Session: 12-07-01: Engineered Materials for Thermal Control

Paper Number: 166772

Electric Field Dependent Phonon Properties and Thermal Conductivity of Gan, Aln, and Algan 

The objective of this project is to study how an external electric field impacts the phonon properties and lattice thermal conductivity of wurtzite gallium nitride (GaN), aluminum nitride (AlN), and aluminum gallium nitride (AlGaN) alloys. Group-III nitride semiconductors such as GaN and AlN are of increasing interest in high-voltage and high-frequency applications due to their large band gap, which leads to a high breakdown voltage and advantageous electronic transport and optical properties. AlGaN alloy-based devices have also gained attention, where the variable Al concentration enables tuning of optical and electronic properties. However, as these devices operate under higher voltages, the electric field within the active region can rise to several MV/cm, approaching the breakdown field. Extreme electric field concentration and excessive Joule heating present thermal management challenges for GaN-, AlN-, and AlGaN-based devices. Quantifying the effect of high electric fields on phonon properties and thermal conductivity in these materials is thus required to understand the impact on device performance.

We first investigate the effect of near-breakdown electric fields on the phonon properties and lattice thermal conductivity of wurtzite GaN and AlN. Using first-principles calculations based on density functional theory (DFT) within the QuantumESPRESSO implementation, we analyze the effects of electric fields applied along the in-plane (x) and out-of-plane (z) directions. The second- and third-order interatomic force constants (IFCs) are computed via the finite displacement method implemented in the PHONOPY and PHONO3PY packages. To account for the long-range Coulomb interactions, non-analytical correction terms derived from density functional perturbation theory are applied. The lattice thermal conductivity is calculated using PHONO3PY based on the single mode relaxation time approximation and the linearized phonon Boltzmann equation. Furthermore, the cumulative lattice thermal conductivity as a function of phonon mean free path and frequency is studied to assess the contributions of different phonon modes and how the mode contributions are modified by an externally applied field.

For GaN, a 5 MV/cm electric field applied along the +z direction increases the z-direction thermal conductivity by 9% at a temperature of 300 K. Applying a 5 MV/cm field along the +x direction induces significant symmetry breaking and reduces the thermal conductivity by 27% in the x-direction and 20% in the z-direction. The lack of inversion symmetry in the wurtzite structure coupled with the associated spontaneous polarization and intrinsic electric field cause the thermal conductivity to vary with the direction of the external field. While an electric field applied in the +z-direction increases the thermal conductivity along z, applying a field in the -z-direction opposes the intrinsic electric field, thereby reducing the thermal conductivity by 12%. For AlN, applying a 10 MV/cm electric field in the +z-direction increases the z-direction thermal conductivity by 7%. Similar to GaN, applying a 10 MV/cm field along the +x-direction induces symmetry breaking, resulting in a 23% reduction in thermal conductivity in the x-direction and a 10% reduction in the z-direction. However, unlike GaN, applying a field along the -z-direction increases thermal conductivity by 5%. This result can be attributed to AlN having a stronger spontaneous polarization, leading to a stronger intrinsic electric field compared to GaN.

We also aim to examine high-field effects in AlGaN alloys. Randomized alloy structures are generated using the special quasirandom structure (SQS) approach. Supercell and band-unfolding techniques are used to obtain the effective phonon band structure with and without an electric field present. The phonon band structures obtained from the SQS structures are compared with results computed via the virtual crystal approximation (VCA) to quantify the impact of local atomic disorder. The VCA is also used to estimate the alloy thermal conductivity across varying Al concentrations, both in the presence and absence of an applied external field.

Presenting Author: Minsung An Carnegie Mellon University

Presenting Author Biography: Minsung An is a graduate student researcher in Mechanical Engineering at Carnegie Mellon. Her research focuses on phonon transport and lattice thermal conductivity in semiconductor nanostructures, using first-principles calculations and computational modeling. Currently, she is investigating the phonon properties and thermal conductivity of AlN and AlGaN alloys under extreme electric fields to improve the understanding of transport phenomena in high-power electronics.

Authors:

Minsung An Carnegie Mellon University
Abhishek Pathak Carnegie Mellon University
Hariharan Ramasubramanian Carnegie Mellon University
Jonathan A. Malen Carnegie Mellon University
Alan J. H. Mcgaughey Carnegie Mellon University