Characterization and Raman investigations on high-quality ZnO thin films fabricated by reactive electron beam evaporation technique

High-quality zinc oxide thin films have been evaporated by reactive e-beam in an oxygen environment. Photoluminescence (PL) on ZnO evaporated at 300 °C reveals an enhancement of the ultraviolet near band edge emission at 3.28 eV and shows the smallest full-width at half-maximum of the NBE peak about 150 meV, proving that the optimum growth temperature is 300 °C. X-ray diffraction measurements have shown that ZnO films are highly c-axis-oriented and that the linewidth of the (0 0 2) peak is sensitive to the variation of substrate temperature. The best crystallinity occurred at 300 °C, correlating with the PL measurements. The optical transmittance is the highest at 300 °C and has been increased after annealing in air, showing an improvement of the optical quality. Raman spectroscopy has been found to be an efficient tool to evaluate the residual stress in the as-grown ZnO films from the position of the E2 (high) mode. On the other hand, the vanishing of the 574 cm−1 Raman feature after annealing has been explained as due to an increase of grain size and the reduction of O vacancy and Zn interstitial. A model has been developed in this work to determinate the normal mode frequencies from Fourier analysis by considering the five force constants starting from the five elastic constants of the material. The vibrational modes at about 100, 435 and 1123 cm−1 have been found from the model for a ZnO thickness of 0.3 μm.