Phonon-Assisted Photoluminescence and Ultrafast Exciton Dynamics in Two-Dimensional Silicon Carbide

Abstract

Phonon assisted photoluminescence provides a direct window into exciton phonon interactions in low dimensional semiconductors. Using fully ab initio many body perturbation theory, including finite momentum Bethe Salpeter calculations, we investigate phonon assisted emission and exciton dynamics in two dimensional hexagonal silicon carbide and benchmark its response against 2D hexagonal boron nitride. By explicitly resolving exciton phonon matrix elements, we identify high energy optical TO LO phonons as the dominant contributors to sideband formation and quantify their spectral weights. h SiC exhibits pronounced phonon assisted sidebands comparable to h BN, despite a smaller exciton phonon energy separation and fewer resolved replicas. The bright K K exciton governs near UV zero phonon emission, while intervalley excitons acquire radiative character through symmetry allowed optical-phonon coupling. Temperature dependent scattering rates reveal an ultrashort bright exciton lifetime of approximately 300 fs at 10 K, highlighting rapid exciton relaxation driven by intrinsic phonon channels. These results establish monolayer SiC as a symmetry-activated platform for efficient, strain-free phonon-assisted emission and provide a quantitative framework for ultrafast exciton dynamics in wide bandgap 2D semiconductors.

Figures (6)

• Figure 1: (a) Quasiparticle band structure of 2D h-SiC computed within the $G_0W_0$ approximation. The dominant interband transitions forming the lowest excitons are indicated: $e_1$ corresponds to the direct K–K transition, while $e_2$ and $e_3$ denote indirect K–$\mathbf{\Gamma}$ and M–K$^{\prime}$ transitions, respectively. The fancy-arrow schematically represent indirect recombination dynamics. (b) Momentum-space schematic of excitonic configurations in the first Brillouin zone, contrasting direct intravalley ($\mathbf{Q}\approx 0$) and intervalley (finite $\mathbf{Q}$) excitons. The corresponding real-space electron–hole probability densities are derived from the excitonic wavefunction. (c) Excitonic dispersion obtained from the solution of BSE, showing the five lowest branches along high-symmetry directions. The bright intravalley K–K exciton ($e_1$) occurs at $\mathbf{Q}=0$ ($\Gamma$), while finite-$\mathbf{Q}$ states near M correspond to momentum-indirect excitons requiring phonon assistance for radiative recombination. (d–f) Real-space electron probability densities for the three lowest excitons ($e_1$, $e_2$, $e_3$), with the hole fixed at the supercell center. The direct exciton ($e_1$) is compact and nearly isotropic, whereas the intervalley excitons ($e_2$, $e_3$) exhibit extended and anisotropic character consistent with their indirect nature.
• Figure 2: (a)-(f): Phonon mode resolved exc-ph coupling strength $\left|G_{\beta\lambda,\nu}(\mathbf{Q},\mathbf{q})\right|^2$ for the first exciton in monolayer SiC. All values are normalized to $10^{-4} \, \mathrm{eV}^2$. The black hexagon marks the boundary of the first BZ, and the color intensity reflects the strength of exc–ph coupling.
• Figure 3: (a) and (b) Phonon-mode-resolved normalized scattering rates of excitons 1 and 5 at 10 K and 300 K, respectively. (c) Relaxation time for the lowest five excitons (1–5) at $\mathbf{Q}=0$ ($\mathbf{\Gamma}$) as a function of excitonic temperature $T_{\mathrm{exc}}$.
• Figure 4: Excitonic temperature (T = T$_{\mathrm{exc}}$) dependence of the linewidths of excitons 1 and 5 at the $\mathbf{\Gamma}$ valley. The red dashed line represents the $\mathbf{\gamma}$ab initio model.
• Figure 5: Phonon-assisted PL plotted on a logarithmic scale. The $x$-axes has been shifted by the energy of the bright exciton (3.013 eV). PL spectra at an excitonic temperature of 10 K. The inset shows the atomic vibrations (Si atoms in blue, C atoms in brown) are of mode 5(A$^{\prime\prime}$) and 6(A$^{\prime}$) by green and red arrows respectively.