Raman and photoluminescence analysis of CVD diamond films: influence of Si-related luminescence centre on the film detection properties

The performance of detectors based on CVD diamond films strongly depends on the material quality, which, in turn, is extremely sensitive to even slight variations of the deposition conditions. In this work, a careful Raman and photoluminescence (PL) study has been carried out on diamond films grown on Si substrates by microwave plasma CVD under nominally identical deposition conditions, but exhibiting, quite surprisingly, different detection sensitivity. Micro-PL measurements performed on the growth surface and on the cross-section of the samples have shown higher incorporation of Si impurities in films having lower detection performance. Optical microscopy on the nucleation side of such films has evidenced a lower nucleation density, leading to diamond grains of greater size, but also to higher incorporation of Si atoms. Thus, a greater grain size does not necessarily guarantee a higher film quality, if the nucleation density is low. On the basis of these results, the nucleation density is identified as the 'hidden' variable in the growth process of our CVD diamond films, determining, ultimately, their detection sensitivity.