The Electronic Structures and Energies of the Lowest Excited States of the Ns0, Ns+, Ns− and Ns-H Defects in Diamond

At a Glance

Metadata	Details
Publication Date	2023-02-28
Journal	Materials
Authors	Alexander Platonenko, W. C. Mackrodt, Roberto Dovesi
Institutions	University of Latvia, University of Turin
Citations	1
Analysis	Full AI Review Included

Technical Documentation & Analysis: Nitrogen Defects in MPCVD Diamond

Executive Summary

This computational study confirms the electronic structure and optical properties of key substitutional nitrogen (N) defects in diamond (N_s⁰, N_s⁺, N_s^-, N_s-H), providing critical data for engineers developing diamond-based optical and electronic devices.

UV Absorption Confirmation: Direct A-SCF calculations confirm that N_s⁰, N_s⁺, and N_s^- defects collectively contribute to the strong 4.59 eV (270 nm) UV absorption band observed in N-doped CVD diamond.
Visible Absorption Source: The weak 2.38 eV (520 nm) absorption peak, relevant for visible light applications, is specifically attributed to the N_s⁺ defect.
Semi-conductivity Mechanism: The activation energy for semi-conductivity (~1.7 eV) in N-doped diamond is validated, resulting from multiple inelastic phonon scattering events involving the thermally excited state of the N_s⁰ donor hybrid orbital.
Defect Localization: The N_s⁰ defect is confirmed to be highly localized, consisting of the N atom and four nearest neighbor C atoms, ensuring the surrounding host lattice remains pristine diamond.
Material Requirement: Replicating and extending this research requires high-purity, precisely controlled Single Crystal Diamond (SCD) with engineered nitrogen concentrations, a core capability of 6CCVD.

Technical Specifications

The following hard data points were extracted from the B3LYP/A-SCF calculations and experimental context cited in the research:

Parameter	Value	Unit	Context
Strong Optical Absorption Peak	4.59 (270)	eV (nm)	Attributed to N_s⁰, N_s⁺, N_s^- defects
Weak Optical Absorption Peak 1	3.44 (360)	eV (nm)	Attributed to N_s-H or other impurity
Weak Optical Absorption Peak 2	2.38 (520)	eV (nm)	Attributed to N_s⁺ defect
Semi-conductivity Activation Energy	~1.7	eV	Associated with N_s⁰ thermal excitation
Semi-conductivity Onset Temperature	~500	K	Equivalent to ~0.04 eV
N_s⁰ Indirect Gap (α→β)	2.22	eV	Virtual transition energy (potential candidate for 2.38 eV absorption)
N_s^- Indirect Gap (E_g)	1.81	eV	Predicted gap, near thermal activation energy
Diamond Host Indirect Gap (E_g)	5.76	eV	B3LYP calculated value
Diamond Host Direct Gap (E_Γ)	7.00	eV	B3LYP calculated value
Typical CVD Growth Temperature	~1100	K	Context for N_s⁰ concentration assessment

Key Methodologies

The computational analysis relied on highly accurate, first-principles methods suitable for simulating localized defects and excited states in covalent materials:

Computational Framework: Calculations were based on the direct A-SCF (Self-Consistent Field) method, as implemented in the CRYSTAL code, designed to calculate ground and excited states separately.
Functional Selection: The hybrid B3LYP functional was used, noted for its reliability in estimating band gaps and excited states in crystalline materials, proving superior to PBE0 and HSE06 for this application.
Basis Sets: Modified Pople 6-21G basis sets were employed for Carbon (C) and Nitrogen (N) atoms.
Defect Simulation: The defective system was simulated using large supercells containing 64 and 128 atoms to accurately model the point defect within the bulk diamond lattice.
Excitation Energy Determination: Excitation energies (∆_SCF) were obtained directly from the difference between the total energy of the fully relaxed ground state and the fully relaxed excited state (Γ-point energy).
Charge and Spin Analysis: Mulliken partition analysis was used to estimate net atomic charges and bond populations, quantifying the substantial redistribution of charge and spin resulting from electronic transitions.

6CCVD Solutions & Capabilities

This research highlights the critical need for high-quality, defect-engineered diamond materials to advance fundamental physics and device development. 6CCVD is uniquely positioned to supply the necessary MPCVD diamond substrates required to replicate and extend these findings, particularly in controlling nitrogen incorporation for specific defect studies.

