Optimization of reactive pulsed magnetron sputtering for epitaxial growth of undoped ZnO films

The process of ZnO thin films deposition by reactive pulsed magnetron sputtering is optimized for epitaxial growth on Al2O3 (0001) substrates at the temperature of 550 °C. The influence of base pressure, target presputtering, chemical substrate cleaning and additional O2 RF plasma cleaning of the substrate on the film structure and properties is investigated. The films are characterized by X-ray diffraction (XRD) phi-scans and spectroscopic ellipsometry (SE). Chemical cleaning results in the formation of epitaxial ZnO layers in a wide range of deposition parameters, with two types of domains, one of them being rotated by 30° relative to the dominating orientation. The additional oxygen plasma cleaning of the substrate allows to form single-domain epitaxial films. Under optimized conditions, these films remain epitaxial for a wide range of the oxygen partial pressures (1.4•10-4 - 3.4•10-3 mbar). The largest grain size of approximately 100 nm and the best (0001)-type texture of ZnO are formed at the oxygen partial pressure of 1•10-3 mbar with a minimum FWHM of the XRD rocking curve of 0.366°. SE indicates the absence of grading of the optical constants across the film, and a significantly lower optical absorption compared to films grown without oxygen plasma pretreatment. The broadening of the Lorentz oscillator used for parameterization of the optical constants is significantly smaller, which confirms the higher degree of ordering.