Epitaxial thin films of the kagome-lattice topological semimetal CoSn grown using an ultrathin Co seed layer

We have fabricated thin films of the kagome-lattice topological semimetal CoSn on Al2O3 (0001) substrates by radio-frequency magnetron sputtering. While the direct growth by simultaneously supplying Co and Sn resulted in CoSn films with weak CoSn 0002 diffraction intensity, the insertion of a Co seed layer as thin as a few unit cells significantly increased the CoSn 000l diffraction intensities. The detailed X-ray diffraction analysis confirmed the single-phase, c-axis oriented epitaxial growth of CoSn. Scanning transmission electron microscopy observation of an annealed CoSn/Co/Al2O3 (0001) structure revealed the absence of the Co seed layer in the film/substrate interface region and the homogeneous compositional distribution, indicating the alloying of CoSn and Co during annealing. By leveraging the virtually single-layer nature of the sample structure, we evaluated the electrical properties in the film plane and found that the resistivity of the a–b plane at 300 K was comparable to those of bulk crystals, where the role of dispersionless flatbands has been discussed. These results provide a pathway to explore the functional properties derived from the topological electronic states of CoSn.