Dependence of electrical, optical, and structural properties on the thickness of GZO films prepared by CRMS

The dependence of electrical, optical, structural, and surface properties on the thickness of Ga-doped ZnO (GZO) thin films deposited on glass substrates by cylindrical rotating magnetron sputtering (CRMS) was investigated. The resistivity of the CMRS-grown GZO films gradually decreased from 7.4×10 ^-3 Ohm-cm to 4.9×10 ^-4 Ohm-cm with increasing its thickness, while the transparency of the GZO films in visible region were kept constant regardless of its thickness. Both x-ray diffraction and high resolution electron microscope examinations showed that the CMRS-grown GZO film has a strongly (002) preferred orientation with increasing thickness even though it was sputtered at low substrate temperature below 230°C. By using spectroscopic ellipsometry, we estimated accurately the dielectric function and band gap energy of the CRMS grown GZO film. Using absorption coefficient α(=4πk/λ) and linear extrapolation method, the band gap energy of the CRMS grown GZO film was estimated to be 3.67 × 0.04 eV, which is independent of its thickness. However, we found that it gave erroneous thickness-dependence of band gap energy when we used the absorbance (=-ln(T)/d) derived from transmission rather than the absorption coefficient.