Modified amorphous GeTe, formed by the pulsed laser irradiation of as-grown GeTe, was analyzed in terms of variations in the local bonding structure using Raman spectroscopy and X-ray absorption fine structure in tandem with first-principles density functional theory. Amorphized GeTe (acquired from the crystalline phase) was compared with the modified amorphous GeTe to investigate the similarities and discrepancies between these two amorphous phases. Raman spectroscopy showed that these materials have a similar distribution of Ge-centered local structure in both phases, which is mainly composed of an octahedral-like structure. However, extended X-ray absorption fine structure results show the presence of a unique second type of Ge-Te bonding in the amorphized GeTe, which can effectively reduce the energy required for recrystallization. A computational study based on molecular dynamics simulations verified our experimental observations, including the existence of a second type of Ge-Te bonding in the amorphized phase. Moreover we distinguished the structural characteristics underlying the different amorphous phases, such as local atomic configurations and structural symmetries.

Charge density differences (CDDs) on Ge–C–Sb bonds in CGST(5%) and Ge–C–Sb in CGST(10%).

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Although some methods to improve phase-change memory efficiency have been proposed, an effective experimental approach to induce a phase-change like process without external heat energy has not yet been reported. Herein we have shown that GeTe is a prototype phase-change material, which can exhibit a non-thermal phase-change-like process under uniaxial stress. Due to its structural characteristics like directional structural instability and resonance bonding under 1% uniaxial stress, we observed that bond switching in the GeTe film between short and long bonds is possible. Due to this phase change, GeTe displays the same phase-change as crystal layer rotation. Crystal layer rotation has not been observed in the conventional phase change process using intermediate states, but it is related to the structural characteristics required for maintaining local coordination. Moreover, since the resonance bonding characteristics are effectively turned off upon applying uniaxial stress, the high-frequency dielectric constant can be significantly decreased. Our results also show that the most significant process in the non-thermal phase transition of phase-change materials is the modulation of the lattice relaxation process after the initial perturbation, rather than the method inducing the perturbation itself. Finally, these consequences suggest that a new type of phase-change memory is possible through changes in the optical properties under stress.

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