Abstract
Epitaxial growth of the half-Heusler alloy TiNiSn onto (1 0 0)-oriented MgO is compromised by interfacial reactions driven by the oxidising potential of titanium. Here, we demonstrate that a few epitaxial monolayers of elemental vanadium are sufficient to act as an impermeable buffer that maintains epitaxy and stoichiometric thin film growth but suppresses interfacial oxidation of the alloy. Electron diffraction and microscopy are used to characterise the thin film morphologies and thereby determine the optimum deposition conditions. Electron energy loss spectroscopy is used to demonstrate the chemical nature of the resulting thin film interfaces and confirms that TiNiSn film quality is improved.
Highlights
In the effort to produce commercially-viable thermoelectric generators that don’t rely on scarce elements, Heusler alloys have attracted much interest owing to their versatility and capacity for tunability
The sputter deposition rate inevitably reduces with gas pressure, we found that the highest Ar sputtering pressure used, 25 mTorr, produced the smoothest V films on MgO, independent of substrate temperature
Little variation in the V film roughness was expected with temperature [13], AFM measurements of films grown between 298 K and 823 K indicated 623 K to produce the smoothest films and still gave rise to sharp reflection high energy electron diffraction (RHEED) features consistent with epitaxial growth
Summary
In the effort to produce commercially-viable thermoelectric generators that don’t rely on scarce elements, Heusler alloys have attracted much interest owing to their versatility and capacity for tunability. For the half-Heusler (hH) TiNiSn, which is a candidate n-type thermoelectric, its rich compositional phase-space has been exploited to improve thermoelectric properties [1,2,3,4,5] and there is considerable scope for tuning the performance through doping [6,7,8]. We reported on the pulsed laser deposition of TiNiSn upon single-crystalline MgO to produce nearstoichiometric, mono-crystalline, epitaxial films [11]. We observed a deleterious reaction that maintained epitaxy but drew Ti out of the alloy. The subsequent formation of a thin TiO interfacial layer limited the availability of Ti for the intended film and introduced a compositional perturbation that took upwards of 10 nm of further deposition to recover. Some Ti was observed to diffuse several nanometres into the substrate, consistent with facile diffusion by cation substitution mechanisms reported elsewhere [12]
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