Tin monoxide as an s‐orbital‐based p‐type oxide semiconductor: Electronic structures and TFT application

Tin monoxide (SnO) is a stable p‐type oxide semiconductor. This paper reports electrical properties, electronic structures, and thin‐film transistors (TFTs) of SnO. Epitaxial films were fabricated by pulsed laser deposition. The Hall mobility and the hole density of the epitaxial films were 2.4 cm² V⁻¹ s⁻¹ and 2.5 × 10¹⁷, respectively. X‐ray photoelectron spectroscopy (PES) indicated that the closed‐shell 5s² orbitals of Sn²⁺ ions heavily contribute to the hole conduction path in SnO. Top gate type TFTs (W/L = 300/50 µm) employing 20 nm thick SnO channels exhibited field‐effect mobilities µ_sat = 0.7 cm² V⁻¹ s⁻¹ and µ_lin = 1.3 cm² V⁻¹ s⁻¹, which are larger by two orders of magnitude than those reported for p‐channel oxide TFTs to date. On/off current ratios were ∼10² and subthreshold voltage swings (S) ∼7 V/decade. The parameters required for TFT simulations were estimated by ultraviolet PES and first‐principles calculations. The TFT simulations indicated that subgap hole trap density in the SnO channel is >10¹⁹ cm⁻³, which limits the TFT mobilities and the S values.