Behavior of Al Impurity in ZnO Films: Influence of Al-Level Doping on Structure, X-Ray Photoelectron Spectroscopy and Transport Properties

Transparent and conductive ZnO:Al thin films have been deposited by a reactive magnetron sputtering using the layer-by-layer growth method. The grown Al-doped ZnO films of about 100 nm thickness on Si and glass substrates have been investigated with respect to the crystalline phase by X-ray diffraction (XRD), surface morphology by atomic force microscopy (AFM), chemical bonding, and the electronic structure by measuring the X-ray photoelectron spectroscopy (XPS) core-level and valence-band spectra of the ZnO:Al films. The influence of Al content in ZnO films on structure, optical, and XPS spectra, transport parameters n, mu, rho have been studied. The weak temperature dependence of conductivity in the temperature range 77-300 K suggests that Al3+ is a fully ionized impurity that provides the values of electron concentration in the range 7 x 10(19)-2.44 x 10(20) cm(-3). Al electroactivity (EA) in ZnO films is diminishing with Al content increases. A strong decrease in Al EA from 60% to 30% is observed for ZnO:Al films within 0.93-1.22 at% of Al that indicates on the enhancement of concentration of acceptor native and/or neutral complex defects in ZnO.