Ultrafast dynamics of phase change in Ge-Sb-Te materials studied
by coherent phonon spectroscopy
Muneaki Hase1, Kotaro Makino1, and Junji Tominaga2
12Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan.

Nanoelectronics Research institute, National Institute of Advanced Industrial Science and Technology,
Tsukuba Central 4, 1-1-1 Higashi, Tsukuba 305-8562, Japan.

ABSTRACT
There is a possibility of manipulating the rapid phase change in chalcogenide in ultrafast time scale
when using 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 theintriguing approaches. We demonstrate in Ge2Te2/Sb2Te3 superlattice that the phase change from
amorphous into crystalline states can be manipulated within only a few cycles of local lattice vibration(≈ 1 picosecond) by coherent excitation of the local lattice vibration using a pair of femtosecond laser
pulses. Furthermore, we introduce a possibility of the bond-selective excitation in Ge2Te2/Sb2Te3
superlattice by tuning the pump polarization of the single pump pulse. In this new approach, p-polarizedpump pulse is found to be more effective to induce the phase change, probably due to the atomic
arrangements along the sample normal.
Key words: coherent phonon, femtosecond, superlattice, nonthermal phase change.
1. INTRODUCTION
Multi-component chalcogenides, such as Ge-Sb-Te and Ag-In-Sb-Te, are potentially used in optical
data storage 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 phasechange materials. A question arising from the dynamics of the phase change in Ge2Sb2Te5 is how fast
the phase transformation between the amorphous and the crystalline phases occurs [1]. Motivated byunderstanding 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 firstprinciple simulations [2-4]. Coherent phonon spectroscopy (CPS) is a powerful tool to study theultrafast dynamics of phase transitions occurring on pico- and femtoseconds time scales (Fig. 1).

Figure 1. Ultrafast phenomena observed after the irradiation of femtosecond laser pulses.

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 alloyfilms, Först et al. applied CPS and found that the appearance of the phonon modes was significantly
modified upon structural change [8]. However, ultrafast control of coherent phonons and subsequent
phase change was not examined. Here, we have used a femtosecond pump-pulse pair to demonstrate
ultrafast switching of Ge2Te2/Sb2Te3 superlattice (SL) from the amorphous to the crystalline states. Inaddition, the bond-selective excitation in Ge2Te2/Sb2Te3 superlattice has been examined by tuning thepump polarization of the single pump pulse.
2. EXPERIMENTAL
We fabricated Ge2Te2/Sb2Te3 SL thin films,
which consist of the GeTe and the Sb2Te3
layers on silicon (100) substrate by using ahelicon-wave
RFmagnetron–sputteringmachine [9]. GST alloy films were also
deposited on silicon (100) substrate as areference sample. To measure time-resolved
reflectivity change ( ΔR R ) of the sample as afunction of the time delay (τ), 20 fs-width
optical pulses (λ = 850 nm) from a
Ti:sapphire laser was utilized. A pair of€pump-pulses was generated through aMichelson-type interferometer, in which thetime interval ( Δt ) of the double pump-pulses
was precisely controlled by moving a piezo
stage (Fig. 2). For the pump polarization Figure 2. Optical layout of the pump-probe spectroscopyexperiments, The incident angle of the pump with a Michelson interferometer.
€and probe beams onto the sample normal was35°, and the polarization angle of the pump beam was varied from 0° (s-polarization) to 180° (spolarization) via 90° (p-polarization), while that of the probe was fixed at 90°.
3. RESULTS & DISCUSSION
Figure 3(a) shows the time-resolved ΔR R signal observed in Ge2Te2/Sb2Te3 SL films withamorphous (as-grown) and crystalline (annealed) phases at room temperature. After the transient
electronic response due to the excitation of carriers (τ = 0), coherent phonon oscillations with a few
picoseconds relaxation time appear. Fourier transformed (FT) spectra are shown in Fig. 3(b), in which€Figure 3. (a) The ΔR R signal observed in amorphous (as-grown) and crystalline (annealed) SLfilms at 295 K. (b) FT spectra obtained from time-domain data in (a).
€two broad peaks are observed at 4.70 THz and 3.66 THz in amorphous film, while a sharp peak at 3.66
THz is observed in crystalline film. These peaks in the amorphous film can be considered to the A1
optical mode due to tetrahedral GeTe4 structure for the 3.66 THz peak [8,10], and the A1 optical modes
due to Sb2Te3 sub-lattice for the 4.70 THz peak [10]. Notice that the relaxation time of the coherent A1
mode is longer in the case of the crystalline film.

