Influence of negative ion resputtering on ZnO:Al thin films

Abstract

The influence of negative ion resputtering (NIR) by oxygen ions during sputter deposition of aluminum doped zinc oxide (ZnO:Al) thin films is investigated. A planar rf diode sputter deposition source with a ceramic ZnO:Al2O3 (98 wt %:2 wt %) target is used for deposition at 250, 500 and 1000 W. The resulting thin films are characterized by four point probe, Hall measurements, secondary ion mass spectrometry (SIMS), x-ray diffraction (XRD), and atomic force microscopy (AFM). The resistivity and carrier concentration show an order of magnitude variation between 2×10−2 and 1.5×10−3 Ω cm and 7.2×1019 and 5.4×1020 cm−3, respectively, with the position on the substrate and deposition power. The Hall mobility changes are small, from 4.7 to 11.5 cm2/V s. Data from SIMS show that the variations in electrical properties do not result from contamination by alkali metals or changes in the Al dopant concentration. Spectra from XRD have one predominant peak near 34.4° indicating that the film is wurtzite and strongly textured in the (002) basal direction. All peaks are lower in 2θ than literature values, which indicates an expanded lattice in the c direction. Grain size estimates based on AFM data suggest that changes in grain size are not large enough to cause the changes in electrical properties. Compensation of free carriers by interstitial oxygen ions due to implantation and Frenkel pair generation resulting from an energetic oxygen ion flux are consistent with the data, and are postulated to be the mechanisms by which NIR influences the electrical properties of ZnO. The flux of negative ions is estimated to be 1% of the total ion flux. The correlation between NIR and the influence of deposition parameters is discussed.