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 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 $cm^{-1}$, 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 ($\theta_D$) 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.