Electric-field effects on defect migration energetics in GaN

Technical Summary: Electric-field effects on defect migration energetics in GaN

Problem Statement
The reliability of Gallium Nitride (GaN) and AlGaN-based devices, particularly in high-power and radiation environments, is compromised by the migration and reconfiguration of point defects. While defect formation under irradiation is well-studied, the behavior of existing defects under the combined influence of operating electric fields and temperature remains less understood. In high-electron-mobility transistors (HEMTs) and power devices, operating fields often approach the breakdown limit (~3 MV/cm), potentially driving defect transport that leads to threshold-voltage instabilities and performance drift. Existing computational approaches face a trade-off: Density Functional Theory (DFT) offers high accuracy but is restricted to small supercells, while classical Molecular Dynamics (MD) with standard interatomic potentials lacks the capability to model dynamic charge transfer and electrostatic coupling required to simulate defect migration under external electric fields.

Methodology
To address these limitations, the authors developed a ReaxFF reactive force field specifically for GaN. The methodology involved three primary stages:

1. Force Field Development: A comprehensive training dataset was generated using DFT calculations (VASP) within the PBE functional framework. This dataset included equation-of-state (EOS) data for various Ga and GaN phases (wurtzite, rocksalt, zincblende, etc.), N2 molecular profiles, and total energies of supercells containing neutral point defects (vacancies, antisites, Frenkel pairs, interstitials, and Schottky defects).
2. Validation: The ReaxFF parameters were optimized against this DFT dataset. The resulting force field was validated by comparing predicted lattice parameters, cohesive energies, bulk modulus, and defect formation/migration energies against prior DFT and experimental results.
3. Electric Field Simulations: Using the validated ReaxFF model within LAMMPS, the authors performed Nudged Elastic Band (CI-NEB) calculations to determine migration barriers for intrinsic point defects ($V_{Ga}$, $V_N$, $Ga_{i1,oct}$, and $N_{i2,split}$) under external electric fields ranging from 0 to 0.04 V/Å (0–40 MV/cm). Simulations accounted for dynamic charge equilibration (QEq) and local lattice distortions.

Key Contributions and Results

• Force Field Validation: The developed ReaxFF model successfully reproduces DFT structural properties (e.g., lattice constants within 0.3% and bulk modulus within 1.5%) and defect energetics. While some deviations exist in absolute defect formation energies (up to 26% for certain configurations), the model captures the relative trends and migration barriers with high fidelity compared to literature values.
• Non-Linear Field Response: Contrary to the simple linear dipole–field approximation ($\Delta E_B = -\mu_{eff} \cdot E$), the study reveals that electric fields modify migration barriers anisotropically and non-linearly. The response depends heavily on the specific defect type and its migration mechanism.
  • Vacancies: The migration of Nitrogen vacancies ($V_N$) exhibits a nearly linear and symmetric response to the field, consistent with a rigid-dipole model due to minimal lattice strain and charge variation. Gallium vacancies ($V_{Ga}$) show a mild asymmetry; the forward barrier increases while the reverse barrier decreases, but with unequal slopes, attributed to Ga's larger atomic size inducing local strain and charge redistribution.
  • Interstitials: Interstitial defects ($Ga_{i1,oct}$ and $N_{i2,split}$) display pronounced nonlinearity and asymmetry. For $Ga_{i1,oct}$, the field is strong enough to shift the rate-limiting saddle-point configuration, causing a crossover in which energy peak dominates the barrier. These defects involve cooperative motion of multiple atoms, leading to complex partial-charge redistribution and strain responses that deviate significantly from linear predictions.
• Mechanism of Deviation: The departure from linear behavior arises from field-induced partial charge redistribution and local lattice distortion along the reaction coordinate. The effective dipole moment is not fixed but evolves continuously as the defect migrates, a phenomenon captured natively by the reactive force field but missed by fixed-dipole models.

Significance and Claims
The paper claims that electric fields do not merely bias defect motion linearly but can complexly modify the defect migration landscape through charge-lattice coupling.

• Kinetic Impact: The study highlights that while field-induced barrier reductions for high-barrier vacancies (>2 eV) may not significantly alter diffusion rates on device timescales, the effects are kinetically significant for interstitial defects with barriers near 1 eV. A reduction of ~0.05 eV can enhance migration rates by an order of magnitude at room temperature, potentially accelerating defect redistribution in high-field regions of operating devices.
• Device Reliability: These findings suggest that in high-field regions (e.g., gate edges in HEMTs), interstitial defects may become substantially more mobile, contributing to trap generation and threshold-voltage drift. The directional asymmetry of the response implies a net drift of defects toward specific device regions, potentially driving long-term reliability issues.
• Methodological Scope: The authors note that their results correspond to neutral defects due to the classical nature of the QEq charge equilibration scheme. They acknowledge that highly charged defects in real operating conditions (e.g., $V_{Ga}^{3-}$) may exhibit even stronger field responses, suggesting the current findings represent a lower bound on field effects. Nevertheless, the work establishes that field-defect coupling in GaN is anisotropic and defect-specific, necessitating reactive simulations for accurate prediction under realistic operating conditions.