Figure 4. (a) The ΔR R signal observed in amorphous SL film with various time intervalsof the pump-pulse pair. (b) The FT spectra obtained from time-domain data in (a).

Figure 4(a)€presents the results of the double-pump-pulse excitation obtained from amorphous
Ge2Te2/Sb2Te3 SL film with the various time intervals Δt , while the fluences of the first and second
pump-pulse were fixed at 76 and 64 µJ/cm2, respectively. Most importantly, as shown in Fig. 4(b) when
the time interval Δt was scanned at the constant pump fluences, the frequency of the A1 mode due toGeTe4 structure, shifted from its original value ( ΩA = 3.83 THz) to that of the crystalline phase ( ΩC ≈€3.67 THz) as Δt changed from 141 to 276 fs. We attribute the frequency shift, from ΩA to ΩC , to thecrystallization.
The results suggest that the ultrafast phase change was induced by the coherent€
excitation of the local A1 mode. A possible mechanism of the phase change observed by the irradiation€€of €the femtosecond pump-pulse pairs is€€
the umbrella-flip of Ge atoms fromoctahedral (GeTe4) into tetrahedral(GeTe6) Ge-coordinations [11,12].
Here, we discuss why twopump-pulses are needed to induce thephase change in Ge2Te2/Sb2Te3 SL.
Recently, Kolobov et al. reported thatthe rapid phase change between theamorphous and crystalline phases wascharacterized by a flip-flop transition ofGe atoms, that is a displacement of Ge
atoms from tetrahedrally Te-coordinatedsite to octahedrally Te-coordinated site[11]. More recent molecular dynamics
simulations have suggested in theamorphous phase of GST that the Figure 5. Schematic of the potential energy surface during themajority of the atoms have tetrahedral pump-pulse pair excitation. EA represents the activation energy
atomic arrangement (GeTe4) with about when the phase change is promoted by thermal (incoherent)
process.

40–50 % occupations [2]. Hence, our results suggest that the pump-pulse pair selectively modulates thebonding of Ge-Te in the GeTe4 units in the amorphous state, making Ge atoms migrate into octahedral
position along the [111] direction, accompanying with surrounding (Sb and Te) atomic rearrangement(Fig. 5).
Finally, Fig. 6 shows the polarization
dependence of the frequency of the coherent A1
mode of the GeTe4 unit. We found that the ppolarized pump pulse (90°) induces the frequency
red-shift, while the s-polarized (0°) does not. Notethat the magnitude of the red-shift (≈ 0.1 THz) is
significantly comparable to the value in the caseof the double-pulse excitation (Fig. 4). We wouldinterpret the red-shift observed by the p-polarizedpump-pulse to the bond-selective excitation
along the sample normal because the Ge-Te bondin Ge2Te2/Sb2Te3 SL is polarized along [111]direction.
Figure 6. Polarization dependence of the FT intensity
4. CONCLUSION(closed triangles) and the frequency of the coherent
local A1 mode (closed circles).

We have observed ultrafast phase change from amorphous into crystalline phases in Ge2Te2/Sb2Te3 SL
films by the irradiation of a pair of femtosecond pump-pulses. In addition, when the polarization ofpump pulse was set to p-polarization, not only the increase in the phonon amplitude but also the redshift of the coherent phonon frequency were clearly observed. The result implies the displacement of
Ge atoms and surrounding Sb and Te atoms along [111] direction due to the anisotropic excitation of
the weaker bond with p-polarized pulses. Our demonstration of nonthermal ultra-high-speed atomic
rearrangements in the Ge2Te2/Sb2Te3 SL films, coupled with ultra-low fluence laser, will be highly
relevant for the next-generation of ultra-high-speed phase-change random access memory (PCRAM),
operated at THz frequency range.
ACKOWLEDGMENT
We thank potential contributions of Dr. A. V. Kolobov and Dr. P. Fons to this work. This work wassupported in part by KAKENHI-22340076 from MEXT, Japan.
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