Femtosecond photo-induced displacive phase transition in Sb$_{2}$Te (group 2) phase-change material

TL;DR

This work addresses whether group 2 PCMs, exemplified by Sb$_2$Te near Sb$_{70}$Te$_{30}$, can undergo ultrafast, non-thermal RESET. The authors combine ultrafast electron diffraction and femtosecond-resolved sum-frequency generation to track lattice and carrier dynamics after femtosecond optical excitation of crystallized Sb$_2$Te. They observe a non-thermal displacive phase transition in which a coherent Sb displacement along $[111]$ reduces the Peierls distortion on a timescale of $\approx 300~\mathrm{fs}$, followed by electron–phonon equilibration on $\approx 2~\mathrm{ps}$ and Debye–Waller-dominated cooling. The process scales linearly with pump fluence, implying a one-photon mechanism and a threshold near $\approx 20~\mathrm{mJ/cm^2}$ for amorphization, pointing to ultrafast, energy-efficient RESET and potential THz-switching in group 2 PCMs. Together, these results establish non-thermal, displacive lattice dynamics as a key pathway for amorphization in Sb$_2$Te and motivate further ultrafast studies of Sb–Te PCMs for fast, low-energy memory applications.

Abstract

Two classes of Phase Change Materials (PCMs) have emerged as the best candidates for applications requiring the fast reading and writing of data: GeTe-Sb$_{2}$Te$_{3}$ pseudobinary alloys (group 1) and doped Sb-Te compounds near the eutectic composition Sb$_{70}$Te$_{30}$ (group 2). Both material classes undergo reversible switching between a low-resistance opaque crystalline phase and a high-resistance but less absorbing amorphous phase through heating, electrical, or optical pulses, achieving (sub-)nanosecond switching speeds. While group 1 compounds are employed in current generation devices and relatively well studied, model systems in group 2 compounds have been found to crystallize more rapidly and thus offer the perspective of improved devices. Despite their superior crystallization speed (SET process), to this point there have been no ultrafast experimental studies on crystallized PCMs of group 2 for the RESET process. Here we perform ultrafast electron diffraction and femtosecond resolved sum frequency non-linear spectroscopy on Peierls distorted Sb$_{2}$Te crystallized thin films (PCM of group 2) following femtosecond optical pulse irradiation. We observe a pump-induced structural change on two distinct timescales: responses with characteristic timescales of $\approx$ 300 fs and 2~ps. We quantified the experimental result by a coherent displacement and the Debye-Waller effect. In particular, the $\approx$ 300 fs UED signal results from the ultrafast release of the Peierls distortion through non-thermal coherent Sb displacement, while the 2~ps response reflects electron-lattice equilibrium. These results reveal the ultrafast non-thermal structural dynamics of Sb$_{2}$Te and suggest energy-efficient switching of group 2 PCMs should be possible on femtosecond time scales.

Figures (4)

• Figure 1: (a) Schematic drawing of the experimental setup for ultrafast electron diffraction on Sb$_{2}$Te. (b) The radial average of the Sb$_{2}$Te ultrafast electron diffraction experimental raw difference results $I_{\text{diff}}$ (top) and the results after subtracting thermal diffuse scattering (TDS) background (bottom) as a function of pump-probe delay time. ($I_{\text{diff}} = I_{\text{pump}}-I_{\text{probe}}$, where $I_{\text{pump}}$ is the total scattering intensity of Sb$_{2}$Te under pump pulse excitation, $I_\text{probe}$ is the total scattering intensity of Sb$_{2}$Te without pump pulse excitation). The pump fluence was set to 1.5 mJ/cm$^2$. (c) The radially averaged Sb$_{2}$Te static electron diffraction result and its global fitted result for subtracting the background (top panel). The background-subtracted radially averaged Sb$_{2}$Te static electron diffraction and indexing of observed peaks (middle panel). The thermal diffuse scattering background subtracted experimental difference results with UV pump pulse 0.6 ps and 10 ps earlier than femtosecond electron probe pulse (bottom panel). (d) Bragg peak relative intensities as a function of pump-probe delay time from -4 ps to 15 ps. See texts for details.
• Figure 2: (a) The structure factor calculated relative intensity change of observed Bragg reflections by displacing the Sb (which is located in the center of the distorted face-centered cubic lattice unit cell) along the [111] direction (to a less distorted cubic lattice symmetry). (b) Comparison of the experimental and calculated results of the Bragg peak relative intensities change as a function of pump-probe delay time from -4 ps to 15 ps. (c) The derived coherent atomic displacement and pump-probe sum frequency generation spectrum intensity as a function of pump-probe delay time (top). The derived atomic mean square displacement $\langle u^{2}\rangle$ and thermal diffuse scattering relative intensity as a function of pump-probe delay time (bottom). (d) The scheme of the pump-probe sum frequency generation experiments. See texts for details.
• Figure 3: (a) The radially averaged experimental difference diffraction data under different pump fluence excitation with a fixed pump-probe delay time at 0.1 ps. (b) The radially averaged experimental difference data under different pump fluence excitation with a fixed pump-probe delay time at 10 ps. (c) The derived coherent atomic displacement amplitude (blue) and atomic mean square displacement (red) at a fixed delay time of 0.1 ps and 10 ps as a function of pump laser fluence. See texts for details.
• Figure 4: The scheme of the observed ultrafast photo-induced displacive phase transition in a corresponding potential energy change diagram. ① shows the electronic excitation, which flattens the double well potential potential energy surface. The distorted Sb atom changes from the minimum position of the ground state potential energy well to the cliff of the flattened excited state potential energy surface. ② shows the distorted Sb atom moves to the bottom of the flattened energy well due to the restoring force. The displacement of the distorted Sb atom to a less distorted (more centered) position happens in a time scale of 300 fs. ③ shows the electron-phonon equilibrium process, which happens in a time scale of 2 ps. ④ represents the complete nonradiative relaxation of the excited state coordinates back to ground states. See text for details.