On the two-phonon absorption of CVD diamond films

It is well known that the absorption coefficient of diamond in the two-phonon region is constant, for example at 2000 cm− 1, the absorption coefficient is 12.3 cm− 1. This means that the infrared absorbance in the two-phonon region is proportional to the thickness of the samples, which is generally used as standard to normalize the infrared absorption spectra of diamond samples according to their thickness. This is true for natural and HPHT synthetic single crystal diamond. However for polycrystalline or nanocrystalline CVD diamond films, we found that the situation may be different. For high quality thick CVD diamond films of thickness > 150 μm, the infrared absorbance in the two-phonon region is proportional to its thickness. While CVD diamond films of equal thickness but of different quality show variable absorbance in the two-phonon absorption region in terms of thickness. Our investigation on this observation primarily indicates that the grain size of CVD diamond films has influence on the two-phonon absorption. In this work, we present this new result and discuss the mechanism of this phenomenon in the light of the growth mechanism of CVD diamond.