Ge2Sb2Te5 Films Research Articles

Reconfigurable wide-angle broadband terahertz wave antireflection using a non-volatile phase-change material.

Actively wide-angle broadband terahertz (THz) antireflection (AR) coatings with a flexible reconfigurability have a great potential for the development of next-generation versatile THz components and systems with high performance. Here, we present a reconfigurable wide-angle broadband THz AR coating using a phase change material Ge2Sb2Te5 (GST) film, which is based on the impedance matching method. The performance of GST-based AR coating can be effectively achieved by a thermal excitation, exhibiting the complete suppression of unwanted THz-wave reflections for incidence angles from 0∘ to 50∘ in the broad frequency range of 0.1-3.0 THz. Simulation and experimental results show that the GST-based AR coating can efficiently eliminate Fabry-Perot interference caused by unwanted THz-wave reflections from the substrate, thereby significantly improving the performances of THz devices. Moreover, the active AR mechanism of the GST-based coating is investigated, which elucidates the essential role of the phase transition between the amorphous and crystalline phases in changing the conductivity of the film to achieve an impedance matching condition under thermal excitation. Additionally, the non-volatile properties of GST can enable the AR coating to retain a long-term stability for optimal wave-impedance matching without power holding requirements. Our work provides a new, to the best of our knowledge, and promising way for realizing high-performance integrated THz components and systems in the future.

Visible color camouflage and infrared, laser band stealth, compatible with dual-band radiative heat dissipation

Infrared stealth technology garners significant attention due to its value in military applications. With the rise of multi-band detection technologies, the infrared stealth target is easily detected. Therefore, it is urgent to study multi-band camouflage devices. Here, this paper proposes a multi-band camouflage device composed of ZnS, Ge, TiO2, crystalline Ge2Sb2Te5 (cGST) and Ag multilayer films, which can realize multi-band camouflage across visible, mid-wave infrared (MWIR, ε3∼5 μm = 0.22), long-wave infrared (LWIR, ε8∼14 μm = 0.16), and laser (R1.06 μm = 0.01) spectra. In addition, effective radiative heat dissipation was realized in two non-atmospheric transparent windows (ε2.5–3 μm = 0.51 and ε5∼8 μm = 0.65). Among them, visible camouflage in different background environments can be realized independently by adjusting the thickness of the ZnS film to produce different structural colors. Notably, this emissivity spectrum remains robust even as the incident angle increases to 60°. These results can guarantee the realization of multi-band infrared camouflage, laser camouflage and radiation heat dissipation. This study not only addresses high-efficiency infrared camouflage and thermal management but also achieves outstanding visible and laser camouflage performance, offering a comprehensive guidance for advancing multi-band camouflage technology.

Growth mechanism of Ge2Sb2Te5 thin films by atomic layer deposition supercycles of GeTe and SbTe

The film with a composition close to Ge2Sb2Te5 was fabricated by the supercycle atomic layer deposition (ALD) of GeTe and SbTe, followed by tellurization annealing. Supercycle processes are widely used for thin film deposition of multicomponent materials and often exhibit non-ideal growth behavior. Since only in situ analysis can reveal the substrate-dependent growth behavior, we used in situ quartz crystal microbalance (QCM) to study the growth mechanism during ALD supercycle processes at 85 °C. GeTe grown on SbTe was more Te-deficient than continuously grown GeTe film. As a result, more Te-deficient Ge-Sb-Te films were formed than expected. By annealing in a Te ambient at 250 °C, the Te-deficient Ge-Sb-Te film was converted to the Ge0.23Sb0.23Te0.54 close to Ge2Sb2Te5 film, which had a high density equivalent to 95 % of the FCC structure of Ge2Sb2Te5. The film showed excellent conformality and uniform composition in a trench pattern, suggesting a uniform crystallization temperature of 118 °C at all locations.

Effect of Ultraviolet Radiation on Properties of Ge2Sb2Te5 Phase Change Films.

