Semiconductor-like and metal-like transport properties of doped ZnO films prepared by using pulsed laser deposition

Al-doped and undoped ZnO films were deposited on (0001) Al ₂O₃ substrates at various oxygen partial pressures by using pulsed laser deposition. With changing oxygen partial pressure, the c lattice parameter, the FWHM in the rocking curve, and the absorption edge in the optical transmission spectra of the Al-doped ZnO films displayed distinctive differences above and below 10 mTorr. The transport property of the Al-doped ZnO films gradually changed from a semiconductor-like one above 10 mTorr to a metal-like one below 1 mTorr while the undoped ZnO showed only a semiconductor-like property even at 0.1 mTorr. These results suggest that donor defects created by a low oxygen pressure play an important role in lowering the resistivity of the Al-doped ZnO film, but they do not provide ZnO with a sufficient condition to exhibit metal-like properties. We conjecture that the metal-like conductivity of Ai-doped ZnO films is due to a combined effect of Al doping and low oxygen pressure.