Dependence of electrical and structural properties on film thickness of undoped ZnO thin films prepared by plasma-assisted electron beam deposition

The dependence of electrical and structural properties on film thickness was studied on polycrystalline undoped zinc oxide (ZnO) thin films on glass substrates at 200 ∘C prepared by plasma-assisted electron-beam deposition. The film thickness was varied from 23 to 316 nm. Hall effect measurements for undoped ZnO films showed that while the Hall mobility increased with increasing film thickness up to almost 130 nm, it remains almost constant on further increasing the film thickness (>130 nm). The carrier concentration changes a little. As a result, the dependence of the resistivity on film thickness was determined by that of Hall mobility. It was clarified by out-of-plane and in-plane X-ray diffraction (XRD) that while the orientation of undoped ZnO with a thickness below 130 nm is random, ZnO films with a larger thickness (>130 nm) change to c-axis orientation. In-plane XRD line broadening analysis, using the Williamson-Hall method, reveals that both the crystallite size and remaining microstrain increase with increasing film thickness. The same tendency of the averaged surface roughness (Ra), from atom force microscopy (AFM) measurement, as that of the structural and physical properties described above was observed.