Codoping of epitaxial zinc oxide thin films

Abstract

Zinc Oxide (ZnO) thin films were intentionally codoped with Group III elements (Aluminium, Gallium and Indium) in order to investigate and understand the effects of codoping on the morphological, electrical and optical properties of Gallium-doped ZnO (GZO) films. The codoped films were grown on MgAl2O4 spinel substrates using a low temperature solution-phase method known as hydrothermal synthesis and annealed at 600°C in Nitrogen ambient to improve their crystalline quality.

The morphological, electrical and optical properties of the films were primarily examined using atomic force microscope, Hall measurement system and UV-VIS-NIR spectrophotometer respectively. Gallium with Indium ZnO (GIZO) codoped films displayed a dramatic improvement in surface morphology as compared to the GZO film due to the size-compensation effect of smaller Gallium dopants with larger Indium dopants which reduced the lattice strain in ZnO.

The annealed 0.0033M Gallium with 3.3x10-4M Indium film exhibited an electron concentration of 3.14x1020cm-3 and resistivity of 7.38x10-4Ωcm which are both enhancements of 1.5 times over the GZO film. This result is comparable with films fabricated by more expensive and complicated vapour-phase methods. An increase in optical transmittance from 37% to 80% was also observed for this GIZO film due to reduced light scattering as a result of smoother film morphology. In addition, at 1.63x10-2sq/Ω, the figure of merit for this film is very close to the expensive and toxic Indium Tin Oxide transparent films extensively used commercially.