Vacancy-Induced Quantum Properties in 2D Silicon Carbide: Atomistic insights from semi-local and hybrid DFT calculations

TL;DR

This work addresses how vacancies in 2D SiC modulate electronic, magnetic, and optical properties by systematically comparing Si and C vacancies with four exchange-correlation functionals ($PBE$, $SCAN$, $r^2$SCAN, $HSE06$) and two charge-correction schemes. The study demonstrates that $V_{Si}$ states are highly localized with strong spin polarization, while $V_C$ states are more delocalized with weaker magnetism, and reveals distinct migration barriers ($E_b\approx0.8$ eV for Si vs ~1.0 eV for C) leading to higher Si vacancy mobility. Optical responses are strongly charge-state dependent, with positively charged vacancies showing the strongest far-infrared absorption (up to $\approx$ $22\%$ for $V^{+1}_{C}$), underscoring the need for accurate functionals to predict defect energetics and spectra. The results provide a comprehensive framework for defect engineering in 2D SiC for quantum technologies and infrared optoelectronics, highlighting the complementary roles of localized and delocalized defect states.

Abstract

Two-dimensional (2D) materials have emerged as promising platforms for quantum technologies and optoelectronics, with defects playing a crucial role in their properties. We present a comprehensive density functional theory study of silicon and carbon vacancies in monolayer silicon carbide (1L-SiC), a wide-bandgap 2D semiconductor with potential for room-temperature quantum applications. Using PBE, SCAN, r$^2$SCAN, and HSE06 functionals, we reveal distinct characteristics between Si and C vacancies. Formation energies and charge transition levels show strong functional dependence, with HSE06 consistently predicting higher values and deeper transition levels compared to PBE calculations. Electronic structure analysis demonstrates contrasting behavior: silicon vacancies create highly localized states with strong spin polarization, while carbon vacancies produce more dispersed states with weaker magnetic properties. Vacancy migration studies reveal significantly lower barriers for silicon vacancies compared to carbon vacancies, indicating higher mobility for Si vacancies at moderate temperatures. Optical properties, calculated using PBE-DFPT, show distinct charge-state dependent absorption in the far-infrared region, with positively charged states of both vacancy types demonstrating the strongest response. The complementary characteristics of Si and C vacancies - localized versus dispersed states, different magnetic properties, and distinct optical responses - suggest possibilities for defect engineering in quantum and optoelectronic applications. Our results highlight the critical importance of advanced functionals in accurately describing defect properties and provide a comprehensive framework for understanding vacancy behavior in 2D materials.

Figures (7)

• Figure 1: Comparison of defect formation energies for (a) Si vacancy and (b) C vacancy in C-rich environments. The FNV charge correction scheme results are displayed by the dashed line, while the KO charge correction scheme results are shown by the solid line. The range of Fermi levels is calculated based on the thermodynamic stability region in C-rich conditions. The slopes of +1 and -1 for the +1 and -1 charge states respectively reflect the physical charge transfer process.
• Figure 2: Electronic band structures of pristine SiC monolayer calculated using PBE, SCAN, r$^2$SCAN, and HSE06 functionals (from left to right). The Fermi level is set to zero. Red dashed lines indicate the indirect bandgap from $\Gamma$ to M. The progressive increase in bandgap from PBE (2.54 eV) to HSE06 (3.39 eV) demonstrates the impact of improved exchange-correlation treatment.
• Figure 3: Electronic band structure of Si vacancies in a 3×3 monolayer SiC computed using PBE, SCAN, r$^2$SCAN and HSE06 functionals (with D3 dispersion correction) for three charge states: (a) V$^0_\text{Si}$, (b) V$^{-1}_\text{Si}$, and (c) V$^{+1}_\text{Si}$. Pink and blue lines represent spin-up and spin-down bands, respectively. The Fermi level is set to zero. Note the progressive increase in spin splitting from PBE to HSE06, with the latter showing the most pronounced spin polarization, particularly for V$^{-1}_\text{Si}$.
• Figure 4: Electronic band structure of C vacancies in a 3×3 monolayer SiC computed using PBE, SCAN, r$^2$SCAN and HSE06 functionals (with D3 dispersion correction) for three charge states: (a) V$^0_\text{C}$, (b) V$^{-1}_\text{C}$, and (c) V$^{+1}_\text{C}$. Pink and blue lines represent spin-up and spin-down bands, respectively. The Fermi level is set to zero. Carbon vacancy states show broader dispersion than silicon vacancies, indicating more delocalized defect states with weaker spin polarization.
• Figure 5: Nudged elastic band (NEB) calculations of vacancy migration pathways in 1L-SiC. (a) Si vacancy and (b) C vacancy migration paths. Path A (green): migration within single hexagon; Path B (pink): across two hexagons; Path C (blue): to nearest neighbor. Energy profiles for (c) Si vacancy and (d) C vacancy migration shown relative to initial state energy. Si vacancies exhibit barriers ranging from 0.8 eV to 1.75 eV, while C vacancies have barriers ranging from approximately 1.0 eV to 1.4 eV. This generally indicates higher mobility for Si vacancies, particularly as the lowest energy path for Si (0.8 eV) is lower than for C (1.0 eV).