DFT-Based Studies on Carbon Adsorption on the wz-GaN Surfaces and the Influence of Point Defects on the Stability of the Diamond–GaN Interfaces

At a Glance

Metadata	Details
Publication Date	2021-10-29
Journal	Materials
Authors	M. Sznajder, Roman Hrytsak
Institutions	Institute of High Pressure Physics, Rzeszów University
Citations	1
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond-GaN Interface Stability

Research Paper Analyzed: DFT-Based Studies on Carbon Adsorption on the wz-GaN Surfaces and the Influence of Point Defects on the Stability of the Diamond-GaN Interfaces (Sznajder et al., Materials 2021, 14, 6532).

Executive Summary

This DFT-based analysis provides critical insights for engineering stable, low-Thermal Boundary Resistance (TBR) diamond-GaN heterointerfaces, essential for next-generation High-Electron-Mobility Transistors (HEMTs).

Core Challenge: Abrupt diamond-GaN interfaces exhibit built-in electric fields and uncompensated electron charge, leading to mechanical and energetic instability.
Optimal Adsorption: Carbon (C) adsorption studies confirm that the N-terminated GaN surface (N-face) is significantly more energetically favorable for forming the initial C monolayer than the Ga-terminated surface (Ga-face).
Stability Mechanism: Energetic stability is maximized through specific substitutional dopant reconstructions in the topmost GaN layer, which compensate for interface charge and reduce strain.
Key Reconstruction Patterns: The most energetically favorable patterns identified are 3N + 1C_N (Carbon substituting Nitrogen) for the N-C interface type, and 3Ga + 1Si_Ga (Silicon substituting Gallium) for the Ga-C interface type.
Strain Management: Dopant selection must prioritize minimizing the atomic radius mismatch (|r_host - r_dopant|), confirming that C and Si are optimal candidates for reducing strain at the interface.
Material Implication: The findings validate the necessity of precise interface engineering, requiring highly controlled, doped, and ultra-smooth diamond substrates for optimal GaN-on-Diamond device fabrication.

Technical Specifications

The following hard data points were extracted from the DFT calculations regarding material properties and interface stability:

Parameter	Value	Unit	Context
Diamond Lattice Parameter ($a$)	3.583967	Å	Calculated (DFT)
wz-GaN Lattice Parameter ($a$)	3.215	Å	Calculated (DFT)
Maximum Energy Gain ($\Delta H$)	0.530	eV/cell	N-C interface, 3N + 1C_N reconstruction
Most Stable C Adsorption Energy ($E_{ads}$)	-9.749	eV/atom	N-terminated GaN, 0.25 ML coverage (H3 site)
N-C Bond Length (Abrupt Interface)	1.48	Å	Relaxed structure
Ga-C Bond Length (Abrupt Interface)	2.02	Å	Relaxed structure
Optimal Dopant Mismatch (C_N)	0.07	Å	For Carbon substituting Nitrogen (3N + 1C_N)
Optimal Dopant Mismatch (Si_Ga)	0.09	Å	For Silicon substituting Gallium (3Ga + 1Si_Ga)
Lowest Migration Barrier ($E_{barrier}$)	0.3	eV	Si_N + V³⁺ complex, a-axis diffusion
Pressure Change ($\Delta P$) for 3N + 1C_N	-3.276	kBar	Small decrease, indicating stability improvement

Key Methodologies

The stability and reconstruction patterns of the diamond-GaN interface were investigated using advanced computational techniques:

Density Functional Theory (DFT): Calculations were performed using the SIESTA code.
Exchange-Correlation Functionals: The Generalized Gradient Approximation (GGA) was utilized, specifically the Perdew-Burke-Ernzerhof (PBE) form for C and H, and the PBEJsJrHEG functional for Ga and N atoms.
Pseudopotentials and Basis Sets: Troullier-Martins-type pseudopotentials were used. Basis functions included Double Zeta (DZ) for C-2s/H-1s, Double Zeta Polarized (DZP) for C-2p, Triple Zeta (TZ) for Ga-4s/4p, and Triple Zeta Polarized (TZP) for N-2p.
Geometry Optimization: Forces acting on all atoms were relaxed to be no larger than 0.001 eV/Å for bulk crystals and 0.02 eV/Å for slab models.
Dispersion Correction: The Grimme correction scheme (DFT-D approach) was applied to accurately model London dispersion interactions between C, N, and Ga atoms.
Slab Models: Isolated slabs (8 double Ga-N layers) with a 30 Å vacuum layer were used to simulate GaN{0001} surfaces (Ga-face and N-face).
Interface Construction: Diamond-GaN heterojunctions were constructed in the [111] growth direction based on the most stable carbon adsorption sites (N-C and Ga-C abrupt interface types).
Defect Migration Analysis: Migration energy barriers for point defects (e.g., C_N + V_N, Si_Ga + V_Ga) were calculated using the Nudged Elastic Band (NEB) method.

