Transport phenomena in high performance nanocrystalline ZnO:Ga films deposited by plasma-enhanced chemical vapor deposition

Nanocrystalline gallium doped zinc oxide (ZnO:Ga) thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD). A statistical design of experiments (DOE) was employed to optimize electrical conductivity. A carrier concentration of 5.5×1020/cm3 and a mobility of 15 cm2/V s yielding a resistivity of 7.5×10−4 Ω cm resulted from the conditions of high pressure, rf power, and electrode gap. X-ray diffraction showed that gallium doping had a profound impact on film orientation. Atomic force microscopy (AFM) revealed that the films were nanostructured, with an average grain size of 80 nm and a surface roughness of ∼2 nm. This unique morphology benefited optical transmission, but limited electrical performance. Average transmission across the visible spectrum was ∼93% as scattering losses were minimized. Temperature dependent Hall and optical transmission measurements demonstrated that structural defects and ionized impurities were equal contributors to electron scattering.