Analysis of Incubation Times for the Self-Induced Formation of GaN Nanowires: Influence of the Substrate on the Nucleation Mechanism

Abstract

Surprisingly long incubation times for the self-induced formation of GaN nanowires on different substrates can reach hundreds of minutes and remain a mystery in GaN crystal growth. Herein, we examine the incubation times of GaN islands that subsequently give rise to nanowires on amorphous AlxOy/Si and SiNx/Si substrates versus the temperature and gallium flux. Experimental data are obtained by in situ monitoring of the surface morphology by reflection high energy electron diffraction during plasma-assisted molecular beam epitaxy. We develop a model that confirms an inverse power-law dependence of the incubation time on the gallium flux and the Arrhenius-type temperature dependence. The power exponent p and the activation energy Einc are related to the nucleation mechanism and the island growth regime. We find the values p ≅ 1, Einc = 6.0 eV for AlxOy, and p ≅ 2, Einc = 10.2 eV for SiNx buffer. The dominant nucleation mechanism on amorphous AlxOy should be heterogeneous. Homogeneous nucleation dominates on SiNx, while the diffusion growth regime of GaN islands occurs in both cases. Overall, the long incubation times are attributed to extremely low effective diffusion lengths of gallium adatoms such that the squared diffusion length times the gallium bonding rate ranges from 10–4 to 1 nm2 in typical cases.