Applicable Materials

To replicate or extend the research on N-related defects, 6CCVD recommends the following materials:

Material Grade	Description & Application	6CCVD Capability
Optical Grade SCD	Required for studies focusing on the 270 nm and 520 nm absorption peaks. Offers extremely low intrinsic defect density, allowing precise control over intentional N doping to isolate N_s⁰, N_s⁺, and N_s^- concentrations.	SCD thickness: 0.1 µm - 500 µm. Ra < 1 nm polishing.
Electronic Grade SCD	Essential for investigating the semi-conductivity activation energy (~1.7 eV) and thermal transition mechanisms, requiring highly controlled, uniform N doping.	High purity, controlled doping (N or B) available.
Polycrystalline Diamond (PCD)	Suitable for high-power electronic applications where the bulk properties of N-doped diamond are utilized, and large area coverage is required.	Plates/wafers up to 125 mm. Ra < 5 nm polishing on inch-size wafers.

Customization Potential

The study focuses on fundamental defect physics, but practical device integration (e.g., creating stable contacts for semi-conductive layers or fabricating NV centers) requires advanced material processing:

Controlled Doping: 6CCVD offers precise control over nitrogen concentration during MPCVD growth, allowing researchers to tune the ratio of N_s⁰, N_s⁺, and N_s^- defects to isolate specific optical or electronic responses.
Custom Metalization: For electronic transport studies related to the semi-conductivity activation energy, reliable contacts are essential. 6CCVD provides in-house metalization services, including Au, Pt, Pd, Ti, W, and Cu layers, customized to specific device geometries.
Advanced Polishing: To minimize surface scattering and ensure accurate optical measurements (especially for UV absorption at 270 nm), 6CCVD guarantees ultra-smooth surfaces with roughness (Ra) < 1 nm for SCD.
Custom Dimensions: While the research is fundamental, 6CCVD can supply substrates in custom dimensions up to 125 mm (PCD) or thick substrates (up to 10 mm) for high-power or high-pressure applications.

Engineering Support

Understanding the complex interplay between growth conditions, defect charge states (N_s⁰, N_s⁺, N_s^-), and resulting optical/electronic properties is critical for successful device fabrication. 6CCVD’s in-house PhD team specializes in defect engineering and can assist researchers with:

Material selection and specification for projects targeting specific optical absorption bands (e.g., minimizing N_s⁺ for UV transparency or maximizing N_s⁰ for semi-conductivity studies).
Designing optimal doping profiles and post-growth processing to stabilize desired defect charge states.
Consultation on integrating N-doped diamond layers into complex electronic or quantum architectures.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

This paper reports the energies and charge and spin distributions of the mono-substituted N defects, N0s, N+s, N−s and Ns-H in diamonds from direct Δ-SCF calculations based on Gaussian orbitals within the B3LYP function. These predict that (i) Ns0, Ns+ and Ns− all absorb in the region of the strong optical absorption at 270 nm (4.59 eV) reported by Khan et al., with the individual contributions dependent on the experimental conditions; (ii) Ns-H, or some other impurity, is responsible for the weak optical peak at 360 nm (3.44 eV); and that Ns+ is the source of the 520 nm (2.38 eV) absorption. All excitations below the absorption edge of the diamond host are predicted to be excitonic, with substantial re-distributions of charge and spin. The present calculations support the suggestion by Jones et al. that Ns+ contributes to, and in the absence of Ns0 is responsible for, the 4.59 eV optical absorption in N-doped diamonds. The semi-conductivity of the N-doped diamond is predicted to rise from a spin-flip thermal excitation of a CN hybrid orbital of the donor band resulting from multiple in-elastic phonon scattering. Calculations of the self-trapped exciton in the vicinity of Ns0 indicate that it is essentially a local defect consisting of an N and four nn C atoms, and that beyond these the host lattice is essential a pristine diamond as predicted by Ferrari et al. from the calculated EPR hyperfine constants.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma16051979

References

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2012 - The C centre isolated nitrogen-related infra-red absorption at 2688 cm−1: Perfect harmony in diamond [Crossref]
1971 - Dispersed paramagnetic nitrogen content of large laboratory diamonds [Crossref]
1979 - Optical absorption and luminescence in diamond [Crossref]