Semiconducting Half-Heusler Based Compounds and Heterostructures Grown by Molecular Beam Epitaxy
Heusler compounds are an exciting class of intermetallics due to their diverse electrical and magnetic properties, including semiconducting, half metallic, and topologically insulating behaviors. With a crystal structure and lattice parameters similar to III-V compound semiconductors, the possibility of Heusler/III-V semiconductor as well as Heusler/Heusler heterostructures with unique properties is achievable. However, the integration of epitaxial Heusler compounds into functional devices still faces a number of challenges. The tunability of the electronic and magnetic properties of Heusler compounds must first be better understood. In addition, Heusler-based device demonstrations have been limited to magnetic Heuslers, leaving the semiconducting and topologically non-trivial variants relatively unexplored. Finally, most experimental studies of the semiconducting half-Heusler compounds have been limited to bulk polycrystalline samples, which cannot be easily used for device applications.
Here, the semiconducting half-Heusler, CoTiSb is grown by molecular beam epitaxy and used to explore the both the tunability of Heusler compound properties as well as the interface properties of Heusler alloys with III-V and other Heusler compounds. A semiconductor to metal transition is examined by substitutionally alloying Ni for Co. The evolution of the electronic structure in the alloy Co1-xNixTiSb is examined by electrical transport, Seebeck measurements, and angle-resolved photoemission spectroscopy. A gradual transition from semiconducting to metallic behavior is observed for films with x≥0.1. The effects of Ni alloying as well as the surface reconstruction on the valence band, conduction band, and Fermi level positions are examined.
Next, the introduction of ferromagnetism into CoTiSb is achieved by substitutionally alloying Fe on the Ti site, a predicted half-metallic compound for intermediate levels of alloying. The magnetic, electronic, and nano-structural properties of the epitaxial thin films are shown to depend strongly on Fe composition. In particular, a large dependence of the magnetic moment on the site which Fe atoms occupy is found. In addition, evidence of nano-level phase separation is observed and shows a clear compositional dependence.
Finally, the band alignments between CoTiSb and the III-V compounds InAlAs and InGaAs as well as another semiconducting half-Heusler, NiTiSn, are explored using a combination of X-ray photoemission spectroscopy, density functional theory (DFT), and electrical transport. Here good agreement is found for the valence band offsets between the three techniques. However, a discrepancy between the conduction band offsets predicted by DFT and those inferred from electrical transport is found. An effective reduction in the CoTiSb bandgap is used to explain this discrepancy.
This work ultimately advances the current understanding of the tunability of Heusler compounds and lays the foundation for future Heusler based heterostructure devices.