This invention relates to CVD diamond of high colour suitable for optical applications including gemstones of high colour grade. In particular, the present invention relates to a method of adding a gaseous source comprising a second impurity atom type to counter the detrimental effect on colour caused by the presence in the CVD synthesis atmosphere of a first impurity atom type.

The described method applies to the production of both single crystal diamond and polycrystalline diamond, particularly to single crystal diamond.

All documents referred to herein are hereby incorporated by reference.

Methods of depositing material such as diamond on a substrate by CVD are now well established and have been described extensively in 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. CHx, CFx 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, such as N2, NH3, air and N2H4, for example. In many processes inert gases such as helium, neon or argon are also present. Thus, a typical source gas mixture will contain hydrocarbons CxHy, wherein x and y can each be 1 to 10, or halocarbons CxHyHaI2, 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: COx, wherein x can be 0,5 to 2, O2, H2, N2, NH3, B2H6 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 12C or 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.

Single crystal CVD diamond has a range of applications including electronic devices and highly engineered optical devices. The properties of the diamond can be tailored specifically for each application, and in so doing limitations are placed on the details of the synthesis process and the cost of producing the material. International application WO 01/96634 describes the synthesis of high purity diamond suitable for electronic applications, which because of the low levels of impurity in the gas phase of the deposition process and subsequently in the solid also show low absorption and are suitable for the production of "high colour" diamond (that is, material with absorption close to the theoretical limit for impurity free diamond, and thus typically providing colours equivalent to the natural diamond colour grades of D to better than K, where these are colour grades on the Gemological Institute of America (GIA) colour scale, see 'Diamond Grading ABC, V. Pagel-Theisen, 9th Edition, 2001 , page 61). However, there are economic penalties in providing the degree of control necessary to achieve the low levels of nitrogen used in the method of that invention.

The colour scale of the Gemological Institute of America (GIA), which is the most widely used and understood diamond colour scale, is shown in Table 1. Table 1 is derived from 'Diamond Grading ABC, The Manual', Verena Pagel-Theisen, 9th Edition 2001 , published by Rubin and Son n.v. Antwerp, Belgium, page 61. The colours are determined by comparison with standards. The determination of the colour of diamonds is a subjective process and can only reliably be undertaken by persons skilled in the art.

Table 1

*colourless for round brilliants less than 0.47 cts.

The clarity scale of the Gemological Institute of America (GIA), which is the most widely used clarity scale, is shown in Table 2. Table 2 is derived from 'Diamond Grading ABC, The Manual', Verena Pagel-Theisen, 9th Edition 2001 , published by Rubin and Son n.v. Antwerp, Belgium, page 61. It takes into account both internal and external flaws on a cut diamond. Typically, examination is made with the aid of a 10x magnifier or loupe by an experienced grader with appropriate illumination for the type of defect that is being sought.

Table 2

By "high clarity" is meant herein a clarity of at least SI 1 as defined in Table 2, preferably at least VS 2.

The GIA diamond gem grading system is the most widely used grading scale for diamond gems and generally considered the definitive grading scale. For the purposes of this application all gem colour grades are based on the GIA colour grades, and other gem properties such as clarity are likewise based on the GIA grading system. For a given quality of diamond, i.e. material with given absorption characteristics, the colour of a gem also varies with the size and cut of gem produced, moving to poorer colours (to colours towards Z in the alphabet) as the stone gets larger. To enable the colour system to be applied as a material property it is thus necessary to further fix the size and type of cut of the gemstone. All GIA colour grades given in this specification are for a standardised 0.5 ct round brilliant cut unless otherwise stated.

In contrast to growing high purity layers with high colour, synthesis of coloured gemstones, in which deliberate controlled levels of impurities are added to the process, is reported in WO 03/052177 and WO 03/052174. These techniques provide a method for producing CVD diamond layers and CVD diamond gemstones of a range of colours, typically in the blue or brown part of the spectrum.

Nitrogen is a significant impurity in CVD diamond processes. The extent to which it plays a key role in determining the colour and quality of the material is emphasised in the earlier mentioned prior art. Nitrogen is very prevalent, forming the majority of the atmosphere, and commonly being the major contaminant of gas supplies, even those specified as 'high purity'. It is expensive to remove nitrogen from high purity gas supplies to the levels necessary for synthesis of high colour diamond using the method described in WO 01/96634, which impacts on the cost of the final material, and it is desirable to identify alternative synthesis methods more tolerant of impurities, which are suitable for the production of the thick layers of high colour necessary for the production of gemstones and other selected optical devices.

Diamond containing nitrogen in the form of single substitution nitrogen, present in sufficient concentration to give observable spectroscopic features, is called Ib diamond. The spectroscopic features 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 Figure 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.

The UV/visible absorption spectrum of homoepitaxial CVD diamond grown in the presence of nitrogen typically contains a contribution from single substitutional nitrogen with the spectral characteristics described above. In addition to single substitutional nitrogen, homoepitaxial CVD diamond grown in the presence of nitrogen typically contains some nitrogen in the form of nitrogen vacancy centres. When the N-V centre is electrically neutral [N-V]0 it gives rise to absorption with a zero phonon line at 575 nm. When the N-V centre is negatively charged [N-V]" it gives rise to absorption with a zero-phonon line at 637 nm and an associated system of phonon bands with an absorption maximum at approximately 570 nm. At room temperature 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.

The UV/visible absorption spectra of low quality homoepitaxial CVD diamond grown in the presence of nitrogen, may also 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.

The absorption spectrum of higher quality homoepitaxial CVD diamond grown in the presence of nitrogen 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.

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.

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.

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 for homoepitaxial CVD diamond grown in the presence of nitrogen, and in the absence of any second impurity used according to the current invention, can generally be deconstructed into the following approximate components.

1) Single substitutional nitrogen component with an absorption coefficient at 270 nm that is generally within the range 0.4 cm"1 and 10 cm"1 and an absorption coefficient at 425 nm that generally lies between 0.04 cm"1 and 1 cm"1.

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"1 at its centre.

3) An absorption band centred at 2.43 eV (510 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"1 at its centre.

4) A small residual wavelength dependent component of the measured • absorption coefficient (in cm"1) that is found to have a wavelength dependence of the following approximate form: c x (wavelength in microns)"3 where c < 0.2 such that the contribution of this component at 510 nm is generally less than 1.5 cm"1.

Figure 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 x λ"3 component (curve C) and two overlapping bands of the kind described above (curve D).

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.

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.

350 nm band: The peak absorption coefficient contribution of this band.

510 nm band: The peak absorption coefficient contribution of this band.

Ramp: The contribution of the c x λ"3 component to the absorption coefficient at 510 nm.

The present invention is hereby described with reference to the following figures in which:

Figure 1 shows the spectral decomposition of UV/visible absorption spectrum of an orangish brown CVD diamond layer, representing a typical CVD diamond layer grown in the presence of nitrogen without applying the method of this invention. Spectrum A shows a type Ib HPHT synthetic diamond, spectrum B shows an original spectrum of orangish brown CVD diamond, spectrum C shows a spectral component with (wavelength)'3 dependence, and spectrum D shows a spectral component composed of two broad absorption bands;
Figure 2 shows a photoluminescence spectrum of a silicon doped CVD diamond sample recorded at 77 K with 785 nm laser excitation; and
Figure 3 shows a low magnification optical microscopy image of a sample described in Example 7.

DETAILED DESCRIPTION OF THE INVENTION

In a first embodiment, the present invention provides a method of producing a CVD diamond layer having high colour comprising:
(i) providing a substrate;
(ii) providing a CVD synthesis atmosphere in which there exists a first gas comprising a first impurity atom type which has a detrimental effect on the colour of the produced diamond layer; and
(iii) adding into the synthesis atmosphere a second gas comprising a second impurity atom type, wherein the first and second impurity atom types are different; the type and quantity of the second impurity atom type is selected to reduce the detrimental effect on the colour caused by the first impurity atom type so as to produce a diamond layer having high colour; and the first and second impurity atom types are independently nitrogen or atoms which are solid in the elemental state at room temperature and pressure.

In this way, the method of the present invention is able to provide a CVD diamond layer having high colour where the synthesis atmosphere comprises a gas having a first impurity atom type that would prevent high colour diamond from being produced. For example, the presence of a gas in the synthesis atmosphere comprising nitrogen would typically cause the synthesised diamond to have a yellow/brown colour, whereas the presence of a gas in the synthesis atmosphere comprising boron would typically cause the synthesised diamond to have a blue colour.

The term "high colour" is defined in this invention in two different ways depending upon the form of the diamond material and the application to which it is put. The definition of "high colour" used herein is that which is most applicable to the form of the diamond layer produced and its application. When the diamond is in the form of a round brilliant (i.e. when the diamond is in the form of a gem stone), the GIA colour scale is generally used. When the diamond is in the form of a plate, etc, to be used in a technological application, the material is generally defined in terms of its absorption characteristics. Absorption characteristics are also used to define polycrystalline diamond.

