Heat‐Transport Mechanisms in Superlattices

Abstract

The heat transport mechanisms in superlattices are identified from the cross‐plane thermal conductivity Λ of (AlN) x –(GaN) y superlattices measured by time‐domain thermoreflectance. For (AlN) 4.1 nm –(GaN) 55 nm superlattices grown under different conditions, Λ varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest Λ, Λ of (AlN) 4 nm –(GaN) y superlattices decreases monotonically as y decreases, Λ = 6.35 W m −1 K −1 at y = 2.2 nm, 35 times smaller than Λ of bulk GaN. For long‐period superlattices ( y > 40 nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y , G ≈ 620 MW m −2 K −1 . For y < 40 nm, the apparent value of G increases with decreasing y , reaching G ≈ 2 GW m −2 K −1 at y < 3 nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN) 4.1 nm –(GaN) 4.9 nm superlattice irradiated by 2.3 MeV Ar ions to a dose of 2 × 10 14 ions cm −2 is reduced by <35%, suggesting that heat transport in these short‐period superlattices is dominated by long‐wavelength acoustic phonons. Calculations using a Debye‐Callaway model and the assumption of a boundary scattering rate that varies with phonon‐wavelength successfully capture the temperature, period, and ion‐dose dependence of Λ.