Phonon propagation in isotopic diamond superlattices

Abstract

The out-of-plane thermal conductivity and elastic constant of epitaxial [100] 12 C/ 13 C superlattice diamonds with layer thicknesses of 1, 30, and 100 nm are evaluated by picosecond ultrasound spectroscopy. The measured elastic constants of the superlattices are equivalent to those of single-layer diamond thin films. This result confirms our success in synthesizing superlattice specimens with few defects at the interfaces. Therefore, the phonon transport behavior is governed by the mass difference, not the interfacial defects. The measured thermal conductivity of the superlattices is lower than that of a pure 12 C isotope diamond thin film. We estimated the lattice thermal conductivity using the lattice dynamics calculation, attributing the lowered thermal conductivity to the decrease in the phonon group velocity in superlattices. We further consider the effect of mini-umklapp scattering in the superlattice, which explains the dependence of the thermal conductivity on the layer thickness. We reveal that the mini-umklapp scattering effect becomes significant only for an isotope diamond superlattice because of the high Debye temperature and large relative mass difference.

Lattice thermal conductivity in isotope diamond asymmetric superlattices

“…20) We have previously studied the behavior of phonon propagation in isotopic diamond superlattices by picosecond ultrasonic spectroscopy and lattice dynamics calculations for symmetric superlattices with the same layer thickness of 12 C and 13 C diamonds. 21,22) Although a significant reduction in thermal conductivity was achieved, it was difficult to control the thermal conductivity more precisely in a symmetric superlattice. In this study, we systematically study the heat conduction in isotope diamond superlattices with various lattice periods, including asymmetric structures, using a lattice thermal conductivity calculation, and discuss the controllability of heat conduction in detail.…”

“…The out-of-plane group velocity v λ,z can be obtained by lattice dynamics. In our lattice dynamics model, 22) each atom is bonded by four first-nearest-neighbor atoms and twelve second-nearest-neighbor atoms, with bond-stretching and bond-bending stiffness. These values are determined by the reported experimental data on the phonon dispersion relationship 23) and the measured elasticity 22) via picosecond ultrasound spectroscopy.…”

“…In our lattice dynamics model, 22) each atom is bonded by four first-nearest-neighbor atoms and twelve second-nearest-neighbor atoms, with bond-stretching and bond-bending stiffness. These values are determined by the reported experimental data on the phonon dispersion relationship 23) and the measured elasticity 22) via picosecond ultrasound spectroscopy. [24][25][26][27] The details of determining the bond stiffnesses, the lattice dynamics model, 22) and the experimental system [28][29][30] are shown elsewhere.…”

Suppression of Brillouin oscillation in transparent free-standing diamond thin films in picosecond ultrasound

“…The details of our optics are shown elsewhere. [17][18][19] We prepared a 350-nm graphite thin film and diamond freestanding thin films. The edge of each specimen was supported by a 0.5-mm silicon plate, remaining the free-standing portion.…”

“…20 Concerning the free-standing diamond specimens, we first synthesized the homoepitaxial [100] diamond thin film on a 50-µm thick [100] diamond substrate by the microwave plasma-assisted chemical-vapor-deposition (CVD) method. 19,22,23 The substrate was then removed by gas etching method to obtain the free-standing monocrystal-diamond thin films. Their thicknesses are between 1.7 and 4 µm.…”