Photostructural Transformations Research Articles

Photoinduced transformations in (As1–xBix)2S3 glass observed by Raman spectroscopy

AbstractRaman spectra of (As1–xBix)2S3 glass samples with x ≤ 0.2 measured at the excitation with above‐bandgap (532 nm) laser light at a relatively low power density (Pexc = 4 kW/cm2) clearly confirm the amorphous character, thereby markedly extending the known compositional interval of existence of the (As1–xBix)2S3 glass previously known (x ≤ 0.06). Spectra measured at an increased Pexc (40 kW/cm2) reveal a photostructural transformation in the illuminated area of the glass leading to an additional contribution of Bi–S bonds as well as to an increasing number of cage‐type As4S4 units with homopolar As–As bonds. A number of new features in a broad range up to about 1,000 cm−1, which emerge in the Raman spectra of the (As1–xBix)2S3 glasses with high (x ≥ 0.14) Bi content and increase in intensity with the exposure time, are related to a photochemical transformation, namely, oxidation of arsenic and sulphur on the (As1–xBix)2S3 glass surface with formation of units containing arsenate AsO43− and sulphate SO42− ions. These processes are irreversible and occur only in the presence of a sufficient amount of bismuth.

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.

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.

The influence of laser radiation on the structure and optical properties of amorphous films in arsenic–antimony–sulphur system

Purpose. As40 − x Sb x S60 amorphous films are suitable for creation of highly efficient, diffraction gratings, optical diffraction elements, optical compact-disks and sensors, waveguides, elements for infrared and nonlinear optics. This paper is devoted to investigation of Raman spectra of glasses and films and transmission spectra of films in As-Sb-S system with the Sb content up to 12 at % and their changes under laser radiation. Methods. Raman and optical spectroscopy, atomic forse microscopy. The transmission spectra were studied in 400-750 nm range at room temperature using a diffraction monochromator “MDR-23”. The spectral resolution was no worse than 10-3 eV. Results and discussion: Increased concentration of antimony in the composition of As40 − x Sb x S60 (0 ≤ x ≤ 12) amorphous films and laser irradiation ( λ =530 nm) of them result in the shift of their absorption edge to the longwave range. In this case the pseudogap width E g decreases, whereas the index n increases. Under the same conditions of irradiation, the largest changes in optical parameters occur in the As36Sb4S60 film. Changes in the optical characteristics of films are caused by photostructural transformations taking place in them under the laser irradiation. The nanoheterogenous structure of the studied films was established. The structural matrix of As40 − x Sb x S60 films is built from As(Sb)S3 pyramidal units connected by bridging S atoms. No molecular fragments with Sb-Sb bonds in the structure of films were detected. However, the large amount of structural groups with homopolar As-As (As4S4, As4S3) and S-S bonds (Sn) are present in the structure of films. The laser irradiation of As40 − x Sb x S60 films leads to the breaking and switching of As-As and S-S bonds in As4S4, As4S3, and Sn type structural fragments. Accompanied with the formation of structural units with heteropolar bonds As-S (AsS3). These structural transformations are related with the decreasing content of molecular fragments possessing homopolar bonds in films matrix. Conclusions. Transmission and Raman spectra of as-prepared and irradiated As40 − x Sb x S60 amorphous films were investigated. The nanoheterogenous structure of the studied films was confirmed by Raman spectra. It was established that the growth of the Sb content in composition of films and irradiation induced the absorption edge shift to the longwave region, E g decreased and n increased. The largest changed in optical parameters of as-prepared and irradiated films occur in the As36Sb4S60 film. These changes are caused by the photostructural transformations, which are accompanied by decreasing in the number of structural groups with homopolar bonds in films matrix.

Structural origin of surface transformations in arsenic sulfide thin films upon UV-irradiation

Photostructural transformations within AsxS100-x (x=30, 33, 35, 40) thin films upon exposure to LED light of different wavelengths, in both air and argon environments have been studied by high resolution XPS, Raman spectroscopy and LEIS methods. These complementary results show that light of energies close to the band gap does not modify chemical composition of the surface, but induces simple photopolymerization reactions. Superbandgap UV light, however, significantly increases S/As ratio on the surface due to formation of S-rich layer under both environmental conditions. It is proposed that photovaporization of both oxide and non-oxide cage-like molecules is responsible for the observed effect.

