Chalcogenide Glass Thin Films Research Articles

Kinetics of laser-induced photodarkening in arsenic based chalcogenide glasses

Photoinduced changes in the optical properties of chalcogenide glass thin films have been studied extensively but usually only permanent changes were recorded through measurements before and after exposure to pump light. To understand the progression of such effects, in this paper we report in situ measurement of transient changes in the optical transmission of amorphous As2Se3, As5Se5 and As4Se3 thin films during their exposure to a He–Ne laser which has photon energy close to the bandgap of the studied compositions. Mathematical expressions are established, which describe accurately both the light intensity and time dependences of photodarkening. The structural origin of the observed changes is discussed briefly.

Chalcogenide glass e-beam and photoresists for ultrathin grayscale patterning

The advantages and applications of chalcogenide glass (ChG) thin film photoresists for grayscale lithography are demonstrated. It is shown that the ChG films can be used to make ultrathin (~600 nm), high-resolution grayscale patterns, which can find their application, for example, in IR optics. Unlike polymer photoresists, the IR transparent ChG patterns can be useful as such on the surface or can be used to transfer the etched pattern into silicon or other substrates. Even if the ChG is used as an etch mask for the silicon substrate, its greater hardness can achieve a greater etch selectivity than that obtained with organic photoresists. The suitability of ChG photoresists is demonstrated with inexpensive and reliable fabrication of ultrathin Fresnel lenses that are transparent in the visible as well as in the IR region. The optical functionality of the Fresnel lenses is confirmed. Application of silver photodissolution in grayscale lithography for microelectromechanical systems (MEMS) applications is also shown. A substrate to ChG/silver thickness etching ratio of ~10 is obtained for the transfer of patterns into silicon using reactive ion etching (RIE), more than a fivefold increase compared to traditional polymer photoresist.

Evolution of chemical structure during silver photodiffusion into chalcogenide glass thin films

The change of chemical structure resulting after X-ray and photo-induced silver diffusion into chalcogenide glass (ChG) thin films is monitored by high resolution X-ray photoelectron spectroscopy (XPS). As 40S 60 and Ge 30Se 70 thin films, which are based on pyramids and tetrahedral structural units, are investigated as model materials. Survey, core level (As 3 d, S 2 p, Ge 3 d, Ge 2 p, Se 3 d, Ag 3 d 5/2, O 1 s, C 1s) and valence band spectra have been recorded and analyzed. Reference point for the binding energy is established by the subsequent deposition of thin gold film on top of the measured samples. The chemical structure gradually changes during diffusion of silver in all the samples. The mechanism of change depends on the chemical composition, thickness of the diffused silver layer and conditions of irradiation. It is revealed that surface oxygen can play important role in the Ag photodiffusion process, leading to phase separation on the surface of the films. Photodiffusion of Ag into As 40S 60 film leads to the formation of a uniform ternary phase and arsenic oxides on the surface. The formation of ethane-like Ge 2(S 1/2) 6 units together with germanium oxidation are the main outcomes of X-ray induced Ag diffusion into Ge 30Se 70 film.

Embossing of chalcogenide glasses: monomode rib optical waveguides in evaporated thin films

Single-mode optical rib waveguides operating at telecommunication wavelengths are successfully patterned via a hot embossing technique in a thermally evaporated chalcogenide glass thin film on a chalcogenide glass substrate. Ellipsometry is used to measure the refractive index dispersion of the pressed film (As(40)Se(60)) and substrate (Ge(17)As(18)Se(65)).

Fabrication of two-dimensional photonic crystals in a chalcogenide glass

Two-dimensional photonic band gap structures consisting of air holes arranged to form a hexagonal lattice were patterned into chalcogenide glass thin films by Focused Ion Beam (FIB) milling. The dimensional parameters for these structures, namely type of lattice, lattice parameter and radius of the holes, were chosen so as to show a photonic band gap relevant to the infrared range, aiming at their subsequent use in the field of chemical sensing and biosensing. The optical behaviour of these structures was determined by calculating the band structure for the TM (magnetic field in-plane) and TE (electric field in-plane) modes. Furthermore, FIB milling was used to fabricate input and output waveguides coupled to the photonic crystals. The transmission spectra of the resulting structures for the TE and TM modes with two different orientations were also calculated.

