Metal–semiconductor transition in undoped ZnO films deposited by spray pyrolysis

ZnO films were deposited on glass substrate by using spray pyrolysis method. Films were deposited at different solution molarities 0.02 and 0.1 M. The films are highly transparent in the visible range of the electromagnetic spectrum with a transmission reaching up values to 90%. Band gaps were calculated as 3.24 and 3.28 eV with the help of transmission spectrums. When the solution molarity of the sprayed solution is increased from 0.02 to 0.1 M, carrier concentrations of the films increase from 1.6×1019 cm−3 to 5.1×1019 cm−3. Temperature-dependent conductivity measurements of these conducting and transparent films also showed, for the first time, a metal–semiconductor transition (MST). The deposited ZnO films show metallic conductivity above ∼420 K and semiconducting behavior at temperatures below it.