Investigation of softer lattice dynamics in defect engineered GeTe crystals

S Majumder and P Singha and CS Kumar and MK Gupta and D Kumar and R Mittal and DK Shukla and MP Saravanan and D Jaiswal-Nagar and VB Kamble, JOURNAL OF PHYSICS-CONDENSED MATTER, 37, 425401 (2025).

DOI: 10.1088/1361-648X/ae0dd5

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: Ge0.8Te (S1) and Ge0.88Te (S2). X-ray diffraction confirms their highly oriented crystalline nature mainly along the h0l plane, while temperature-dependent Raman spectroscopy reveals pronounced anharmonicity in S1, 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-nGen 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 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 +/- 1.5 K in Ge0.8Te and 176.6 +/- 1.7 K for Ge0.88Te respectively, confirming defect-induced lattice softening. Electrical resistivity analysis further corroborates this, indicating reduced effective phonon frequencies in S1. 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.

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