DISCOVERY AND DISTRIBUTION OF THE [SI-V]- DEFECT IN HPHT-GROWN GEM-QUALITY DIAMONDS

Defect of [Si-V]^- is common in CVD synthetic diamonds, and its occurrence was also reported in some rare natural diamonds (Breeding and Wang, 2008). It is an important feature employed for gem diamond identification, and also has great potential for applications in industry. However little is known about how the silicon impurity gets into diamond lattice either in synthetic or natural diamonds. In this study, we discovered the occurrence of [Si-V]^- in HPHT synthetic diamonds and the correlation between its precipitation and diamond growth sectors was successfully determined.

Total 20 samples, HPHT grown diamond wafers from NDT (New Diamond Technology) were studied in addition to one type IIb HPHT synthetic diamond submitted to GIA Laboratory for grading. Distributions of defects in these samples were carefully mapped using infrared microscopy at room temperature and an imaging Raman microscope at liquid nitrogen temperature.

Defect of [Si-V]^- has doublet emissions at 736.6/736.9 nm (Clark et al., 1995), and can be effectively excited using 633 nm laser. Analyses were conducted at Liquid Nitrogen temperature as the detection of the Si related emissions peak is temperature dependent (Feng and Schwartz 1993). Additionally, the solvent catalysts used in the HPHT methods to grow synthetic diamond either intentionally or unintentionally contain nickel in varying quantities. Nickel impurity creates optical centers which emit a doublet peak at 882.6/884.3nm, and can be easily excited using 780 nm laser.

The [SiV]^- is clearly observed in only certain growth sectors of the synthetic crystal and the distribution is not homogeneous. By comparing the two acquired maps one acquired at 633nm excitation showing the [Si-V]^- distribution and one acquired with 780nm excitation showing the nickel defect distribution, it was found that the [Si-V]^- is confined to the same growth sector as Ni related defect with higher concentrations/intensity at the edges of these sectors. Since it is well known that the Ni defect is confined exclusively to the octahedral growth sectors {111} of diamond (Lawson et al., 1993), this study for the first time confirmed that [Si-V]^- is confined to the {111} octahedral growth sectors.

This new discovery leads to discussion as to the incorporation of silicon in diamond and the relationship to other impurities.