Deep Impurity Levels in Semiconductors. Current State of Our understanding

Abstract

A “deep” level in a semiconductor is a localized state with energy within the forbidden gap, which cannot be described in terms of effective mass theory. Instead of being spread out over a large volume of the crystal, the wave function is concentrated within one or two atomic spacings of the center (impurity atom, lattice defect, or complex) with which it is associated. The energy level is usually near the middle of the gap, rather than near one or other band edge. Because of the extreme localization of the charge on the center, there is often a substantial change in the local atomic configuration when the charge is changed by capture or emission of an electron or hole. This leads to a very strong electronphonon interaction at the center. These centers are important technically for two reasons: they provide non-radiative paths for electron-hole recombination; and by pinning the Fermi level near the middle of the gap, they can produce “semi-insulating” material, whose resistivity is close to intrinsic even though the impurity content is quite high. The most important deep centers are those produced by transition metal impurities, and by lattice defects such as vacancies. The Watkins model of the vacancy, in which the wave function is a linear combination of the dangling bond orbitals of the four nearest neighbor atoms [1], is not only an aid to visualization, but gives a remarkably accurate description of the ground state in a wide variety of centers. Recent calculations by Baraff, Schluter and their coworkers [2] have put this model on a firm theoretical basis and provided useful predictions of excited states and electronphonon interactions at lattice defects.