Surface biofunctionalization of nanostructured GeSbSe chalcogenide glass thin films

Thin nanostructured chalcogenide films were grown using the oblique angle deposition (OAD) technique and subsequently polymerized with thin poly(amino-p-xylylene) (PPX) films. Our objective was twofold, i.e., to use deposited polymeric thin films to allow the attachment of biomolecules to chalcogenide glass thin films, and at the same time, to increase surface area by OAD to enhance surface functionality. The effectiveness of this approach was evaluated by Fourier transform infrared spectroscopy (FTIR), together with a combination of fluorescent protein immobilization and confocal microscopy characterization. It is shown that the presence of amine groups on the surface of the polymer coated chalcogenide thin films yield a notable increment of surface coverage with proteins at large evaporation oblique angles which is expected to enhance detection performance of the film in biosensor applications.

Relief and trench formation on chalcogenide thin films using electron beams

Electron beam irradiation of amorphous GeSe4 thin films reveals formation of reliefs and trenches as a function of electron beam dose. Mounds as high as 115nm are observed at low electron beam dose and trenches as deep as 180nm at high dose. The geometry of trench and mound formation is examined as a function of electron beam dose, film thickness, conductive overlayer thickness, exposure count, and beam step size. Thicker films are found to more likely form trenches than thinner ones. Physical mechanisms for trench and mound formation are proposed. Both types of deformations provide a route toward the direct write of optical waveguides, gratings, and waveguide-cavity coupled structures onto thin films of chalcogenide glass.

포토닉 크리스탈 응용을 위한 비정질 칼코게나이드 As-Ge-Se-S 박막의 특성 연구

In this paper, we investigated the properties of chalcogenide glass thin films formed by photo-inducing for use in 1-dimensional photonic crystals. We used Ag-doped amorphous As-Ge-Se-S thin films which belongs in the chalcogenide materials having sensitive photoluminescence properties. The purpose of this experiment is to form the holographic lattice for 1-dimensional photonic crystals. The way in which photo-induce into the amorphous chalcogenide thin films is holographic lithography method. We confirmed the formation of diffraction lattice by sensing the existence of diffraction beam and measured the diffraction efficiency. The results suggest that there is an application possibility with photonic crystals.

Photoplastic effect in non-crystalline materials: a nanoindentation study

Negative photoplastic effect (NPhE), i.e. the reversible influence of light on the flow stress and hardness in non-crystalline materials (mainly As–S binary chalcogenide glasses for thin-film samples), has been studied through the changes in nanoscale mechanical properties using the nanoindentation technique for various loading procedures. It has been shown that the opposite changes in local mechanical properties (Young's modulus and nanohardness) take place due to band-gap light illumination and that a pronounced non-linear (non-Hookean) mechanism of the formation of strain response is detected. This behaviour has been described in terms of conception which assumes that the heterogeneous (two-phase) structure of chalcogenide glasses undergoes modification with light. A comparative short overview of nanoindentation study of the NPhE in thin films of chalcogenide glasses and azobenzene-containing polymer films is presented in this paper.

In Situ Measurements of X‐Ray‐Induced Silver Diffusion into a Ge30Se70 Thin Film

High‐resolution X‐ray photoelectron spectroscopy is used to identify the mechanism of X‐ray‐induced Ag diffusion into Ge30Se70 chalcogenide glass thin films, which are prepared in situ to avoid oxygen contamination. From the analysis of Ge 3d, Se 3d, and Ag 3d core levels, and valence band spectra, changes in the electronic structure are determined as Ag diffuses gradually with increasing irradiation. The ternary phase based on Ge2Se6 units, which contains homopolar Ge–Ge bonds, forms when diffusion approaches equilibrium where Ag content ∼30 at.%. The formation of a Se‐rich composition is indicated in the near‐surface region at the initial stage of the process, but the previously assumed Ag2Se phase is not detected.

