Donor-acceptor-pair photoluminescence in Ga-doped ZnO thin films grown by plasma-assisted molecular beam epitaxy

Abstract

Three dominant emission lines: neutral GaZn donor bound exciton I8 (3.359 eV); ionized GaZn donor bound exciton I1 (3.368–3.371 eV), and donor-acceptor-pair (DAP) IDA (3.313–3.321 eV) were observed in the 9 K photoluminescence (PL) spectra from a series of Ga-doped ZnO thin films with electron carrier concentration (n) ranging from 3.6×1018 to 3.5×1019 cm−3. As n increases, the dominant PL line changes from I1 to IDA, and finally to I8. Characteristic blueshifts of IDA PL lines were observed with increasing n, with increasing excitation power in power-dependent PL spectra, and with increasing temperature in temperature-dependent PL spectra. The experimental results of IDA lines in Ga-doped ZnO are generalized to a proposed model to explain the possibilities of the widely observed 3.30–3.32 eV PL lines in ZnO as DAP transitions, which are associated with a shallow donor (e.g., Ga, Al, In, H, etc) with an ionization energy of ∼44–65 meV and a deep acceptor VZn with an ionization energy of ∼180 meV.