Heat-transport mechanisms in superlattices

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-(CaN)_55nm superlattices grown under different conditions, A varies by a factor of two; this is attributed to differences in the roughness of the AlN/CaN 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 A of bulk CaN. For long-period superlattices (y > 40nm), the mean thermal conductance C of AlN/GaN interfaces is independent of y, G ≈ 620 MW m^-2 K ^-1. For y < 40 nm, the apparent value of C increases with decreasing y, reaching C » 2 CW 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-(CaN)_4.9 nm superlattice irradiated by 2.3 MeVAr ions to a dose of 2 × 10¹⁴ 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 A.