Towards High-Quality Nitrogen Doped Single Crystal Diamond

The nitrogen vacancy defect (NV^-) in diamond is a strong candidate for quantum sensing applications. Many researchers have demonstrated success in utilizing the NV^- defect for sensing temperature [1] and magnetic field [2], however, the challenge of repeatably manufacturing NV^- defects has not been adequately addressed. Here, a parametric study of a chemical vapor deposition (CVD) reactor was performed to understand the effects of substrate temperature, reactor pressure, and substrate surface preparation on the incorporation of nitrogen in single crystal diamond (SCD). Eight (100) SCD substrates (MTI) were used for deposition. The pressure was varied from 250 mbar to 370 mbar and substrate offcut from 0.5 degrees to 2 degrees with respect to the (100) surface while holding the feed gas composition constant at 1238 ppm N:C with 4% CH₄ in H₂.<br/>The study was further expanded to determine the effect of lower methane concentrations on growth quality and defect incorporation. Previous CVD work on SCD demonstrated the operation of a high-power density and high pressure microwave plasma assisted chemical vapor deposition (MPACVD) reactor and studied operation conditions such as pressure, substrate temperature, and methane (CH₄) composition [3]. Because this reactor was designed to increase growth rate, higher gas phase methane concentrations were explored, from 3 percent to 9 percent. However, the effect of methane concentrations below 3 percent on SCD growth quality were not explored.<br/>To investigate methane concentrations of 3 percent and below in the gas phase, three (100) CVD diamond substrates (MTI) were used for three depositions with varying methane percentages from 1-3% of the total gas phase composition. All depositions were performed at 320 mbar, 2000-2100 W, and a total gas flow rate of 400 SCCM. Each sample was characterized using differential interference contrast microscopy (DICM), birefringence imaging, Fourier Transform Infrared (FTIR) spectroscopy, ultra-violet and visible spectroscopy (UV-Vis), and Raman spectroscopy to determine the quality of the growth. Vertical growth rate was determined by thickness measurements taken before and after deposition using a linear encoder.<br/>We found that the substrate surface roughness was correlated with the growth quality and nitrogen incorporation. Two (100) HPHT substrates (NDT) were used to optimize hydrogen etching conditions to prepare the surface for deposition such that the effects of substrate surface conditions on defect incorporation can be isolated from process variable effects. The results of these experiments will be utilized to re-evaluate the effects of reactor pressure and offcut as well as establish a repeatable technique for growth of high quality, nitrogen doped single crystal diamond.<br/><br/><b>References</b><br/>[1] Y.Wu, et al., <i>Nano Lett</i>., (2021), <b>21</b>, 3780-3788<br/>[2] D. R. Glenn, et al., <i>Geochem. </i><i>Geophys. Geosyst., </i><b>18</b> (2017) pg 3254-3267<br/>[3] Y. Gu, et al., Diam. Relat. Mater., <b>24</b>, (2012). 210-214