POLYTYPES AND CRYSTALLINITY OF ULTRATHIN EPITAXIAL-FILMS OF LAYERED MATERIALS STUDIED WITH GRAZING-INCIDENCE X-RAY-DIFFRACTION

The structure of ultrathin epitaxial films of layered NbSe2 and TaSe2 grown on Se-terminated GaAs((111) over bar) substrates was determined by grazing incidence X-ray diffraction. It was found that the crystall ographic polytypes of the films were dependent on the growth temperatu res. The temperature range for the growth of octahedrally coordinated TaSe2 was different from that of the bulk. Disappearance of 3R-type po rtion in NbSe2 at a high growth temperature will be favorable to fabri cate superconducting ultrathin epitaxial films.