Epitaxial Sr(Sn, Ge)$_{x}$Ti$_{1-x}$O$_{3}$ buffer layers for continuous strain engineering on SrTiO$_{3}$ substrates

TL;DR

This work introduces a continuous strain-engineering approach for perovskites by using a tunable buffer layer, Sr(Sn,Ge)_xTi_{1-x}O_3, on SrTiO_3 substrates to modulate in-plane lattice parameters from $a \approx 3.88$ Å to $a \approx 4.01$ Å. By pairing SSTO/SGTO buffers with BaTiO_3 overlayers, the authors demonstrate controlled transitions between fully relaxed, highly compressively strained, and inverted epitaxy states, as evidenced by XRD, STEM, and PFM analyses. The study provides a chemically tunable, high-quality route to continuous strain states, enabling precise tuning of ferroelectric transitions (e.g., BTO $T_C$) and offering industrially compatible integration on STO-based platforms. The findings open avenues for applying similar buffer-layer concepts to other perovskites and oxide systems, expanding the toolkit for strain-engineered functionalities in complex oxides.

Abstract

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 for the growth of perovskite materials by involving a single SrTiO$_{3}$ substrate (the most available perovskite in single crystal form) and a buffer layer that consists of the solid solution Sr(Sn, Ge)$_{x}$Ti$_{1-x}$O$_{3}$, of which the lattice parameter can be tuned in a continuous fashion, from 3.880 Å up to 4.007 Å, while maintaining coherent epitaxial growth on SrTiO$_{3}$ with high quality interfaces. Using a BaTiO$_{3}$ overlayer as a model system, we show that changes to the buffer layer composition, i.e. increase of in-plane lattice parameter, change the strain state of BaTiO$_{3}$ 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.

Figures (5)

• Figure 1: Lattice parameter of bulk SrSn_xTi_1-xO3 (SSTO) and SrGe_xTi_1-xO3 (SGTO) as a function of tin and germanium concentration. The inset shows the corresponding x-ray diffraction patterns around the (211) peak for all compositions.
• Figure 2: a) AFM images of BaTiO3 films grown on buffer layers of SrSn_xTi_1-xO3 with various concentrations. The composition of the buffer layers and the roughness of the heterostructure surface are denoted under each image. b) Piezoresponse force microscopy (PFM) on a BTO/SSTO film, with $x = 0.75$. From top to bottom: Topography (height) image of the selected area; Written box-in-box pattern with alternating positively and negatively polarized squares and maximum amplitude of 9 V; PFM amplitude signal of the same area; and at the bottom, PFM image of the same area. The images are consistent with a upward intrinsic polarization and a switching voltage between 6 and 7.
• Figure 3: a) BTO out-of-plane lattice parameter as a function of temperature for different SSTO buffer layer compositions. The dashed lines show the thermal expansion in the paraelectric phase, following the data from the x= 0.90 composition. The phase transition temperature $T_\text{C}$ increases with increasing compressive strain. b) Out-of-plane lattice parameter at room temperature for different buffer layer compositions. An opposite trend is observed in the BTO lattice parameters for $x < 0.5$ and $x > 0.5$. c) Transition temperature $T_\text{C}$versus out-of-plane lattice parameter of BTO. d) A $2\theta - \omega$ scan of a BTO film on SrSn_0.75Ti_0.25O3 on STO.
• Figure 4: STEM images of the a-d) BTO/SSGTO and e-h) SSGTO/STO interfaces for four compositions of the SSGTO buffer layer. The films with intermediate compositions $x = 0.15$ and $x = 0.45$ show coherent epitaxial growth at both interfaces. The SGTO film shows coherent growth at the SGTO/STO interface but defective and highly strained growth at the BTO/SGTO interface. Conversely, the SSTO film with $x = 0.9$ has a coherent BTO/SSTO interface and an SSTO/STO interface with regions of coherent growth and regions where the strain is relaxed through interfacial defects (i). In both cases, the appearance of these defects can be understood by considering the lattice mismatch between respective layers.
• Figure 5: Profiles of the out-of-plane (a-d) and in-plane (e-h) lattice parameters as a function of relative depth in the film as calculated using STEMfit. The vertical axis shows the relative vertical position in the image as shown in Figure \ref{['fig:filtered-stem']}, i.e. 1 corresponds to the top of the image, 0 to the bottom. The light gray markers show the local lattice parameter value of each atomic column detected in the image. The colored line represents a moving average of this data along the vertical axis. The different colors of this line correspond to the different layers of the film, as indicated in the legend. The length of each of the scale bars in (a-d) represents 10 unit cells in each corresponding image.