Wide-gap II-VI superlattices

At this time, there is evident a sharp increase of interest in the II-VI class of semiconducting compounds largely due to recent success in the growth of these materials by molecular beam epitaxy (MBE). One of the more important areas for application of high-quality II-VI films is infrared imaging where CdTe deposited by MBE onto GaAs substrates is proposed as the substrate for subsequent HgCdTe and HgTe/CdTe superlattice deposition. Moreover, interest in the wide-gap II-VI compounds is stimulated by the need for electronically addressable flat panel display devices, and for the development of wide-gap (blue) LED and injection laser devices. For applications in the blue portion of the visible spectrum, ZnSe and ZnS have long been favored candidates. Very high quality ZnSe has recently been grown by MBE. The photoluminescence spectra of the MBE-grown ZnSe samples grown at Purdue University and elsewhere strongly suggests that, in many cases, the film quality exceeds that obtainable in bulk form. In addition to the Conventional II-VI materials, a new class of materials called diluted magnetic semiconductors (DMS) are currently receiving considerable attention. DMS are II-VI semiconductors such as CdTe or ZnSe with a fraction of the group II element substituted by a magnetic transition element such as Mn. The incorporation of Mn leads to very large magnetooptic effects, on the Order of several hundred times that exhibited by conventional semiconductors of a comparable bandgap. An especially significant feature of II-VI DMS materials is the increase in bandgap resulting from Mn incorporation. The band-gap increases with Mn mole fraction in a manner similar to the effect of Al in the (Ga, Al)As system, and with similar implications to the creation of quantum well structures and superlattices. The first part of this paper reviews our recent work involving quantum well structures in the (Zn, Mn)Se system, with the second part emphasizing (Cd, Mn)Te. Included in the discussion of superlattice structures in (Cd, Mn)Te is a description of recent results concerning the heteroepitaxial growth of CdTe on GaAs-materials having a lattice constant mismatch of 14.6 percent.