Annealing-induced modifications in sol–gel spin-coated Ga:ZnO thin films

Abstract

Transparent conductive gallium-doped zinc oxide (Ga:ZnO) thin films were prepared by the optimized sol–gel spin-coating technique on glass substrate. After deposition, the films were annealed at temperatures ranging from 200 to 500 °C in the air. The effect of post-annealing temperature on structural, optical and electrical behaviors of Ga:ZnO (GZO) thin films was investigated. Experimental results showed the polycrystalline nature of the films with a hexagonal (wurtzite) crystal structure and a preferred orientation with c-axis, perpendicular to the substrate. The average optical transmittance of all the annealed films is over 89 %. The grain size and optical transmittance of the GZO films increased with increasing annealing temperatures. The photoluminescence spectra of GZO films showed the near-band-edge (NBE) emissions at 390 nm and blue emissions at 456 nm. The NBE emissions might originate from excitonic recombination, and the blue emission peak can be attributed to the zinc interstitial and oxygen vacancies in the ZnO lattice. The lowest resistivity of 2.23 × 10−3 Ω cm and the highest figure of merit are achieved for a GZO film annealed at 500 °C. The new results obtained from this study indicate that the structural, optical and electrical properties of GZO thin films can all be improved through an appropriate post-annealing operation. The obtained results confirm that the thermal annealing has considerable effects on the properties of GZO thin films deposited by sol–gel spin-coating method and can be used as a promising TCO for optoelectronic applications. AFM surface morphology of GZO thin films annealed at (a) 200 °C, (b) 300 °C, (c) 400 °C, (d) 500 °C