High-speed optical recording in vitreous chalcogenide thin films

Thin films of glassy chalcogenide semiconductor are widely used as recording media in optical data storage. To obtain relief micro- and nanoscale structures on the surface of optical master discs inorganic photoresists based on chalcogenide glassy semiconductors can be used. They have high resolution and allow for exposure by short laser pulses. Implementation of such exposure is promoted by increasing the speed of photostructural transformations at high powers of exposing radiation. This increase in the sensitivity is associated with both local heating by illumination and a high density of excited electron-hole pairs. The exposure mode of the inorganic photoresists based on glassy chalcogenide semiconductor pulses of 10 -8 -10 -9 s is close to the threshold of local photothermal destruction. Significant impact on the value of the threshold of photothermal destruction effects the choice of the substrate material which determines the rate of heat removal from the irradiation area. Moreover, one also needs to consider the effect of pulsed annealing of the inorganic photoresist material on the process of selective etching. We have established an inversion of the selective etching of the inorganic negative photoresist based on As2S3 in the center of the irradiated zone. The diameter of this zone is about 20% of the diameter of exposing beam. After the selective etching in alkaline solution in the center of protrusions being formed on the substrate, there observed are some dimples with the depth of 30-50 nm. Prior to the processing of irradiated inorganic photoresist by the selective etching these dimples were absent and their appearance is not due to possible local material evaporation of the inorganic photoresist. A possible reason for the inversion of solubility of the inorganic photoresist could be pulsed annealing in the recording process.

Wavelength Dependence of Photostructural Transformations in As2S3 Thin Films

Abstract Spectral dependence of photostructural transformations on the surface and in the interior of thermally evaporated As 2 S 3 thin films is studied under the influence of bandgap, sub-bandgap and super-bandgap light. For the given light intensity (W~15 mW/cm 2 ) the irradiation by bandgap and super-bandgap light leads to partial restoration of pyramidal structural units in the film, which is a characteristic of the bulk glasses of this stoichiometric composition. The mechanism of photostructural transformations on the surface of the film exhibits strong spectral dependence. S- rich layer is formed upon irradiation by super-bandgap UV light. The sub-bandgap light does not cause any noticeable photostructural transformations at this light intensity.

Noise properties of cadmium selenide based photoresistors under background illumination

Investigations of the noise properties of CdSe based photoresistor demonstrate that under background illumination of definite power, a noise minimum is observed in its dependence on the bias voltage. Dependences of the main parameters of the established noise minimum on the background illumination power are investigated. Based on the calculated noise and photoconductivity voltages, it is demonstrated that the detected noise minimum can be due to only the special features of forming variances of the number of free charge carriers. It is assumed that a decrease in the number of fluctuations of the number of charge carriers in the allowed bands is probably due to reversible photostructural transformations in the semiconductor lattice caused by capture of nonequilibrium holes.

Enhancement of photoinduced transformations in amorphous chalcogenide film via surface plasmon resonances

Laser-matter coupling results specific structural changes in amorphous chalcogenide semiconductor layers which originate from electron-hole excitations, defect creation or modification and subsequent atomic motions. These changes can be influenced by plasmon fields. Plasmon enhanced photo-darkening and bleaching, optical recording in thin As xSe 1 − x films have been demonstrated in this paper, specifically in As 20Se 80 and As 2Se 3 compositions which revealed the best effects of stimulated expansion or optical darkening respectively due to the He–Ne laser (λ = 633 nm) illumination. Gold nanoparticles deposited on the silica glass substrate and covered by an amorphous chalcogenide film satisfy the conditions of efficient surface plasmon resonance in this spectral region. These experimental results support the importance of localized electric fields in photo-structural transformations of chalcogenide glasses as well as suggest better approaches for improving the performance of these optical recording media.

Photostructural transformations in chalcogenide films

The results of an investigation of photostructural transformations in thin layers of chalcogenide materials obtained by X-ray diffraction analysis are presented. The dynamics of the process can be described by an exponential temperature dependence with the activation energy ΔE = 0.2 eV. The X-ray diffraction results allow us to attribute the amorphous chalcogenide semiconductor layers to recording materials with a spatial resolution at the molecular level.

Chalcogenide glassy photoresists: History of development, properties, and applications

The history of the development of photoresists based on the photostructural transformations in chalcogenide glassy semiconductors is discussed in this paper. The properties of such photoresists including the possibility of an essential increase of their photosensitivity and also applications are examined. Together with the photoresists, the chalcogenide glassy roentgen-, electron beam-, and plasma-resists are briefly considered.

Charging of glassy chalcogenide semiconductors in corona discharge and its effect on holographic grating formation

Recording of optical holographic gratings based on photostructural transformations in thin (≈ 1 μm) As2S3 and As2S3 semiconductor layers in the presence and absence of a corona discharge and also chemical etching of these gratings are studied. Initiation of a corona at the stage of interference grating recording is shown to improve the exposure contrast of metal-glassy chalcogenide semiconductor thin-film structures. The holographic sensitivity, diffraction efficiency, dynamic range, and contrast are also improved severalfold. When phase relief gratings formed in these layers are selectively etched in a chemical etchant in the presence of a corona, their profile becomes more regular and deeper by 25–30% and the diffraction efficiency increases by 30–50%.

