Abstract
Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, particularly thermoelectric materials. Generally, the main role of chemical doping lies in optimizing the carrier concentration, but there can potentially be other important effects. Here, we show that chemical doping plays multiple roles for both electron and phonon transport properties in half-Heusler thermoelectric materials. With ZrNiSn-based half-Heusler materials as an example, we use high-quality single and polycrystalline crystals, various probes, including electrical transport measurements, inelastic neutron scattering measurement, and first-principles calculations, to investigate the underlying electron-phonon interaction. We find that chemical doping brings strong screening effects to ionized impurities, grain boundary, and polar optical phonon scattering, but has negligible influence on lattice thermal conductivity. Furthermore, it is possible to establish a carrier scattering phase diagram, which can be used to select reasonable strategies for optimization of the thermoelectric performance.
Highlights
Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, thermoelectric materials
Temperature dependences of the mobility in these polycrystalline samples were studied over the temperature range of 5 K to 315 K. μ(T) of Sb-doped samples monotonically decreases with temperature (Fig. 2a), since the acoustic phonon and alloy scatterings contribute importantly to the electron scattering
Similar to n-type Mg3(Sb,Bi)[2] system, this kind of increasing trend of μ(T) raises the issue of the carrier scattering mechanism in underdoped TE materials with low carrier concentration or with small grain size[44,45], i.e., does the increased μ(T) below the crossover temperature stems from the grain boundary scattering or ionized impurity scattering44,45?
Summary
Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, thermoelectric materials. We show that chemical doping plays multiple roles for both electron and phonon transport properties in half-Heusler thermoelectric materials. The optimization of TE performance is complex, requiring manipulation and optimization of both the electrical and thermal transport[11] This includes controlling the underlying scattering mechanisms, e.g. alloy scattering[12], ionized impurity scattering[13], carrier-carrier scattering[14], acoustic and optical phonon scatterings[15] for the charge carriers, and phonon anharmonicity[16,17,18], atomic rattling[19], nanostructuring[20], and glass-like or superionic behavior[21,22,23] for the phonons. The carrier mobility follows μac ∝ T−3/2 for acoustic phonon dominated scattering, μpe ∝ T−1/2 for piezoelectric scattering, or μi ∝ T3/2 for ionized impurity scattering[24], while the lattice thermal conductivity follows κU ∝ T−1 for Umklapp scattering or κal ∝ T−1/2 for alloy scattering[25,26]
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