Intrinsic to extrinsic phonon lifetime transition in a GaAs–AlAs superlattice

Abstract

We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon-phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the 'interfacial atomic disorder' model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness.

Boundary scattering of phonons: Specularity of a randomly rough surface in the small-perturbation limit

“…On the other hand, modeling interfacial roughness as atomic disorder [26,27], which implies a small correlation length, will typically underestimate boundary scattering. A case in point is the attenuation of sub-THz coherent phonons is GaAs-AlAs superlattices [47]: experimentally measured extrinsic scattering rates (i.e., scattering by interface roughness and defects) were orders of magnitude greater than the atomic disorder model predicted. Incidentally, the experimental scattering rate scaled with frequency as interfaces without much loss at 79 K [47].…”

“…A case in point is the attenuation of sub-THz coherent phonons is GaAs-AlAs superlattices [47]: experimentally measured extrinsic scattering rates (i.e., scattering by interface roughness and defects) were orders of magnitude greater than the atomic disorder model predicted. Incidentally, the experimental scattering rate scaled with frequency as interfaces without much loss at 79 K [47]. An rigorous analysis of phonon scattering by interfacial roughness in a superlattice will require a separate treatment as the problem is different from scattering by a free surface of a bulk material; however, general trends are expected to be similar to the ones discussed here.…”

“… , indicating an intermediate case between the limits of 2  and 4  scaling. It should be noted that scattering of sub-THz phonons by interface roughness in a superlattice is one case where the small perturbation approach would be well justified as losses in a single scattering event are typically small: for example, one can see ~0.3 THz coherent phonon wavepackets cross over 400 interfaces without much loss at 79 K [47]. An rigorous analysis of phonon scattering by interfacial roughness in a superlattice will require a separate treatment as the problem is different from scattering by a free surface of a bulk material; however, general trends are expected to be similar to the ones discussed here.…”

Decoherence in semiconductor cavity QED systems due to phonon couplings

“…This property of the deformation potential interaction also implies a nonbroadened zero phonon line (ZPL) in QD absorption spectra [40]; however it is well known that several mechanisms lead to a finite width of the ZPL, e.g., the coupling to excited QD states [42] and finite phonon lifetimes [49][50][51][52] due to, e.g., surfaces, crystal impurities, or anharmonic interactions leading to decay of one phonon into two of smaller energy. The simplest mechanism to implement in our model, and one that will always be present, is the finite lifetime of the phonons.…”

Two-colour high-speed asynchronous optical sampling based on offset-stabilized Yb:KYW and Ti:sapphire oscillators

“…The fit yields a decay time of (76 ± 1) ps. Recent pump-probe studies on a different GaAs/AlAs superlattice reveiled room temperature decay times of 1020 ps and 300 ps at frequencies of 320 GHz and 640 GHz, respectively [34]. This indicates a strong difference in the extrinsic scattering rates, which are the driving mechanisms for phonon decay at room temperature.…”