Nonequilibrium Layered PbS Stabilized by Sn Doping: Bipolar Semiconductors with Low Thermal Conductivity

Layered Sn- and Ge-based monochalcogenides have been known as promising semiconductor materials with appropriately narrow band gaps close to those of Si and GaAs. On the other hand, Pb-based ones possess much narrower band gaps and adopt the cubic rock-salt (RS)-type structure under ambient conditions, and their layered structures are considered thermodynamically unstable. We recently succeeded in the stabilization of GeS-type layered structures in lightly Sn-doped PbS by the combination of a high-temperature solid-state reaction with thermal quenching. In this paper, we have comprehensively investigated the relationship between the crystal structures, electronic structures, and also electronic and thermal transport properties of (Pb_1–xSn_x)S (x = 0–1). It is experimentally confirmed that an equilibrium phase of layered GeS-type Sn-rich (Pb_1–xSn_x)S is a p-type semiconductor at x ≥ 0.7, whereas n-type conduction is observed at x = 0.5 and 0.6. In contrast, the stabilized nonequilibrium layered phase with 0.2 ≤ x ≤ 0.4 is an n-type semiconductor with the band gaps of 1.18–1.22 eV, and the electron density increases up to 6.4 × 10¹⁷ cm^–3 in (Pb_0.8Sn_0.2)S. Furthermore, the layered nonequilibrium phase exhibits an ultralow room-temperature thermal conductivity of 0.40–0.65 W/(mK), much lower than those of both end members, i.e., GeS-type SnS (x = 1) and RS-type PbS (x = 0). Based on first-principles electron and phonon transport calculations, layered n-type (Pb_0.75Sn_0.25)S potentially shows a high thermoelectric figure of-merit of 0.34 even at 300 K under an optimized electron concentration. The controllability of bipolar carrier polarity in layered (Pb_1–xSn_x)S alongside the low thermal conductivity is an advantageous characteristic for applications based on p–n homojunctions, such as photovoltaics and thermoelectrics.