Interpretation of the Changing the Band Gap of Al2O3 Depending on Its Crystalline Form: Connection with Different Local Symmetries

The valence and conduction bands of am- and gamma-Al2O3 films grown by the atomic layer deposition technique were studied simultaneously in identical experimental conditions using high-resolution near -edge X-ray absorption fine structure and soft X-ray photoelectron spectroscopy. The valence band maximum was found to be centered at 3.64 +/- 0.04 eV for am-Al2O3 and 3.47 +/- 0.04 eV for gamma-Al2O3. The band gap of Al2O3 was determined to be 7.0 +/- 0.1 and 7.6 +/- 0.1 eV for measured am- and gamma-Al2O3, respectively. The main role in changing the band gap belongs to a shift of the bottom of conduction band depending on Al2O3 crystalline form. The position of the bottom of the conduction band is governed by the charge transfer from Al atom to the oxygen that depends strongly on the Al atom coordination symmetries. A strong p-d hybridization allowed for T-d symmetry but forbidden for Oh symmetry plays the decisive role in the formation of the bottom of the conduction band.