High-Mobility Metastable Rock-Salt Type (Sn,Ca)Se Thin Film Stabilized by Direct Epitaxial Growth on a YSZ (111) Single-Crystal Substrate

Metastable cubic (Sn_1–xPb_x)Se with x ≥ 0.5 is expected to be a high mobility semiconductor due to its Dirac-like electronic state, but it has an excessively high carrier concentration of ∼10¹⁹ cm^–3 and is not suitable for semiconductor device applications such as thin film transistors and solar cells. Further, thin films of (Sn_1–xPb_x)Se require a complicated synthesis process because of the high vapor pressure of Pb. We herein report the direct growth of metastable cubic (Sn_1–xCa_x)Se films alloyed with CaSe, which has a wider bandgap and lower vapor pressure than PbSe. The cubic (Sn_1–xCa_x)Se epitaxial films with x = 0.4–0.8 are stabilized on YSZ (111) single crystalline substrates by pulsed laser deposition. (Sn_1–xCa_x)Se has a direct-transition-type bandgap, and the bandgap energy can be varied from 1.4 eV (x = 0.4) to 2.0 eV (x = 0.8) by changing x. These films with x = 0.4–0.6 show p-type conduction with low hole carrier concentrations of ∼10¹⁷ cm^–3. Hall mobility analysis suggests that the hole transport would be dominated by 180° rotational domain structures, which is specific to (111) oriented epitaxial films. However, it, in turn, clarifies that the in-grain carrier mobility in the (Sn_0.6Ca_0.4)Se film is as high as 322 cm²/(Vs), which is much higher than those in thermodynamically stable layered SnSe and other Sn-based layered semiconductor films at room temperature. Therefore, the present results prove the potential of high mobility (Sn_1–xCa_x)Se films for semiconductor device applications via a simple thin-film deposition process.