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METHOD FOR MAKING FANCY ORANGE COLOURED SINGLE CRYSTAL CVD DIAMOND AND PRODUCT
OBTAINED
This invention relates to a method of making a fancy orange single crystal
diamond material by post-growth treatment of a diamond material that has been
grown by a CVD (chemical vapour deposition) process, and to CVD single crystal
diamond material which is fancy orange in colour.
The term "fancy-coloured diamond" is a well-established gem trade classification
and is used to refer to unusual coloured diamonds. A useful history and
background to the grading of fancy coloured diamond gemstones, including the
use of Munsell colour charts is given by King et al, in Gems & Gemology, Vol. 30,
No. 4, 1994 (pp.220-242).
Examples of fancy coloured synthetic and natural diamonds made by introducing
colour centres into the diamond are known in the prior art. For example,
EP0615954A and EP0316856A describe irradiation of synthetic diamond material
with an electron beam or a neutron beam to form lattice defects (interstitials and
isolated vacancies) in the crystal. Thereafter the diamond crystal is annealed in a
prescribed temperature range to form colour centres. Neither of these
publications discloses orange diamond material.
Another publication describing the formation of fancy coloured diamond material
is "Optical Absorption and Luminescence" by John Walker in "Reports on
Progress in Physics", Volume 42, 1979. That publication similarly describes the
steps of forming lattice defects in crystals by electron beam irradiation, and if
necessary annealing to cause the lattice defects to combine with nitrogen atoms
contained in the crystals. There is no disclosure of orange diamond material in
this publication.
US 2004/0175499 (Twitchen et al) describes a method starting with a coloured
CVD diamond, usually brown or near-brown, and applying a prescribed heat
treatment to produce another and desirable colour in the diamond. The prior art
reference notes that the relative strengths of the absorption bands in the visible
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region of the spectrum of btown single crystal CVD diamond can be altered by
annealing, with concurrent changes in the Raman spectrum, and that changes in
the absorption spectrum are observed at much lower temperatures than are
required to alter the colour of brown natural diamond. Significant colour changes
are said to be achieved by annealing at atmospheric pressure in an inert
atmosphere at temperatures of 1600oc or less. One example describes a grown
CVD diamond polished into a round brilliant of 0.51 carat that was graded as light
brown. After annealing at 1700° C for 24 hours it was graded as light orangish
pink. Another example describes a grown CVD diamond slice which had an
orange brown colour, and after annealing this colour becomes colourless. A
further example describes a grown CVD diamond layer polished into a
rectangular cut gemstone of 1.04 carats which is graded fancy dark orangey
brown colour. After annealing at 160ooc for four hours this becomes a fancy
intense brownish pink colour.
We have found that a fancy orange colour can be introduced into synthetic CVD
diamond material by irradiating synthetic CVD diamond material for a time
sufficient to introduce a specified concentration of isolated vacancies into the
diamond material, and then annealing that isolated-vacancy-containing CVD
diamond material for a sufficiently long time at a low temperature to produce a
fancy orange coloured diamond material. It is thought that during the low
temperature anneal at least some of those isolated vacancies are converted into
vacancy chains in the CVD diamond material and that the vacancy chains are
responsible for the perceived fancy orange colour of the diamond material.
A first aspect of the present invention provides a method of making fancy orange
synthetic CVD diamond material, the method comprising: (i) providing a single
crystal diamond material that has been grown by a CVD process and has a [N5°]
concentration less than 5 ppm; (ii) irradiating the provided CVD diamond material
so as to introduce isolated vacancies V into at least part of the provided CVD
diamond material such that the total concentration of isolated vacancies [Vr] ,=
([V~ + [V-]) in the.irradiated .diamond material is at least the greater of (a) 0.5 ppm
and (b) 50% higher than the [N5°] concentration in ppm in the provided CVD
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diamond material, and (iii) annealing the irradiated diamond material at a
temperature of at least 700°C and at most 900°C for a period of at least 2 hours,
optionally at least 4 hours, optionally at least 8 hours thereby forming vacancy
chains from at least some of the introduced isolated vacancies.
According to the first aspect of the invention, the total concentration of isolated
vacancies [Vr1 = ([\fl1 + rv-n in the irradiated diamond material is at least the
greater of (a) 0.5 ppm and (b) 50% higher than the [Ns01 concentration in ppm in
the provided CVD diamond material. This means that the total concentration of
isolated vacancies [Vr1 = ([\fl1 + rv-n always has a minimum value of 0.5 ppm
even for low or zero [Ns 01 concentrations in the provided CVD diamond material.
Above [Ns 01 concentrations in the provided CVD diamond material of about 0.33
ppm the minimum value of the concentration of isolated vacancies [Vr1 = ([\fl1 +
rv-n in the irradiated diamond material is given by calculating 50% higher than the
[Ns01 concentration in ppm in the provided CVD diamond material, since that will
result in a value for the concentration of isolated vacancies [Vr1 = ([\fl1 + rv-n
greater than 0.5 ppm.
In some embodiments according to the invention the irradiation and annealing
steps are carried out so as to reduce the concentration of isolated vacancies, [Vr1
to a concentration of< 0.3 ppm.
Fancy orange coloured diamond material made by the method according to the
present invention may be used as gemstones. Other applications, for example
use as a colour filter or cutting tool for example a scalpel are also envisaged.
The diamond material that is provided by step (i) of the method is referred to in
this specification as the "provided diamond". The step of actually growing the
CVD diamond material may or may not form part of the method of embodiments
of the invention. Providing a CVD diamond material may simply mean, for
example, selecting a pre-grown CVD diamond material. The diamond material
after irradiation step (ii) is referred to as the "irradiated diamond", and· ·the ·
diamond material after·the irradiation and annealing step is referred to as the
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"treated diamond material" or as the "irradiated and annealed diamond material".
Steps (i) to (iii) of embodiments of methods of the present invention, describing
the diamond material at each stage of the method are illustrated in the flow
diagram of Figure 1.
