Defects in wide band gap II-VI crystals

Abstract

Defects, in all materials, tend to cause properties which are not ‘desired’. Defects are intensively studied, certainly partly because of intrinsic interest, but probably more so in the hope that by understanding their structure, properties, and/or origin, one can reduce or eliminate them. II-VI materials, and particularly the wide band gap ones, are presently of high interest, and there has been a concomitant extensive study of their defects. One high interest application of widegap materials (defined here, arbitrarily, as those with a band gap greater than 1.5 eV) is in short wavelength optical devices, such as the recently developed blue-green diode laser. At present, ZnSe—and related alloys—are used for this laser, and this material will therefore be emphasized in the present article. A second high interest application of widegap materials is in solar cells, where 1.5 eV provides a good match to the peak of the solar spectrum. The primary II-VI material for this application is CdTe, and this will thus be the second material emphasized in the present article. In the present review of defect properties, we shall consider defects both in terms of their ‘individual’ properties, and in terms of their ‘interactions’; the latter can take place with the material, dopants and/or other defects. Although the distinction between these two aspects may sometimes be blurred, we will use it to provide a convenient framework. The behavior as ‘individuals’ will be emphasized in the first part, which will cover structural and electronic information as deduced from experiment, and from theory. Regarding the interaction part, we shall discuss the pervasive problems of self-compensation and dopant solubility, which are impeding desired doping levels in all the widegap II-VI compounds; it can be noted that it is in this area where defects tend to be far more deleterious in the widegap II-VI materials (and probably widegap materials in general) than they are in the common elemental and III-V materials. C 1997 Elsevier Science S.A.