Recent technological advances have seen the development of semiconductor nanowires which have considerable advantages over their predecessors in that they utilize planar semiconductors, as novel building blocks for the next generation of semiconductor devices. Shell growth is the fundamental way to fabricate both lateral homo- and heterojunctions. This has been heavily reported in literature where foreign-metal-catalyzed nanowire cores were developed. Conversely, the use of foreign metal catalysts such as gold has been observed not only to introduce the risk of contaminating the nanowire crystal, but also induce problems during the shell growth. This therefore implies that the knowledge obtained from foreign-metal-catalyzed growth cannot be transferred unswervingly to the self-catalyzed method. Presently, self-catalyzed nanowires have been seen to contain defects due to the changes in droplets properties during crystal growth. Such defects extend to the shell and degrade its quality. Unfortunately, a detailed theory on how to suppress the formation of these defects is still lacking and therefore more detailed studies on how to grow high quality shells are needed.
Dr. Yunyan Zhang and colleagues from the Department of Electronic and Electrical Engineering at University College London in the United Kingdom, conducted a study to grow core-shell nanowires using the self-catalyzed nanowires in the GaAs(P) material system. The hoped to investigate the influence of the defects at the nanowire tip region on the shell growth. Their work is published in the research journal, Nano Letters.
The research techniques entailed the direct growth of self-catalyzed gallium arsenide and GaAs(P) nanowires on p-type silicon substrates by solid-source molecular beam epitaxy. Next, the team ensured that gallium arsenide shell and the nanowires were independently grown with a gallium beam equivalent pressure. The researchers then used an ion beam−scanning electron microscope system to determine the nanowire morphology. Eventually, transmission Electron Microscopy and photoluminescence measurements were performed.
The authors observed that the defects from the core were able to extend into the shell thereby degrading the crystal quality. In addition, the defects were seen to regionally alter the surface properties of the nanowires which affected the shell uniformity. Moreover, the researchers noted that defect-free core nanowire up to the topmost bilayer were achieved due to the formation of Berylium−Gallium alloy droplets, which resulted in growth of shells with highly regular morphology and pure-zinc blende crystal quality. These high-quality nanowires demonstrated an enhancement of one order of magnitude for room-temperature emission compared to that of the defective nanowires.
The Yunyan Zhang and colleagues study has successfully presented a systematic cross-examination of the growth of self-catalyzed core−shell nanowire using MBE grown gallium arsenide and GaAs(P) nanowire. It has been seen that elimination of defects in the core nanowire is essential to achieve high-quality shell growth, which can be achieved by introducing beryllium doping during the growth. The results obtained in their study solved a long-term defect-formation issue in self-catalyzed nanowire growth and shed light on fabricating high-quality core−shell nanowire devices.
Yunyan Zhang, H. Aruni Fonseka, Martin Aagesen, James A. Gott, Ana M. Sanchez, Jiang Wu, Dongyoung Kim, Pamela Jurczak, Suguo Huo, Huiyun Liu. Growth of Pure Zinc-Blende GaAs(P) Core−Shell Nanowires with Highly Regular Morphology. Nano Lett. 2017, volume 17, pages 4946−4950
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