Abstract
Traditionally, fabrication of GaAs nanowires (NWs) has mainly relied on the use of gold as catalyst through the vapor-liquid-solid (VLS) mechanism. However, the incorporation of gold may generate deep level traps and degrade the electronic and optical property of the nanowires. In this work, we report on a novel method for the growth of vertical GaAs NWs on Si (111) substrates by molecular beam epitaxy without the use of gold. The synthesis is based on gallium-assisted VLS growth, where gallium is selectively pre-deposited in the pinholes in the native oxide prior to the growth of nanowires. This method is expected to grow GaAs nanowires on Si substrates with enhanced electronic and optical property, and hence offer possible integration of high performance GaAs nanoscale devices with the mature Si-based technology. We have systematically studied the influences of Ga pre-deposition condition, native oxide, beam flux, and growth temperature on the growth of GaAs NWs. Furthermore, by eliminating the Ga droplets at the NWs’ tip first and then well controlling the beam fluxes and temperature, we have achieved the growth of GaAs/AlGaAs core-shell nanowires. Besides, we investigated the growth rate of the AlGaAs shell for the benefit of solar cell fabrication in the future. Finally, we studied the incorporation of the commonly dopants Si and Be, and investigated the I-V properties of NWs doped with various Si and Be concentrations. The morphology of nanowires was investigated using scanning electron microscopy. It was found that the Ga pre-deposition time and temperature would control the diameter and density of the nanowires, respectively. On the other hand, the diameter of NWs would increase with increasing Ga flux under constant As flux, while the axial growth rate of NWs is proportional to the As flux under constant Ga flux. Analysis based on the observation of transmission electron microscopy shows that the NWs grown under high V/III flux ratio exhibit a perfect Zinc-Blende structure in the main body region, while twin structure for the tip and bottom. We also observed a clear interface of the GaAs core and AlGaAs shell. The crystal quality of GaAs NWs was also investigated using Raman spectroscopy.