With the expanding utilization of space technology, the stability of electronic components' performance in radiation environments has garnered significant attention. In this study, we prepared Ge2Sb2Te5 phase change films and memory units on silicon substrates to explore the influence of ultraviolet (UV) radiation on their characteristics. The experimental findings revealed that UV irradiation at a power density of 450 mW/cm2 decreased the amorphous resistance and thermal stability of Ge2Sb2Te5 films, impeding their multistage storage performance. Nevertheless, the amorphous state could still undergo effective transformation into a crystalline state. Furthermore, UV irradiation triggered the photoelectric effect, narrowing the band gap and causing a redshift of the Raman peak in amorphous films. Remarkably, the surface properties of Ge2Sb2Te5 films remained unchanged under irradiation. The phase change memory device based on Ge2Sb2Te5 film retained its SET-RESET conversion capability at a pulse width of 100 ns post-UV irradiation, demonstrating resilience against UV radiation. This study offers the practical insights for the application of phase change memory in space radiation environments.

Reversible Laser Imprinting of Phase Change Photonic Structures in Integrated Waveguides.

Formation of laser-induced periodic surface structures (LIPSS) is known as a fast and robust method of functionalization of material surfaces. Of particular interest are LIPSS that manifest as periodic modulation of phase state of the material, as it implies reversibility of phase modification that constitute rewritable LIPSS, and recently was demonstrated for chalcogenide phase change materials (PCMs). Due to remarkable properties of chalcogenide PCMs─nonvolatality, prominent optical contrast and ns switching speed─such novel phase change LIPSS hold potential for exciting applications in all-optical tunable photonics. In this work we explore phase change LIPSS formation in thin films of Ge2Sb2Te5 (GST) integrated with planar and rib waveguides. We demonstrate that by fine-tuning laser radiation, the morphology of phase change LIPSS can be controlled, including their period and fill factor, and investigate the limitations of multicycle rewriting of the structures. We also demonstrate the formation of phase change LIPSS on a 1D waveguide, which has potential for use as tunable Bragg filters or structures for on-demand light decoupling into the far-field. The presented concept of applying phase change LIPSS offers a promising approach to enable fast and simple tuning in integrated photonic devices.

Tunable high-Q resonances based on the asymmetric nanohole array of phase-change material

Dielectric metasurfaces have made great progress over the past decade to enhance light-matter interaction. Recently, dielectric nanohole structures have been employed for the creation of dielectric metasurfaces. However, the optical characteristics of most dielectric nanohole structures remain fixed once they are manufactured. This study investigates the optical properties of Ge2Sb2Te5 (GST) film perforated with a periodic dual nanohole array. The GST dual nanohole array is capable of supporting multiple guided resonances and bound states in the continuum (BICs). By introducing asymmetry in the radius, BICs can be transformed into quasi-BICs with high-Q resonances. The wavelengths of guided resonances and quasi-BICs can be dynamically controlled through the phase transition of GST. Furthermore, modifying the gap allows for the achievement of active high-Q electromagnetically induced transparency, resulting from the interaction between one guided resonance and one quasi-BIC mode. The GST asymmetric nanohole array holds potential for applications in optical modulators, slow-light devices, and nonlinear optical devices.

Research on Improved Crystallization Properties and Underlying Mechanism of the Sb2Te3 Phase-Change Thin Film by Inserting Sn15Sb85 Layers.

As a non-volatile semiconductor memory technology, phase-change memory has a wide range of application prospects as a result of the excellent comprehensive performance. In this paper, multilayer thin films based on Sb2Te3 material were designed and prepared by inserting the Sn15Sb85 layer. The thermal and electrical properties of superlattice-like Sb2Te3/Sn15Sb85 phase-change films can be adjusted by controlling the thickness ratio of Sb2Te3/Sn15Sb85. In comparison to the monolayer Sb2Te3 film, the multilayer layer Sb2Te3/Sn15Sb85 materials have a higher crystallization temperature, larger crystallization activation energy, and longer data lifetime, indicating the great improvement of thermal stability. The changes in the phase structure and vibrational modes of multilayer Sb2Te3/Sn15Sb85 films were characterized by X-ray diffraction and Raman spectroscopy, respectively. The presence of Sn15Sb85 layers restrains grain growth and refines the grain size. The multilayer Sb2Te3/Sn15Sb85 films exhibit better surface flatness, smaller surface potential undulation, and lower thickness variations than single-layer Sb2Te3. Phase-change memory cells based on the [Sb2Te3 (1 nm)/Sn15Sb85 (9 nm)]5 thin film has a lower threshold voltage (1.9 V) and threshold current (3.1 μA) compared to the Ge2Sb2Te5 film. Meanwhile, the electrical heating model of a phase-change memory cell based on a [Sb2Te3 (1 nm)/Sn15Sb85 (9 nm)]5 multilayer structure was established by multiphysics coupling analysis, proving the great improvement in heat transfer performance and efficiency. The experimental and theoretical studies confirm that the insertion of the Sn15Sb85 layer can significantly improve the crystallization properties of Sb2Te3 films, paving the way for optimizing the phase-change materials by regulating the microstructural parameters.

