Yu. V. Babich

Distribution of N+ centers in synthetic diamond single crystals

The distribution of N+ centers in synthetic diamond grown by the temperature-gradient method in a nickel-containing metal-carbon system has been studied using FTIR mapping. The results clearly demonstrate that the distribution has a complex nature and is directly related to the sectorial structure of the crystal and linear growth rate, whose effects are analyzed with consideration for literature data. The aggregation rate of N+ single nitrogen is slower than that of neutral substitutional nitrogen (C centers).

Distribution of nitrogen-related defects in diamond single crystals grown under nonisothermal conditions

Experimental data are presented on the distribution of nitrogen-related defects in synthetic diamond single crystals grown by the temperature-gradient method in the Fe-Ni-C system at p = 6.0 GPa and a temperature varied stepwise in the range 1370 to 1550°C. The sectorial and zonal structures of plates cut from the crystals are investigated, and the concentration profiles of C, A, and N+ centers are obtained using Fourier transform IR spectroscopy. The zonal and sectorial variations in the concentrations of nitrogen centers and total nitrogen are analyzed. The total nitrogen concentration in the structure of diamond is shown to increase by 50–60 ppm as the temperature is lowered by 100°C. The conclusion is drawn that, at a constant composition of the system, the temperature (as well as its variation with time) plays a key role in determining the rate of nitrogen incorporation into the structure of diamond and subsequent transformations of nitrogen-related defects.

Nitrogen Incorporation in Octahedral Diamonds Grown in the Fe-Ni-C System

The extension of the experimental database on the formation and transformation of nitrogen defects in diamond [1–7] provides a basis for the interpretation of the real features of this widespread structural impurity in natural diamonds. At the same time, a number of questions about nitrogen and conditions of crystal growth remain poorly understood even in simple model systems. In particular, the influence of growth rate, which depends on supersaturation, on the intensity of incorporation of single nitrogen atoms in octahedral diamonds synthesized in the Fe–Ni–C system is not still determined. Based on the analysis of differences in the content of nitrogen defects in various growth sec tors, Satoh et al. [3] suggested that low growth rates are favorable for the entrapment of nitrogen. Kiflawi et al. [8] performed IR spectroscopy and also observed a decrease in nitrogen content in response to a short term increase in growth rate, which was interpreted as the result of the mutual influence of admixtures (N and Ni) on the growth surface. On the other hand, a comparison of the total contents of nitrogen defects in adjoining octahedral growth sectors formed at different growth rates did not reveal any clear tendency [9]. Additional capability for the elucidation of this problem was pro vided by the detection of positively charged single sub stitutional nitrogen N+ in the diamond structure [10] and information that nitrogen in such a state can be used as a peculiar qualitative indicator of supersatura tion and, correspondingly, growth rate [11]. This is pos sible, because part of nitrogen transformed into N+ serves as a charge compensator for substitutional nickel in the diamond structure [12, 13], the intensity of entrapment of which is known to depend on growth rate [8, 14, 15]. It is obvious that, in the case of the existence of any relation between the intensity of nitrogen incor poration in the diamond structure and the growth rate, a similar (positive or negative) relation must be observed between bulk nitrogen and N+ contents. Therefore, in order to establish the relation between the incorpora tion of single nitrogen and growth rate, we used IR microspectroscopy to evaluate the correlation between total nitrogen and N+ contents for synthetic diamonds grown in the nickel bearing metal–carbon (Fe–Ni–C) system.

Modeling of theA-Defect distribution in diamond crystals grown by the temperature-gradient method

Based on the available data on the kinetics ofC-to-A nitrogen aggregation, theA-defect profile is calculated for a particular growth sector in diamond crystals grown by the temperature-gradient method. The effects of the growth duration and temperature on the contents ofC andA nitrogen are examined. The formation ofA defects from single substitutional nitrogen is shown to occur even under typical growth conditions, at temperatures of 1300-1500°C, and to have a significant effect on the structural perfection of diamond.

9.3: Sub-bandgap photoemission correlated with adsorbed molecules and bulk impurities in diamond

We report on photoemission images from NEA diamond using sub-bandgap and near-bandgap photon energies. Emission can be stimulated with photon wavelengths as low as 365 nm, but the low-energy emission disappears after heating the diamond to 700°C, while emission stimulated with 254 and 224 nm photons remains. In both cases the emission occurs only where the concentration of bulk point defects is low. We explain this behavior as the result of photon absorption both in the bulk and at the surface competing with bulk recombination.

Growth Rate Effect on Nitrogen Aggregation in Synthetic Diamonds: Analysis of C- and A-Defect Distributions

The distribution of C and A defects in the octahedral growth sectors of a diamond crystal grown by the temperature-gradient method at 6.0 GPa and 1470–1495°C in the Fe–Ni–C system was studied by Fourier transform IR spectroscopy. The use of the induced-zonality technique made it possible to determine the postgrowth annealing duration, evaluate the rate constants K of nitrogen aggregation in the framework of second-order kinetics, and to relate them to the growth rate vg at a particular point. The results demonstrate that the aggregation rate varies widely over the octahedral growth sector and that K varies nonlinearly with vg . The more rapid nitrogen aggregation at higher growth rates is tentatively attributed to an increase in nickel concentration in diamond.

Linear growth rate and sectorial growth dynamics of diamond crystals grown by the temperature-gradient techniques (Fe–Ni–C system)

The paper reports data on the linear growth rates of synthetic diamond single crystals grown at high P–T parameters by the temperature-gradient technique in the Fe–Ni–C system. Techniques of stepwise temperature changes and generation of growth microzoning were applied to evaluate the growth rates of various octahedral and cubic growth sectors and variations in these rates with growth time. The maximum linear growth rates of the order of 100–300 µm/h were detected at the initial activation of crystal growth, after which the growth rates nonlinearly decreased throughout the whole growth time to 5–20 µm/h. The fact that the linear growth rates can broadly vary indicates that the inner structure and growth dynamics of single diamond crystals grown by the temperature-gradient technique should be taken into account when applied in mineral–geochemical studies (capture of inclusions, accommodation of admixture components, changes of the defective structure, etc.).

Manifestation of nitrogen interstitials in synthetic diamonds obtained using a temperature gradient technique (Fe–Ni–C system)

The IR-peak 1450 cm–1 (H1a-center) associated with nitrogen interstitials have been studied in nitrogen-bearing diamonds synthesized at high P-T parameters in the Fe–Ni–C system. FTIR study shows that manifestation of this nitrogen form is restricted to the regions of active transformation of C-defects into A-defects, which confirms the connection of its formation with C => A aggregation process. An examination of the dependence of the 1450 cm–1 peak on the degree of nitrogen aggregation indicates that H1a-centers are not only formed during C/A aggregation but also disappear simultaneously with the end of C => A transformation. Established facts suggest direct involving of nitrogen as interstitials in the C => A aggregation and serve as strong experimental argument in support of the “interstitial” mechanism of nitrogen migration during aggregation in diamonds containing transition metals.