Phonon scattering in chemical-vapor-deposited diamond

TLDR

The study employs the Callaway model to analyze thermal conductivity in CVD diamond samples that vary in defect density, grain size, and thickness, incorporating normal three‑phonon scattering processes. Measured in‑plane thermal conductivity of CVD diamond (5–400 K) reaches a record 20 W cm⁻¹ K⁻¹ at room temperature, with higher‑conductivity samples exhibiting weak long‑wavelength phonon scattering at grain boundaries, while overall conductivity is limited by point and extended defects, grain boundaries, microcracks, and phonon‑phonon scattering, and anisotropy is higher along columnar grains, indicating that growth conditions rather than CVD type dictate performance.

Abstract

The in-plane thermal conductivity ${\mathrm{\ensuremath{\kappa}}}_{\mathrm{\ensuremath{\parallel}}}$ has been measured over the temperature range 5--400 K for samples of chemical-vapor-deposited (CVD) diamond made by both the microwave and hot-filament processes. The samples span a range of defect level, grain size, and degree of thinning. Comparison with a model of heat transport suggests that ${\mathrm{\ensuremath{\kappa}}}_{\mathrm{\ensuremath{\parallel}}}$ is limited by scattering of phonons from point defects, extended defects of \ensuremath{\sim}1.5 nm diameter, dislocations, grain boundaries, and microcracks, as well as by phonon-phonon scattering at high temperatures. The Callaway model of thermal conductivity is used to include the effects of normal three-phonon scattering processes. In the higher-conductivity samples, scattering of long-wavelength phonons is very weak even at grain boundaries, indicating relatively smooth boundaries. The value of ${\mathrm{\ensuremath{\kappa}}}_{\mathrm{\ensuremath{\parallel}}}$=20 W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ at room temperature for some of the microwave CVD samples is the highest reported to date for CVD diamond. Measurements of the anisotropy in conductivity obtained from the measured perpendicular conductivity ${\mathrm{\ensuremath{\kappa}}}_{\mathrm{\ensuremath{\perp}}}$ consistently show a higher conductivity along the (columnar) grains. The hot-filament-CVD sample measured exhibits a room-temperature conductivity approaching that of the best microwave-plasma samples, indicating that the thermal conductivity is determined more by the specific conditions of growth than by the type of CVD growth (microwave or hot filament).