Molecular-beam epitaxy of ultrathin Si films on sapphire

Molecular-beam (MBE) epitaxy of silicon on sapphire (1 (1) over bar 02) was studied by growing films from 30 nm up to 1 mkm thick in ultra-high vacuum (10(-7) Torr). The substrate temperature during deposition was 450-750 degrees C. Before deposition the high-temerature (similar to 1400 degrees C) substrate annealing procedure was performed. The growth rate was similar to 2.5-5 angstrom/s. Surface morphology was studied by means of atomic-force microscopy (AFM), structure of the films was controlled by reflection of high energy electrons (RHEED). Formation of the three-dimensional islands (clusters) of two types (square shape and hemispherical) was observed. The "square" clusters appeared mostly under low growth temperatures (450-600 degrees C). The hemispherical clusters had the larger sizes and were observed under long deposition times and high growth temperatures. The minimal thickness of continuous Si film was about 32 with 2.0 nm Surface roughness. The electron diffraction patterns contained spots, what proved the single-crystal structure of the Si layer.