Tuning the transport and thermoelectric properties of In2O3 bulk ceramics through doping at In-site

Abstract

Bulk ceramics In2−xMxO3, with a metal-like behavior, have been synthesized in air for M=Ti4+, Zr4+, Sn4+, Ta5+, and Nb5+, with rather low solubility limits xℓ, ranging from 0.01 to 0.1. An abrupt increase in the electrical conductivity and of the carrier concentration with x is observed in the monophasic region (x<xℓ), whereas in the biphasic region (x>xℓ) these values do not vary significantly. These results show that the valence of the doping element plays a crucial role in such properties, similar to degenerated semiconductors. Similarly, the thermopower |S| value is correlated with this evolution decreasing as x increases for x<xℓ. For the Sn doped samples, the maximum carrier concentration n=10.8×1020 cm−3 and electrical conductivity σ=5×103 S cm−1 are both achieved at x=0.06–0.1 (∼3–5 at. %) for a thermopower S of −20 μV/K at room temperature. In comparison, the minimum |S| and maxima σ and n of In2O3 compounds doped with the other cations occur at lower doping levels, e.g., xℓ∼0.02 and xℓ∼0.015 for Ti and Zr/Nb/Ta doped In2O3, respectively. The relationship between the values of room temperature Hall mobility and carrier concentration shows that numbers and/or effects of multicharged scattering centers of electrons are different depending on the doping element. In2O3 doped with M(IV) elements shows maximum values of mobility close to x=0.006, whereas the doping with M(V) cations induces a decrease in the Hall mobility even for small doping levels. The thermoelectric performances are significantly improved at these low doping levels, with ZT values close to 0.3 at 1000 K in the doped materials against 0.1 mW/mK2 for the undoped phase.