Configurable dual-topological-interface-states induced reflection in hybrid multilayers consisting of a Ge2Sb2Te5 film

Active control of induced reflection is crucial for many potential applications ranging from slowing light to biosensing devices. However, most previous approaches require patterned nanostructures to achieve controllable induced reflection, which hinders their further applications due to complicated architectures. Herein, we propose a lithography-free multilayered structure to achieve the induced reflection through the coupling of dual-topological-interface-states. The multilayers consist of two one-dimensional (1D) photonic crystals (PCs) and an Ag film separated by a Spacer, topological edge state (TES) and topological Tamm state (TTS) can be excited simultaneously and their coupling induces the reflection window. The coupled-oscillator model is proposed to mimic the coupling between the TES and TTS, and the analytical results are in good agreement with finite element method (FEM). In addition, the TES-TTS induced reflection is robust to the variation of structural parameters. By integrating an ultra-thin phase-change film of Ge2Sb2Te5 (GST) into the multilayers, the induced reflection can be switched through the phase transition of the GST film. The multipole decomposition reveals that the vanished reflection window is arising from the disappearance of TTS associated with the toroidal dipole (TD) mode.

Improved thermoelectric properties of Ge2Sb2Te5 films by Bi doping: Experiments and first-principles calculation

The high carrier concentration and low Seebeck coefficient limit the thermoelectric properties of Ge2Sb2Te5 materials. In this paper, Bi-doped Ge2Sb2Te5 thin films with different doping amounts were prepared by magnetron sputtering. The thermoelectric properties of Bi-doped Ge2Sb2Te5 were measured by experiments and predicted by density functional theory and Boltzmann transport theory. The results show that Bi atoms occupy Ge vacancies and bond with Te atoms. The replacement of Ge2+ vacancy with Bi3+ effectively modulates the carrier concentration and improves the Seebeck coefficient. Bi doping increases the degeneracy of the valence band and density of state near the valence band maximum (VBM), and thereby beneficial for the thermoelectric properties of p-type Ge2Sb2Te5, while not desirable for n-type. At 673 K, the power factor of the sample at 20 W doping power reaches 549.56 μWK−2m−1, which is nearly fifteen times that of the intrinsic Ge2Sb2Te5.

Optically Tunable Ultrafast Broadband Terahertz Polarimetric Device Using Nonvolatile Phase‐Change Material

AbstractActively tunable ultrafast broadband terahertz (THz) polarimetry using a reconfigurable phase‐change material holds great potentials and prospects for the achievement of next‐generation versatile integrated THz components and systems in a variety of THz applications. Here, an optically tunable ultrafast broadband THz polarimetric device (THz‐PoD) composed of a phase‐change material Ge2Sb2Te5 (GST) and a thin mica substrate is demonstrated. This proposed novel THz‐PoD is verified for a frequency range of 0.1–2.5 THz, exhibiting broadband and ultrafast determination of polarization states for linearly polarized THz waves at polarization angles from −90° to 90°. It is shown that optical excitation with ns pulses allows easy and efficient control of the polarimetric properties of such THz‐PoD. The essential role of the GST film in switching the phase transition between the amorphous and crystalline phases is emphasized by the theoretical investigation of the optically tunable ultrafast polarimetric mechanism of the device. This phase transition allows optically changing the THz‐PoD's properties by ns‐pulsed laser in a controlled way to achieve THz polarimetry for linearly polarized THz waves. The combined advantages of this strategy can open up a new and promising way for realizing versatile reconfigurable and integrated THz devices, which may further promote the development of novel THz systems and applications.

Regulation of laser-induced nanogratings by tuning the Marangoni-plasmon-coupled effect.

