Dependence of the optical properties on the substrate temperature in ZnO thin films grown on flexible polyimide substrates

The optical properties of ZnO thin films grown on flexible polyimide (PI) substrates at various temperatures were investigated. X-ray diffraction patterns showed that the ZnO thin films grown on PI substrates had a c-axis preferential orientation. Photoluminescence (PL) spectra showed that the peaks at 3.32 and 3.36 eV could be attributed to the near band-edge emissions of the excitons bound to acceptors and of excitons bound to donors, respectively and that the peak at 3.24 eV was related to the donor-acceptor pair transitions. The activation energies of the excitons bound to neutral native donors, as obtained from the temperature-dependent PL spectra, were 10 and 85 meV, respectively.