Ultrafast dynamics of coherent phonons in phase change materials:
Evaluation of thermal conductivity
Muneaki Hase1 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, JapanABSTRACT
We present the evaluation of lattice thermal conductivity of GeTe/Sb2Te3 superlattice (SL) by using acoherent phonon spectroscopy. The time-resolved pump-probe reflectivity measurement was performed
at various lattice temperatures. The transient reflectivity obtained in amorphous and crystalline
GeTe/Sb2Te3 SL films exhibits the coherent A1 (optical) modes at terahertz (THz) frequencies, whosedephasing time is picosecond range. The relaxation time and frequency of the coherent A1 modes areused to compute the lattice thermal conductivity based on the Debye theory, including scattering by
grain boundary and point defect, umklapp process, and phonon resonant scattering. The results indicatethat the thermal conductivity in the amorphous SL film is less temperature dependent, due to thedominant phonon-defect scattering, while in the crystalline SL it is temperature dependent because of
the main contributions from umklapp and phonon resonant scatterings.
Keywords: Coherent phonon, phase change, thermal conductivity, superlattice

1. Introduction
There is a possibility of manipulating the rapid phase change in chalcogenides in ultrafast time spans if
one uses ultrashort laser pulses, whose pulse duration is less than the time period of lattice vibrations,being typically hundreds of femtosecond. This idea is based on the coherent control of local lattice
vibrations, whose atomic motions play an important role in the rapid phase change in chalcogenide
films [1].
One of the advantages of using Ge2Sb2Te5 (GST) films as the optical recording media is its highspeed switching characteristics, whose time scale is less than 1 nanosecond [2, 3]. In the last decade,however, most of the literatures have studied nanosecond dynamics of the phase change in GST
materials using nanosecond and picosecond laser (or electrical) pulses [4]. Hence thermal properties ofGST materials have been found to govern the phase change characteristics in GST materials when it ispromoted by laser heating. There, thermal conductivity is important to engineer the performance of thephase change[5], such that lower thermal conductivity enables one to realize low power operation of theswitching, where focused laser irradiation causes lattice heating. In the present manuscript, weinvestigate the thermal conductivity of GeTe/Sb2Te3 SL using coherent phonon spectroscopy.

2. Experimental techniques
Coherent phonon spectroscopy (CPS) [6] has recently been applied to GST materials of alloy andsuperlatticed films [7, 8]. In those study, the observed local phonon modes in the amorphous GST films
were found to be strongly damped modes, with those relaxation time of less than a few picoseconds dueto the scattering by lattice defects [8]. Wang et al. have recently proposed to use CPS as a powerful
method to obtain lattice thermal conductivity, where the relaxation time and frequency of the coherent
optical modes were used to compute the conductivity based on the Debye theory [9]. In the presentstudy, a reflection-type pump-probe measurements using a mode-locked Ti:sapphire laser (pulse width= 20 fs and a central wavelength = 850 nm) was employed at the temperature range of 5 – 300 K. The
average power of the pump and probe beams were fixed at 120 and 3 mW, respectively. The excitation

of the GeTe/Sb2Te3 SL films with the 850 nm (= 1.46 eV) laser pulse generates photo-carriers acrossthe narrow band gap of ≈ 0.5 - 0.7 eV. The transient reflectivity (TR) change (ΔR/R) was measured as afunction of the time delay between the pump and probe pulses (Fig. 1).

Fig. 1. Optical layout of the pump-probe reflectivity measurement.

We have chosen GeTe/Sb2Te3 SL as a sample after the proposal of a class of superlattice-like
GeTe/Sb2Te3 [10]. Significantly lower SET and RESET programming current for the SL cells has been
discovered [10, 11] and thus GeTe/Sb2Te3 SL will be a potential candidate for the future PRAM devises.
The samples used in the present study were thin films of [(Ge2Te2)1/(Sb2Te3)1]20 and SLs fabricatedusing a helicon-wave RF magnetron sputtering machine on Si (100) substrate. The annealing of the asgrown SL films at 503 K (230 ºC) for ten minutes changed the SL films from amorphous (a-) intocrystalline (c-) states.

