Chalcogenide Semiconductor Films Research Articles

Створення активних оптичних метаповерхонь на плівках халькогенідних напівпровідників зі зміною фазового стану

The development of meta-optics is due to human aspirations for the maximum miniaturization of optical elements, the design and control of light fluxes, as well as the improvement of visualization and image processing. Metasurfaces, i.e. arrays with subwavelength distances (smaller than the wavelength of light), and optically thin elements trigger new physical mechanism and phenomena that are very different from those observed in three-dimensional bulk materials. Thus, meta-devices perform complete control and management of the characteristics of the light flux (phase, amplitude, polarization) with the help of one flat layer. Compared to traditional bulky lenses, metasurface lenses have advantages such as flatness, light weight, and compatibility with semiconductor manufacturing technology. The use of active (reconfigurable) metasurfaces, the characteristics of which can be dynamically rearranged after manufacturing, makes it possible to significantly expand the capabilities of meta-optics. The paper presents the results of the analysis of the properties and technologies of creating optically active metasurfaces for optical image processing and transformation systems. Generalized methods of forming metasurfaces are described: self-organization, selective chemical etching, holographic and lithographic. To implement the work of active (reconfigured) metasurfaces based on materials with a change in phase state (amorphous/crystalline), heating technologies with electric current pulses of various amplitudes and durations and the action of direct optical radiation are used. The analysis of materials for the formation of optically active metasurfaces and devices based on them that simulate the front of a light wave and work on reflection and transmission is presented. Special attention is paid to the use of photosensitive chalcogenide semiconductors as metamaterials with a phase change. Examples of materials such as Ge2Sb2Te5 (GST) and AgxInSb2Te (AIST), which have been used for decades in optical data storage and electronic memory devices, are given. A series of novel compositions of optical phase change materials such as Ge2Sb2Se4Te (GSST), Sb2S3, Sb2Se3, Ge2Sb2Te3S2 and In3SbTe2 for optical and photonic applications are also proposed. Direct laser recording on photosensitive films of chalcogenide semiconductors with the use of technological equipment for laser recording of master disks is proposed as a promising method of forming arrays with submicron distances and realizing the work of active metasurfaces.

Chalcogenide optomemristors for multi-factor neuromorphic computation

Neuromorphic hardware that emulates biological computations is a key driver of progress in AI. For example, memristive technologies, including chalcogenide-based in-memory computing concepts, have been employed to dramatically accelerate and increase the efficiency of basic neural operations. However, powerful mechanisms such as reinforcement learning and dendritic computation require more advanced device operations involving multiple interacting signals. Here we show that nano-scaled films of chalcogenide semiconductors can perform such multi-factor in-memory computation where their tunable electronic and optical properties are jointly exploited. We demonstrate that ultrathin photoactive cavities of Ge-doped Selenide can emulate synapses with three-factor neo-Hebbian plasticity and dendrites with shunting inhibition. We apply these properties to solve a maze game through on-device reinforcement learning, as well as to provide a single-neuron solution to linearly inseparable XOR implementation.

Дисперсия плазменных колебаний в аморфных халькогенидных полупроводниках

Thin films of amorphous chalcogenide semiconductors on a silicon crystal are studied by the method of plasma vibration dispersion and the asymmetry of the number of electrons in the zone of formation of the total external reflection of X-rays and the excitation of plasmons is calculated. Loop-like dispersion curves were observed and the average plasmon energies and the associated internal mechanical stresses and polarization of the studied films were determined. The absence of internal mechanical stresses and polarization in an amorphous semiconductor film of molybdenum sulfide is found.

