Low-temperature electrical conductivity and optical absorption edge of ZnO films prepared by chemical vapour deposition

Abstract

The optical and electrical properties of c-axis-oriented ZnO films prepared by chemical vapour deposition (CVD) in oxygen atmosphere containing water vapour are investigated. The minimum dc resistivity gives 2.20 Ω cm at a film formation temperature of 550°C. The temperature dependence of electrical conductivity for the films indicates the band conduction obeying Arrhenius plots for temperatures between 170 and 297 K, which involves thermionic and thermal field emissions over grain boundaries in the films. The donor levels, Ed, produced by interstitial Zn atoms are between 0.02 and 0.20 eV. It is found that the variable-range hopping conduction occurs in the films at temperatures lower than about 170 K. The films are transparent in the visible range and have sharp ultraviolet absorption edges at 380 nm wavelength. The optical band gap energy of the films lies in a range of 3.19 to 3.23 eV. From the analysis of the Urbach tail at the absorption edge the width of the tail of localized states Ee = 0.09 to 0.10 eV is obtained. It is concluded that the localized levels are situated in a range Ee extending from near the edge of the conduction band into the band gap.