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Register a new userThe Ultrafast Optical Response of the Amorphous and Crystalline States of the Phase Change Material Ge$_2$Sb$_2$Te$_5$
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We examine the ultrafast optical response of the crystalline and amorphous phases of the phase change material Ge$_2$Sb$_2$Te$_5$ below the phase transformation threshold. Simultaneous measurement of the transmissivity and reflectivity of thin film samples yields the time-dependent evolution of the dielectric function for both phases. We then identify how lattice motion and electronic excitation manifest in the dielectric response. The dielectric response of both phases is large but markedly different. At 800 nm, the changes in amorphous GST are well described by the Drude response of the generated photo-carriers, whereas the crystalline phase is better described by the depopulation of resonant bonds. We find that the generated coherent phonons have a greater influence in the amorphous phase than the crystalline phase. Furthermore, coherent phonons do not influence resonant bonding. For fluences up to 50% of the transformation threshold, the structure does not exhibit bond softening in either phase, enabling large changes of the optical properties without structural modification.
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High-speed electrical switching of Ge2Sb2Te5 (GST) remains a challenging task due to the large impedance mismatch between the low-conductivity amorphous state and the high-conductivity crystalline state. In this letter, we demonstrate an effective doping scheme using nickel to reduce the resistivity contrast between the amorphous and crystalline states by nearly three orders of magnitude. Most importantly, our results show that doping produces the desired electrical performance without adversely affecting the films optical properties. The nickel doping level is approximately 2% and the lattice structure remains nearly unchanged when compared with undoped-GST. The refractive indices at amorphous and crystalline states were obtained using ellipsometry which echoes the results from XRD. The materials thermal transport properties are measured using time-domain thermoreflectance (TDTR), showing no change upon doping. The advantages of this doping system will open up new opportunities for designing electrically reconfigurable high speed optical elements in the near-infrared spectrum.
We report the creation of Ge$_2$Sb$_2$Te$_5$ metasurfaces on sapphire substrates by the ablation method and the study of their structural properties by scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical diffraction. The main emphasis is on the optical technique, which boils down to obtaining bright Laue diffraction patterns on a screen, observing them with the naked eye, and analyzing the fine structure of diffraction reflections. It has been demonstrated that in one simple optical experiment it is possible to assess the quality of fabricated metasurfaces, determine the structure symmetry, and, moreover, determine the number of structural elements and lattice constants of the micron-sized metasurface. The accuracy of the optical technique is confirmed by comparison with the results of studies by SEM and AFM methods.
Ge2Sb2Te5 and related phase change materials are highly unusual in that they can be readily transformed between amorphous and crystalline states using very fast melt, quench, anneal cycles, although the resulting states are extremely long lived at ambient temperature. These states have remarkably different physical properties including very different optical constants in the visible in strong contrast to common glass formers such as silicates or phosphates. This behavior has been described in terms of resonant bonding, but puzzles remain, particularly regarding different physical properties of crystalline and amorphous phases. Here we show that there is a strong competition between ionic and covalent bonding in cubic phase providing a link between the chemical basis of phase change memory property and origins of giant responses of piezoelectric materials (PbTiO3, BiFeO3). This has important consequences for dynamical behavior in particular leading to a simultaneous hardening of acoustic modes and softening of high frequency optic modes in crystalline phase relative to amorphous. This different bonding in amorphous and crystalline phases provides a direct explanation for different physical properties and understanding of the combination of long time stability and rapid switching and may be useful in finding new phase change compositions with superior properties.
A long-standing question for avant-grade data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved X-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The novel approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process, meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in phase-change materials.
Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST) has been widely used as a popular phase change material. In this study, we show that it exhibits high Seebeck coefficients 200 - 300 $mu$V/K in its cubic crystalline phase ($it{c}$-GST) at remarkably high $it{p}$-type doping levels of $sim$ 1$times$10$^{19}$ - 6$times$10$^{19}$ cm$^{-3}$ at room temperature. More importantly, at low temperature (T = 200 K), the Seebeck coefficient was found to exceed 200 $mu$V/K for a doping range 1$times$10$^{19}$ - 3.5$times$10$^{19}$ cm$^{-3}$. Given that the lattice thermal conductivity in this phase has already been measured to be extremely low ($sim$ 0.7 W/m-K at 300 K),citep{r51} our results suggest the possibility of using $it{c}$-GST as a low-temperature thermoelectric material.
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