Control of the spontaneous formation of oxide overlayers on GaP nanowires grown by physical vapor deposition

Growth of gallium phosphide nanowires by vapor deposition of simple thermally evaporated inorganic precursors is generally accompanied by unintentional formation of thick oxide coating, which may compromise the optical and electrical properties of the nanowires. Controlling and eliminating this outer layer during thermal evaporation growth of GaP nanowires represents a barrier to simple and scalable preparation of this technologically important material. In this article, we systematically investigated the role of different parameters (temperature, hydrogen flow rate, and starting Ga/P ratio) in the synthesis of GaP nanowires, and mapped out the conditions for the growth of oxide-layer-free nanowires. Increase in temperature, hydrogen flow, and phosphorus concentration led to diminished oxide layer thickness and improved nanowire morphology. Long and straight nanowires with the near perfect stoichiometry and complete absence of oxide outer layer were obtained for 1050 °C, 100 sccm hydrogen flow rate, and Ga/P flux ratio of 0.5. In contrast to other reports, this work emphasizes the importance of introducing hydrogen flow and excess phosphorus, which provide for reducing environment and reduced rate of the reaction of Ga with O in the growth process, respectively. The ability to control dielectric medium around GaP NWs by controlling the formation of oxide overlayer was demonstrated by Raman spectroscopy. The results of this work demonstrate a full control of the multi-parameter space in the simple, inexpensive, and scalable synthesis of GaP NWs, and may provide a guideline for rational improvement of the growth conditions for other types of semiconductor nanowires.