Chalcogenide Glassy Semiconductors Research Articles

Glass transition and crystallization of Se95Te5 chalcogenide glassy semiconductor

The study is dedicated to the investigation of thermo-physical characteristics of Se95Te5 chalcogenide glassy semiconductor during its glass formation and crystallization processes, employing various scanning rates of 5, 10, 15 and 20 K/min in non-isothermal modes through DSC measurement. Analysis of the structural relaxation kinetics involves the Kissinger’s, Augis and Bennett's, as well as Matusita’s approaches. Experimental data yield contains the determination of crucial parameters such as glass transition (𝑇𝑇𝑔𝑔), crystallization(𝑇𝑇𝑐𝑐), and melting temperatures alongside factors like reduced temperature of glass transition (𝑇𝑇𝑟𝑟𝑟𝑟), Hruby’s parameter (𝐾𝐾𝑔𝑔𝑔𝑔), fragility index (𝐹𝐹𝑖𝑖), Avrami exponents (n, m), glass transition (140.24 kJ/mol) and crystallization (Ec = 95.11 kJ/mol) energies, respectively. The results confirm that Se95Te5 chalcogenide system as an efficient glass former. Matusita’s method reveals that the crystallization mechanism (n = 2.51, m = 1.9) corresponds to volumetric nucleation with two-dimensional growth.

Current Filamentation and Switching Effect in Chalcogenide Glassy Semiconductors: A Review

A review on current filamentation and switching effect in chalcogenide glassy semiconductors (CGSs), which are promising materials for the development of phase change memory devices, is presented. First, a history of the research on CGSs and their properties is considered. Next, formation of a current filament in CGSs is discussed and the scale of heat release in the material as well as the geometric shape of the filament is analyzed. Finally, various hypotheses developed for the explanation of the switching effect in CGSs are reviewed. It is shown that the most relevant model of the switching effect in CGS is the model of multi-phonon tunneling ionization of the so-called ‘negative-U centers’. This model is based on the assumption that an avalanche-like increase in current at a certain point in time is associated with mass tunneling of electrons located on atoms, occuring due to thermal vibrations of atoms.

Висмут қоспасы бар радиацияға төзімді наноөлшемді GE2SB2TE5 қабықшалардағы ауысу және жады эффектісінің ерекшеліктері

The physics and technology of noncrystalline materials is one of the rapidly developing areas of condensed matter physics, materials science, and nanotechnologies. Among the variety of materials with a non-crystalline structure, a special place is occupied by the class of chalcogenide glassy semiconductors (CGS) with unique properties and phenomena, such as the switching effect, photostructural transformations. The switching effect is observed in thin CGS films and is associated with a fast-reversible phase transition of the structure from the glassy state to the crystalline state under the action of voltage pulses or laser radiation. Based on this effect, a separate class of information carriers has been formed for non-volatile storage devices such as PCM (Phase Change Memory) with high performance and reliability. Materials for use in PCM devices are CGSs of complex compositions lying on the GeTe-Sb2Te3 quasi-binary boundary, and thin films of Ge2Sb2Te5 composition are the most promising for use in PCM devices. The ability to control the electronic properties of such functional materials based on CGS significantly expands the scope of their application and is an important scientific and practical task, since traditional methods for purposefully changing the electronic properties of crystalline semiconductors, such as doping during synthesis or the thermal diffusion method, turned out to be ineffective for CGS. Impurities introduced into CGS at low concentrations by the methods indicated above usually do not show electrical activity, since they are compensated by their charged structural defects, and an increase in the impurity concentration leads to significant crystallization. In this work, the atomic composition of Ge2Sb2Te5 nanosized films with a bismuth content of more than 12 at.%, obtained by ion-plasma deposition in the direct current mode, was studied by the method of energy dispersive analysis using a scanning electron microscope. In addition, the current–voltage characteristics of the samples were measured. It has been found that the transition voltage and the time, which characterize the switching and memory effects, depend significantly on the film thickness and composition. The addition of bismuth reduces the film switching time. A decrease in the film thickness leads to a decrease in the threshold voltage Uth and the switching time. We believe that the results of this work are useful for the development of optical memory based on phase change materials.

