IdentificationofCVDSyntheticDiamond(1).doc

Identification of CVD Synthetic DiamondCVD化学气相沉积法合成钻石的鉴别Gemmological Association of All Japan, Research Laboratory:Hiroshi KITAWAKI (FGA, CGJ), Ahmadjan Abduriyim (Ph D), Makoto OKANO (CGJ)We conducted gemmological study on synthetic diamonds produced by CVD technique by Apollo Diamond Inc., and synthetic diamonds produced for research purposes by CVD technique by Element Six (De Beers Industrial Diamond Division). The stones we tested were type IIb of brown to near colourless, and it is usually very hard to distinguish these stones from natural diamonds only by common gemmological tests. However, their characteristic orange fluorescence under ultraviolet light and absence of anomalous double refraction due to strain called “Tatami” structure, which is commonly seen in type II natural diamonds, indicate their synthetic origin. Combination of laboratory techniques such as photoluminescence analysis (PL) and cathode luminescence analysis (CL) can identify between them clearly. Introduction At the beginning of 1990s, synthetic diamonds became available on the gem market and then the new material became widely known to client in the industry. Most of the synthetic diamonds are typeIb with yellow colour and usually under 2ct weight, however, typeIIa of colourless and typeIIb of blue colour materials are also existence. In recent years, pink to red or purple colour stones have been produced by irradiation and heat treatment. Such stones have all been synthesized by high-pressure-high-temperature (HPHT) process and their identification method is based on the growth environment and crystal morphology that are essential to the HPHT process. According to Rapaport News, Apollo Diamond Inc. of Boston, Massachusetts in the United State announced in August 2003 its project to sell synthetic diamonds produced by CVD method for gem use. Apollo Diamond Inc. allegedly have developed technique to produce facetable high-quality CVD diamond over 2mm in thickness and the material is comparable in quality to rare natural typeIIdiamond. While the company has stated that it would fully disclose all the information, for instance, by inscribing on faceted products by laser, it also announced that the CVD diamonds, similar to HPHT treated diamonds, cannot be distinguished by standard gemmological instruments. Although CVD diamonds are not much paid attention by gem industry, researchers in recent years have already known that CVD method can produce high quality diamonds in several millimetre thicknesses. It is unlikely that CVD diamonds dominate gem industry right away. However, as Chatham Created Gems announced over ten years ago that it was going to sell synthetic diamonds produced by HPHT method and it lead us to the current situation, not only gemmologists but also all those who related to the gem industry should pay attention on the direction of development of CVD diamond hereafter. What is CVD synthesis? Techniques to grow crystal by means of adsorption reaction and thermal decomposition of atoms and molecules contained in vapour are called vapour deposition methods. When this crystal growth process involves chemical reaction, it is called CVD (Chemical Vapour Deposition), and when a crystal grows physically, it is called PVD (Physical Vapour Deposition). In diamond synthesis process, when diamond, a crystal of carbon, is synthesised under high temperature and high pressure in which the diamond is thermodynamically stable, it is generally called high-pressure synthesis or high temperature high pressure method. Contrary to this, in CVD process, gas of organic compound such as methane containing carbon that is ingredient of diamond is dissociated in low temperature low pressure region (usually a tenth of atmospheric pressure), under which diamond is thermodynamically unstable, to make it into active status called radical, and then is deposited as diamond on a substrate maintained at between 800 to 1000 C. When the substrate is made of a single crystal of diamond (regardless natural or synthetic by HPHT method), a single crystal of diamond grows, whereas when the substrate is made of silicon, tungsten or molybdenum, polycrystalline diamond is produced. Quite large energy is required to activate the source gas, and there are several methods to do so, such as heating by filament (hot filament method), plasma heating by microwave (microwave plasma method), plasma heating by high frequency (high frequency plasma method) and heating by combustion-flame (combustion-flame method). Among these, microwave plasma method is suitable to synthesise high-quality single crystals in larger size (Figure 1) although it is not a successful CVD method to synthesise in large area. Figure 1:CVD synthesis (the Apollo way) (quotations from WIRED 2003) Historical background In 1953, Eversole of Union Carbide Crop. in the United State proved that diamond showed homoepitaxial growth from carbon-bearing gas under low pressure. In 1956, Derjaguin of former Soviet Union announced that he successfully synthesised diamond by CVD method. In these methods growth rate of diamond was extremely low, and graphite also grew simultaneously with diamond so that in each case chemical reaction of the process had to be suspended. Fundamental research had been repeated thereafter and finally Matsumoto of NIRIM (the National Institute for Research in Inorganic Materials), Japan, made a breakthrough by succeeding development of a hot filament CVD method in 1981, followed by development of a microwave CVD method by Kamo of NIRIM. This method drew attention, as its experiment was reproducible and could produce crystals in good quality. On the 1st January 1986, major national newspapers reported an article of Mr. Hirose of Nippon Institute of Technology under the title “Diamond made from Alcohol”. This