Direct observation of Al-doping-induced electronic states in the valence band and band gap of ZnO films

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We present a synchrotron radiation hard x-ray photoemission spectroscopy study of the electronic structure of Al-doped ZnO films. Doping-induced states appear between the Zn3$d$ and O2$p$ levels and in the band gap just below the conduction band minimum (CBM). Ab initio calculations confirm the Al impurity origin of these induced states. The drop in the film resistivity with Al doping is not due to the progressive shifting of the Fermi level above the CBM, but rather to the filling of the Al impurity band state, which pins the Fermi level just below the CBM.