Mechanism of photoinduced nanodimensional expansion/contraction in glassy thin layers of As2S3

The mechanism of photoinduced expansion/contraction in glassy As2S3 has a photochemical nature. Photostructural transformation is connected with rearrangement of As4S4 molecules and similar non-stoichiometric compounds, As4S4+x. These molecules are believed to exist in clusters which correspond to the second glass-forming region of As–S systems, including the As56S44 eutectic.

Molecular design of light-sensitive nanodimensional systems

A universal approach to the molecular design of light-sensitive nanodimensional systems with specific characteristics was developed on the basis of unsaturated and macrocyclic compounds. It was possible to construct various nanodimensional systems within the scope of a single class of compound using a limited number of components. The use of photostructural transformations to control complex formation and mechanical movements in molecular devices and machines is discussed.

Molecular meccano for light-sensitive and light-emitting nanosized systems based on unsaturated and macrocyclic compounds

The review presents the results of the development of an universal approach to the molecular design of light-sensitive and light-emitting nanosized systems with desired properties based on unsaturated and macrocyclic compounds. Within the same class of compounds, various nanosized systems were constructed using a limited number of structural fragments. These nanosized systems are susceptible to all main types of photoprocesses, such as fluorescence, photodissociation, photoisomerization, photocycloaddition, photoelectrocyclization, excimer formation, charge-transfer complex formation, the formation of the twisted intramolecular charge-transfer state (TICT state), and the electron transfer. The use of photostructural transformations for controlling the complexation and mechanical movements in molecular devices and machines is discussed. The prospects of application of the new strategy are exemplified by the design of the previously unknown types of molecular switches, materials for optical chemosensors, optical data recording and storage media, photoswitchable molecular devices, and photocontrolled molecular machines.

Photoluminescence, photostructural transformations and photoinduced anisotropy in rare-earth-doped chalcogenide glassy films

Thin films fabricated by coevaporation in vacuum of chalcogenide glass and a rare-earth (RE) containing material are shown to have strong rare-earth photoluminescence, photostructural transformations and photoinduced anisotropy. The photoluminescence spectra of the films are similar to that of very different Nd- and Er-doped solids. Effects of concentration quenching of luminescence and luminescence fatigue are observed and studied. Using the photoresist effect of the doped films, binary diffraction gratings possessing strong IR photoluminescence were prepared.

Photoluminescence and photostructural transformations in neodymium-doped glassy chalcogenide films

Abstract Nd-doped glassy As2Se3 and As2S3 films are prepared using vacuum co-evaporation of crushed glassy chalcogenide material and Nd2S3 powder. The films display strong Nd photoluminescence with maxima at ∼1080 nm and ∼900 nm and photostructural transformations (photodarkening and photoresist effect). Fabrication of luminescent diffraction gratings using Nd-doped films is also demonstrated.

Reversible photoinduced changes in the spectrum of localized states in AsSe films

Study of the temperature dependence of the carrier drift mobility in AsSe films undergoing photostructural transformations has revealed a reversible photoinduced change in the mobility and its activation energy. It has been established that the drift mobility and its activation energy depend on the conditions of sample annealing. The shape of the drift pulses indicates that the transport becomes more dispersive in irradiated films and those annealed after irradiation. The concentrations and energies of the localized states controlling the transport in AsSe films before and after irradiation have been determined. The data obtained indicate that the main parameters of traps for charge carriers can be controlled.

Photoluminescence and photodarkening effect in erbium-doped chalcogenide glassy films

Er-doped glassy As 2S 3 and As 2Se 3 films are prepared by vacuum co-evaporation of chalcogenide glass and Er chalcogenide powder. The films display strong 1.54 μm photoluminescence (PL) and keep the possibility of the photostructural transformations and photoinduced anisotropy. The used method is believed to be applied for preparation of different chalcogenide glassy films doped by various rare-earth elements.

Study of mechanical amorphization of mineral orpiment (As2S3) and arsenic and germanium selenide crystals by optical methods

Reversible photostructural transformations (darkening and clearing) accompanied by a shift of the edge transmission and a fatigue effect of luminescence have been found in deformed As2S3, GeSe2, and As2Se3 single crystals and powders at T = 77 K. The experimental results are interpreted based on the configurational model of two stable states of molecular groups.