Improved electrical properties of laser annealed In and Ga co-doped ZnO thin films for transparent conducting oxide applications

In this research, dopants with same stoichiometric composition of In and Ga were varied in the range of 1-4 mol % to increase the electrical conductance and optical transmittance. The total sum of 2 mol% of In and Ga codoped ZnO thin films showed the lowest sheet resistance of 15.9 k Omega/sq. and high transmittance of more than 90% at 380 nm after rapid thermal annealing (RTA) and CO2 laser annealing. Based on scanning electron microscopy images and calculations using the Scherrer formula, the grain sizes were measured and were compared for the increase of In and Ga dopants. The RTA and CO2 laser annealed ZnO thin films co-doped with 2 mol% of In and Ga exhibited a higher energy band gap of 3.29 eV than those of other specimens. The root mean square (RMS) values of the surface roughness were extracted from atomic force microscopy (AFM) images, and the lowest RMS value of 22 nm was obtained for the RTA and CO2 laser annealed ZnO thin films co-doped with 2 mol % of In and Ga. We believe that this increase in the energy band gap is related to the blue shift in the dopant ionization process of In and Ga dopants. And these RTA and CO2 laser annealing can improve the ionization of In and Ga dopants, which is related to the increased energy band gap.