Nanowires (NWs) have a wide range of innovative applications due to their unique properties as compared to their counterparts such as thin films. As a result, a lot of research work has been recently shifted towards the study of NWs with the aim of improving their functionalities in the various fields. Currently various methods are available for fabricating the nanowires. Vapor-liquid-solid (VLS) method, for example, has been considered very complex due to the numerous procedures involved. It is a droplet-catalyzed type of growth and thus affects the crystal structure, morphology as well as the dopant of the nanowires. Especially, the doping mechanism is far from clear, as the participation of the nanoscale liquid phase makes the doping environment highly complex and significantly different from that of the thin-film growth.
Traditionally, the catalyst consists of the use of external metals. Recent years, self-catalyzed growth has drawn great attention as it does not require the use of foreign materials hence associated with minimal contamination as compared to its counterpart. Researchers at the University College London in the United Kingdom: Dr. Yunyan Zhang, Dr. Jiang Wu, Ph.D. student Dongyoung Kim, Ph.D. student Pamela Jurczak, Professor Lincoln Lauhon and Professor Huiyun Liu in collaboration with Dr. Zhiyuan Sun at Northwestern University in the United States and Dr. Ana Sanchez from University of Warwick and Dr. Manfred Ramsteiner at Paul-Drude-Institut in Germany investigated methods by which self-catalyzed NWs can be doped using GaAs NWs and beryllium dopants. Another objective of their study was to investigate the changes during the growth process and effects that the formed droplets had on the self-catalyzed nanowires. This research work is published in the journal, Nano Letters.
In the experiments conducted by the authors, the self-catalyzed GaAs NWs were grown by solid-source molecular beam epitaxy. Other factors taken into consideration during the experiment included temperature of the substrate, the concentration of the nominal doping, V/III flux ratio and the beam pressure.
The authors successfully observed for the first time in the world that NWs comprise of beryllium atoms through the Ga droplets. Ga droplets are also of great significance during the growth process as they aid the formation of thermodynamic equilibrium at the growth front. In this way, most Be dopants are incorporated substitutionally to Ga sites and resulted in effective doping. High beryllium fluxes only lead to its saturation and retention in the droplets. This is in stark contrast to thin-film vapor−solid growth at high dopant fluxes that can result in segregation and precipitation of dopants, leading to a deterioration in material quality.
The researchers also studied the influence of beryllium doping on the whole NW growth process. During self-catalyzed growth, the formation of Be-Ga alloy droplet is very useful, because it helps in the suppression of WZ nucleation and thus leads to the growth of pure ZB crystal throughout the process.
The research by Yunyan Zhang and colleagues created the much-needed insight into the NW doping and provided essential knowledge to control the electrical properties of NWs. Besides, this research is also valuable for understanding the basic physics of doping nanoscale materials through a liquid phase.
Zhang, Y., Sun, Z., Sanchez, A., Ramsteiner, M., Aagesen, M., & Wu, J. et al. (2017). Doping of Self-Catalyzed Nanowires under the Influence of Droplets. Nano Letters, 18(1), 81-87.
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