Ga-related photoluminescence lines in Ga-doped ZnO grown by plasma-assisted molecular-beam epitaxy

Low-temperature photoluminescence (PL) and temperature-dependent Hall-effect (T-Hall) measurements were carried out in undoped and Ga-doped ZnO thin films grown by molecular-beam epitaxy. As the carrier concentration increases from 1.8x10(18) to 1.8x10(20) cm(-3), the dominant PL line at 9 K changes from I-1 (3.368-3.371 eV) to I-DA (3.317-3.321 eV), and finally to I-8 (3.359 eV). The dominance of I-1, due to ionized-donor bound excitons, is unexpected in n-type samples but is shown to be consistent with the T-Hall results. We also show that I-DA has characteristics of a donor-acceptor-pair transition, and use a detailed, quantitative analysis to argue that it arises from Ga-Zn donors paired with Zn-vacancy (V-Zn) acceptors. In this analysis, the Ga-Zn(0/+) energy is well-known from two-electron satellite transitions, and the V-Zn(0/-) energy is taken from a recent theoretical calculation.