The provided CVD diamond material in the method of the present invention has a
[Ns0
] concentration (that is a concentration of neutral single substitutional nitrogen
defects) less than 5 ppm, optionally less than 4 ppm, optionally less than 3 ppm,
optically less than 2 ppm, optionally less than 1 ppm. The colour of the provided
CVD diamond material may vary according to the [Ns 0
] concentration, and the
manner in which the diamond material has been grown. It is known that [Ns0
]
defects themselves introduce a yellow colouration into diamond material,
particularly at concentrations greater than 0.3 ppm, but the skilled person will
recognize that the observation of colour is related to both the concentration and
the optical path length through the diamond. It is also known that the presence of
the low concentrations of nitrogen in a CVD growth environment can affect the
nature and concentration of other defects that are incorporated in a CVD
synthetic diamond material as the diamond material grows, and that at least
some of these other defects provide colour centres contribute to the colour of
CVD diamond material, typically introducing a brown colouration to the diamond
material. All measurements to calculate the concentration of Ns0 are done
following UV excitation.
It is thought that the colour centres that contribute to the brown colouration of
CVD diamond grown in the presence of low concentrations of nitrogen are unique
to single crystal CVD diamond, or to pieces cut or produced from layers of single
crystal CVD diamond. It is furthermore known that the colour centres contributing
to brown colouration in CVD diamond are different from those contributing to any
brown colouration observed in natural diamond because the defects in the CVD
diamond material cause absorption bands in the absorption spectra of the grown
. CVD diamond material that are not found in the absorption spectra of natural
···diamond. Evidence for this comes from' Raman scattering from non-diamond ···carbon
observable with an infrared excitation source (e.g. 785 nm or 1064 nm)
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which is not observed for brown natural diamond. Further, it is known that these
colour centres in natural diamond material anneal at a different temperature to
those in CVD diamond material.
It is believed that some of the colour centres contributing to the brown colouration
seen in CVD synthetic diamond grown in processes in which low concentrations
of nitrogen are introduced relate to localised disruption of the diamond bonding
· within the single crystal CVD diamond. The exact nature of the defects is not fully
understood, but the use of electron paramagnetic resonance (EPR) and optical
absorption spectroscopy techniques have been used to study the nature of the
defects and improve our understanding somewhat. The presence of the nitrogen
in the grown CVD synthetic diamond material can be evidenced by looking at
absorption spectra for the grown CVD diamond material, and analysis of these
spectra gives some indication of the relative proportions of different types of
defect present. A typical spectrum for grown CVD synthetic diamond material
grown with nitrogen added to the synthesis environment shows a peak at about
270 nm, which is generated by the presence of neutral single substitutional
nitrogen (Ns0
) atoms in the diamond lattice. Additionally peaks have been
observed at about 350 nm and approximately 510 nm corresponding to other
defects characteristic and unique to CVD synthetic diamond material, and
furthermore a so-called "ramp", that is a rising background of the form c x A.-3 has
been observed, where c is a constant and A. is the wavelength. While the Ns0 is
primarily identifiable by its peak at 270 nm, it also contributes in smaller amounts
to the absorption spectrum at higher wavelengths, in particular at wavelengths in
the visible part of the spectrum, which is generally considered to cover the
wavelength- range 350 nm to 750 nm.
It is the combination of features evident in the visible part of the absorption
spectrum of the CVD diamond material, i.e. (a) the Ns 0 contribution in the visible
part of the spectrum, (b) the 350 nm peak, (c) the 510 nm peak and (d) the ramp
feature, thataffect the. perceived colour of the diamond material and are believed
to be"responsible for-·the brown colour typically seen in nitrogen doped CVD
synthetic diamond material. The peaks at 350 nm and 510 nm, are not seen in
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the absorption spectra of natural diamonds, nor in the absorption spectra of other
synthetic diamonds, for example synthetic HPHT diamonds of the type described
in EP615954A. For the purposes of this specification, all defects other than the
N5° defects that contribute to the absorption spectrum in the visible part of the
spectrum, which we have discussed above as the 350 nm, 510 nm and ramp
features, will be referred to collectively as "X defects". As noted above, at this
time the structural nature of these defects at an atomic level is not understood,
merely their effect on the grown diamond material's absorption spectra. Without
binding the invention in any way, it is thought that the nature of the defects
responsible for the brown colouration might be related to the presence of multivacancy
clusters (each cluster being made up of tens of vacancies e.g. 30 or 40
vacancies or more) that are grown-in under large growth rates, concomitant with
the addition of nitrogen to the plasma to a hydrogen I methane (H2/CH4) source
gas. Such clusters are thermally unstable and may be removed to some degree,
by high-temperature treatment (i.e. annealing). It is thought that smaller vacancyrelated
defects, such as a NVH (nitrogen-vacancy-hydrogen) defects that are
made up of nitrogen and hydrogen and a missing carbon atom, may be partially
responsible for the brown colour and these defects may also be removed by hightemperature
treatment.
In preferred methods according to the invention, the absorption coefficients at
350 nm and 510 nm for the provided diamond material are less than 3 cm-1 and 1
cm-1 respectively, optionally less than 2 cm-1 and 0.8 cm-1 respectively.
Depending on the method of manufacture, and the [Ns 0
] concentration in the asgrown
CVD diamond material, the provided CVD diamond material used in
methods according to the invention may typically appear colourless, near
colourless, or yellow or brown with weak to medium saturation C* and very light
to medium lightness L * (C* and L * are discussed in detail later in this
specification). The [Ns0
] concentration in the provided diamond material is less
than 5 ppm. which limits any yellow colouration of.the diamond material. For
certain embOdiments according to the invention the -aesorption coefficient at 350
nm and 510,nm are less than 3 cm-1 and 1 cm-1 respectively, optionally less than
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2 cm-1 and 0.8 cm-1 respectively, the absorption coefficients at these wavelengths
being a measure of the brownness of the diamond material since the X defects
are thought to be responsible for much of the brown colouration due to abovementioned
X defects in diamond material grown by a CVD process incorporating
nitrogen in the source gas.
According to different embodiments of methods according to the present
invention, the provided CVD diamond may or may not contain Ns0
. Where it does
contain N5°, the concentration of [Ns0
] present in the synthetic CVD diamond
material of the present invention may be measured using EPR for levels <5x1015
cm-3 and using UV visible optical absorption techniques for higher concentrations.
These techniques are applied to samples post exposure to UV light.
