Water-soluble hexagonal BaAl₂O₄ as sacrificial layer for freestanding crystalline membranes and flexible devices

Freestanding functional membranes open a promising avenue to the fabrication of flexible electronic devices. To date, research has mainly focused on perovskite-like oxides with pseudocubic structures. Investigation of freestanding hexagonal oxide materials is severely restricted due to the lack of a proper sacrificial layer. Here we present our discovery of water-soluble crystalline hexagonal BaAl₂O₄, which can serve as an excellent sacrificial layer for obtaining membranes with six-fold or three-fold symmetry. Remarkably, BaAl₂O₄ can rapidly dissolve in water (<1 min), but is stable in air, O₂ and NH₃, even at very high temperatures, thus allowing in situ or ex situ growth of high-quality materials for integrated devices. To demonstrate the generic nature of this sacrificial layer, we tested a large collection of oxide and nitride films, including YMnO₃ (0001), LiCoO₂ (0001), α-Fe₂O₃ (0001), In₂O₃ (111), NiO (111), β-Ga₂O₃ ( $\overline{2}01$ ) and TiN (111). Furthermore, integrated devices based on such crystalline membranes demonstrate a substantially improved performance.