The IV-VI semiconductor system Pb1−xSnxTe with high Sn content is a new type of topological insulator (TI) material system, namely a topological crystalline insulator (TCI), in which the band structure is inverted and crystal symmetry (rather than time-reversal symmetry) determines the topology and protects the Dirac-like surface states. The symmetry of these states and the number of equivalent metallic surface states is determined by the orientation of surfaces. We will develop molecular beam epitaxy (MBE) for the heteroepitaxial growth of thin Pb1−xSnxTe layer and multilayer structures on nearly lattice-matched Cd(Zn)Te with different surface orientations. The interface of Pb1−xSnxTe with rock-salt structure and CdTe with zinc-blende structure is little understood and will be studied with respect to structure and charge distribution. Both of these affect the electronic transport properties of thin layers and can be tuned by dopants like group V element Bi. Thin epitaxial Pb1−xSnxTe layers with inverted band alignment embedded in PbTe (or CdTe) with normal band alignment will enable 2D TCI structures, whose TI properties are determined by the substrate orientation. The various topologies of surface states and rather large band gaps, which are widely tunable through the inversion point by alloying, layer thickness, strain, temperature, and electric fields promise deeper insight into bandstructure and electronic transport of such TCI structures. High electron mobilities and large dielectric constants promote electronic quantum effects in PbSnTe transport devices. Efficient interband transitions in the mid-infrared (MIR) spectral range also promise spectroscopy studies of optical properties of TCI structures. We anticipate that the realization of such high-quality Pb1−xSnxTe layer structures in this project will result in 2D and 3D structures of a novel TCI material system for basic research, like the spin quantum Hall effect in a TCI 2D layer, and possibly for spintronic and optoelectronic applications. Besides our structural and conductivity studies for optimizing MBE techniques and material quality, angle-resolved photoelectron spectroscopy (ARPES), advanced magnetotransport, and scanning probe studies of surface states combined with theoretical studies of TCIs within the CRC are important for exploring the properties of PbSnTe TCIs.
 J. Liu et al., Nat. Mat. 13, 179 (2014)