U.S. patent application number 10/655581 was filed with the patent office on 2004-10-07 for coloured diamond.
Invention is credited to Cooper, Michael Andrew, Dorn, Baerbel Susanne Charlotte, Martineau, Philip Maurice, Scarsbrook, Geoffrey Alan, Twitchen, Daniel James.
Application Number | 20040194690 10/655581 |
Document ID | / |
Family ID | 9927670 |
Filed Date | 2004-10-07 |
United States Patent
Application |
20040194690 |
Kind Code |
A1 |
Twitchen, Daniel James ; et
al. |
October 7, 2004 |
Coloured diamond
Abstract
A diamond layer of single crystal CVD diamond which is coloured,
preferably which has a fancy colour, and which has a thickness of
greater than 1 mm.
Inventors: |
Twitchen, Daniel James;
(Sunningdale, GB) ; Martineau, Philip Maurice;
(Littlewick Green, GB) ; Scarsbrook, Geoffrey Alan;
(Ascot, GB) ; Dorn, Baerbel Susanne Charlotte;
(Bracknell, GB) ; Cooper, Michael Andrew;
(Wokingham, GB) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
9927670 |
Appl. No.: |
10/655581 |
Filed: |
September 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10655581 |
Sep 5, 2003 |
|
|
|
10318111 |
Dec 13, 2002 |
|
|
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Current U.S.
Class: |
117/68 |
Current CPC
Class: |
C30B 25/105 20130101;
Y10T 428/30 20150115; C01B 32/28 20170801; C30B 29/04 20130101;
C30B 25/105 20130101; C30B 29/04 20130101 |
Class at
Publication: |
117/068 |
International
Class: |
C30B 007/00; C30B
021/02; C30B 028/06 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 2001 |
GB |
0130004.5 |
Claims
We claim:
1. A diamond layer of single crystal CVD diamond which is coloured
and which has a thickness greater than 1 mm.
2. A diamond layer according to claim 1 which has a fancy
colour.
3. A diamond layer according to claim 2 wherein the colour is a
fancy colour with a dominant brown component.
4. A diamond layer according to claim 1 wherein the colour is a
fancy orangey brown, orange-brown, pinkish brown, pink-brown or
dark brown.
5. A diamond layer according to claim 1 wherein the hue angle is
less than 80 degrees.
6. A diamond layer according to claim 1 wherein the hue angle is
less than 75 degrees.
7. A diamond layer according to claim 1 wherein the hue angle is
less than 70 degrees.
8. A diamond layer according to claim 1 which has a thickness
greater than 2 mm.
9. A diamond layer according to claim 1 which has a thickness
greater than 3 mm.
10. A layer of single crystal CVD diamond according to claim 1
which has one or more of the characteristics (i), (ii), (iii)
observable in the majority volume of the layer, which comprises at
least 55 percent of the whole volume of the layer: (i) The majority
volume of the layer contains one or more defect and impurity
related colour centres that contribute to the absorption spectrum
of the diamond as set out in the absorption coefficient column
below:
13 Absorption coefficient Designation Starts Ends Peak (at peak)
270 nm 220 nm 325 nm 270 nm 0.1 cm.sup.-1-30 cm.sup.-1 350 nm band
270 nm 450 nm 350 nm 0.3 cm.sup.-1-20 cm.sup.-1 510 nm band 420 nm
640 nm 510 nm 0.1 cm.sup.-1-10 cm.sup.-1 570/637 nm 500 nm 640 nm
570 nm 0.1 cm.sup.-1-5 cm.sup.-1 Designation Form of Curve
Absorption Coefficient Ramp Rising background of form Contribution
at 510 nm Absorption coefficient (cm.sup.-1) = is: <3 cm.sup.-1
C .times. .lambda..sup.-3 (C = constant, .lambda.in .mu.m)
(ii) The majority volume of the layer contains defect and impurity
related centres that contribute to the luminescence spectrum as set
out in the Normalised luminescence intensity column of the table
below, when measured in the manner described herein using Ar ion
514 nm laser excitation at 77 K:
14 Normalised luminescence intensity of zero phonon line
Designation Starts Ends Peak at 77 K 575 nm 510 nm 680 nm 575 nm
0.02-80 637 nm 635 nm 800 nm 637 nm 0.01-300
(iii) The majority volume of the CVD diamond layer exhibits a ratio
of normalised 637 nm1575 nm luminescence, measured in the manner
described herein, which is in the range 0.2-10.
11. A diamond layer according to claim 10 wherein the majority
volume comprises at least 80 percent of the whole volume of the
layer.
12. A diamond layer according to claim 10 wherein the majority
volume comprises at least 95 percent of the whole volume of the
layer.
13. A diamond layer according to claim 10 wherein the majority
volume of the layer is formed from a single growth sector.
14. A diamond layer according to claim 10 wherein the colour centre
that contributes to the absorption spectrum of a diamond at 270 nm
has the characteristics:
15 Absorption Designation Starts Ends Peak coefficient (at peak)
270 nm 235 nm 325 nm 270 nm 0.4 cm.sup.-1-10 cm.sup.-1
15. A diamond layer according to claim 10 wherein the colour centre
that contributes to the absorption spectrum of a diamond at 270 nm
has the characteristics:
16 Absorption Designation Starts Ends Peak coefficient (at peak)
270 nm 235 nm 325 nm 270 nm 0.8 cm.sup.-1-6 cm.sup.-1
16. A diamond layer according to claim 10 wherein the colour centre
that contributes to the absorption spectrum of a diamond at 350 nm
has the characteristics:
17 Absorption Designation Starts Ends Peak coefficient (at peak)
350 nm band 270 nm 450 nm 350 nm 1.0 cm.sup.-1-8 cm.sup.-1
17. A diamond layer according to claim 10 wherein the colour centre
that contributes to the absorption spectrum of a diamond at 350 nm
has the characteristics:
18 Absorption Designation Starts Ends Peak coefficient (at peak)
350 nm band 270 nm 450 nm 350 nm 1.5 cm.sup.-1-6 cm.sup.-1
18. A diamond layer according to claim 10 wherein the colour centre
that contributes to the absorption spectrum of a diamond at 510 nm
has the characteristics:
19 Absorption Designation Starts Ends Peak coefficient (at peak)
510 nm band 420 nm 640 nm 510 nm 0.2 cm.sup.-1-4 cm.sup.-1
19. A diamond layer according to claim 10 wherein the colour centre
that contributes to the absorption spectrum of a diamond at 510 nm
has the characteristics,
20 Absorption Designation Starts Ends Peak coefficient (at peak)
510 nm band 420 nm 640 nm 510 nm 0.4 cm.sup.-1-2 cm.sup.-1
20. A diamond layer according to claim 10 wherein the colour centre
that contributes to the absorption spectrum of a diamond at 570/637
nm has the characteristics:
21 Absorption Designation Starts Ends Peak coefficient (at peak)
570/637 nm 500 nm 640 nm 570 nm 0.3 cm.sup.-1-3 cm.sup.-1
21. A diamond layer according to claim 10 wherein the colour centre
that contributes to the absorption spectrum of a diamond at 570/637
nm has the characteristics:
22 Absorption Designation Starts Ends Peak coefficient (at peak)
570/637 nm 500 nm 640 nm 570 nm 0.3 cm.sup.-1-1.5 cm.sup.-1
22. A diamond layer according to claim 10 wherein the ramp has the
characteristics:
23 Absorption coefficient Designation Form of Curve (at peak) Ramp
Rising background of form Contribution at Absorption coefficient
(cm.sup.-1) = C .times. .lambda..sup.-3 510 nm is: (C = constant,
.lambda.in .mu.m) <1.5 cm.sup.-1
23. A diamond layer according to claim 10 wherein the ramp has the
characteristics:
24 Absorption coefficient Designation Form of Curve (at peak) Ramp
Rising background of form Contribution at Absorption coefficient
(cm.sup.-1) = C .times. .lambda..sup.-3 510 nm is: (C = constant,
.lambda.in .mu.m) <0.8 cm.sup.-1
24. A diamond layer according to claim 10 wherein the colour centre
that contributes to the luminescence spectrum of a diamond at 575
nm has the characteristics:
25 Normalised luminescence intensity of zero phonon line
Designation Starts Ends Peak at 77K 575 nm 570 nm 680 nm 575 nm
0.05-60
25. A diamond layer according to claim 10 wherein the colour centre
that contributes to the luminescence spectrum of a diamond at 575
nm has the characteristics:
26 Normalised luminescence intensity of zero phonon line
Designation Starts Ends Peak at 77K 575 nm 570 nm 680 nm 575 nm
0.2-40
26. A diamond layer according to claim 10 wherein the colour centre
that contributes to the luminescence spectrum of a diamond at 637
nm has the characteristics:
27 Normalised luminescence intensity of zero phonon line
Designation Starts Ends Peak at 77K 637 nm 635 nm 800 nm 637 nm
0.02-200
27. A diamond layer according to claim 10 wherein the colour centre
that contributes to the luminescence spectrum of a diamond at 637
nm has the characteristics:
28 Normalised luminescence intensity of zero phonon line
Designation Starts Ends Peak at 77K 637 nm 635 nm 800 nm 637 nm
0.03-100
28. A diamond layer according to claim 10 wherein the ratio of
normalised 637 nm/575 nm luminescence is in the range 0.5 to 8.
