Ultrafast Dephasing of Coherent Optical Phonons in Atomically Controlled GeTe/Sb2Te3 Superlattices
 
One of the most common materials for optical recording media is Ge2Sb2Te5 (GST), in which phase transition between crystalline and amorphous phases serve rewritable recording. Recently, extensive theoretical investigation on the mechanism of the phase change in GST have been made using molecular dynamics simulations. In addition, experimental studies using extended x-ray absorption fine structure (XAFS) and Raman scattering measurements have examined dynamics of phase transition in GST, suggesting that the structure of amorphous GST can be described as a cross-section of a distorted rocksalt structure with vacancies and the amorphization of GST is due to an umbrella flip of Ge atoms from an octahedral position into a tetrahedral one.

One of the advantages of GST as the optical recording media is its high speed switching of read-write characteristics, whose time scale has been believed to be less than a nanosecond. In order to understand and to control the rapid phase change in GST, a time-resolved study of phonon dynamics in GST is strongly demanded, however, the time-resolved study is still very few. Moreover, a new class of semiconductor superlattices (GeTe/Sb2Te3) with three different states have recently been proposed, which will enable us to realize reversible transition among the three states by means of the irradiation of laser pulses

Here, we have studied ultrafast dynamics of coherent optical phonons in GeTe/Sb2Te3 SLs to show the damping of the coherent A1 mode is temperature dependent in crystalline, while that in the amorphous phase does not. These facts can be understood in terms of phonon anharmonic decay in the crystalline phase, but phonon-defect (vacancy) scattering in the amorphous phase. Thus the existence of disordered vacancies (or voids) is evident in amorphous phase, while the vacancies (or voids) in crystalline phase are highly ordered. The frequency shift of the A1 mode observed in the amorphous phase relative to the crystalline phase is suggestive to the local structural change of GeTe4 into GeTe6. The disordering of the vacancies plays dominant role in the volume expansion in amorphous GST film, resulting in the frequency red-shift relative to the GeTe/Sb2Te3 SLs. We believe that the present study has uncovered the vivid information on the arrangement of the vacancies as well as ultrafast dephasing dynamics of lattice vibrations in GeTe/Sb2Te3 SLs toward the application of laser induced optical switching using this unique materials. This method can be applied to all the other Ge-Sb-Te systems to understand fundamental lattice dynamics.

*Collaboration with Dr. J. Tominaga.
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Fig.1 The transient reflectivity signal observed in (a) amorphous and in (b) crystalline GeTe/2Te3 SLs at various temperatures.
PRB 79, 174112 (2009).
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Fig. 2. FT spectra obtained from the time-domain data in Fig. 2; (a) amorphous and (b) crystalline GeTe/Sb2Te3 SLs at various temperatures. The solid lines are the fit to the data with Lorentz functions. The inset in (b) represents magnified FT spectra at 5 K, in which two sharp peaks (*) and a broad peak (**) are detected.
PRB 79, 174112 (2009).