Thus, when the diamond layer of the invention is in the form of a gem stone, 'High colour' is generally defined as being colour better than K on the Gemological Institute of America (GIA) gem diamond colour scale (described above) as determined for a 0.5 ct round brilliant. Such colour grades are perceived by a skilled diamond grader as being nearly colourless or colourless. The diamond may have colour better than J, preferably better than I, preferably better than H, preferably better than G, preferably better than F, or preferably better than E. The diamond layer of the invention has "very high colour" where the colour is D to F on the GIA gem diamond colour scale as determined for a 0.5 ct round brilliant.

For technological applications and for polycrystalline diamond layers of the present invention, "high colour" is generally defined as the majority volume of the material having at least one of the following absorption coefficients at the following specific wavelengths in the near ultraviolet and visible part of the electromagnetic spectrum (that is wavelengths) in the range approximately 270 nm to 800 nm) when measured at room temperature:
(i) at 270 nm, is less than 2.9 cm"1, preferably less than 1.9 cm"1, preferably less than 1.0 cm"1, preferably less than
0.40 cm"1;
(ii) at 350 nm, is less than 1.5 cm"1, preferably less than 0.90 cm'1, preferably less than 0.50 cm"1, preferably less than
0.20 cm"1;
(iii) at 520 nm, is less than 0.45 cm"1, preferably less than
0.30 cm"1, preferably less than 0.14 cm"1, preferably less than 0.06 cm"1; and
(iv) at 700 nm, is less than 0.18 cm"1, preferably less than
0.12 cm"1, preferably less than 0.06 cm"1, preferably less than 0.03 cm"1.

Material of the invention can have sharp absorption features in the range 720-750 nm, but these contribute little to the colour and are thus not restricted by these definitions.

To derive the absorption coefficient the reflection loss must first be subtracted from the measured absorbance spectrum. When subtracting the reflection loss, it is important to take account of the spectral dependence of the reflection coefficient. This can be derived from the wavelength dependence of the refractive index of diamond given by F. Peter in Z. Phys. 15, 358-368 (1923). Using this and standard formulae for the dependence of reflection loss for a parallel-sided plate on the refractive index, the effect of reflection losses on the apparent absorbance can be calculated as a function of wavelength and subtracted from measured spectra to allow absorption coefficient spectra to be calculated more accurately.

Alternatively, "high colour" may be defined using the CIELAB colour system as outlined later in this specification. This colour modelling system allows colour grades to be determined from absorption spectra.

As used herein, the term "majority volume" means at least 50% of the diamond layer, preferably at least 55%, preferably at least 60%, preferably at least 70%, preferably at least 80%, preferably at least 90%, preferably at least 95% of the diamond layer.

The second gas is deliberately added to the CVD diamond synthesis atmosphere. Preferably, the second gas is added in a controlled manner. The presence of the second gas may be controlled such that the concentration of the second gas is stable to better than 20%, preferably better than 10%, preferably better than 3%.

Without being bound by any particular theory, it is believed that the second impurity atom type suppresses the roughening effect that the first impurity atom type otherwise has on the growth surface. By keeping the growth surface smooth the uptake of a wide range of defects is suppressed which otherwise degrade the colour. Addition of a gaseous source comprising an impurity atom type (such as nitrogen) to a single crystal CVD diamond synthesis process, can change the reactions occurring on the diamond growth surface in such a way that the roughness of the surface is increased, giving the surface a greater propensity for incorporation of defects. This is particularly the case when an impurity catalyses the nucleation of new layers of diamond in different regions of a {100} surface, leading to the formation of macrosteps consisting of terraces with inclined risers that offer different kinds of sites for defect incorporation, such as is described in Martineau et a/., Gems & Gemology, 40(1), 2 (2004).

There are many kinds of defects that may potentially be involved. For example, single substitutional impurity defects may be incorporated. These involve the substitution of a carbon atom by an impurity atom. Hydrogen is always present in the CVD growth environment and may become incorporated either on its own or in combination with one or more impurity atoms. Vacancies (unoccupied sites in the diamond lattice which would normally be occupied by a carbon atom) may become incorporated in combination with one or more neighbouring impurity atoms (e.g. nitrogen-vacancy defects), or one or more hydrogen atoms (e.g. vacancy-hydrogen complexes). Some defect complexes involve impurity atoms, hydrogen atoms and vacancies (e.g. nitrogen-vacancy-hydrogen complexes). Clusters of vacancies may be formed with or without bonded hydrogen and in some cases may be associated with impurity atoms.

The wide ranging set of defects incorporated once the surface is roughened is generally found to have an undesirable effect on the optical and electronic properties of the material. For example, the set of defects may contain some which give the material undesirable optical properties because of the way they absorb light in, for example, the visible region of the spectrum. They will degrade electronic properties because they reduce the mobility and lifetime of carriers.

One general mechanism that is believed to underlie the current invention is that the deleterious effect of one gaseous source comprising a first impurity atom type can be suppressed by the addition of a second gaseous source comprising a second impurity atom type which suppresses the roughening effect that the first impurity atom type would otherwise have on the growth surface. By suppressing the roughening of the surface the addition of the second impurity also suppresses the incorporation of the wide range of defects outlined above that degrade the properties of the material grown.

In the presence of both impurity atom types, with growth taking place on a smooth surface, the two impurity atom types will generally be incorporated but with a lower efficiency than would be observed for growth on a rough surface. It is significant, however, that many of the defects discussed above (e.g. vacancy clusters and hydrogen-related defects) are not observed at all when growth has taken place on a smooth surface as a result of the addition of the second impurity atom type. The outcome is that the two impurity atom types may be incorporated into the diamond material at moderate concentrations that are measurable but without the wide range of defects that have the strongest adverse effects on the properties of the diamond layer produced, such as its optical transmission.

It is also believed that the method of the present invention may additionally be based on a second general principle in which the two impurity atom types are incorporated in such a way that they mutually compensate each other. As such, the two impurity atom types are chosen so that, within particular concentration ranges in the diamond layer, they do not have a substantial adverse effect on the material properties that are desired. According to prior teaching in the art, there would be an assumption that this would exclude any benefit from compensation using nitrogen, which is normally associated with a range of defects that degrades the colour and other properties. However, in light of the first general principle outlined above that defects can be decreased on addition of a second impurity atom type, additional advantage can be taken of the mutual compensation effect between the two impurity atom types. This will generally be partly because one impurity atom type compensates for the effect that the other would have in its absence and vice-versa. Compensation can be illustrated using the example of nitrogen and boron. By themselves substitutional nitrogen and boron give diamond yellow/brown and blue colour, respectively. However, the inventors of the present application have found that when present together in approximately the same concentrations, colourless material can result because the substitutional nitrogen defects donate electrons to the substitutional boron defects and the resultant ionised defects do not give rise to significant optical absorption.

For a given set of growth conditions (such as, substrate temperature, pressure and plasma temperature) the inventors have found that there is a threshold nitrogen concentration that can be tolerated by the CVD diamond synthesis process before the surface roughens and the grown diamond becomes brown. However, the threshold nitrogen concentration tends to be so low that considerable time and expense is involved to achieve a nitrogen concentration below the threshold in order to avoid the incorporation of defects affecting the material's optical and other properties.

The inventors of the present invention have found that the addition of a second impurity atom type (such as boron or silicon) to the growth gases can significantly increase the threshold nitrogen concentration to levels that might be present in growth environments when relatively little attention is given to nitrogen elimination. This allows diamond to be grown in the presence of relatively high concentrations of nitrogen without the degradation of the optical and other properties that would otherwise result because of the incorporation of defects such as vacancy clusters and hydrogen-related defects. In addition it has been surprisingly found that this is possible even though there may be significant incorporation into the grown diamond of both nitrogen and the second impurity atom type.

It is known in the prior art (WO 2005/061400) to deliberately add certain dopants to the synthesis atmosphere of CVD single crystal diamond in order to provide the single crystal CVD diamond with a "tag", that is, a mark of origin or fingerprint. The dopant is selected such that the mark of origin or fingerprint is not readily detectable or does not affect the perceived quality of the diamond material under normal viewing conditions but which is detectable or rendered detectable under specialised conditions, such as when exposed to light or radiation or a specified wavelength.

In contrast, the present invention relates to the use of a second impurity atom type to counter the detrimental effect on colour of a first impurity atom type present in the CVD synthesis atmosphere. In this way, the present invention enables a CVD diamond to be produced which has high colour even though the CVD synthesis atmosphere comprises an amount of a first impurity atom type which would otherwise produce a diamond not having high colour. This has the advantage of removing the need to take special steps to eliminate impurity atom types known to adversely affect the colour of diamond from the synthesis atmosphere merely by adding a particular type and amount of a second impurity atom. Consequently, the synthesis of CVD diamond can be simplified and is more efficient in both time and cost.

