Ultra High-Concentration Nitrogen-Doped CVD Diamond with Highest Crystallinity

High impurity doping in diamond using microwave plasma chemical vapor deposition (MPCVD) has been actively investigated with many dopants such as boron, nitrogen, and phosphorus. In particular, high concentration doping as high as 2×10²²cm^-3 has been achieved for boron doping [1], whereas the highest concentration for nitrogen is only 2×10²⁰cm^-3 [2]. The fabrication of highly nitrogen-doped diamond is expected to realize highly sensitive magnetic sensing by NV centers and superconductivity theoretically predicted [3]. In general, increasing the proportion of supplied nitrogen gas degrades the film quality of diamond [4]. On the other hand, it has been reported that supplying carbon and oxygen at a ratio of 1:1 during deposition improves the film quality [5]. Diamonds doped with high concentrations of boron, which has a larger ionic radius than carbon, exhibit crystal expansion in vertical direction [1]. Up to now, there are no reports on crystal distortion in highly nitrogen doped diamonds where nitrogen, has a smaller ionic radius than carbon. In this study, we fabricated diamond and characterized the physical properties of the thin films by Reciprocal Space Mapping (RSM) using a XRD (Bruker D8 DISCOVER) and cross-sectional imaging using a TEM (JEOL JEM-2100F).<br/>CVD synthesis was performed using a waveguide confined MPCVD system with quartz tube where plasma was confined within waveguide, with a pressure of 110Torr, a microwave power 300W and at a temperature of 850°C. CH₄, N₂, and CO₂ gases were used for homoepitaxial growth. The gas mixture conditions are [C,O]=1.2% (common value for C and O), [C,O]=1.5, 2.0%, and [C,O]=4.0% (common value for C and O). The total gas flow rate was fixed at 200 sccm, and samples were prepared by varying the ratio of gases. HPHT synthetic (111)-oriented diamond substrates were used.<br/>As a result, the nitrogen concentration of [C,O]=4.0% was 8×10²⁰cm^-3 from SIMS measurements. It is the world's highest concentration of nitrogen existed in diamond. From the cross-sectional TEM images, it was confirmed that twinning defects were introduced into the epitaxial layer in [C,O]=1.5, 2.0%, and stacking defects were introduced into the epitaxial layer in [C,O]=1.2%.On the other hand, [C,O]=4.0% showed no twins or stacking faults, but dark spots which appeared to be nitrogen aggregation [6] in the epitaxial layer. The origin of these spots must be further investigated. In the Raman spectrum measurement (RENISHAW inVia Basis, 532nm), a peak at 1333cm^-1, the Raman peak of diamond, was confirmed for all three samples. [C,O]=4.0% showed the FWHM of 8.6cm^-1 at 1333cm^-1. The FWHM is comparable to that of HPHT diamond with few defects. High quality nitrogen-doped diamond was produced despite of containing as much as 8×10²⁰cm^-3 nitrogen. The lattice expansion was calculated from RSM. [C,O]=1.2% exhibited 0.66% expansion in the horizontal direction and 0.20% shrinkage in the perpendicular direction. [C,O]=1.5, 2.0% exhibited 1.47% expansion in the horizontal direction and 0.50% shrinkage in the perpendicular direction. This anisotropic expansion and shrinkage is opposite to those of heavily boron doped samples [1] and is caused by high concentration of nitrogen above 5×10²⁰cm^-3. However, [C,O]=4.0% exhibited no crystal lattice expansion or shrinkage. The well-crystalline CVD diamond has been fabricated with the highest nitrogen concentration of 8×10²⁰cm^-3 by using [C,O]=4.0%．It is the most promising condition for high nitrogen incorporation with high quality CVD diamond equivalent to HPHT diamond and can be applied high sensitive NV magnetometry.<br/>[1]T. Kageura, H. Kawarada, <i>et al.</i>, Diam. Mater. <b>90</b>, 181-187(2018).<br/>[2]Y. Nakano, N. Tokuda, <i>et al.</i>, Diam. Mater. <b>125</b>, 108997(2022).<br/>[3]Y. Ma, G. Zou, <i>et al.</i>, Phys. Rev. B <b>72</b>, 014306(2005).<br/>[4]E.Boettger, C. P. Klages <i>et al.</i>, J. Appl. Phys.<b> 77</b>, 6332(1995).<br/>[5]P. K. Bachmann, H. Lydtin <i>et al.</i>, Diam. Mater. <b>1</b>, 1-12(1991).<br/>[6]T. Evans, J. Maguire, <i>et al.</i>, J. Phys. C <b>14</b>, L379(1981).