29. A diamond layer according to claim 10 wherein the ratio of
normalised 637 nm/575 nm luminescence is in the range 2 to 5.
30. A layer of single crystal diamond which is coloured and which
has, observable in the majority volume of the layer wherein the
majority volume comprises at least 55 percent of the whole volume
of the layer, a low ramp as set out in the table below:
29 Absorption Designation Form of Curve Coefficient Ramp Rising
background of form Contribution at Absorption coefficient
(cm.sup.-1) = C .times. .lambda..sup.-3 510 nm is: (C = constant,
.lambda.in .mu.m) <3 cm.sup.-1
and wherein the majority volume contains one or more of the defect
and impurity related colour centres that contribute to the
absorption spectrum of diamond as set out in the absorption
coefficient column of the table below:
30 Absorption coefficient Designation Starts Ends Peak (at peak)
270 nm 220 nm 325 nm 270 nm 0.1 cm.sup.-1-30 cm.sup.-1 350 nm band
270 nm 450 nm 350 nm +/- 10 nm 0.3 cm.sup.-1-20 cm.sup.-1 510 nm
band 420 nm 640 nm 510 nm +/- 50 nm 0.1 cm.sup.-1-10 cm.sup.-1
570/637 nm 500 nm 640 nm 570 nm 0.1 cm.sup.-1-5 cm.sup.-1
31. A method of producing a coloured single crystal diamond layer
includes the steps of providing a diamond substrate having a
surface which is substantially free of crystal defects, providing a
source of gas, dissociating the source gas to produce a synthesis
atmosphere which contains 0,5 to 500 ppm nitrogen, calculated as
molecular nitrogen, and allowing homoepitaxial diamond growth on
the surface which is substantially free of crystal defects.
32. A method according to claim 31 wherein the synthesis atmosphere
contains 1 to 100 ppm nitrogen, calculated as molecular
nitrogen.
33. A method according to claim 31 wherein the synthesis atmosphere
contains nitrogen in an amount suitable to enhance the size of the
{100} growth sector and reduce the size of competing growth
sectors.
34. A method according to claim 31 wherein the density of defects
is such of surface etch features related lo defects is below
5.times.10.sup.3mm.sup.2.
35. A method according to claim 31 wherein the density of defects
is such that the density of surface etch features related to
defects is below 10.sup.2/mm.sup.2.
36. A method according to claim 31 wherein the surface or surfaces
of the diamond substrate on which CVD diamond growth occurs is
selected from the {100}, {110}, {113} and {111} surfaces.
37. A diamond layer produced by a method according to claim 31.
38. A gemstone produced from a diamond layer according to claim 1
or claim 37.
39. A gemstone according to claim 38 with a quality grading of SI1
or better.
40. A gemstone according to claim 38 with a quality grading of VS1
or better.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a method of producing by chemical
vapour deposition (hereinafter referred to as CVD) coloured single
crystal diamond, and in one aspect a method of producing fancy
coloured diamond, these coloured diamonds being suitable, for
example, for preparation for ornamental purposes or applications in
which colour is a secondary parameter that may influence market
acceptance.
[0002] Intrinsic diamond has an indirect band gap of 5.5 eV and is
transparent in the visible part of the spectrum. Introducing
defects or colour centres, as they will be called, which have
associated energy levels within the band gap gives the diamond a
characteristic colour which is dependent on the type and
concentration of the colour centres. This colour can result from
either absorption or photoluminescence or some combination of these
two. One example of a common colour centre present in synthetic
diamond is nitrogen which, when sitting on a substitutional lattice
site in the neutral charge state, has an associated energy level
.about.1.7 eV below the conduction band--the resulting absorption
gives the diamond a characteristic yellow/brown colour.
[0003] Methods of depositing material such as diamond on a
substrate by CVD are now well established and have been described
extensively in the patent and other literature. Where diamond is
being deposited on a substrate, the method generally involves
providing a gas mixture which, on dissociation, can provide
hydrogen or a halogen (e.g. F,Cl) in atomic form and C or
carbon-containing radicals and other reactive species, e.g.
CH.sub.x, CF.sub.x wherein x can be 1 to 4. In addition, oxygen
containing sources may be present as may sources for nitrogen, and
for boron. Nitrogen can be introduced in the synthesis plasma in
many forms; typically these are N.sub.2, NH.sub.3, air and
N.sub.2H.sub.4. In many processes inert gases such as helium, neon
or argon are also present. Thus, a typical source gas mixture will
contain hydrocarbons C.sub.xH.sub.y wherein x and y can each be 1
to 10 or halocarbons C.sub.xH.sub.yHal.sub.z wherein x and z can
each be 1 to 10 and y can be 0 to 10 and optionally one or more of
the following: CO.sub.x, wherein x can be 0,5 to 2, O.sub.2,
H.sub.2, N.sub.2, NH.sub.3, B.sub.2H.sub.6 and an inert gas. Each
gas may be present in its natural isotopic ratio, or the relative
isotopic ratios may be artificially controlled; for example
hydrogen may be present as deuterium or tritium, and carbon may be
present as .sup.12C or .sup.13C. Dissociation of the source gas
mixture is brought about by an energy source such as microwaves, RF
(radio frequency) energy, a flame, a hot filament or jet based
technique and the reactive gas species so produced are allowed to
deposit onto a substrate and form diamond.
[0004] CVD diamond may be produced on a variety of substrates.
Depending on the nature of the substrate and details of the process
chemistry, polycrystalline or single crystal CVD diamond may be
produced.
[0005] It is well known that post growth treatment such as
irradiation with sufficiently energetic particles (electron,
neutron etc) to produce lattice defects (interstitials and
vacancies) and suitable annealing can result in the formation of
colour centres, such as the nitrogen vacancy [N-V] colour centre,
which can give the diamond a desirable colour (see for example EP 0
615 954 A1. EP 0 326 855 A1 and the references cited therein).
Further characteristics and artificial production of colour centres
are discussed in detail by John Walker in the Reports on Progress
in Physics, Vol. 42 1979. The artificial production method of
colour centres outlined therein comprises 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. However, there are
limitations to the colours and uniformity that can be produced as a
consequence of competitive defect formation and because of the
strong sector dependence associated with defects such as nitrogen
in diamond.
[0006] The colour of a diamond produced by utilising this post
growth colour centre formation method is the colour of the rough
diamond combined with the colour of the colour centre produced. In
order to obtain the ornamental value desired, and thus achieve a
combination of high transparency and fancy colour, it has been
usual practice to use diamonds that were initially either
transparent or light yellow.
[0007] There are three visual attributes to colour: hue, lightness
and saturation. Hue is the attribute of colour that allows it to be
classified as red, green, blue, yellow, black or white, or a hue
that is intermediate between adjacent pairs or triplets of these
basic hues (Stephen C. Hofer, Collecting and Classifying Coloured
Diamonds, 1998, Ashland Press, New York).
[0008] White, grey and black objects are differentiated on a
lightness scale of light to dark. Lightness is the attribute of
colour that is defined by the degree of similarity with a neutral
achromatic scale starting with white and progressing through darker
levels of grey and ending with black.