The CVD diamond layer produced by the method of the present invention may be a single crystal.

Alternatively, the CVD diamond layer may be polycrystalline. Polycrystalline CVD diamond layers are well known in the art. They are generally grown on a non-diamond substrate (for example silicon, silicon carbide, tungsten, molybdenum and other carbide forming metals). Growth from multiple randomly located and oriented nuclei combined with a growth mechanism in which the growth rate varies with crystallographic direction results in polycrystalline layer in which the grains have growth directions that are more-or-less aligned with a single crystallographic direction (e.g. parallel to <100> or <110>), but randomly oriented perpendicular to the growth direction (i.e. in the plane of the layer). Such a disc is described by those skilled in the art as having a 'wire texture'.

Alternatively, the CVD diamond layer may heteroepitaxial. Heteroepitaxial CVD diamond layers are well known in the art. They are generally grown on single crystal substrates of non-diamond materials including silicon, silicon carbide, strontium titanate and iridium. Often complex interlayer structures are used between the substrate and the CVD diamond layer to control strain and reduce the impact of thermal expansion mismatch. The nuclei of a heteroepitaxial diamond layer initially form with a specific orientation relationship with the substrate and then grow into 'domains' of diamond that are more-or-less in the same crystallographic orientation, normally with a definite relationship to a direction in the single crystal substrate. The domains are normally separated by low angle boundaries. Domains with lateral sizes of several hundred micrometers have been reported for layers a few tens of micrometers thick.

In the method of the present invention, the first impurity atom type is preferably nitrogen and the second impurity atom type is selected from silicon, boron, phosphorus or sulphur. In this way, the addition of a gaseous source comprising silicon, boron, phosphorus or sulphur impurity atom types counters the detrimental effect on colour of diamond that nitrogen would otherwise have. More preferably, the second impurity atom type is silicon and hence the silicon impurity atoms counter the detrimental effect on colour of diamond of nitrogen impurity atoms. Alternatively the second impurity atom type is boron and hence the boron impurity atoms counter the detrimental effect on colour of diamond of nitrogen impurity atoms.

Alternatively, the first impurity atom type is silicon, boron, phosphorus or sulphur and the second impurity atom type is nitrogen. In this way, the addition of a gaseous source comprising nitrogen impurity atoms counters the detrimental effect on colour of diamond that either silicon, boron, phosphorus or sulphur would have. More preferably, the first impurity atom type is silicon and hence the nitrogen impurity atoms counter the detrimental effect on colour of diamond of silicon impurity atoms. Alternatively, the first impurity atom type is boron and hence nitrogen impurity atoms counter the detrimental effect on colour of diamond of boron impurity atoms.

Where the first or second impurity atom type is nitrogen, the first or second gas may be any gaseous species which contains nitrogen including N2, NH3 (ammonia), N2H4 (hydrazine) and HCN (hydrogen cyanide). Preferably, the first or second gas is N2, NH3, or N2H4. Preferably, the first or second gas is N2 or NH3, preferably the first or second gas is N2. The nitrogen present in the synthesis atmosphere is calculated as parts per million (ppm) or parts per billion (ppb) of molecular nitrogen (ie N2) as a molecular fraction of the total gas volume. Thus 100 ppb of nitrogen added as molecular nitrogen (N2) is equivalent to 200 ppb of atoms of nitrogen or 200 ppb of ammonia (NH3).

For impurity additions other than nitrogen, the gas phase concentration in ppm or ppb refers to the concentration in synthesis atmosphere of the impurity added as the preferred gaseous species.

Where the first or second impurity atom type is boron, the first or second gas is preferably B2H6, BCI or BH3. Preferably, the first or second gas is B2H6.

Where the first or second impurity atom type is silicon, the first or second gas is preferably SiH4, or Si2H6. Preferably, the first or second gas is SiH4.

Where the first or second impurity atom type is sulphur, the first or second gas is preferably H2S.

Where the first or second impurity atom type is phosphorus, the first or second gas is preferably PH3.

For silicon, boron, sulphur and phosphorus if gaseous species other than the preferred species (ie B2H6, SiH4, H2S and PH3) are used to add the impurity atom type to the synthesis environment, the number of atoms of the impurity atom type in the molecular species added must be accounted for in determining the concentration of that species in the synthesis environment.

The impurity atom types are added to the synthesis atmosphere as gases. Although it is possible, with the exception of nitrogen, to add all the impurity atom types as single element solids, it is extremely difficult, if not impossible, to accurately and reproducibly control the rate at which such additions are made. For example, additions of boron have been made by exposing solid boron to the synthesis atmosphere; the same applies to silicon where solid sources have been used. However, gaseous sources of the impurity atom types are used in the method of the present invention because the gaseous source of an impurity atom type may be prepared in a highly pure form, diluted gravimetrically with a carrier gas and then analysed post-manufacture to accurately determine the exact concentration. Given the gas concentration, precise and reproducible additions can be added using gas metering devices such as mass-flow controllers.

The first impurity atom type may be nitrogen and the second impurity atom type may be sulphur. The first impurity atom type may be nitrogen and the second impurity atom type may be phosphorus. The first impurity atom type may be sulphur and the second impurity atom type may be nitrogen. The first impurity atom type may be phosphorus and the second impurity atom type may be nitrogen. The first impurity atom type may be phosphorus and the second impurity atom type may be sulphur. The first impurity atom type may be sulphur and the second impurity atom type may be phosphorus. The first impurity atom type may be boron and the second impurity atom type may be silicon. The first impurity atom type may be silicon and the second impurity atom type may be boron. The first impurity atom type may be boron and the second impurity atom type may be phosphorus. The first impurity atom type may be phosphorus and the second impurity atom type may be boron. The first impurity atom type may be boron and the second impurity atom type may be sulphur. The first impurity atom type may be sulphur and the second impurity atom type may be boron. The first impurity atom type may be silicon and the second impurity atom type may be phosphorus. The first impurity atom type may be phosphorus and the second impurity atom type may be silicon. The first impurity atom type may be silicon and the second impurity atom type may be sulphur. The first impurity atom type may be sulphur and the second impurity atom type may be silicon.

The incorporation of an impurity atom type from the synthesis atmosphere into the solid diamond is highly dependent upon the exact details of the synthesis process. Such matters have been well detailed in the prior art and are well known to those skilled in the art. Parameters that influence the level of incorporation include the nature of the molecular species used to provide the impurity atom, the temperature of the synthesis atmosphere, the pressure of the synthesis atmosphere, the temperature of the surface of the substrate, the crystallographic nature of the surface and the gas flow conditions with the synthesis system.

Where the first or second gas source comprises nitrogen, the concentration of the gas comprising nitrogen in the synthesis atmosphere may be greater than 300 ppb, greater than 500 ppb, greater than 600 ppb, greater than 1 ppm, greater than 2 ppm, greater than 3 ppm, greater than 5 ppm, greater than 10 ppm, greater than 20 ppm, greater than 30 ppm. The concentration of the gas comprising nitrogen may be in the range from 300 ppb to 30 ppm, 500 ppb to 20 ppm, 600 ppb to 10 ppm, 1 ppm to 5 ppm, or

2 ppm to 3 ppm.

When the first or second gas source comprises boron, the concentration of the gas comprising boron in the synthesis atmosphere may be greater than 0.5 ppb, greater than 1.0 ppb, greater than 2 ppb, greater than 5 ppb, greater than 10 ppb, greater than 20 ppb, greater than 50 ppb, greater than 0.1 ppm, greater than 0.2 ppm. The concentration of the gas comprising boron in the synthesis atmosphere may be from 0.5 ppb to 0.2 ppm, from 1.0 ppb to 0.1 ppm, from 2 ppb to 50 ppb, from 10 ppb to 20 ppb. The concentration of the gas comprising boron in the synthesis atmosphere may be less than 1.4 ppm, less than 0.1 ppm, or less than 0.05 ppm.

When the first or second gas source comprises silicon, the concentration of the gas comprising silicon in the synthesis atmosphere may be greater than 0.01 ppm, greater than 0.03 ppm, greater than 0.1 ppm, greater than 0.2 ppm, greater than 0.5 ppm, greater than 1 ppm, greater than 2 ppm, greater than 5 ppm, greater than 10 ppm, greater than 20 ppm. The concentration of the gas source comprising silicon in the synthesis atmosphere may be from 0.01 ppm to 20 ppm, 0.03 ppm to 10 ppm, 0.1 ppm to 5 ppm, 0.2 ppm to 2 ppm, or 0.5 ppm to 1 ppm.

