Photoluminescence studies of transparent conductive ZnO films to identify their donor species

We studied the correlation between defect species, as probed by using photoluminescence (PL), and the transparent conductive properties of undoped ZnO films sputter-deposited on glass substrates at room temperature. The near-stoichiometric but slightly oxygen-deficient ZnO films had resistivities of 3×10−3 Ωcm and optical transmittances of 85% at visible wavelengths. The PL spectra exhibited only a band-edge emission peaking at 380 nm, suggesting that intrinsic defects were not the origin of the n-type conduction. Post annealing at 500°C in a vacuum reduced the carrier concentration by five orders of magnitude. However, the spectral features of the PL remained intact; i.e., the change was only attenuated band-edge emission. These observations can be consistently explained if we suppose that the donors are hydrogen impurities. Colored ZnO films deposited under a reducing condition had resistivities of 2−4×10−3 Ωcm, and their optical transmittances were 50−70% because of Zni atoms. Post annealing at 500°C desorbed some Zni atoms, and consequently transparency increased. The resultant PL spectra exhibited an emission at 396 nm accompanied with a deep-level emission at 400−500 nm, each corresponding to transitions from the conduction band to VZn and from Zni to the valence band. Thus, Zni was the primary donor in the Zn-rich films. Deposition under a flow of O2 gas produced resistive ZnO films. Incorporating excess oxygen atoms disordered the crystal lattice, as indicated by the broad deep-level emissions from Zni, Oi, VZn, and VO. The high resistivity was due to charge compensation between donors (Zni) and acceptors (Oi and VZn).