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

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.

Figure 1

HAXPES VB spectra of the undoped and Al-doped ZnO films. Symbols are as follows: red (dark gray) dots, ZnO; black hollow circles, AZO 0.1%; black full squares, AZO 0.7%; blue (gray) hollow squares, AZO 2%. The inset shows the VBM for the ZnO and the AZO 2%. The VBM maximum values obtained from linear extrapolation are indicated.

Figure 2

Details of the VB spectra (a) in the gap between the Zn3$d$ and O2$p$ levels, and (b) at the FL region. Symbols and colors are as in Fig. 1.

Figure 3

Spherically projected densities of states of AZO with an aluminum content of 4% as obtained from GGA + $U$ calculations. The VBM has been set to the experimental value. The dark areas represent the total DOS and the white areas the projected DOS on Zn (upper panel), on O (middle panel), and in an expanded view on Al (lower panel). The red arrow identifies the Al-related DOS in the gap between the Zn3$d$ and O2$p$.

Figure 4

Density of states assuming 1% Al and 0.05% ${\text{V}}_{\text{O}}$ concentrations. The bulk valence and conduction bands are in black. The defect states (Al and ${\text{V}}_{\text{O}}$) are in gray. The occupied DOS as measured by HAXPES is symbolized by the white vertical stripes. The DOS are broadened by experimental resolution.