Characterization of Undoped ZnO Films Post-Annealed by Using Helium Gas

In this study we used helium gas for the first time for annealing undoped ZnO films at different temperatures in the range of 500 similar to 800 degrees C and we report their structural, optical and electrical properties. All measurements were carried out at, room temperature. The undoped ZnO samples exhibited a (002) preferential orientation with the c-axis perpendicular to the substrate, suggesting that samples of high quality were prepared. The PL spectra of the ZnO samples revealed a strong near-band-edge ultraviolet (UV) emission peak at 3.26 eV, a strong red emission peak at 1.91 eV and a weak green emission peak at 2.36 eV, which were attributed to a free electron-acceptor transition, a zinc interstitial and a single ionized oxygen vacancy, respectively. All peak strengths in the PL spectra, increased with increasing post-annealing temperature. We also confirmed from PL and Hall measurements that, all samples revealed n-type conductivity. With increasing post-annealing temperature, the electron carrier concentration and the strength of UV emission peak increased, suggesting that they are related to each other.