Synthesis and transparent conductivity of crack-free La:BaSnO₃ epitaxial flexible sheets

La-doped BaSnO₃ (LBSO), which exhibits both high electron mobility and visible-light transparency, is a promising transparent electrode/transistor material that does not require expensive elements such as indium. However, because a high crystal orientation is necessary for high mobility, the development of a synthetic technique is crucial for next-generation optoelectronic applications. One promising method for achieving this is the lift-off and transfer method. Epitaxial films are first deposited on single-crystal substrates, peeled off from the substrates, and subsequently transferred onto other substrates. However, such transferred sheets typically contain a high density of cracks. Therefore, LBSO sheets with flexibility, high mobility, and transparency have not yet been reported. In this study, we successfully synthesized crack-free LBSO epitaxial sheets via a lift-off and transfer method using a water-soluble Sr₃Al₂O₆ sacrificial layer and amorphous (a-)Al₂O₃ protection layer. The LBSO sheet simultaneously exhibited a high electron mobility of 80 cm² V^-1 s^-1 and a wide optical bandgap of 3.5 eV owing to the epitaxial crystallinity of the sheet. Moreover, two types of LBSO sheets were prepared, flat and rolled sheets, by tuning the lift-off process. The flat sheet had a lateral size of 5 mm × 5 mm, whereas the rolled sheet had a tube shape with a height of 5 mm and a diameter of 1 mm. Such large crack-free area and flexibility were achieved in LBSO sheets owing to the use of the a-Al₂O₃ protection layer.