Controlling thermoelectric transport via native defects in the diamond-like semiconductors Cu2HgGeTe4 and Hg2GeTe4

Abstract

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 Cu₂HgGeTe₄ and related ordered vacancy compound Hg₂GeTe₄, 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. Cu₂HgGeTe₄ 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 Hg₂GeTe₄ 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 carrier concentrations, dopability, and transport properties. This study elucidates the doping asymmetry between two solid-solution forming DLS phases Cu₂HgGeTe₄ and Hg₂GeTe₄ by revealing the defect chemistry of each compound, and suggests design strategies for defect engineering of DLS phases.