Studies on temperature- and excitation-power-dependent photoluminescence of ZnO thin film grown by plasma-assisted molecular beam epitaxy

The temperature- and excitation-power-dependence of near-edge emission in a ZnO thin film were studied. The near-band-edge emission peaks at 3.370, 3.355, 3.338, 3.305, 3.254, 3.184, 3.112, and 3.042 eV were attributed to the emission of free excitons (FX); neutral-donor-bound excitons (D0X); neutral-acceptor or deep donor excitons; two-electron satellites (TES); donor–acceptor pairs (DAP); and the first-order longitudinal optical (1LO) phonon replica of DAP (DAP-1LO), DAP-2LO, and DAP-3LO, respectively. According to Haynes's rule, the factors a and b are 0.3 and 5 meV, respectively (Eloc = 0.3ED + 5 meV). The temperature dependence of the FX transition energy was analyzed using both Varshni's formula and Cody's formula for the variation in the band gap with temperature. Varshni's formula yielded a good fit for FX emission. The dependence of the photoluminescence intensity I on the excitation power L can be described by a power law, i.e., I ∼ Lα, where α is an exponent. Our results show that α is 0.86 for D0X and 0.63 for DAP.