Understanding charge carrier mobility in Hg₂GeTe₄

Abstract

High charge carrier mobility in semiconductor materials is desirable across a broad range of fields ranging from light-emitting devices to thermoelectrics. Electronic mobility is driven by both the intrinsic electronic band structure of the material as well as the energy dependent electron scattering mechanisms. Semiconductors with excellent mobility span a large chemical space: transparent conductor CdO, topological insulator HgTe, and Zintl compound KAlSb4. Therefore, engineering high mobility from chemistry alone is difficult if not impossible. Relating chemistry and synthetic processing to their impact on mobility is highly desirable, but experimentally difficult. Adding a fourth thermomagnetic measurement, the Nernst coefficient, to the traditional thermoelectric transport measurement suite (resistivity, Hall coefficient, Seebeck), allows the experimentalist to derive a carrier lifetime/scattering parameter as a function of temperature. We design a custom apparatus to measure the Nernst effect and perform initial model measurements to address the question of what scattering mechanisms limit the mobility of several potential thermoelectric materials. In our design, we test different sample and sample holder geometries to optimize reproducibility. For the model materials we measure the Nernst signal at low magnetic field (µB < 1) in addition to traditional Hall coefficient, Seebeck, and resistivity. We employ the method of four coefficients to determine four electronic parameters: µ, n, m*DOS, and λ (scattering factor). By utilizing the method of four coefficients, we can decouple effects from electronic band structure from energy-dependent scattering effects, and therefore design optimal thermoelectric materials and validate the scattering predictions from computational methods.