[0009] Saturation is the attribute of colour that is defined by the
degree of difference from an achromatic colour of the same
lightness. It is also a descriptive term corresponding to the
strength of a colour. The diamond trade uses adjectives such as
intense, strong and vivid to denote different degrees of saturation
assessed visually. In the CIELAB colour system, saturation is the
degree of departure from the neutral colour axis (defined by
saturation=[(a*).sup.2+(b*).sup.2].sup.1/2, see hereinafter).
Lightness is a visual quality perceived separately from
saturation.
[0010] The dominant colour of much of the diamond of the invention
described hereinafter is brown. Brown is generally a darker, less
saturated version of orange. As brown becomes lighter and more
saturated it becomes orange. Brown colours also underlie a portion
of the yellow hue family so that orange-yellow and orangish yellows
in their darker and weaker variants may fall into the brown
region.
[0011] For diamonds, intermediate colour descriptions between brown
and orange are used. In order of decreasing browness and increasing
orangeness, the description of the colour goes through the
following sequence: brown, orangish brown, orangebrown,
brown-orange, brownish orange, orange. Similar sequences apply for
the transitions from brown to orangeyellow or orangish yellow. In
three-dimensional colour space the region of brown colours is also
bordered by pink colour regions and on moving from brown to pink
the following sequence is followed: brown, pinkish brown,
pink-brown, brown-pink, brownish pink, pink.
[0012] Fancy coloured diamonds are diamonds with an obvious and
unusual colour. When the dominant component of that colour is brown
they are described as fancy brown. This term covers a complex range
of colours, defined by a three dimensional region of colour space.
It covers large ranges in the values of lightness, hue and
saturation.
[0013] The inherent colour of a cut diamond, sometimes called the
body colour, can best be judged if the diamond is viewed from the
side for typical cuts. The apparent colour seen in the faceup
direction (ie looking towards the table) can be greatly affected by
the cut of the stone because of the effect that this has on the
path length within the stone for the light subsequently reaching
the eye. For example, inherently orangebrown diamond can be cut in
such a way that its face-up colour appears brighter, resulting in a
reversal of the dominant colour to brown-orange.
SUMMARY OF THE INVENTION
[0014] According to a first aspect of the invention, there is
provided a coloured single crystal diamond layer, preferably a
fancy coloured single crystal diamond layer, and more preferably a
fancy coloured single crystal diamond layer where brown is the
dominant colour, synthesised by CVD and prepared or suitable for
preparation as a cut stone for ornamental application or for other
applicatons where colour may influence market acceptance. The CVD
diamond layer of the invention preferably has a hue angle of less
than 80.degree., preferably a hue angle of less than 75.degree. and
more preferably a hue angle of less than 70.degree.. The hue angle
for a particular hue can be found by extending the line back from
the point representing that hue on the a* b* colour plot as
described more fully hereinafter, and shown on FIG. 3.
[0015] The CVD diamond layer of the invention has a thickness
greater than 1 mm, and preferably greater than 2 mm and more
preferably greater than 3 mm.
[0016] The CVD diamond layer of the invention may also have one or
more of the following characteristics (i), (ii) and (iii)
observable in the majority volume of the layer, where the majority
volume comprises at least 55%, and preferably at least 80%, and
more preferably at least 95% of the whole volume of the layer.
Preferably the majority volume of the layer is formed from a single
growth sector.
[0017] (i) The majority volume of the CVD diamond layer contains
one or more of the defect and impurity related colour centres that
contribute to the absorption spectrum of the diamond as set out in
the absorption coefficient column of the table below:
1 Desig- Absorption coefficient nation Starts Ends Peak (at peak)
270 nm 220 nm 325 nm 270 nm 0.1 cm.sup..sup.-1-30 cm.sup.-1
preferably 0.4 cm.sup.-1-10 cm.sup.-1 more 0.8 cm.sup.-1-6
cm.sup.-1 preferably 350 nm 270 nm 450 nm 350 nm 0.3 cm.sup.-1-20
cm.sup.-1 band +/- 10 nm preferably 1.0 cm.sup.-1-8 cm.sup.-1 more
1.5 cm.sup.-1-6 cm.sup.-1 preferably 510 nm 420 nm 640 nm 510 nm
0.1 cm.sup.-1-10 cm.sup.-1 band +/- 50 nm preferably 0.2
cm.sup.-1-4 cm.sup.-1 more 0.4 cm.sup.-1-2 cm.sup.-1 preferably
570/ 500 nm 640 nm 570 nm 0.1 cm.sup.-1-5 cm.sup.-1 637 nm
preferably 0.3 cm.sup.-1-3 cm.sup.-1 more 0.3 cm.sup.-1-1.5
cm.sup.-1 preferably Desig- nation Form of Curve Absorption
Coefficient Ramp Rising background Contribution at 510 nm is: of
form <3 cm.sup.-1 Absorption coefficient preferably <1.5
cm.sup.-1 (cm.sup.-1) = C .times. more preferably <0.8 cm.sup.-1
.lambda..sup.-3 (C = constant, .lambda.in .mu.m)
[0018] (ii) The majority volume of the CVD diamond layer contains
defect and impurity related centres that contribute to the
luminescence spectrum as detailed in the normalised luminenscence
intensity column of the table below, when measured in the
prescribed manner using Ar ion 514nm laser excitation at 77K and
normalised relative to the Raman scattering intensity:
2 Normalised luminescence intensity of zero phonon Designation
Starts Ends Peak line at 77K 575 nm 570 nm 680 nm 575 nm 0.02-80
preferably 0.05-60 more 0.2-40 preferably 637 nm 635 nm 800 nm 637
nm 0.01-300 preferably 0.02-200 more 0.03-100 preferably
[0019] (iii) The majority volume of the CVD diamond layer exhibits
a ratio of normalised 637 nm/575 nm luminescence, measured in the
manner described herein, which is in the range 0.2-10, and
preferably in the range 0.5-8, and more preferably in the range
2-5.
[0020] The present invention provides, according to another aspect,
coloured single crystal CVD diamond which has a low ramp value as
defined above in combination with a defect and impurity related
colour centre that contributes to the absorption spectrum of the
diamond at one or more of 270 nm, 350 nm band, 510 nm band and
570/637 nm, as set out in the table forming part of characteristic
(i). The low ramp value in combination with one or more of the
absorption spectrum characteristics provides the diamond with a
desirable colour. The diamond will generally be in layer form. The
thickness of the layer may range from a few microns to several mm
in thickness.
[0021] The present invention provides a coloured single crystal CVD
diamond which is desirable. A particular aspect of the invention is
the provision of fancy coloured diamond suitable to produce
gemstones, the term fancy referring to a gem trade classification
of stronger and more unusual colours in diamond. Even more
particularly the invention can provide a range of fancy brown
colours, an example being fancy light pink brown. This invention
further provides for a thick (>1 mm) diamond layer with uniform
properties through its thickness so that any desirable colour is
not quenched or hidden by defects related to low crystalline
quality. The fanciness of the colours was originally not
anticipated, nor the degree to which they could be controlled by
choosing appropriate synthesis and substrate conditions. No post
growth treatment is needed to produce these colours. In fact many
of these colours are impossible to produce using post growth
treatments, as a consequence of the relative colour centre
formation mechanisms that compete during irradiation and annealing.
In addition, characteristics associated with the CVD growth
mechanism can result in absorption bands at .about.350 nm and
.about.510 nm. These are important for the final colour produced,
but the centres responsible are not present in natural or other
synthetic diamond. Consequently the colours achieved are unique to
CVD diamond, and more particularly to CVD diamond of the
invention.
[0022] Further, there is no post growth treatment as the colour
centres are introduced by a careful selection of growth conditions.
There are many reports in the literature of homoepitaxial CVD
growth on high pressure high temperature (HPHT) synthetic and
natural diamond substrates. Although there are only a few reports
of thick layers (>100 .mu.m), these tend to have an unattractive
brown colour which results mainly from absorption related to low
crystalline quality defects and graphitic/metallic inclusions and
which tend to increase with growth thickness. Even if growth
conditions were chosen to allow incorporation of colour centres
that would give the diamond a desirable colour, this desirable
colour would be masked by the dominant absorption relating to the
low quality nature of the diamond crystal structure.
