First Principles Design of High Hole Mobility p-Type Sn–O–X Ternary Oxides: Valence Orbital Engineering of Sn²⁺ in Sn²⁺–O–X by Selection of Appropriate Elements X

The development of high-performance p-type oxides with good hole mobilities is critical for the application of metal–oxide semiconductors in back-end-of-line complementary metal–oxide semiconductor devices. [S. Salahuddin et al. Nat Electron. 1, 442 (2018)] Sn2+-based oxides have been proposed as high-mobility p-type oxides due to their relatively low effective hole masses resulting from the hybridized O–2p/Sn–5s orbital character at the valence band edge. Nonetheless, the most investigated Sn2+-based oxide, SnO, has a very small band gap (∼0.7 eV) for practical p-type oxide devices. Here, we report a systematic search and identification of high-mobility p-type oxide candidates with a wide band gap and robust phase stability by extending beyond the SnO binary phase into Sn–O–X ternary compounds with appropriate elements X. The large repository of density functional theory calculations within the Materials Project was employed to screen candidates. Using a step-by-step filtering procedure, we have identified several promising candidates including Rb2Sn2O3, TiSnO3, Ta2SnO6, and Sn5(PO5)2. By balancing the phase robustness and hole mobility, Ta2SnO6 was recommended for an initial experimental p-type oxide synthesis validation. We demonstrate that introducing an element X into the binary SnO can tune its electronic band gap and phase stability. A general design principle for Sn2+–O–X compounds and a broader concept for M–O–X ternary oxides are developed that provide fundamental material insights into the rational design of high-performance p-type oxides.