Influence of the oxygen partial pressure on the opto-electrical properties of ZnO thin films deposited by reactive evaporation

ZnO films with transmittances greater than 80% in the blue spectral region and electrical resistivities of about 8.5 x 10(-4) Omega cm were deposited by reactive evaporation. Semiconductor thin films with these characteristics are very attractive to use as transparent contact in thin film solar cells. A parameter study revealed that the main deposition parameters affect significantly the opto-electrical properties of the ZnO films, being the oxygen partial pressure the parameter which most affects both, the transmittance and the conductivity. Actually high conductive ZnO films with blue transmittances greater than 80% are routinely prepared by using oxygen partial pressures greater than 0.2 mbar and evaporation temperatures of Zn of about 540 degrees C. Measurements of the Hall coefficient indicated that the changes in the electrical resistivity of the ZnO films induced by changes in the deposition parameters, are mainly due to changes in the free carrier density. On the other hand, it was found through AFM (Atomic Force Microscopy) measurements that the high values of blue transmittance obtained with ZnO films deposited at high O-2 partial pressures are in part controlled by morphological effects.