I01(Invited)

Ultrafast dynamics and control of phase change in Ge-Sb-Te
materials by the excitation of coherent phonons
Muneaki Hase
1

Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan.

ABSTRACT
There is a possibility of manipulating the rapid phase change in chalcogenide in ultrafast time scale whenusing femtosecond laser pulses. Especially, coherent control of local vibrations, whose atomic motion plays
an important role in the rapid phase change, has been considered to be one of the intriguing approaches. We
demonstrate in Ge2Te2/Sb2Te3 superlattice that the phase change (or precursor of phase change) fromcrystalline into amorphous states can be observed within a few picoseconds by coherent phonon spectroscopy.
Non-thermal nature of the phase change in superlattice and conventional Ge2Sb2Te5 alloy will be discussed.
Key words: Coherent phonon, Femtosecond, Nonthermal phase change, GeSbTe.

1. INTRODUCTION
Multi-component chalcogenides, such as Ge-Sb-Te and Ag-In-Sb-Te, are potentially used in optical datastorage media in the forms of rewritable CDs, DVDs, and Blu-ray discs. Especially, Ge2Sb2Te5 (GST)
has proven to be one of the highest-performance alloys among commercially available phase-changematerials. A question arising from the dynamics of the phase change in Ge2Sb2Te5 is how fast the phasetransformation between the amorphous and the crystalline phases occurs [1]. Motivated by understanding
the mechanism of the rapid phase change, extensive investigations on GST have been carried out using
electrical and optical measurements, and the ab initio molecular dynamics and first principle simulations[2-4].
Coherent phonon spectroscopy (CPS) is a powerful tool to study the ultrafast dynamics of phasetransitions occurring on pico- and femtoseconds time scales. In the CPS, a pump pulse impulsively
generates coherent lattice motion through impulsive stimulated Raman scattering [5-7]. To investigate
the dynamics of the phase transition in GST alloy films, Först et al. applied CPS and found that theappearance of the phonon modes was significantly modified upon structural change [8]. We have so far
used a weak femtosecond pump-pulse pair (< 100 µJ/cm2) to demonstrate ultrafast switching ofGe2Te2/Sb2Te3 superlattice (SL) from the amorphous to crystalline states [9]. In addition, bond-selective
excitation in Ge2Te2/Sb2Te3 superlattice has been examined by tuning the pump polarization of the single
pump pulse with the similar weak fluence [10]. However, little is known by coherent phonon
spectroscopy about the ultrafast process from the crystalline to amorphous states because one canimagine the potential in the crystalline states is more deep and stable than the amorphous [11] andtherefore at least several mJ/cm2 fluences will be needed to induce such reverse process.
Here we would discuss a possibility of observation of a metastable state in photoexcited
crystalline GST-SL and coherent control of the phase change on the bases of coherent phonon excitation
with more than 10 mJ/cm2 fluences.
2. EXPERIMENTAL
The sample used were Ge2Sb2Te5 (GST) alloy film and Ge2Te2/Sb2Te3 SL thin films, which consist of theGeTe and the Sb2Te3 layers on silicon (100) substrate by grown by using a helicon-wave RF magnetron–
sputtering machine [9,12]. GST alloy films were also deposited on silicon (100) substrate as a reference
sample. To measure time-resolved reflectivity change ( ΔR R ) of the sample as a function of the timedelay (τ), 40 fs-width optical pulses (λ = 800 nm) from a Ti:sapphire regenerative amplifier system wereutilized. A pair of the pump-pulses was generated through a Michelson-type interferometer, in which theseparation time ( Δt ) of the double pump-pulses was precisely controlled by moving one arm of the€mirror.

