10.26456/pcascnn/2024.16.351 Abstract: This paper presents the results of numerical modelling of optical radiation propagation in a SU-8 polymer waveguide and signal modulation at different phase states of an array of nanoparticles of the phase-change material Ge2Sb2Te5 (GST). It is shown how the transmitted radiation is modulated for different numbers of nanoparticles when placed on the top and at the edge of the waveguide. The simulation results show that in addition to the influence of the phase states (crystalline or amorphous) on the properties of the transmitted signal, in the case of nanoparticles not only reflection and absorption but also scattering of the material play a prominent role. The basic possibility of controlling the optical signal of telecommunication range passing through the interface by switching the optical active element based on nanoparticles of phase-change material is demonstrated. The concept of developing photonic integrated circuits proposed in this work is the cheapest of all known planar technologies of developing waveguide devices and allows realizing computing elements and architectures on its basis with a high degree of heterogeneous integration.

This paper presents and investigates a new architecture of a computational cell based on nanoparticles of the phase-change material Ge2Sb2Te5. Such a cell is a chaotic array of nanoparticles deposited between closely spaced electrical contacts. The state of such a structure is determined by the resistance of the nanoparticle array, which depends on the phase state of each particle of the material. Simulation results show that the proposed structure has a number of electrical states switching features that cannot be achieved using a thin film architecture. The proposed architecture allows for smoother and more controlled switching of the resistance by electrical pulses. Simulation of the evolution of the cell state using complex control actions showed that the proposed structure can behave as an artificial convolutional neuron with horizontal connections and also as a multi-level memory cell. In addition, the proposed design is technologically simple to achieve and inexpensive to manufacture.

The paper presents the results of theoretical analysis of crystallization of GeTe and Ge2Sb2Te5 thin films under the influence of pulse laser radiation. The phase transformations and the fraction of the crystalline phase was estimated on basis of the change of the probe optical reflection coefficient from the film sample surface. The formalism based on the Kolmogorov-Johnson-Mehl-Avrami theory was used to evaluate the kinetic behaviors of the phase transformation under the action of laser radiation. On the basis of experimental data of reflection changes during crystallization process of the researched materials, graphs were plotted and Avrami constants were determined. It is shown that GeTe exhibits a single step crystallization process associated with a high rate of nucleation and crystallite growth in all directions. The Ge2Sb2Te5 alloy is characterized by a two-step crystallization process with a change in the Avrami constant due to the influence of many factors such as the film geometry, sputtering characteristics, etc. Such type of crystallization is explained by the predominance of the high-stochastic nucleation.

Experimental results on the synthesis of nanoparticles of Ge2Sb2Te5 phase-change material by the direct laser-induced transfer method are presented. Thin films obtained by the thermal vacuum deposition were used as a donor material and silicon wafers as an acceptor. The laser-induced forward transfer was carried out using the sub-nanosecond pulsed laser irradiation. The morphology, topology, and size of the obtained nanoparticles were analyzed by scanning electron microscopy. Structural studies were performed by Raman scattering. A quasi-uniform distribution of nanoparticles on the substrate and a quasi-uniform size distribution were achieved. It was shown that it is possible to achieve a nanoparticle diameter of less than 100 nm. Raman spectra show that the nanoparticles obtained are in the crystalline state. The results show the possibility of creating an element based on nanoparticles with a specific distribution in size as a technological alternative to devices based on thin films. The use of nanoparticles will make it possible to achieve the energy efficiency, greater flexibility, and smoothness of switching as well as to realize neuromorphic and stochastic computation.

The paper presents the results of modelling the phase shift of a passing optical beam caused by the formation of a layered structure in a controllable cell made of the phase-change material Ge2Sb2Te5 induced by the controlling influence of pulsed laser radiation of short and ultrashort duration. The crystallization of a thin film of Ge2Sb2Te5 is analyzed on the basis of the thermokinetic approach, taking into account the kinetic properties of the material, the energy and duration of the applied laser radiation, the amorphization of the upper layers of the film during rapid heating and the temperature dependence of the kinetic properties. Graphs of the thickness of the crystalline layer in the film material were plotted for each influencing pulsed radiation. From the data on the position of the crystalline layer, the phase shift of the transmitted optical radiation is calculated. Based on the modelling data of the investigated cell, a phase shifter can be constructed to transform an optical beam of arbitrary aperture. The proposed method of controlling the optical beam front by changing the structural state of a thin film can be very promising when accurate and fast tuning of the optical phase transparency is required.

The work presents the results of comprehensive studies of the optical and transport properties, phase transition heats, the X-Ray diffraction and Raman spectra of Ge2Sb2Se4Te1, Sb2Se3 and Bi2Se3 samples obtained by vacuum thermal deposition. We demonstrate a high contrast in the refractive index and extinction coefficient in the spectral range in wavelengths of 500-1800 nm and transmissivity and reflectivity spectra in the range in wavelengths of 500-3000 nm. The investigated materials show low optical losses and high figure of merit in much of the IR-spectrum. Changes in structural properties are analyzed by XRD and Raman methods. Features of phase transformations are described by thermogravimetry and differential scanning calorimetry.

We present the experimental study of free-space optical control of the optical beam phase shift caused by the formation of a layered structure in an elementary controllable cell made of phase-change material Ge2Sb2Te5 subjected to the controlling effect of pulsed laser radiation. The phase change of the signal optical beam passing through the controlled cell from phase-change material relative to the control beam in the Jamin interferometer is demonstrated.

