Effects of rf power on surface-morphological, structural and electrical properties of aluminium-doped zinc oxide films by magnetron sputtering

Abstract

Aluminium-doped zinc oxide (ZnO:Al) films were prepared by magnetron sputtering at different radio-frequency powers ( P rf) of 50, 100, 150 and 200 W. The properties of the films were characterised by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Raman microscopy, and spectrophotometry with the emphasis on the evolution of compositional, surface-morphological, optical, electrical and microstructural properties. XPS spectra showed that within the detection limit the films are chemically identical to near-stoichiometric ZnO. AFM revealed that root-mean-square roughness of the films has almost linear increase with increasing P rf. Optical band gap E gopt of the films increases from 3.31 to 3.51 eV when P rf increases from 50 to 200 W. A widening E gopt of the ZnO:Al films compared to the band gap (∼3.29 eV) of undoped ZnO films is attributed to a net result of the competition between the Burstein–Moss effect and many-body effects. An electron concentration in the films was calculated in the range of 3.73 × 10 19 to 2.12 × 10 20 cm −3. Raman spectroscopy analysis indicated that well-identified peaks appear at around 439 cm −1 for all samples, corresponding to the band characteristics of the wurtzite phase. Raman peaks in the range 573–579 cm −1 are also observed, corresponding to the A 1 (LO) mode of ZnO.