Compensation in Al-Doped ZnO by Al-Related Acceptor Complexes: Synchrotron X-Ray Absorption Spectroscopy and Theory

The synchrotron x-ray absorption near edge structures (XANES) technique was used in conjunction with first-principles calculations to characterize Al-doped ZnO films. Standard characterizations revealed that the amount of carrier concentration and mobility depend on the growth conditions, i.e. H-2 (or O-2)/Ar gas ratio and Al concentration. First-principles calculations showed that Al energetically prefers to substitute on the Zn site, forming a donor Al-Zn, over being an interstitial (Al-i). The measured Al K-edge XANES spectra are in good agreement with the simulated spectra of Al-Zn, indicating that the majority of Al atoms are substituting for Zn. The reduction in carrier concentration or mobility in some samples can be attributed to the Al-Zn-V-Zn and 2Al(Zn)-V-Zn complex formations that have similar XANES features. In addition, XANES of some samples showed additional features that are the indication of some alpha-Al2O3 or nAl(Zn)-O-i formation, explaining their poorer conductivity. DOI: 10.1103/PhysRevLett.110.055502