Theory of 3d Transition Atom Impurities in Semiconductors

Abstract

The utility of semiconductors in many device applications, such as high­ speed logic circuits ( 1-2), optoelectronic devices (3-4), microwave devices (5), and solar cells (6), rests on the characteristics of the intentionally introduced impurities (dopants) as much as on the properties of trace amounts of unintentional contaminations. Transition atom (T A) impurities in semiconductors form a special class of such contaminants. They were studied experimentally in great detail (e.g. review articles in References 7-10) using a broad range of techniques, including optical absorption, luminescence, photocapacitance, photoconductivity, electron para­ magnetic resonance (EPR), electron nuclear double resonance (ENDOR), deep level transient spectroscopy (DLTS), and Hall effect. This review article is concerned with the theoretical understanding of the electronic properties of T A impurities in Si, III-V, and II-VI semiconductors. First, we establish the nomenclature. When a transition atom takes up a substitutional site, say on a cation, its formal oxidation state when neutral (labeled A 0) becomes that of the site it replaces, e.g. T A 3 + if it replaces a column III element. When the impurity captures an electron its charge state becomes negative (denoted A ), and the oxidation state is TA2+. Conversely, when the impurity loses an electron its charge state becomes positive (labeled A +), and the oxidation state becomes TA4+. The tenfold degenerate atomic d orbitals can split in the cubic environment into a