Evidence for the AlZn-Oi defect-complex model for magnetron-sputtered aluminum-doped zinc oxide: A combined X-ray absorption near edge spectroscopy, X-ray diffraction and electronic transport study

Al-doped ZnO and Zn1 - xMgxO films, deposited by magnetron sputtering at substrate temperatures from about 25 °C to 500 °C and exhibiting a strong variation of the carrier concentration, were investigated by X-ray absorption spectroscopy (XAS), analyzing the X-ray absorption near-edge structures. The near-order structure, derived from the X-ray absorption spectra, shows that the dopant Al was built-in onto Zn lattice sites, independently of the deposition temperature up to 450 °C. FEFF calculations confirm the lattice location of the dopant on zinc lattice sites. The formation of secondary (spinel) phases occurs only at deposition temperatures above 450 °C, more pronounced for the Zn1 - xMgxO:Al films. In contrast to the XAS measurements, the electrical parameters of the films - carrier concentration and Hall mobility - exhibit a strong dependence on the substrate temperature, with a variation of the electron concentration by a factor of 4.5 from 25 °C to about 300 °C. This shows that the XAS method is significantly less sensitive compared to electrical measurements for the characterization of electrically active dopant elements. This fact is tentatively explained by the formation of oxygen interstitial defects Oi during deposition, annealing out at low temperatures, and by the formation of defect complexes of dopant atoms with intrinsic lattice defects, like Al-Oi or Al-Mg-Oi, which induce the observed electrical effects but are not detectable by XAS methods.