Influence of Ar/O2 gas ratios on the crystal quality and band gap of Zn1−xCdxO thin films

Zn1−xCdxO (x=0.1,0.2) thin films with highly (0 0 2)-preferred orientations were deposited on glass and Si(1 1 1) substrates by DC reactive magnetron-sputtering method in the atmospheres with different Ar/O2 ratios. The properties were investigated by X-ray diffraction, optical absorption spectra, XPS, scanning electron microscopy and atomic force microscopy. When the Ar/O2 ratios change from 1:4 to 1:1, the full-width at half-maximum of the films deposited on glass substrates of Zn0.9Cd0.1O films decreases (from 0.36°) gradually and reaches a minimum value of 0.29° at the ratio of 1:1; the band gap (Eg) decreases (from 3.149 eV) gradually and reaches a minimum value of 3.099 eV at the same ratio of 1:1.When the Ar/O2 ratios continue to increase up to 2:1, the FWHM increases to 0.35°; the band gap (Eg) increases to 3.114 eV. The variations for Zn0.8Cd0.2O films are the same as for Zn0.9Cd0.1O films. A mechanism for the influence of Ar/O2 gas ratios on the band gap of Zn1−xCdxO thin films is proposed.