Chemistry, growth kinetics, and epitaxial stabilization of Sn²⁺ in Sn-doped SrTiO₃ using (CH₃)₆Sn₂ tin precursor

PbTiO₃-based ferroelectrics have impressive electroactive properties, originating from the Pb²⁺ 6s² electron lone-pair, which cause large elastic distortion and electric polarization due to cooperative pseudo Jahn-Teller effect. Recently, tin-based perovskite oxide (SnTiO₃) containing Sn²⁺ and a chemistry similar to that of the 6s² lone-pair has been identified as a thermally stable, environmentally friendly substitute for PbTiO₃-based ferroelectrics. However experimental attempts to stabilize Sn²⁺ on the A-site of perovskite ATiO₃ have so far failed. In this work, we report on the growth of atomically smooth, epitaxial, and coherent Sn-alloyed SrTiO₃ films on SrTiO₃ (001) substrates using a hybrid molecular beam epitaxy approach. With increasing Sn concentration, the out-of-plane lattice parameter first increases in accordance with the Vegard’s law and then decreases for Sn(Sr+Ti+Sn) at. % ratio > 0.1 due to the incorporation of Sn²⁺ at the A-site. Using a combination of high-resolution X-ray photoelectron spectroscopy and density functional calculations, we show that while majority of Sn is on the B-site, there is a quantitatively unknown fraction of Sn being consistent with the A-site occupancy making SrTiO₃ polar. A relaxor-like ferroelectric local distortion with monoclinic symmetry, induced by A-site Sn²⁺, was observed in Sn-doped SrTiO₃ with Sn(Sr+Ti+Sn) at. % ratio = 0.1 using optical second harmonic generation measurements. The role of growth kinetics on the stability of Sn²⁺ in SrTiO₃ is discussed.