The Influence On Growth Quality Of The Single Crystal Diamonds By Microwave Plasma CVD Synthetic Technology

With the development of Chemical Vapor Deposition?CVD?synthetic technology,diamonds have become more and more widespread in the field of high and new technology.Although the growth rate of CVD diamonds has been gradually improved,it is still difficult to avoid the polycrystalline diamonds formed at the periphery.The polycrystalline diamonds at the periphery limit the size of CVD diamonds and reduce the growth quality.In addition,the CVD diamonds can easily crack during the periphery growth or mosaic growth because of the internal stress.Thus,if we want to grow large size and high quality CVD single crystal diamonds,we should restrain the polycrystalline diamonds formed at the periphery and reduce the internal stress during the growth,which is helpful to maintain the crystalline integrity of CVD diamonds.At present,some foreign institutions have realized the industrial production of CVD single crystal diamonds,but the technology is still highly classified.In order to break down the technical barriers and master the synthetic technology of CVD diamonds,we have used the MPCVD equipment to study the synthetic process of CVD single crystal diamonds.Firstly,the influence of gas proportion,growth temperature and gas pressure on the growth quality of CVD single crystal diamond has been systematically studied based on the CH₄-H₂ growth system.The results show that the saturation and diffusion rate of the growth radicals have been affected by methane concentration and growth temperature,thus affecting the growth rate and surface morphology of CVD diamonds.At relatively low methane concentration and moderate growth temperature,CVD diamonds usually grow in the“step flow”mode,with better growth quality and less impurities.In this paper,CVD diamonds have been grown with 3%CH₄/H₂ ratio and the growth temperature 1200?by“step flow”mode.The FWHM of Raman shift and XRD Rocking Curve are respectively 3.9 cm^-1 and 41.76 arc sec,which indicates the great growth quality of the CVD diamonds.At higher pressure,the hydrogen etching is more effective and the growth rate can be higher without the growth quality decreasing.Secondly,in the paper,some of the technical details of the growth process have been improved,such as the selection of the seed crystals,the modification of the substrate holders,the nitrogen addition and the change of the periphery morphology of the seed crystals,etc.The results show that:?1?There are many defects in natural diamonds and the lattice constants vary greatly.The internal stress and crystalline quality show great difference as well.Compared with the natural diamonds,the HPHT and CVD diamonds have better crystal structure and show little difference in crystalline quality and stress characteristics,which are more suitable for the homoepitaxial growth of CVD single crystal diamonds.?2?The periphery overgrowth of the CVD diamonds can be effectively controlled by the application of the enclosed type substrate holders with circular recess.The polycrystalline diamonds at the periphery can be restrained and the growth quality as well as the surface flatness can be improved.For the seed crystals with 1 mm thickness,when the recess depth is 1mm or 1.5 mm,the crystalline quality at the central region and periphery is quite good with less impurities and lower internal stress.?3?The addition of nitrogen can obviously improve the growth rate of CVD diamond,but it can raise the impurity content.When the nitrogen flow is relatively low??0.6 sccm?,the addition of nitrogen has little negative effect on the crystalline quality of CVD diamonds.Among them,when the nitrogen flow is 0.3 sccm,the growth surface of the CVD diamond is quite smooth with less impurities and better crystalline quality.The FWHM of the Raman shift and the XRD Rocking Cure are respectively 4cm^-1 and 35.1 arc sec.?4?In the growth system of CH₄-H₂-N₂,the?100?periphery is more attractive for nitrogen atoms and can easily form the N-V defects.As a result,the lateral growth of the?100?periphery is quite strong with poorer crystalline quality and more impurities.By modifying the periphery morphology of the seed crystal,the growth quality of the?100?can be absolutely improved,which is promising for the mosaic growth of CVD diamonds.Lastly,a large CVD single crystal diamond with the size of 7.1 mm×7 mm×2.2mm and weight of 1.626 ct has been obtained in the enclosed type substrate holder by periodic repeat growth with 0.3 sccm nitrogen added.In the high resolution Raman spectra,the FWHM of the CVD bulk is as low as 2.2 cm^-1,while the FWHM of the HPHT seed is 2.7 cm^-1.The growth of the large CVD single crystal diamond shows that,as the growth cycle progresses,the impurities and defects gradually accumulate.Therefore,it is necessary to adjust the growth parameters of CVD diamond in different growth cycles so as to obtain the large size CVD single crystal diamond with better crystalline quality and fewer defects.