Piezoelectric Carrier Confinement by Lattice Mismatch at ZnO/Zn_(0.6)Mg_(0.4)O Heterointerface

摘要

This paper describes a strong piezoelectric carrier confinement at a ZnO/Zn_(0.6)Mg_(0.4)O heterointerface grown on an a-plane sapphire substrate by molecular beam epitaxy. A ZnO/Zn_(0.6)Mg_(0.4)O double-heterojunction structure was grown without intentional doping and the formation of a deep potential well for electrons was confirmed using cathodoluminescence and transmittance spectra. Photoluminescence spectra at 4.5 K consisted of an intense near-band-edge emission at 3.359 eV and a broad and weak peak on the low-energy side. The results of Hall effect measurement revealed that the conduction is n-type with a high carrier concentration of ～1.2 x 10~(13) cm~(-2). The mobilities are ～170 cm~2/V·s at 300 K and ～400 cm~2/V·s at 77 K without deterioration at lower temperatures; these values are much higher than those of a thick single-layer ZnO film grown on an a-plane sapphire substrate. We attribute these optical and electrical properties to the formation of two-dimensional electron gas at the ZnO/Zn_(0.6)Mg_(0.4)O heterointerface by the piezoelectric polarization in a strained ZnO well.