LOW-TEMPERATURE ELECTRICAL-CONDUCTIVITY AND OPTICAL-ABSORPTION EDGE OF ZNO FILMS PREPARED BY CHEMICAL-VAPOR-DEPOSITION

The optical and electrical properties of c-axis-oriented ZnO films pre pared by chemical vapour deposition (CVD) in oxygen atmosphere contain ing water vapour are investigated. The minimum de resistivity gives 2. 20 Omega cm at a film formation temperature of 550 degrees C. The temp erature dependence of electrical conductivity for the films indicates the band conduction obeying Arrhenius plots for temperatures between 1 70 and 297 K, which involves thermionic and thermal field emissions ov er grain boundaries in the films. The donor levels, E(d), produced by interstitial Zn atoms are between 0.02 and 0.20 eV. It is found that t he variable-range hopping conduction occurs in the films at temperatur es lower than about 170 K. The films are transparent in the visible ra nge 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 E(e) = 0.09 to 0.10 eV is obtai ned. It is concluded that the localized levels are situated in a range E(e) extending from near the edge of the conduction band into the ban d gap.