Stacking of SrTiO₃ membranes Towards Atomically-Flat Bicrystals

Single crystalline films can be synthesized on an epitaxially-grown sacrificial layer which is selectively dissolved in solvents, enabling release and transfer of single crystalline membranes on arbitrary substrates. However, commonly used sacrificial layers (such as SrCa₂Al₂O₆) pose complex stoichiometry and synthesis challenges in molecular beam epitaxy (MBE). In this study, we discuss two distinct but MBE-friendly, fast and facile approaches to synthesize single-crystalline oxide membranes using hybrid MBE [1,2]. In particular, we show epitaxial growth of single-crystalline SrTiO₃membranes in layer-by-layer growth mode on a solution-processed amorphous SrCa₂Al₂O₆ and in a step-flow growth mode on SrO sacrificial layer. Films grown on SrO layer dissolve rapidly (< 5 minutes) in water, resulting in millimeter-sized membranes. Further, with emphasis on creating atomically-flat bicrystals, we stack SrTiO₃ membranes at rational angles to create artificial Moiré and quasi-crystalline lattices. The twist angle is verified using x-ray diffraction (XRD), and scanning transmission electron microscopy (STEM). By combining scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy, we understand the surface morphology, bond formation and properties at the interface of the two stacked membranes.