Formation of holographic diffraction gratings in thin films of chalcogenide glassy semiconductors

The paper presents a study of the formation of holographic diffraction gratings in thin films of chalcogenide glassy semiconductors. The recording process of holographic gratings at the argon-laser radiation wave length 488 nm and the process of chemical etching that enables the formation of а relief holographic grating are analysed. The optimum conditions for the formation of diffraction gratings in films of arsenic sulfide As2S3 are defined. It is shown that at the 488 nm wave length of an argon laser the optimum exposure comes to ∼5–8 J/cm2. At the recording stage a quasi-phase (relief-phase) grating is formed, with the diffraction efficiency on the order of a few per cent. Etching of the exposed sample with a solution of NaOH alkali in deionised water and isopropanol makes it possible to increase considerably the relief depth and to improve the diffraction efficiency of a thin diffraction grating approximately up to 20 % for the red spectral region, and to approach the maximal value ∼34 % for the near infra-red region. The results of the study considered look promising for the creation of relief holographic gratings which are essential in present-day optical instrument building (production of spectral devices, holographic sights, and the like).

Оцінювання впливу зовнішніх факторів на параметри бістабільних перемикальних пристроїв на базі аморфних напівпровідників

The influence of temperature and radiation on the threshold voltage and switching delay time of the switching device based on ChGS is evaluated. It is shown that when the temperature changes from 250 to 350 oК, the threshold voltage changes within 5 mV, and the temperature voltage coefficient is about 0,03 %/deg, which is much less compared to similar devices based on single-crystal semiconductors. Analytical expressions were obtained and a study of the dependence of the switching delay time on the temperature and density of capture traps near the Fermi level was performed. The results of studies of the dependence of the switching delay time on the temperature show that when the temperature changes from 250 to 350 К, the delay time changes by 0,35 ps, and the temperature coefficient of the delay time does not exceed 0,42 %/deg. Moreover, with increasing mobility of the charge carriers, the delay time decreases. The study of the dependence of the switching delay time on the density of the capture traps showed that this dependence is practically linear and when the density of the capture traps increases by an order of magnitude, the delay time also changes by an order of magnitude. Analytical dependences of charge carrier mobility and switching delay time on neutron flux density are obtained. It is shown that the change in neutron flux density up to 1016 N/cm2 the mobility of charge carriers slowly decreases from 10–2 to 0,99 10–2 cm2/V·s, and with a further increase in the dose to 1018 N/cm2 the mobility decreases twice and is 0,5 10–2 cm2/V·s. The switching delay time with increasing irradiation dose from 1010 to 1016 N/cm2 increases from 26,2 ns to 26,8 ns, and with a further increase in the neutron flux density, the delay time increases rapidly and is 57 ns at a neutron flux density of 1018 N/cm2. The results of the research indicate that switching devices based on chalcogenide glassy semiconductors are characterized by higher radiation resistance in comparison with devices based on single-crystal bipolar semiconductor components, the amplifying properties of which change at a neutron flux to 1012 N/сm2 decrease more than twice.

WIDTH OF ENERGY BAND GAP OF NANOPOROUS SEMICONDUCTOR FILMS

The aim of this work is to experimentally clarify the reasons for the appearance of jumps in the current and memory of semiconductor nanoporous structures.Porous nanostructures were obtained by electrochemical etching. The current-voltage characteristics of the samples were measured for porous silicon and on thin films of a chalcogenide glassy semiconductor. The existence of jump-like switching and current hysteresis in porous silicon nanofilms under laser illumination is shown experimentally.A connection between the switching voltage values and the dependence of the band gap on the porosity of nanofilms is found. These results make it possible to construct a theory of current switching and its hysteresis based on the concepts of the theory of second-order phase transitions.

