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

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 Ge 3 Sb 2 Te 6 (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.

Reconfiguring Magnetic Infrared Resonances with the Plasmonic Phase-Change Material In3SbTe2

“…It was recently demonstrated that the local optical properties of the PCM Ge 3 Sb 2 Te 6 do not change between amorphous and reamorphized or crystalline and recrystallized states in the infrared spectral range. 64 Therefore, we also assume the permittivity of reamorphized or recrystallized IST to be the same as amorphous and crystalline IST, respectively. The SRR geometry is idealized into a rectangular shape with rounded edges (see insets in Figure 2C).…”

“…We also support our measurements with full-wave simulations of the antenna arrays in Figure C (see the Methods section, Supporting Information). It was recently demonstrated that the local optical properties of the PCM Ge 3 Sb 2 Te 6 do not change between amorphous and reamorphized or crystalline and recrystallized states in the infrared spectral range . Therefore, we also assume the permittivity of reamorphized or recrystallized IST to be the same as amorphous and crystalline IST, respectively.…”

Near-field localization of the boson peak on tantalum films for superconducting quantum devices

“…The direct observation and identification of the boson peak using THz nanospectroscopy opens new doors to explore amorphous materials at the nanoscale. Finally, we note that these methods are particularly relevant to 2D materials, where reduced dimensionality strongly enhances the near-field optical response, and may be applied to disordered or amorphous forms of these materials such as bilayer SiO 2 [60], h-BN [61], graphene, and the metal dichalcogenides [62,63]. fitting model that accounts for details seen in angle resolved XPS data across all treatments, we implement an additional doublet at intermediate binding energy to account for suboxide contributions.…”

“…In contrast, THz nanospectroscopy provides a frequency-dependent imprint of the interrogated material within the nanofocus [39,40]. Therefore, THz s-SNOM is able to bypass the typical Rayleigh diffraction limit and provides nanoscale insight into lattice dynamics and electronic processes, including the characterization of amorphous and crystalline material phases [62,63].…”

Imaging the Terahertz Nanoscale Conductivity of Polycrystalline CsPbBr3 Perovskite Thin Films

“…Continuous-wave (cw) THz-sSNOM systems provide improved imaging speed and spatial resolution. However, most methods, which are often laser-based, operate at higher THz frequencies − where mobile charge carrier contrasts in semiconductors diminish or are explored in a different context. ,, In contrast, all-electronic continuous-wave THz nanoscopy , enables comparatively fast and sensitive amplitude- and phase-resolved THz near-field imaging as demonstrated at 600 GHz for doped Si, where the phase information is conveniently given in a single pass measurement using heterodyne detection.…”