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High transparent (about 90 %) and low resistivity (2 x 10-3 cm) n-type ZnO thin films were prepared by RF diode sputtering from ZnO+2 % Al2O3 target on glass substrates at constant Ar pressure (1.3 Pa), different RF power density and substrate temperatures. The ZnO:Al thin films was characterized by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Only one sharp (002) diffraction line appeared on XRD spectra, which is a clear evidence for the preferential c-axis orientation of the films. Nevertheless, the profile of the observed diffraction line is asymmetric towards to the higher diffraction angles, which indicates that the heterogeneous regions are present in the films. This heterogeneity is caused by presence of Al atoms in various lattice positions and is more evident in films deposited at lower RF power densities. The crystallinity of the films improved with the increase of the RF power density and at higher substrate temperatures. The increase of the RF power density from 1.13 W/cm2 to 4.52 W/cm2 caused the increase of the grain size from about 57 nm to 120 nm. The (002) line shifted toward higher angles, which indicates the increased Al dopant concentration into the film. Al substitutes for Zn in the lattice and having the smaller ionic radius than Zn causes the reduction of the lattice parameter and change the lattice distortion in ZnO:Al films from compressive to tensile lattice stresses. The reduction of the lattice parameter leads to the shift of (002) line toward higher angles. The (002) diffraction line has maximum intensity and the lowest FWHM (Full Width Half Maximum) for ZnO:Al deposited at 200ºC indicating the best crystallinity of the films grown at this temperature. The shift of the (002) line toward higher angles was observed as well. Surface roughness evaluated by SEM measurements decreased and smoother films were obtained at higher temperatures and RF powers. The electrical and optical properties fully correlated to the results from XRD and SEM measurements and to the changes of the structure, which are represented from these measurements. Blue shift of the absorption edge is observed with the increase of either RF power or temperature. |