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_0.8Te (S₁) and Ge_0.88Te (S₂). X-ray diffraction confirms their highly oriented crystalline nature mainly along theh0lplane, while temperature-dependent Raman spectroscopy reveals pronounced anharmonicity inS₁, indicated by stronger three-phonon scattering in the in-planeE-mode. A suppressed Raman feature at 239 cm^-1inS₂suggests fewer disordered GeTe4-nGe_ntetrahedra, 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^-1mode affected by defects at the Ge-site. Complementary calculation of phonon linewidth via MLMD and temperature-dependent effective potential methods affirm the predominance of three-phonon scattering below 300 K. Specific heat measurements, modeled using Debye-Einstein formalism, show lower Debye temperatures (θD) of 172.3 ± 1.5 K in Ge_0.8Te and 176.6 ± 1.7 K for Ge_0.88Te respectively, confirming defect-induced lattice softening. Electrical resistivity analysis further corroborates this, indicating reduced effective phonon frequencies inS₁. 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.