Localized defects in semiconductors

Abstract

In recent years there has been much interest in the general problem of point imperfections in solids, particularly in the context of the technologically important semiconducting materials. Quite apart from the possible practical implications there are also challenging academic reasons for pursuing studies of this subject. A number of features characteristic of low symmetry defect centres within an otherwise perfect crystal lattice await more gull understanding by theoreticians. One of the most fundamental problems, and one which has e to be dealt with in a totally satisfactory manner, is that of determining the response of the valence electrons to the presence of localised defects. (We shall treat this many-body problem by resource to the perfect crystal screening function but this is obviously an appropriate means.) In addition it would be valuable to better appreciate the precise numerical significance of the Jahn-Teller effect and other distortion-dependent phenomena such as the Stoke’s shift and various aspects of the electron-phonon interaction. Unfortunately, much of the theory required for a completely details understanding of these topics, and others, has yet to be developed in a satisfactory form. Although some characteristics of defect systems have been accounted for by relatively simple approaches the necessity for large-scale computer studies is increasingly apparent if good numerical agreement is to be obtained. However, even with the aid of powerful computing facilities there are still many difficulties to overcome. A vast amount of experimental information concerning semiconductor properties has been accumulated but much of this has still to be explained on a firm theoretical basis. This situation will almost certainly prevail for some time to come. Meanwhile, we must continually employ theoretical methods and models which may ultimately lead to more complete understanding. The task is not an easy one, however, and even some of the more accepted results are continually in need of revision. Consequently it is not surprising that some of the less well understood properties of semiconductors, particularly those in relation to chemical impurities and intrinsic defects, are still subject to considerable discussion and indeed confusion.