Electronic-structure investigation of oxidized aluminum films with electron-momentum spectroscopy

Electron-momentum spectroscopy (EMS) or ($e, 2e$) measurements with oxidized aluminum thin films have been performed. Due to the surface sensitive nature of the EMS spectrometer employed, the measured ($e, 2e$) events come from the front oxidized layer as viewed by the electron detectors. The measurements show clearly two major features in the spectral momentum density distribution and they are related to the upper valence band and the lower valence band of aluminum oxide. The first is a "dual parabola" energy-momentum dispersion pattern spanning about 8 eV in the upper valence band. This dual parabola pattern has been qualitatively reproduced by a linear muffin-tin orbital (LMTO) calculation on spherically averaged $\alpha$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ with nearly the same energy span. In the lower valence band, the LMTO calculation indicates a dispersion spanning about 5 eV, and the measured spectral momentum density plot shows a similar "bowl" shape but with less dispersion. The possible causes that blur the dispersion in the lower valence band are discussed. Other features in the spectral momentum density distribution are also discussed and compared with the LMTO calculation.