[Ns 0
] in the neutral charge state content can be measured by using electron
paramagnetic resonance (EPR). Whilst the method is well-known in the art, for
completeness it is summarised here. In measurements conducted using EPR,
the abundance of a particular paramagnetic defect (e.g. the neutral singlesubstitutional
nitrogen defect) is proportional to the integrated intensity of all the
EPR absorption resonance lines originating from that centre. This permits the
concentration of the defect to be determined by comparing the integrated
intensity to that which is observed from a reference sample, provided care is
taken to prevent or correct for the effects of microwave power saturation. Since
continuous wave EPR spectra are recorded using field modulation, double
integration is required to determine the EPR intensity and hence the defect
concentration. To minimise the errors associated with double integration, base
line correction, finite limits of integration, etc., especially in cases where
overlapping EPR spectra are present, a spectral fitting method (using a NelderMead
simplex algorithm (J. A. Neider and R. Mead, The Computer Journal, 7
(1965), 308)) is employed to determine the integrated intensity of the EPR
centres present in the example of interest. This entails fitting the experimental
spectra with simulated, spectra of the defects present in · the example and
determining the ir:itegrated intensity· of each from the simulation::,Experimentally it
is observed that neither a Lorentzian nor Gaussian line shape provides a good fit
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to the experimental EPR spectra, therefore a Tsallis function is used to produce
the simulated spectra (D.F. Howarth, J.A. Weil, Z. Zimpel, J. Magn. Res., 161
(2003), 215). Furthermore, in the case of low nitrogen concentrations, it is often
necessary to use modulation amplitudes approaching or exceeding the line width
of the EPR signals to achieve a good signal/noise ratio (enabling accurate
concentration determination within a reasonable time frame). Hence pseudomodulation
is employed, with the Tsallis line shape in order to produce a good fit
to the recorded EPR spectra (J.S. Hyde, M. Pasenkiewicz-Gierula, A.
Jesmanowicz, W.E. Antholine, Appl. Magn. Reson., 1 (1990), 483). Using this
method the concentration in ppm can be determined with a reproducibility of
better than ±5%.
The technique of UV-visible absorption spectroscopy for measuring higher [Ns 0
]
concentrations is well-known in the art, and involves measurements using the
270 nm peak of the absorption spectrum of the diamond material.
The provided diamond material according to the present invention may be grown
using a conventional CVD process, for example of the type disclosed in WO
03/052177. Such a process, as noted above may result in a diamond material
having some brown colouration, but providing this brown colouration is not too
strong, ·it can be masked by the introduced orange colouration resulting from the
post growth irradiation and annealing treatment of the method of the present
invention.
Another CVD growth process that may be used to produce the provided CVD
diamond material is a CVD growth process in which the source gas contains
carbon, hydrogen, nitrogen and oxygen, rather than the more usual carbon
hydrogen and nitrogen. For example, oxygen may be added to the process gas
at a concentration of at least 10000 ppm in the gas phase. In particular, the
provided CVD diamond material in step (i) of the method according to the first
aspect of.the invention may,be grown directly by the process described in GB
:application GB0922449'!'4. ·and ·us provisional application .USSN':-6-1/289,282 the:
entire disclosures of which are incorporated herein by reference. ·specifically the
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method involves providing a substrate; providing a source gas; and allowing
homoepitaxial diamond synthesis on the substrate; wherein the synthesis
environment comprises nitrogen at an atomic concentration of from about 0.4
ppm to about 50 ppm; and wherein the source gas comprises: (a) an atomic
fraction of hydrogen Ht, from about 0.4 to about 0.75; (b) an atomic fraction of
carbon, C1, from about 0.15 to about 0.3; (c) an atomic fraction of oxygen, 0 1,
from about 0.13 to about 0.4; wherein Ht + Ct + Ot = 1; wherein the ratio of atomic
fraction of carbon to the atomic fraction of oxygen Ct:Ot, satisfies the ratio of
about 0.45:1 < Ct:Ot < about 1.25:1; wherein the source gas comprises
hydrogen atoms added as hydrogen molecules, H2, at an atomic fraction of the
total number of hydrogen, oxygen and carbon atoms present of between 0.05 and
0.4; and wherein the atomic fractions Ht, Ct and Ot are fractions of the total
number of hydrogen, oxygen and carbon atoms present in the source gas. This
method of growing CVD diamond material shall be referred to in the specification
as the "added oxygen CVD growth process". It typically results (depending on
the nitrogen concentration) in a provided CVD diamond material which is
colourless, near colourless or has low brown colouration.
The colour of the irradiated diamond material is a combination of the starting
colour, if any, of the provided diamond material and the orange colour introduced
by the irradiation and annealing steps to introduce vacancy chains. Other
impurities that could introduce colour into the provided diamond material may in
certain embodiments be minimised. For example, uncompensated boron
(isolated boron) may itself introduce a blue colour to the diamond material. For
some embodiments the atomic boron concentration [B] in the provided diamond
material is less than 5x1 015 cm-3
.
It is known that if there is uncompensated boron in a diamond material this may
be compensated for by irradiating to introduce isolated vacancies, the isolated
vacancies combining with the boron so that neither the boron, nor those
compensating isolated . vacancies ·impart any. colour. to the diamond. material.
Therefore in· some:; ·embodirri'ents ·according· to the present' in·ventiofi·,·· if ·the
diamond material does contain.· uncompensated boron · (for example in a
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concentration of> 5 x 1015 cm-3
), then the irradiation step may be carried out so
as to introduce sufficient isolated vacancies not only to compensate the boron but
also to achieve the specified isolated vacancy concentration [Vr]. The level of
additional irradiation needed for boron-compensation could be determined
empirically by the person skilled in the art. Thus in some embodiments of method
according to the invention, uncompensated boron is present in the provided
diamond material in a concentration of >5x1 015 cm-3
, and the irradiation step
introduces sufficient isolated vacancies into the diamond material so that total
concentration of isolated vacancies [Vr] in the irradiated diamond material, after
isolated vacancies have been used to compensate the boron, is at least the
greater of 0.5 ppm or 50% higher than the [Ns ~ concentration in ppm in the
provided CVD diamond material. The level of additional irradiation needed for
boron compensation could be determined empirically by the person skilled in the
art. Total boron in the material may be quantified using techniques known to the
skilled man. Secondary ion mass spectroscopy (SIMS) may be used for example
to ascertain the total boron concentration. The uncompensated boron may be
ascertained using either the induced absorption measured in the infrared part of
the diamond spectrum, or through Hall or electrical transport measurements, in a
manner known to the skilled person.
