Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons

Multicomponent chalcogenides, such as quasi-binary GeTe–Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs. Ge2Sb2Te5 (GST) in particular has proven to be one of the best-performing materials, whose reliability allows more than 106 write–erase cycles. Despite these industrial applications, the fundamental kinetics of rapid phase change in GST remain controversial, and active debate continues over the ultimate speed limit. Here we explore ultrafast structural transformation in a photoexcited GST superlattice, where GeTe and Sb2Te3 are spatially separated, using coherent phonon spectroscopy with pump–pump–probe sequences. By analysing the coherent phonon spectra in different time regions, complex structural dynamics upon excitation are observed in the GST superlattice (but not in GST alloys), which can be described as the mixing of Ge sites from two different coordination environments. Our results suggest the possible applicability of GST superlattices for ultrafast switching devices.

pulse. The insets represent the FT spectra obtained before and after irradiation by the different fluences, recorded using the single P2 pulse at 3.2 mJ cm -2 . The FT spectrum presented by the dashed line on the top inset was obtained from a reflectivity signal with P2 = 10.6 mJ cm -2 , meaning it reflects the state during the irreversible phase change. The peak frequency presented by red text is obtained by the FT spectra after irradiation by the P2 pulse. The peak position of the optical mode, involving covalently bonded Ge atoms, is found to be irreversible when P2 ≥ 8.0 mJ cm -2 . (b) The same as (a), but obtained by using irradiation by a two-pulse sequence with ∆t = 270 fs for a total pump fluence ranging from 8.0 to 10.9 mJ cm -2 (a constant ratio of P1:P2 = 10:6.5 was maintained). The FT spectrum presented by the dashed line was obtained from the reflectivity signal just after the second pump pulse with Ftotal = 10.9 mJ cm -2 , meaning it reflects the state during irreversible phase change. The peak frequency presented by red text is obtained by the FT spectra after the irradiation by the double pump-pulse sequence. The peak position of the optical mode was found to be irreversible when Ftotal ≥ 9.5 mJ cm -2 . Note that the FT spectra "during" the irreversible phase change exhibit different peak positions and intensities as shown in (a) and (b), implying that the pre-phase transformation state from the RESET phase is different for the case of a single pulse and a double pump-pulse sequence.

Supplementary Figure 4. Comparison of reflectivity change (carrier) responses for the SET and RESET phases of iPCM and GST alloy films obtained before and after irradiation by a P1
pulse. P1 = 10.6 mJ cm -2 and P2 = 6.9 mJ cm -2 was maintained for all samples of the (a) SET phase of iPCM, (b) RESET phase of iPCM, (c) poly-crystalline (fcc) GST alloy, and (d) amorphous GST alloy. In the RESET phase of the iPCM, (b), there was a difference between the reflectivity response before and after irradiation by the P1 pulse. The frequency of the optical mode was not changed before and after the quadruple pulse excitation, staying at 3.55 THz. The FT spectra presented by dashed lines were obtained from the reflectivity signal during the quadruple pulse excitation. They exhibit emergence of the double peak spectra and blue-shift of the optical mode at Ftotal = 13.2 mJ cm -2 (P1 = 4.0 mJ cm -2 , P2 = 2.6 mJ cm -2 , P3 = 4.0 mJ cm -2 , and P4 = 2.6 mJ cm -2 ) but the final state is still the same as that before the irradiation, meaning a reversible pre-phase transformation took place.

