Phonon scattering in diamond films

Abstract

Abstract We have studied the thermal conductivity κ of free standing diamond films with different quality and sizes of the crystallines in bulk. The measurements were performed in the temperature range 10–350 K by steady-state heat flux technique. We have estimated the value and temperature dependence of the phonon mean free path lp using the simple gas kinetic equation for thermal conductivity: κ≈1/3 l p C p v. The values of heat capacity Cp and mean sound velocity v are known from literature for massive crystals. At high (room) temperatures the mean free path of phonons is limited by the phonon–phonon Umklapp interaction and phonon scattering on point defect in bulk. Both these processes lead to increasing of lp(T) with cooling the sample but they have different temperature dependencies. So from lp(T) dependence one can judge the quality (purity) of the sample. At low temperatures the phonon-boundary scattering dominates and lp is limited by the crystalline sizes. The mean free path of phonons in the best of samples has reached the value of a few microns at temperatures 20–70 K, which agrees with the middle sizes of crystallites estimated from X-ray measurements and from microphotos. At lower temperatures the value of lp is increasing again. It can be attributed to the phonon penetration through the boundaries between crystallines. The temperature dependence of λ in this process follows l(T)∼T−(1−2).