Epitaxial Sr(Sn,Ge) x Ti1– x O3 buffer layers for continuous strain engineering on SrTiO3 substrates

Epitaxial strain plays a key role in determining the structure and functionality of thin films, with the choice of substrate being traditionally used to control the magnitude of the applied strain. However, even in the large family of perovskite materials, this allows for only a limited, discrete set of strain states to be achieved. Here, we report on an approach to controlling epitaxial strain during the growth of perovskite materials by using a single SrTiO3 substrate (the most available perovskite in single crystal form) and a buffer layer consisting of the solid solution Sr(Sn,Ge)xTi1–xO3, whose lattice parameter can be tuned continuously from 3.880 Å to 4.007 Å while maintaining coherent epitaxial growth on SrTiO3 with high quality interfaces. Using a BaTiO3 overlayer as a model system, we show that changes to the buffer layer composition, i.e., an increase in the in-plane lattice parameter, change the strain state of BaTiO3 from fully relaxed, through highly compressively strained, to an exotic state showing ’inverted’ epitaxy in which the buffer layer is relaxed from the substrate but lattice matched to the overlayer.