Vibrational, Electronic, and Magnetic Excitations in II–VI Quantum Well Structures

Abstract

The tetrahedrally coordinated II-VI compound semiconductors represent an important class of semiconductors in the physics of quantum-well structures and their optoelectronic applications. The majority of II-VI’s are direct band gap semiconductors, with energy gaps spanning an impressive range of values (from zero for the Hg-based compounds to 3.8 eV for ZnS).¹ The plot of ionicity vs. bond-charge for the tetrahedrally coordinated semiconductors of the zinc blende or the wurtzite structure underscores the dramatic increase in ionicity of the II-VI’s and their predisposition to assume a rocksalt structure.² Indeed, the first-order Raman spectrum of CdTe, which at ambient pressure exhibits the characteristic zone center LO-TO pair, disappears at a pressure of ~ 30 kbar; above that pressure, CdTe transforms to a rocksalt structure with the characteristic absence of the first-order Raman spectrum.³ The increased ionicity translates to an increased Fröhlich interaction (polaron coupling constant). An examination of the lattice parameters and the band gaps characterizing the II-VI’s and the III-V’s reveals that a variety of lattice-matched, and hence strain-free, II-VI/II-VI as well as II-VI/III-V heterostructures can be visualized. They offer a wide range of total band off-sets. A judicious use of molecular beam epitaxy (MBE) also allows the fabrication of strained layer superlattices or quantum-well structures.¹ The successful incorporation of transition metal ions of the iron group (e.g., Mn²⁺, Co²⁺, Fe²⁺,...) into the II-VI’s results in a diluted magnetic semiconductor (DMS) like Cd_1-xMn_xTe.