Unified disorder induced gap state model for insulator–semiconductor and metal–semiconductor interfaces

Abstract

The energy location for the interface state density Nss minimum of the insulator–semiconductor (I–S) interface and the Fermi-level pinning position at the metal–semiconductor (M–S) interface are shown to coincide and to lie at the same position of 5.0 eV from the vacuum level for major tetrahedral semiconductors. Neither the unified defect model nor the metal induced gap state model can explain the novel striking correlation between the I–S and M–S interfaces. The correlation as well as the observed peculiar photoionization behavior of the I–S interface are explained by the novel unified disorder induced gap state (DIGS) model where DIGS pin or restrict the movement of the surface Fermi level. The above characteristic energy, EHO, is shown to be the Fermi energy of the DIGS spectrum which is given by the hybrid orbital energy of the sp3 bond of the host. The DIGS model explains remarkably well the behavior of the M–S interface formed on the bare or oxide covered semiconductor surface as well as the various features of Nss distribution of the I–S interface. The correlation between the DIGS-free heterojunction (S–S) interface and M–S/I–S interface is explained by the fact that EHO is a universal reference energy level of the host which is invariant under any off-diagonal interactions, as is evidenced by the alignment of transition metal deep levels, DX centers and EL2 with respect to EHO. Band offset at the S–S interface is proposed to be determined by the alignment of EHO which inevitably involves formation of interface dipole when two EHO levels lie at different positions from the vacuum level.