ZnO-based epitaxial films
Single-domain ZnO epitaxial films are grown over a wide range of oxygen pressures and at substrate temperatures down to 100 °C on c-plane sapphire substrates using reactive pulsed magnetron sputtering (RPMS). The a-axis of the single-domain film is rotated by 30° with respect to the a-axis of the sapphire substrate, so that in-plane epitaxial relationships are ZnO[11 -20] || Al2O3[11-2 0] and ZnO[10-10] || Al2O3[11-20]. The lowest rocking curve full width on half maximum for the ZnO films is of 0.366°.
Typical aligned and random Rutherford backscattering / ion channelling spectra of epitaxial ZnO are shown above. The aligned spectrum exhibits a large reduction in the backscattered yield of the film. The χmin value is 6% as measured in the near-surface region, and increases to ~17% in the vicinity of the film/substrate interface. A χmin value of ~6% is indicative of a good crystalline quality. For comparison, 2% have been reported in literature for films produced by pulsed laser deposition at a significantly higher temperature of 750 °C.
Al-doped ZnO films grown by the same method using metallic Zn-Al targets with Al concentration of 0.7 and 8.7 at. % show substantially deteriorated in-plane ordering, the stronger, the higher Al concentration is. However, the films grown using the targets with with Al concentration of 0.7 at. % still show free electron mobility as high as 46 cm2V-1s-1.
M. Vinnichenko, N. Shevchenko, A. Rogozin, R. Grötschel, A. Mücklich, A. Kolitsch, and W. Möller: Structure and dielectric function of two- and single-domain ZnO epitaxial films. J. Appl. Phys. 102, 113505 (2007).