Strain‑Tunable Band Gaps and Exceptional Carrier Mobility in SiAs and SiAs₂ Monolayers: First‑Principles Insights

Abstract

Identifying atomically thin two‑dimensional (2D) semiconductors that combine a sizable band gap with high carrier mobility remains a key challenge in materials science. Recent synthesis of layered SiAs single crystals has shown that few‑layer SiAs can be mechanically exfoliated, suggesting the feasibility of monolayer production. Using density functional theory (DFT), we demonstrate that both SiAs and SiAs₂ monolayers are dynamically and thermodynamically stable semiconductors with indirect band gaps of 2.39 eV and 2.07 eV, respectively. Applying biaxial strain converts these indirect gaps into direct gaps (1.75 eV for SiAs and 1.60 eV for SiAs₂) and, at larger strains, induces a metal transition. Carrier‑mobility calculations reveal values up to 3.9 × 10³ cm² V⁻¹ s⁻¹ for holes in SiAs, surpassing MoS₂ and exhibiting anisotropy similar to black phosphorene. These findings highlight SiAs and SiAs₂ as promising candidates for next‑generation optoelectronic devices.

Background

Two‑dimensional materials have become a cornerstone of contemporary condensed‑matter research, offering tunable electronic, optical, and mechanical properties. After graphene’s discovery, a diverse family of 2D crystals—including silicene, boron nitride, transition‑metal dichalcogenides (TMDs), black phosphorus, borophene, arsenene, tellurene, and many isoelectronic derivatives—has been experimentally realized, each bringing unique advantages and challenges [1–23].

While graphene boasts exceptional carrier mobility, its zero band gap limits digital applications. TMDs provide intrinsic gaps but suffer from low mobility [26–28], and phosphorene offers both a band gap and high mobility yet is chemically unstable in air [13, 29]. The recent successful growth of SiAs and SiAs₂ layered crystals opens a new pathway to stable, exfoliable 2D semiconductors with potentially superior properties [30–32].

In this study, we employ first‑principles DFT to predict the structural, electronic, and transport characteristics of SiAs and SiAs₂ monolayers. We confirm their mechanical stability, calculate their band structures, and evaluate strain‑induced band‑gap engineering and carrier mobilities, positioning these materials as strong contenders for high‑performance optoelectronics.

Computational Methods

All calculations were performed with the Vienna Ab‑initio Simulation Package (VASP) using the Perdew‑Burke‑Ernzerhof (PBE) exchange‑correlation functional within the generalized gradient approximation (GGA). The projector augmented‑wave (PAW) method described the core‑valence interaction. A 20 Å vacuum gap prevented interlayer coupling, and a 500 eV plane‑wave cutoff ensured convergence. Brillouin‑zone sampling employed a 15 × 5 × 1 Monkhorst–Pack grid for 2D sheets. Structural relaxations converged to 10⁻⁴ eV in total energy and 0.01 eV/Å in forces.

Phonon dispersions were obtained via the supercell approach in PHONOPY, with force constants derived from density‑functional perturbation theory (DFPT) in VASP. To confirm thermal stability, 6 ps NVT molecular‑dynamics simulations at 300 K were run on 3 × 3 × 1 supercells (time step 2 fs). Raman spectra were computed at the PBE level using CASTEP [39–41].

Results and Discussions

The relaxed monolayer geometries of SiAs and SiAs₂ are shown in Figure 1. Bulk SiAs (C2/m symmetry) and SiAs₂ (Pbam symmetry) are held together by weak van‑der‑Waals forces, with interlayer spacings of 3.06 Å and 1.66 Å, respectively. The SiAs monolayer adopts a rhombic lattice (a₁ = 3.69 Å, b₁ = 10.83 Å, φ = 99.81°) with six Si and six As atoms per unit cell; Si–Si and Si–As bonds measure 2.35 Å and 2.39–2.43 Å, and the buckling height is 4.86 Å. SiAs₂ forms a rectangular lattice (a₂ = 3.68 Å, b₂ = 10.57 Å) containing four Si and eight As atoms; As–As bonds are 2.50 Å, Si–As bonds 2.41–2.45 Å, and the buckling height is 5.09 Å. Electron‑density maps reveal that As atoms, due to their higher electronegativity, accumulate electron density at the expense of Si, confirming strong covalent bonding.