INCREASING RADIATION RESISTANCE OF MEMORY DEVICES BASED ON AMORPHOUS SEMICONDUCTORS

A memory cell structure is proposed that uses a Schottky barrier thin film transistor based on an amorphous semiconductor as a junction element, and a chalcogenide glassy semiconductor film as a switching element. A physical storage cell model has been developed. The dependence of the transistor and memory cell parameters on the dose of neutron flux and γ - quanta was investigated. It is shown that when the dose of neutron irradiation is changed, the steepness of the drain-gate characteristic (DGC) decreases by 10% at a dose of the order of 1015 n/s, at the same time, the transfer coefficient of the bipolar n-p-n transistor decreases by 20% already at doses of 1013 n/s, indicating a significant increase in the radiation resistance of the proposed memory cell. In the case of irradiation with γ - quanta in the range up to 2.6 MRad, the steepness of the DGC of the proposed structure changes by only 10%. When used as an isolation element, a field-effect transistor with an insulated gate, the slope of the DGC is reduced by 50%. It is shown that the current of recording information of the proposed structure when changing the dose of γ - quantum flux to 2.6 MRad changes by about 10%, at the same time, in the case of using a field-effect transistor with an isolated cover, the information recording current changes by 50%. The study of the dependence of the gate current on the dose of γ – quanta is showed. When the radiation dose changes from 0 to 2.6 MRad, the gate current changes only by 10%, which indicates the high resistance of the proposed structure to the action of permeable radiation. Also, studies of the dependence of the conductivity of single-crystal semiconductors on the radiation dose ɣ by quanta and neutron flux show that a significant increase in the specific resistivity of AS occurs at doses 2-3 orders of magnitude larger than in the case of single-crystal n-type conductivity semiconductors.

Використання халькогенідних склоподібних напівпровідників для створення мікро- та нанорозмірних структур

The results on the analysis of methods for the formation of nanoscale elements on thin films of chalcogenide glassy semiconductors and their application to create optical diffraction elements are presented. When creating micro- and nanoelements of optoelectronic devices, micro / nanoelectromechanical systems (MEMS/NEMS) and diffractive optical elements, chalcogenide glassy semiconductors (CGS) can be effectively used. The use of CGS is mainly based on their sensitivity to various types of radiation, which cause structural changes in them and transparency for infrared light. Numerous studies have been carried out aimed at studying the processes of formation of nanostructures on CGS films. A number of photoinduced changes are observed in CGS, which are associated with structural transformations, phase transitions, defect formation, and atom diffusion. It is of interest to determine the technologies for the formation of micro- and nanoscale structures on CGS films that can be used to create diffractive optical elements for optoelectronic devices. Increasing the resolution of recording media based on vitreous chalcogenide semiconductors can be achieved by choosing recording modes and glass compositions, in which the nonlinearity of the exposure characteristics of photosensitive material is most pronounced, as well as the introduction of noble metal nanoparticles for excitation of precious metals for excitation. A promising area of application of chalcogenide glassy semiconductors can be direct laser recording of diffractive optical elements at upgraded laser beam recorder of master disks. The unique properties of chalcogenide glassy semiconductors allow the formation of microrelief images without selective chemical etching due to the irradiation of thin films with rays of different polarization. Fig.: 8. Refs: 41 titles.

Formfactor of a hologram on a chalcogenide glassy semiconductor and azopolymer

In this work, the formfactor influence on the diffraction efficiency of 2D and 3D phase holograms was analyzed. Experimental results showed that holograms recorded in a chalcogenide glassy semiconductor and an azopolymer have limitations on maximum achievable diffraction efficiency. The coefficient of optimal exposure increase that is necessary to achieve maximum achievable diffraction efficiency was obtained. Due to the difference between the values of the formfactor in the case of the Raman-Nath diffraction and of that in the case of the Bragg diffraction for diffraction on thin (2D) holograms, the value of the formfactor turned out to be larger than that for diffraction on volume (3D) phase holograms.

