MBE Growth of Tin-Telluride Thin Films on (001) GaAs Substrates

Topological insulators (TIs) have attracted a great deal of interest due to their unique properties. Band inversion occurs at high symmetry points in the Brillouin zone, and it becomes metallic state on the surface, although the inside is an insulator. This specific surface states are called topological surface states (TSSs). Recent research has led to the discovery of a new type of TIs which termed topological crystalline insulators (TCIs). TCIs have been investigated intensively for Tin-based materials. SnSe is known as the orthorhombic GeS type structure that is a thermodynamically stable phase. The lattice parameter of rock-salt SnSe is 4.23Å. It was reported that the rock-salt structure epitaxial SnSe (111) thin films less than 20nm were grown on Bi₂Se₃ substrates at around 300^oC, and its TSSs have been confirmed by angle-resolved photoemission spectroscopy (ARPES). Tin-Telluride (SnTe) is also expected as a material for TCIs. The lattice parameter is 6.303Å. Molecular beam epitaxy (MBE) growth of SnTe has been performed using the compound source, and BaF₂ substrates. TSSs of SnTe was also confirmed by ARPES. Among various TCIs and TSSs materials, SnTe has advantages in MBE growth. The vapor pressure of Sn is lower than that of Bi. The vapor pressure of Te is also lower than that of Se. These facts suggest that the MBE growth is easier than other materials. In this paper, SnTe thin films were prepared by MBE using elemental tin (6N) and tellurium (6N Super) as source materials. The GaAs substrate was used because its crystal structure is one of the cubic crystal systems, and also relatively easy to obtain. The lattice constant of GaAs is 5.65Å., and the lattice mismatch between SnTe and GaAs is 11.6%. The effect of the molecular beam intensity ratio (J_Te/J_Sn) and growth temperature (T_g) on the crystallinity were investigated. After the oxide desorption of the (001) oriented GaAs substrate, SnTe layers were directly grown for 90 min. The molecular beam intensity ratio (J_Te/J_Sn) was varied by changing the molecular beam intensity of Tellurium while fixing that of Tin, and varied from 1.3 to 2.9. Growth temperatures were varied from 225^oC to 240^oC. Structural analysis of the sample was mainly performed by means of the x-ray diffraction (XRD) θ-2θ measurement. Diffraction peaks from the GaAs substrate and the rock-salt type structure SnTe were observed from all samples. A diffraction peak associated with hexagonal crystalline Te was also observed from some samples. The relative diffraction peak intensity of the Te gradually decreased as the molecular beam intensity ratio decreased. Since the substrate temperature was low, excess Te atoms would be easily adsorbed on the growth surface, and incorporated into the film without forming compounds. The diffraction peaks of 002 and 222 SnTe were observed from samples. The relative peak intensity of 222 SnTe was smaller for the sample grown at the lower substrate temperature. It implied that the formation of SnTe (111) domain was suppressed at low temperatures. Transmission electron microscopy images were measured for the substrate/epilayer interfacial region, and it was confirmed that the cubic structure SnTe layers were formed on the GaAs substrate.<br/>This work was supported in part by the Waseda University Grant for Special Research Projects.