Secondary Ion Mass Spectrometry (SIMS) measurements have shown that, for a given concentration of silicon in the growth gases, in the absence of nitrogen, the concentration of silicon in the grown diamond is higher for {111}, {110} or {113} growth than for {100} growth. For growth on a substrate with {100} orientation, although gaseous silicon impurity tends to increase the threshold nitrogen concentration for surface roughening, addition of high concentrations of nitrogen to the growth gases will eventually cause the surface to roughen and the efficiency of silicon incorporation to increase dramatically. When this happens SIMS measurements indicate that the concentration of silicon in the diamond can significantly exceed that of nitrogen and in such cases the diamond will generally show a grey colour resulting from high concentrations of the defect responsible for a spectroscopic feature at 945 nm in the absorption spectrum (currently believed to be a neutral silicon-vacancy defect). In general, as the concentration of gaseous silicon is increased the grey colour is perceived earlier for material grown on {111}, {110} or {113} than for {100} growth.

When silicon is the first or the second impurity atom type, the concentration of silicon in the majority volume of the diamond layer produced may be less than or equal to 2 x 1018 atoms/cm3. The concentration of silicon in the majority volume of the diamond layer may be in the range from 1014 atoms/cm3 to 2 x 1018 atoms/cm3, from 3 x 1014 atoms/cm3 to 1017 atoms/cm3, from 1015 atoms/cm3 to 3 x 1016 atoms/cm3, or from 3 x 1015 atoms/cm3 to 1016 atoms/cm3. The concentration of silicon in the majority volume of the diamond layer may be greater than 1013 atoms/cm3, greater than 1014 atoms/cm3, greater than 3 x 1014 atoms/cm3, greater than 1015 atoms/cm3, greater than 3 x1015 atoms/cm3, greater than 1016, greater than 3 x 1016 atoms/cm3, greater than 1017 atoms/cm3.

When nitrogen is the first or the second impurity atom type, the concentration of nitrogen in the majority volume of the diamond layer may be from 1 x 1014 atoms/cm3 to 5 x 1017 atoms/cm3, from 5 x 1015 atoms/cm3 to 2 x 1017 atoms/cm3, or from 1 x 1016 to 5 x 1016 atoms/cm3. The concentration of nitrogen in the majority volume of the diamond layer may be greater than 2 x 1015 atoms/cm3, greater than 5 x 1015 atoms/cm3, greater than 1016 atoms/cm3, greater than 3 x 1016 atoms/cm3, greater than 1017 atoms/cm3.

When boron is the first or the second impurity atom type, the concentration of boron in the majority volume of the diamond layer may be from 1014 atoms/cm3 to 1018 atoms/cm3, from 3 x 1014 atoms/cm3 to 1017 atoms/cm3, from 1015 atoms/cm3 to 1016 atoms/cm3, or from 3 x 1015 atoms/cm3 to 1016 atoms/cm3. The concentration of boron in the majority volume of the diamond layer may be greater than 1013 atoms/cm3, greater than 1014 atoms/cm3, greater than 3 x 1014 atoms/cm3, greater than 1015 atoms/cm3, greater than 3 x1015 atoms/cm3, greater than 1016, greater than 3 x 1016 atoms/cm3, greater than 1017 atoms/cm3.

Typically, the concentration of the first and second impurity atom types, as well as the concentration of any other impurities in the diamond layer, may be measured using secondary ion mass spectroscopy (SIMS). Detection limits for impurity atoms vary depending on the SIMS conditions used. However, SIMS detection limits for the first and second impurity atom types of the present invention typically lie in the range 1014 to 1017 atoms/cm3. In particular, for elements such as boron and silicon the detection limits are typically about 1015 atoms/cm3, whereas for nitrogen they are typically about 1016 atoms/cm3. Other techniques, such as combustion analysis, absorption, EPR, can give higher sensitivity in some instances.

When the first and second impurity atom types are nitrogen and silicon, respectively or vice versa, the concentration of nitrogen in the majority volume of the diamond layer is preferably less than or equal to 2 x 1017 atoms/cm3 and the concentration of silicon in the majority volume of the diamond layer is preferably less than or equal to 2 x 1018 atoms/cm3. In this way, high colour in the synthesised diamond may be more readily achieved.

When the first and second impurity atom types are nitrogen and silicon, respectively or vice versa, the ratio of the concentration of nitrogen to silicon in the majority volume of the diamond layer produced may be 1 :20 to 20:1 , 1 :10 to 10:1, 1:9 to 9:1 , 1:8 to 8:1, 1:7 to 7:1 , 1 :6 to 6:1 , 1 :5 to 5:1, 1:4 to 4:1 , 1 :3 to 3:1, 1:2 to 2:1 , preferably 1:1.

When the first and second impurity atom types are nitrogen and silicon, respectively or vice versa, the gas comprising nitrogen may be present in the synthesis atmosphere at a concentration of greater than 100 ppb, greater than 200 ppb, greater than 300 ppb and the gas comprising silicon may be present in the synthesis atmosphere at a concentration of greater than 10 ppb.

When the first and second impurity atom types are nitrogen and boron, respectively or vice versa, the ratio of the concentration of nitrogen to the concentration of boron in the majority volume of the diamond layer may be in the range from 1 :2 to 2:1 , from 2:3 to 3:2, from 3:4 to 4:3, from 4:5 to 6:5, from 9:10 to 11 :10, preferably the ratio is 1:1. Preferably, the ratio of nitrogen to boron is greater than 1 :5.

When single substitutional boron and nitrogen are present in diamond in approximately the same concentrations, colourless material can result because the nitrogen defects donate electrons to the boron defects and the resultant ionised defects do not give rise to significant optical absorption. Thus, not only does boron have a beneficial effect on growth in the presence of nitrogen because of the fact that it suppresses roughening of the growth surface, boron and nitrogen incorporated into the diamond can compensate each other to give material with low optical absorption.

When the first and second impurity atom types are nitrogen and boron, respectively or vice versa, the gas comprising nitrogen may be present in the synthesis atmosphere at a concentration of greater than 100 ppb, preferably greater than 200 ppb, preferably greater than 300 ppb and the gas comprising boron may be present in the synthesis atmosphere at a concentration of greater than 0.5 ppb.

Preferably, the CVD diamond layer produced by any of the above methods has an increased normalized free exciton intensity compared to a method where the second gas comprising a second impurity type atom is not added. Preferably, there is a strong free exciton luminescence in the cathodoluminescence spectrum measured at 77 K, with the integrated intensity of the free exciton luminescence exceeding 0.3, preferably exceeding 0.4, preferably exceeding 0.5, preferably exceeding 0.6, preferably exceeding 0.7, preferably exceeding 0.8, preferably exceeding 0.9 of the integrated free exciton luminescence intensity for a homoepitaxial CVD diamond sample grown under high purity conditions.

The CVD diamond layer produced by any of the above methods may have an increase in carrier mobility, carrier lifetime, charge collection distance and/or charge collection efficiency compared to a method where the second gas comprising a second impurity type atom is not added. The charge collection distance of the produced diamond layer may be greater than 100 μm, greater than 150 μm, greater than 200 μm, greater than 300 μm, greater than 500 μm, or greater than 1000 μm when measured with an applied electric field of 1.0 V/μm. A method of measuring charge collection distance in diamond is described in WO 01/96633, for example. The carrier mobility of the produced diamond layer may be 1200 Cm2V1S'1, preferably 1500 CmV1S-1, preferably 1800 cmW1, preferably 2200 cmVV1, preferably 2500 cmW1. Preferably the charge collection efficiency of the produced diamond layer is 30%, preferably 50%, preferably 70%, preferably 80%, preferably 90%, preferably 95%, preferably 97%. The carrier lifetime of the produced diamond layer may be greater than 1 ns, greater than 3 ns, greater than 10 ns, greater than 30 ns, or greater than

100 ns.

Nitrogen as an impurity is known to affect the electronic properties of single crystal CVD diamond, in particular the charge collection distance, carrier mobilities and carrier lifetimes. In the absence of nitrogen the electronic properties of single crystal CVD diamond can be very good (see for example lsberg et al, Science, volume 297, pages 1970-1672, where methods of measurement and results are disclosed). As nitrogen is progressively added to the synthesis atmosphere, the electronic properties of the resultant material are progressively degraded.

Previous experimentation has shown that the intensity of the free exciton emission at 235 nm measured at 77 K is a good proxy for the electronic properties (WO 01/96633). Using this proxy, we are able to propose the following expected behaviour for combined nitrogen and silicon additions to diamond.

If silicon is added with nitrogen, the deleterious effects of the nitrogen on electronic properties are ameliorated, with the amount of amelioration increasing, but the rate of amelioration decreasing, as the concentration of silicon added is increased, until at some fraction of the concentration of nitrogen being incorporated, adding further silicon ceases to have a further ameliorating effect at which point the properties start to degrade once more.

