Far-Infrared Near-Field Optical Imaging and Kelvin Probe Force Microscopy of Laser-Crystallized and -Amorphized Phase Change Material Ge3Sb2Te6

Julian Barnett; Lukas Wehmeier; Andreas Heßler; Martin Lewin; Julian Pries; Matthias Wuttig; J Michael Klopf; Susanne C Kehr; Lukas M Eng; Thomas Taubner

doi:10.1021/acs.nanolett.1c02353

Far-Infrared Near-Field Optical Imaging and Kelvin Probe Force Microscopy of Laser-Crystallized and -Amorphized Phase Change Material Ge₃Sb₂Te₆

Nano Lett. 2021 Nov 10;21(21):9012-9020. doi: 10.1021/acs.nanolett.1c02353. Epub 2021 Oct 19.

Authors

Julian Barnett¹, Lukas Wehmeier^{2

3}, Andreas Heßler¹, Martin Lewin¹, Julian Pries¹, Matthias Wuttig¹, J Michael Klopf⁴, Susanne C Kehr², Lukas M Eng^{2

3}, Thomas Taubner¹

Affiliations

¹ I. Institute of Physics (IA), RWTH Aachen, 52074 Aachen, Germany.
² Institute of Applied Physics, Technische Universität Dresden, 01062 Dresden, Germany.
³ ct.qmat, Dresden-Würzburg Cluster of Excellence-EXC 2147, Technische Universität Dresden, 01062 Dresden, Germany.
⁴ Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany.

PMID: 34665620
DOI: 10.1021/acs.nanolett.1c02353

Abstract

Chalcogenide phase change materials reversibly switch between non-volatile states with vastly different optical properties, enabling novel active nanophotonic devices. However, a fundamental understanding of their laser-switching behavior is lacking and the resulting local optical properties are unclear at the nanoscale. Here, we combine infrared scattering-type scanning near-field optical microscopy (SNOM) and Kelvin probe force microscopy (KPFM) to investigate four states of laser-switched Ge₃Sb₂Te₆ (as-deposited amorphous, crystallized, reamorphized, and recrystallized) with nanometer lateral resolution. We find SNOM to be especially sensitive to differences between crystalline and amorphous states, while KPFM has higher sensitivity to changes introduced by melt-quenching. Using illumination from a free-electron laser, we use the higher sensitivity to free charge carriers of far-infrared (THz) SNOM compared to mid-infrared SNOM and find evidence that the local conductivity of crystalline states depends on the switching process. This insight into the local switching of optical properties is essential for developing active nanophotonic devices.

Keywords: GST; Kelvin probe force microscopy; free-electron laser; metavalent bonding; optical switching; phase change material; scattering-type scanning near-field optical microscopy.