Evolution of low-frequency vibrational modes in ultrathin GeSbTe films

Phase change materials (PCM) are compounds employed in non-volatile random-access memory and rewriteable optical storage media thanks to the rapid and reversible transformation between the amorphous and crystalline states. GeSbTe alloys are the most studied PCMs and consist of lamellae separated by van der Waals gaps in the ordered crystalline phase. Here we report a methodology for the study of the two-dimensional (2D) character of epitaxial GeSbTe by looking at the weak inter-lamellae interactions. Our unique approach is the use of molecular beam epitaxy for the controlled synthesis of ultrathin films combined with Raman spectroscopy for the investigation of the PCM properties. The shift of the vibrational modes in the low-frequency range results in a direct probe of the film thickness.

This cover figure shows the experimental and theoretical evolution of the in-plane low frequency vibrational modes of Ge₂Sb₂Te₅ from bulk down to a single lamella. The energy shift is mainly related to the weak van der Waals interactions between the layers.

This methodology has been selected for the cover page of a Special Issue on Phase Change and Ovonic Materials in Physica Status Solidi Rapid Research Letters (https://doi.org/10.1002/pssr.202170014). In this work, layered Ge₂Sb₂Te₅ films were grown by solid source molecular beam epitaxy on Sb-passivated Si(111) surfaces with a control down to a single lamella. By means of Raman spectroscopy, a non-destructive characterization tool, the evolution of the peak positions with film thickness was investigated. This unveils the unexplored low frequency region, namely the in-plane E_g and out-of-plane A_1g modes between 30 and 40 cm^-1, which mostly modulate the interactions across the van der Waals gap and not those inside the lamella. Calculations based on density functional theory allow a clear identification of the experimental Raman peaks, confirming the trends observed experimentally. Similar dependencies of the Raman peaks are shown also for Ge₁Sb₂Te₄ obtained in turn by post-growth annealing of the epitaxial GeSbTe series. In contrast to transmission electron microscopy techniques, which requires cumbersome preparation and statistical analysis, the method is quick and robust and highlights the advantages of the molecular beam technique for the precise control of the layer size and the importance of the vibrational modes for probing the 2D nature of the material.

1	Author	E. Zallo , D. Dragoni , Y. Zaytseva , S. Cecchi , N. I. Borgardt , M. Bernasconi , R. Calarco
	Title	Evolution of Low-Frequency Vibrational Modes in Ultrathin GeSbTe Films
	Source	Phys. Status Solidi-Rapid Res. Lett. , 15 , 200434 ( 2021 )
DOI : 10.1002/pssr.202000434 \| 3214 Cite : Bibtex RIS @article{ author = { E. Zallo and D. Dragoni and Y. Zaytseva and S. Cecchi and N. I. Borgardt and M. Bernasconi and R. Calarco }, title = { Evolution of Low-Frequency Vibrational Modes in Ultrathin GeSbTe Films }, journal = { Physica Status Solidi Rapid Research Letters }, volume = { 15 }, year = { 2021 }, pages = { 200434 } } E. Zallo, D. Dragoni, Y. Zaytseva, S. Cecchi, N. I. Borgardt, M. Bernasconi, and R. Calarco TY - JOUR AU - E. Zallo AU - D. Dragoni AU - Y. Zaytseva AU - S. Cecchi AU - N. I. Borgardt AU - M. Bernasconi AU - R. Calarco TI - Evolution of Low-Frequency Vibrational Modes in Ultrathin GeSbTe Films JF - Physica Status Solidi Rapid Research Letters JA - Phys. Status Solidi-Rapid Res. Lett. J1 - VL - 15 SP - 200434 PY - 2021/03/16

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Evolution of low-frequency vibrational modes in ultrathin GeSbTe films

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