Heat-Transport Mechanisms in Superlattices

The heat transport mechanisms in superlattices are identified from the cross-plane thermal conductivity Lambda of (AlN)(x)-(GaN)(y) superlattices measured by time-domain thermoreflectance. For (AlN)(4.1) (nm)-(GaN)(55) (nm) superlattices grown under different conditions, A 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 Lambda, Lambda of (AlN)(4) (nm)-(GaN)(y) superlattices decreases monotonically as y decreases, Lambda = 6.35 W m(-1) K-1 at y = 2.2 nm, 35 times smaller than A of bulk GaN. For long-period superlattices (y > 40 nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G approximate to 620 MW m(-2) K-1. For y < 40 nm, the apparent value of G increases with decreasing y, reaching G approximate to 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 x 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 A.