Radiation-Resistant Memory Device Based on Chalcogenide Glassy Semiconductor

A memory cell structure is proposed that uses a Schottky barrier thin film transistor based on an amorphous semiconductor as a junction element, and a chalcogenide glassy semiconductor film as a switching element. A physical storage cell model has been developed. The dependence of the transistor and memory cell parameters on the dose of neutron flux and γ - quanta was investigated. It is shown that when the dose of neutron irradiation is changed, the steepness of the drain-gate characteristic (DGC) decreases by 10% at a dose of the order of 1015 n/s, at the same time, the transfer coefficient of the bipolar n-p-n transistor decreases by 20% already at doses of 1013 n/s, indicating a significant increase in the radiation resistance of the proposed memory cell. In the case of irradiation with γ - quanta in the range up to 2.6 Mrad, the steepness of the DGC of the proposed structure changes by only 10%. When used as an isolation element, a field-effect transistor with an insulated gate, the slope of the DGC is reduced by 50%, that it is bad result. It is shown that the current of recording information of the proposed structure when changing the dose of γ - quantum flux to 2.6 Mrad changes by about 10%, at the same time, in the case of using a field-effect transistor with an isolated cover, the information recording current changes by 50%. The study of the dependence of the gate current on the dose of γ – quanta is showed. When the radiation dose changes from 0 to 2.6 Mrad, the gate current changes only by 10%, which indicates the high resistance of the proposed structure to the action of permeable radiation.

Application of the Method of X-Ray Fluorescence Analysis to Determine the Composition of Glassy and Crystalline Alloys of the Systems As<sub>x</sub>S<sub>1-x </sub>and As<sub>x</sub>Se<sub>1-x</sub>

The article presents an X-ray fluorescence analysis of chalcogenide glassy semiconductors. The standard method is applied to determine the concentration of arsenic, selenium, and sulfur in alloys. This technique, the quantitative composition of the glasses is defined with an accuracy of ± 0.0002.

The Effect of the Chemical Composition on the Structure and Physical Properties of Chalcogenide Glassy Semiconductors of the As–Ge–Se System

Using the methods of X-ray diffraction and density measurement (in the short- and middle-order scale), the influence of the chemical composition on the parameters of the local structure and the physical properties of chalcogenide glassy semiconductors (CGS) of the As–Ge–Se system is studied. The changes occurring in the numerical values of the correlation length and quasi-period in the middle-order region, as well as the packing coefficient, compactness, average atomic volume, and the number of constraints, are interpreted within the cluster-void model and the theory of topological constraints by the existence of the chemical percolation threshold.

Effect of the Samarium Impurity on the Local Structure of Se95Te5 Chalcogenide Glassy Semiconductor and Current Passage through Al–Se95Te5〈Sm〉–Te Structures

The effect of samarium doping on the local structure and morphological features of the surface of a Se95Te5 chalcogenide glassy semiconductor film is investigated by X-ray diffraction analysis and atomic force microscopy, and the effect of doping on the current flow through Al–Se95Te5〈Sm〉–Te structures is examined by measuring the I–V characteristics in the steady-state mode. The results obtained are interpreted using Lampert’s theory of injection currents, the Elliott void-cluster model, and the Mott and Street charged defects model proposed for chalcogenide glasses.

Structure of Se95As5 Chalcogenide Glassy Semiconductor Doped by EuF3 Impurity

The local structure of film samples of the chalcogenide glassy semiconductors Se95As5 and Se95As5(EuF3)x (x = 0.01–1 at %) are studied by X-ray diffraction and Raman scattering. The “quasiperiod” of the structure, the correlation length, the structural elements, and the chemical bonds, which form the amorphous matrix of the investigated materials, are determined. The obtained results are interpreted within the framework of the Elliott void–cluster model taking into account the chemical activity of europium ions.

Structure and Optical Properties of Chalcogenide Glassy Semiconductors of the As–Ge–Se System

The types of structural elements and chemical bonds forming an amorphous matrix of chalcogenide glassy semiconductors of the As–Ge–Se system and changes occurring in them depending on the chemical composition are determined using a method for studying the Raman and transmission spectra and density measurements. It is shown that a transition from the elastic to isostatic state and then to the stressed-rigid state occurs in the system under study with increasing arsenic and germanium contents. The observed changes in the Raman spectra and the features of the dependences of optical and other parameters on the chemical composition are explained within the chemical-ordering model, taking into account the existence of local states near the allowed band boundaries.

