AZO thin films conductivity increases with the Al content and reaches a maximum at about 3 % doping level; higher doping results in a decrease of conductivity. The density of carrier in the films is determined from the C-V measurements for different Al contents in AZO thin films deposited by chemical spray pyrolysis on p-Si. The carrier concentration is, for all Al concentration, lower than the corresponding theoretical values and, therefore, not all Al+ ions include in the ZnO structure donors electrons. Nevertheless, we have experimental evidence that the gap does not collapse and, thereby, the conductivity increase is not due to a metal-insulator transition. We have measured, using synchrotron radiation in the VUV range, a reduction in the band gap of about 150 meV for the 3% Al doped ZnO film with respect to the undoped and the 1% Al doped ones. This reduction has also been detected in E.E.L.S. measurements which show a reduction in the band gap of about 200meV for same films. This is due to the hybridization of the Al dopants with the ZnO host lattice. This additional number of electrons is enough to enhance the conductivity by two orders of magnitude in the 3% Al-doped ZnO films because the transport properties in a solid are controlled exclusively by the mafia of electrons at, or, very near to the Fermi surface. Moreover, the electrical conductivity dependence of temperature, measure by the four point method, shows a variable hopping range for the conduction mechanism.

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