[0023] In addition, the majority volume of the CVD diamond layer of
the invention may exhibit one or more of the following
properties:
[0024] 1. High crystalline quality as determined by a low density
of extended defects, related factors such as narrow Raman line
width, relatively featureless X-ray topography and narrow rocking
curve, mechanical integrity, strength and mechanical processability
of the material to form highly polished surfaces and edges. In this
context high quality excludes quality factors normally requiring
the absence of N, including features such as: the N impurities
themselves and also associated point defects including H related
defects and vacancies, electronic based properties such as mobility
and charge collection distance which are very sensitive to
scattering centres and traps, and the specific optical absorption
and luminescence characteristics induced by the presence of the
added nitrogen and the associated defects.
[0025] 2. A level of any single impurity: Fe, Si, P, S, Ni, Co, Al,
Mn of not greater than 1 ppm and a total impurity content of not
greater than 5 ppm. In the above, "impurity" excludes hydrogen and
its isotopic forms.
[0026] 3. In EPR, a spin density <1.times.10.sup.17 cm.sup.-3
and more typically <5.times.10.sup.16 cm.sup.-3 at g=2,0028. In
single crystal diamond this line at g=2.0028 is related to lattice
defect concentrations and is typically large in natural type IIa
diamond, in CVD diamond plastically deformed through indentation,
and in poor quality homoepitaxial diamond.
[0027] 4. X-ray topography showing features related to growth where
<100> edges of the original substrate are grown out to form
<110> edges.
[0028] The coloured CVD diamond layer of the invention may be on a
surface of a substrate, typically a diamond substrate, and will
generally be a free standing layer. A gemstone can be produced from
the composite CVD diamond layer/diamond substrate or from the free
standing layer.
[0029] The coloured single crystal CVD diamond of the invention may
be made by a method that forms yet another aspect of the invention.
This method includes the steps of providing a diamond substrate
having a surface which is substantially free of crystal defects,
providing a source gas, dissociating the source gas to produce a
synthesis atmosphere which contains 0,5 to 500 ppm nitrogen,
calculated as molecular nitrogen, and allowing homoepitaxial
diamond growth on the surface which is substantially free of
crystal defects.
[0030] In the method of the invention, the source gas which is used
to produce the synthesis atmosphere in which homoepitaxial growth
on the diamond substrate occurs contains a suitable amount of
nitrogen. The nitrogen may be included in the source gas or added
to a source gas which contains substantially no nitrogen. The
nitrogen, either in the source gas or added to the source gas, must
be such as to produce a synthesis atmosphere which contains 0,5 to
500 ppm, preferably 1 to 100 ppm of nitrogen, calculated as
molecular nitrogen. The nitrogen in the source gas or added to it
may be molecular nitrogen or a nitrogen containing gas such as
ammonia.
[0031] The nitrogen in the synthesis atmosphere or plasma, in
addition to producing colour centres in the diamond, can be used
beneficially to cause morphological changes to the growing single
crystal CVD diamond. Specifically, the addition of nitrogen to the
gas phase can be used to enhance the size of the {100} growth
sector and reduce the size of competing growth sectors such as the
{111}. This means that, for growth on a {100} plate, the addition
of nitrogen enables the growth to remain substantially {100} growth
sector.
[0032] Coloured gemstones and more particularly fancy coloured
gemstones may be produced from the CVD diamond of the invention and
CVD diamond produced by the method described above. Such gemstones
may be of high quality. In gem quality grading, one of the four key
quality parameters is the clarity of the diamond gemstone. The
clarity grades used are generally those defined by the GIA
(Gemological Institute of America) and run on a scale from FL
(flawless), IF, VVS1 (very very slightly included), VVS2, VS1 (very
slightly included), VS2, SI1 (slightly included), SI2, I1
(imperfect), I2 and I3.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1. Spectral decomposition of UV/visible absorption
spectrum of an orangish brown CVD diamond layer
[0034] Spectrum A; Type Ib HPHT synthetic diamond
[0035] Spectrum B: Original spectrum of orangish brown CVE)
diamond
[0036] Spectrum C: Spectral component with (wavelength).sup.-3
dependence
[0037] Spectrum D: Spectral component composed of two broad
absorption bands
[0038] FIG. 2 UV/visible absorption spectra for a set of brown CVD
layers
[0039] FIG. 3 CIELAB a*b* diagram for brown CVD diamond
[0040] FIG. 4 CIELAB L*C* diagram for brown CVD diamond
[0041] FIG. 5 CIELAB a*b* plot for diamond layers grown in
different kinds of CVD process
[0042] FIG. 6 CIELAB L*C* plot for diamond layers grown in
different kinds of CVD process
[0043] FIG. 7 Spectral decomposition of UV/visible spectrum of
FN-1
[0044] Spectrum A: FN-1
[0045] Spectrum B: Type Ib HPHT synthetic diamond
[0046] Spectrum C: Spectral component with (wavelength).sup.-3
dependence
[0047] Spectrum D: Spectral component composed of two broad
absorption bands
[0048] FIG. 8 CIELAB a*b* diagram from FN-1
[0049] FIG. 9 CIELAB lightness saturation diagram for FN-1
DETAILED DESCRIPTION OF THE INVENTION
Absorption spectroscopy of homoepitaxial CVD diamond
[0050] The UV/visible absorption spectrum of type Ib diamond
contains features associated with single substitutional nitrogen.
These include an absorption coefficient maximum at 270 nm and, to
longer wavelengths, a gradual decrease in absorption coefficient
between approximately 300 nm and 500 nm, with signs of a broad
absorption band at approximately 365 nm. These features can be seen
in absorption spectra of a type Ib high pressure high temperature
diamond such as spectrum A in FIG. 1. Although the effect of single
substitutional nitrogen on the absorption spectrum is greatest in
the ultra-violet, it is the weaker absorption that extends into the
visible region of the spectrum that affects the colour of the type
Ib diamond and gives it a characteristic yellow/brown colour. This
particular colour when strong and displaying the brown element is
generally judged to be undesirable in a gemstone.
[0051] The UV/visible absorption spectrum of homoepitaxial CVD
diamond doped with nitrogen typically contains a contribution from
single substitutional nitrogen with the spectral characteristics
described above. In addition to single substitutional nitrogen,
nitrogen doped homoepitaxial CVD diamond typically contains some
nitrogen in the form of nitrogen vacancy centres. When the N-V
centre is electrically neutral [N-V].sup.0 it gives rise to
absorption with a zero phonon line at 575 nm. When the N-V centre
is negatively charged [N-V].sup.- it gives rise to absorption with
a zerophonon line at 637 nm and an associated system of phonon
bands with an absorption maximum at approximately 570 nm. At room
temperature, the normal temperature for observation of gemstones,
the absorption bands of these two charge states of the N-V centre
merge into a broad band from about 500 nm-640 nm. This absorption
band is in the yellow part of the visible spectrum, and when it is
strong the crystals can exhibit a complementary pink/purple colour.
This absorption can play an important part in determining the
colour of the diamond of this invention.
[0052] The UV/visible absorption spectra of low quality
homoepitaxial CVD diamond, show a gradual rise in measured
absorption from the red to the blue region of the spectrum and into
the ultra-violet. There may also be contributions from scattering.
The spectra generally contain no other features, apart from those
related to single substitutional nitrogen. This absorption spectrum
gives an undesirable brown colour and such diamond often contains
clearly visible graphitic inclusions. Such diamond is unsuitable as
a gemstone material for these reasons and because it cannot in
general be grown to substantial thicknesses without severe
degradation of the crystal quality.
[0053] The coloured single crystal CVD diamond of the invention is
of high crystalline quality and is substantially free of extended
crystal defects and defects that tend to degrade the colour. The
absorption spectrum of the nitrogen-doped diamond of the current
invention contains additional contributions that are not present in
natural, HPHT synthetic diamond or low quality CVD diamond. These
include two broad bands centred at approximately 350 nm and 510 nm.
The band at approximately 350 nm is distinct from the broad feature
in that region of the spectrum of ordinary type Ib spectrum and
distorts the spectrum of ordinary type Ib diamond to an extent
dependent on the concentration of the centre responsible relative
to the single substitutional nitrogen.
[0054] Similarly the band centred at approximately 510 nm can
overlap absorption relating to negative nitrogen-vacancy centres
and the visible absorption relating to single substitutional
nitrogen.
