Phase-change materials (PCM) have gained considerable interest in the photonics community as nonvolatile tunable materials for reconfigurable devices. Here, we experimentally demonstrate the continuous electrical control of the crystalline fraction of Ge2Sb2Te5 (GST) via voltage pulses in platinum micro-heaters. Using co-designed electrical micro-heaters and a simple optical stack of thin films, including GST, we demonstrate the programming of more than 50 distinct levels of optical reflection in the near-infrared, spanning a three orders of magnitude reflection modulation. This continuous electrically programmed phase transition is a promising strategy for PCM integration in industrial foundries and shows promise for individual pixel addressing in multi-level applications such as neuromorphic computing or spatial light modulators.

Photonic integrated devices are progressively evolving beyond passive components into fully programmable systems, notably driven by the progress in chalcogenide phase-change materials (PCMs) for non-volatile reconfigurable nanophotonics. However, the stochastic nature of their crystal grain formation results in strong spatial and temporal crystalline inhomogeneities. Here, the concept of spatially-controlled planar guided crystallization is proposed, a novel method for programming the growth of optically homogeneous low-loss Sb2S3 PCM, leveraging the seeded directional and progressive crystallization within confined channels. This guided crystallization method is experimentally shown to circumvent the current limitations of conventional PCM-based nanophotonic devices, including a multilevel non-volatile optical phase-shifter exploiting a silicon nitride-based Mach-Zehnder interferometer, and a programmable metasurface with spectrally reconfigurable bound state in the continuum. Precisely controlling the growth of PCMs to ensure optically uniform crystalline properties across devices is the cornerstone for the industrial development of non-volatile reconfigurable photonic integrated circuits.

We experimentally demonstrate the all-optical reversible switching of Sb2S3, an emerging phase change material suitable for low-loss active nanophotonics. We then propose approaches to optimize this local, multi-level and reversible phase transition using multiphysics considerations.

Phase change materials (PCM) have gained a considerable interest in the photonics community as active materials in reconfigurable devices. Their significant change of optical properties up on the amorphous to crystalline reversible phase transition enables a large and dynamical tuning of devices' response. Among the various PCM, Ge2Sb2Te5 (GST) is the most widely used thanks to its fast phase change as well as the large modulation of its refractive index (Delta n > 1). Although this PCM has been mainly exploited for simple binary on-off functions, it presents an additional degree of freedom: the multi-level partial change of state. Indeed, we show in this paper multi-level continuously tunable reflectivity allowed by the electrically-controlled partial crystallization of GST. This is achieved via the Joule effect, by injecting short voltage pulses (few tens of mu s) through platinum microheaters underneath GST patches. This electrically-induced phase transition is the most promising strategy for PCM integration in industrial foundries and shows promises for individual pixel addressing in multi-level applications such as neuromorphic computing or grayscale displays.

Phase change materials (PCMs) have gained a tremendous interest as a means to actively tune nanophotonic devices through the large optical modulation produced by their amorphous to crystalline reversible transition. Recently, materials such as Sb2S3 emerged as particularly promising low loss PCMs, with both large refractive index modulations and transparency in the visible and near-infrared. Controlling the local and reversible phase transition in this material is of major importance for future applications, and an appealing method to do so is to exploit pulsed lasers. Yet, the physics and limits involved in the optical switching of Sb2S3 are not yet well understood. Here, the reversible laser-induced phase transition of Sb2S3 is investigated, focusing specifically on the mechanisms that drive the optically induced amorphization, with multi-physics considerations including the optical and thermal properties of the PCM and its environment. The laser energy threshold for reversibly changing the phase of the PCM is determined through both theoretical analysis and experimental investigation, not only between fully amorphous and crystalline states but also between partially recrystallized states. Then, the non-negligible impact of the material's polycrystallinity and anisotropy on the power thresholds for optical switching is revealed. Finally, the challenges related to laser amorphization of thick Sb2S3 layers are addressed, as well as strategies to overcome them. These results enable a qualitative and quantitative understanding of the physics behind the optically-induced reversible change of phase in Sb2S3 layers.

Neuromorphic nanoelectronic devices that can emulate the temperature-sensitive dynamics of biological neurons are of great interest for bioinspired robotics and advanced applications such as in silico neuroscience. In this work, we demonstrate the biomimetic thermosensitive properties of two-terminal V3O5 memristive devices and showcase their similarity to the firing characteristics of thermosensitive biological neurons. The temperature-dependent electrical characteristics of V3O5-based memristors are used to understand the spiking response of a simple relaxation oscillator. The temperature-dependent dynamics of these oscillators are then compared with those of biological neurons through numerical simulations of a conductance-based neuron model, the Morris-Lecar neuron model. Finally, we demonstrate a robust neuromorphic thermosensation system inspired by biological thermoreceptors for bioinspired thermal perception and representation. These results not only demonstrate the biorealistic emulative potential of threshold-switching memristors but also establish V3O5 as a functional material for realizing solid-state neurons for neuromorphic computing and sensing applications.