achievement sent messages to public widely that not only academic significance of the method but also the fact that diamond could be made from gas or liquid under low pressure. Compared to HPHT method, the CVD method uses small and simple device and it can be used for coating in any form with polycrystalline diamond, therefore it is expected to be applied to coating of machine equipment, heat sink of semiconductor equipment, window material for infrared and radial ray or to the electronics field. Sample and method Nine pieces of CVD diamonds in total were used for this study. Three of them were produced by Element Six (De Beers Industrial Diamond Division) for research purposes, and were loaned by DTC Research Centre (Photo 1), two of which were square-cut brownish stones and the rest was nearly colourless, polished into tabular from rough. Element Six has launched research on the possibility of use of CVD diamond for gem purpose and on its detection since late 1980s. Another six pieces were produced by Apollo Diamond Inc., which were not for sale and we borrowed from Branko Deljanin of European Gemological Laboratory (EGL) (Photo 2). Among them four pieces were faceted but other two were rough as grown (Photo 3). The rest of the two had been HPHT treated to lighten the colour (Photo 4). Photo 1: CVD diamonds produced by Element Six (De Beers Industrial Diamond Division) for research purposes. 0.86, 0.39 and 0.50 carats. Photo 2: CVD diamonds produced by Apollo Diamond Inc. 1.70, 0.36 and 0.21 caratsPhoto 3: A CVD diamond rough crystal produced by Apollo Diamond Inc., 0.189ct.Photo 4: An HPHT-treated CVD diamond produced by Apollo Diamond Inc., 0.226ct.Other than general identification tests such as magnification test by a microscope, UV fluorescence test and spectroscopic test, spectrometry (UV-visible, near-infrared and infrared region), X-ray fluorescence analysis, Cathode luminescence (CL) analysis and photo luminescence (PL) analysis were carried out on these samples. Identification of CVD diamonds -Basic theory- Compared to HPHT method, a reaction chamber of the device for CVD method can be enlarged easily, and this means that the method may be developed as a synthesis of lager-size diamonds. However, it is difficult for this method to grow diamond in thick layer because its growth rate is extremely low. With the technique of early 1980s, it is calculated to take four months to grow diamond layer thick enough to obtain a 0.5ct brilliant cut stone, and six months for one carat. To solve the problem of growth rate, epitaxial growth has to carry out on a 100 substrate while retaining the condition (types and concentration of source gas and temperature of a substrate) to grow 100 face in preference to 111 face. From recent research, it is known that the growth rate can be accelerated by adding nitrogen gas, and it can achieve high speed growth rate of over 100m/h under a good condition. Such differences of growth environment between CVD diamond and natural diamond become key points to identify them, and the examples observed are different crystal morphology of diamonds grown in both environment and different impurity elements contained in those diamonds. Figure 2: Crystal morphology of natural diamonds and CVD diamonds.Most of natural diamonds are composed of only 111 growth sector,whereas CVD diamonds (single crystals) are composed of one large growth sector 100 and smaller sector 111. Standard Identification Tests Appearance A rough crystal of CVD diamond synthesized by Apollo Diamond inc. has totally different appearance from a rough crystal of natural diamond. Most of natural diamonds show crystal forms based on an octahedron, while CVD diamonds show tabular form (Fig 2). Those crystal forms are also different from cube-octahedral form of previous HPHT synthetic diamond. Two pieces of rough stones tested this time were both completely separated from their substrates (Apollo Diamond inc. uses HPHT synthetic diamond for their substrates, which are removed by laser after the CVD crystal growth.). Outer rim of the rough crystals show dark brown area of low quality, which is non-diamond carbon (Photo 3/ front page). Two pieces of Element Six and one piece of Apollo Diamond inc. were faceted, with the latter have been HPHT treated and thus its transparency was lowered and facet surface was roughened (Photo 4/ front page). According to Mr. Branko Deljanin of EGL, brown tint of the stone has been slightly removed by the HPHT treatment. Inclusion CVD diamonds tested this time were generally good in clarity, and two faceted pieces of Element Six were in VS class*. A few pinpoints were observed under 10X magnification in a tabular polished stone of Apollo Diamond inc. (Photo 5). Further magnifying observation revealed that those pinpoints were dark brown irregular-shaped substance, which is though to be non-diamond carbon. As CVD synthesis does not involve metal solution, metal inclusion, which is often seen in synthetic diamond produced by HPHT process, will not exist. Therefore, CVD diamond does not havemagnetism, which has been announced as one of the simple identifying features for synthetic diamonds.Photo 5: A small number of pinpoints observed in a tabular polished stone of Apollo Diamond inc. under 10X magnification. Colour Zoning All CVD diamonds tested this time except one piece, which is near colourless of Element Six, have a brown hue. They are graded as Fancy Light Brown and Fancy Brown*, with a slight yellow hue. Brown colour distribution in the CVD diamonds was observed evenly throughout each stone, however some stones showed several brown lines. These are probably parallel to 100 and may have been caused by deposition of non-diamond carbon due to variation of temperature or gas pressure during its growth. Brown colour zoning seen in natural diamonds has been formed by