Relation of optical and electrical properties to the microstructure of intrinsic transparent conducting ZnO thin films

The construction of transparent electronic devices based on zinc oxide depend on the availability of high performance p type ZnO. This paper addresses the origin of n type conductivity in undoped ZnO which would require compensation before p-type material is possible. ZnO films were prepared by magnetron sputtering and filtered cathodic arc deposition with and without PIII. The intrinsic free carrier properties of have been analyzed by infrared ellipsometry and temperature dependent conductivity measurements. The correlation of intrinsic carrier density and the crystal size and orientation as assessed by XRD shows that the free carriers originate from charged intrinsic defects. Even an undoped ZnO film with large and well oriented grains can exhibit substantial defect conductivity. Temperature dependent conductivity measurements lead to the conclusion that the charged defect sites give rise to electronic subbands in the band gap. The defect conductivity of undoped ZnO is comparable to values in the literature for Al-doped ZnO.