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

Aluminium-doped zinc oxide (ZnO:Al) films were prepared by magnetron sputtering at different radio-frequency powers (Prf) 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 Prf. Optical band gap Egopt of the films increases from 3.31 to 3.51 eV when Prf increases from 50 to 200 W. A widening Egopt 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 × 1019 to 2.12 × 1020 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 A1 (LO) mode of ZnO.