Photoinduced Ultrafast Transition of the Local Correlated Structure in Chalcogenide Phase-Change Materials

Revealing the bonding and time-evolving atomic dynamics in functional materials with complex lattice structures can update the fundamental knowledge on rich physics therein, and also help to manipulate the material properties as desired. As the most prototypical chalcogenide phase change material, ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ has been widely used in optical data storage and nonvolatile electric memory due to the fast switching speed and the low energy consumption. However, the basic understanding of the structural dynamics on the atomic scale is still not clear. Using femtosecond electron diffraction, structure factor calculation, and time-dependent density-functional theory molecular dynamic simulation, we reveal the photoinduced ultrafast transition of the local correlated structure in the averaged rocksalt phase of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. The randomly oriented Peierls distortion among unit cells in the averaged rocksalt phase of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ is termed as local correlated structures. The ultrafast suppression of the local Peierls distortions in the individual unit cell gives rise to a local structure change from the rhombohedral to the cubic geometry within $\sim 0.3\mathrm{ps}$. In addition, the impact of the carrier relaxation and the large number of vacancies to the ultrafast structural response is quantified and discussed. Our Letter provides new microscopic insights into contributions of the local correlated structure to the transient structural and optical responses in phase change materials. Moreover, we stress the significance of femtosecond electron diffraction in revealing the local correlated structure in the subunit cell and the link between the local correlated structure and physical properties in functional materials with complex microstructures.

Figure 1

The crystalline structure and the transient structural dynamics of GST-225. (a) Top: a supercell structure of the rocksalt phase GST-225. The length of three axes of single unit cell is 6.01 Å. Bottom: a schematic illustration of randomly oriented Peierls distortions with longer ($\sim 3.15\AA$) and shorter ($\sim 2.85\AA$) Ge(Sb)—Te bonds in the subunit cell. The gray arrows indicate the orientation of the adjacent local rhombohedral structures. (b) Top: the transmission diffraction pattern and the radially averaged intensity distribution (background subtracted) at a negative delay. The inset displays a better separation of (111) and (200) reflections by controlling the current of the magnetic lens positioned after the sample. Bottom: the change in diffraction intensity from $-0.2$ to 60 ps, by subtracting the intensity distribution at negative delay with 2300 nm and $4.6\mathrm{mJ}/{\mathrm{cm}}^2$ laser pump. (c),(d) The temporal evolution of the normalized Bragg reflection intensities with 2300 nm ($4.6\mathrm{mJ}/{\mathrm{cm}}^2$) and 500 nm ($0.61\mathrm{mJ}/{\mathrm{cm}}^2$) laser pump. The solid lines are the fits with a monoexponential function.

Figure 2

(a) and (b) with 2300 nm laser pump. Left: normalized intensity change of the (111) reflection as a function of the time delay at room temperature and 112 K, respectively. The solid line is the fit with a monoexponential function. Right: overall change in diffraction intensity from negative to positive delays. (c) The 2D square nets indicate the suppression of the local distortions with random orientation among unit cells. The $\mathrm{Ge}/\mathrm{Sb}$ atoms (purple balls) move toward the high symmetry position (the corresponding movement of the Te atom is not shown for simplification). In the subunit cell, the sketch of the atomic displacement along the [111] direction and the corresponding potential energy surface change are shown. (d) Time constants of the intensity decay of the (111) reflection with different pump fluences for 500 and 2300 nm laser excitation at room temperature and 112 K. The error bars correspond to 68.3% confidence intervals of the fit. The measurements carried out at 112 K are marked with green balls.

Figure 3

Calculated and experimental anisotropic intensity change associated with the suppression of the local rhombohedral structure. (a) By structure factor calculation, the relative intensity change of Bragg reflections ($|\mathrm{F}{|}^2$) with (top) only the offset displacement of Ge atom and (bottom) the opposite displacement of $\mathrm{Ge}/\mathrm{Sb}$ and Te atom along the [111] direction. The zero displacement position indicates the site in an ideal rocksalt phase. The offset displacement of atoms along the [111] direction represents the local rhombohedral structure in individual unit cell. (b) The measured anisotropic intensity change within 0.3 ps, i.e., the larger intensity change of the (111) reflection than that of (200) and (220) reflection. (c) Calculated intensity change with the displacement of Ge from $-0.15$ to 0 Å, Sb from $-0.08$ to 0 Å, Te from 0.08 to 0 Å, and the vibrations of Te atoms (${\mu }_{\mathrm{Te}}\sim 0$ to 0.2 Å).

Figure 4

Simulation results of bond length in linear triatomic bonding geometry in the supercell of GST-225 before and after the electronic excitation. (a) Left: the bond length distribution within the supercell before laser excitation. The tilted distribution indicates a shorter and a longer bond in a linear triatomic bonding geometry. Right: the bond length distribution at 0.84 ps (integration from 0.72 ps to 0.96 ps) after 6% electronic excitation. (b),(c) The temporal evolution of the ratio between the length of line $a$ and line $b$ [indicated in (a)] with 1% and 6% electronic excitation, respectively. The decrease of the ratio, i.e., the ellipticity, indicates the suppression of the bond length discrepancy in a linear triatomic bonding geometry.