Abstract
A detailed study of the effect of doping on the electrical transport characteristics of ZnO thin films is presented. This is performed for undoped ZnO and for ZnO doped with Al and Ga. The films have been characterized by conventional direct current, DC, Hall transport measurements, impedance spectroscopy, IS, and confocal Raman spectroscopy, CRS. The variation of the transport properties caused by doping is related to a variation of the built-in electric field at the grain boundaries. This variation is observed through a variation of confocal Raman spectroscopy localized bands that are related to structural ZnO defects. This technique allows one to measure the Raman spectra as electrical current is being fed to the film. The comparison between DC Hall transport measurements and impedance spectroscopy data confirms a semiconductor like behavior for ZnO and ZnO:Al and a metallic-like behavior of ZnO:Ga. This is due to the different amount of ions that enter substitutionally in the ZnO matrix and thus introduce carriers that compensate charge traps at the grain boundaries. The variation of this charge trap density and the associated electric field is probably the most relevant limiting factor of these films' transport properties.
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