Incoherent phonon transport dominates the out-of-plane thermal conductivity of epitaxial van der Waals Bi2Se3/In2Se3 superlattices

The increase in thermal conductivity, κ, at small period lengths in covalently bonded semiconductor and oxide epitaxial superlattices (SLs) is a hallmark of coherent phonon transport and a route for improving heat dissipation in the thin limit. Here, we show that, on the contrary, the phonon coherence length remains shorter than the minimum SL period in van der Waals (vdW) SLs. We measured the cross-plane κz of epitaxial Bi2Se3/β-In2Se3 vdW SLs grown by molecular beam epitaxy; our results show that κ decreases monotonically with increasing interface density down to the two-layer period limit. This suggests that the weak interface vdW bonding of these SLs results in diffusive phonon transport over the whole period range. We discuss further reduction to single-layer SLs and its effects on crystal and interface quality. The results presented in this paper highlight a constraint for thermal management in vdW-based nanoelectronic and thermoelectric devices.