Interaction of Dopants with the I3-Type Basal Stacking Fault in Hexagonal-Diamond Si
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-06-05 |
| Journal | The Journal of Physical Chemistry C |
| Authors | Marc TĂșnica, Perpetua Wanjiru Muchiri, Alberto Zobelli, Anna Marzegalli, Emilio Scalise |
| Institutions | Technical University of Kenya, Centre National de la Recherche Scientifique |
| Citations | 1 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study uses Density Functional Theory (DFT) to analyze the interaction between extrinsic dopants (Group III, IV, V elements) and the I3-type basal stacking fault (I3-BSF) in hexagonal-diamond silicon (hex-Si), a material critical for next-generation optoelectronics.
- Acceptor Repulsion: Neutral and negatively charged p-type dopants (acceptors: Al, Ga, In) exhibit a strong thermodynamic tendency to segregate away from the I3-BSF, preferring the pristine hexagonal bulk region. Segregation energies (DSE) reach 80-140 meV.
- Donor Neutrality: Neutral and positively charged n-type dopants (donors: P, As, Sb) and isovalent impurities (Ge) show negligible DSE variation or slight attraction, indicating the I3-BSF is not a significant thermodynamic barrier for these species.
- Mechanism: The repulsion of acceptors is primarily attributed to the large positive covalent radius mismatch (steric effect) and the energetic cost of forcing the dopant wave function to transition from its stable C3v symmetry (hex-Si) to Td symmetry (cubic-like fault region).
- Charge State Effect: Ionization (charging) significantly reduces the DSE barrier for both p-type (43-80% reduction) and n-type dopants, suggesting charged ions interact less strongly with the fault.
- Boron Exception: Boron (B) and Carbon (C) are exceptions; B- and neutral C atoms show negative DSE values, indicating a tendency to segregate into the stacking fault plane.
- Validation: Results are confirmed by analyzing the extreme case of an abrupt hex/cub-Si interface, where acceptors strongly segregate toward the hexagonal phase.
- Implication: These findings enable defect-dopant engineering, allowing the I3-BSF to be used as a tool to spatially control doping profiles in hex-Si nanowires and heterostructures.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Optimized Lattice Parameter (aSi) | 3.84 | A | Hexagonal Si (hex-Si) |
| Optimized Lattice Parameter (cSi) | 6.34 | A | Hexagonal Si (hex-Si) |
| Stacking Fault Energy (gamma I3-BSF) | -85 | mJ/m2 | Calculated value, compatible with hex-Si metastability. |
| Simulated Doping Concentration | 1.3 x 1020 | cm-3 | Concentration used in 384-atom bulk supercell. |
| Max DSE (Neutral Acceptors: Al, Ga, In) | 80-140 | meV | Maximum repulsion energy from I3-BSF (Region II). |
| Max DSE (Neutral Donor: N) | 70 | meV | Maximum repulsion observed for N (Region I/II boundary). |
| DSE (Neutral Isovalent: Ge) | less than 20 | meV | Negligible interaction with I3-BSF. |
| DSE (Ionized Acceptor: B-) | -40 | meV | Indicates segregation/attraction to the I3-BSF. |
| DSE Reduction (Ionized Acceptors) | 43-80 | % | Reduction in DSE compared to neutral state near I3-BSF. |
| Force Convergence Threshold | 10-2 | eV/A | DFT structural optimization criteria. |
| Stress Convergence Threshold | 10-1 | GPa | DFT structural optimization criteria. |
Key Methodologies
Section titled âKey MethodologiesâThe study employed first-principles DFT simulations to calculate the Dopant Segregation Energy (DSE) and analyze structural and electronic properties.
- Computational Framework: Spin-polarized ab initio DFT simulations were performed using the SIESTA code.
- Exchange-Correlation Functional: The Generalized Gradient Approximation (GGA) with the Perdew, Burke, and Ernzerhof (PBE) functional was utilized.
- Basis Sets and Pseudopotentials: A double-zeta polarized basis set was used for Si valence electrons, and a double-zeta basis set plus two polarization orbitals was used for dopant atoms. Inner core electrons were replaced by norm-conserving Troullier-Martins pseudopotentials.
