Optical study of defects in thick undoped CVD synthetic diamond layers

The growth of thick single crystal synthetic diamonds by plasma-assisted chemical vapour deposition (PACVD) that are colourless and with a low impurity content is an important challenge to achieve gem-quality material. To this aim, advanced optical imaging and spectroscopy techniques are useful tools to optimize the growth process as well as to identify CVD-made diamond gems. In this paper, two thick synthetic diamond crystals with unique structural and spectroscopic properties were grown by PACVD without intentional addition of nitrogen and without any post-treatment to enhance their colour. While the first one, sample A, was 760 mu m thick and exhibited a grey to greyish brownish colour quite unusual for undoped material, the second one, sample B, was exceptionally thick (3300 mu m) and colourless. It was laser-cut and polished to obtain a high gem-quality round brilliant of 0.44 carat with G colour and VVS2 clarity grade. The amount, distribution and nature of defects in these synthetic crystals is discussed using a broad range of optical characterisation techniques towards growth optimization and getting a better understanding of the material properties (and their possible integration to the gem market). (C) 2014 Elsevier B.V. All rights reserved.