Investigation of Softer Lattice Dynamics in Defect Engineered GeTe Crystals

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 h0l 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 cm −1 in S2 suggests fewer disordered GeTe 4 − nGe n tetrahedra, correlating with reduced Ge-Ge bonding signatures. Machine-Learned Molecular Dynamics (MLMD) simulations show dominant Te contributions below 100 cm−1 , while Ge dominates above, particularly influencing the 120 cm−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 (θ D ) of 172.3 ± 1.5 K in Ge 0.8 Te and 176.6 ± 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.