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Optical response of undoped and doped zinc oxide thin films

Harriman, Tres Allan
The optical response of undoped and doped ZnO thin films was investigated. Epitaxial films were deposited on c-plane and r-plane sapphire using pulsed laser deposition and were characterized with a combination of photoluminescence (PL) spectroscopy, Raman spectroscopy, high resolution transmission electron microscopy (HR-TEM), x-ray diffraction (XRD), and Hall effect measurements. To investigate the effect of residual stress, undoped ZnO was used and the magnitude of residual stress was changed by varying the nominal film thickness from 5 nm to 200 nm. X-ray diffraction was used to measure the out of plane strain, which was used to estimate the residual stress. The residual stress and ZnO E2high Raman mode frequency were found to be related linearly by a factor of approximately -170 MPa/cm-1, which is in agreement with high pressure investigations of bulk ZnO. PL experiments were performed at temperatures from 4.2 K to 300 K. Films created with a lower laser energy were found to have strong near band edge (NBE) emission, compared to those created with a higher laser energy that had poor emission intensity. For films on c-plane sapphire, fine PL structure including emission related to free-excitons, neutral-donor-bound-excitons, defects, a donor acceptor pair, and also phonon replicas of emissions were observed and identified. For films on r-plane sapphire the biggest difference in the PL emission compared to those on c-plane sapphire was the appearance of a free-electron to bound-acceptor emission that was attributed to acceptor states created by basal plane stacking faults. Doped films containing between 0 wt. % and 50 wt. % ReO2 were also investigated. Low doping concentrations (5 and 10 wt. %) were found to improve the overall crystal quality and increase the conductivity. Films with higher concentrations of ReO2 were found to have separate ReO2 phases present. It remains unclear where the Re atoms are incorporated into the films at lower doping concentrations.