Determination of the Nitrogen Vacancy as a Shallow Compensating Center in GaN Doped with Divalent Metals
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2015-01-07 |
| Journal | Physical Review Letters |
| Authors | John Buckeridge, C. Richard A. Catlow, David O. Scanlon, Thomas W. Keal, Paul Sherwood |
| Institutions | University of Bath, Daresbury Laboratory |
| Citations | 85 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Defect Engineering in Wide Bandgap Semiconductors
Section titled âTechnical Documentation & Analysis: Defect Engineering in Wide Bandgap SemiconductorsâReference Paper: Buckeridge et al., âDetermination of the nitrogen vacancy as a shallow compensating center in GaN doped with divalent metalsâ (arXiv:1412.1694v1, 2014).
Executive Summary
Section titled âExecutive SummaryâThis analysis focuses on the fundamental defect physics governing conductivity and optical properties in GaN, a critical wide-bandgap semiconductor. The findings regarding defect compensation and shallow donor levels are highly relevant to advanced material engineering in diamond (SCD/PCD).
- Core Finding: Doping GaN with divalent metals (e.g., Mg) does not primarily result in free hole formation (p-type conductivity) but is instead compensated by the spontaneous formation of Nitrogen Vacancies (V$_{\text{N}}$).
- Methodology: High-fidelity multiscale modeling (Hybrid QM/MM embedded cluster method using DFT with 42% exact exchange) was employed to accurately calculate defect formation and ionization energies relative to a well-defined vacuum level.
- Conductivity Barrier: The energy required to dissociate a hole from Mg$_{\text{Ga}}$ is highly unfavorable (1.404 eV), confirming the thermodynamic instability of free holes and explaining the native n-type nature of GaN.
- Optical Signature Explained: The V$_{\text{N}}$ defect acts as a shallow donor, with a calculated vertical ionization energy of 44 meV below the Conduction Band Minimum (CBM), which successfully accounts for the ubiquitous 3.466 eV photoluminescence (PL) peak observed in lightly doped and undoped GaN.
- Implication for Wide Bandgap Materials: This research underscores the critical role of native point defects (like V$_{\text{N}}$) in compensating desired dopants, a challenge central to achieving controlled conductivity and stable quantum centers in all wide-bandgap materials, including diamond.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the theoretical calculations and experimental comparisons presented in the paper.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| GaN Band Gap (Low T) | 3.503 | eV | Experimental reference value [6] |
| Mg$_{\text{Ga}}$ Equilibrium Level | 0.307 | eV | Calculated level above Valence Band Maximum (VBM) |
| V$_{\text{N}}$ Shallow Donor Level (Calculated) | 44 | meV | Vertical Ionization Energy (IE) below CBM |
| V$_{\text{N}}$ Shallow Donor Level (Experimental) | 51 ± 13 | meV | Measured via TAS/Electron Irradiation [20, 21] |
| Hole Dissociation Energy (Mg$_{\text{Ga}}$) | 1.404 | eV | Energy required for Mg${\text{Ga}}$ $\rightarrow$ Mg${\text{Ga}}^{+}$ + h$^{+}$ |
| Compensation Reaction Energy | -1.245 | eV | Energy for h$^{+}$ + V${\text{N}}^{3+}$ $\rightarrow$ V${\text{N}}^{+}$, indicating instability of free holes |
| Key PL Peak (ABE/V$_{\text{N}}$) | 3.466 | eV | Attributed to Acceptor-Bound Exciton (ABE) or V$_{\text{N}}$ compensation |
| Blue Luminescence (BL) Peak | 2.9 | eV | Attributed to ionised V$_{\text{N}}$ relaxation/recombination |
| Computational Exchange Percentage | 42 | % | Exact exchange used in the BB1K hybrid functional |
Key Methodologies
Section titled âKey MethodologiesâThe study relied on advanced computational techniques to accurately model localized defects and their interaction with the host lattice.
- Multiscale Embedded Cluster Method: Employed the Hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) approach [22] to model defects in GaN, ensuring accurate treatment of localized defects without supercell image-charge interactions.
- QM Region Definition: The defect and its immediate surrounding (approx. 100 atoms) were treated using Density Functional Theory (DFT) with a triple-zeta-plus-polarisation Gaussian basis set.
- Hybrid Functional Selection: A hybrid exchange and correlation functional (BB1K) employing 42% exact exchange was used to accurately reproduce the dielectric, elastic, and structural properties of the bulk material [24].
- MM Region Embedding: The QM region was embedded within a larger cluster (10,000 - 20,000 atoms) treated at the MM level using polarizable-shell interatomic potentials, fully accounting for the dielectric response of the surrounding material.