[0055] The overlapping of the various contributions to the
absorption spectra can cause the bands at approximately 350 and 510
nm to give rise to broad shoulders in the absorption spectrum
rather than distinct maxima. These contributions to absorption do
however have a very significant effect on the relative absorption
coefficients of the diamond at wavelengths in the spectral region
between 400 and 600 nm where the eye is very sensitive to small
differences. They therefore make an important contribution to the
perceived colour of the diamond. Together with the luminescence
characteristics noted below, these absorption characteristics can
give diamond gemstones produced from such diamond desirable fancy
brown colours, including fancy dark brown, orange brown and pink
brown.
[0056] The width and position in the spectrum of these bands can
vary. The position of peak maxima is most easily ascertained by
using the second differential of the spectrum. It has been found
that absorption spectra can generally be deconstructed into the
following approximate components.
[0057] 1) Single substitutional nitrogen component with an
absorption coefficient at 270 nm that is generally within the range
0.4 cm.sup.-1 and 10 cm.sup.-1 and an absorption coefficient at 425
nm that generally lies between 0.04 cm.sup.-1 and 1 cm.sup.-1.
[0058] 2) An absorption band centred at 3.54 eV (350 nm) +/-0.2 eV
with a FWHM of approximately 1 eV and a maximum contribution to the
absorption spectrum generally between 1 and 8 cm.sup.-1 at its
centre.
[0059] 3) An absorption band centred at 2.43 eV (610 nm) +/-0.4 eV
with a FWHM of approximately 1 eV and a maximum contribution to the
absorption spectrum generally between 0.2 and 4 cm.sup.-1 at its
centre.
[0060] 4) A small residual wavelength dependent component of the
measured absorption coefficient (in cm.sup.-1) that is found to
have a wavelength dependence of the following approximate form; c x
(wavelength in microns).sup.-3 where c<0.2 such that the
contribution of this component at 510 nm is generally less than 1.5
cm.sup.-1.
[0061] FIG. 1 shows the absorption spectrum of a brown CVD diamond
layer (curve B) and the components into which it can be decomposed.
The first step in such a spectral decomposition is the subtraction
of the spectrum of a type Ib HPHT synthetic diamond (curve A),
scaled so that the residual shows no 270 nm feature. The residual
spectrum can then be decomposed into a c.times..lambda..sup.-3
component (curve C) and two overlapping bands of the kind described
above (curve D).
[0062] It has been found that the form of UV/visible spectra of CVD
diamond grown using a range of different processes can be well
specified by sums of the components described above, with different
weighting factors for the components in different cases. For the
purposes of specifying the shape of the spectrum the contributions
of the different components are given in the following ways.
[0063] 270 nm: The peak 270 nm absorption coefficient of the type
Ib component is measured from a sloping baseline connecting the
type Ib spectrum either side of the 270 nm feature that extends
over the approximate range 235 nm-325 nm.
[0064] 350 nm band: The peak absorption coefficient contribution of
this band.
[0065] 510 nm band: The peak absorption coefficient contribution of
this band.
[0066] Ramp: The contribution of the c.times..lambda..sup.-3
component to the absorption coefficient at 510 nm.
CIELAB chromaticity Coordinate Derivation
[0067] 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 CIELAB chromaticity coordinates quoted in
this patent application have been derived in the way described
below. Using a standard D65 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
[0068] S.sub..lambda.=transmittance at wavelength .lambda.
[0069] L.sub..lambda.=spectral power distribution of the
illumination
[0070] x.sub..lambda.=red response function of the eye
[0071] y.sub..lambda.=green response function of the eye
[0072] z.sub..lambda.=blue response function of the eye
X=.SIGMA..sub..lambda.[S.sub..lambda.x.sub..lambda.L.sub..lambda.]/Y.sub.0
Y=.SIGMA..sub..lambda.[S.sub..lambda.y.sub..lambda.L.sub..lambda.]/Y.sub.0
Z=.SIGMA..sub..lambda.[S.sub..lambda.z.sub..lambda.L.sub..lambda.]/Y.sub.0
[0073] Where
Y.sub.0=.SIGMA..sub..lambda.y.sub..lambda.L.sub..lambda.
L*=116 (Y/Y.sub.0).sup.1/3-16=Lightness (for
Y/Y.sub.0>0.008856)
a*=500[(X/X.sub.0).sup.1/3-(Y/Y.sub.0).sup.1/3](for
X/X.sub.0>0.008856, Y/Y.sub.0>0.0088 56)
b*=200[(Y/Y.sub.0).sup.1/3-(Z/Z.sub.0).sup.1/3](for
Z/Z.sub.0>0.008856)
C*=(a*.sup.2+b*.sup.2).sup.1/2=saturation
h.sub.ab=arctan(b*/a*)=hue angle
[0074] Modified versions of these equations must be used outside
the limits of Y/Y.sub.0, X/X.sub.0 and Z/Z.sub.0. The modified
versions are given in a technical report prepared by the Commission
Internationale de L'Eclairage (Colorimetry (1986)).
[0075] It is normal 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.
[0076] 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 CIELAB
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.
[0077] L*, the lightness, forms the third dimension of the CIELAB
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. This can
be illustrated on a colour tone diagram in which L* is plotted
along the y-axis and C* is plotted along the x-axis (such as FIG.
4). 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.
[0078] The C* (saturation) numbers can be divided into saturation
ranges of 10 C* units and assigned descriptive terms as below.
3 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
[0079] Similarly the L* numbers can be divided up into lightness
ranges as follows:
4 5-15 very very dark 15-25 very dark 25-35 dark 35-45 medium/dark
45-55 medium 55-65 light/medium 65-75 light 75-85 very light 85-95
very very light
[0080] There are four basic colour tones defined by the following
combinations of lightness and saturation:
5 Bright: Light and high saturation, Pale: Light and low
saturation, Deep: High saturation and dark, Dull: Low saturation
and dark.
[0081] FIG. 2 shows absorption spectra for four samples with
orangish brown to orange-brown colour and grown to 1.7 mm thickness
with differing growth conditions. These spectra have similar shapes
but display a range of different absorption strengths. Thus, by
altering the growth conditions, it is possible to tune the strength
of absorption to achieve different colours for a given thickness of
CVD layer. Similarly for a gemstone produced with a given size and
cut, the colour can be tuned by altering the growth conditions.
[0082] The table below lists the strengths of the different
contributions to the four spectra shown in FIG. 2, defined in the
way described earlier, together with the CIELAB information derived
from the spectra. The hue angle, as given earlier, is defined as
h.sub.ab=arctan(b*/a*).
6 Table of absorption contributions and CIELAB values Spectrum A B
C D 270 nm band (cm.sup.-1) 0.93 1.3 1.6 1.6 350 nm band
(cm.sup.-1) 0.45 1.5 2.0 4.0 510 nm band (cm.sup.-1) 0.3 0.6 0.8
1.2 Ramp (cm.sup.-1) 0.19 0.46 0.60 1.26 a* 1.2 1.7 2.7 4.0 b* 2.8
6.4 7.9 14.5 C* 3.0 6.7 8.3 15 L* 84 82 79 72 Hue angle (degrees)
68 75 71 75
[0083] FIGS. 3 and 4 show respectively an a*b* plot and an L*C*
plot derived, in the way discussed above, from the absorption
spectrum of one of the 1.7 mm thick orangish brown CVD diamond
layers (C). It can be seen that the L*C* curve runs between regions
corresponding to pale, moderately bright, deep and finally dull.
Although this layer had a pale tone, the optical properties of the
diamond are such that thicker layers of such diamond, after
skillfully polishing, can yield gemstones with a range of different
possible tones and colours. This is illustrated by the polished
gemstones of examples 1, 3 and 4 that were given colour grades of
fancy light pink-brown, fancy dark orangish brown and fancy pink
brown.
[0084] FIGS. 5 and 6 show CIELAB a*b* and L*C* plots for a range of
samples of similar thickness. They show that significant variations
in hue, saturation and lightness result from differences in growth
conditions. Thus the CVD process can be adjusted to control the
colour that will result for a polished stone of a given size and
cut.