€3. RESULTS & DISCUSSIONFigure 1. The ΔR R signal observed in crystalline GST-SL film at three different separation times(∆t) of the double pump-pulse.
€Figure 1 shows
the time-resolved ΔR R signal observed at room temperature in Ge2Te2/Sb2Te3 SL
films in the crystalline (annealed) phase. After the transient electronic response due to the excitation ofcarriers (τ = 0), coherent phonon oscillations with a few picoseconds relaxation time appear in the case of
the single pump pulse excitation. When the second pump pulse is applied, the relaxation time of the€ changes depending on the separation time (∆t). Fourier transformed (FT)
coherent phonon significantly
spectra taken in crystalline state with single pump excitation (15 mJ/cm2) exhibit one sharp peak at ≈ 3.5THz, which slightly red-shifts from the peak position under lower photo-excitation [9]. Most importantly,
the FT spectra taken only from the time-domain data (∆t = 290 fs) after the 2nd pump arrival explore thedouble peak structure peaking at ≈ 2.7 THz and ≈ 3.6 THz. The new peak at 2.7 THz is not due to theappearance of the amorphous phase, but would be a new local structure or bond softening being observed
under high density photo-excitation. Strong lattice anharmonicity in the crystalline phase has been
confirmed by thermal properties studies [11]. The main peak at 3.5 THz blue-shifts by 0.1 THz. Thisblue-shift may imply the amorphous phase appear under the strong photo-excitation. The separation timeof ∆t = 290 fs (3.4 THz) nearly corresponds to the time period of the coherent A1 mode in the crystalline
phase, so the excitation by such second pump pulse excite the lattice coherently. This case can be
explained by the umbrella flip model of the phase change [13], where the local atomic arrangement
accompanying phonon motion is crucial to switching the phase.

4. SUMMARY
We have observed a possibility of ultrafast phase change from crystalline into amorphous phases in
Ge2Te2/Sb2Te3 SL films by the irradiation of a pair of femtosecond pump-pulses. To confirm suchultrafast phase change, time-resolved X-ray diffraction measurements have been carried out at SACLA in
SPring-8 using 10-fs XFEL, although such measurements are still underway.

ACKOWLEDGMENT
I thank potential contributions of Dr. Junji Tominaga, who fabricated the superlattice samples, and Dr. A.
V. Kolobov and Dr. P. Fons for their theoretical and experimental collaborations to this work. This work
was supported in part by the X-ray Free Electron Laser Priority Strategy Program (MEXT).
REFERENCES
1. J. Hegedüs and S. R. Elliott, Nature Mat. Vol. 7, 399 (2008).
2. M. Konishi, H. Santo, Y. Hongo, K. Tajima, M. Hosoi, and T. Saiki, Applied Optics, Vol. 49, 3470 (2010).
3. J. Akola and R. O. Jones, Phys. Rev. B. Vol. 76, 235201 (2007).
4. W. Welnic, A. Pamungkas, R. Detemple, C. Steimer, S. Blügel, and M. Wuttig, Nature Mat. 5, 56 (2006).
5. T. P. Dougherty, G. P. Wiederrecht, K. A. Nelson, M. H. Garrett, H. P. Jensen, and C. Warde, Science Vol.
258, 770 (1992).
6. G. A. Garrett, T. F. Albrecht, J. F. Whitaker, and R. Merlin, Phys. Rev. Lett. Vol. 77, 3661 (1996).
7. M. Hase, M. Kitajima, S. Nakashima, and K. Mizoguchi, Phys. Rev. Lett. Vol. 88, 067401 (2002).
8. M. Först, T. Dekorsy, C. Trappe, M. Laurenzis, H. Kurz, and B Bèchevet. Appl. Phys. Lett. Vol. 77, 1964
(2000).
9. K. Makino, J. Tominaga, and M. Hase, Optics Express Vol. 19, 1260 (2011).
10. K. Makino, J. Tominaga, A. V. Kolobov, P. Fons, and M. Hase, Appl. Phys. Lett. Vol. 101, 232101 (2012).
11. T. Mastunaga, N. Yamada, R. Kojima, S. Shamoto, M. Sato, H. Tanida, T. Uruga, S. Kohara, M. Tanaka,P. Zalden, G. Bruns, I. Sergueev, H. Christian, R. P. Hermann, and M. Wuttig, Adv. Func. Mater. Vol. 21,2232 (2011).
12. J. Tominaga, P. Fons, A. V. Kolobov, T. Shima, T. W. Chong, R. Zhao, H. K. Lee, and L. Shi, Jpn. J.
Appl. Phys. Vol. 47, 5763 (2008).
13. A. V. Kolobov, P. Fons, A. I. Frenkel, A. L. Ankudinov, J. Tominaga, and T. Uruga, Nature Mat. Vol. 3,703 (2004).