Diamond like semiconductors (DLS) have emerged as candidates for thermoelectric energy conversion. Towards understanding and optimizing performance, we present a comprehensive investigation of the electronic properties of two DLS phases, quaternary Cu 2 HgGeTe 4 and related ordered vacancy compound Hg 2 GeTe 4 , including thermodynamic stability, defect chemistry, and transport properties. To establish the thermodynamic link between the related but distinct phases, the stability region for both is visualized in chemical potential space. In spite of their similar structure and bonding, we show that the two materials exhibit reciprocal behaviors for dopability. Cu 2 HgGeTe 4 is degenerately p-type in all environments despite its wide stability region, due to the presence of low-energy acceptor defects V Cu and Cu Hg and is resistant to extrinsic n-type doping. Meanwhile Hg 2 GeTe 4 has a narrow stability region and intrinsic behavior due to the relatively high formation energy of native defects, but presents an opportunity for bi-polar doping. While these two compounds have similar structure, bonding, and chemical constituents, the reciprocal nature of their dopability emerges from significant differences in band edge positions. A Brouwer band diagram approach is utilized to visualize the role of native defects on carriermore »
Point defects in Cu 2 ZnSnSe 4 (CZTSe): Resonant X-ray diffraction study of the low-temperature order/disorder transition: Point defects in Cu 2 ZnSnSe 4 (CZTSe)
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The new, quaternary diamond-like semiconductor (DLS) Cu 4 MnGe 2 S 7 was prepared at high-temperature from a stoichiometric reaction of the elements under vacuum. Single crystal X-ray diffraction data were used to solve and refine the structure in the polar space group Cc. Cu 4 MnGe 2 S 7 features [Ge 2 S 7 ] 6− units and adopts the Cu 5 Si 2 S 7 structure type that can be considered a derivative of the hexagonal diamond structure. The DLS Cu 2 MnGeS 4 with the wurtz-stannite structure was similarly prepared at a lower temperature. The achievement of relatively phase-pure samples, confirmed by X-ray powder diffraction data, was nontrival as differential thermal analysis shows an incongruent melting behaviour for both compounds at relatively high temperature. The dark red Cu 2 MnGeS 4 and Cu 4 MnGe 2 S 7 compounds exhibit direct optical bandgaps of 2.21 and 1.98 eV, respectively. The infrared (IR) spectra indicate potentially wide windows of optical transparency up to 25 μm for both materials. Using the Kurtz–Perry powder method, the second-order nonlinear optical susceptibility, χ (2) , values for Cu 2 MnGeS 4 and Cu 4 MnGe 2 S 7 were estimated to bemore »
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The optimization and application of new functional materials depends critically on our ability to manipulate the charge carrier density. Despite predictions of good n-type thermoelectric performance in the quaternary telluride diamond-like semiconductors ( e.g. Cu 2 HgGeTe 4 ), our prior experimental survey indicates that the materials exhibit degenerate p-type carrier densities (>10 20 h + cm −3 ) and resist extrinsic n-type doping. In this work, we apply the technique of phase boundary mapping to the Cu 2 HgGeTe 4 system. We begin by creating the quaternary phase diagram through a mixture of literature meta-analysis and experimental synthesis, discovering a new material (Hg 2 GeTe 4 ) in the process. We subsequently find that Hg 2 GeTe 4 and Cu 2 HgGeTe 4 share a full solid solution. An unusual affinity for Cu Hg and Hg Cu formation within Cu 2 HgGeTe 4 leads to a relatively complex phase diagram, rich with off-stoichiometry. Through subsequent probing of the fourteen pertinent composition-invariant points formed by the single-phase region, we achieve carrier density control ranging from degenerate (>10 21 h + cm −3 ) to non-degenerate (<10 17 h + cm −3 ) via manipulation of native defect formation. Furthermore, thismore »
