Thermal transport in a semiconductor heterostructure measured by time-resolved x-ray diffraction

We report studies of thermal transport across the interface of a semiconductor heterostructure using x-ray diffraction to measure the time-dependent lattice expansion after ultrafast laser excitation. Femtosecond laser pulses are used to rapidly and locally heat the substrate at the buried interface of an ${\text{Al}}_{0.3}{\text{Ga}}_{0.7}\text{As}/\text{GaAs}$ heterostructure grown by molecular-beam epitaxy. High-resolution time-resolved x-ray diffraction is used to study the heating and cooling of the film and substrate independently. The data are compared with a simple model for the thermal transport incorporated into dynamical diffraction calculations allowing us to extract the room-temperature cross-plane film thermal conductivity. The value is 40% lower than that extrapolated from prior results on liquid-phase epitaxy grown samples of varying concentrations.

Figure 1

(Color online) X-ray rocking curve from an ${\text{Al}}_{0.3}{\text{Ga}}_{0.7}\text{As}/\text{GaAs}$ heterostructure showing a laser-excitation-induced shifts in the angle of the 004 Bragg reflections.

Figure 2

(Color online) Time-resolved Bragg shift for excitation with 0.95 W average power. (a) Propagation of a coherent acoustic phonon pulse from the near instantaneous stress due to photoexcitation. (b) Data showing the heating and cooling of the film and substrate and comparison with simulations. The arrows indicate periodic coherent acoustic echoes.

Figure 3

(Color online) Calculated temperature distribution over the film and the first $10\mu \text{m}$ of the substrate at different times, assuming instantaneous substrate heating across a $20\mu \text{m}$ depth and the best fit parameters at 0.95 W.