- Ionization Energy Determination: Ionization energies were calculated relative to a well-defined âquasi-vacuumâ reference level, providing a robust method for determining defect levels relative to the band edges (VBM and CBM).
- Defect States Analyzed: Formation energies ($E_f$) and vertical ionization energies (IEs) were calculated for various charge states of the Nitrogen Vacancy (V${\text{N}}$) and divalent metal substitutions (Mg${\text{Ga}}$, Be${\text{Ga}}$, Zn${\text{Ga}}$, Cd${\text{Ga}}$, Hg${\text{Ga}}$).
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe challenges faced in GaNâspecifically controlling conductivity and managing compensating defectsâare directly analogous to the challenges 6CCVD solves in the diamond platform. Diamond (SCD/PCD) is the ultimate wide-bandgap semiconductor, and our expertise lies in precise defect engineering via MPCVD growth.
Applicable Materials for Defect Engineering
Section titled âApplicable Materials for Defect EngineeringâTo replicate or extend research into defect compensation and conductivity control, 6CCVD recommends the following materials:
| Research Requirement | 6CCVD Material Solution | Application Focus |
|---|---|---|
| Conductivity Control (p-type) | Boron-Doped Diamond (BDD) | BDD provides stable, high-concentration p-type conductivity, overcoming the compensation issues seen in GaN. Available in both SCD and PCD formats. |
| High-Purity Defect Studies | Optical Grade Single Crystal Diamond (SCD) | Ultra-low impurity SCD is essential for controlled creation of specific point defects (e.g., NV, SiV centers) for quantum sensing and computing, where background native defects must be suppressed. |
| Large-Area Electronic Devices | Polycrystalline Diamond (PCD) | PCD plates up to 125 mm diameter, suitable for high-power electronics and thermal management applications requiring large, uniform conductive or insulating layers. |
Customization Potential
Section titled âCustomization PotentialâThe paper highlights the necessity of precise control over dopant concentration and defect environments. 6CCVDâs MPCVD capabilities are uniquely suited to meet these stringent requirements:
- Custom Dimensions and Thickness: We provide SCD and PCD plates/wafers up to 125 mm in diameter. Thicknesses are customizable from 0.1 ”m to 500 ”m for active layers, and substrates up to 10 mm thick.
- Ultra-Low Roughness Polishing: For optical and epitaxial studies, our SCD surfaces achieve Ra < 1 nm, and inch-size PCD achieves Ra < 5 nm, ensuring minimal surface scattering and high-quality interfaces.
- Advanced Metalization: The electrical characterization of defect levels (like the 44 meV V$_{\text{N}}$ donor level) requires reliable contacts. 6CCVD offers in-house metalization services, including deposition of Au, Pt, Pd, Ti, W, and Cu, tailored to specific device architectures.
- Targeted Doping: We specialize in controlling the incorporation of dopants (like Boron and Nitrogen) during growth, allowing researchers to precisely manage the concentration of compensating centers, analogous to controlling V$_{\text{N}}$ in GaN.
Engineering Support
Section titled âEngineering SupportâThe detailed theoretical analysis presented in this paper requires deep material science expertise to translate into successful device fabrication.
- Defect Physics Consultation: 6CCVDâs in-house PhD team specializes in wide-bandgap defect physics and can assist researchers in selecting the optimal diamond material (SCD, PCD, or BDD) and growth recipe for projects involving conductivity control, quantum defect creation, or thermal management.
- Global Logistics: We ensure reliable, global shipping (DDU default, DDP available) for sensitive, high-value diamond materials, supporting international research collaborations.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
We report accurate energetics of defects introduced in GaN on doping with divalent metals, focusing on the technologically important case of Mg doping, using a model that takes into consideration both the effect of hole localization and dipolar polarization of the host material, and includes a well-defined reference level. Defect formation and ionization energies show that divalent dopants are counterbalanced in GaN by nitrogen vacancies and not by holes, which explains both the difficulty in achieving p-type conductivity in GaN and the associated major spectroscopic features, including the ubiquitous 3.46 eV photoluminescence line, a characteristic of all lightly divalent-metal-doped GaN materials that has also been shown to occur in pure GaN samples. Our results give a comprehensive explanation for the observed behavior of GaN doped with low concentrations of divalent metals in good agreement with relevant experiment.
Tech Support
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
Section titled âReferencesâ- 2008 - Handbook of Nitride Semiconductors and Devices
- 2013 - III-Nitride Based Light Emitting Diodes and Applications
- 2004 - Semiconductors: Data Handbook [Crossref]