2, Colorado School of Mines, Golden, Colorado, United States
3, Stanford University, Palo Alto, California, United States
The search for new thermoelectric materials requires optimizing a set of interrelated, complex, and often conflicting material properties. Amongst the many desirable features needed to achieve good thermoelectric performance in semiconductors, two critical ones are low thermal conductivity and high carrier concentrations (dopability). This presentation will highlight recent combined computational and experimental efforts to tailor and understand thermal and electronic properties of a chemically-diverse set of telluride-based diamond-like semiconductors (DLS) within the Cu2IIBIVTe4 (IIB = Zn, Cd, Hg, IV = Si, Ge, Sn) material space and other related compounds. We use combined computational first-principles methods, synthesis, and characterization to comprehensively assess this space for thermoelectric performance. To address thermal properties, first-principles modeling together with thermal transport measurements together suggest that substantial site disorder arising from the prevalence of IIB-Cu and CuIIB antisite defects gives rise to ultra-low thermal conductivities. The measured trend in the thermal conductivities corresponds well to the spectral mismatch of the Cu and group IIB species in the phonon spectrum, suggesting the key role of these antisite defects in scattering thermal carriers. To address dopability, we use density functional theory to investigate the intrinsic defect chemistry of several telluride DLS including Cu2HgGeTe4 and the related ordered oxygen-vacancy compound Hg2GeTe4, and predict achievable carrier concentrations. Experimentally, carrier density control has been demonstrated within the Cu2HgGeTe4-Hg2GeTe4 solid solution. Our first-principles analysis shows that Cu2HgGeTe4 can range from degenerate p-type to highly n-type under different thermodynamic environments. In agreement with high-temperature x-ray diffraction and resonant spectroscopy experiments, the predominant defects in the quaternary are found to be antisite defects with Cu and Hg. On the other hand, as Hg2GeTe4 does not contain Cu-related antisites, native defects exhibit relatively high defect formation energies, and Hg2GeTe4 is predicted to have an equilibrium Fermi energy near mid-gap for all growth environments and a wide dopability window. To overcome the small intrinsic carrier concentrations in Hg2GeTe4, we use first-principles to screen and recommend a set of extrinsic dopants that can be used to tune the carrier type from p-type to n-type.