Engineering strontium aluminate sacrificial layers for fabricating monocrystalline complex oxide freestanding membranes

There was a long history of releasing various monocrystalline semiconductor structures from their hosting substrates to form “freestanding” structures, in order to change the substrates and for other special purposes. The release was achieved by breaking the bonds between the film and the substrate, through methods such as forming interfacial gas bubbles (“smart-cut” technology for fabricating semiconductor-on-insulator wafers) or chemical etching (selectively etching epitaxial AlAs underlayer for fabricating GaAs-on-silicon photonic devices). The exfoliation of layered van der Waals materials in recent decades also produced another class of freestanding monocrystalline materials—two-dimensional (2D) materials. In addition to changeable substrates, being freestanding also allowed unique methods to manipulate the 2D materials; for example, transferring them on flexible substrates and directly stretching them controls the strain in their lattice, as well as their strain-dependent physical properties.

And now it is the turn for complex oxides. Complex oxides manifest exotic physical properties such as multiferroicity, various magnetic orders, and high-Tc superconductivity, which are generally believed to be the results of the interplay between the charge, the orbital, and the spin of the electrons, as well as the lattice structures of this type of material. These oxides often hold perovskite (ABO3) or perovskite-related crystal structures; their similar atomic arrangements and lattice constants enable epitaxial growth on each other, yielding different epitaxial heterostructures and various accompanying emergent phenomena. The combination of such rich features from complex oxides and the advantages from freestanding membranes requires the fabrication of oxide freestanding membranes, which would bring tremendous opportunities in both fundamental research and device applications.