Optical transmission of ZnO thin films

ZnO films were prepared by d.c. reactive cathodic sputtering in a planar magnetron. The structural properties of d.c. planar magnetron sputtered ZnO films are studied as a function of deposition parameters: substrate type, substrate temperature, sputter gas pressure, growth rate and sputtering power. ZnO films growth has been carried out in an oxygen and argon atmosphere. In dependence on the O2 partial pressure in the argon sputtering gas there exists a narrow process window around a po/(pAr+po) ratio of 5 - 10% which yields transparent, low- resistance layers. The discharge voltage dependence on the oxygen partial pressure is a sensitive indicator for the oxidation state of the target surface and can be used for the regulation of the deposition process. Lower O2 partial pressures yield metallic-like, opaque, but highly resistant layers. Higher oxygen partial pressures lead to transparent but highly resistant ZnO layers. Layers of lowest resistivity (5 X 10-4 (Omega) cm) and highest optical transmission (90%) have a stoichiometric ratio Zn:O of 1.0 and exhibit the largest grains. These films are applied as piezoelectric transducers in micromechanical sensors and actuators.