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Stannous oxide (SnO) and stannic oxide (SnO2) are both important wide-band-gap semiconductors. To study the conductive mechanism in detail, high quality epitaxial films are essential. Here we propose a simple method to grow high quality epitaxial films of either stannous oxide or stannic oxide on an r-plane sapphire substrate by using pulsed laser deposition with a metallic tin target. The valence state of tin is controlled by tuning the oxygen pressure during the deposition procedure. Metal tin impurities and the transition phase of Sn3O4 are avoided and the growth windows from stannous oxide to stannic oxide are confirmed. For single-crystalline SnO epitaxial film, a rocking curve half-width of 0.22° is obtained, which is better than the 0.46° of that on a YSZ substrate. The minimum roughnesses achieved were 0.37 nm for the SnO2 epitaxial film and 0.84 nm for the SnO epitaxial film. The epitaxial relationship between SnO and the r-sapphire substrate was determined to be SnO [ 1 ¯ 1 0 ] //sapphire [ 1 ¯ 2 1 ¯ 0 ] and SnO [1 1 0]//sapphire [ 1 0 1 ¯ 1 ] . For SnO2, the epitaxial relation is SnO2 [0 1 0]//sapphire [ 1 ¯ 2 1 ¯ 0 ] and SnO2 [ 1 ¯ 0 1 ] //sapphire [ 1 0 1 ¯ 1 ] . The band schematics, deduced by combined XPS valence band spectroscopy and optical transmittance spectroscopy, indicated p-type bands of SnO and n-type bands of SnO2. Raman spectroscopy is suggested to be a superior fingerprint of SnO single-crystalline quality, due to its greater sensitivity than X-ray diffraction.

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.