Typically the [Ns0
] concentration in ppm in the provided CVD diamond material
will remain substantially unchanged by the irradiation step (step (ii) of methods
according to the invention). It will be changed by the annealing step (step (iii) of
methods according to the invention, as explained later in this specification.
The provided CVD diamond material used in the method according to the first
aspect of the present invention preferably may have at least about 50%,
alternatively at least about 80%, alternatively at least about 90%, alternatively at
least about 95% of the volume of the synthetic CVD diamond material formed
from a single growth sector. This single growth sector is preferably a {1 00} or a
.{1.1 0} growth sector. The. material of the single growth sector preferably has Ns 0
·· levels within ·±10% of the mean for"'greater than about-50% of the volume ol~'the
growth sector, alternatively greater than about 60% of the volume of the growth
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sector, alternatively greater than about 80% of the volume of the growth sector.
Using a provided synthetic CVD diamond material that has been grown from a
single growth sector is advantageous as the CVD diamond material will have
fewer surfaces with different crystallographic orientation (which are the surfaces
corresponding to differing growth sectors). Surfaces with different
crystallographic orientations exhibit strongly differential uptake of nitrogen
impurity and a synthetic CVD diamond material comprising more growth sectors
therefore tends to show more undesirable zones with different colour, resulting
from the different concentrations of N5° in different growth sectors.
Tbe colour of a diamond material coloured by using a post growth treatment
method is the colour of the diamond material prior to post growth treatment
combined with the effect on colour of any defect produced during the post growth
treatment. According to the method of the present invention, we have found that
if we apply a particular post-CVD-growth treatment we can introduce an orange
colour in to the diamond material. Small to moderate amounts of yellow or brown
in the starting material may be tolerated and the treated diamond (post irradiation
and anneal according to the invention) will still appear orange, since the orange
colouration introduced by the post growth treatment has moderate to strong
saturation (C* as hereinafter described), and medium to light lightness (L * as
hereinafter described) and is therefore able to mask small to moderate amounts
of yellow or brown in the provided CVD diamond. For certain embodiments
according to the invention we start with a provided CVD diamond that has
minimal or no colour, i.e. is substantially colourless; for other embodiments
according to the invention we start with a provided diamond material that has
some colour, usually some yellow or brownness. For example, for some
embodiments, to produce a light orange diamond material, which has a low C*
and/or high L * value, (e.g. C* < 10, and/or L * > 65) it would be necessary to start
with a colourless or pale yellow material.
According to the method of the present invention, the irradiation.step introduces a_
'·~total isolated vacancy concentration [Vt]"which is at least· the greater of .(a) 0.5 '
.ppm and (b) 50% higher than the [Ns0
] concentration in the provided diamond
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material. The isolated vacancy concentration [VT] is given by the sum of ['11 and
[V-], where [V0] is the concentration of isolated neutral vacancies, and [Vl is the
concentration of negatively charged isolated vacancies, both in ppm. Both [VO]
and [Vl concentrations are determined from the GR1 and ND1 absorption
features of the absorption spectrum of the irradiated diamond in a manner
described hereinafter. It is possible that said irradiation might introduce
vacancies in other forms, e.g. as pairs or in possible isolated positive vacancies.
The inventors did not observe any obvious features in the material that could be
associated with such defects, but do not rule out this possibility. In certain
embodiments according to the invention the total isolated vacancy concentration
[VT] is more than the greater of (a) 0.5 ppm and (b) 50% higher than the [Ns0
]
concentration in the provided diamond material. For example the total isolated
vacancy concentration [VT] may be at least 0.7 ppm, or at least 0.9 ppm, or at
least 1.0 ppm greater than the [Ns0
] concentration in the provided diamond
material.
In general, the greater the irradiation dose, the greater the number of isolated
vacancies created. The number of isolated vacancies can depend not only on
the period of the irradiation dose but also on the number and nature of defects in
the provided CVD diamond material. Therefore in order to calculate the desired
dose of electronic radiation, the isolated vacancy production rate is also
calculated for the given irradiation conditions, as will be known to those skilled in
the art.
Factors such as diamond temperature, beam energy, beam flux, and even the
starting diamond's properties can affect the [VT] produced for a fixed
experimental irradiation set-up and time. Irradiation is typically carried out with
the sample mounted under ambient conditions -300 K with only minimal
temperature rise during the irradiation dose (e.g. less than 100 K). However,
factors such as beam energy and beam flux can lead to sample heating .
. Preferably the sample. is held as cold as possible (with even cryogenic cooling at
77 K··being ·advantageous under some circumstances) to enable·. high dose rates.
without compromising temperature control and thus minimize the irradiation time.
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This is advantageous for commercial reasons. Calibration of the dose applied
against the vacancies produced for the particular provided diamond being used in
order to satisfy these limits on [Vr] concentration introduced will form part of the
skilled person's responsibilities before carrying out the method of the present
invention. Such calibration techniques are routine for the person skilled in the art.
A larger sample could be rotated and irradiated from two or more sides in order to
introduce vacancies through the whole thickness of the diamond material.
Optionally the provided diamond material may be annealed in the temperature
range 1400-2500°C prior to the first irradiation step.
Step (iii) of methods according to the invention comprises annealing irradiated
diamond material at a temperature of at least 700°C and at most 900°C for a
period of at least 2 hours. The effect that this annealing step has on the isolated
vacancies in the irradiated diamond material depends on whether, and how
many, Ns 0 defects are present in the irradiated diamond material. If there are Ns 0
defects in the diamond material, then initially annealing at 700oc to 900°C will
form NV centres, each NV centre being the result of a Ns 0 defect joining with a
single isolated vacancy. In this case, when Ns0 defects are present, it is
predominantly after the maximum number of NV centres has formed that vacancy
chains start to form. However, not all Ns0 defects are converted to NV centres,
this being thought be due to the distribution of some of the Ns0 defects. Once the
concentration of NV centres have saturated out, any isolated vacancies that have
not been used up to form NV centres are available to combine with each other to
form vacancy chains. It is these vacancy chains that are thought to give the
orange colour in the treated diamond material according to the present invention.
