Unraveling the electronic structure of silicon vacancy centers in 4H-SiC

Abstract

Point defects in silicon carbide (SiC), particularly the negatively-charged silicon vacancy ($\mathrm{V_{Si}^{-}}$) in 4H-SiC, are leading candidates for scalable quantum technologies due to their favorable spin-optical properties and compatibility with industrial semiconductor fabrication processes. Comprehensive knowledge of a defect's electronic structure is essential for interpreting spin-optical dynamics and for the reliable design and optimization of defect-based quantum devices. Despite extensive study, our knowledge of the electronic structure of $\mathrm{V_{Si}^{-}}$\ is limited since key excited-state manifolds have remained inaccessible to conventional steady-state spectroscopy. In this study, transient absorption spectroscopy is utilized to probe non-equilibrium electronic transitions of $\mathrm{V_{Si}^{-}}$\ and to uncover previously unobserved excited states. The first direct observation of the elusive V2' quartet transition is presented, with its broad spectral signature attributed to nonadiabatic vibronic coupling. Within the spin-doublet manifold, which is central to optically detected magnetic resonance (ODMR) but has remained unresolved spectroscopically, multiple optical transitions are identified. The complete electronic level structure in the relevant energy range is elucidated by combining polarization-resolved spectroscopy, group-theoretical analysis, quantum embedding calculations and first-principles optical lineshape modeling. Collectively, these results provide a microscopic understanding of the $\mathrm{V_{Si}^{-}}$\ electronic structure. Our approach also establishes a general framework for resolving and understanding complex excited-state manifolds in wide-bandgap color centers.

Figures (24)

• Figure 1: Overview.a Crystal structure of the $\text{V}_{\text{Si}}$ center in 4H-SiC, with the basal-plane hexagonal (h) and quasi-cubic (k) lattice sites indicated. b Single-particle energy levels of $\mathrm{V_{Si}}^-$. Left: Idealized $T_d$ symmetry; Right: crystal-field splitting resulting in $C_{3v}$ symmetry. Red arrow: excitation to $^4A_2'$ excited state. c Electronic states in the quartet ($S=3/2$) and doublet ($S=1/2$) spin channels and schematic of the transient absorption (TA) measurement scheme. d Top: Schematic of the probing geometry in both the oblique and normal incidence cases of $\mathrm{V_{Si}^{-}}$ electronic transitions relative to the c-plane, with a coordinate system for the transition dipole moments defined by $\mu_x$, $\mu_y$ and $\mu_z$. Purple arrows indicate the electric field vectors of the probe beam. Bottom: Front view of sample ($c$-plane), i.e., in the laser beam direction, with summary of the transition dipole moments (or "dipoles") excited by the 0$^\circ$ or 90$^\circ$ polarization of the electric field vector.
• Figure 2: Spin-quartet electronic states and optical transitions.a$V_{h}$ and b$V_{k}$: TA spectra at pump-probe delay of $t = 4$ ns (black curves), calculated PSBs of V1 and V2 in emission (orange shaded) and absorption (blue shaded); insets: enlarged version of the ZPL spectral regions (right) and electronic level scheme with optical transitions and transition energies in eV (left); c TA spectra of $\text{V}_{k}$ for 0$^\circ$ and 90$^\circ$ probe polarizations with the energy axis referenced to the V2 ZPL; heuristic model simulating the broadening of the V2$^{\prime}$ ZPL in the inset; d TA spectra of $\text{V}_{k}$, focusing on the GSB by comparing the V2 PSB with absorption lineshape simulation (shaded) and V2$^{\prime}$ PSB red-shifted by 25 meV.
• Figure 3: Spin-doublet electronic states and optical transitions.a Spectrally-resolved TA dynamics for $V_{h}$ and $V_{k}$; colored arrows indicate transitions whose dynamics are shown in panel c. b Extracted electronic level scheme and relaxation dynamics of $V_{h}$ and $V_{k}$. Gray dashed arrows denote optical transition energies (in eV), while blue dashed arrows indicate relaxation pathways with time constants (in ns). c TA dynamics of spin-quartet and spin-doublet transitions and PL decay (right axis) of spin-quartet transitions of $V_{h}$ and $V_{k}$; d TA spectra of spin-doublet transitions of $V_{h}$ and $V_{k}$ at a pump–probe delay of 90 ns. The insets show magnified views of selected doublet features.
• Figure 4: Electronic structure and optical selection rules of spin-doublet and spin-quartet channels.a Electronic levels for $\text{V}_{k}$ calculated using quantum embedding. b,c Electronic structure and optical selection rules of the spin-quartet and spin-doublet channels, respectively. d,e TA probe polarization dependence of spin-doublet transitions for the $V_{h}$ and $V_{k}$ configurations, measured under oblique incidence (blue and black traces) and normal incidence (orange traces).
• Figure S1: Schematic of the sample sandwiched between two high refractive index rutile prisms and optical path.