- Supercell Design (I3-BSF): A 4x4x6 bulk supercell (384 atoms) was constructed along the [0001] direction, containing ten hex-Si layers interspersed with two cubic-diamond stoichiometry planar defects (I3-BSF).
- Supercell Design (Interface): An abrupt hex/cub-Si interface supercell (448 atoms) was built along the [0001] direction for comparative validation.
- Structural Optimization: Atomic positions were optimized using a conjugate gradient algorithm until forces were less than 10-2 eV/A and stress was lower than 10-1 GPa.
- K-point Sampling: An 8x8x8 k-point grid was used for hex-Si crystal optimization, and a 2x2x1 Monkhorst-Pack k-point grid was used for the defective supercells.
- Charged Defect Calculation: For charged impurity calculations (ionized dopants), a compensating jellium background was included to maintain charge neutrality and avoid divergence of the electrostatic energy.
- DSE Calculation: DSE was defined as the difference between the total energy of the defective system with the dopant at position z (EtotI3-BSF(z)) and the reference energy when the dopant is at the farthest atomic plane (z0), minimizing interaction with the fault.
Commercial Applications
Section titled âCommercial ApplicationsâThe findings provide crucial insights for engineering Group IV semiconductor materials, particularly for applications requiring precise control over doping profiles in nanostructures.
- High-Performance Optoelectronics: Hex-Si and hex-Ge nanowires are candidates for infrared light emission and absorption (photovoltaics). Controlling dopant placement relative to common I3-BSFs is essential for optimizing p-n junction performance and minimizing defect-related recombination.
- Defect-Assisted Doping Profile Control: The I3-BSF can be intentionally used as a ârepulsive barrierâ for p-type dopants (Al, Ga, In) during growth, leading to self-organized dopant depletion zones near the fault plane. This enables the creation of functional, anisotropic doping profiles.
- Hex/Cub-Si Heterostructure Design: The strong segregation of acceptors toward the hexagonal phase at abrupt hex/cub interfaces can be exploited to build heterostructures with highly localized p-type regions, potentially enhancing carrier confinement or mobility.
- Nanowire Field-Effect Transistors (NW-FETs): In NW-FETs, dopant accumulation near extended defects can degrade electrical activation and mobility. Understanding the DSE allows for material processing strategies that ensure donors (n-type) are preferentially incorporated into the fault region (where they are stable) or that acceptors (p-type) are kept in the bulk, optimizing device reliability.
- Group IV Materials Processing: The results inform growth recipes (e.g., temperature, precursor flow) that influence the formation kinetics of I3-BSFs and subsequent dopant incorporation, leading to application-tailored semiconductor technologies.
View Original Abstract
Recently synthesized hexagonal-diamond silicon, germanium, and silicon-germanium nanowires exhibit remarkable optical and electronic properties when compared to cubic-diamond polytypes. Because of the metastability of the hexagonal-diamond phase, I3-type basal stacking faults are frequently observed in these materials. Understanding and modulating the interaction between these extended defects and dopants are essential for advancing the design and performance of these novel semiconductors. In the present study, we employ density functional theory calculations to investigate the interaction of extrinsic dopants (group III, IV, and V elements) with the I3-type basal stacking fault in hexagonal-diamond silicon. Contrary to the behavior observed in cubic-diamond silicon with intrinsic stacking faults, we demonstrate that neutral and negatively charged p-type impurities exhibit a marked tendency to occupy lattice sites far from the I3-type basal stacking fault. The interaction of acceptors with the planar defect reduces their energetic stability. However, this effect is much less pronounced for neutral or positively charged n-type dopants and isovalent impurities. The thermodynamic energy barrier to segregation for these dopants is small and may even become negative, indicating a tendency to segregate into the fault. Through a detailed analysis of structural modifications, ionization effects, and impurity-level charge density distribution, we show that the origin of this behavior can be attributed to variations in the impurityâs steric effects and its wave function character. Finally, all these results are validated by considering the extreme case of an abrupt hexagonal/cubic silicon interface, where acceptor segregation from the cubic to the hexagonal region is demonstrated, confirming the behavior observed for p-type dopants near the I3-type defect.