Determination of the composition of chalcogenid glasses ASxSE1-x by the method of X-ray fluorescent analysis

The features of X-ray fluorescence analysis of chalcogenide glassy semiconductors are considered in the research. The standard method was used to determine the concentrations of arsenic and selenium AsxSe1-xalloys. The use of this method allows determining the quantitative composition of glasses with an accuracy of ± 0.0002.

Підвищення радіаційної стійкості енергонезалежних запам’ятовувальних пристроїв на базі халькогенідних склоподібних напівпровідників

The work is aimed at increasing the radiation resistance of storage devices, which are one of the main elements in the construction of information processing systems. The proposed structure of a radiation-resistant memory cell, in which a film of a chalcogenide vitreous semiconductor is used as a switching element, and a unipolar transistor serves as a decisive element. The switching element is resistant to radiation, and the radiation stability of a unipolar transistor is several orders of magnitude lower. Therefore, a method for increasing the radiation stability of unipolar transistors is proposed. To eliminate the change in the parameters of unipolar transistors under the action of ionizing radiation on the processes occurring in the subgap layer of the dielectric and at the interface between silicon and silicon dioxide, it helps to accumulate a positive charge in the manufacture of such a transistor, prior to the high-temperature formation of the subgap layer of the dielectric, to conduct the subgap region of the semiconductor substrate ion implantation of fluorine, which diffuses into this layer. In addition, not silicon dioxide, but silicon nitride — Si3N4 is used as the sub-barrier layer of the dielectric, which also contributes to an increase in radiation stability. A physical model of a memory cell based on chalcogenide glassy semiconductors has been developed, and analytical expressions have been proposed to calculate the dependence of the model parameters on the radiation dose.

Влияние примеси самария на локальную структуру халькогенидного стеклообразного полупроводника Se-=SUB=-95-=/SUB=-Te-=SUB=-5-=/SUB=- и механизм прохождения тока через структуры Al-Se-=SUB=-95-=/SUB=-Te-=SUB=-5-=/SUB=-<Sm>-Te

Effect of samarium doping on the local structure and morphological features of films surface of the Se95Te5 chalcogenide glassy semiconductor system have been investigated by X-ray analysis and atomic force microscopy methods, and the influence of doping on the current passing mechanism through the Al-Se95Te5 <Sm> -Te structures have been also considered by measuring the current-voltage characteristics in a stationary mode. The results obtained are interpreted on the basis of the theory of Lampert injection currents, the Elliott void-cluster model and the Mott and Street charged defect model proposed for chalcogenide glasses.

Cтруктура и оптические свойства халькогенидных стеклообразных полупроводников системы As-Ge-Se

AbstractThe types of structural elements and chemical bonds forming an amorphous matrix of chalcogenide glassy semiconductors of the As–Ge–Se system and changes occurring in them depending on the chemical composition are determined using a method for studying the Raman and transmission spectra and density measurements. It is shown that a transition from the elastic to isostatic state and then to the stressed-rigid state occurs in the system under study with increasing arsenic and germanium contents. The observed changes in the Raman spectra and the features of the dependences of optical and other parameters on the chemical composition are explained within the chemical-ordering model, taking into account the existence of local states near the allowed band boundaries.

Структура халькогенидного стеклообразного полупроводника Se-=SUB=-95-=/SUB=-As-=SUB=-5-=/SUB=-, легированного примесью EuF-=SUB=-3-=/SUB=-

The local structure of film samples of chalcogenide glassy semiconductor Se95As5 and Se95As5(EuF3)x (x = 0.01 ÷ 1 at%) have been studied by X-ray diffraction and Raman scattering. The ‘‘quasi-period’’ of the structure, the correlation length, the structural elements and chemical bonds that form the amorphous matrix of the materials studied have been determined. Interpretation of results obtained has been carried out within the framework of the Elliot voidscluster model, taking into account the chemical activity of europium ions.