Investigation of the optical constants of arsenic chalcogenide films in the wavelength range 0.5–2.5 μm

Thin films of chalcogenide glasses deposited on quartz glass substrates by thermal evaporation in vacuum have been investigated. The dependences n(λ) and k(λ) for films of different composition have been determined from the transmission spectra. Expressions of the n = A + BL + CL 2 + Dλ 2 + Eλ 4 type (L = (λ 2 − 0.028)−1 and A, B, C, D, and E are constants) for calculating the refractive indices of As2Se3, AsSe4, AsS4, and AsS16.2Se16.2 films in the wavelength range from 0.5 to 2.5 μm are reported.

Preparation and optical dispersion and absorption of Ag-photodoped GexSb40−xS60 (x = 10, 20 and 30) chalcogenide glass thin films

We have analysed the effect of silver content on the optical properties of Ag-photodoped amorphous GexSb40−xS60 (with x = 10, 20 and 30 at.%) chalcogenide thin films; the chalcogenide host layers were prepared by vacuum thermal evaporation. Films of composition Agy(GexSb40−xS60)100−y, with y ≲ 10 at.%, were prepared by successively photodissolving about 10 nm thick layers of silver. The film thickness and optical constants have been accurately determined by a refined envelope method, based upon the two envelope curves of the optical-transmission spectrum, obtained at normal incidence. The dispersion of the linear refractive index of the Ag-photodoped chalcogenide films is analysed in terms of the Wemple–DiDomenico single-effective-oscillator model. We have found that the maximum change in the index of refraction, between the Ag-photodoped and the undoped material, is about 0.08, when the Ag-concentration reaches the level of saturation of around 10 at.%. On the other hand, the Tauc gap, , decreases notably, with increasing Ag-content: for instance, in the particular case of x = 10 at.%, the smallest Ge-content, decreases from 1.97 down to 1.67 eV.

Structural heterogeneity in chalcogenide glass films prepared by thermal evaporation

GeSe 2 and Ge 28Sb 12Se 60 chalcogenide glass thin films have been deposited on single crystal silicon substrates by vacuum thermal evaporation. The surface morphology of these films has been investigated by field emission-scanning electron microscopy and atomic force microscopy, revealing heterogeneities in their microstructure consisting of granular regions ∼15–50 nm in size, which were coarser in the case of the GeSe 2 films. Typical RMS film surface roughness values were ∼0.9–1.3 nm.

Why are chalcogenide glasses the materials of choice for Ovonic switching devices?

Non-destructive switching from a high resistance OFF state to a highly conducting ON state occurs at a critical field between 0.5 and 0.7 MV/cm in thin films of chalcogenide glasses placed between two non-reacting contacts as discovered by S.R. Ovshinsky in the 1960s. The switching device returns to its original high resistance state when the ON state current falls below a holding current value. We present a theory of this electronic switching and of the conducting ON state, which allows many billion reproducible switching cycles. In particular the question is addressed why the unique properties and the defect chemistry of chalcogenide glasses lead to this non-destructive and reproducible switching effect. It is proposed that the switching at the critical field is initiated by a modified Zener tunneling breakdown. In the ON state most of the holding voltage drop occurs near the electrodes and the current is confined to a current filament. The high current density of the ON state is supplied by Fowler–Nordheim field emission from the contacts. This is made possible by the narrowness of the potential barriers at the contacts which in turn is a consequence of the defect chemistry of chalcogenide glasses. Electronic switching is also observed in Ovonic memory devices having a preferred composition of Ge 2 Sb 2 Te 5 . The large atomic sizes of Sb and Te result in a partial 3-fold covalent coordination of Te which might affect the defect chemistry of this and related memory glasses.

