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 1.8×1018 to 1.8×1020 cm−3, the dominant PL line at 9 K changes from I1 (3.368–3.371 eV) to IDA (3.317–3.321 eV), and finally to I8 (3.359 eV). The dominance of I1, 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 IDA has characteristics of a donor-acceptor-pair transition, and use a detailed, quantitative analysis to argue that it arises from GaZn donors paired with Zn-vacancy (VZn) acceptors. In this analysis, the GaZn0/+ energy is well-known from two-electron satellite transitions, and the VZn0/− energy is taken from a recent theoretical calculation.