3. Experimental results
Figures 2(a) and (b) show the ΔR/R signal observed at 5 and 300 K in [(Ge2Te2)1/(Sb2Te3)1]20 SL films
with the amorphous and crystalline phases, respectively. After the transient electronic response due tothe excitation of nonequilibrium carriers at the time delay zero, coherent oscillations with several
picoseconds relaxation time appear. As shown in (c) and (d), Fourier transformed (FT) spectra are
obtained from the time-domain data, in which two broad peaks are observed at ≈ 5.1 THz and ≈ 3.78
THz in amorphous film, while a sharp peak at 3.68 THz is observed in crystalline film at 300 K [8].
These peaks in the amorphous film were assigned to the A1 optical modes due to the local GeTe4 unit
(3.78 THz peak) [7, 8], and that due to the local pyramidal SbTe3 unit (5.1 THz peak) [8]. The red shiftof the local A1 mode frequency in the crystalline phase (3.78 THz → 3.68 THz) has been attributed to
the local structural change from tetrahedral GeTe4 into octahedral GeTe6 species [8].
To investigate the effect of the scattering by these local phonon modes on lattice thermalconductivity, the parameters of the coherent A1 mode (the€ frequency and the decay rate) are used tocompute the lattice thermal conductivity based on the Debye theory, combined with the resonant
scattering model [12]. Lattice thermal conductivity is expressed as,κ (T ) =31k ⎛ k T ⎞
CV υ 2τ c = B2 ⎜ B ⎟
32 π υ ⎝  ⎠ΘD T

∫0x 4ex()τ c−1 e x −1dx .

(1)where x = ω kBT , CV is the lattice specific heat, υ the sound velocity, Θ D the Debye temperature,ω the€phonon €frequency, τ c the acoustic phonon relaxation time, whose inverse can be given bycontributions from various €scattering mechanisms [12, 13]:€ €€€€€τ c−1 =υ+ Aω 4 + Bω 2Te −Θ D 3T +LCω 22.

(2)(Ω − ω )22where L, A, B, and C characterize grain boundary, phonon-defect scattering, phonon-phonon umklapp€ phonon resonant scattering, respectively. Ω is the optical phonon frequency observed
scattering, andin the CPS and the€last term in Eq. (2) represents the resonant scattering between the localized opticalmodes and acoustic phonon modes. From the low temperature limit of the decay rate of the coherent A1
modes (0.253 ps-1 for a-[(Ge2Te2)1/(Sb2Te3)1]20 €and 0.026 ps-1 for c-[(Ge2Te2)1/(Sb2Te3)1]20), we canestimate the ratio of the phonon-defect scattering rate in the amorphous to the crystalline Aa/Ac to be ≈10 for the [(Ge2Te2)1/(Sb2Te3)1]20 SL film. The same ratio of Ba/Bc = Ca/Cc = 10 has been applied in thesimulation. We take the resonant phonon frequency ( Ω ) at 300 K from the FT spectra [8] obtained by
the time domain data in Fig. 2. The magnitudes of all the parameters (L, A, B, and C) are determined togive the experimental value of κ for a-[(Ge2Te2)1/(Sb2Te3)1]20 SL (κ ≈ 0.33 Wm-1K-1 at 300 K) [14].
€Fig. 2. (a) Transient reflectivity trace for a–[(Ge2Te2)1/(Sb2Te3)1]20 film recorded at 120 mW(286 µJ/cm2). (b) Transient reflectivity trace for c–[(Ge2Te2)1/(Sb2Te3)1]20 film with the same
fluence. (c) and (d) are the corresponding Fourier transformed spectra.

4. Conclusion
In conclusion, our results on ultrafast coherent phonon spectroscopy have illustrated temperaturedependence of lattice thermal conductivity in GeTe/Sb2Te3 SL films. These data show that the Debye
model, including scatterings by grain boundary and point defect, umklapp process, and phonon resonant
scattering, well reproduces the experimental value of thermal conductivity measured by using thermoreflectance. The thermal conductivity in the a–SL films is less temperature dependent, due to thedominant phonon-defect scattering, while in the c–SL films it is strongly temperature dependent
because of the main contributions from umklapp and phonon resonant scattering.

Fig. 3. Lattice thermal conductivity calculated by the Debye theory. We found that the thermal
conductivity in a-GeTe/Sb2Te3 SL film is less temperature dependent.

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