The growth of transparent conducting ZnO films by pulsed laser ablation

The structure of undoped, Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films grown on sapphire and NaCl substrates by 193 nm pulsed laser ablation of a ZnO target in a low background pressure of oxygen was investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The films on sapphire grew with the polar (0002) orientation. The samples deposited on NaCl, at substrate temperatures above 570 K, presented a mixture of polar and non-polar orientations. All samples demonstrated improved crystalline quality, as measured by the FWHM of the ZnO (0002) rocking curve, with increasing substrate temperature. The best crystalline quality was observed for the undoped films. The inclusion of Al or Ga into the lattice degraded the crystallinity of the films, but allowed production of highly conductive films. AZO and GZO film resistivities were measured using a four-point probe method and were found to decrease with increasing deposition temperature. Film thickness was determined using variable angle spectroscopic ellipsometry.