Crystal Growth and Characterization of the Ultrahigh-TemperatureSubstrate Ta1– x Hf x C0.5

Incorporation of AlyGa1–yN semiconductors into high-power electronics offers efficiency improvements in power transmission, generation, and use, if approaches to eliminate the defects arising from film-lattice mismatch can be established. Here, we report the optical floating zone crystal growth of Ta1–xHfxC0.5 (x = 0.2), a new metallic substrate material family lattice matched to the ultrawide-band gap, Al-rich side (y = 0.91) of the AlyGa1–yN solid solution. Laue diffraction demonstrates large single-crystal domains in the as-grown boule. Single-crystal X-ray diffraction at T = 213 K in conjunction with first-principles calculations shows that the material adopts a layered crystal structure with AA-type stacking of (Ta/Hf)-C-(Ta/Hf) trilayers described in the trigonal space group P3̅m1 (#164), with a = 3. 1168(4) Å, c = 4.9644(4) Å, and β = 120.0°. X-ray photoelectron spectroscopy (XPS) measurements show the Hf:Ta ratio to be close to the nominal value of 0.8:0.2 in the grown crystal. Density Functional Theory calculations reveal that this structure is stabilized by the low energy of carbon-vacancy formation of a hypothetical (Ta/Hf)1C1 anti-NiAs structure type, and imply flexibility in interface structure with an overlayer nitride film. A surface preparation/polishing procedure is developed that reduces root-mean-square (RMS) surface roughness from as-cut 130 to 7 nm as measured by atomic force microscopy. Scanning electron microscopy shows the presence of a native surface oxide, removed by polishing, along with carbon-rich pits. Time-domain thermoreflectance measurements show a room temperature thermal conductivity of κ = 18.1(4) W m–1 K–1. These results provide key first steps for utilizing metallic, lattice-matched substrates for the growth of Al-rich AlyGa1–yN semiconductors.