研究目的
Investigating the effects of temperature and pressure on the phonons of GaN in both wurtzite and zinc-blende structures, and calculating the elastic anisotropies of GaN as a function of pressure at 0 K.
研究成果
The study concludes that pressure determines the degree to which temperature alters the lifetimes of optical phonons, owing to a decrease in the number of three-phonon downscattering channels for optical modes. The elastic anisotropy was found to increase with pressure in both wurtzite and zinc-blende GaN, with elastic instabilities at 40 GPa (zinc blende) and 65 GPa (wurtzite), providing upper bounds for the possible metastability of either phase.
研究不足
The quasiharmonic approximation (QHA) gave reasonable results for the temperature dependence of the phonon DOS at zero pressure but unreliably predicted the combined effects of temperature and pressure. The study also highlights the need for calculations of thermal conductivity at the high T and P of interest.
1：Experimental Design and Method Selection:
Ab initio calculations were performed on GaN using density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP). Helmholtz free energies were calculated for each material and temperature as a function of volume. Finite-temperature phonon properties were calculated using a version of the temperature-dependent effective potential (TDEP) method.
2：Sample Selection and Data Sources:
Phonon properties were calculated for wurtzite and zinc-blende GaN at 0 and 1120 K at each of three pressures: 0, 30, and 60 GPa in wurtzite GaN and 0, 15, and 30 GPa in zinc-blende GaN.
3：List of Experimental Equipment and Materials:
VASP software was used for DFT calculations, with a projector augmented-wave method, the local-density approximation, and a plane-wave cutoff of 600 eV.
4：Experimental Procedures and Operational Workflow:
Phonon properties were calculated on supercells of wurtzite and zinc-blende GaN held at the corresponding volumes. In wurtzite GaN, aspect ratios were determined quasiharmonically by relaxing a unit cell of wurtzite GaN at volumes of interest with ground-state DFT.
5：Data Analysis Methods:
Phonon densities of states (DOSs) and mode Grüneisen parameters were calculated and analyzed to understand the effects of temperature and pressure on phonon frequencies and lifetimes.
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