Therefore there will be an optimum amount of silicon that can be added for a given amount of nitrogen in the solid, but the optimum value is dependent on the exact property that is being considered and the amount of nitrogen incorporated. The inventors expect that the optimum value of silicon addition with regard to its effect on the electronic properties is generally somewhat less than the silicon addition at which the colour of the diamond starts degrading (ie the greyness caused by silicon begins to become apparent).

Thus, it is possible for a series of diamonds containing a given concentration of nitrogen and different concentrations of silicon (ranging from just above zero to well beyond the optimum) to have electronic properties that can be slightly better, much better, the same or worse than an otherwise identical diamond containing no silicon.

It is equally possible for a diamond of the invention to have poor electronic properties (i.e. the silicon concentration is well beyond the optimum), but good optical properties as the greyness caused by the silicon has not yet become sufficient to be perceived as a colour change or caused a significant change to the optical absorption spectrum.

A similar situation pertains when boron is added to a synthesis atmosphere containing diamond. Initially the boron ameliorates the deleterious effects of the nitrogen and the electronic properties improve. As the amount of boron added is increased, at some point, probably when the amounts of nitrogen and boron are approximately equal, the improvement in the electronic properties will stop and then, with higher rates of addition, begin to decline. This behaviour can be understood with a classical semiconductor compensation model. The rate of improvement and subsequent decline in the properties is expected to be much sharper than for the case of nitrogen and silicon.

The first gas comprising a first impurity atom type may be deliberately added to the synthesis atmosphere. Alternatively, the first gas may be present in the synthesis atmosphere unintentionally, including being present in the synthesis atmosphere because it has not been removed even though it affects properties of the diamond layer produced. Preferably, the synthesis atmosphere comprises a concentration of the first gas, which has not been added deliberately, of greater than 0.1 ppb, preferably greater than 1 ppb, preferably greater than 10 ppb. An example of such a situation is where nitrogen remains in the synthesis atmosphere in the form of NH3, air or N2H4, for example, and it is considered too expensive or time consuming to adopt extra measures to remove such gases from the synthesis atmosphere. Preferably, the synthesis atmosphere comprises a concentration of the gas comprising nitrogen, which has not been added deliberately, of greater than 300 ppb.

The first gas may be present in the synthesis atmosphere in a manner which is controlled or in a manner which is not controlled. Where the first gas is present in a manner which is not controlled, the first impurity type atom may be present as an impurity of one of the gases required for diamond synthesis. Alternatively, where the first gas is added in a manner which is controlled, this may be such that there is only an upper limit of the amount of gas that may be introduced into the synthesis atmosphere. Alternatively, the presence of the first gas may be controlled such that the concentration of the first gas is stable to better than 20%, preferably better than 10%, preferably better than 3%.

Preferably, the diamond layer is greater than 0.1 mm thickness, preferably greater than 0.5 mm thickness, preferably greater than 1 mm thickness, preferably greater than 2mm thickness.

In the method of the first embodiment:
(1 ) the substrate may be a diamond substrate having a surface which is substantially free of crystal defects such that a revealing plasma etch would reveal a density of surface etch features related to defects below 5 x 103/mm2;
(2) the duration of the synthesis of the diamond layer may be at least 50 hours; and/or
(3) the substrate may comprises multiple separated single crystal diamond substrates.

The method may comprise at least one, preferably at least two, preferably all three of features (1 ) to (3). The method may comprise feature (1 ), feature (2), feature (3), features (1) and (2), features (1) and (3), features (2) and (3), or features (1), (2) and (3).

By using a diamond substrate having a surface which is substantially free of crystal defects, the quality of the grown diamond can be greatly improved. In particular, fewer defects will be present in the grown diamond layer.

The substrate may be a single diamond substrate, such as in 1) above. Alternatively, the substrate may be a plurality of separated single crystal diamond substrates. Preferably, the plurality of substrates are separated laterally. The plurality of separated single crystal diamond substrates may each be substantially free of crystal defects as for feature 1) above. The plurality of laterally separated single crystal diamond substrates may be grown on simultaneously in the same synthesis system under substantially the same growth conditions.

The method may comprise using separated multiple single crystal diamond substrates. Preferably there are greater than 5, greater than 20, greater than 50, greater than 80, greater than 100, greater than 120, greater than 150, greater than 200 single crystal substrates. Use of such multiple separated single crystal diamond substrates may produce multiple single crystal diamond layers. Alternatively, polycrystalline diamond layers may be produced which extend laterally in at least one direction greater than 30 mm, preferably greater than 60 mm, preferably greater than 90 mm, preferably greater than 110 mm, preferably greater than 130 mm.

The duration of the synthesis of the diamond layer may be at least 50 hours, at least 75 hours, at least 100 hours, at least 150 hours.

In a second embodiment of the present invention there is provided a method of producing a CVD diamond layer, comprising:
(i) providing a substrate;
(ii) providing a CVD synthesis atmosphere in which there exists a concentration of nitrogen which is not deliberately added of greater than 300 ppb; and (iii) adding into the synthesis atmosphere a second gas comprising a second impurity atom type other than nitrogen,
wherein the second impurity atom type is added in a controlled manner in an amount that reduces the detrimental effect on the colour caused by the nitrogen so as to produce a diamond layer having high colour; and the second impurity atom type is solid in the elemental state.

In this way, a CVD diamond layer having high colour may be produced even though the synthesis atmosphere comprises an amount of nitrogen that, in the absence of the second gas, would have an undesirable affect on the colour of the produced diamond such that the produced diamond would not have high colour. The method of the second embodiment enables high colour CVD diamond to be synthesised without having to take any additional steps to remove the undesirable nitrogen from the synthesis atmosphere. The term "high colour" is as defined previously. Preferably, the diamond layer has very high colour, as defined previously.

In the second embodiment of the invention, the CVD diamond layer may be a single crystal. Alternatively, in the second embodiment of the invention, the CVD diamond layer may be polycrystalline.

The preferred features of the first embodiment of the method of the present invention outlined above apply equally to the second embodiment of the method of the present invention as long as the first source gas comprises nitrogen.

In particular, the second impurity atom type may be boron, silicon, phosphorus or sulphur. Preferably, the second impurity atom type is silicon.

In the method of the second embodiment of the present invention, the concentration of nitrogen that is not deliberately added to the synthesis atmosphere may be added in an uncontrolled manner. In addition to the concentration of nitrogen that is not deliberately added to the synthesis atmosphere, additional nitrogen may be added deliberately to the synthesis atmosphere.

In a third embodiment of the present invention there is provided a method of producing a CVD diamond layer comprising the steps of:
(i) providing a substrate; and
(ii) adding into a CVD synthesis atmosphere a gaseous source comprising silicon.

In this way a silicon doped diamond layer is provided. Preferably, in the third embodiment of the invention, the CVD diamond layer is a single crystal. Alternatively, in the third embodiment of the invention, the CVD diamond layer is polycrystalline.

In the method of the third embodiment:
(1 ) the layer may be grown to greater than 0.1 mm thickness;
(2) the substrate may be a diamond substrate having a surface which is substantially free of crystal defects such that a revealing plasma etch would reveal a density of surface etch features related to defects below 5 x 103/mm2;
(3) the duration of the synthesis of the single crystal diamond layer may be at least 50 hours; and/or
(4) the substrate may comprise multiple separated single crystal diamond substrates.

The method may comprise at least one, at least two, at least three, preferably all four of features (1) to (4). The method may comprise feature (1), feature (2), feature (3), feature (4), features (1) and (2), features (1) and (3), features (1) and (4), features (2) and (3), features (2) and (4), features (3) and (4), features (1), (2) and (3), features (1), (3) and (4), features (2), (3) and (4),

The layer may be grown to a thickness of greater than 0.5 mm, greater than 1 mm, greater than 2 mm.

The duration of the synthesis of the diamond layer may be at least 50 hours, at least 75 hours, at least 100 hours, at least 150 hours.

The method may comprise using multiple separated single crystal diamond substrates. Preferably there are greater than 5, preferably greater than 20, preferably greater than 50 single crystal substrates. Use of such multiple separated single crystal diamond substrates may produce multiple single crystal diamond layers. Alternatively, polycrystalline diamond layers may be produced which extend laterally in at least one direction greater than 30 mm, preferably greater than 60 mm, preferably greater than 90 mm, preferably greater than 110 mm, preferably greater than 130 mm.

The preferred features of the first embodiment of the method of the present invention outlined above apply equally to the third embodiment of the method of the present invention as long as the first or second source gases comprise silicon.

The concentration of silicon in the majority volume of the diamond layer produced by the third embodiment of the method of the present invention may be up to 2 x 1018 atoms/cm3, from 1014 atoms/cm3 to 2 x 1018 atoms/cm3, from 3 x 1014 atoms/cm3 to 1017 atoms/cm3, from 1015 atoms/cm3 to 3 x 1016 atoms/cm3, from 3 x 1015 atoms/cm3 to 1016 atoms/cm3, from 2 x1017 to 2 x 1018 atoms/cm3.

