P-Type Nonpolar a-ZnO:N Thin Films on r-Sapphire Substrates Grown by Molecular Beam Epitaxy

Abstract

We have grown nonpolar nitrogen (N) doped a-plane zinc oxide (ZnO) films on r-plane sapphire substrates in order to eliminate the self-polarization component in the growth direction, which decreases the doping efficiency of N acceptors. Nonpolar a-ZnO:N films were grown by plasma-assisted molecular beam epitaxy (PA-MBE) using Zn metal and a plasma source of O2 and NO mixed gas. It was confirmed by reflection high-energy electron diffraction and x-ray diffraction that single phase a-plane ZnO:N films were grown on the r-plane sapphire substrates. After the PA-MBE growth, the post-annealing was performed in an oxygen atmosphere. Photoluminescence experiments showed donor–acceptor pair emissions increase with increasing the annealing temperature (≤ 700°C). AC magnetic field Hall effect measurements revealed that n-type conduction of the as-grown films clearly changed to the p-type at the annealing temperature of 650°C. The resistivity, hole concentration, and mobility were ρ = 3.4 Ω cm, p = 8.0 × 1017 cm−3, and μ = 2.3 cm2/Vs, respectively.