DEFECTS IN WIDE-BAND GAP II-VI CRYSTALS

Defects, in all materials, tend to cause properties which are not 'des ired'. Defects are intensively studied, certainly partly because of in trinsic interest, but probably more so in the hope that by understandi ng their structure, properties, and/or origin, one can reduce or elimi nate them. II-VI materials, and particularly the wide band gap ones, a re presently of high interest, and there has been a concomitant extens ive study of their defects. One high interest application of widegap m aterials (defined here, arbitrarily, as those with a band gap greater than 1.5 eV) is in short wavelength optical devices, such as the recen tly developed blue-green diode laser. At present, ZnSe-and related all oys-are used for this laser, and this material will therefore be empha sized in the present article. A second high interest application of wi degap 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 emphas ized in the present article. In the present review of defect propertie s, we shall consider defects both in terms of their 'individual' prope rties, and in terms of their 'interactions'; the latter can take place with the material, dopants and/or other defects. Although the distinc tion between these two aspects may sometimes be blurred, we will use i t to provide a convenient framework. The behavior as 'individuals' wil l be emphasized in the first part, which will cover structural and ele ctronic information as deduced from experiment, and from theory. Regar ding the interaction part, we shall discuss the pervasive problems of self-compensation and dopant solubility, which are impeding desired do ping levels in all the widegap II-VI compounds; it can be noted that i t is in this area where defects tend to be far more deleterious in the widegap II-VI materials (and probably widegap materials in general) t han they are in the common elemental and III-V materials. (C) 1997 Els evier Science S.A.