[0085] Collectors of natural fancy colour diamonds acknowledge that
these are desirable colours. In his book "Collecting and
Classifying Coloured Diamonds" (Ashland Press, New York, 1998),
Stephen Hofer describes the Aurora Collection, one of the largest
collections of natural fancy coloured diamonds. These diamonds are
acknowledged to have desirable colours and amongst them there are
several with colours similar to those which can be achieved in CVD
synthetic diamond using the method of this invention. Some of these
are listed below. In the two cases where the CIELAB data are given,
the hue angles are very close to that for CVD synthetic diamond of
this invention.
7 Table of colour descriptions of selected diamonds from the Aurora
Collection Hue angle Aurora no. Colour a* b* C* L* (degrees) 259
Light pinkish orangish 5.1 11.1 12.2 70 65 (topaz) brown 231 Medium
dark pinkish 8.3 18.3 20.1 43 66 (cinnamon) orangish brown 48
Medium pinkish (cinnamon) orangish brown 171 Dark orangish brown
(cognac) 130 Very dark orangish (chestnut) brown 78 Medium-dark
pinkish (cinnamon) orangish brown
[0086] Luminescence
[0087] Although the colour of a diamond is principally dependent on
its absorption spectrum, it can also be influenced by its
luminescence properties. This may be particularly the case for
certain viewing conditions. For example, the luminescence will have
the greatest effect when the diamond is viewed from a small
distance under illumination with light that contains a strong
component in a wavelength range that excites the luminescence most
efficiently.
[0088] The diamond of the present invention can show strong
luminescence from nitrogen-vacancy colour centres. The neutral and
negatively charged N-V centres have their zero-phonon lines at 575
nm and 637 nm, respectively, and have absorption band systems on
the shorter wavelength side of these zero-phonon lines. Light of
wavelengths within the range covered by these absorption bands can
be absorbed by these colour centres and give rise to luminescence
with a spectrum which is characteristic of these centres. The
luminescence from the neutral N-V centre is predominantly orange.
That from the negatively charged N-V centre is red.
[0089] The negatively charged N-V centre is a relatively strong
absorber, giving rise to an absorption band system with a maximum
at around 570 nm. Some of the energy absorbed at these centres is
re-emitted as luminescence. In contrast, the neutral N-V centre has
a very small effect on the absorption spectrum and the energy
absorbed is typically converted to luminescence with a high
efficiency.
[0090] N-V centres in the vicinity of an electron donor such as
single substitutional nitrogen, are negatively charged, while
isolated N-V centres are neutral. The effect of a given
concentration of N-V centres on the colour of a diamond therefore
depends on the concentration and relative distribution of electron
donors. For example, N-V centres in diamond containing a high
concentration of N will contribute to the colour predominantly via
absorption of light by negatively charged N-V centres with a
smaller contribution coming from luminescence. In the case of
diamond containing low concentrations of electron donors such as
nitrogen, luminescence from neutral N-V centres can make a more
important contribution.
[0091] Luminescence Measurement and Quantification
[0092] As a result of variations in the importance of non-radiative
paths, luminescence properties of diamond samples cannot in general
be deduced directly from the concentrations of the various
contributing centres as determined by absorption spectroscopy.
Quantitative luminescence properties of diamond samples can,
however be specified by normalising the integrated intensities of
relevant luminescence lines or bands relative to the integrated
intensity of diamond Raman scattering (nominally at 1332 cm.sup.-1)
collected under the same conditions.
[0093] The table below lists the results of quantitative
luminescence measurements made on a range of single crystal CVD
diamond samples of the invention. In each case, the measurements
were made after removal of the {100} substrate on which they were
grown The growth conditions favoured the formation of predominantly
<100> sector diamond samples with uniform luminescence
properties as judged by luminescence imaging. Any small additional
sectors with different luminescence properties were removed before
the measurements were made.
[0094] The luminescence was excited at 77K with a 300 mW 514 nm
argon ion laser beam and spectra were recorded using a Spex 1404
spectrometer equipped with a holographic grating (1800 grooves/mm)
and a Hamamatsu R928 photomultiplier. The data were corrected to
allow for spectral response function of the spectrometer system,
derived using a standard lamp with a known spectral output.
8 Normalised Normalised Sample I(575) I(637) I(637)/I(575) 404
1.929 6.880 3.566 407 5.808 17.65 3.039 409 3.116 10.07 3.233 410
1.293 4.267 3.299 412 2.703 7.367 2.725 414 17.09 52.29 3.058 415
19.06 41.92 2.198 416 17.02 70.00 4.111 417 32.86 69.77 2.123 418
29.34 61.31 2.089 423 6.985 7.019 1.004 424 51.41 101.8 1.981 425
68.22 277.4 4.067 426 16.17 29.23 1.807 434 4.929 4.378 0.8883 435
0.4982 1.223 2.455 437 0.3816 0.2224 0.5828 439 4.24 2.891 0.6818
505 0.00954 0.04031 4.225 507c 0.3455 2.347 6.793 507b 0.106
0.03252 0.3068 511b 4.611 4.211 0.9134 501 2.586 1.959 0.7577 512
7.282 7.686 1.055 515 0.01886 0.01932 1.024 520 0.1802 0.5421 3.008
521 0.0402 0.03197 0.7936 513 0.0243 0.01765 0.7240 509 25.22 13.87
0.5498 511c 0.0371 0.01112 0.2997 513b 1.091 1.262 1.155 513c
0.1717 0.2224 1.295 513d 1.992 0.7645 0.3836 510b 0.3922 0.6963
1.775 510c 0.1643 0.6268 3.815 510d 1.091 0.6811 0.6238 514a 126.6
56.57 0.4466 514b 101.3 50.79 0.5012 514c 141.6 67.83 0.4789
[0095] It is important for the production of high crystalline
quality (as herein defined) thick single crystal CVD diamond with
properties suitable for coloured gem stones that growth takes place
on a diamond surface which is substantially free of crystal
defects. In this context defects primarily mean dislocations and
micro cracks, but also include twin boundaries, point defects not
intrinsically associated with the dopant N atoms, low angle
boundaries and any other extended disruption to the crystal
lattice. Preferably the substrate is a low birefringence type Ia
natural, Ib or IIa high pressure/high temperature synthetic diamond
or a CVD synthesised single crystal diamond.
[0096] The quality of growth on a substrate which is not
substantially free of defects rapidly degrades as the layer grows
thicker and as the defect structures multiply, causing general
crystal degradation, twinning and renucleation.
[0097] The defect density is most easily characterised by optical
evaluation after using a plasma or chemical etch optimised to
reveal the defects (referred to as a revealing plasma etch), using
for example a brief plasma etch of the type described below. Two
types of defects can be revealed:
[0098] 1) Those intrinsic to the substrate material quality. In
selected natural diamond the density of these defects can be as low
as 50/mm.sup.2 with more typical values being 10.sup.2/mm.sup.2,
whilst in others it can be 10.sup.6/mm.sup.2 or greater.
[0099] 2) Those resulting from polishing, including dislocation
structures and microcracks forming chatter tracks along polishing
lines. The density of these can vary considerably over a sample,
with typical values ranging from about 10.sup.2/mm.sup.2, up to
more than 10.sup.4/mm.sup.2 in poorly polished regions or
samples.
[0100] The preferred low density of defects is such that the
density of surface etch features related to defects, as described
above, is below 5.times.10.sup.3/mm.sup.2, and more preferably
below 10.sup.2/mm.sup.2.
[0101] The defect level at and below the substrate surface on which
the CVD growth takes place may thus be minimised by careful
preparation of the substrate. Included here under preparation is
any process applied to the material from mine recovery (in the case
of natural diamond) or synthesis (in the case of synthetic
material) as each stage can influence the defect density within the
material at the plane which will ultimately form the substrate
surface when preparation as a substrate is complete. Particular
processing steps may include conventional diamond processes such as
mechanical sawing, lapping and polishing (in this application
specifically optimised for low defect levels), and less
conventional techniques such as laser processing or ion
implantation and lift off techniques, chemical/mechanical
polishing, and both liquid and plasma chemical processing
techniques. In addition, the surface R.sub.Q (root mean square
deviation of surface profile from flat measured by stylus
profilometer, preferably measured over 0,08 mm length) should be
minimised, typical values prior to any plasma etch being no more
than a few nanometers, i.e. less than 10 nanometers.
