Dispersive phonon imaging in III-V semiconductors

Low-temperature transport properties of high-frequency acoustic phonons are investigated in GaAs, InSb, InP and InAs using the phonon-imaging technique. In this method, a focused laser beam provides a movable heat source on one side of a cooled crystal (~ 2 K). A single small phonon detector on the opposite face records the transmitted heat flux as a function of propagation direction. Ballistic phonons channel along directions in the crystal which are completely determined by the detailed shape of constant-energy surfaces in wavevector space. The resulting focusing patterns are characterized by sharp phonon caustics which are clearly identified from the continuous background due to scattered phonons. In the dispersive regime, where phonon wavelength is comparable to atomic spacing, the angular positions of these caustic lines are very sensitive to phonon frequency, thus providing a novel test for lattice dynamics theories. Experiments are performed with superconducting tunnel junctions and Al bolometers to probe both the high-frequency and low-frequency regimes, respectively. We find that large-k ballistic phonons give rise to distinct focusing patterns in all four types of crystals, with thicknesses varying between 0.4 and 0.8 mm. Due to isotope scattering in the bulk, tunnel-junction experiments yield well-defined caustic patterns with a dominant frequency given by the detector gap 2.6.. In InSb, where zone boundary frequencies are small ( VT A "' 1.2 THz), the frequency dependence of the dispersive phonon focusing patterns are Ill measured using PbTl (0.43, 0.59 THz) and PbBi (0.69, 0.73, 0.78, 0.82 THz) tunnel junction detectors. The results are interpreted with Monte Carlo calculations based on rigid, dipole, shell, and bond-charge models. Although each model yields satisfactory fits to the previously measured dispersion curves, the predicted patterns show remarkable differences in the caustic structures. This result underscores the utility of phonon imaging in providing new information about the elastic forces between atoms in crystals.U of I OnlyThesi