Band gap variation and lattice, surface, and interface ‘‘instabilities’’ in Hg1−xCdxTe and related compounds

Abstract

This paper brings into focus the differences between Hg1−xCdxTe (MCT) and ‘‘ordinary’’ semiconductors such as column 4, 3–5, and most 2–6 compounds. The key characteristic of MCT which makes it both different and of practical interest is the variation of band gap with composition (0 to 1.5 eV). This paper shows that this benefit is not without its difficulties. In particular, the same unusual characteristic of the Hg atomic states which leads to the advantageous movement of band gap with composition weakens the bonding of Hg in the lattice. This bonding weakness exhibits itself in many ways in the characteristics of MCT, both at surfaces and interfaces and in the bulk. Because of this there is a potential instability associated with the surface or interface; however, this instability depends critically on the crystalline perfection of the MCT to depths far below the surface. The reason for this is defects move from the bulk to the surface (or in the opposite direction) with surprising ease—even at room temperature. As will be discussed in this paper, this is another result of the weakness of the Hg bonding in bulk MCT. In this matter of movement of defects from bulk to surface and vice versa, MCT differs greatly from the other 3–5 and elementary semiconductors studied in detail to date. This difference will be discussed in more detail in this paper. The extreme susceptibility of MCT to mechanical damage is also related to the weakness of the Hg bonding. One object of this paper is to make a first attempt at showing how the Hg bonding affects the electronic structure, lattice bonding, and defect structure of MCT. A key phenomenon in the electronic structure is the breakdown of the virtual crystal potential approximation which has worked to a good approximation in all other covalent semiconductors studied to date. This breakdown is selective and involves only those valence states near zone boundaries containing large cation s wave function admixtures. This again reflects the unusual character of the Hg atomic 6 s2 valence state. Thus, a focal point for understanding of the wide range of unusual phenomena in MCT developed in this paper is understanding of the Hg atomic structure and its effect on MCT electronic structure, bonding, defect formation, and related phenomena. This characteristic of Hg is common to elements with high atomic number and similar or related characteristics can be expected from semiconductor compound alloys containing such heavy atoms.