Influence of the nanoscale structural features on the properties and electronic structure of Al-doped ZnO thin films: An X-ray absorption study

Transparent Al-doped ZnO thin films were deposited by reactive magnetron sputtering with different oxygen flow rates. The electronic resistivity, measured by the 4 point-probe method, is very sensitive to the sample position relative to the magnetron axis: the closer the magnetron from the axis the higher the resistivity. This is more pronounced for the films deposited under higher oxygen flow rate. Neither Rutherford backscattering spectroscopy nor Zn–K edge X-ray absorption near-edge structure (XANES) analyses evidenced any change in chemical composition such as a measurable variation of the oxygen stoichiometry. XANES at the Al–K and O–K edges show that (i) a portion of the aluminum atoms get positioned in octahedral conformation with oxygen, consistent with the formation of an Al2O3(ZnO)m nanolaminate structure, (ii) the films exhibit relaxed O-terminated (0 0 0 1) surfaces with a higher density of empty states in more resistive samples. These two findings are believed to play a significant role on the electrical measurements by dopant deactivation and by creating an insulating barrier at the film surface, respectively.