The optical blueshift saturation behavior of Mg$_{x}$Zn$_{1-x}$O films

A phenomenon of blueshift saturation was observed from cathodoluminescence (CL) spectra of Mg$_{x}$Zn$_{1-x}$O films (0.069 $\le x\le$ 0.8) that were grown by pulsed laser deposition technology. By analyzing the results of composition-dependent X-ray diffraction spectra, scanning electron microscopy images, absorption spectra, and CL spectra, the crystalline structural disorder was determined via Urbach energy value. Furthermore, a competition mechanism between band-filling effect and band tail states was proposed in the composition-dependent CL spectra. This competition is believed to be responsible for the composition-induced blueshift saturation often observed in the disordered alloy semiconductors. The results of this research can provide important reference in energy-band engineering. Keywords: Mg$_{x}$Zn$_{1-x}$O, luminescence, Urbach energy, band tail state.