Laser-induced subwavelength nanogratings on films find widespread applications in enhancing a spectrum through surface plasmon excitation. It is challenging to achieve high uniformity, diversity, and controllability due to the intricate interplay between two basic mechanisms in laser nanostructuring: the Marangoni effect and surface plasmon polaritons (SPPs). We tune the coupled effect on Ge2Sb2Te5 films by adjusting the laser polarization, whose component controls the two effects' strength ratio. The Marangoni effect dominates when the SPPs' direction mismatches with the growing direction of nanogratings. Tuning this competition relationship helps to create nanogratings with tunable duty cycle and distribution, which are significant for light modulation applications. A highly efficient direct writing method with a line-shaped laser beam is employed to create large-area regular nanogratings by enhancing the effect tuning. We demonstrate diverse Au nanogratings with the aid of a lift-off operation and apply them in surface plasmon-coupled emission (SPCE), showcasing exceptional enhancement and narrowing performance.

Metasurface with all-optical tunability for spatially-resolved and multilevel thermal radiation

Abstract Manipulating the thermal emission in the infrared (IR) range significantly impacts both fundamental scientific research and various technological applications, including IR thermal camouflage, information encryption, and radiative cooling. While prior research has put forth numerous materials and structures for these objectives, the significant challenge lies in attaining spatially resolved and dynamically multilevel control over their thermal emissions. In this study, a one-step ultrafast laser writing technique is experimentally demonstrated to achieve position-selective control over thermal emission based on the phase-change material Ge2Sb2Te5 (GST). Ultrafast laser writing technique enables direct fabrication and manipulation of laser-induced crystalline micro/nano-structures on GST films. Thermal emission can be precisely controlled by adjusting the pulse energy of the ultrafast laser, achieving a high thermal emissivity modulation precision of 0.0014. By controlling thermal emission, the ultrafast laser writing technique enables multilevel patterned processing. This provides a promising approach for multilevel IR thermal camouflage, which is demonstrated with emissivity-modulated GST emitters. Remarkably, ultrafast laser-induced crystalline micro/nano-structures display geometric grating features, resulting in a diffraction-based structural color effect. This study demonstrates the effective use of laser-printed patterns for storing information in both visible and infrared spectrum.

Realization of Selector‐Memory Bi‐Functionality with Self‐Current Regulation Utilizing Poly‐Crystalline Based GST Electrolyte for Memristor Hardware Development

AbstractConductive bridge random‐access memory (CBRAM) are two terminal devices that offer excellent switching performance. In addition, CBRAM shows various switching modes, including volatile threshold switching (TS) and nonvolatile threshold switching (N‐TS). These properties expand its applications to memory, selector, biological synapses, and neurons. However, due to the uncontrollable behavior of stochastic switching between TS and N‐TS in CBRAM devices, a novel approach is needed to improve the switching performance of CBRAM. Moreover, conventional devices that have different stacking between TS and N‐TS increase fabrication cost and worsen the device yield. Here, the selector‐memory bi‐functionality with self‐current regulation effect of Ag‐inserted Ge2Sb2Te5 (GST) thin films is demonstrated. Selector‐memory bi‐functionality, having TS behavior with an adjustable on/off current, with confined conductive filaments (CFs) improves the uniformity and reduces the fabrication cost by implementing TS/N‐TS in a single stack. From the material analysis, it becomes evident that confined Ag‐based CFs within GST films are key factors for realizing selector‐memory bi‐functionality. The selector‐memory bi‐functionality is achieved through the reaction of Ag metal cations with non‐bonded Te atoms in GST film depending on field polarity. These results suggest that the Ag‐inserted GST film contributes to the development of large‐scale nonvolatile memory and neuromorphic application.

Laser Dynamic Control of the Thermal Emissivity of a Planar Cavity Structure Based on a Phase-Change Material.

Tailoring the thermal emission of a material in the long-wave infrared (IR) range of 8-13 μm is crucial for many IR-adaptive applications, including personal thermal management, IR camouflage, and radiative cooling. Although various materials and surface structures have been proposed for these purposes, space-selective and dynamic control of their emissivity is challenging. In this study, we present a planar surface cavity structure consisting of a Ge2Sb2Te5 (GST) film on top of a thin metal reflector to modulate its emissivity by using an ultraviolet laser beam. A laser-induced phase change in GST allowed for the local control of emissivity. The average emissivity in the long-wave IR range was tunable from 0.15 to 0.77 simply by changing the laser energy deposited on the GST film. This enabled the laser printing of high-contrast emissivity patterns, which were erasable by subsequent thermal annealing. Emissivity-modulated GST cavities could be fabricated on not only rigid substrates but also flexible plastic substrates such as polyimide. The GST surface cavity was highly flexible and remained stable upon repeated bending to a curvature radius of 0.5 cm. This study provides a promising route for realizing scalable and flexible thermal emitters with tunable surface emissivity.

