Scandium nitride (ScN), a transition-metal nitride with a rocksalt structure, has recently attracted attention as a promising material for thermoelectric applications due to its combination of high electrical conductivity and the potential to reduce thermal conductivity (TC) in nanostructured forms ^1,2,3. Controlling thermal transport in ScN is critical for improving its thermoelectric performance, as bulk and pure ScN exhibit intrinsically high TC, which limits the thermoelectric properties and overall efficiency. Thin-film configurations, however, enable suppression of phonon transport through enhanced boundary scattering⁵. Furthermore, thin-film alloys such as ScₓCr₁₋ₓN (0 < x < 1) can exhibit further reductions in TC due to enhanced phonon scattering from alloy-induced mass and strain disorder and can be considered as an additional approach to modify thermal transport properties⁶. In this study, we investigated the thickness, temperature, and alloy composition dependence of the TC of ScN and ScₓCr₁₋ₓN thin films. The ScN samples had nominal thicknesses of 85 nm, 200 nm, and 500 nm, and were grown on Al₂O₃(0001) substrates using plasma-assisted molecular beam epitaxy (PAMBE)¹. TC was measured using a home-built two-color time-domain thermoreflectance (TDTR) setup. At room temperature, a clear thickness dependence was observed, with the 500 nm film exhibiting 24.6 W/mK, the 200 nm film 15.8 W/mK, and the 85 nm film 12.3 W/mK. Temperature-dependent measurements indicated an Umklapp peak in TC at approximately 150 K for the 500 nm film (34.7W/mK) and 200 K for the 200 nm film (23.1 W/mK). This upward shift in peak temperature with decreasing thickness reflects the dominance of phonon-boundary scattering, as the phonon mean free path (MFP) approaches the film thickness. Further reducing the temperature results in a decrease in TC due to the diminished phonon population, which limits the number of effective heat carriers. Additionally, we measured a series of eight ScₓCr₁₋ₓN alloy thin films with a thickness of approximately 80 nm and compositions ranging from pure CrN to pure ScN. The results showed a minimum in TC at around 45% Cr and 55% Sc, which was notably lower than the values measured for both pure CrN and pure ScN thin films. This minimum is consistent with maximum phonon scattering caused by alloy disorder, where mass and strain field fluctuations disrupt phonon propagation most effectively at intermediate compositions⁷.
References

1. Dinh D. V. et al. DOI: 10.1063/5.0164058
2. Kerdsongpanya S. et al. DOI: 10.1063/1.3665945
3. Rao D. et al. DOI: 10.1063/5.0004761
4. Saha B. et al. DOI: 10.1063/1.3291117
5. Minnich A. J. DOI: 10.1063/1.4935160
6. Kerdsongpanya S. et al. DOI: 10.1063/1.4968570
7. Gurunathan R. et al. DOI: 10.1039/C9MH01990A