6CCVD Solutions & Capabilities

This research confirms that achieving high-performance GaN-on-Diamond devices hinges on precise control over the diamond substrate’s surface chemistry, doping, and physical quality. 6CCVD is uniquely positioned to supply the specialized MPCVD diamond materials required to replicate and advance these critical interface engineering studies.

Applicable Materials for Interface Engineering

To achieve the controlled substitutional doping and ultra-smooth surfaces necessary for stable GaN-on-Diamond integration, 6CCVD recommends the following materials:

High-Purity Single Crystal Diamond (SCD):
- Application: Ideal substrate for heteroepitaxial GaN growth or direct bonding experiments requiring the highest thermal conductivity and lowest defect density.
- Relevance: Provides the stable, high-quality diamond structure (preserved in the relaxed heterostructure, Figure 10) necessary to minimize strain effects.
Boron-Doped Diamond (BDD) Substrates:
- Application: While the paper focuses on C and Si dopants, BDD demonstrates 6CCVD’s capability for controlled substitutional doping in the diamond lattice, crucial for simulating and testing charge compensation mechanisms (like 3N + 1C_N).
- Relevance: Allows researchers to study the impact of controlled p-type doping on interface charge neutrality and electric field penetration.
Polycrystalline Diamond (PCD) Wafers:
- Application: Cost-effective option for large-area GaN-on-Diamond bonding studies, available up to 125mm diameter.
- Relevance: Our PCD material offers thermal conductivity approaching SCD, suitable for high-power HEMT thermal management research.

Customization Potential for Low-TBR Interfaces

The energetic stability of the diamond-GaN interface is highly sensitive to surface preparation and intermediate layers. 6CCVD offers capabilities directly addressing the requirements highlighted by this DFT study:

Research Requirement	6CCVD Capability	Technical Specification
Ultra-Smooth Surface	Advanced Polishing Services	Ra < 1 nm (SCD) or Ra < 5 nm (Inch-size PCD)
Custom Dimensions	Large-Area Substrates	Plates/wafers up to 125 mm (PCD)
Interfacial Layers	Custom Metalization	In-house deposition of Au, Pt, Pd, Ti, W, Cu
Thickness Control	SCD/PCD Layer Precision	SCD (0.1 µm - 500 µm), PCD (0.1 µm - 500 µm)
Substrate Thickness	Mechanical Stability	Substrates available up to 10 mm thick

Engineering Support

The successful implementation of the charge compensation patterns (3N + 1C_N and 3Ga + 1Si_Ga) requires precise control over the diamond surface termination and subsequent bonding or growth processes.

Interface Design Consultation: 6CCVD’s in-house PhD engineering team specializes in MPCVD growth and surface science, offering expert consultation on material selection, surface termination (e.g., H-termination for bonding), and doping strategies for GaN-on-Diamond HEMT projects.
Global Logistics: We ensure reliable, global delivery of highly sensitive diamond substrates, with DDU default shipping and DDP options available.

For custom specifications or material consultation regarding GaN-on-Diamond integration, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Integration of diamond with GaN-based high-electron-mobility transistors improves thermal management, influencing the reliability, performance, and lifetime of GaN-based devices. The current GaN-on-diamond integration technology requires precise interface engineering and appropriate interfacial layers. In this respect, we performed first principles calculation on the stability of diamond-GaN interfaces in the framework of density functional theory. Initially, some stable adsorption sites of C atoms were found on the Ga- and N-terminated surfaces that enabled the creation of a flat carbon monolayer. Following this, a model of diamond-GaN heterojunction with the growth direction [111] was constructed based on carbon adsorption results on GaN{0001} surfaces. Finally, we demonstrate the ways of improving the energetic stability of diamond-GaN interfaces by means of certain reconstructions induced by substitutional dopants present in the topmost GaN substrate’s layer.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma14216532

References

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2021 - Interface Engineering Enabling Next Generation GaN-on-Diamond Power Devices [Crossref]
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2020 - Boosted ultraviolet electroluminescence of InGaN/AlGaN quantum structures grown on high-index contrast patterned sapphire with silica array [Crossref]
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1981 - Hole-drift velocity in natural diamond [Crossref]