Radio frequency magnetron sputtered ZnO/SiO2/glass thin film: Role of ZnO thickness on structural and optical properties

Optically transparent zinc oxide (ZnO) thin films having thickness of 70, 150, 200 and 270 nm are deposited on heated glass substrates using the radio frequency (rf) magnetron sputtering method to examine their structural and waveguide properties. The XRD analyses show that the ZnO films on SiO2/Glass substrate are poly-oriented and have a hexagonal wurtzite structure. All thin films exhibit optical transmittance of >80% in the visible range. The increase in film thickness to 270 nm leads to growth of crystallinity and grain size to 35 nm, increase in the transmittance and refractive indexes for both TM and TE modes, and decrease in the bandgap energy. The room temperature photoluminescence (PL) exhibit UV (∼3.20-3.23 eV) and green (∼2.37 eV) emission for all samples. However, the peaks intensity for UV emission gradually decreases which is attributed to decrease in the concentration of photo-carriers, their aggregations and larger amount of energy transfer to the down layer. Achieving the high optical transmittance of 95%, high refractive indexes and high level of crystallinity for 270 nm ZnO films prepared by socio economic rf magnetron sputtered make it appropriate transparent conductors for waveguide applications. The excellent features of the results suggest that our systematic growth and analysis methods may establish a basis for the tunable growth of ZnO/SiO2/glass hetero-structure thin films suitable in nanophotonics particularly waveguide application.