Patent Application: US-201113109528-A

Abstract:
the invention relates to single crystal diamond with high optical quality and methods of making the same . the diamond possesses an intensity ratio of the second - order raman peak to the fluorescence background of around 5 or greater .

Description:
the inventors have developed a microwave plasma assisted chemical vapor deposition ( mpcvd ) technique to produce large diamond single - crystals at very high growth rates ( up to 200 μm / hr ) and have succeeded in fabricating single crystals above 12 mm thickness ( or 10 carats in size ) and having a variety of optical and mechanical properties . importantly , the mpcvd techniques of the present invention can be used to produce large , high optical quality single crystal diamond without a subsequent annealing step . recently , these techniques have been refined to produce large , high purity single crystal diamond anvils . the example shown in fig1 was cut from a 13 . 5 carat sc - cvd diamond block grown at high growth rates ( around 50 μm / hr ) in the absence of impurities other than hydrogen . high - purity cvd single crystal material is technically more difficult to grow than brown material . one challenge the industry has faced is finding ways to routinely and reliably produce synthetic diamonds larger than 1 carat . certain u . s . patents and patent applications provide information related to methods for producing single crystal cvd diamond . applicants thereby incorporate the following patents and patent applications by reference : u . s . pat . no . 6 , 858 , 078 ; u . s . ; u . s . application ser . no . 11 / 438 , 260 and u . s . application ser . no . 12 / 624 , 768 . a table of diamonds made by the methods of the present invention is provided below . uv - vis absorption , raman / photoluminescence spectroscopy , cathodoluminescence , birefringence topography were used to characterize the diamond anvil and other counterparts . the measurements show that the material has high optical quality and clarity . the testing has shown that high purity cvd single crystal material has been grown at high growth rates ( around 50 μm / hr .). the material has high optical quality and clarity without a layer structure . the high purity single crystal cvd diamond has a large intensity ratio ( above 5 ) of the second - order raman peak to the background . such diamond shows promise as a material for a high pressure window as well as a material in other applications including , but not limited to , gems , high pressure anvils and other electronic and mechanical devices . growth conditions include taking advanced measures to preclude the introduction of nitrogen into the cvd chamber . the nitrogen content in the reaction gases ( e . g ., h 2 , ch 4 , co 2 , o 2 ) should be below 1 ppm to produce colorless d to j colored diamond . precautions are also taken to preclude the introduction of other impurities . in certain embodiments of the invention , the crystal orientation of the substrate is 0 - 15 degrees off { 100 }. without being bound by theory , this is thought to increase the nucleation rate and reduce the level of impurities . in a preferred embodiment , the crystal orientation of the substrate is about 5 - 10 degrees off { 100 }. in another preferred embodiment , the crystal orientation of the substrate is about 7 degrees off { 100 }. in one embodiment of the present invention , the growth temperature of the diamond is from about 1000 to about 1400 degrees c . in another embodiment , the growth temperature is from about 1250 to about 1350 degrees c . in another embodiment , the growth temperature of the diamond is about 1300 degrees c . without being bound by theory , it is thought that operating at this optimum growth temperature range increases the toughness of the growing diamond crystal , thus reducing the potential for growth - limiting cracks . diamond grown in the absence of nitrogen is more susceptible to cracks than nitrogen - doped diamond . it is therefore important to prevent cracks from forming to achieve the large growth size of single crystal diamond . the diamond of the present invention was synthesized at pressures ranging from about 100 - 200 torr . in one embodiment , the pressure was about 150 torr . in embodiments of the invention , gas flow rates ranged from about 30 - 80 sccm ch 4 ( used as a carbon source ) and 300 - 500 h 2 . the inventors have developed microwave plasma assisted chemical vapor deposition ( mpcvd ) techniques to produce large diamond single - crystals at very high growth rates ( up to 200 μm / h ) and have succeeded in fabricating single crystal diamonds above 12 mm thickness ( or 10 carats in size ) and having a variety of optical and mechanical properties [ 1 - 5 ]. recently , these techniques have been refined to produce large , high - purity single crystal diamond anvils . a 2 . 3 carat colorless round cvd diamond has been successfully fabricated at high growth rates without annealing . it was cut from a 13 . 5 carat rough diamond block grown at around 50 μm / h in the absence of impurities other than hydrogen . microscopic and spectroscopic measurements show that this material has high optical - quality and clarity without visible layers . as incidated above , high - purity cvd single crystal material is technically more difficult to grow than brown material . one challenge the industry faced has been finding ways to routinely and reliably produce synthetic diamonds larger than 1 carat [ 7 ], especially diamonds that are near - colorless and colorless . one of the limitations has been the very slow growth fate that was necessary to obtain high quality material , limiting the thicknesses of synthesised crystals to a few micrometers [ 14 ]. nitrogen is the most common impurity in synthetic diamonds . if nitrogen is present in the diamond growth environment , it is easily included in the lattice of the growing crystal [ 13 ]. improved vacuumn is necessary for growth without nitrogen . another limitation is that the stress inside cvd diamond grown without nitrogen makes it crack very easily , which also hinders the achievement of big volume , high - purity diamond crystals . a 5 kw , 2 . 45 ghz astex mpcvd system was used for single crystal diamond synthesis in the present invention . hpht synthetic type - ib and single - crystal cvd diamond with { 100 } surfaces and minimum surface defects were used as substrates for diamond growth . a hydrogen generator with a palladium purifier was used to produce clean hydrogen with 7n purity . high purity methane ( 99 . 