Wurtzite Structure Semiconductors

Abstract

The wurtzite structure II-VI semiconductors are closely related to the II-VI zinc-blendes. In analogy with the diamond and zinc-blende structures, the wurtzite structure can be constructed by considering two interpenetrating lattices. However, in this case they are hexagonal close packed lattices. The nearest neighbors and next nearest neighbors are the same in the ideal wurtzite and zinc-blende structures. For the ideal wurtzite case, the lattice parameters are cia = 2V6/3 = 1.633, and u = 3/8 [9.1]. When the nearest neighbor distance is the same in the wurtzite and zinc-blende structures, the lattice constants are related by az =-./2aw. For wurtzite, there are four atoms per unit cell in contrast to two for zinc-blende. Since the volume per atom is the same, a given volume of reciprocal space for wurtzite contains approximately twice as many reciprocal lattice points. The lattice constants for several wurtzite structure semiconductors are given in Table 9.1. Some common wurtzite semiconductors are BeO, ZnO, ZnS, ZnSe, CdS, and CdSe. The last four of these compounds can also be crystallized in the zinc-blende structure. The most studied materials in this group are ZnO, ZnS, CdS, and CdSe, and we will focus on EPM calculations for these materials. The first EPM application to the wurtzite structure was the calculation of the electronic properties of ZnS [9.2]. This material is a particularly instructive one to study since there is considerable information available for both the zinc-blende and wurtzite modifications. The next applications were to CdS and CdSe [9.3,4]. The calculations were done in a similar manner to the early applications of the EPM to the diamond and zinc-blende semiconductors [9.5]. The pseudopotential for wurtzite can be written in a similar form to that of zinc-blende V(r) = E[Ss(G)Vs(G) +iSA(G)VA(G)]e iGor (9.1) G where the symmetric and antisymmetric form factors Vs(G) and VA(G) are given by half the sum and difference respectively of the form factors for the two types of atoms in the unit cell. The symmetric and antisymmetric structure factors can be written as (9.2) 140 M. L. Cohen et al., Electronic Structure and Optical Properties of Semiconductors