Phonon scattering in diamond films

We have studied the thermal conductivity re of free standing diamond films with different quality and sizes of the crystallines in bulk. The measureme nts were performed in the temperature range 10-350 K by steady-state heat f lux technique. We have estimated the value and temperature dependence of th e phonon mean free path l(p) using the simple gas kinetic equation for ther mal conductivity: kappa approximate to 1/3 l(p) C(p)upsilon. The values of heat capacity C-p and mean sound velocity are known from literature for mas sive 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 l(p)(T) with cooling the sample but they have different temperature dependencies. So from l(p)(T) de pendence one can judge the quality (purity) of the sample. At low temperatures the phonon-boundary scattering dominates and l(p) is li mited by the crystalline sizes. The mean free path of phonons in the best o f samples has reached the value of a few microns at temperatures 20-70 K, w hich agrees with the middle sizes of crystallites estimated from X-ray meas urements and from microphotos. At lower temperatures the value of l(p) is increasing again. It can be attr ibuted to the phonon penetration through the boundaries between crystalline s. The temperature dependence of %. in this process follows l(T) similar to T-(1-2). (C) 1999 Elsevier Science B.V. All rights reserved.