Abstract
Polycrystalline CVD diamond has been annealed in vacuum at temperatures t~an~ = 1550 Β°Cβ1690 Β°C to investigate an influence of modifications of grain boundaries (GB) and defects on thermal conductivity and optical properties of originally optical quality material. While the sample was converted by this heat treatment to βblack diamondβ, excitons still could be observed in cathodoluminescence spectra. Photoluminescence spectra revealed an aggregation of nitrogen (formation of H3 optical center) after annealing at 1690 Β°C. Inβplane thermal conductivity k (T) measured by a heated bar method in the temperature range T = 5β410 K shows a strong (up to 5 times) and monotonic decrease at T < 45 K upon annealing, while room temperature k (298 K) value remains almost constant (ca. 19.0 W/cm K) till t~an~ = 1650 Β°C, rapidly degrading at higher t~an~. The experimental data on thermal conductivity are analyzed in the framework of full Callaway theory taking into account threeβphonon scattering processes, phonon scattering on point defects, sample boundaries, and GB. The proposed model reproduces well the observed k (T) behavior, the phonon scattering on the modified GB playing an important role. (Β© 2008 WILEYβVCH Verlag GmbH & Co. KGaA, Weinheim)