FeTe/FeSe Superlattices: Epitaxial Growth and Transport Properties

Tetragonal FeSe is a well-known superconductor with a transition temperature T_C of 8 K in bulk form and shows a huge enhancement in T_C up to 65 K by reducing thickness to the monolayer level. Whereas β-phase FeTe, a parent compound of FeSe, does not exhibit superconductivity at either ambient or high hydrostatic pressure. According to current findings, non-superconducting FeTe might become superconducting via alloying with FeSe, annealing in oxygen, and/or heterostructuring with topological insulators. However, till now, the maximum T_C values of FeTe films have been so far from room temperature. In this work, the concept of interface superconductivity and superconducting proximity of FeSe on FeTe layers are included to design FeTe/FeSe superlattice system. [(FeTe)_m/(FeSe)_n]_N superlattices with varying thicknesses down to sub-unit cell of each constituent layer were successfully grown on As-terminated GaAs (111) substrates with 300-nm-thick CdTe buffer layers via molecular beam epitaxy. The observation of streaky reflection high energy electron diffraction patterns indicates the layer-by-layer epitaxial growth mode. X-ray diffraction patterns show clear satellite peaks demonstrating the periodic stacking structures of FeSe and FeTe. Interestingly, the superlattice films exhibit the onset superconducting transition temperatures around 15 K. Our results offer a possible way to tune the superconducting properties of Fe(Se,Te) thin films.