Raman spectra (Figure S2) exhibit clear shifts between monolayer and bulk phases, attributable to the loss of interlayer van‑der‑Waals interactions.

Stability was quantified through cohesive energies: 5.13 eV/atom for SiAs and 4.98 eV/atom for SiAs₂, exceeding those of arsenene (2.99 eV/atom) and silicene (3.71 eV/atom) [18, 43]. Phonon calculations (Figure 2a) show no imaginary modes except a softened transverse acoustic branch near Γ, confirming dynamic stability. 300 K molecular‑dynamics (Figure 2b) display minimal energy fluctuation and preserved structure, indicating thermal robustness.

Band‑structure analysis (Figure 3) indicates that both monolayers are indirect semiconductors: SiAs (E_g = 1.72 eV) with VBM at Y and CBM at Γ; SiAs₂ (E_g = 1.42 eV) nearly direct with VBM near Y and CBM slightly displaced. HSE06 hybrid functional calculations increase the gaps to 2.39 eV (SiAs) and 2.07 eV (SiAs₂). Orbital decomposition shows VBM dominated by p_y orbitals and CBM by s orbitals, suggesting strain‑induced shifts can drive indirect‑to‑direct transitions.

Carrier mobilities were evaluated using deformation‑potential theory. Effective masses (m*) vary anisotropically: for SiAs, m*_e = 0.15 m₀ (x) and 0.80 m₀ (y); m*_h = 0.86 m₀ (x) and 0.22 m₀ (y). For SiAs₂, m*_e = 0.14 m₀ (x) and 2.05 m₀ (y); m*_h = 0.65 m₀ (x) and 1.82 m₀ (y). Elastic constants and deformation potentials (Table S1) yield electron mobilities of 0.66 × 10³ cm² V⁻¹ s⁻¹ (x) and 0.54 × 10³ cm² V⁻¹ s⁻¹ (y) for SiAs, and 0.26 × 10³ cm² V⁻¹ s⁻¹ (x) and 0.11 × 10³ cm² V⁻¹ s⁻¹ (y) for SiAs₂. Hole mobilities reach 3.90 × 10³ cm² V⁻¹ s⁻¹ (x) and 0.30 × 10³ cm² V⁻¹ s⁻¹ (y) for SiAs, surpassing MoS₂ and mirroring the anisotropic transport of black phosphorene.

Partial density‑of‑states (PDOS) plots (Figure 4) reveal strong s–p hybridization between Si and As, with localized p_z lone‑pair states contributing to buckling. Charge‑density maps confirm that VBM is a hybrid of Si/As p orbitals, while CBM derives mainly from s orbitals.

Strain engineering dramatically tunes electronic properties. Biaxial compressive strain increases buckling height and induces a sequence of indirect‑to‑direct transitions in SiAs (Fig. 5c) and, at larger strains, a metal transition. For SiAs₂, tensile strains of 8–10 % convert the material into a direct‑gap semiconductor (E_g = 1.60 eV, HSE) before a metallic state appears at higher strain (Fig. 5d). Detailed band‑structure evolution under strain is shown in Figures 6 and 7. Representative direct‑gap configurations are displayed in Figure S7.

Conclusions

First‑principles DFT predicts that SiAs and SiAs₂ monolayers are both dynamically and thermodynamically stable semiconductors. Their indirect band gaps (2.39 eV and 2.07 eV) are highly tunable by strain, enabling indirect‑to‑direct and metal transitions. Carrier mobilities exceed those of MoS₂ and exhibit pronounced anisotropy akin to black phosphorene. These properties position SiAs and SiAs₂ as compelling platforms for high‑performance optoelectronic devices at the nanoscale.

Abbreviations

2D:

Two‑dimensional

CASTEP:

Cambridge sequential total energy package

CBM:

Conduction band minimum

DFT:

Density functional theory

DFPT:

Density functional perturbation theory

DP:

Deformation potential

GGA:

Generalized gradient approximation

MD:

Molecular dynamics

NVT:

Moles‑volume‑temperature

PAW:

Projector augmented wave

PBE:

Perdew‑Burke‑Ernzerhof

PDOS:

Partial density of states

TMDs:

Transition‑metal dichalcogenides

VASP:

Vienna ab initio simulation package

VBM:

Valence band maximum