This paper presents a thermokinetic computational model of phase transitions in GST225 (germanium–antimony–tellurium) thin films [as well as other phase change materials (PCMs)] induced and initiated by the impact of nano- and femtosecond laser pulses in a wide energy fluence range according to the results of experimental studies using Raman spectra and thin-film samples of TEM cross-sectional image analysis. Applying this phase transition model makes it possible to understand the mechanism of the induced phase transition regarding the usage of PCMs in photonics and optoelectronic devices, which require precise control of the phase state of their PCM-based active elements for their functioning. The proposed model shows the internal structure of the sample, generating both the profile of the crystalline fraction distribution over the sample's depth, providing images of virtual TEM sections, as well as the volume distribution of the crystalline phase.

The work is devoted to the study of the characteristics of the state control of a thin-film element based on a phase-change GeTe material. The properties of such an element have been controlled by the action of sequences of ultrashort laser pulses. This action leads to a rapid heating of the thin film element and provides a phase transition between states with a resistance different by several orders of magnitude. The dynamics of the resistance was studied using a high speed oscilloscope according to the scheme where the element under study was the voltage divider arm of a highly stable source. Three different types of conductivity switching were observed for 100 nm thin films. For low energy laser radiation, several distinct states were obtained in which the material film has predominantly semiconducting properties. As the energy of the optical pulses increases, the number of possible stable states determined by the specific conductivity of the material decreases to two, one of which (low resistance) is exclusively metallic properties. In all cases, the time taken to switch to a stable state does not exceed a few tens of nanoseconds for films up to 100 nm thick. The study has demonstrated that the structures described can be used to implement optically controlled memristive elements. In addition, the large number of possible allowable specific resistances of the element will make it possible to use it to increase the information capacity of memory cells based on phase-change materials or to implement optoelectronic neuromorphic systems.

This paper presents optical transmission and reflection coefficients of Ge2Sb2Te5 thin films changing dynamics during the femtosecond laser radiation induced phase transitions studying results. The authors propose a predictive model based on the thermokinetic approach allowing to qualitatively and quantitatively determine the fraction of the crystalline phase and the depth of its occurrence in the GST film. The research demonstrates that in a series of controlling femtosecond laser pulses for 100 nm or more thin films, the most accurate control of the crystalline phase fraction is ensured at energy fluences in the range of 10-20 mJ/cm2. At energy fluences higher than 25 mJ/cm2 in the two-pulse mode, a significant part of the film is reamorphized. For 100-200 nm thin films, the phase transformation begins a few nanoseconds after the onset of exposure and ends in no more than 100 ns under the described control mode.

Crystallization of as-deposited amorphous Ge2Sb2Te5 films (180 nm) by femtosecond pulsed laser radiation have been studied using in-situ electrical resistance measurements with temporal resolution. The results show that a double pulses impact is optimal for the controllability of the crystallization process. In this case, the switching time of the film resistance is less than 100 ns and is associated with the formation of conducting crystalline paths in the material after the pulse laser impact. We have proposed a model of the electrical conductivity dynamics caused by impact of the ultrashort laser radiation, which is well related to experimental data.

Various materials that ensure the creation of volatile and nonvolatile memristor structures are under study in the development of elements of neuromorphic systems. TaOx thin films and memristor structures in the crossbar geometry based on them are synthesized by pulsed laser deposition in the droplet-free mode using mask technologies. TaOx thin films are obtained at different temperatures of the sapphire substrate (25 and 350°C), oxygen pressure in the vacuum chamber (from 0.5 to 80 mTorr), and wavelength of the ablation radiation (248 and 532 nm). High-resolution X-ray diffraction studies of films make it possible to determine the conditions for the preparation of films with the formation of nanocrystallites and the dependence of the size of nanocrystallites on the preparation conditions. The composition of the films and the degree of oxidation of tantalum are determined by X-ray photoelectron spectroscopy. The two-probe method is used to study the current–voltage characteristics (I—V characteristics) of the films at unipolar and bipolar voltage scanning. The nonvolatile memristive effect in Ta/TaOх/Ta/с-Аl2O3 thin-film structures is revealed during measurements of the I—V characteristics in the planar and crossbar geometry.

The work presents the results of comprehensive studies of the electrical resistivity and optical transmission coefficient temperature dynamics, phase transition heats, the phase composition, and Raman spectra of GeTe, Ge2Sb2Te5 (GST225), and Ge2Sb2Se4Te1 (GSST(2241)) samples obtained by vacuum thermal deposition. The phase transition from the amorphous to the crystalline state with intense crystallization for GSST2241 at ?(alpha) = 595 K which was first determined in our work. It was shown that the stepwise change in the electrical resistivity for a thin-film GeTe was 4.5 orders of magnitude and for GST(225) - 5 orders and as for GSST(2241) this value reached 6 orders of magnitude, the change in the relative optical transmittance delta T/T reached almost 100%. The temperature regions with the maximum values of the derivatives of resistivity and optical transmittance are in good agreement with the temperatures of crystallization (phase transitions) determined by the DSC method.

В работе рассмотрены исследования управляемого многоуровневого переключения на основе сильного контраста оптических свойств для разных фаз некоторых халькогенидных материалов. Приведенные результаты позволяют создать полностью оптические волноводные и нейроморфные устройства и системы на их базе.