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
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-10-29 |
| Journal | Materials |
| Authors | M. Sznajder, Roman Hrytsak |
| Institutions | Polish Academy of Sciences, Rzeszów University |
| Citations | 1 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This DFT-based study investigates interface engineering strategies for stable Diamond-GaN heterostructures, critical for improving thermal management in high-power GaN devices (HEMTs).
- Core Problem Addressed: Abrupt Diamond-GaN interfaces (N-C and Ga-C types) exhibit poor energetic stability due to built-in electric fields (polarization) and uncompensated electron charge (monopole behavior).
- Adsorption Preference: Carbon (C) adsorption is energetically favored on the N-terminated GaN surface (N-face, 000-1) over the Ga-terminated surface (Ga-face, 0001) by approximately 2 eV/atom.
- Monolayer Formation: A flat carbon monolayer (precursor to diamond growth) is only stably formed on the N-face via the “on top” adsorption site.
- Reconstruction Strategy: Energetic stability is significantly improved by introducing specific substitutional dopants in the topmost GaN layer to achieve charge neutrality (monopole-to-dipole transition) and reduce strain.
- Optimal Dopants: The most energetically favorable reconstructions utilized Carbon substituting Nitrogen (CN) at the N-C interface (pattern 3N + 1CN) and Silicon substituting Gallium (SiGa) at the Ga-C interface (pattern 3Ga + 1SiGa).
- Energy Gain: The maximum energy gain achieved through reconstruction was 0.53 eV/cell for the N-C interface type (3N + 1CN).
- Diffusion Feasibility: The migration energy barrier for the charged SiN + VN3+ complex is low (0.3-0.9 eV), suggesting that vacancy-mediated diffusion of compensating dopants is feasible in bulk GaN crystals.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Lattice Parameter (Calculated) | 3.58 | A | Bulk crystal (a) |
| wz-GaN Lattice Parameter (Calculated) | 3.215 | A | Bulk crystal (a) |
| wz-GaN c/a Ratio (Calculated) | 1.6236 | Dimensionless | Bulk crystal |
| C Adsorption Energy (Ga-face, 1 ML) | -7.713 | eV/atom | Most stable site (T4, under Ga layer) |
| C Adsorption Energy (N-face, 1 ML) | -6.760 | eV/atom | Only site enabling flat monolayer (On top) |
| N-C Interface Bond Length (Abrupt) | 1.48 | A | Relaxed heterostructure |
| Ga-C Interface Bond Length (Abrupt) | 2.02 | A | Relaxed heterostructure |
| Maximum Energy Gain (N-C Interface) | 0.53 | eV/cell | Reconstruction pattern 3N + 1CN |
| Total Pressure Change (N-C, 3N + 1CN) | -3.276 | kBar | Energetically favorable reconstruction |
| Total Pressure Change (Ga-C, 3Ga + 1SiGa) | 0.735 | kBar | Energetically favorable reconstruction |
| SiN + VN3+ Migration Barrier (a-axis) | 0.30 | eV | Vacancy-mediated diffusion |
| SiN + VN3+ Migration Barrier (c-axis) | 0.93 | eV | Vacancy-mediated diffusion |
| Diamond Thermal Conductivity | 2000 | W/mK | Reference value |
| GaN Bandgap | 3.4 | eV | Reference value |
Key Methodologies
Section titled “Key Methodologies”The study relied on Density Functional Theory (DFT) calculations using the SIESTA code, focusing on structural relaxation, adsorption energy calculation, and defect migration barriers.
- DFT Setup: Generalized Gradient Approximation (GGA) was used for exchange-correlation. PBE functional was used for C and H atoms, and PBEJsJrHEG functional was applied for Ga and N atoms.
- Pseudopotentials and Basis Sets: Troullier-Martins pseudopotentials were employed. Basis sets included Double Zeta (DZ) for C-2s, Double Zeta Polarized (DZP) for C-2p, Triple Zeta (TZ) for Ga-4s/4p, Single Zeta (SZ) for Ga-3d, and Triple Zeta Polarized (TZP) for N-2p.
- Adsorption Modeling:
- GaN slabs (8 double Ga-N layers) were modeled along the [0001] direction, passivated by pseudo-hydrogen atoms (0.75e or 1.25e).
- A 30 A vacuum layer was used. Lateral cell sizes included 1x1, 2x2, and 3x3.
- London dispersion interactions were included using the Grimme correction scheme (DFT-D).
- Adsorption energy (Eads) was calculated relative to the clean surface and isolated C atom energy.
- Interface Modeling:
- Abrupt N-C and Ga-C interfaces were constructed by overlaying the diamond structure onto the most stable C adsorption sites (on top) of the respective GaN surfaces.
- The diamond part was strained to the GaN lateral unit cell.
- Electrostatic potential (Vel) and valence electron charge density (Qval) profiles were computed to analyze electric fields and charge compensation.
- Defect and Reconstruction Modeling:
- Substitutional dopants (C, Si) were introduced into the topmost GaN layer to test charge compensation patterns (e.g., 3N + 1CN).
- Energy gain (Delta H) due to reconstruction was calculated based on the total energy difference between abrupt and reconstructed slabs, accounting for chemical potentials.
- Migration Barrier Calculation: The Nudged Elastic Band (NEB) method was used on a 5x5x3 wz-GaN supercell (300 atoms) to determine the energy barriers for vacancy-mediated diffusion of selected substitutional dopants (e.g., CN + VN).
Commercial Applications
Section titled “Commercial Applications”The findings directly support the development and optimization of high-performance electronic and optoelectronic devices requiring superior thermal management, primarily through the GaN-on-Diamond architecture.
- High-Power Radio Frequency (RF) Devices:
- GaN-based High-Electron-Mobility Transistors (HEMTs) operating in the 5G/6G and tetrahertz communication bands.
- Radar detection systems and high-power RF applications where heat dissipation is critical for reliability and lifetime.
- Power Electronics:
- Next-generation high-power switching devices and converters utilizing GaN, benefiting from the high thermal conductivity of the diamond substrate.
- Optoelectronics:
- Innovative GaN-based Light-Emitting Diodes (LEDs) designed for high current densities, requiring low thermal resistance to maintain efficiency and longevity.
- Heterogeneous Integration Technology:
- Informing the choice of optimal interfacial layers (e.g., C-rich or Si-rich) during heteroepitaxial diamond growth on GaN or during GaN-diamond bonding processes (SAB).
- Guiding the intentional doping of the GaN substrate surface to create stable, charge-compensated interfaces, reducing thermal boundary resistance (TBReff).
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
Section titled “Tech Support”Original Source
Section titled “Original Source”References
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