Effects of thiourea dosage on the structural, optical and electrical properties of one-step solution processed Cu3BiS3 film for photovoltaics

Recently, Cu3BiS3 has emerged as an attractive photovoltaic material with low cost and excellent light absorption. However, its device application is still hindered by relatively poor material quality. Here, a one-step solution method based on dimethyl sulfoxide (DMSO) and thiourea (TU) was used to prepare Cu3BiS3 thin films. The effects of TU dosage on the structural, optical and electrical properties of Cu3BiS3 were systematically discerned. The obtained Cu3BiS3 shows the band gap ranging from 1.40 to 1.68 eV and the carrier concentration in the order of 1016 cm−3. Although a stoichiometric overdose of TU was usually applied in preparing high-quality chalcogenide semiconductor films, our results illustrate that properly increasing concentration of TU in precursor solution enhances the grain size and carrier mobility, while much higher TU supply tends to deteriorate them. Correspondingly, the efficiency of device with the structure of Mo/Cu3BiS3/CdS/ZnO/ITO/Al exhibits the similar tendency. The best PCE of 0.291% occurs for the precursor solution with TU concentration of 3.0 M, and the corresponding VOC, JSC and FF values are 233 mV, 2.867 mA/cm2 and 43.552%, respectively.

Прямий лазерний запис мікрорельєфних структур в халькогенідних склах лазерним променевим записувачем мастер-дисків

Methods to create microrelief phase elements for diffractive optics and ophthalmology were analyzed. Direct laser writing, by a laser beam recorder used for master discs is an effective method for the formation of flat microrelief phase elements on thin films of chalcogenide semiconductors, where photo-structural transformations occur under the action of laser radiation.

Comparative analysis of the thickness and electrical conductivity of thin chalcogenide semiconductor films

The structure and thickness of zinc and cadmium chalcogenide semiconductor films are studied by X-ray radiography. The film thickness is shown to be comparable with the half-value layer depth. The electrical conductivity of the films increases upon heating in the hydrogen atmosphere and decreases upon heating in carbon oxide. The opposite trend is observed in the ratio between the electrical conductivity and band gap of the initial and oxidized film surfaces.

Study on optical investigations and DC conduction mechanism in polycrystalline chalcogenide (Cd, Zn) semiconductor films grown by screen-printing method

The use of thin films of polycrystalline chalcogenide semiconductors has attracted much interest in an extensive variety of applications in various electronic, optoelectronic and photovoltaic devices due to their favorable electrical and optical properties. In this paper II–VI compound (cadmium and zinc chalcogenide) binary and ternary semiconductor thick films were deposited on ultraclean glass substrates by screen-printing method followed by sintering process. The polycrystalline nature of these films was confirmed by X-ray diffraction studies. The stoichiometry of the films was confirmed using energy dispersive X-ray analysis. The optical band gap and absorption coefficient of the films were determined by reflectance measurement in wave-length range 350–900 nm using UV/Vis spectrophotometer. The conduction mechanism in these films were studied by measuring the dark DC electrical conductivity in 300–400 K temperature range by using standard two probe method in vacuum. It was observed that conductivity of these films increases with increase in temperature. The linear nature of plot ln σDC against 1000/T for these films indicates that conductivity of films is in close agreement with the Arrhenius relation and conduction in these films is through thermally activated process.

Surface pattering of Ge–As–Se thin films by electric charge accumulation

We deposited electric charge in periodic manner in form of square lattices of points in Ge–As–Se chalcogenide semiconductor films by electron beam. Various types of surface reliefs are observed as a result of interaction of electron beam with the semiconductor thin films. For the films where charge removal is facilitated by conducting gold underlayer shape of surface relief is similar for all deposited doses per point. For the films with no gold underlayer, where charge removal is hindered, the shape of local relief depends on the dose per point and at higher doses also on the lattice period. When the lattice period is larger than interaction volume of electron in the material the identical surface reliefs consisting of spire and crater are formed during each charge deposition. When the lattice period is smaller than the interaction volume of the electron identical surface reliefs are formed only in every second charge deposition event. As a result highly organized checkerboard-like lattices are formed. We suggest that a reason for the appearance of such complex periodic structures is charge accumulation in these films. When the ratio between the size of space charge region and lattice period is small enough there would be influence of already deposited charge on the charge deposition in the next step. This opens possibility to fabricate structures on the surface of these materials by combining charge already accumulated in the material with charge deposited in consecutive step. We also show that it is possible to experimentally define the size of space charge region in chalcogenide thin films.

