Thick diamond layers angled by polishing to reveal defect and impurity depth profiles

Diamond layers were grown top a thickness of 25 to 35 μm either on type-Ib synthetic or type-IIa natural diamond substrates by using high or low microwave-power densities. To evaluate defects and impurities depth profiles, the samples were angled by polishing and characterized by cathodoluminescence. The first important finding is that boron, nitrogen and structural defects seem to be concentrated in a 7–10-μm-thick region near the substrate/layer interface, as evidenced by strong defect or impurity-related luminescences and a weak free-exciton peak. The diamond quality is thus much improved in the bulk or near the surface, especially when high microwave-power densities are used, due to the inhibition of unepitaxial crystals formation. The second important finding is that using type-IIa natural substrates, which contain extremely low levels of impurities, results in a strong decrease in the amount of boron and the intensity of the nitrogen-related peak both in the bulk and at the interface of the CVD layer. These results suggest that most impurities in our CVD layers could originate from the substrate itself.