Experimental determination of the top of the valence band in amorphous Al$_2$O$_3$ and $\gamma$-Al$_2$O$_3$

Investigation of the energy distribution of occupied states in the valence band and determination of its upper level in films of amorphous Al$_2$O$_3$ and $\gamma$-Al$_2$O$_3$, synthesized by atomic layer deposition on a silicon substrate, was performed by X-ray photoelectron spectroscopy. The top of the valence gap in $\gamma$-Al$_2$O$_3$ was found to shift by 0.8 eV towards larger electron binding energies with respect to amorphous Al$_2$O$_3$.