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  Boron doped diamondRelated Patent Categories: Coating Processes, Coating By Vapor, Gas, Or Smoke, Carbon Or Carbide Coating, Boron And Carbon Containing Coating (e.g., Boron Carbide, Etc.)The Patent Description & Claims data below is from USPTO Patent Application 20070092647. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This is a divisional application of U.S. application Ser. No. 10/653,419, filed Sep. 3, 2003, which is a continuation of U.S. application Ser. No. 10/319,573, filed Dec. 16, 2002. BACKGROUND OF THE INVENTION [0002] This invention relates to doped diamond and more particularly to doped diamond produced by chemical vapour deposition (hereinafter referred to as CVD diamond). [0003] There are a range of applications of diamond for which a doped diamond layer of significant dimensions, with a uniform dopant concentration and associated electronic and/or optical properties would be advantageous. Dependent on the detailed application, this material needs to substantially exclude detrimental electronic or optically active traps or defects. To date, material of this type has not been available. [0004] Applications such as high power electronics require bulk free standing diamond with thicknesses ranging from 50 to 1000 .mu.m and lateral sizes varying from 1.times.1 mm.sup.2 to 50.times.50 mm.sup.2. For viable production in a competitive market it is beneficial that the diamond used for these structures is grown as a bulk material and processed into the final devices. In addition, wafer scale processing is possible with larger pieces, further reducing device fabrication costs. For optical applications, such as filters and absorbed power measurement devices, the large size and thickness of the raw material can be an intrinsic requirement of the device. Thus there are a range of benefits to synthesising thick layers. [0005] Boron is the only known dopant in diamond which has well characterised relatively shallow dopant behaviour. Other potentially shallow dopants reported in the literature to be under investigation include S, P, O, Li, but these are not yet available as reliable bulk dopants. There are many electronic applications which need doped diamond, often over relatively large areas and with very uniform properties. However, the incorporation of boron during synthesis is a very sensitive property of the particular growth sector. Polycrystalline diamond contains a random selection of growth sectors, and although the average boron concentration may be uniform on a scale much larger than the grain size, at the same scale as the grain size the local boron concentration varies substantially from point to point. [0006] Dopants can also be put into diamond by post growth treatment. The only currently reliable post growth treatment applicable to diamond is ion implantation, and this provides a method of producing layered diamond structures, but not uniform bulk doping. For instance, a `p-i` (p-type--intrinsic) structure can be produced by using an appropriate dose and energy for boron implantation into a high quality natural type Ia diamond. Unfortunately residual damage (vacancies and interstitials) is always created under conditions of ion implantation. This damage is impossible to remove completely, although annealing treatments can reduce it. The damage leads to degraded charge carrier properties resulting from defect scattering and compensation of boron acceptors. [0007] Methods of depositing or growing material such as diamond on a substrate by chemical vapour deposition (CVD) are now well established and have been described extensively in the patent and other literature. Where diamond is being deposited on a substrate by CVD, 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. 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 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. [0008] 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. [0009] Obtaining incorporation of boron into the solid during deposition is less difficult than for many other potential dopants. The incorporation ratio for boron, which is the ratio of the dopant boron (B) to carbon (C) concentration in the solid ([B]/[C]:solid), compared to that in the depositing gas ([B]/[C]:gas) is generally about 1 (in the {100} growth sector) although it varies with many factors. There are many methods by which CVD diamond may be doped during synthesis with boron. With microwave plasma, hot filament and arc jet techniques, diborane (B.sub.2H.sub.6) or some other appropriate gas may be added to the gas stream, the incoming gases may be bubbled through methanol or acetone containing boria (B.sub.2O.sub.3), boron powder may be placed in the chamber, or a boron rod inserted into the plasma. For growth by the combustion flame method a fine mist of methanol containing boric acid can be injected into the gas stream with an atomiser. Diamond films have also been doped unintentionally when, for example, the plasma has decomposed a substrate holder fabricated from hexagonal boron nitride. [0010] Nitrogen can also 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. [0011] Although high purity single crystal (SC) CVD diamond has an important role in potential high power electronics, the number of potential applications would be substantially increased if a CVD doped diamond with uniform and advantageous electronic properties was available. In addition, there are other applications of boron doped diamond where uniformity in the colour, luminescence, or other properties associated with B doping is advantageous. SUMMARY OF THE INVENTION [0012] According to a first aspect of the invention, there is provided a layer of single crystal boron doped diamond produced by CVD wherein the total boron concentration is uniform, with a variation over the majority volume which is less than 50%, and preferably less than 20%, measured with a lateral resolution at each measurement point of less than 50 .mu.m, and preferably with a lateral resolution at each measurement point of less than 30 .mu.m, and having at least one of the characteristics (i) to (iii): [0013] (i) the layer is formed from a single growth sector, which is preferably one of the {100}, the {113}, the {111} and the {110}, and more preferably the {100}, sectors, [0014] (ii) the layer thickness exceeds 100 .mu.m, and preferably exceeds 500 .mu.m, and [0015] (iii) the volume of the layer exceeds 1 mm.sup.3, and preferably exceeds 3 mm.sup.3, and more preferably exceeds 10 mm.sup.3, and even more preferably exceeds 30 mm.sup.3. [0016] The term "majority volume" as used herein and in the claims represents at least 70%, preferably greater than 85%, and more preferably greater than 95%, of the total volume of the diamond layer. [0017] The CVD single crystal boron doped diamond layer of the invention may also contain nitrogen as a dopant. The diamond