Deep levels in semiconductors

All defects which are dominated by short-range forces belong to the family of ‘deep’ impurities and exhibit distinctly different properties from the familiar shallow donors and acceptors, where the decisive term is the Coulomb potential. Whereas formation of the shallow states relates to a small part of the Brillouin zone and can be described within the effective mass theory, the opposite is true of the deep states. However, it has recently been shown that the formation of deep localized states in semiconductors can be described with speed and accuracy, and in a self-consistent manner, by exploiting the localized character of the main part of the defect potential. It is possible to project the interaction between the localized potential and the rest of the crystal upon a limited number of localized functions, spanning the range of the potential, with an uncertainty which is small compared to the magnitude of the forbidden gap. This is achieved without truncating proper characterization of the electronic structure of the host or the chemical identity of the imperfection. An important step forward has also recently been made in devising novel spectroscopic tools tailored to the problem of detection of deep levels. With the advent of deep level transient spectroscopy, combined with optical and electron-microscope techniques, the defect signature can be established in an interactive manner, instead of relying on a single-event data. New phenomena were brought to light (e.g. multiphonon capture and recombination) and the gap separating theory and experiment narrowed. It is the purpose of this article to review this development, with a view of focusing upon quantitative guidelines resulting from computer modelling of deep level systems in semiconductors, e.g. GaP:N, GaP:O, vacancies and vacancy-related defects in Si and GaAs, Si:Co, etc. Although a truly quantitative comparison with experiment is not yet possible because of the remaining uncertainties in the form of the potential in a reconstructed lattice and incompleteness of the experimental data, the way is open for a comprehensive examination of the problem of deep levels.