Transition in the preferred orientation of RF sputtered ZnO/Si thin films by thermal annealing: Structural, morphological, and optical characteristics

Abstract

Polycrystalline ZnO thin films having preferential growth along the polar (002) plane are prepared on Si (111) substrate using RF sputtering technique. A gradual build-up of the semipolar (103) plane with the dominant (002) polar plane of 400–700 °C annealed films is revealed by the Glancing incidence angle X-Ray diffraction (GXRD) pattern. However, a change in preferred orientation from polar (002) to semi-polar (103) and non-polar (100) planes is observed with further annealing in the temperature range 800–1200 °C. This occurs due to an increase in the mobility of adatoms due to annealing which encourages diffusion across grain boundaries and competitive growth among planes leading to the growth of (103) plane. This structural deviation of film from (002) plane to (103) plane leads to the change in surface morphology of annealed films around 800 °C as depicted in Field Emission Scanning Electron Microscopy (FESEM) images. The observed changes in surface morphology of films are further correlated with the variation in optical band gap values and diffuse reflectance spectra. The low reflectance values of the samples represent their anti-reflection property which makes ZnO/Si thin film a conceivable applicant in optoelectronics.