layer will generally contain a nitrogen concentration no greater than 1/5 of that of the boron concentration, and preferably less than 1/50 of that of the boron concentration. [0018] The diamond layer is preferably of "high crystalline quality". In this context "high crystalline quality" allows the presence of the dopant boron atoms and nitrogen atoms and associated point defects such as those including vacancies, hydrogen and the like. [0019] The single crystal boron doped diamond layer may also have one or more of the following characteristics in the majority volume of the diamond, where that majority volume is defined as above: [0020] (a) the layer contains an uncompensated boron concentration greater than 1.times.10.sup.14 atoms/cm.sup.3 and less than 1.times.10.sup.20 atoms/cm.sup.3, preferably an uncompensated boron concentration greater than 1.times.10.sup.15 atoms/cm.sup.3 and less than 2.times.10.sup.19 atoms/cm.sup.3, and more preferably an uncompensated boron concentration greater than 5.times.10.sup.15 atoms/cm.sup.3 and less than 2.times.10.sup.18 atoms/cm.sup.3, [0021] (b) a hole mobility (.mu..sub.h) measured at 300K which exceeds .mu..sub.h=G.times.2.1.times.10.sup.10/N.sub.h.sup.0.52) for N.sub.h not exceeding 8.times.10.sup.15 atoms/cm.sup.3 (Equation (1) .mu..sub.h=G.times.1.times.10.sup.18/N.sub.hfor N.sub.h greater than 8.times.10.sup.15 atoms/cm.sup.3 (Equation (2) [0022] where Nh is the concentration of holes (or equivalently, the concentration of ionised boron acceptors), the functional relationship between .mu..sub.h and N.sub.h is based on current models and the value of G represents the gain over the best values of .mu..sub.h currently reported. G has a value of greater than 1. 1, and preferably a value greater than 1.4, and more preferably a value greater than 1.7, and even more preferably a value greater than 2.0. [0023] (c) Low or absent luminescent features at 575 and 637 nm, relating to nitrogen-vacancy (N-V) centres. Specifically, the ratio of integrated intensities of the nitrogen vacancy centre zero-phonon lines at 575 nm and 637 nm centre is less than 1/50, and preferably less than 1/100, and more preferably less than 1/300 the integrated intensity of the diamond Raman line at 1332 cm.sup.-1 when measured at 77 K with 514 nm Ar ion laser excitation. [0024] (d) A Raman line width as measured at 300K of less than 4 cm.sup.-1 FWHW (full width at half maximum height) and preferably less than 3 cm.sup.-1 and more preferably less than 2.5 cm.sup.-1, under 514 nm Ar ion excitation. [0025] (e) A high degree of uniformity in the uncompensated boron concentration as measured by FTIR using the method described below. In particular, the frequency distribution of uncompensated boron measurements taken by FTIR over a representative sample taken from the layer must be such that 90% of the measurements vary by less than 50%, and preferably by less than 30%, expressed as a percentage of the mean. [0026] (f) A uniform bound exciton emission (BE) at 238 nm consistent with the concentration of uncompensated substitutional boron atoms in the solid, measuring the BE at 77 K under UV excitation using the method described below. In particular, the frequency distribution of the BE taken by this method over any representative surface of the layer or sample taken from the layer must be such that 90% of the measurements vary by less than 50%, and preferably by less than 30%, expressed as a percentage of the mean. [0027] (g) A strong free exciton (FE) intensity measured at 77 K under UV excitation, with a high degree of uniformity measured using the method given below. In particular, the frequency distribution of FE measurements taken by this method over any representative surface of the layer or sample taken from the layer must be such that 90% of the measurements vary by less than 50%, and preferably by less than 30%, expressed as a percentage of the mean. [0028] The high mobility found in the CVD diamond of the invention is surprising. The current model for the variation of the mobility with concentration of carriers (or ionised acceptors), in the domain where the carrier concentration is greater than 8.times.10.sup.15 atoms/cm.sup.3, is based on the belief that the acceptor boron atoms are the dominant scattering mechanism, and that their contribution is essentially intrinsic to their presence. Consequently this model suggests that values higher than this cannot be achieved. In contrast therefore, the results of the work described herein show the model to be in error, in that other factors, which can be removed, have previously limited the mobility in doped diamond reported in the literature. [0029] The single crystal boron doped CVD diamond layer of the invention may be free standing or form a layer or region of a larger diamond body or layer. That larger diamond layer or body may be single crystal or polycrystalline diamond produced by CVD or other synthetic method. That larger diamond layer or body may be doped with boron, nitrogen or other elements. [0030] The diamond layer or body of the invention may take the form of a gemstone. [0031] According to another aspect of the invention there is provided a method of producing a layer of boron doped single crystal CVD diamond. This method includes the steps of providing a diamond substrate having a surface which is substantially free of crystal defects, providing a source gas, such source gas including a source of boron, dissociating the source gas and allowing homoepitaxial diamond growth on the surface which is substantially free of crystal defects thereby producing a layer of single crystal boron doped diamond, preferably of the type described above. Essential to this method is that the diamond growth takes place on a diamond surface that is substantially free of crystal defects. [0032] The method of the invention may additionally include the use of controlled nitrogen additions to the source gas. The nitrogen in the source gas provides an additional means of control of the morphology developed by the growing single crystal, and the incorporation ratio for nitrogen is substantially lower than that for boron. Thus nitrogen additions, calculated as molecular nitrogen, in the range greater than 0.5 ppm and less than 10000 ppm, and preferably in the range greater than 1 ppm and less than 1000 ppm, and more preferably in the range greater than 3 ppm and less than 200 ppm do not adversely affect the electronic properties of the boron doped layer significantly, since the doped material intentionally has boron present as a scattering centre, but does 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. Those skilled in the art will appreciate that the stage of using nitrogen to modify the morphology, and the stage of growing the uniformly boron doped layer may be separated or sequential. Continue reading... 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