| AVS 60th International Symposium and Exhibition | |
| Magnetic Interfaces and Nanostructures | Monday Sessions |
| Session MI-MoM |
| Session: | Topological Materials, Rashba Systems, and Heusler Alloys |
| Presenter: | J.K. Kawasaki, UCSB |
| Authors: | J.K. Kawasaki, UCSB L.M. Johansson, Lund University, Sweden J. Shabani, UCSB A. Rice, UCSB M. Hjort, Lund University, Sweden R. Timm, Lund University, Sweden B.D. Schultz, UCSB T. Balasubramanian, Lund University, Sweden A. Mikkelsen, Lund University, Sweden C.J. Palmstrom, UCSB |
| Correspondent: | Click to Email |
The Half Heuslers are an attractive family of compounds for high temperature thermoelectrics, and recently there has been renewed interest in these compounds since some are proposed to be topological insulators. We report the epitaxial growth, transport, and angle resolved photoemission spectroscopy (ARPES) measurements of epitaxial films of the Half Heusler compounds CoTiSb and NiTiSn. Both belong to the subset of Half Heuslers with 18 valence electrons per formula unit that are predicted to be trivial insulators despite being composed entirely of metallic components. Here the CoTiSb and NiTiSn films were grown by molecular beam epitaxy on lattice matched InAlAs/InP (001) and (111), or on MgO (001), respectively. The films are epitaxial and single crystalline, as measured by reflection high-energy electron diffraction, low energy electron diffraction, and X-ray diffraction. Both CoTiSb and NiTiSn films also show surface reconstructions that vary with anneal temperature.
For the CoTiSb, scanning tunnelling spectroscopy and temperature-dependent transport measurements reveal that the films are insulating, with unintentionally doped carrier concentrations and mobilities comparable to that of highly doped conventional compound semiconductors (n = 1018 cm-3 and μ = 500 cm2/Vs at 300 K). The CoTiSb films also show a peak in the low temperature (1.8-10 K) magnetoresistance that may result from localization or some other mechanism. ARPES measurements reveal that CoTiSb is a bulk insulator but has surface states within the band gap.
Stoichiometric NiTiSn films also show semiconducting-like transport. Additionally, composites of Full Heusler Ni2TiSn inclusions within a Half Heusler NiTiSn matrix have been grown by codeposition with excess Ni. Despite the large lattice mismatch (2.9%) between the Ni2TiSn and NiTiSn, the Ni1+δTiSn films remain epitaxial for compositions in excess of δ > 50%. These Half Heusler / Full Heusler nanocomposites show promise for phonon scattering in thermoelectric applications.
This work was supported by the ARO, ONR, and NSF.