Two-dimensional SnO ultrathin epitaxial films: Pulsed laser deposition growth and quantum confinement effects

Two-dimensional (2D) ultrathin oxides are crucial for modern quantum electronic devices. However, the well -controlled growth of 2D oxide materials is highly challenging. In this work, ultrathin single-crystal SnO epitaxial films were achieved on r-sapphire by pulsed laser deposition. The quasi-2D SnO films have thicknesses ranging from 3.4 to 25.4 nm, allowing observation of quantum confinement effects in the optical bandgap. The critical thickness where the films relax in plane is around 19 nm. Below 19 nm, the films are strained compressively in plane and tensilely along the c-axis. Quantum confinement effects are observed by UV-IR spectroscopy for the optical transition at the direct bandgap. By fitting Brus equation, the reduced effective mass is deduced to be 0.137 +/- 0.016 me. The exciton radius and binding energy are 4.62 +/- 0.61 nm and 10.8 +/- 1.2 meV, respectively. These parameters are crucial for the design and application of SnO-based quantum devices.