We propose and demonstrate a simple method to accurately monitor and program arbitrary states of partial crystallization in phase-change materials (PCMs). The method relies both on the optical absorption in PCMs as well as on the physics of crystallization kinetics. Instead of raising temperature incrementally to increase the fraction of crystallized material, we leverage the time evolution of crystallization at constant temperatures and couple this to a real-time optical monitoring to precisely control the change of phase. We experimentally demonstrate this scheme by encoding a dozen of distinct states of crystallization in two different PCMs: GST and Sb2S3. We further exploit this time-crystallization for the in-situ analysis of phase change mechanisms and demonstrate that the physics of crystallization in Sb2S3 is fully described by the so-called Johnson-Mehl-Avrami-Kolmogorov formalism. The presented method not only paves the way towards real-time and model-free programming of non-volatile reconfigurable photonic integrated devices, but also provides crucial insights into the physics of crystallization in PCMs.

Temperature mapping by in situ thermoreflectance thermal imaging (TRTI) or midwave infrared spectroscopy has played an important role in understanding the origins of threshold switching and the effect of insulator-metal transitions in oxide-based memrsitive devices. In this study, we use scanning thermal microscopy (SThM) as an alternative thermal mapping technique that offers high spatial resolution imaging (∼100 nm) and is independent of material. Specifically, SThM is used to map the temperature distribution in NbOx-based cross-bar and nanovia devices with Pt top electrodes. The measurements on cross-bar devices reproduce the current redistribution and confinement processes previously observed by TRTI but without the need to coat the electrodes with a material of high thermo-reflectance coefficient (e.g., Au), while those on the nanovia devices highlight the spatial resolution of the technique. The measured temperature distributions are compared with those obtained from physics-based finite-element simulations and suggest that thermal boundary resistance plays an important role in heat transfer between the active device volume and the top electrode.

The effect at the nanoscale of a Ti interfacial layer on the performances of TiN/HfZrO/TiN capacitors is reported. Ferroelectric hafnium zirconium oxide (HZO) is synthesized by magnetron sputtering of a ceramic target. Titanium nitride top and bottom electrodes are grown by reactive magnetron sputtering. The insertion of an ultra‐thin Ti layer at the top electrode/HZO interface impacts the crystalline phase and the electrical properties of the ferroelectric HZO. Following post‐deposition annealing, the Ti layer is oxidized and becomes titanium oxide. Compositional and structural characterization is performed using glancing incidence X‐Ray diffraction and electron energy‐loss spectroscopy. The TiO z layer is clearly distinguishable at the top electrode/HZO interface. Electrical characterization is conducted by positive‐up‐negative‐down (PUND) technique. The remnant polarization reaches a maximum value of for 6 nm thick HZO. The results are discussed in the framework of structural, compositional, and physical properties of the electrode/HZO interfaces and their effect on the electrical performances of thin HZO‐based junctions, which could subsequently be considered for the demonstration of synaptic learning mechanisms for neuromorphic applications.

Herein, the effect of a 2 nm thin aluminum layer inserted between the ferroelectric layer and the top electrode in a TiN//TiN stack deposited by reactive magnetron sputtering is investigated. The oxidation of the interfacial layer during annealing due to scavenging of the impacts both the ferroelectric properties and the electrical conductivity of the junction. It is shown that the overall conductivity of the junction is boosted 20 folds while the resistance ratio between the positive and negative polarization states is increased from 1.3 up to 3.7. Through a systematic analysis of programming conditions, pulse duration, and height, we show that both the remanent polarization and On/Off current ratio can be enhanced at the expanse of the endurance leading to a trade‐off.

Oxides that exhibit an insulator-metal transition can be used to fabricate energy-efficient relaxation oscillators for use in hardware-based neural networks but there are very few oxides with transition temperatures above room temperature. Here the structural, electrical, and thermal properties of V3 O5 thin films and their application as the functional oxide in metal/oxide/metal relaxation oscillators are reported. The V3 O5 devices show electroforming-free volatile threshold switching and negative differential resistance (NDR) with stable (<3% variation) cycle-to-cycle operation. The physical mechanisms underpinning these characteristics are investigated using a combination of electrical measurements, in situ thermal imaging, and device modeling. This shows that conduction is confined to a narrow filamentary path due to self-confinement of the current distribution and that the NDR response is initiated at temperatures well below the insulator-metal transition temperature where it is dominated by the temperature-dependent conductivity of the insulating phase. Finally, the dynamics of individual and coupled V3 O5 -based relaxation oscillators is reported, showing that capacitively coupled devices exhibit rich non-linear dynamics, including frequency and phase synchronization. These results establish V3 O5 as a new functional material for volatile threshold switching and advance the development of robust solid-state neurons for neuromorphic computing.