Stability, Electronic Structure, and Potential as Spin Qubits of Oxygen Related Defects in 4H-SiC +

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4H-SiC has shown great potential in quantum technology due to its mature crystal growth technology, controllable n-type and p-type doping, and micro nano processing capabilities. Similar to diamond, the point defects present in 4H-SiC can serve as spin qubits and enable spin photon interfaces for applications in quantum computing, cryptography, and sensing. Among numerous defects, oxygen related defects have regained research interest in recent years due to their unique electronic structure and optical properties.

1. Classification and Stability of Oxygen Defects in 4H-SiC

According to Iwamoto et al.’s research, oxygen related defects in 4H–SiC can be divided into four categories:

Group-I: Alternative oxygen defects (such as O_C, O_Si, O_CO_Si)

Group II: Oxygen gap defects (O_i,1, O_i,2)

Group III: Oxygen vacancy complexes (such as O_CV_Si, O_SiV_C)

Group IV: Alternative oxygen carbon complexes (such as O_CC_Si)

Under n-type conditions, the three most stable defects are:

O_i,1 (Triple coordination, bonded to three Si atoms)

O_C (quadruple coordination, C_3v symmetry)

O_CV_Si (oxygen silicon vacancy complex)

Among them, the formation energy of O_CV_Si is relatively low, especially under n-type conditions, it is easy to form and stably exists, which is the focus of subsequent research.

2. Formation and Detection of Oxygen Defects in 4H-SiC

O_CV_Si defects can be formed in 4H-SiC through oxygen ion implantation and subsequent annealing. If the Fermi level is regulated to the range of EV+0.96-2.07eV through Schottky contact, O_CV_Si can maintain the S=1 spin state, while O_i,1, and O_C have no spin, thereby suppressing spin decoherence.

The O_III-O_v deep energy levels discovered in early DLTS measurements were within the range of the (-1/-2) acceptor energy levels (EV+2.40-2.52eV) of O_CV_Si, and the observed O_III-O_V signal may be related to defects in O_CV_Si. In addition, the D_I series defects observed in the experiment (ZPL≈1.23 eV) are highly consistent with the theoretically predicted O_CV_Si ZPL energy, indicating that this defect may have been observed in the experiment.

Fig. 1 Formation energies of oxygen related defects in 4H-SiC under (a), (b) Si-rich and (c), (d) C-rich conditions

3. Structure and Electronic Properties of O_CV_Si Defects

Kobayashi et al. conducted a detailed study on O_CV_Si defects. This defect exhibits C_3v symmetry in all four lattice configurations of kk, hh, hk, and kh, with oxygen atoms located on the symmetry axis of three carbon atoms, forming three equally long Si-O bonds (bond length of approximately 1.83 Å).

Fig. 2 Optimized 4H-SiC oxygen defect structure

This defect has a high spin ground state (S=1) in the neutral charge state (q=0), with spin mainly localized on the dangling bond orbitals of three carbon atoms. In its single particle energy level diagram, the energy gap between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) is 1.61-1.80eV, slightly higher than the actual optical transition energy.

Fig. 3 Single particle energy levels of O_CV_Si⁰ defects in 4H-SiC

4. Optical Properties and Near-Infrared Emission of Oxygen Defects

By using the Constrained Occupancy Method (ASCF), the zero phonon line (ZPL) energy of O_CV_Si⁰ is calculated to be between 1.11-1.24eV, corresponding to a wavelength of 1004-1117nm, belonging to the near-infrared band.

Fig. 4 PL spectra of O_CV_Si⁰ defects with kk configuration in 4H-SiC

In addition, the radiation lifetime of this defect is extremely short (12.5ns in kk mode), comparable to diamond NV centers and V_Si defects in SiC. More noteworthy is that its Debye Waller factor is as high as 13.4%, indicating that ZPL dominates the photoluminescence spectrum and is suitable as an efficient quantum light source.

5. Spin Properties of Oxygen Defects and Quantum Bit Potential

The ground state spin of O_CV_Si⁰ is S=1, with obvious zero field splitting (ZFS). The axial component D is between 1.61-1.74GHz, and the transverse component E is non-zero in low symmetry configurations. In addition, the hyperfine coupling constant shows significant interactions with the spin of surrounding ¹³C, ²⁹Si, and ¹⁷O nuclei, which can be used for spin manipulation and recognition.

The structure, formation energy, spin, and optical properties of various oxygen related defects in 4H-SiC were systematically studied through first principles calculations. Among them, O_CV_Si defects have low formation energy, high spin ground state (S=1), near-infrared ZPL emission, short radiation lifetime, and high Debye Waller factor under n-type conditions, making them a highly promising candidate defect for spin photon interfaces. In the future, through oxygen injection and band regulation, it is expected to achieve its application in quantum devices.

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References:

1. Iwamoto, S., Shimura, T., Watanabe, H., & Kobayashi, T. (2024). Oxygen-related defects in 4H-SiC from first principles. Applied Physics Express, 17(5), 051008.
2. Kobayashi, T., Shimura, T., & Watanabe, H. (2023). Oxygen-vacancy defect in 4H-SiC as a near-infrared emitter: An ab initio study. Journal of Applied Physics, 134(14).