6.2 The Samples and the Growth Methods
6.5.2 Nitrogen Concentration Changes
The nitrogen concentration, as measured by FTIR, decreases across each sample, as shown in figure 6.18. The sample with the largest concentration of nitrogen is also the one with the largest concentration of 13C; the amorphous 13C may
have a higher nitrogen contamination. However, across each sample the nitrogen concentration decreases whereas the 13C concentration increases.
Figure6.19shows the nitrogen concentration multiplied by the surface area at each point during growth. This figure shows an increase in nitrogen uptake followed by a plateau. This may be the point at which an equilibrium is reached between the melt and the growing diamond.
These results quantified the decrease in nitrogen which was suspected from the colour of the samples. However, even given these high changes in nitrogen concen- tration, they are not large enough to cause a change in the Raman line position. The SIMS results did not agree quantitatively with the concentration of nitrogen across the diamond studied although the results did indicate a fall in concentra- tion. This discrepancy is likely to have arisen from the lack of a reference for measuring the nitrogen count rate. Normally, when SIMS is performed to mea- sure the nitrogen concentration of a diamond, the count rate is compared to that of12C. However this was not possible with these samples as the12C concentration
was also changing. As a result the nitrogen was measured by count rate alone and thus has been over estimated, by approximately 3.5 times.
Additionally other minor problems have previously been identified when measuring nitrogen using SIMS including leaks, contamination and surface impurities [168]. This is usually avoided by comparing the SIMS count rate to that of a diamond with a known nitrogen concentration [169], however this was not done on this occasion.
despite different nitrogen concentrations. The samples with higher13C concentra- tions also had higher nitrogen concentrations which may have originated from13C
source contamination. However, within each sample increasing13C was related to
decreasing nitrogen.
These diamonds show a distinctive increase of 13C moving away from the seed. Within error the Raman results and the SIMS results agree. There is not an in- crease in sample NL636-01, sample NL636-02 increases from 1.5% to 3.5%, sample NL636-03 increases from 4.4% to 5.7% and sample NL636-04 increases from 5.7% to 10.4%. This is indicative of changes in the melt during the HPHT process. An increase in 13C was caused by the amorphous 13C source slowly dissolving. The
steeper gradients of the samples with different 13C enrichment in the source and
solvent also indicated that the carbon source mixes slowly with the solvent during growth.
The main findings of this chapter are:
• It has been demonstrated that a non-uniform13C enrichment is created when
the source of 12C and 13C are in different forms, in this work graphitic and amorphous.
• It has been demonstrated that, due to slow mixing of the carbon source with the solvent, a gradient of 13C enrichment will be created if the source and
solvent are differently 13C enriched.
• An equation has been calculated which links the Raman line position to the
13C enrichment, in the 0% to 10% region, shown below.
13C (%) = 2.61×(1332.9−Raman line position)±0.5 (6.3)
before they are of device quality. Large scale manufacturing of devices requires isotopic uniformity, and the samples investigated here do not display this. Im- provements could be made by ensuring the 12C and 13C source are of the same
form, either graphitic or amorphous. Additionally, the same isotopic enrichment is required for the solvent and the carbon source.
An interesting new way to make single crystal diamond is heteroepitaxial chem- ical vapour deposition (CVD) on a lattice matched silicon substrate. The first CVD growth was onto non-diamond substrates but this created polycrystalline diamond [170]. Heteroepitaxial growth is a technique commonly used in industry for gallium nitride grown on a silicon carbide seed [23]. The huge advantage of the heteroepitaxial method for diamonds is that the substrate used can be larger and significantly less expensive than for traditional homoepitaxial CVD. The main issue with the method is lattice mismatch which may result in highly defective single crystal or polycrystalline diamond.
Many applications of diamond make use of its unique properties including: high thermal conductivity, high carrier mobility and high breakdown field. These prop- erties are all diminished by the grain boundaries present in polycrystalline dia- mond, hence there is a large incentive to make commercially affordable single crystal diamond. A method by which large single crystal diamond could be grown on lattice matched silicon substrates could prove very advantageous in the future and may create many new commercial possibilities.
In this chapter new single crystal heteroepitaxial samples were studied with the aim of identifying the differences between this material and homoepitaxial CVD diamonds. Additionally, the material was investigated to understand the possi- bility of making devices for commercial applications. Due to the different strain
Figure 7.1: Substrate used for heteroepitaxy growth showing lattice constant matching. Starting with silicon, then yttria-stabilised zirconia, then iridium and finally diamond. The mismatch at each layer is 5.39%, 25.4% and 7.19% starting from the bottom. Image from [171].
environment arising from the lattice mismatch, point defects and extended defects may be present in differing quantities to homoepitaxial CVD. Two different sam- ples were studied; one ‘intrinsic’ and one ‘nitrogen doped’. These were investigated with a variety of different techniques, including electron paramagnetic resonance and fluorescence imaging.