In the third embodiment of the method of the present invention, the addition of silicon may reduce an adverse effect on a property of the produced diamond layer caused by the presence of an impurity atom type. Preferably, the impurity atom type is nitrogen. The impurity atom type may be introduced into the synthesis atmosphere as a gas in a controlled or uncontrolled manner, as described previously. Preferably, the impurity atom type is nitrogen and the synthesis atmosphere comprises a concentration of nitrogen which is not deliberately added of greater than 300 ppb.

The property may be colour and adding silicon may produce a CVD diamond layer having high colour, wherein "high colour" is as defined above. Preferably, the CVD diamond layer has very high colour, wherein "very high colour" is as defined above.

The property may be free exciton emission of the diamond layer and adding silicon may produce a CVD diamond layer with an increased normalized free exciton intensity compared to a method where silicon is not added. There may be a strong free exciton luminescence in the cathodoluminescence spectrum measured at 77 K, with the integrated intensity of the free exciton luminescence exceeding 0.3, preferably exceeding 0.4, preferably exceeding 0.5, preferably exceeding 0.6, preferably exceeding 0.7, preferably exceeding 0.8, preferably exceeding 0.9 of the integrated free exciton luminescence intensity for a homoepitaxial CVD diamond sample grown under high purity conditions.

The property may be at least one of: carrier mobility; carrier lifetime; and charge collection distance and adding silicon may produce a CVD diamond layer with an increase in carrier mobility, carrier lifetime and/or charge collection distance compared to a method where silicon is not added. The charge collection distance of the produced diamond layer may be greater than 100 μm, greater than 150 μm, greater than 200 μm, greater than 300 μm, greater than 500 μm, greater than 1000 μm when measured with an applied electric field of 1.0 V/μm. The carrier mobility of the produced diamond layer may be 1200 cm2V"V1, preferably 1500 cm2V"1s"1, preferably 1800 Cm2V1S"1, preferably 2200 cmW, preferably 2500 cmW1. The charge collection efficiency of the produced diamond layer may be 30%, preferably 50%, preferably 70%, preferably 80%, preferably 90%, preferably 95%, preferably 97%. The carrier lifetime of the produced diamond layer may be greater than 1 ns, greater than 3 ns, greater than 10 ns, greater than 30 ns, greater than 100 ns.

In any of the methods described above (that is, for the first, second and third embodiments), when the CVD diamond layer is a single crystal, the majority volume of the diamond layer may have at least one of the following features:
a) an absorption spectrum measured at room temperature such that the colour of a standard 0.5 ct round brilliant would be better than K;
b) an absorption coefficient at 270 nm measured at room temperature which is less than 1.9 cm"1;
c) an absorption coefficient at 350 nm measured at room temperature which is less than 0.90 cm"1;
d) an absorption at 520 nm of less than 0.30 cm"1 ; or
e) an absorption at 700 nm of less than 0.12 cm"1.

The majority volume of the diamond layer may comprise at least 55%, preferably at least 60%, preferably at least 70%, preferably at least 80%, preferably at least 90%, preferably at least 95% of the diamond layer.

The single crystal diamond layer may have at least two, at least three, at least four, preferably all five of the features (a) to (e). The diamond layer may have features a) and b); features a) and c); a) and d); a) and e); b) and c); b) and d); b) and e); c) and d); c) and e); d) and e); a), b) and c); a), b) and d); a), b) and e); a), c) and d); a), c) and e); a), d) and e); b), c) and d); b), c) and e); b), d) and e); c), d) and e); a), b), c) and d); a), b), c) and e); a), b), d) and e); a), c), d) and e); b), c), d) and e); or a), b), c), d) and e).

Preferably, for feature a), the diamond layer has an absorption spectrum measured at room temperature such that the colour of a standard 0.5 ct round brilliant would be better than J, preferably better than I; preferably better than H, preferably better than G, preferably better than F, preferably better than E, preferably D.

Preferably, for feature b), the diamond layer has an absorption coefficient at 270 nm measured at room temperature which is less than 1.0 cm"1; preferably less than 0.4 cm"1.

Preferably, for feature c), the diamond layer has an absorption coefficient at 350 nm measured at room temperature which is less than 0.5 cm"1; preferably less than 0.2 cm"1.

Preferably, for feature d), the diamond layer has an absorption coefficient at 520 nm measured at room temperature which is less than 0.14 cm"1; preferably less than 0.06 cm"1.

Preferably, for feature e), the diamond layer has an absorption coefficient at 700 nm measured at room temperature which is less than 0.06 cm"1; preferably less than 0.03 cm"1.

In any of the methods described above (that is, relating to the first, second and third embodiments), when the CVD diamond layer is polycrystalline, the majority volume of the diamond layer may have at least one of the following features:
a) an absorption coefficient at 270 nm measured at room temperature which is less than 1.9 cm"1;
b) an absorption coefficient at 350 nm measured at room temperature which is less than 0.90 cm"1;
c) an absorption at 520 nm of less than 0.30 cm"1 ; and
d) an absorption at 700 nm of less than 0.12 cm"1.

The polycrystalline diamond layer may have feature a), feature b), feature c), feature d), features a) and b), features a) and c), features a) and d), features b) and c), features b) and d), features c) and d), features a) b) and c), features a) b) and d), features a) c) and d), features b) c) and d), or features a), b), c) and d).

Preferably, for feature a), the diamond layer has an absorption coefficient at 270 nm measured at room temperature which is less than 1.0 cm"1; preferably less than 0.4 cm"1.

Preferably, for feature b), the diamond layer has an absorption coefficient at 350 nm measured at room temperature which is less than 0.5 cm"1; preferably less than 0.2 cm'1.

Preferably, for feature c), the diamond layer has an absorption coefficient at 520 nm measured at room temperature which is less than 0.14 cm"1; preferably less than 0.06 cm"1.

Preferably, for feature d), the diamond layer has an absorption coefficient at 700 nm measured at room temperature which is less than 0.06 cm"1; preferably less than 0.03 cm"1.

In any of the methods outlined above where the CVD diamond layer is a single crystal, the diamond layer is preferably formed into a gemstone having three orthogonal dimensions greater than 2 mm, where at least one axis lies either along the <100> crystal direction or along the principle symmetry axis of the gemstone.

According to the present invention there is provided a CVD diamond layer produced by any one of the methods disclosed above.

Where the CVD diamond layer is a single crystal, the majority volume of the diamond layer may be formed from a single growth sector.

In view of the reduction in defects in the diamond layer produced by any of the methods of the invention described above because of a reduction in surface roughening during growth, the diamond layer may also have improved mechanical and chemical properties, including wear resistance and thermal stability. The wear properties of a material are the result of very complex interactions between a wide range of the macroscopic properties of the material including, for example, its hardness, strength, stiffness, toughness, grain size, thermal conductivity, grain orientation etc. It is well known in the art that diamond has exceptional wear properties and these are widely exploited: it is used as a tool material in a wide range of applications including cutting tools, rock drill, wire dies and many others.

The performance of a diamond tool in a particular application is strongly influenced by its microstructure and in the particular case of single crystal diamond, the point and extended defect densities. A particular example is a wire drawing die as disclosed in WO2004/074557, where reducing the strain by controlling extended defect density is shown to be particularly effective at improving the wear properties. Since the methods of the invention can provide single crystal diamond material with reduced point and extended defect densities compared with diamond prepared using generally the same method without the added second impurity, it can be reasonably expected that the material of the invention will have improved wear properties.

According to the present invention there is also provided a CVD diamond layer comprising an impurity atom type selected from silicon, sulphur or phosphorus, wherein the diamond layer has high colour.

According to the present invention there is provided a CVD diamond layer comprising an impurity atom type selected from silicon, sulphur or phosphorus wherein the concentration of the impurity atom type in the majority volume of the diamond layer is from 1014 to 2 x 1018 atoms/cm3. The concentration of silicon in the majority volume of the diamond layer may be greater than 1013 atoms/cm3, greater than 1014 atoms/cm3, greater than 3 x 1014 atoms/cm3, greater than 1015 atoms/cm3, greater than 3 x1015 atoms/cm3, greater than 1016, greater than 3 x 1016 atoms/cm3, greater than 1017 atoms/cm3. The concentration of the impurity atom type may be from 3 x 1014 atoms/cm3 to 1017 atoms/cm3, from 1015 atoms/cm3 to 3 x 1016 atoms/cm3, or from 3 x 1015 atoms/cm3 to 1016 atoms/cm3, from 1016 to 2 x 1017 atoms/cm3, from 2 x 1016 to 1017 atoms/cm3, greater than 2 x 1017 atoms/cm3. Preferably, the majority volume of the CVD diamond layer comprises from 2 x 1017 to 2 x 1018 atoms/cm3 of an impurity atom type selected from silicon, sulphur or phosphorus. Preferably, the impurity atom type is silicon. The CVD diamond layer may be a single crystal. Alternatively, the CVD diamond layer may be polycrystalline.