Phase transitions in thin Ge2Sb2Te5 chalcogenide films according to Raman spectroscopy data

Data on the Raman spectra of thin Ge2Sb2Te5 chalcogenide semiconductor films are reported. The study is performed with the purpose of determining the temperatures of phase transitions initiated by laser radiation.

Spin coating-Co-reduction approach: A general strategy for preparation of oriented chalcogenide thin film on arbitrary substrates

A new and general spin coating-co-reduction strategy for synthesis of c-axis oriented (Bi,Sb)2(Te,Se)3 chalcogenide thin films and multi-layered structures on different substrates was reported. This method combines spin coating technique and co-reduction chemical reaction processes using commercial nitrates and oxides as raw materials. This simple and reliable method provides a new approach for fabricating chalcogenide semiconductor or semi-metal thin films, and will have wide application potential in opto-electric, thermoelectric, phase-change memory, chemical sensor, and spintronic devices.

Electrical conductivity of CuI-AsI3-As2Se3 and CuI-SbI3-As2Se3 chalcogenide films prepared by chemical deposition

The electrical conductivity of chalcogenide semiconductor films in the CuI-AsI3-As2Se3 and CuI-SbI3-As2Se3 systems, which have been prepared by chemical deposition from mono-n-butylamine, has been studied as a function of the temperature and film composition. It has been established that the electrical conductivity of the CuI-AsI3-As2Se3 and CuI-SbI3-As2Se3 films is predominantly determined by the copper iodide content. It has been demonstrated that the electrical properties of the chalcogenide glasses and the related films are characterized by the same values to within the experimental error, which is explained by the same model of dissolution of vitreous semiconductors in amines with the retention of the electrical properties of chalcogenide glasses after the deposition of films from their solutions.

Electrical conductivity of CuI-Cu2Se-As2Se3 chalcogenide films prepared by chemical deposition

The electrical conductivity of CuI-Cu2Se-As2Se3 chalcogenide semiconductor films prepared through the chemical deposition from an organic solvent has been investigated as a function of the temperature and composition of the films. It has been established that the electrical properties of the chalcogenide glasses and related films are characterized by the same values within the limits of the experimental error. This result is in agreement with the model of dissolution of vitreous semiconductors in organic bases (amines), according to which the main properties of bulk (cast) chalcogenide glasses are retained in films prepared from these glasses.

Small-size spectrometer for emission analysis of low-temperature plasma flows

Calculation data for characteristics of concave holographic arrays are reported, allowing development of low-size spectrometers with medium and high spectral resolution. The holographic arrays were fabricated around new subnanostructured films of chalcogenide semiconductors. A medium-resolution, small-size spectrometer was used to measure the distribution of temperature in the gas flow emanating from a DC plasmatorch with sectioned inter-electrode insert by the relative-intensity method. Presently available methods for measuring the temperature of non-uniform optically thin plasma were employed. A comparison of spectral and thermophysical methods for temperature measurements is given.

Effect of electric field on photoinduced changes in the optical properties of chalcogenide glassy semiconductors

The influence of an applied electric field on the photoinduced changes in the optical properties of glassy chalcogenide semiconductor films of the (As2S3)x(As2Se3)1−x solid solution system has been studied in thin-layer structures of the following types: metal-semiconductor-metal, metal-semiconductor-insulator-metal, metal-semiconductor-ionic electrode, and metal-semiconductor-insulator-ionic electrode. The ionic electrodes were created on polymer substrates in a xerographic regime. It is established that the applied electric field significantly influences the photoinduced changes in the optical properties of chalcogenide glassy semi-conductors. Possible mechanisms of the observed electrophotoinduced effect are formulated and its potential applications are indicated.