Therefore in methods according to the present invention the irradiation of the
provided CVD diamond material such as to introduce isolated vacancies V into at
least part of the provided CVD diamond material is such that the total
concentration of ~sola ted vacancies [Vr] in the irradiated ·diamond material is at
least the ~Yeater of (a) OB ppm and (b) 50% higher than the [N·lJ concentration in
ppm, so that there are sufficient excess·isolated vacancies over and above those ·
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that combine to form NV centres, available to join together to form vacancy
chains.
If there are no Ns 0 defects in the provided CVD diamond material (and provided
there is no other un-compensated element such as boron present) then when the
annealing step (iii) of the methods according to the invention is carried out the
isolated vacancies formed during the irradiation step will start to group into
vacancy chains immediately.
Any isolated vacancies remaining in the lattice typically result in a flatter UV Vis
spectrum, which typically results in a more grey diamond material. Therefore for
some embodiments, the concentration of isolated vacancies after the annealing
process is complete is substantially reduced, and may be minimised. For some
embodiments, after the annealing process the total concentration of isolated
vacancies is <0.3 ppm, optionally <0.2, optionally <0.1, or optionally <0.05 ppm
for a 0.5 carat (ct) Round Brilliant Cut (rbc).
For many embodiments, the optimum anneal its one that will produce the highest
conversion of isolated vacancies to vacancy chains possible. Such embodiments
may result in a vivid orange material, with a high C* value, typically C* > 20.
Whilst not in the visible part of the spectra itself, an increase in absorption
centred at 250 nm is characteristic of the orange colouration and the saturation of
the orange colour scales with this feature. Therefore, a measure of the vacancy
chain concentration is the absorption at 250 nm. For some embodiments, after
irradiation and annealing, for a 0.5 ct round brilliant cut diamond stone, the
absorption at 250 nm when measured at room temperature is >5 cm-1
, optionally
> 7 cm-1
, optionally >1 0 cm-1 when the spectra has been scaled to 0 cm-1 at
800 nm. Those skilled in the art will know that vacancy concentrations will need
to be altered for diamond stones with different path lengths to result in the stated
the absorption coefficients~
15
An additional benefit to irradiating the CVD diamond material is typically the
colour of the material will be more stable to low-temperature annealing and
exposure to UV light (energy having an energy of at least 5.5 eV) compared with
untreated CVD diamond. This stabilisation effect is discussed in GB application
number 0911075.0 and US provisional application number 61/220,663, both filed
26th June 2009, and in GB application number 0917219 .8 and US provisional
application number 61/247,735, both filed 1st October 2009, the entire disclosures
of which are incorporated herein by reference.
In some embodiments according to the invention the provided diamond material
shows a measurable difference in at least one of its absorption characteristics in
first and second states, the first state being after exposure to irradiation having an
energy of at least 5.5 eV (typically UV light) and the second state being after
thermal treatment at 798 K (525°C), and after the irradiation and annealing steps
of methods according to the invention, the change in colour saturation value C*
between the diamond material in the first and second states is reduced by at least
0.5. This colour stabilisation may sometimes occur after simply irradiating ..
Optionally, after the irradiation and annealing steps of methods according to the
invention, the change in C* of the diamond material in the said first and second
states is less than 1 .
In general, the annealing step will be carried out after the irradiation step is
complete. However, it is also envisaged that there may be some overlap of the
irradiation and annealing processes, for example the annealing step may start
before the irradiation step is complete, or the two processes may be carried out,
and start and finish, at substantially the same time.
Typically annealing is carried out in an inert atmosphere e.g. an argon
atmosphere or under vacuum. Annealing is typically carried out at <1 00 mBar.
The present invention also provides the diamond material whenever.made by a
· .. method according to the first aspect of the invention.
16
A second aspect of the present invention provides a CVD diamond material which
when in the form of a 0.5 carat rbc is graded fancy orange.
The terminology fancy orange diamond material is defined as diamonds which
have a clear and distinct orange colour (Diamond grading ABC The Manual, by
Verena Pagel-Theisen, Rubin & Son, Belgium, 9th Edition, 2001, Page 67).
A third aspect of the present invention provides a CVD synthetic single crystal
diamond having a hue angle in the range 69-90 for an equivalent 0.5 ct Round
Brilliant Cut (rbc) diamond.
The perceived colour of any particular diamond stone depends on the size and
cut of the diamond. Therefore, where reference is given to the hue angle (which
determines colour), or to any colour, it is usual in the field to quote this in terms of
a standard size, usually 0.5 carat, and a standard cut, usually round brilliant cut
(often known as RBC or rbc) diamond stone. For any given diamond stone, be it
larger or smaller than 0.5 carat, or round brilliant cut or any other cut, models are
available to adjust the colour to that for the standard size and cut. Therefore, the
provided diamond material used in the method according to the first aspect of the
invention may have any size or cut, but colour parameters where specified are
adjusted to those for an equivalent material diamond stone of standard 0.5 carat
size, and standard round brilliant cut for comparison of quoted values.
Embodiments of the invention may have one or more of the following colour
characteristics for an equivalent 0.5 ct Round Brilliant Cut (RBC) diamond stone.
Table 1
Characteristic Range
Hue angle a 68°-90°
optionally 69° - 85°
:
optionally 70° - 80° ..
Saturation C* ···;-,:) ., 2-70. . ry~·.·
optionally 3 - 65
17
optionally 4 - 60
Lightness L * >45
optionally >50
optionally >55
Material of this invention can be differentiated from as grown orange material,
which has had no treatment, by features that are introduced during the irradiation.
These include small but measurable features in absorption or PL when measured
at 77 K or below. For example features at 741 nm, 673 nm, 575 nm or 503 nm
may be enhanced.
