Carrier Transport through Grain Boundaries in Highly Transparent Conductive Ga-Doped ZnO Polycrystalline Films

摘要

We investigated the effects of grain boundaries on the carrier mobility of polycrystalline highly transparent conductive Ga-doped ZnO (GZO) films with thicknesses in the range from 100 to 500nm on glass substrates at a temperature of 200°C. GZO films were prepared by ion-plating deposition with DC arc discharge. A systematic study has been made of the thickness dependence of the structural, electrical and optical properties of GZO films. The full width at half maximum (FWHMω) of the (0002) rocking curve was found to decrease with increasing thickness, whereas the grain size increased with increasing thickness. The comparison of the Hall mobility with the optical mobility calculated by analysis using the simple Drude model combined with the Tauc-Lorentz model of data obtained by spectroscopic ellipsometry (SE) demonstrates that grain boundaries present a significant obstacle to free carriers in GZO films with thicknesses of up to 344nm. In 344-nm-thick GZO films with a high carrier concentration of 1.23×10~(21) cm~(-3), the Hall mobility of 29cm~2/Vs is close to the optical mobility, i.e. the carrier mobility in the grain bulk. In such films, a very low resistivity of 1.87×10~(-4) Ωcm was obtained. This indicates the very small contribution of grain boundaries to the total resistivity of the GZO films.