Electronic structure of a metal-insulator interface: Towards a theory of nonreactive adhesion

Abstract

With the aim of studying metal-insulator adhesion, we have performed an analytical description of the electronic structure of a flat and defectless metal-insulator interface, for both rocksalt and zinc-blende crystallographic structures of the insulator and, respectively, (100) and (110) orientations of the interface. We model the metal by a jellium, and the AB-type insulator by a tight-binding Hamiltonian with one atomic orbital per site. A matching procedure involving a Green’s-function method yields the local density of states of the metal-induced gap states (MIGS), which are found to be in good agreement with previous numerical estimations on specific materials. By analytically solving the Poisson equation in a self-consistent way, we are able to determine the position of the Fermi level of the whole system for any value of the insulator ionicity. Our results depend upon the density of electrons in the metal, and upon the penetration length and the density of MIGS at midgap. They do not depend much upon the crystallographic structure and orientation of the interface. The two relevant parameters are the Fermi energy of the metal and a ratio that represents the ionocovalent character of the insulator. This latter quantity can be allowed to vary from zero to infinity, thus describing the whole range of compounds from covalent semiconductors to highly insulating materials. We produce an analytical expression of the Schottky-barrier height and of the index of interface behavior, S, valid in the whole range of ionicity. S is found to fit well the available experimental data. We demonstrate that the capacitor model to estimate S is restricted to strongly ionic insulators, while it was generally used in the opposite limit. We suggest finally that the above electronic parameters also drive the strength of adhesion and wetting in nonreactive metal-insulator systems.