The colour of the irradiated and annealed diamond can be quantified in a well
established manner using "CIE L *a*b* Chromaticity Coordinates". The use of
CIE L *a*b* Chromaticity Coordinates in diamond is described in WO
2004/022821, the entire disclosure of which is incorporated herein by reference.
a* and b* are plotted as x and y axes of a graph and the hue angle is measured
from the positive a* axis towards the positive b* axis. Thus a hue angle of greater
than 90° and less than 180° lies in the upper left quadrant of the a*b* graph. In
this scheme for describing colour L * is the lightness and a fourth coordinate C* is
the saturation.
The perceived colour of an object depends on the transmittance/absorbance
spectrum of the object, the spectral power distribution of the illumination source
and the response curves of the observer's eyes. The CIE L *a*b* chromaticity
coordinates (and therefore hue angles) quoted herein have been derived in the
way described below. Using a standard 065 illumination spectrum and standard
(red, green and blue) response curves of the eye (G. Wyszecki and W. S. Stiles,
John Wiley, New York-London-Sydney, 1967) CIE L *a*b* chromaticity
coordinates of a parallel-sided plate of diamond have been derived from its
transmittance spectrum using the relationships below, between 350 nm and
800 nm with a data interval of 1 nm:
s~.. = transmittance at wavelength A.
18
Lt..= spectral power distribution of the illumination
xA. = red response function of the eye
YA. =green response function of the eye
zA. = blue response function of the eye
Where Yo= r,.. y,.. L,..
L* = 116 (YNo)113 -16 =Lightness
a*= 500[(X/Xo)113
- (YNo)113
]
b* = 200[(YNo)113
- (Z/Zo)113
]
C* = (a*2 + b*2
)
112 =saturation
hab = arctan (b* I a*) = hue angle
(for YNo > 0.008856)
(for X/Xo > 0.008856, YNo > 0.008856)
(for Z/Zo > 0.008856)
Modified versions of these equations must be used outside the limits of YN0 ,
X!Xo and Z/Zo. The modified versions are given in a technical report prepared by
the Commission lnternationale de L'Eclairage (Colorimetry (1986)).
It is standard to plot a* and b* coordinates on a graph with a* corresponding to
the x axis and b* corresponding to the y axis. Positive a* and b* values
correspond respectively to red and yellow components to the hue. Negative a*
and b* values correspond respectively to green and blue components. The
positive quadrant of the graph then covers hues ranging from yellow through
orange to red, with saturations (C*) given by the distance from the origin.
19
It is possible to predict how the a*b* coordinates of diamond with a given
absorption coefficient spectrum will change as the optical path length is varied. In
order to do this, the reflection loss must first be subtracted from the measured
absorbance spectrum. The absorbance is then scaled to allow for a different path
length and then the reflection loss is added back on. The absorbance spectrum
can then be converted to a transmittance spectrum which is used to derive the
CIE L *a*b* coordinates for the new thickness. In this way the dependence of the
hue, saturation and lightness on optical path length can be modelled to give an
understanding of how the colour of diamond with given absorption properties per
unit thickness will depend on the optical path length.
L*, the lightness, forms the third dimension of the CIE L *a*b* colour space. It is
important to understand the way in which the lightness and saturation vary as the
optical path length is changed for diamond with particular optical absorption
properties. The method described in the preceding paragraph can also be used
to predict how the L *C* coordinates of diamond with a given absorption
coefficient spectrum depend on the optical path length.
The C* (saturation) numbers can be divided into saturation ranges of 10 C* units
and assigned descriptive terms as below.
0-10 weak
10-20 weak-moderate
20-30 moderate
30-40 moderate-strong
40-50 strong
50-60 strong-very strong
60-70 very strong
70-80+ very very strong
Similarly the L * numbers can be divided up into lightness ranges as follows:
5-15 very very dark
20
15-25 very dark
25-35 dark
35-45 medium/dark
45-55 medium
55-65 lighUmedium
65-75 light
75-85 very light
85-95 very very light
There are four basic colour tones defined by the following combinations of
lightness and saturation:
Bright: Light and high saturation, Pale: Light and low saturation,
Deep: High saturation and dark, Dull: Low saturation and dark.
The preferred hue angle, and a*, b*, C* and L* values provide a quantitative
measure the quality and colour of synthetic CVD diamond material of the present
invention. These colour properties are advantageous because they give the
diamond an orange colour and can be used for ornamental purposes such as
gemstones for jewellery, or for use as coloured filters or similar.
For all samples used in this specification absorption peak heights quoted in this
specification are measured using a UV/visible absorption spectrum of the
synthetic CVD diamond material taken at room temperature.
All room temperature absorption spectra mentioned herein were collected using a
Perkin Elmer Lambda-19 spectrometer. A reflection loss spectrum was created
using tabulated refractive index data and standard expressions for the reflection
loss for a parallel-sided plate. The refractive index was determined according to
Peter's equation [Z. Phys., 15 (1923), 358-368)] and subsequent reflection loss
derived using the standard Fresnel equation. The reflection loss spectrum was
subtracted'irom the measured absorbance data and an absorption coefficient
21
spectrum for the sample is created from the resulting spectrum. Absorption
coefficient data were shifted so that absorption coefficient was zero at 800 nm.
Concentrations in ppm given in the present specification for the different defects,
[NV+'l and ['1'-], may be calculated in a known standard manner by integrating
the area of peaks from the absorption spectrum of the diamond usually collected
at liquid nitrogen temperatures and using published coefficients for comparison to
calculate concentration. For concentrations of NV centres and isolated
vacancies, the spectra are advantageously obtained at 77 K, using liquid nitrogen
to cool the samples, since at that temperature sharp peaks at -741 nm and -394
nm attributable to V0 and v- and at 575 nm and 637 nm are seen attributable to
NV0 and Nv- defects respectively. The coefficients that are used for the
calculations of concentrations of NV centres and isolated vacancies in the
present specification are those set out by G. Davies in Physica B, 273-274
(1999), 15-23, as detailed in Table 2 below.
Table 2
In Table 2, "A" is the integrated absorption (meV cm-1
) in the zero phonon line of
the transition, measured at 77 K, with the absorption coefficient in cm-1 and the
photon energy in meV. The concentration is in cm-3
.
