Theory of metal—ionic-insulator interfaces

A theory of the interfacial dipole energy and the threshold for photoemission from metals covered with a thin layer of ionic insulator was developed. The calculated thresholds are found to be in good agreement with recent measured values for metal-Ba${\mathrm{F}}_2$ and metal-LiF systems. Moreover, the calculations agree with the observed interface parameter $S$, the systematic dependence of the thresholds on the insulator, and the observation that the thresholds tend to be smaller than the metal work function. The model includes a multiband free-electron description of the metal and a two-band model of the ionic insulator. The multiband feature was required for the agreement between theory and experiment, which suggests the sensitivity of the observed thresholds to band-structure effects in the metal. We also found the results to be sensitive to the choice of electron effective mass in the insulator.