[0102] One specific method of minimising the surface damage of the
substrate, is to include an in situ plasma etch on the surface on
which the homoepitaxial diamond growth is to occur. In principle
this etch need not be in situ, nor immediately prior to the growth
process, but the greatest benefit is achieved if it is in situ,
because it avoids any risk of further physical damage or chemical
contamination An in situ etch is also generally most convenient
when the growth process is also plasma based. The plasma etch can
use similar conditions to the deposition or diamond growing
process, but with the absence of any carbon containing source gas
and generally at a slightly lower temperature to give better
control of the etch rate. For example, ft can consist of one or
more of:
[0103] (i) an oxygen etch using predominantly hydrogen with
optionally a small amount of Ar and a required small amount of
O.sub.2. Typical oxygen etch conditions are pressures of
50-450.times.10.sup.2 Pa, an etching gas containing an oxygen
content of 1 to 4 percent an argon content of 0 to 30 percent and
the balance hydrogen, all percentages being by volume, with a
substrate temperature 600-1100.degree. C. (more typically
800.degree. C.) and a typical duration of 3-60 minutes.
[0104] (ii) a hydrogen etch which is similar to (i) but where the
oxygen is absent.
[0105] (iii) alternative methods for the etch not solely based on
argon, hydrogen and oxygen may be used, for example, those
utilising halogens, other inert gases or nitrogen.
[0106] Typically the etch consists of an oxygen etch followed by a
hydrogen etch and then moving directly into synthesis by the
introduction of the carbon source gas. The etch time/temperature is
selected to enable remaining surface damage from processing to be
removed, and for any surface contaminants to be removed, but
without forming a highly roughened surface and without etching
extensively along extended defects such as dislocations which
intersect the surface and thus cause deep pits. As the etch is
aggressive, it is particularly important for this stage that the
chamber design and material selection for its components be such
that no material is transferred by the plasma into the gas phase or
to the substrate surface. The hydrogen etch following the oxygen
etch is less specfic to crystal defects rounding off the
angularities caused by the oxygen etch which aggressively attacks
such defects and providing a smoother, better surface for
subsequent growth.
[0107] The surface or surfaces of the diamond substrate on which
the CVD diamond growth occurs are preferably the {100}, {110},
{113} or {111} surfaces. Due to processing constraints, the actual
sample surface orientation can differ from these ideal orientations
up to 5.degree., and in some cases up to 10.degree., although this
is less desirable as it adversely affects reproducibility.
[0108] It is also important in the method of the invention that the
impurity content of the environment in which the CVD growth takes
place is properly controlled. More particularly, the diamond growth
must take place in the presence of an atmosphere containing
substantially no contaminants other than the intentionally added
nitrogen which should be controlled to better than 500 parts per
billion (as a molecular fraction of the total gas volume) or 5% in
the gas phase, whichever is the larger, and preferably to better
than 300 parts per billion (as a molecular fraction of the total
gas volume) or 3% in the gas phase, whichever is the larger, and
more preferably to better than 100 parts per billion (as a
molecular fraction of the total gas volume) or 1% in the gas phase,
whichever is the larger. Measurement of absolute and relative
nitrogen concentration in the gas phase at concentrations as low as
100 ppb requires sophisticated monitoring equipment such as that
which can be achieved, for example, by gas chromotography. An
example of such a method is now described:
[0109] Standard gas chromatography (GC) art consists of: a gas
sample stream is extracted from the point of interest using a
narrow bore sample line, optimised for maximum flow velocity and
minimum dead volume, and passed through the GC sample coil before
being passed to waste. The GC sample coil is a section of tube
coiled up with a fixed and known volume (typically 1 cm.sup.3 for
standard atmospheric pressure injection) which can be switched from
its location in the sample line into the carrier gas (high purity
He) line feeding into the gas chromatography columns. This places a
sample of gas of known volume into the gas flow entering the
column; in the art, this procedure is called sample injection.
[0110] The injected sample is carried by the carrier gas through
the first GC column (filled with a molecular sieve optimised for
separation of simple inorganic gases) and is partially separated,
but the high concentration of primary gases (e.g. H.sub.2, Ar)
causes column saturation which makes complete separation of, for
example nitrogen difficult. The relevant section of the effluent
from the first column is then switched into the feed of a second
column, thereby avoiding the majority of the other gases being
passed into the second column, avoiding column saturation and
enabling complete separation of the target gas (N.sub.2). This
procedure is called "heart-cutting".
[0111] The output flow of the second column is put through a
discharge ionisation detector (DID), which detects the increase in
leakage current through the carrier gas caused by the presence of
the sample. Chemical structure is identified by the gas residence
time which is calibrated from standard gas mixtures. The response
of the DID is linear over more than 5 orders of magnitude, and is
calibrated by use of special calibrated gas mixtures, typically in
the range of 10-100 ppm, made by gravimetric analysis and then
verified by the supplier. Linearity of the DID can be verified by
careful dilution experiments.
[0112] This known art of gas chromatography has been further
modified and developed for this application as follows: The
processes being analysed here are typically operating at
50-500.times.102 Pa. Normal GC operation uses the excess pressure
over atmospheric pressure of the source gas to drive the gas
through the sample line. Here, the sample is driven by attaching a
vacuum pump at the waste end of the line and the sample drawn
through at below atmospheric pressure. However, whilst the gas is
flowing the line impedance can cause significant pressure drop in
the line, affecting calibration and sensitivity. Consequently,
between the sample coil and the vacuum pump is placed a valve which
is shut for a short duration before sample injection in order to
enable the pressure at the sample coil to stabilise and be measured
by a pressure gauge. To ensure a sufficient mass of sample gas is
injected, the sample coil volume is enlarged to about 5 cm.sup.3.
Dependent on the design of the sample line, this technique can
operate effectively down to pressures of about 70.times.10.sup.2
Pa. Calibration of the GC is dependent on the mass of sample
injected, and the greatest accuracy is obtained by calibrating the
GC using the same sample pressure as that available from the source
under analysis. Very high standards of vacuum and gas handling
practice must be observed to ensure that the measurements are
correct.
[0113] The point of sampling may be upstream of the synthesis
chamber to characterise the incoming gases, within the chamber to
characterise the chamber environment, or downstream of the
chamber.
[0114] The source gas may be any known in the art and will contain
a carbon-containing material which dissociates producing radicals
or other reactive species. The gas mixture will also generally
contain gases suitable to provide hydrogen or a halogen in atomic
form.
[0115] The dissociation of the source gas is preferably carried out
using microwave energy in a reactor examples of which are known in
the art. However, the transfer of any impurities from the reactor
should be minimised. A microwave system may be used to ensure that
the plasma is placed away from all surfaces except the substrate
surface on which diamond growth is to occur and its mount
(substrate carrier). Examples of a preferred mount materials are:
molybdenum, tungsten, silicon and silicon carbide. Examples of
preferred reactor chamber materials are stainless steel, aluminium,
copper, gold and platinum.
[0116] A high plasma power density should be used, resulting from
high microwave power (typically 3-60 kW, for substrate carrier
diameters of 25300 mm) and high gas pressures
(50-500.times.10.sup.2 Pa, and preferably 100-450.times.10.sup.2
Pa).
[0117] Using the above conditions it has been possible to produce
thick high quality single crystal CVD diamond layers with a
desirable fancy colour using nitrogen additions, calculated as
molecular nitrogen, to the gas flow in the range 0.5 to 500 ppm.
The range of nitrogen concentrations for which growth of fancy
brown diamond is possible has a complex dependence on other
parameters such as substrate temperature, pressure and gas
composition.
[0118] Suitable conditions for synthesis of the material of the
invention are best illustrated by way of example.
EXAMPLE 1
[0119] Substrates suitable for synthesising single crystal CVD
diamond of the invention may be prepared as follows:
[0120] i) Selection of stock material (type Ia natural stones and
type Ib HPHT stones) was optimised on the basis of microscopic
investigation and birefringence imaging to identify substrates
which were free of strain and imperfections.
[0121] ii) Laser sawing, lapping and polishing to minimise
subsurface defects using a method of a revealing plasma etch to
determine Fe defect level being introduced by the processing.
[0122] iii) After optimisation it was possible routinely to produce
substrates in which the density of defects measurable after a
revealing etch is dependent primarily on the material quality and
is below 5.times.10.sup.3/mm.sup.2, and generally below
10.sup.2/mm.sup.2. Substrates prepared by this process are then
used for the subsequent synthesis.