The provided CVD diamond material used in the method according to the present
invention, and also the irradiated CVD diamond material resulting from the
method of the present invention may, or may not, form part of a larger piece of
diamond material. For example part only of the larger piece of diamond material
may be irradiated, and/or part only of the larger piece of diamond material may
have the defined absorption· characteristics. As would be ·apparent to the person
skilled in the art multiple layers could also be irradiated and/or have the required
22
absorption characteristics, so that the provided CVD diamond material used in
the method according to the invention may form part, e.g. one or multiple layers
of a larger piece of diamond material. It is well known that the depth of
penetration of irradiation is dependent on the energy of the irradiation. So in
preferred embodiments an irradiation energy is selected such that the irradiation
penetrates only part of the depth of a CVD diamond material. This means that
isolated vacancies would be introduced only in the penetrated part of the
irradiated CVD diamond material, and hence that penetrated part of the CVD
diamond material would be the "diamond material" used formed by the method of
the present invention.
Where the provided CVD diamond material provides only part of a larger piece of
diamond material, as discussed above that provided CVD diamond material alone
may have the advantageous optical properties described for certain embodiments
of the invention. Thus for example a top or embedded layer or layers of a large
piece of CVD diamond material may have an orange colouration. Where any
other non-orange layers are substantially colourless the colour of the larger piece
of diamond material is dominated by the orange layer(s).
In some embodiments according to the invention at least 50% or at least 60% or
at least 70% or at least 80% or at least 90% or substantially the whole diamond
stone may have substantially the same colour.
In other embodiments according to the invention of diamond stone may comprise
layers or pockets of diamond material of the same colour.
Embodiments of the invention will now be described, by way of example, with
reference to the accompanying drawings and examples, wherein:
Figure 1, which has been referred to hereinbefore, is a flow chart which shows
routes for methods a~cording to the invention for obtaining .ora~ge diamond
material;
23
Figure 2 are UV visible absorption spectra measured at room temperature for
examples 1 and 2, post irradiation and anneal; and
Figure 3 are UV Visible absorption spectra measured at 77 K for examples 3, 5
and 6 post irradiation and anneal.
Examples
HPHT diamond substrates suitable for synthesising single crystal CVD diamond
of the invention were laser sawn, lapped into substrates, polished to minimise
subsurface defects such that the density of defects is below 5 x 103 /mm2, and
generally is below 102 /mm. Polished HPHT plates 3.6 mm x 3.6 mm square by
500 !Jm thick, with all faces {1 00} having a surface roughness Ra at this stage of
less than 1 nm were mounted on a refractory metal disk, and introduced into a
CVD diamond growing reactor.
Growth stages
1) The CVD diamond reactor was pre-fitted with point of use purifiers,
reducing unintentional contaminant species in the incoming gas stream to
below 80 ppb.
2) An in situ oxygen plasma etch was performed using 50/40/3000 seem
(standard cubic centimetre per second) of 02/Ar/H2 and a substrate
temperature of 760°C.
3) This moved without interruption into a hydrogen etch with the removal of
the 02 from the gas flow.
4) This moved into the growth process by the addition of the carbon source
(in this case CH4) and dopant gases. For these examples the CH4 flowing
at 165 seem, nitrogen was present in the process gas at different levels for
the different samples, provided from a calibrated source, for example a
source containing 100 ppb N2 either as Air in Ar or N2 in H2, and for some
examples 02 was also present in the process gas as shown in Table 3.
Table 3
24
Example Nitrogen dopant present in Oxygen flow present in
the process gas (ppm) the process gas (ppm)
1 and 2 0.7 0
3 1.8 9160
4-6 1.1 13657
5) On completion of the growth period, the substrate was removed from the
reactor and the CVD diamond layer removed from the substrate by laser
sawing and mechanical polishing techniques.
6) This produced a CVD sample which had typical dimensions -3.1 x5x5 mm.
This grown CVD diamond is the "provided diamond" defined by the claims of the
present specification.
The examples were electron irradiated a 4.5 MeV electron beam at 50% scan
width and 20 rnA beam current using an electron beam source such as that found
at lsotron pic. Diamond samples to be irradiated are mounted in indium on a
water cooled copper block to prevent the samples being heated above 350K. The
samples were then annealed in an Elite tube furnace (model THS 16/50/180-
2416CG and 27160/T). Typically to make an orange diamond material a dose of
5.8 x 1018 e-/cm2 (equivalent to 6 hours irradiation with a 4.5 MeV electron beam
at 50% scan width and 20 rnA beam current) followed by an 8 hour anneal at
800°C was used.
Table 4 records the CVD growth chemistry, the [Ns0
] concentration in the
provided diamond material, the absorption coefficients at 350 nm and 510 nm
and the colour, of the provided diamond material, the irradiation dose, the
vacancy concentration post irradiation, the annealing time and temperature, the
colour of the diamond material post irradiation and anneal, the colour
characteristics, the [NV]. [V0
] and [V-] concentrations and the absorption at
250 nm related to vacancy chains-,. all post irradiation and anneal. The results
table 4 includes not only examples falling within the scope of the present
invention, but also a number of comparative examples. For example, if the
25
irradiation dose is not high enough, the number of isolated vacancies available to
combine to form chains upon annealing, irrespective of the length of the anneal
will not be large enough to form a significant concentration of vacancy chains; this
is the case for comparative examples 2 and 6, which fall outside the scope of the
present invention as the concentration of isolated vacancies incorporated during
the irradiation step is less than the greater of (a) 0.5 ppm and (b) 0.5 ppm more
than the [Ns 0
] concentration, and the absorption at 250 nm in the treated sample
is <5 cm-1
. This is also illustrated with reference to Figure 2 which is a room
temperature UV visible absorption spectrum for examples 1 and 2, post
irradiation and anneal. This figure shows strong absorption at 250 nm for
example 1, indicating the presence of vacancy chains, whereas in example 2 the
absorption in the 250 nm range is less than 5 cm-1
, showing a low concentration
of vacancy chains has been formed. Similarly we have found that if the annealing
time is not long enough then total concentration of isolated vacancies remaining
in the treated sample is > 0.3 ppm; this is the case in comparative example 5,
which is annealed for only 1 hour and results in a grey coloured diamond material
as compared with the orange colour achieved with example 4, which is an
identical diamond material sample to that of example 5 in terms of composition
and irradiation, but is annealed for a longer time.
