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 (Pb1–xSnx)S (x = 0–1). It is experimentally confirmed that an equilibrium phase of layered GeS-type Sn-rich (Pb1–xSnx)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 (Pb0.8Sn0.2)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 (Pb0.75Sn0.25)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 (Pb1–xSnx)S alongside the low thermal conductivity is an advantageous characteristic for applications based on p–n homojunctions, such as photovoltaics and thermoelectrics.