Selective area molecular beam epitaxial growth of n-GaN nanowires on SiO2 hole-mask patterned sapphire and graphene substrates

Abstract

The growth conditions of silicon-doped GaN (n-GaN) nanowire (NW) selective area growth (SAG) are explored using plasma-assisted molecular beam epitaxy on SiO₂ hole-mask patterned substrates: sapphire (SAP) and graphene on SAP (GoS). To achieve high selectivity, the growth temperature and Ga flux windows in which the nucleation on the SiO₂ mask is highly suppressed are investigated and found to be quite narrow. By tuning the mask etching process, n-GaN NW morphologies ranging from vertical and widely separated NWs grown through SiO₂ pinholes to densely positioned NWs with coalesced upper surface in wider mask holes are demonstrated on SAP.

Using optimized growth conditions for SAG on SAP substrates, successful growth of vertical n-GaN NWs was achieved on similarly hole-mask patterned GoS substrates. Micro-Raman spectra indicate that the graphene is well protected from the nitrogen plasma by the SiO₂ mask, whereas the graphene is damaged in the holes of the mask. X-ray diffraction spectra reveal an epitaxial relationship with the SAP and resistance measurements show conductivity between NWs of different mask holes. From this, we conclude that n-GaN NWs after nucleating on SAP through nanoholes in the graphene overgrow it and that this part of the graphene, at least close to the edge of the hole areas, can conduct well to the SiO₂ mask covered graphene, indicating that graphene in such configuration can be used as a bottom electrode in a device structure. The impact of Ga diffusion and desorption on SAG is demonstrated for growth temperatures of 810 and 815 °C for SiO₂ hole-mask patterns with nominal hole diameter of 2.5 µm and hole pitches of 3.0–10.0 µm. For hole pitches above 5 µm, Ga diffusion from the mask is the major contribution to the NW growth.