Investigation of Softer Lattice Dynamics in Defect Engineered GeTe Crystals

Saptak Majumder; Pintu Singha; Sharath Kumar C.; Mayanak K. Gupta; Dharmendra Kumar; R. Mittal; D. K. Shukla; M. P Saravanan; Deepshikha Jaiswal-Nagar; Vinayak B. Kamble

paper

Investigation of Softer Lattice Dynamics in Defect Engineered GeTe Crystals

arXiv:2502.10745

Abstract

The impact of Ge vacancies on the low-temperature lattice dynamics of single-crystalline GeTe was investigated through a comparative study of two off-stoichiometric samples: Ge $_{0.8}$ Te (S $_{1}$ ) and Ge $_{0.88}$ Te (S $_{2}$ ). X-ray diffraction confirms their highly oriented crystalline nature mainly along the $h 0 l$ plane, while temperature-dependent Raman spectroscopy reveals pronounced anharmonicity in S $_{1}$ , indicated by stronger three-phonon scattering in the in-plane E-mode. A suppressed Raman feature at 239 $c m^{- 1}$ in S $_{2}$ suggests fewer disordered GeTe $_{4 - n}$ Ge $_{n}$ tetrahedra, correlating with reduced Ge-Ge bonding signatures. Machine-Learned Molecular Dynamics (MLMD) simulations show dominant Te contributions below 100 $c m^{- 1}$ , while Ge dominates above, particularly influencing the 120 $c m^{- 1}$ mode affected by defects at the Ge-site. Complementary calculation of phonon linewidth via MLMD and Temperature-Dependent Effective Potential (TDEP) methods affirm the predominance of three-phonon scattering below 300 K. Specific heat measurements, modeled using Debye-Einstein formalism, show lower Debye temperatures ( $I^¸_{D}$ ) of 172.3 $\pm$ 1.5 K in Ge $_{0.8}$ Te and 176.6 $\pm$ 1.7 K for Ge $_{0.88}$ Te respectively, confirming defect-induced lattice softening. Electrical resistivity analysis further corroborates this, indicating reduced effective phonon frequencies in $S_{1}$ . Thus, our results establish that higher Ge vacancies lead to softer, and hence more anharmonic lattice dynamics in GeTe, with its relevance in designing superior thermoelectric and phase-change memory applications.