A characteristic feature of crystalline thin-film growth of Si on a 7x7 superlattice surface of Si(111)

A Si film has been grown on a 7 x 7 superlattice surface of a Si(111) substrate at 250 and 380 degrees C. Reflection high-energy electron diffraction patterns of the Si film at 250 degrees C consist of a diffuse 2 x 2 structure and a weak 7 x 7 structure at 380 degrees C. The experimental results indicate that the initial growth of crystalline nuclei is caused by the crystallization of amorphous-like islands at 250 degrees C and by the formation of an island composed of three 5 x 5 structural units with a corner hole at their center at 380 degrees C. The formation of crystalline nuclei is discussed.