On the mechanism of gray scale patterning of Ag-containing As 2S 3 thin films

We demonstrate an application of photo-induced silver diffusion into chalcogenide glass thin film for gray scale lithography. The gray scale chalcogenide glass masking layer generated in the present experiments was dry etched using reactive ion etching. The etching rate increases almost linearly with the total dose of absorbed light, thus forming the basis of gray scale lithography. Chemical composition as well as electronic structure on the surface of chalcogenide glassy film has been determined by high-resolution X-ray photoelectron spectroscopy (XPS) of the film at different stages of the patterning process. Influence of thermal annealing of chalcogenide film before Ag deposition has been investigated using scanning electron microscopy (SEM) and XPS techniques. It is observed that thermal annealing of the chalcogenide film slows the process of silver diffusion during the proposed processing procedure. A mechanism is proposed to explain the stages of gray scale lithography based on chalcogenide glass photoresists.

Mass Transport in Chalcogenide Electrolyte Films — Materials and Applications

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Mass transport in chalcogenide electrolyte films – materials and applications

Certain metals can be added to thin films of chalcogenide glasses by photodissolution to create materials with unique morphology and properties. When Ag is combined in this fashion with Ge–Se or Ge–S glasses, the resulting ternary contains a dispersed nanocrystalline Ag 2S(e) phase that has large quantities of mobile metal ions. The presence of these ions allows the ternaries to act as solid electrolytes. If an anode which has an oxidizable form of the ionic metal and an inert cathode are applied in contact with such a phase-separated electrolyte, an ion current can flow under an applied bias in excess of a few hundred millivolt. Electrons from the cathode reduce the excess metal due to the ion flux and an electrodeposit forms on or in the electrolyte. Utilizing this effect, we developed programmable metallization cell (PMC) technology which offers new functionality for such materials. Based on mass transport driven by electrochemical processes, PMC technology may be applied in solid state electronics, integrated optics, microelectromechanical systems (MEMS), and microfluidics. This paper is a review of the unique materials aspects of thin film solid electrolytes formed by photodissolution of metal into a chalcogenide base glass and the demonstrated applications of this technology.

Selective wet-etching of undoped and silver photodoped amorphous thin films of chalcogenide glasses in inorganic alkaline solutions

The unexposed/exposed, Ag-rich (exposed)/Ag-poor (unexposed) and amorphous (unexposed)/crystallized (exposed) parts of vacuum evaporated amorphous chalcogenide films of composition As33S67, As33S50Se17, As33S33.5Se33.5, As33S17Se50, As33Se67, Agx(As33S67)100−x and Agx(As33S33.5Se33.5)100−x were selectively etched in water solutions of inorganic alkaline substances (NaOH, Na2S, (NH4)2S, Na2CO3 and NaCN). Different etching rates for exposed and unexposed parts were found depending upon the sample composition, type of etchant and its concentration. A resolution coefficient, γ, was confirmed by selectively etched films after their exposure through a gray-scale photomask with microlenses array motives.

Spectroscopy studies of structural phase transitions of chalcogenide glass thin films (Ge 2S 3) x(As 2S 3) 1− x at coordination number 2.67

We have investigated Fourier transform infrared (FTIR) spectra, Raman spectra and ultraviolet and visible absorption spectra of the (Ge 2S 3) x (As 2S 3) 1− x chalcogenide glass thin films with 0.1≤ x≤0.9 and the average coordination number Z between 2.52 and 2.76. The relation of the structural and optical properties to the average coordination number is demonstrated by varying the composition parameter x of the glass. All measured quantities energy gap, sound velocity, IR and Raman peak intensities, position and the intensity of the boson peak showed the extreme values at Z=2.67, indicating that structural phase transition occur at this coordination number.

Photoinduced integrated-optic devices in rapid thermally annealed chalcogenide glasses

Photosensitivity in chalcogenide glasses has been exploited to demonstrate a variety of photoinduced optical devices. Of particular interest have been rapid-thermally annealed thin films of chalcogenide glasses, because they afford very large photo-induced index changes and compatibility with traditional platforms for optical devices such as Silicon and GaAs. The materials compatibility enables hybrid integration schemes, while the large photo-induced index changes facilitate novel optical circuits and devices. The ability to induce large index changes facilitates a variety of devices such as high-density optical interconnections and unique index tapered waveguides among others, and this paper will review this technology and the device design space afforded by it.