Ab initio Studies of Electronic Structure of Defects in PbTe
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Ab initio studies of electronic structure of defects in PbTe Salameh Ahmad*, Daniel Bilc*, S.D. Mahanti*, and M.G. Kanatzidis** *Department of Physics and Astronomy,** Department of Chemistry Michigan State University, East Lansing, MI 48824 ABSTRACT Ab initio electronics structure calculations have been carried out in a series of RPb2n-1Te2n, n=16, compounds to understand the nature of “defect” states introduced by R where R = vacancy, monovalent Na, K, Rb, Cs, Ag atoms and divalent Cd atoms. We find that the density of states (DOS) near the top of the valence band and the bottom of the conduction band get significantly modified. The Na atom seems to perturb this region least (ideal acceptor in PbTe) and the other monovalent atoms enhance the DOS near the top of the valence band. Cd is an interesting case, since it introduces a strong resonance state near the bottom of the conduction band. INTRODUCTION Narrow Band-gap IV-VI semiconductors are of great interest since last four decades because of their fundamental properties in solid state physics and for their practical applications. Lead chalcogenide salts PbSe and PbTe are IV-VI narrow gap semiconductors whose study over several decades has been motivated by their importance in infrared detectors, light-emitting devices, infrared lasers, thermophotovoltaics and thermoelectrics [1, 2, 3]. In fact PbTe was one of the first materials studied by Ioffe and his colleagues in the middle of the last century when there was a revival of interest in thermoelectricity [4]. This compound, its alloys with SnTe and PbSe, and related compounds called TAGS (alloys of AgSbTe2 and GeTe) were for many years the best thermoelectric materials at temperatures ~ 700 K [5]. In recent years quantum wells of PbTe/Pb1-xEuxTe, PbSe0.98Te0.02/PbTe superlattices [6] and novel quaternary compounds AgSbPb2n-2Te2n (n = 9, 10) [7] have attracted considerable attention because of their large thermoelectric figure of merit. Other compounds where Ag is replaced by an alkali atom and cadmium atom are also promising thermoelectrics. Since the thermoelectric performance of a semiconductor depends sensitively on its electronic structure [8, 9], it is important to understand the nature of the electronic states associated with the substitution of Pb by other atoms and vacancies. Shallow and deep defect states in semiconductors are known to profoundly alter their electronic structure near the band gap and control their transport properties. It is believed that in narrow band gap semiconductors like PbTe deep defect states are more likely to occur [10, 11, 12]. Unlike shallow impurity levels, which are produced by the long-range Coulomb potential, deep levels are produced by the short-range atomic-like defects potential. The detail understanding of the shallow defect states in common semiconductors traces back to the classic work of Kohn and Luttinger [13] and can be regarded as basically understood. However the problem of defects in narrow-gap semiconductors, particularly with regard to the physics
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