Electrical conductivity and optical properties of ZnO films annealed in hydrogen atmosphere after chemical vapor deposition

The d.c. electrical conductivity and optical properties of polycrystalline zinc oxide films (220–450 nm thick) annealed in hydrogen after chemical vapor deposition are investigated. A minimum film resistivity after the annealing gives 0.31 cm for the film as-deposited at a substrate temperature of 823 K. From the temperature dependence of conductivity, band conduction is confirmed for the films at temperatures above 250 K. The effect of grain-boundary scattering is due to thermionic emission of electrons over grain boundary barriers. At temperatures below 250 K, variable-range hopping transport is found to be dominant. The films are transparent in the wavelength range 400 to 1000 nm and have sharp ultraviolet absorption edges at 380 nm. The absorption edge analysis reveals the optical band gap energy for the films to be 3.18–3.23 eV. The Urbach tail analysis gives the width of localized states E_e=0.06-0.14eV.