Photoluminescence spectroscopy offers a sensitive method for detecting the presence of silicon-related defects in diamond. A silicon-related photoluminescence line at 737 nm can generally be detected with 633 nm HeNe laser excitation at 77 K. Research by the present inventors has indicated that the photoluminescence spectrum of silicon-doped diamond, excited at 77 K with 785 nm laser radiation, also often shows a line at 946 nm. This is generally accompanied by another line at 975 nm. These two photoluminescence lines have not been reported before. Figure 2 shows a typical photoluminescence spectrum for silicon-doped diamond excited with 785 nm laser radiation.

The present inventors have also investigated silicon-doped samples using EPR (Electron Paramagnetic Resonance). This offers a sensitive method for detecting and characterising silicon-related defects. The current detection limits allow defect concentrations as low as one part per billion to be measured. A neutral silicon-vacancy defect has recently been detected and characterised using EPR, and work is proceeding to identify other silicon related defects in the same way. Current results suggest that the 946 nm photoluminescence line may be an optical signature of the neutral silicon vacancy defect identified using EPR.

Preferably, the CVD diamond layer has high colour, wherein "high colour" is as defined above.

Preferably, the CVD diamond layer has a thickness of greater than 0.1 mm, preferably greater than 0.5 mm, preferably greater than 1 mm, preferably greater than 2 mm.

The CVD diamond layer produced by any of the methods of the present invention may have a birefringence of less than 1x10~3, preferably less than 1x10'4, preferably less than 3x10"4, preferably less than 1x10"5 over a volume greater than 0.1 mm3, preferably greater than 0.5 mm3, preferably greater than 1 mm3, preferably greater than 3.4 mm3, preferably greater than 8 mm3, preferably greater than 27 mm3, preferably greater than 64 mm3, preferably greater than 125 mm3, preferably greater than 512 mm3, preferably greater than 1000 mm3. Birefringence may be characterized using, for example, Metripol® apparatus.

For an isotropic medium, such as stress-free diamond, the refractive index is independent of the direction of the polarization of light. If a diamond sample is inhomogeneously stressed, either because of grown-in stress or local defects or because of externally applied pressure, the refractive index is anisotropic. The variation of the refractive index with direction of polarization may be represented by a surface called the optical indicatrix that has the general form of an ellipsoid. The difference between any two ellipsoid axes is the linear birefringence for light directed along the third. This may be expressed as a function involving the refractive index of the unstressed material, the stress and opto-elastic coefficients.

Metripol® (Oxford Cryosystems) gives information on how the refractive index at a given wavelength depends on polarization direction in the plane perpendicular to the viewing direction. An explanation of how the Metripol® works is given by A. M. Glazer et a/, in Proc. R. Soc. Lond. A (1996) 452, 2751-2765.

The Metripol® instrument determines the direction of the "slow axis", i.e. the polarization direction in the plane perpendicular to the viewing direction for which the refractive index is a maximum. It also measures |sin δ| where δ is the phase shift given by

d = (2 pi / lambda) Dn L

where λ is the wavelength of the light, L is the thickness of the specimen and Δn is the difference between the refractive index for light polarized parallel to the slow and fast axes i.e. the birefringence. Δn L is known as the Optical retardation'.

For retardation in first order, with L = 0.6 mm and λ = 589.6 nm, then: when sin δ = 1 and Δn L = λ / 4, it can be deduced that Δn = 2.45 x 10"4 . when sin δ = 0.5 and Δn L = λ / 12, it can be deduced that Δn = 0.819 x 10"4.

Metripol® produces three colour-coded images showing the spatial variations of a) the "slow axis", b) |sin δ| and c) the absorbance at the wavelength of operation.

Samples are prepared as optical plates of known thickness and analysed over an area of at least 1.3 mm x 1.3 mm, and preferably 2.5 mm x 2.5 mm, and more preferably 4 mm x 4 mm. Metripol® |sin δ| images are then analysed and the maximum value of jsin δ| in each frame over the whole of the analysis area and use these values to characterise the maximum value of Δn can be calculated of the whole of the area analysed.

The behaviour of sine δ is the property of a particular plate of material, constrained here to plates of useful thickness by application of a minimum thickness. A more fundamental property of the material can be obtained by converting the sine δ information back to a value averaged over the thickness of the sample of the difference between the refractive index for light polarised parallel to the slow and fast axes, Δn[aVerage]-

Instrument resolution and noise sets a lower limit to the value of |sin δ| and hence the retardation Delta-n.d measurable by Metripol®. This in turn sets a lower limit on the measurable birefringence, although the limit on this parameter depends on the specimen thickness. For illustration, if the lower limit on |sin deltaj is 0.03, for light of wavelength 550 nm, this corresponds to a lower limit on the measurable birefringence of Δn = 1.05x10"5 for a sample of thickness 500 microns; or a lower limit on the measurable birefringence of Δn = 7.5x10"7 for a sample of thickness 3500 microns.

Birefringence values may be determined in 3 orthogonal directions which effectively enable a volume measurement. This may be particularly important in some applications such as spherical optics etc. The limits defined below are calculated based on measurements and assuming a 3 mm path length.

Preferably, the methods of this invention provide for the fabrication of diamond material such that birefringence measurements in at least one, preferably two, preferably all three orthogonal directions show values of Δn such that:

preferably Δn is less than 2x10"6 over areas greater than 1x1 mm, preferably over areas greater than 2x2 mm, preferably over areas greater than 4x4 mm, preferably over areas greater than 7x7 mm, preferably over areas greater than 15x15 mm;

preferably Δn is less than 5x10"6 over areas greater than 1x1 mm, preferably over areas greater than 2x2 mm, preferably over areas greater than 4x4 mm, preferably over areas greater than 7x7 mm, preferably over areas greater than 15x15 mm;

preferably Δn is less than 1x10"5 over areas greater than 1x1 mm, preferably greater than 2x2 mm, preferably greater than 4x4 mm, preferably greater than 7x7 mm, preferably greater than 15x15 mm.

Where birefringence values lie below a given threshold for each of three orthogonal directions of a particular volume of diamond, then for the purposes of this specification that volume is deemed to have a birefringence value below that threshold.

The present invention also provides a CVD diamond layer produced according to any of the methods outlined above for use as an optical element.

The present invention also provides a CVD diamond layer produced according to any of the methods outlined above for use as an electrical or electronic element. The present invention also provides a CVD diamond layer produced according to any of the methods above for use as a cutting tool or wire drawing die or other wear-resistant part.

The present invention also provides a CVD diamond layer produced according to any of the methods outlined above wherein the diamond layer has a thickness greater than 0.1 mm, preferably greater than 0.5 mm, preferably greater than 1 mm, preferably greater than 2 mm.

The present invention also provides a CVD single crystal diamond layer produced according to any of the methods outlined above wherein the diamond layer is in the form of a gemstone.

Preferably, the CVD single crystal diamond has three orthogonal dimensions greater than 2 mm, wherein at least one axis lies along the <100> crystal direction or along the principle symmetry axis of the gemstone. Preferably, the three orthogonal dimensions are greater than 2.5 mm, preferably greater than 3.0 mm, preferably greater than 3.5 mm. Preferably, the CVD single crystal diamond layer is of high clarity, with clarity of at least SM on the GIA gem grading scale, as defined above. Preferably, the CVD single crystal diamond layer has clarity of at least VS2, preferably at least VVS2, preferably at least WS1 on the GIA gem grading scale.

The methods of the present invention may be used to produce CVD polycrystalline diamond, in which the detrimental effects of the presence of nitrogen, for example, can be ameliorated. In particular, the detrimental effect of having small amounts of nitrogen on the electronic properties of the polycrystalline diamond, such as charge collection efficiency and carrier lifetime, may be countered. In this way, because of the improvement in electronic properties, polycrystalline diamond containing nitrogen can be used as a detector. This has the advantage of enabling detectors to be made from polycrystalline diamond containing nitrogen rather than having to rely on "pure" polycrystalline diamond that is much more expensive to synthesise.

CLAIMS

1. A method of producing a CVD diamond layer having high colour comprising:
(i) providing a substrate;
(ii) providing a CVD synthesis atmosphere in which there exists a first gas comprising a first impurity atom type which has a detrimental effect on the colour of the produced diamond layer; and
(iii) adding into the synthesis atmosphere a second gas comprising a second impurity atom type,
wherein the first and second impurity atom types are different; the type and quantity of the second impurity atom type is selected to reduce the detrimental effect on the colour caused by the first impurity atom type so as to produce a diamond layer having high colour; and the first and second impurity atom types are independently nitrogen or atoms which are solid in the elemental state.

