Dynamic properties of THz-phonons in semiconductor nano-structures

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The recent development in fabricating small structures makes it possible to control phononic properties by utilizing these structures of a nanometer scale. Thus the dynamic properties of phonons in nano-structures in a THz-frequency range is very important topic to be studied urgently, Experimentally, the picosecond light pulse method to generate and detect acoustic phonons in the 0.1 to 1 THz range (called as picosecond ultrasonics) has been developed. This method is specially powerful in studying the acoustic properties of thin films and multilayered elastic structures for a wide range of temperatures . However, theoretical studies on the new, interesting topics found by the picosecond ultrasonics are very few. The purpose of the present project is to elucidate theoretically, [I] anomalous reduction of vertical thermal conductivity in GaAS/AlAS superlattices measured by the picosecond ultrasonics and [II] the resonant interaction of the extended and surface localized phonons in multi … More layered periodic structures, which has been expected to be observed in the picosecond ultrasonics. The published work supported by this grant has been listed in a separate sheet. Here we briefly summarize the important results we obtained: (1) Different from the scattering in bulk materials the isotope scattering of phonons is anisotropic and mode dependent. We found that the longitudinal (L) phonons are scattered much weaker than the transverse phonons in superlattices for wide ranges of frequencies. Thus, the L phonons play important role in the heat transport in superlattices. (2) The reduction of thermal conductivity in superlattice is predicted due to the zone-folding and associated flattening of phonon dispersion curves near the mini-Brillouin zone center and boundary. However, the effects are smaller than those measured for GaAS/AlAS samples. This suggests that anharmonic mini-Umklapp scattering on phonons should be the main source for the measured large reduction of the vertical thermal conductivity in superlattices. (3) The transmission and reflection of a rectangular phonon packet for the GaAs-(GaAs/AlAsィイD2NィエD2-HィイD22ィエD2O and Al-(Al/polymer)ィイD2NィエD2-HィイD22ィエD2O systems are studied. A prominent feature predicted is the double-peak structure of the reflected packet. The physical origin is the resonance of the incident packet with the localized vibrational mode at the surface of the superlattice. Less