Realization of a photoswitchable anapole metasurface based on phase change material Ge2Sb2Te5

The electromagnetic anapole mode originates from the phase cancellation interference between the far-field radiation of an oscillating electric dipole moment and toroidal dipole moment, which presents a radiation-free state of light while enhancing the near-field, and has potential applications in micro- and nanophotonics. The active control of the anapole is crucial for the design and realization of tunable photonic devices. In this paper, we realize dynamic tuning of an anapole metasurface and metasurface optical switching based on the phase change material Ge2Sb2Te5 (GST). By utilizing the destructive interference of the electric dipole moment and ring dipole moment, we design the non-radiative anapole mode. At the same time, we introduce the phase change material GST to dynamically regulate the intensity and position of the far-field scattering, electric field, and transmission spectra, and to realize the transition from anapole mode to electric dipole mode. At the same time, the modulation of the transmission spectrum by the metasurface after the addition of GST film is achieved. A relative transmission modulation of 640.62% is achieved. Our study provides ideas for realizing effective active modulation of active micro- and nanophotonic devices, and promotes active modulation of active micro- and nanophotonic devices in lasers and filters and potential applications in dynamic near-field imaging.

Ultrafast SET/RESET operation for optoelectronic hybrid phase-change memory device cells based on Ge2Sb2Te5 material using partial crystallization strategy

Combination of nonvolatile storage and in-memory computing promises to break through the “memory bottleneck” that computing device adopts von Neumann architecture with individual computing and memory unit. Thus, the advanced nonvolatile memory device with ultrafast operation speed is urgently required. Here, the optoelectronic hybrid phase-change memory based on the Ge2Sb2Te5 material is proposed, where the picosecond laser induced reversible phase-change is utilized to write and erase the information while the resistance difference is adopted to realize the accurate information readout. Due to the significant difference in resistance between crystalline and amorphous states, a partial crystallization strategy can be adopted to achieve ultrafast SET operation. Results indicate that SET operation speed of the Ge2Sb2Te5 film and device unit can be as fast as 52 and 130 ps, respectively, while the RESET speed reaches 13 ps. In parallel, the resistance ratio of RESET to SET state is still as high as two orders of magnitude. By using partial crystallization strategy, the phase-change induced by picosecond laser only occurs from amorphous to face-centered-cubic crystalline state with low crystallinity and the defective octahedral motif is observed in the Ge2Sb2Te5 film, which is beneficial to achieve the ultrafast operation speed. At the same time, the ordered clusters existed in the as-deposited and picosecond laser induced RESET films can accelerate the nucleation process of the Ge2Sb2Te5 film, which is one of the important reasons for achieving ultrafast SET speed. The optoelectronic hybrid phase-change memory with ultrafast operation speed may be one of the promising solutions for the in-memory computing.

Flexible germanium monotelluride phase change films with ultra-high bending stability for wearable piezoresistive sensors

Although non-stoichiometric chalcogenide phase change materials (e.g., Ge2Sb2Te5, Sb2Te3) have been used in wearable electronics, their non-stoichiometric vacancies will inevitably degrade their bending stability and electrical contrast. In this study, we demonstrated that stoichiometric germanium monotelluride films deposited on flexible polyimide substrates exhibited excellent phase transition characteristics and ultra-high bending stability under different bending states. The GeTe films showed improved bending stability under compressive strain than under tensile strain, and the GeTe films still exhibited high conductivity contrast (6 orders of magnitude) after 2000 bending cycles, with potential use in flexible memory devices. As a proof-of-concept, the bending stability of the stoichiometric GeTe films was shown to be significantly better than the non-stoichiometric Ge2Sb2Te5 films. Notably, we fabricated a flexible piezoresistive sensor based on the GeTe film, which exhibited good sensitivity (gauge factor is 38), low detection limit (0.1%), short response time (83.3 ms), good reliability and repeatability (more than 1200 cycles) and high heat resistance (200 ℃), and was used to track human pulse, as well as finger and face movements. The results showed that the piezoresistive sensor had good sensitivity and a stable time-resolved electrical response, and could be used to detect the movement of different human body parts and the small signals of different pressure points. These findings indicated that GeTe-based wearable piezoresistive sensors hold significant promise for medical health monitoring and biosensing applications.

