ICYS Annual Report 2023

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ICYS Annual Report 2023Takeyuki TSUJI1. Outline of Research2. Research ActivitiesFig.1. (a) Structure of nitrogen vacancy center in diamond. (b) Schematic of CVD diamond film grown on the (111) diamond substrate with θmis. Fig. 2. (a) The shift of energy level of NV center as a function of the Z-direction (growth direction) at each θmis.(b) The diamond Raman peak shift as a function of the growth direction at each θmis.Reference1) Bernien, H. et al. Nature 497, 86-90 (2013) 2) Bhaskar, M. K. et al. Nature 580, 60-64 (2020)3) Dolde, F. et al. Nat. Phys. 7, 459-463 (2011).4) J.F. Barry et.al., Rev. Mod. Phys. 92 (2020). 5) M.P. Gaukroger et al., Diam. Relat. Mater. 17 (2008) 6) M. Mermoux et.al., J. Appl. Phys. 97 043530 (2005) 7) P. van Enckevort, et.al, 8) D.A. Broadway et.al, Nano Lett. 19 4543-4550 (2019) 9) T. Tsuji et.al., Applied Physics Express 17, 115502 (2024). 10) T. Tsuji et.al. Adv. Quantum Technol, 7, 2300194 (2024)Diamond is promising material for quantum devices. Nitogen-Vacancy (NV) centers (Fig.(a)), which is point defect in diamond are expected to be used for quantum communication [1], quantum computation [2] and quantum sensor [3]. Diamonds typically contain stress due to residual strain and defect such as dislocations. However, these stresses degrade the performance of quantum devices. Stress in diamond causes inhomogeneous energy levels of NV center. This causes a reduce in the spin dephasing time of NV center which determine the performance of quantum devices [4]. Therefore, reducing stress in diamonds is important for realizing quantum devices.In three-dimensional space on the x, y, and z axes, the stress is applied to the x, y, and z with respect to the x, y, and z planes in a diamond crystal. Thus, the direction and magnitude of stress is thus represented by a 3 × 3 tensor known as stress tensor. Stress tensor has 6 independent components since stress tensor is a symmetric matrix. However, conventional stress imaging technique such as x-ray topography [5], Raman spectroscopy [6], and birefringence [7] have been sensitive to only one or a convolution of the stress tensor components. In contrast, all six components of the stress tensor can be determined by employing a NV center aligned in four crystallographic directions in diamond [8]. The purpose of this research is to develop crystal growth techniques to reduce stress in diamonds and to evaluate the direction and magnitude of the stress using NV center.We found that mis-angle (θmis) was a key to decrease stress in (111) diamond film. As shown in the Fig.1 (b), when diamond is synthesized by chemical-vapor-deposition (CVD) method, the diamond substrate is polished at an angle, which is defined as the mis-angle (θmis). CVD diamond films of approximately 60 µm was synthesized on each of (111) diamond substrates polished with θmis of 2.0, 3.7, 5.0, and 10 degrees. We evaluated the stress in these diamond films using NV centers and Raman spectrum.The Fig.2 (a) shows the shift of energy level of NV center as a function of the Z-direction, growth direction, at each θmis. It is clearly seen that the energy level shift was more suppressed as θmis was increased. This result means that stress on NV center was reduced as θmis was increased. The Fig.2(b) shows the diamond Raman peak shift as a function of the growth direction at each θmis . When the θmis was 2 degrees, the Raman peak decreased as it approached the CVD surface. On the other hand, the change in the Raman peak was within the measurement error at θmis ≥ 3.7°. Therefore, we found that stress on diamond lattice was also reduced at θmis ≥ 3.7°. Analyzing the direction and magnitude of the stress from these results, compressive stress of 33 MPa in the [110] and [-1-12] directions, and 2 MPa in the [111] direction dominantly existed in the diamond film at θmis of 2.0 degrees. In addition, these stresses were reduced by more than about 1/10 by increasing the θmis to 10 degrees [9].Furthermore, we found that the spin dephasing time of NV center which determines the performance of quantum devices was improved to the theoretical value by increasing θmis to 10 degrees thanks to the reduction in stress in diamond films [10]. Research Digest 27Growth of Low-Strain Diamond for High Performance Quantum Devices