Microwave-Assisted Spectroscopy of Vacancy-Related Spin Centers in Hexagonal $\mathrm{Si}\mathrm{C}$

Optically active spin centers associated with atomic scale defects in $\mathrm{Si}\mathrm{C}$ are promising candidates for quantum technology, owing to their outstanding optical and spin properties. Photoluminescence as a mature optical investigating tool is widely used for the identification of spin defects and exploration of their properties. However, in the case of spectrally overlapping contributions from different types of defects, traditional photoluminescence measurements cannot be used to separately obtain their optical and vibrational properties, such as the local phonon energy and the Debye-Waller factor. Here, we apply spin-resonant microwave-assisted spectroscopy to investigate the optical and vibrational properties of silicon vacancies in 6H-$\mathrm{Si}\mathrm{C}$ and divacancies in 4H- and 6H-$\mathrm{Si}\mathrm{C}$. We isolate contributions from each type of defect, investigate their local vibrational modes, and obtain the Debye-Waller factor. This work proves that microwave-assisted spectroscopy is a suitable tool for the investigation of optical and vibrational properties of a large variety of spin defects.

Figure 1

Schematic representation of VV defects in 4H- (a) and 6H-$\mathrm{Si}\mathrm{C}$ (b). Cubic and hexagonal sites are labeled as k and h, respectively.

Figure 2

(a),(b) Simplified schemes of the $V_{\mathrm{Si}}$ and VV optical pumping processes, respectively. Defects are excited by an 808 or 940 nm laser, which is represented by a black arrow. Red dashed arrows represent the radiative recombination process. Black short dashed arrows indicate the spin-dependent relaxation process. Green arrows show spin transitions induced by the MW field. Because the basal and axial VV defects have different symmetry (C_3v and C_1h), their zero-field resonance structures are different. Left and right dashed boxes represent the spin resonance for the axial and basal VV defect spins at MW frequencies D and D ± E, respectively. ES, excited state; GS, ground state; MS, metastable states. (c) Scheme of our homebuilt microwave-assisted spectroscopy setup. LP, long pass.

Figure 3

(a) Chopper-modulated laser excitation and PL signal. (b) MW sequences and modulated PL signal (MW-assisted spectroscopy).

Figure 4

Low-temperature PL spectra from the $V_{\mathrm{Si}}$ and VV centers in 4H-$\mathrm{Si}\mathrm{C}$ (a) and 6H-$\mathrm{Si}\mathrm{C}$ (b). Blue and red curves are measured with 808 and 940 nm excitation, respectively. QL5 and QL6 centers in 6H-$\mathrm{Si}\mathrm{C}$ reveal spectrally indistinguishable ZPLs.

Figure 5

(a)–(c) ODMR spectra of the VV centers PL4, PL3, and PL2, respectively, in 4H-$\mathrm{Si}\mathrm{C}$. Peak position indicates the resonance frequency of a certain divacancy at zero magnetic field. For basal VV centers PL4 and PL3, there are two zero-field resonance frequencies, corresponding to D ± E. (d)–(f) Low-temperature MW-induced ΔPL spectra under the resonance frequency. ZPL, phonon energy (ΔE), and DW factors can be acquired directly from these spectra.

Figure 6

(a) ODMR spectrum of V2 and V3 centers in electron-irradiated 6H-$\mathrm{Si}\mathrm{C}$ at a fluence of 10¹⁷ cm⁻² at 100 K. Peak position indicates the resonance frequency of 129 and 27 MHz at zero magnetic field. (b),(c) Low-temperature MW-induced ΔPL spectra under the resonance frequency. ZPL, phonon energy (ΔE), and DW factor can be acquired directly from this spectrum.

Figure 7

(a)–(e) ODMR spectra of divacancies QL2-QL6 in 6H-$\mathrm{Si}\mathrm{C}$. Peak position indicates the MW resonance frequency of a certain divacancy at zero magnetic field. For basal VV, QL6 and QL4, there are two zero-field resonance frequencies D + E and D-E. (f)–(j) Low-temperature ΔPL spectra corresponding to QL2-QL6 in 6H-$\mathrm{Si}\mathrm{C}$. For each defect, ZPL and PSBs emerge clearly. Because the zero-field splitting is individual for each divacancy, the ΔPL spectrum contains information about a certain divacancy, the ground-state-spin splitting is equal to the applied MW frequency. Local vibrational energy is obtained from two adjacent phonon peaks and the DW factor can be directly acquired from the ΔPL spectrum.

Figure 8

Normalized ΔPL spectrum of basal divacancies QL4 (a) and QL 6 (b). Owing to lower symmetry (C_1h), there are two MW resonance frequencies, corresponding to GS zero-field spin-transition energies D + E and D-E. Two different resonance frequencies give the same phonon energy and DW factor.

Figure 9

ΔPL spectrum of V2 from 10¹⁷ cm⁻² electron-irradiated sample at 30 K (a), 100 K (b), 140 K (c), and 180 K (d). As the temperature increases, the ratio of ZPL intensity to the first-phonon replica decreases, but the DW factor has no obvious change.

Figure 10

(a) Normalized ΔPL spectra of V2 from three samples with irradiation fluences of 10¹⁵, 10¹⁷, and 10¹⁸ cm⁻². (b) DW factor as a function of fluence at 15 K.