plastic deformation that the diamond received while it reached to the earths surface from deep under the ground where it had originally formed. It is usually observed as brown grains (planete brown colour zoning) arranged in one direction or two directions intersecting to one another. The brown line pattern seen in CVD diamond appears not being related to anomalous double refraction due to strain that is going to be mentioned later, while the brown colour zoning seen in natural diamond is commonly associated with the anomalous double refraction showing interference colour. Anomalous Double Refraction due to Strain The CVD diamonds we tested this time showed characteristic streaky pattern of anomalous double refraction due to strain. They run in parallel to the growth direction of the crystal, which can be clearly observed from the direction perpendicular to the growth plane of the tabular crystal (i.e., mainly from the girdle). These patterns are thought due to dislocation caused during growth of the crystal (Photo 6). Anomalous double refraction due to strain seen in natural diamond can be roughly devided into two categories, one of which is caused during the growth and the other due to plastic deformation, the latter strongly indicates natural origin of the stone. Typical anomalous double refraction due to strain caused by plastic deformation is so-called Tatami structure seen in a typeII diamond (Photo 7). Although CVD diamond belongs to this type II, it does not show the Tatami structure. However, when a CVD diamond is observed from the direction parallel to the growth direction (usually along the direction perpendicular to the table), the stone shows Tatami -like structure on first glance so that care should be taken (Photo 8) not to be deceived.Photo 6: Characteristic streaky pattern of anomalous double refraction due to strain observed under crossed polarised lights. They run in direction parallel to the direction of crystal growth, which will appear distinct by observation from direction perpendicular to the growth plane of a tabular crystal (mainly in direction of the girdle).Photo 7: Anomalous double refraction called Tatami structure observed in natural diamonds.Photo 8: Anomalous double refraction resembling Tatami -like structure at first glance seen in CVD diamonds (observed from the direction parallel to the growth direction). UV FluorescenceMost pieces of the CVD diamonds tested this time showed characteristic orange UV fluorescence (Photo 9). They were generally more clearer in LWUV than in SWUV, but if intensity of the fluorescence is weak, it should be observed in a completely dark room. Those fluorescence colour is thought due to NV centre (575nm), which is rarely seen in a type II natural diamond. In CVD diamond, injection of nitrogen gas to accelerate growth rate and formation of void associated with generation of dislocation may be involved in production of the NV centre.Photo 9: Characteristic orange UV fluorescence seen in CVD diamonds. Laboratory Techniques UV-Visible Spectrum Spectral analysis between UV-visible region under room temperature showed gradually stronger absorption from LWUV towards SWUV. The absorption end in the UV region is between 220nm and 240nm, which is a typical spectrum pattern of type II. In samples with a brown hue, broad absorptions were observed between 500nm and 550nm. The intensity of absorption and the depth of the brown hue are in proportion, and this absorption was not seen in a near colourless sample. Some stones showed a slight absorption on 270nm, which is due to isolated single atoms of nitrogen. N3 centre (415nm), which is seen in almost all natural diamonds, was not recognised. The Diamond Trading Companys Diamond Verification Instruments- DiamondSure is also a very convenient technique to detect the N3 centre as a factor to determine the type of diamond. Natural Type Ia diamond indicates as “pass” and various Type II as “refer”, it means that the CVD synthetic diamond will be certainly refer and further testing is required. Infrared Spectrometry (FTIR) Spectral analysis in near-infrared to infrared region under room temperature was examined, and in the infrared region, most of the samples were in the type II category which does not show absorption in nitrogen region in diamond. Some samples showed a very weak absorption on 1344cm-1 when the resolution power was set to 1cm-1 and its elapsed time was increased. This is related to isolated single atom of nitrogen and is typically seen in type Ib. In the near-infrared region, some showed a slight absorption on 7354cm-1. According to Wuyi Wang et.al, (2003), CVD diamonds may show absorptions on 8753, 6856, 6425 and 5564cm-1 relating to hydrogen impurity when they are analysed with elapsed time of 1024. Cathode luminescence technique can observe growth history of a diamond and this is the most effective means to reveal the origin of a diamond. In this study, two types, a luminoscope ELM-3 and a combination of an electron microscope and a spectroscope, were used. The former can observe emission colour directly, while the latter have higher resolving power of CL figure. The Diamond View developed by DTC employs shorter wave ultraviolet light instead of electron beam in CL technique, and its principle is very similar to a luminoscope. The Diamond View has better operationality but CL method can provide clearer image of fluorescence image. CL images of natural diamond are variable and they can even be applied to individual identification (Photo 10). Synthetic diamonds produced by HPHT technique show sector zoning (Photo 11), which makes distinction from natural diamonds easier. Photo 10: CL figure (by a luminoscope) of natural diamond; natural diamonds show various CL figures that can even be used for individual identification (left: colourless type Ia; right: yellow ty