires increases with increasing the growth temperature however the growth rate remains nearly constant under a constant Ga flux in the optimal growth temperature. Nanowires grown by our method adopt WZ structure unlike conventional VLS nanowires which mostly adopt ZB structure, this is likely to be a result from different surface energy of substrate surface. Growth evolution shows that nanowires started forming as small dots before elongating to nanowire structure. Dots that unable to become nanowires are crystallized under As pressure to ring-like structure before getting buried by buffer layer. A simple growth model was proposed for better understanding of the self-catalyzed growth mode mechanism with using Ga as seed particles.