X-Ray diffraction and Raman spectroscopy for a better understanding of ZnO:Al growth process

ZnO:Al thin films were prepared on glass substrates by radio frequency RF magnetron sputtering from a ceramic ZnO target mixed with Al2O3 (1 wt%) in pure argon atmosphere with a power of 250 W. Two series of samples were deposited, the first one as a function of the substrate temperature (between 20 and 325 °C) at 0.12 Pa, the second one as a function of the working pressure (between 0.01 and 2.2 Pa) at room temperature. The influence of these deposition parameters was studied by a detailed microstructural analysis using X-Ray diffraction and Raman spectroscopy. Their electrical properties were characterized by Hall effect measurements. As the substrate temperature is increased, crystallite size increases while the strain and the electrical resistivity decrease. An improvement of the crystallinity is deduced from Raman spectra and X-Ray diffraction patterns. From the pressure series, an electrical optimum is observed at 0.12 Pa. Above 0.12 Pa, there is a degradation of the crystallinity, attributed to the low mobility of the adatoms and below 0.12 Pa, high-energy sputtered particles damage the growing films. The trends given by Thornton structure zone model are also consistent with X-Ray diffraction results.