Thermal stability and high speed for optoelectronic hybrid phase-change memory based on Cr doped Ge2Sb2Te5 thin film

Non-volatile memory, which can simultaneously store data and compute in memory, is a promising candidate to break through “memory bottleneck”. However, its operation speed and thermal stability are still contradictory and difficult to meet the requirement of in-memory computing especially in high-temperature applications. In this work, optoelectronic hybrid phase-change memory based on Cr doped Ge2Sb2Te5 material is proposed, where the picosecond laser is used to perform the information recording/erasing operation based on the reversible phase-change characteristics while resistance signal is detected to realize the information readout. Results imply that the Cr doped Ge2Sb2Te5 film with Cr concentration of 10.7 at. % has high crystallization temperature (289 °C) and ten-year data-retention temperature (221 °C), low density-change rate (2.3%) and small resistance drift coefficients (0.022 at 85 °C, 0.056 at 120 °C, 0.070 at 150 °C, and 0.00131 for the laser induced SET state). The corresponding memory device has the SET/RESET operation speeds of as fast as 1820 ps/13 ps accompanied by the RESET/SET resistance ratio of higher than two orders of magnitude. The structural analyses indicate that Cr doping can suppress the growth of grains and reduce the grain size, improving the thermal stability of amorphous Ge2Sb2Te5 thin film while the octahedral/defective octahedral motif ensures its fast phase-change speed. Therefore, Cr doped Ge2Sb2Te5 based optoelectronic hybrid phase-change memory with superior thermal stability and ultrafast operation speed may be one of the promising solutions for in-memory computing.

Terahertz-infrared spectroscopy of Ge2Sb2Te5 films on sapphire: Evolution of broadband electrodynamic response upon phase transitions

Phase-change alloy Ge2Sb2Te5 (GST) forms a favorable material platform for modern optics, photonics, and electronics thanks to a pronounced increase in conductivity with thermally induced phase transitions from amorphous (a-GST) into cubic (c-GST) and then hexagonal (h-GST) crystalline states at the temperatures of ≃150 and ≃300°C, respectively. Nevertheless, the data on broadband electrodynamic response of distinct GST phases are still missing, which hamper the design and implementation of related devices and technologies. In this paper, a-, c-, and h-GST films on a sapphire substrate are studied using broadband dielectric spectroscopy. For all GST phases, complex dielectric permittivity is retrieved using Drude and Lorentz models in the frequency range of 0.06–50 THz or the wavelength range of ≃5000–6 μm. A contribution from the free charge-carriers conductivity and vibrational modes to the broadband response of an analyte is quantified. In this way, the Drude model allows for estimation of the static (direct current—DC) and dynamic (at 1.0 THz) conductivity values, caused by motions of free charges only, which are as high as σDC≃15 and 40 S/cm and σ1.0THz≃8.8 and 28.6 S/cm for the c- and h-GSTs, respectively. This overall agrees with the results of electrical measurements of GST conductivity using the four-point probe technique. The broadband electrodynamic response models obtained for the three GST phases are important for further research and developments of GST-based devices and technologies.

Influence of bismuth on optical and electrical properties of thin Ge2Sb2Te5 films prepared by DC ion plasma sputtering

The structure of Ge2Sb2Te5 thin films doped with Bi has been studied using TEM, Raman spectroscopy, and Optical Transmission Spectroscopy. Thin films were obtained by direct current (DC) ion–plasma sputtering with thicknesses from 60 to 200 nm. TEM images revealed an amorphous film structure with bismuth nanocrystallites with a size of ∼1 to ∼20 nm. A shift in the Raman peaks of Bi–doped Ge2Sb2Te5 films compared to undoped samples was observed. A shift of the fundamental absorption edge to the long wavelength region of the spectrum along with an increase in the concentration of bismuth were detected in the optical transmission spectra of the films. A decrease in the optical band gap of bismuth-doped films was found. It has been shown that with increasing bismuth content in the films, their conductivity also increases. It has been found from studies of the current–voltage film characteristics reversible switching effect takes place.