Propelled by the development of material synthesis, the evolution of single crystal materials kept creating transformative applications and providing platforms for novel fundamental studies in the past several decades - representative examples include the production of silicon single crystals for modern electronics and epitaxial III-V semiconductor films for light-emitting devices. In recent years, there was a trend of releasing single crystal films, such as semiconductor nanomembranes and two-dimensional van der Waals materials, from their substrates or bulk materials to yield a new type of single crystal material, freestanding membranes. Due to their low bending stiffness, such membranes become candidates of flexible electronics for biological, tissue-compliant applications. For some materials such as Si membranes, the reduced thicknesses lowers the time of biodegradation, enabling environmental-friendly devices as well as bioresorbable medical devices. Pick-and-place techniques also allow van der Waals stacking of membranes with different crystal structures. However, such opportunity of transforming films on substrates to freestanding membranes were mostly limited to materials mentioned above.
Recently, we have accomplished the fabrication of single crystal freestanding membranes of another material family - perovskite oxides.[1] Perovskite oxides possess exotic physical properties such as ferroic behavior, high-Tc superconductivity and metal-insulator transitions that are potentially suitable for many novel applications including non-volatile memories and high performance transistors. By subsequently growing an epitaxial sacrificial layer Sr3Al2O6 and the desired oxide film on a single crystal substrate followed by the etching of the sacrificial layer using room temperature water (an etchant benign to most of the oxides), the top film can be released from the substrate. Such process produces high-quality single crystal membranes and heterostructures of designed composition and vertical structure, with an extreme aspect ratio up to 1,000,000 - mm-scale lateral dimension and nm-scale vertical dimension. As an example of the applications, the fabrication of ultrathin (~ 16 nm) oxide ferroelectric tunnel junctions (FTJs) on silicon by membrane pick-and-place has been demonstrated.[2] Epitaxial oxide FTJs are memory devices featuring nondestructive readout and scalability that can exceed current commercial high-speed, nonvolatile ferroelectric memories, yet their fabrication is difficult to adapt to silicon substrates for integration into complementary metal-oxide-semiconductor electronics due to the existence of their oxide substrates. The performance of the transferred FTJ memory devices on Si is comparable to epitaxial devices on the oxide substrate, suggesting a viable route toward next-generation nonvolatile memories.
ZnO-based biodegradable light emitting diodes (LEDs) has also been achieved as a first step towards the applications of non-perovskite oxide freestanding heterostructures.[3] When exposed to aqueous conditions, ZnO and other components in the LED vanish into compounds benign to the environment or living organisms, paving the way for future “green” consumer devices and temporary biomedical implants for various light-based diagnostic and therapeutic applications.
[1] D. Lu et al., Nature Mater. 15, 1255 (2016).
[2] D. Lu et al., Nano Lett. 19, 3999 (2019).
[3] D. Lu et al., Adv. Mater. DOI: 10.1002/adma.201902739 (2019).