2. The method of claim 1 wherein the CVD diamond layer is a single crystal.

3. The method of claim 1 wherein the CVD diamond layer is polycrystalline.

4. The method of any of claims 1 to 3 wherein the first impurity atom type is nitrogen and the second impurity atom type is selected from silicon, boron, phosphorus or sulphur.

5. The method of claim 4 wherein the second impurity atom type is silicon.

6. The method of claim 4 wherein the second impurity atom type is boron.

7. The method of any of claims 1 to 3 wherein the first impurity atom type is silicon, boron, phosphorus or sulphur and the second impurity atom type is nitrogen.

8. The method of claim 7 wherein the first impurity atom type is silicon.

9. The method of 7 wherein the first impurity atom type is boron.

10. The method of claim 5 or claim 8 wherein the concentration of nitrogen in the majority volume of the diamond layer is less than or equal to

2 x 1017 atoms/cm3 and the concentration of silicon in the majority volume of the diamond layer is less than or equal to 2 x 1018 atoms/cm3.

11. The method of any of claim 5, 8 or 10 wherein the ratio of the concentration of nitrogen to the concentration of silicon in the majority volume of the diamond layer is from 1 :20 to 20:1.

12. The method of any of claims 5, 8, 10 or 11 wherein the gas comprising nitrogen is present in the synthesis atmosphere at a concentration of greater than 100 ppb and the gas comprising silicon is present in the synthesis atmosphere at a concentration of greater than 10 ppb.

13. The method of claim 6 or claim 9 wherein the ratio of the concentration of nitrogen to the concentration of boron in the majority volume of the diamond layer is from 1 :2 to 2:1.

14. The method of any of claims 6, 9 or 13 wherein the gas comprising nitrogen is present in the synthesis atmosphere at a concentration of greater than 100 ppb and the gas comprising boron is present in the synthesis atmosphere at a concentration of greater than 0.5 ppb.

15. The method of any of the preceding claims wherein the produced CVD diamond layer has an increased normalized free exciton intensity compared to a method where the second gas comprising a second impurity type atom is not added.

16. The method of any of the preceding claims wherein the produced CVD diamond layer has an increase in carrier mobility, carrier lifetime and/or charge collection distance compared to a method where the second gas comprising a second impurity type atom is not added.

17. The method of any of the preceding claims wherein the synthesis atmosphere comprises a concentration of the gas comprising nitrogen which has not been added deliberately of greater than 300 ppb.

18. The method of any of the preceding claims wherein the first gas is not added in a controlled manner.

19. The method of any preceding claim wherein the diamond layer is greater than 0.1 mm in thickness.

20. A method of producing a CVD diamond layer, comprising:
(i) providing a substrate;
(ii) providing a CVD synthesis atmosphere in which there exists a concentration of nitrogen which is not deliberately added of greater than 300 ppb; and
(iii) adding into the synthesis atmosphere a second gas comprising a second impurity atom type other than nitrogen,
wherein the second impurity atom type is added in a controlled manner in an amount that reduces the detrimental effect on the colour caused by the nitrogen so as to produce a diamond layer having high colour; and the second impurity atom type is solid in the elemental state.

21. A method of producing a CVD diamond layer comprising the steps of:
(i) providing a substrate; and (ii) adding into a CVD synthesis atmosphere a gaseous source comprising silicon.

22. The method of claim 21 wherein the CVD diamond layer is a single crystal.

23. The method of claim 21 wherein the CVD diamond layer is polycrystalline.

24. The method of any of claims 21 to 23 wherein the method comprises one or more of the following features:
(1) the layer is grown to greater than 0.1 mm thickness;
(2) the substrate is a diamond substrate having a surface which is substantially free of crystal defects such that a revealing plasma etch would reveal a density of surface etch features related to defects below 5 x 103/mm2;
(3) the duration of the synthesis of the single crystal diamond layer is at least 50 hours; and
(4) the substrate comprises multiple separated single crystal diamond substrates.

25. The method of any of claims 21 to 24 wherein the concentration of silicon in the majority volume of the diamond layer is less than or equal to 2 x 1018 atoms/cm3.

26. The method of any of claims 21 to 25 wherein the addition of silicon reduces an adverse effect on a property of the produced diamond layer caused by the presence of an impurity atom type.

27. The method of claim 26 wherein the impurity atom type is nitrogen.

28. The method of claim 26 or claim 27 wherein the property is colour and adding silicon produces a CVD diamond layer having high colour.

29. The method of claim 26 or 27 wherein the property is free exciton emission of the diamond layer and adding silicon produces a CVD diamond layer with an increased normalized free exciton intensity compared to a method where silicon is not added.

30. The method of claim 26 or 27 wherein the property is at least one of: carrier mobility; carrier lifetime; and charge collection distance and adding silicon produces a CVD diamond layer with an increase in carrier mobility, carrier lifetime and/or charge collection distance compared to a method where silicon is not added.

31. The method of any of claims 1 , 2, 4 to 22 and 24 to 30 wherein the CVD diamond layer is a single crystal, and wherein the majority volume of the diamond layer has at least one of the following features:
a) an absorption spectrum measured at room temperature such that the colour of a standard 0.5 ct round brilliant would be better than K;
b) an absorption coefficient at 270 nm measured at room temperature which is less than 1.9 cm"1;
c) an absorption coefficient at 350 nm measured at room temperature which is less than 0.90 cm"1;
d) an absorption at 520 nm of less than 0.30 cm"1 ; or
e) an absorption at 700 nm of less than 0.12 cm"1.

32. The method of any of claim 1 , 3 to 21 and 23 to 30 wherein the CVD diamond layer is polycrystalline, and wherein the majority volume of the diamond layer has at least one of the following features:
a) an absorption coefficient at 270 nm measured at room temperature which is less than 1.9 cm"1;
b) an absorption coefficient at 350 nm measured at room temperature which is less than 0.90 cm'1;
c) an absorption at 520 nm of less than 0.30 cm'1 ; and
d) an absorption at 700 nm of less than 0.12 cm"1.

33. The method of any of claims 1, 2, 4 to 22 and 24 to 31 wherein the CVD diamond layer is a single crystal and wherein the diamond layer is formed into a gemstone having three orthogonal dimensions greater than 2 mm, where at least one axis lies either along the <100> crystal direction or along the principle symmetry axis of the gemstone.

34. A CVD diamond layer produced by the method of any one of the preceding claims.

35. A CVD diamond layer comprising an impurity atom type selected from silicon, sulphur or phosphorus, wherein the diamond layer has high colour.

36. A CVD diamond layer wherein the majority volume of the diamond layer comprises from 1015 to 2 x 1018 atoms/cm3 of an impurity atom type selected from silicon, sulphur or phosphorus.

37. The CVD diamond layer of claim 35 or claim 36 wherein the impurity atom type is silicon.

38. The CVD diamond layer of any of claims 36 or 37 wherein the diamond layer has high colour.

39. The CVD diamond layer of any of claims 35 to 38 wherein the layer has a thickness of greater than 1 mm.

40. The CVD diamond layer of any of claims 35 to 39 wherein the layer has a birefringence of less than 1 x 10"3 over a volume greater than 0.1 mm3.

41. The CVD diamond layer of any of claims 35 to 40 wherein the diamond layer is a single crystal.

42. The CVD diamond layer of any of claims 35 to 40 wherein the diamond layer is polycrystalline.

43. A CVD diamond layer produced according to the method of any of claims 1 to 33 for use as an optical element.

44. A CVD diamond layer produced according to the method of any of claims 1 to 33 for use as an electrical or electronic element.

45. A CVD diamond layer produced according to the method of any of claims 1 to 33 having a thickness greater than 0.1 mm.

46. A CVD single crystal diamond layer produced according to the method of any of claims 1 , 2, 4 to 22 and 24 to 31 in the form of a gemstone.

47. A CVD single crystal diamond layer according to claim 46 having three orthogonal dimensions greater than 2 mm, wherein at least one axis lies along the <100> crystal direction or along the principle symmetry axis of the gemstone.

48. A CVD single crystal diamond layer according to claim 46 or claim 47 having a clarity of at least SH on the GIA gem grading scale.

49. Use of a sufficient quantity of a gaseous source in a method of producing a CVD diamond layer having high colour, wherein the gaseous source comprises a second impurity atom type to counter the detrimental effect on colour of diamond of a first impurity atom type.

50. Use of a gaseous source of silicon in a method of CVD diamond production, wherein the gaseous source is added to a reaction chamber comprising a substrate and a diamond synthesis atmosphere such that the silicon counters the detrimental effect of a first impurity atom type.