Figure 3 shows UV visible spectra taken at 77 K post irradiation and anneal and
illustrates for example 3 strong absorption at 250 nm and no peak at 741 nm or
394 nm remaining, showing that substantially all of the isolated vacancies have
been annealed out. Figure 3 also illustrates why comparative example 5 (which
has been annealed for an insufficient time) appears grey post irradiation and
anneal since there are peaks at 7 41 nm and 394 nm indicating the presence of
isolated vacancies and also at 575 nm and 637 nm showing the presence of NV
centres. Similarly Figure 3 illustrates why comparative example 6 (which has
been subjected to insufficient irradiation dose) appears pale pink, since there are
peaks at 575 nm and 637 nm, showing the presence of NV centres, a small
concentration of isolated vacancies remaining, and weak absorption at 250 nm.
indicating a low concentration of vacancy chains ..
26
All of the orange diamond samples according to the invention (examples 1, 3 and
4, show strong absorption at around 250 nm. This absorption is believed to be
due to the presence of vacancy chains. For example, the measured absorption at
250 nm is >5 cm·1 for both samples 1 and 3, whereas for comparative sample 2 it
is <5 cm·1
.
As noted above an additional benefit of irradiating the CVD diamond material is
that typically the colour of the material will be more stable to low temperature
annealing and exposure to UV light compared to untreated CVD diamond. We
found that upon heating example 1 the change in C* between the two states was
<1 which illustrates this benefit.
Example CVD Colour in Absorption at
Number Growth Ns0 cone Abs Abs the provided Irradiation Vacancy Anneal time Observed Colour [NV] [VOJ and pvC] 250 nm
chemistry in at at diamond dose concentration and colour of characteristics concentration concentration related to
provided 350 510 pre- (e/cm2
) post temperature diamond (ppm) (ppm) vacancy
CVD nm nm irradiation irradiation (hours) after L* chains at RT
diamond (Colour (ppm) (OC) irradiation c· Post irradiation Post Post
(ppm) grade if 0.5 and a and anneal irradiation and irradiation and
ct RBC) annealing anneal anneal
1 Traditional 0.1 1.09 0.45 Colourless 5.8 X 10"' yu = 1.41 8 hrs at Bright vivid L • = 63.1 NV'= 0.035 1'~' = <0.003 22.01
CVD v- = o.o3 8oo·c orange C* = 52.6 Nv- = o.ooo4 v = <0.003
growth a= 78_2
process
2* Traditional 0.1 1.09 0.45 Colourless 2.6 X 10 yu = 0.17 8 hrs at Dull pinkish L* = 80.2 NV'= 0.031 I'~'= o.o46
CVD v- = o.033 8oo·c brown C* = 5.05 Nv- = o.oo86 v- = o.oo86
growth a= 67.7 2.04
process
3 Added 0.6 1.85 0.60 Pale yellow 3.9x10'• yu = 1.95 8 hrs at Bright vivid L*=59.1 NV"= 0.19 1 V" = 5x1 015 cm-3
, and the irradiation step (ii) introduces sufficient isolated
vacancies into the diamond material so that total concentration of isolated
vacancies [Vr] in the irradiated diamond material, after isolated vacancies
have been used to compensate the boron, is at least the greater of (a) 0.5
ppm and (b) 50% higher than the [Ns~ concentration in ppm in the provided
diamond material.
8. A method according to any preceding claim, wherein the provided diamond
material is irradiated from two or more sides.
9. A method according to any preceding claim, wherein at least 50 % of the
provided CVD diamond has been formed from a single growth sector.
10. A method according to any preceding claim, wherein, after the irradiation
and annealing steps (ii) and (iii), the absorption in the 250 nm region of the
irradiated and annealed diamond material, when measured at room
temperature, is >5 cm-1
.
11. A method according to any preceding claim wherein the provided diamond
material shows a measurable difference in at least one of its absorption
characteristics in first and second states, the first state being after exposure
to irradiation having an energy of at least 5.5 eV and the second state being
after thermal treatment at 798 K (525°C), and wherein after the irradiation
and annealing steps of the method the change in colour saturation value C*
between the diamond material in the said first and second states is reduced
by at least 0.5 compared to the change in colour saturation value C*
between the diamond material in the said first and second states for the
provided diamond material.
12. A method according to any preceding claim, wherein after the irradiation
and annealing steps of the method, the change in colour saturation C* of the
diamond material irFfirst and second states is less than 1, ·the Jirst state
30
being after exposure to irradiation having an energy of at least 5.5 eV and
the second state being after thermal treatment at 798 K (525°C).
13. A method according to any preceding claim, wherein the provided diamond
material is annealed in the temperature range 1400°C - 2500°C prior to the
irradiation step.
14. A method according to any preceding claim, wherein the annealing step (iii)
of method claim 1 is carried out after irradiation step (ii) of method claim 1 is
complete.
15. CVD diamond material when made by a method according to any of claims
1-14.
16. CVD diamond material which for an equivalent 0.5 carat Round Brilliant Cut
(RBC) is graded fancy orange.
17. CVD synthetic single crystal diamond material having the following colour
characteristics measured for an equivalent 0.5 ct Round Brilliant Cut (RBC)
diamond.
Characteristic Range
Hue angle a 68°-90°
Saturation C* 2-70
Lightness L * >45
18. CVD diamond material according to any of claims 15-17, wherein the
concentration of isolated vacancies is< 0.3 ppm.
19. CVD diamond material according to any of claims 15-18, wherein for an
equivalent 0:5 ctround brilliant cut diamond stone, the absorption in the 250
nm region when :measured at room temperature is >5 cm-1
. :~ · ·
31
20. CVD diamond material according to any of claims 15-19, wherein the
change in saturation C* of the diamond material in first and second states is
less than 1 , the first state being after exposure to irradiation having an
energy of at least 5.5 eV and the second state being after thermal treatment
at 798 K (525°C).
21. Jewellery comprising diamond material according to any of claims 15-20
and a setting for the diamond material.
22. A round brilliant cut diamond gemstone comprising diamond material
according to any of claims 15-20.