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IV-VI compounds (PbTe, PbSe, PbS, SnTe, GeTe) and their alloys are narrow-gap semiconductors crystallizing in the rock-salt structure and known for very good thermoelectric and infrared optoelectronic properties exploited, e.g. in mid-infrared p-n junction lasers and detectors. Recently, these materials have been recognized as a new class of topological materials - topological crystalline insulators (TCI) [1]. The properties of TCI surface states will be discussed for bulk crystals, crystalline bulk nanocomposites, epitaxial multilayers and quantum dot heterostructures. The TCI surface states were discovered by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) as well as observed in magneto-transport and magneto-optical studies [1,2]. These states constitute a new type of two-dimensional (2D) electron system with unique properties brought about by strong relativistic effects (spin-orbit interaction). Their electron structure exhibits metallic electronic structure with linear Dirac-like dispersion and spin–momentum locking. In Pb1-xSnxTe (x=0-1) and Pb1-xSnxSe (x=0-0.4) substitutional alloys the chemical composition, temperature and hydrostatic pressure induced band inversion is observed between conduction and valence bands. The terminal compounds SnTe and SnSe (in the rock-salt crystal structure) exhibit the inverted band ordering whereas in PbTe and PbSe the band ordering is topologically trivial. Particularly important technological path involves spontaneous formation of nanoscale two-phase coherent crystalline structures, e.g. in PbTe-SnTe-CdTe or PbSe-SnSe-CdSe semiconductor systems. It permits the growth of high crystalline quality composite thermoelectric or optoelectronic nanostrucutres. As the refractive index of IV-VI compounds is very high (typically n≈6) these materials show excellent optical contrast to other semiconductors as demonstrated, e.g. in very efficient Bragg-mirrors composed of just four layers of PbTe and CdTe. By designing the IV-VI topological/trivial heterostructures (superlattices) in the form of 2D multilayers, 1D nanowires or 0D quantum dots one can also exploit topological Dirac interface states in new class of infrared metamaterials [3-5].
[1] P. Dziawa, B.J. Kowalski, K. Dybko et al., Nature Materials 11, 1023 (2012).
[2] P. Sessi, D. Di Sante, A. Szczerbakow et al., Science 354, 1269 (2016).
[3] M. Szot, K. Dybko, P. Dziawa et al., Crystal Growth & Design 11, 4794 (2011).
[4] G. Karczewski, M. Szot, S. Kret et al., Nanotechnology 26, 135601 (2015).
[5] J. Sadowski, P. Dziawa, A. Kaleta et al., Nanoscale (2018) doi: 10.1039/c8nr06096g.
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