Recording information in thin films of chalcogenide semiconductors by using photoinduced transformations

This paper discusses how the half-width and the depth of the phototransformed region of a chalcogenide semiconductor depend on the power of the recording Gaussian laser beam. It is shown that the shape of the region is close to parabolic or trapezoidal, depending on the parameters of the photosensitive material and the power of the recording beam. The results can help to select the necessary conditions for optimizing the information-recording regime in thin films of chalcogenide semiconductors. The proposed model qualitatively describes the characteristics of the pits obtained in thin films of As40S60 after positive selective etching. © 2005 Optical Society of America

The Study on the Characteristic of Phase Transition in Differential Thickness of Se1Sb2Se2Thin Films

The phase transition between amorphous and crystalline states in chalcogenide semiconductor films can be controlled by electrical or pulsed laser beam; hence some chalcogenide semiconductor films can be applied to electrically write/erase nonvolatile memory devices, where the low conductive amorphous state and the high conductive crystalline state are assigned to binary states. In this letter, the characteristics of phase transition in differential chalcogenide thin film are investigated. Al was used for the electrode as the thickness of 100, 300, 500 nm, respectively.

Amorphous As–S–Se semiconductor resists for holography and lithography

The photo- and electron-beam induced changes in solubility of thin films of the amorphous chalcogenide semiconductors As–S–Se and As 2S 3 have been studied. The possibilities of practical application of these materials as resists for the production of relief holograms and holographic optical elements are discussed. It is shown that the self-enhancement (SE) phenomenon of holographic recording in amorphous chalcogenide semiconductor films by light or thermal treatment can be used to increase the diffraction efficiency (DE) of the holograms.

Electrical conduction in epitaxial BeSeTe/Si

Epitaxial wide-band gap II–VI beryllium chalcogenide semiconductor films BeTe, BeSe, and BeSeTe were grown on arsenic-terminated silicon substrates. Electrical characteristics of Schottky contacts to the films were measured. Current-temperature measurements at elevated temperatures showed conduction band offsets of 1.2 eV for the BeSe0.41Te0.59/As/Si and 1.3 eV for the BeSe/As/Si heterostructures. At room temperature, the current density through BeSe/Si and BeSe0.41Te0.59/Si films was mid-10−9 A/cm2 at 0.1 MV/cm, similar to previously reported values for ZnS/Si, while BeTe/Si films had orders of magnitude higher current density possibly due to interfacial recombination.

Growth characteristics of vacuum coated thick a-Se films for device applications

There has been a resurgence of interest in the growth characteristics of amorphous Se (a-Se) films due to their recently commercialized applications in x-ray imaging as x-ray photoconductors. Electronic quality a-Se films require the substrate temperature during vacuum deposition to be as high as possible but increasing the substrate temperature leads to amorphous films containing crystalline inclusions and eventually to polycrystalline films which have high conductivity and are therefore not useful as photoconductors. We prepared a variety of thick a-Se films on Al substrates by conventional vacuum deposition in which only the substrate temperature Tsb and boat temperature Tbt were varied. Experiments indicated the appearance of crystalline zones (crystallites) in the amorphous Se film as the substrate temperature was increased. We identified an experimental critical Tbt−Tsb boundary between amorphous films with and without crystalline inclusions. A model has been developed for the critical relationship between Tsb and Tbt, i.e., Tbt=f(Tsb) based on two competing rate mechanisms: the rate of deposition (or material condensation on the substrate) and rate of crystal growth in the film assuming heterogeneous nucleation at favorable substrate sites. Over the narrow substrate temperature range, we approximated the Vogel–Tamman–Fulcher behavior of the crystal growth rate by an Arrhenius rate with an apparent activation energy Ecr that accounts for most crystallization studies on a-Se. The condensation rate was assumed to follow an Arrhenius rate with the activation energy determined by the enthalpy of evaporation, Eev. The Tbt=f(Tsb) model was particularly sensitive to the Ecr/Eev ratio. By using the presently accepted values for Eev and Ecr, it was shown that the experimental results could be well fitted to the Tbt=f(Tsb) model. The results have a wider significance than a-Se and can be easily extended to apply to a number of other chalcogenide semiconductor films.

Crystal-glass phase transition induced by pulses of electric field in chalcogenide semiconductors

It is shown that the minimum power of an electric field which induces the crystal-glass phase transition in micrometer-thick films of chalcogenide semiconductors does not depend on pulse duration τ for τ>10 µs and increases, as τ decreases to τ<1–3 µs. The dependence obtained is similar to that observed for crystal-glass phase transition induced by pulses of laser radiation.