Structural and optical properties of highly (110)-oriented non-polar ZnO evaporated films on Si substrates

Abstract

Zinc oxide (ZnO) films with thicknesses from (9.1±0.7) to (145±7)nm were deposited on silicon substrates by ion-assisted electron beam evaporation (EBE), and transformed to a highly (110)-oriented non-polar phase through thermal treatment at temperatures of 300, 500, and 700°C. The structural, morphological and optical properties of the ZnO thin films were systematically characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and photoluminescence (PL). The XRD and AFM results illustrate that the deposited ZnO films evolved from an X-ray amorphous structure to the polycrystalline hexagonal wurtzite structure with increasing thickness and experienced a crystallographic plane improvement from (102) to the (110) non-polar phase during annealing. The dielectric functions of the whole layer are mainly affected by the dielectric responses near the surface of ZnO nanocrystals and show an upward trend with nanocrystal growth, induced by thickness and annealing temperature. The optical band gaps were also investigated and correlated with the structural properties. In the PL spectra, a strong ultraviolet luminescence (UVL) band and a weak visible green luminescence (GL) band were observed. The intensity ratio of UVL and GL—associated with crystal quality—agrees with the results acquired from other measurements. The two determinants for highly (110)-oriented non-polar ZnO films are the suppression of growth in the (002) direction by ion bombardment and the promotion of the (110) plane induced by diffusion energy during annealing.