MBE-grown tetragonal FeTe consisting of c-axis-aligned nanocrystals

Tetragonal FeTe grown on c-plane sapphire by the molecular beam epitaxy technique is found to result in a new structural phase consisting of c-axis-aligned nanocrystals. Their reflection high-energy electron diffraction patterns display two sets of streaks simultaneously at all rotation angles of the sample. High-resolution x-ray diffraction studies confirm that the nanocrystals are tetragonal FeTe with their c-axes aligned to the growth direction. Atomic force microscopy imaging reveals that further growth of these nanocrystals involves a cannibalism process resulting in nanocrystal pillars with sizes of about 0.5–1 µm. The temperature-dependent resistance of these thin films displays an overall semiconducting behavior, however, with a non-measurable state or jumps and falls depending on their nominal thickness, which can be attributed to the thermal responses of the nanocrystals during cooling and heating processes. This discovery provides an approach to form inhomogeneous heterostructures with all possible twisted angles.