Controlling the p-type conductivity of α-SnO thin films by potassium doping

Development of a high-performance, p-type oxide channel is crucial to realize all-oxide complementary metal–oxide semiconductor technology that is amenable to 3D integration. Among p-type oxides, α-SnO is one of the most promising owing to its relatively high hole mobility {as high as 21 cm2 V−1 s−1 has been reported [M. Minohara et al., J. Phys. Chem. C 124, 1755–1760 (2020)]}, back-end-of-line compatible processing temperature (≤400 °C), and good optical transparency for visible light. Unfortunately, doping control has only been demonstrated over a limited range of hole concentrations in such films. Here, we demonstrate systematic control of the hole concentration of α-SnO thin films via potassium doping. First-principles calculations identify potassium substitution on the tin site (KSn) of α-SnO to be a promising acceptor that is not (self)-compensated by native vacancies or potassium interstitials (Ki). We synthesize epitaxial K-doped α-SnO thin films with controlled doping concentration using suboxide molecular-beam epitaxy. The concentration of potassium is measured by secondary ion mass spectrometry, and its incorporation into the α-SnO structure is corroborated by x-ray diffraction. The effect of potassium doping on the optical response of α-SnO is measured by spectroscopic ellipsometry. Potassium doping provides systematic control of hole doping in α-SnO thin films over the 4.8 × 1017 to 1.5 × 1019 cm−3 range without significant degradation of hole mobility or the introduction of states that absorb visible light. Temperature-dependent Hall measurements reveal that the potassium is a shallow acceptor in α-SnO with an ionization energy in the 10–20 meV range.