Supplementary Note 1.
Transient reflectivity studies of the RESET phase of iPCM. In contrast to the reversible dynamics observed in the SET phase, the phase transformation from the RESET phase of the iPCM was irreversible, as reported in Supplementary Fig. 2. Given the fact that this is an irreversible process, the sample has most probably been transformed following the first pair of pulses. Therefore, the initial state may be actually characterized by the top spectra in Supplementary Fig. 2d if one could observe the phonon spectra with a single shot, and not the bottom one in Supplementary Fig. 2b. From the spectra and systematics one could speculate that this state is actually a SET state, however according to Supplementary Fig. 4 and the related discussion this is not the case. If the phase transformation by the first pair of pulses was indeed the case, this would mean that multiple pump-pulse sequences could be used to switch iPCM from a new metastable structure, whose frequency is similar to the case of the SET phase ( Supplementary Fig.   2d), after the first pair of pump-pulses, to the transient double-peak structure ( Supplementary Fig. 2c) and back again to the metastable state similar to the SET phase ( Supplementary Fig. 2d), within a single cycle of the pump-pump-probe sequence.
Fluence dependence in the RESET phase of iPCM. Supplementary Fig. 3 demonstrates the experimental results to check if only single pulse excitation can induce the irreversible phase change in the RESET phase of iPCM. It was found that a single pulse with a fluence higher than 8.0 mJ cm -2 could switch the RESET phase to a metastable state similar to the SET phase ( Supplementary Fig. 3a). On the other hand, for the double pump-pulse sequence with a total fluence of ≥ 9.5 mJ cm -2 one can also switch the RESET phase to the same final state (Supplementary Fig. 3b). Thus, both the single and twopump sequence can switch the RESET phase with similar total fluences.
The technique presented here can only be used to study reversible processes, and therefore to explore the precise pathway of the irreversible phenomena from the RESET phase of iPCM, a single-shot experiment taken for example by a technique such as X-ray diffraction or X-ray absorption will be required in the future.

Supplementary Note 2.
Reflectivity carrier response in different phases. To explore if the structural change observed is reversible, we have measured the reflectivity response for the amorphous and crystalline phases of a GST alloy film, and the SET and RESET phases of a iPCM film, before and after irradiation by a strong P1 pulse for Δt = 290 fs (for the SET phases) or 270 fs (for the RESET phases) as shown in Supplementary Fig. 4. The two different phases of the iPCM and GST alloy films exhibited very different carrier response. For the case of the response from the RESET phase of iPCM after irradiation by the P1 pulse ( Supplementary Fig. 4b), a significantly different response from that obtained before the irradiation was observed, while in other cases the responses exhibit a reversible nature as demonstrated in Supplementary Figs. 4a, 4c, and 4d. The small but significant deviation of the response in the RESET phase of the iPCM from that before the P1 pulse irradiation implies that the superlattice structure may be preserved with small local atomic rearrangements during irradiation, although a more independent analysis of that state is required. Thus, the carrier responses shown in Supplementary Fig. 4 suggest the metastable state after irradiation by the P1 pulse to the RESET phase of iPCM is different from that of the so-called laser-crystallized (LC) structure [2] , which should show much different carrier response from the original phase.

Supplementary Note 3.
Transient reflectivity studies of the poly-crystalline (SET phase) GST alloy. To compare the ΔR/R signal observed in iPCM with that occurring in conventional GST alloy films, Supplementary Fig. 5a presents the transient reflectivity detected in a thin film (30 nm) of the poly-crystalline (fcc) phase of the GST alloy after irradiation with different Δt with slightly higher pump fluences. The coherent phonon oscillation exhibited a shorter lifetime even without prepump-pulse (P1) excitation (see the bottom curve), and a further strong damping of the phonon oscillation was observed when the P1 pulse was applied for different Δt. For the case of the poly-crystalline (SET phase) GST alloy, phonon frequency blue-shifting was not observed in the excited state, but broadening and redshift (phonon softening) of the main peak (3.1 THz in Supplementary Fig. 5b) down to 2.7 THz was observed at Δt = 290 fs ( Supplementary Fig. 5c). This phonon softening relaxes within a few picoseconds, followed by the full recovery of the original crystalline phase ( Supplementary Fig. 5d). The broadening of the 3.1 THz peak in the excited state suggests transient disordering of the crystal lattice [3] and interactions with phonon modes associated with lattice defects or grain boundaries [4] leading to strong anharmonicity in the SET phase of the GST alloy [5] , which results in faster phonon damping. From the fact that in the poly-crystalline GST alloy no frequency blue-shifting of the optical phonon was