Density-functional theory study of the thermodynamic, Optical, and Dynamical Properties of the stable and metastable phases of MnTe*

MnTe is a semiconductor phase-change material that has been shown to exhibit a reversible transition between stable and metastable states in response to Joule heating in a device structure[1]. in-situ device-based switching occurs between the (NiAs) α-MnTe and a metastable form of the high temperature β-MnTe phase, the β’-MnTe phase, due to the device structure. While the stable α-MnTe has been studied in the literature, the electronic properties of the metastable phases β‑MnTe and β’-MnTe phases are largely unknown. The wide band gap of these MnTe phases suggests they may be viable candidates for interferometric-based terahertz switching devices over more traditional phase-change materials such as Ge₂Sb₂Te₅ which suffer from large optical absorption.

We have used density-functional theory (DFT) to study the static and dynamical structure of the stable and metastable phases and report on both optical, pressure-induced and dynamic properties including simulated Raman and infrared spectra. All phases were found to be antiferromagnetic and self-consistent Hubbard U corrections were employed to correct for d-electron self-repulsion effects. The band structure and optical absorption were computed in addition to the phonon dispersion using the plane wave density-functional theory code Vasp 6. We have also calculated the infrared absorption and Raman spectra using the auxiliary codes phonopy and phono3py. The effects of temperature on both the infrared absorption and Raman spectra were accounted for by the calculation of three-phonon interactions. We have also calculated enthalpic contributions to the free energy taking into account the presence of strain on the different phases as might be present in a device structure in order to understand the role of pressure induced stability on the free energy of the stable  -MnTe phase and the metastable β-MnTe and β’-MnTe phases. In addition, we have also calculated the hypothetical energy barrier for the transition from the –MnTe phase and the metastable β’-MnTe phase using the nudged elastic band formalism.

[1] S. Mori et al.. Nat. Commun., 11(1):85, 2020.