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Ultralow-fluence for Phase-Change Process

Ultrafast active materials with tunable properties are currently investigated for producing successful memory and data-processing devices. Among others, Phase-Change Materials (PCMs) are eligible for this purpose. They can reversibly switch between a high-conductive crystalline state (SET) and a low-conductive amorphous state (RESET), defining a binary code. The transformation is triggered by an electrical or optical pulse of different intensity and time duration. 3D Ge-Sb-Te based alloys, of different stoichiometry, are already employed in DVDs or Blu-Ray Disks, but they are expected to function also in non-volatile memories and RAM. The challenge is to demonstrate that the scalability to 2D, 1D up to 0D of the GST alloys improves the phase-change process in terms of lower power threshold and faster switching time. Nowadays, GST thin films and nanoparticles have been synthetized and have beenshown to function with competitive results.
A team of researchers from the University of Trieste and the MagneDyn beamline at Fermi demonstrated the optical switch from crystalline to amorphous state of Ge2Sb2Te5nanoparticles (GST NPs) with size <10 nm, produced via magnetron sputtering by collaborators from the University of Groeningen. Details were reported in the journal Nanoscale.
This work aims at showing the very low power limit of an optical pulse needed to amorphize crystalline Ge2Sb2Te5 nanoparticles. Particles of 7.8 nm and 10.4 nm diameter size were deposited on Mica and capped with ~200nm of PMMA. Researchers made use of a table-top Ti:Sapphire regenerative amplified system-available at the IDontKerr (IDK) laboratory (MagneDyn beamline support laboratory) to produce pump laser pulses at 400 nm, of ~100 fs and with a repetition rate from 1kHz to single shot. 

A grid of single-shot laser spots with increasing average fluence was created on the GST NPs samples. According to the pump average fluence applied, researchers measured -with an overlapping probe beam at 800 nm- a difference in optical transmittivity before and after the laser pump treatment. Optical microscope images revealed how the laser spotted areas have a brighter contrast with respect to the “virgin” ones. Then, by annealing the sample for 20 min at 150°C (crystallization temperature) the spots disappeared. 

In addition, X-Ray Diffraction (XRD) spectra -taken at the XRD1 beamline at Elettra- indicated the signature of the structural difference between the two NPs states. 

All these evidences lead to the conclusion that the single shot pulses, above a specific average fluence, amorphize the crystalline GST NPs. By measuring the amorphized areas, the switching threshold fluences were extrapolated for both-size NPs. The comparison with thin film cases showed a x5 times lower power request and thus the advantage of using 0D materials with respect to higher dimensions materials.


Figure 1.  Trasmission Electron Microscopy image of the nanoparticles sample. Ultafast single-shot optical process with fs-pulse at 400 nm. Microscope images of amorphized and amorphized/ablated areas obtained on the nanoparticles sample. Comparison of amorphization threshold fluences between thin films and nanoparticles cases. 

 

 

This research was conducted by the following research team:

Barbara Casarin1, Antonio Caretta2, Roberta Ciprian2, Marco Malvestuto2, Fulvio Parmigiani3Bin Chen4and, Bart J. Kooi4

 

Università degli Studi di Trieste, Trieste, Italy and Elettra Sincrotrone Trieste S.C.p.A., Trieste, Italy
Elettra Sincrotrone Trieste S.C.p.A., Trieste, Italy
Università degli Studi di Trieste, Trieste, Italy and Elettra Sincrotrone Trieste S.C.p.A., Trieste, Italy and University of Cologne, Cologne, Germany
University of Groeningen, Groeningen, Netherlands


Contact persons:

Marco Malvestuto, email: marco.malvestuto@elettra.eu

Reference

Barbara Casarin, Antonio Caretta, Bin Chen, Bart J. Kooi, Roberta Ciprian, Fulvio Parmigiani and Marco Malvestuto, “Ultralow-Fluence Single-Shot Optical Crystalline-to-Amorphous Phase Transition in Ge-Sb-Te Nanoparticles”, Nanoscale, 2018, DOI: 10.1039/C8NR04350G

 
Last Updated on Tuesday, 20 November 2018 16:28