9995 %) was also used . diamond was synthesized at pressures ranging from about 100 - 200 torr . in one embodiment , the pressure was about 150 torr . gas flow rates ranged from about 30 - 80 sccm ch 4 ( used as a carbon source ) and 300 - 500 h 2 . the growth temperature ranged from about 1250 to about 1350 degrees c . in another embodiment , the growth temperature of the diamond is about 1300 degrees c . it must be noted that most of the faceted synthetic diamonds previously produced have been small ( 0 . 30 ct and less ) and highly colored . few near - colorless or colorless laboratory - grown diamonds have been encountered { 12 }. in may 2010 , gia lab announced their first identification of cvd synthetic diamond over one carat ( 1 . 05 carat pear shape near - colorless ) { 10 }. later in november , 2010 , gemesis claimed to produce colorless cvd diamond mostly over a half carat , the largest stone being 1 . 11 carat { 11 }. these diamonds stand in contrast to the diamond produced by the methods of the present invention , characterization of which is provided below . uv - vis absorption , micro - raman / photoluminescence spectroscopy and cathodoluminescence , and birefringence microscopy were used to characterize the diamond anvil of the present invention and other counterparts . the measurements show that the material has high optical quality . when observed under a microscope , the high purity cvd diamond is water clear and relatively free of inclusions and cracks . there are no visible layers ( or growth interfaces ) and striations from a cross - sectional view within the thickness above 5 mm . as shown in fig1 , the as - grown 2 . 3 - carat round brilliant cut diamond , measuring 8 . 5 × 8 . 5 × 5 . 2 mm , was color graded as near colorless ( j grade ). its clarity grade was equivalent to si 1 . no inclusions are observed in the center , and the diamond contains only a few inclusions on the girdle . another characteristic optical property is the absence of layers in the single crystal diamond of the present invention . in most cvd diamond layers or striations form as a result of a differing uptake of defects on the risers and terraces of growth steps { 6 }. this is characteristic in cvd diamonds , especially in nitrogen - doped cvd diamond . such layers are invisible in this high - purity diamond . fig2 shows a visible transmission spectra at 300 k for a high purity cvd diamond plate and the 2 . 3 carat anvil . the plate does not exhibit any absorption lines or bands in the visible region . with an optical path of 5 mm , the anvil displays relatively high transmission in the visible range ( from 400 to 800 nm ) and has much lower absorption than nitrogen - doped diamond . the lower transparency compared to the plate may be due to unintentional incorporation of nitrogen by leaking ( discussed below ). the absorption at 737 nm from the silicon - vacancy - related center ( si - v ) is invisible or very weak . raman / photoluminescence ( pl ) spectra were measured at room temperature using a 457 nm laser ( fig3 ) and a 532 nm laser ( fig4 ) for excitation . there was no photoluminescence at 575 nm and 637 nm caused by nv 0 and nv − centers . a sharp diamond first - order raman peak and a second - order raman peak were observed as shown in fig3 . the intensity ratio of the second - order raman peak to the fluorescence indicates very good crystal quality , thus rendering the diamond favorable for high - pressure optical windows . cvd diamond incorporating nitrogen usually has 575 nm and 637 nm luminescence . this leads to the characteristic orange red fluorescence which will overlap with the optical signal from the sample inside the diamond anvil cell . in the absence of nv centers , high - purity cvd diamond does not show the orange / orange - red fluorescence . there is an si - v peak at 737 nm in the pl spectrum with 532 nm laser excitation as shown in fig4 . cathodoluminescence microscopy was performed on the 2 . 3 ct anvil . unlike nitrogen - added cvd diamond , which displays a strong orange emission , this sample has no emission , except for an orange fine line ( around 20 ˜ 50 um ) in a quarter section ( fig5 ). photoluminescence ( pl ) spectra were measured along the growth direction from culet to girdle of the anvil ( fig4 ). all of the spectra were normalized to have an identical height of the diamond raman peak . higher fluorescence was detected around the area of the orange cl line . without being bound by theory , this is likely due to an unintentional incorporation of nitrogen caused by leaking . moreover , it might lead to the color j of the anvil . inclusions are also characterized using confocal raman imaging spectroscopy . fig6 shows the raman imaging results on a small ( abound 30 um ) black inclusion close to the girdle . the first image is the map of diamond peak intensity . the second image is the map of the broad g band caused by amorphous carbon . the third image is the map of the 1610 cm − 1 caused by graphite or nanocystalline diamond . as a result , the inclusion is composed of amorphous - carbon , graphite and nanocystalline diamond . high - purity cvd single crystal material has been grown at high growth rates ( around 50 μm / h ). this material has high optical quality and clarity without visible layers . the high - purity single crystal cvd diamond has a large intensity ratio ( above 5 ) of the second - order raman peak to the background . it is a promising material for high pressure optical windows . the color may be due to a leak of nitrogen . the inclusion composed of amorphous - carbon , graphite and nanocystalline diamond . the 737 nm si peak in pl , the striations in cl microscopy , and the inclusions composed of graphite , amorphous - carbon , and nanocrystalline diamond can be the features to identify these high - purity cvd diamonds from natural diamonds . with this enhanced rapid growth , high optical quality single crystal cvd diamond anvils over 1 carat can be routinely fabricated . it is to be understood that both of the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed . yan c . s et al ., proc . natl . acad . sci . 2002 , 99 , 12523 - 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