[0123] A high temperature/high pressure synthetic type 1b diamond
was grown in a high pressure press, and as a substrate using the
method described above to minimise substrate defects to form a
polished plate with lateral dimensions 5 mm.times.5 mm and
thickness 500 .mu.m, with all faces {100}. The surface roughness
R.sub.Q at this stage was less than 1 nm. The substrate was mounted
on a tungsten substrate using a high temperature diamond braze.
This was introduced into a reactor and an etch and growth cycle
commenced as described above, and more particularly:
[0124] 1) The 2.45 GHz reactor was pre-fitted with point of use
purifiers, reducing unintentional contaminant species in the
incoming gas stream to below 80 ppb.
[0125] 2) An in situ oxygen plasma etch was performed using
15/75/600 sccm (standard cubic centimetre per second) of
O.sub.2/Ar/H.sub.2 at 263.times.10.sup.2 Pa and a substrate
temperature of 730.degree. C.
[0126] 3) This moved without interruption into a hydrogen etch with
the removal of the O.sub.2 from the gas flow.
[0127] 4) This moved into the growth process by the addition of the
carbon source (in this case CH.sub.4) and dopant gases. In this
instance was CH.sub.4 flowing at 42 sccm and 3 ppm N.sub.2
(calculated as [N.sub.2]/[All gases] where [N.sub.2] represents the
number of moles of N.sub.2 and [All gases] represents the number of
moles of all gases present) in the gas phase. The substrate
temperature was 830.degree. C.
[0128] 5) On completion of the growth period, the substrate was
removed from the reactor and the CVD diamond layer removed from the
substrate.
[0129] 6) This layer, identified as FN-1, was then polished to
produce a 6.times.6.times.3 mm square cut synthetic diamond with
weight 1.1 carats and certified by a professional diamond grader to
have a desirable fancy light pink brown colour and a quality grade
of VS1.
[0130] 7) FN-1 was further characterised by the data provided
below:
[0131] i) An optical absorption spectrum showing the characteristic
broad bands at 270 nm and approximately 355 nm and 510 nm. FIG. 7
shows the decomposition of the original spectrum (curve A) into a
type Ib spectrum (curve B), a ramp component with a
(wavelength).sup.-3 dependence (curve C) and the two overlapping
bands centred at 355 and 510 nm (curve D). The peak 270 nm
absorption coefficient of the type Ib component above a sloping
baseline connecting the type Ib spectrum either side of the 270 nm
peak, is 0.67 cm.sup.-1. The (wavelengths) component and the 510 nm
band contribute 0.11 cm.sup.-1 and 0.21 cm.sup.-1 respectively at
510 nm. The 355 m band contributes 0.32 cm.sup.-1 at its peak.
FIGS. 8 and 9 show CIELAB hue and tone diagrams respectively for
diamond with the FN-1 absorption spectrum. The CIELAB coordinates
derived from the absorption spectrum of FN-1 were as follows;
a*=1.8, b*=3.9, L*=81, C*=4.3 and hue angle=65 degrees.
[0132] ii) Luminescence excited at 77 K with a 300 mW 514 nm Ar ion
laser showing the zero phonon lines at 575 and 637 nm with Raman
normalised intensities of 6.98 and 7.02 respectively
[0133] iii) The EPR spectra showing single substitutional nitrogen
with concentration 0.3 ppm.
[0134] iv) X-ray rocking curves map, showing the angular spread of
the sample to be less than 20 arc sec.
[0135] v) Raman spectrum showing a line width (FWHM) to be 2
cm.sup.-1.
[0136] vi) SIMS showed a total nitrogen concentration of 0.35
ppm
EXAMPLE 2
[0137] A 3.0 mm thick layer of CVD diamond was grown on a type Ib
HPHT synthetic diamond substrate prepared in the same way as
described in example 1 except with the following growth
conditions:
[0138] (i) Etch temp of 718.degree. C.
[0139] (ii) Growth conditions consisted of 32125/600 sccm (standard
cubic centimetre per second) of CH.sub.4/Ar/H.sub.2 at
180.times.10.sup.2 Pa and a substrate temperature of 800.degree. C.
with 24 ppm added N.sub.2.
[0140] After growth, the substrate was removed and the top and
bottom surfaces of the were polished. UV/visible absorption spectra
of the resulting CVD layer, designated FN-2, were recorded and
analysed into the components discussed in the detailed description
of the invention. The results are listed in the table below.
9 Sample 270 nm 360 nm band 510 nm band Ramp FN-2 1.35 cm.sup.-1
1.05 cm.sup.-1 0.55 cm.sup.-1 0.31 cm.sup.-1
[0141] The layer had a pale orangish brown colour and when the
CIELAB coordinates were derived from the absorption spectrum, in
the way described in the detailed description of the invention, the
following results were obtained.
10 Hue angle Sample a* b* C* L* (degrees) FN-2 1.9 4.8 5.2 81
69
EXAMPLE 3
[0142] A 2.84 mm thick layer of CVD diamond was grown on a type Ib
HPHT synthetic diamond substrate prepared in the same way as
described in example 1 except with the following growth
conditions:
[0143] (i) Etch temp of 710.degree. C.
[0144] (ii) Growth conditions consisted of 42/25/600 sccm (standard
cubic centimetre per second) of CH.sub.4/Ar/H.sub.2 at
420.times.10.sup.2 Pa and a substrate temperature of 880.degree. C.
with 24 ppm added N.sub.2.
[0145] The substrate was removed and resulting CVD layer,
designated FN-3, was polished into a rectangular cut CVD gemstone
of 1.04 carats which was certified by a professional diamond grader
to have a desirable fancy dark orangey brown colour and a quality
grade of SI1.
[0146] The luminescence excited at 77 K with a 300 mW 514 nm Ar ion
laser showing the zero phonon lines at 575 and 637 nm with Raman
normalised intensities of 27.7 and 44.1 respectively.
EXAMPLE 4
[0147] A 3.53 mm thick layer of CVD diamond was grown on a type Ib
HPHT synthetic diamond substrate prepared in the same way as
described in example 1 except with the following growth
conditions
[0148] (i) Etch temp of 740.degree. C. .
[0149] (ii) Growth conditions consisted of 38/25/600 sccm (standard
cubic centimetre per second) of CH.sub.4/Ar/H.sub.2 at
283.times.10.sup.2 Pa and a substrate temperature of 860.degree. C.
with 21 ppm added N.sub.2.
[0150] The substrate was removed and resulting CVD layer,
designated FN-4, was polished into a rectangular cut CVD gemstone
of 1.04 carats which was certified by a professional diamond grader
to have a desirable fancy pink brown colour and a quality grade of
SI3.
[0151] The luminescence excited at 77 K with a 300 mW 514 nm Ar ion
laser showing the zero phonon lines at 575 and 637 nm with Raman
normalised intensities of 15.26 and 21.03 respectively.
EXAMPLE 5
[0152] A 1.7 mm thick layer of CVD diamond was grown on a type Ib
HPHT synthetic diamond substrate prepared in Me same way as
described in example 1 except with the following growth
conditions:
[0153] (i) Etch temp of 716.degree. C.
[0154] (ii) Growth conditions consisted of 160/40/3000 scar
(standard cubic centimetre per second) of CH.sub.4Ar/H.sub.2 at
260.times.10.sup.2 Pa and a substrate temperature of 823.degree. C.
with 3.8 ppm added N.sub.2.
[0155] After growth, the substrate was removed and the top and
bottom surfaces of the CVD diamond layer were polished. A
UV/visible absorption spectrum of the resulting CVD layer,
designated FN-5, was recorded (spectrum C in FIG. 2) and analysed
into the components discussed in the detailed description of the
invention. The results are listed in the table below.
11 Sample 270 nm 360 nm band 510 nm band Ramp FN-5 1.60 cm.sup.-1
2.0 cm.sup.-1 0.80 cm.sup.-1 0.60 cm.sup.-1
[0156] The layer had a pale orangish brown colour and when the
CIELAB coordinates were derived from the absorption spectrum, in
the way described in the detailed description of the invention, the
following results were obtained.
12 Hue angle Sample a* b* C* L* (degrees) FN-5 2.7 7.9 8.3 79
71
* * * * *