Identification of the Nitrogen Interstitial as Origin of the 3.1 eV Photoluminescence Band in Hexagonal Boron Nitride
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-05-06 |
| Journal | physica status solidi (b) |
| Authors | Elham Khorasani, Thomas Frauenheim, Balint Aradi, Peter Deak, Elham Khorasani |
| Institutions | Shenzhen Institute of Information Technology, Beijing Computational Science Research Center |
| Citations | 7 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”- Defect Identification: The Nitrogen Interstitial (Ni) was definitively identified as the intrinsic defect responsible for the prominent 3.1 eV Photoluminescence (PL) band observed in hexagonal Boron Nitride (hBN).
- PL Confirmation: The calculated PL energy for the Ni defect is 3.0 eV, which aligns strongly with the experimentally measured N-sensitive band position of 3.1 eV.
- Stability and Compensation: Ni possesses the lowest formation energy among intrinsic defects under n-type and N-rich growth conditions. Its deep acceptor level (E(0/-) = 2.68 eV above VBM) confirms it acts as a highly efficient compensating center in n-type samples.
- VN Exclusion: The Nitrogen Vacancy (VN) was ruled out as the origin of the Three Boron Center (TBC) Electron Paramagnetic Resonance (EPR) signal. The calculated average hyperfine coupling (Aave = 34.23 MHz) is significantly less than the experimental value (117.06 MHz).
- Growth Implications: VN is less stable than Ni in n-type samples, even under extreme N-poor conditions, suggesting VN can only be created through non-equilibrium processes like irradiation, not thermal equilibrium growth.
- Methodology: Calculations utilized an optimized screened hybrid functional (HSE(alpha=0.3, mu=0.4)) that satisfies the generalized Koopmans condition, ensuring high accuracy for defect levels and formation energies.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Calculated PL Energy (Ni) | 3.0 | eV | Recombination energy for the Ni defect. |
| Experimental PL Band Position | 3.1 | eV | N-sensitive band observed in hBN samples. |
| Ni Acceptor Level E(0/-) | 2.68 | eV | Charge transition level relative to Valence Band Maximum (VBM). |
| Ni Donor Level E(+/0) | 0.94 | eV | Charge transition level relative to VBM. |
| Ni-N Bond Length | 1.38 | Angstrom | Corresponds to a single bond in the lowest energy Ni configuration. |
| VN Hyperfine Coupling (Aave, Calculated) | 34.23 | MHz | Average principal value for the VN defect model. |
| TBC Hyperfine Coupling (Aave, Experimental) | 117.06 | MHz | Measured value for the Three Boron Center (TBC). |
| Hybrid Functional Mixing Parameter (alpha) | 0.3 | Dimensionless | Optimized parameter for HSE functional in bulk hBN. |
| Hybrid Functional Screening Parameter (mu) | 0.4 | Angstrom-1 | Optimized parameter for HSE functional in bulk hBN. |
| Energy Cutoff (Wave Functions) | 420 | eV | Applied for wave function expansion (VASP). |
| Energy Cutoff (Charge Density) | 840 | eV | Applied for charge density expansion (VASP). |
| Force Convergence Criterion | 0.01 | eV/Angstrom | Used for geometry relaxation. |
| High-Frequency Dielectric Constant (epsiloninfinity) | 4.10 | Dimensionless | Used for charged defect correction (SLABCC). |
Key Methodologies
Section titled “Key Methodologies”- Computational Framework: Calculations were performed using Density Functional Theory (DFT) via the Vienna Ab initio Simulation Package (VASP 5.4.4) and the Projector Augmented Wave (PAW) method.
- Functional Optimization: An optimized screened hybrid functional (HSE) was employed, using a mixing parameter alpha = 0.3 and a screening parameter mu = 0.4, specifically tuned to reproduce the hBN band gap and satisfy the generalized Koopmans theorem (gKT).
- Supercell and Relaxation: Defects were modeled in an orthogonal 120-atom supercell (5a1, 3a1 + 6a2, a3). Geometries were relaxed at fixed lattice constants using a force criterion of 0.01 eV/Angstrom.
- Interlayer Interactions: Van der Waals (vdW) interactions between hBN layers were included using the Tkatchenko-Scheffler method (sR = 0.96).
- Charged Defect Correction: Total energies for charged defects were corrected a posteriori using the SLABCC code to eliminate artificial interactions between repeated charges in the periodic boundary conditions, utilizing the high-frequency dielectric constant (epsiloninfinity = 4.10).
- Photoluminescence (PL) Calculation: PL energy was calculated as the difference between the total energy of the initial state (a bound exciton: electron in the Conduction Band Minimum (CBM) and a hole trapped at Ni) and the final state (relaxed geometry after recombination).
- Hyperfine Interaction: Hyperfine splitting calculations for VN were performed and compared to experimental data from 11B-enriched samples, using the nuclear gyromagnetic ratios for Boron (11B) and Nitrogen.
Commercial Applications
Section titled “Commercial Applications”- Quantum Emitters and Photonics: The Ni defect, identified as a stable color center source (3.1 eV), is critical for engineering single-photon emitters (SPEs) in hBN for use in quantum cryptography, quantum sensing, and integrated quantum circuits.
- Deep UV Optoelectronics: hBN is a wide-bandgap material used in Deep UV (DUV) LEDs and detectors. Controlling the concentration of Ni is essential, as this defect acts as a recombination center that can reduce efficiency or introduce unwanted emission bands.
- High-Purity Material Synthesis: The finding that Ni is the most stable intrinsic defect in n-type/N-rich conditions provides a crucial guide for optimizing growth techniques (e.g., MOCVD) to minimize compensating defects and achieve desired n-type doping levels.
- Advanced Insulators and Dielectrics: As Ni is a deep acceptor and strong compensating center, understanding its electronic levels is necessary for predicting and controlling the electrical properties of hBN used as a dielectric layer in 2D material heterostructures.
View Original Abstract
Nitrogen interstitials () have the lowest formation energy among intrinsic defects of hexagonal boron nitride (hBN) under n‐type and N‐rich conditions. Using an optimized hybrid functional, which reproduces the gap and satisfies the generalized Koopman’s condition, an configuration is found, which is lower in energy than the ones reported so far. The (0/-) charge transition level is also much deeper, so acts as a very efficient compensating center in n‐type samples. Its calculated photoluminescence (PL) at 3.0 eV agrees well with the position of an N‐sensitive band measured at 3.1 eV. It is also found that the nitrogen vacancy () cannot be the origin of the three‐boron‐center (TBC) electron paramagnetic resonance (EPR) center and in thermal equilibrium it is even unlikely to exist in n‐type samples.