Direct fabrication of surface relief gratings in chalcogenide glasses by excimer laser interference lithography

In this paper we study the possibility to realise surface relief gratings in thin chalcogenide glass films by holographic exposure using a pulsed KrF excimer laser. Gratings with a period of 540 nm and depths of 100–300 nm were patterned at the surface of 1 μm thick films. Due to coupling of an incident near-infrared laser beam to waveguide modes a resonance-like polarisation dependent decline of transmission was observed at specific incidence angles. Just one laser pulse with a fluence of 12 mJ/cm² per beam was sufficient to achieve the required grating parameters in sulphide glasses with low T _g.     

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 As₂₀Se₈₀ and As₂Se₃ 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.     

Photo-induced transformations in amorphous chalcogenide nano-multilayers

Photo-stimulated interdiffusion in a-Se/As₂S₃ amorphous chalcogenide nano-multilayers (ANML) is known as a useful method for amplitude-phase optical relief formation besides the known amorphous–amorphous or amorphous–crystalline photo-induced structural transformations (PST) in homogeneous chalcogenide layers, but it has a relatively narrow sensitivity spectral range and small amplitude modulation. Experimental evidences of improvement of optical recording processes were obtained in Te-, Bi-, Sb-containing nano-layered structures based on As₂S₃ matrix. The influence of nano-structuring and combination of components on the sensitivity, type of the recorded relief is discussed.     

Holographic recording in amorphous chalcogenide thin films

A review of the recent advances and developments in the practical application of chalcogenide materials is presented, focusing special attention on holography and lithography using amorphous chalcogenide thin films.     

6. Conductivity effects in amorphous chalcogenide films

Thin amorphous films of chalcogenide Te₄₈As₁₈Ge₆Si₂₈ were prepared and their current-voltage (I–V) characteristics were examined. Memory switching effects were observed which were stable and reproducible, but the switching voltage was found to depend on the restoring voltage in a regular way for cyclic switching. Furthermore, the time delay for an applied voltage pulse to cause switching may be related to the I–V curve prior the switching. This dependence seems to support an electronic mechanism of switching.     

Auto-correlation function analysis of phase formation in iron ion-implanted amorphous silicon layers

High-resolution transmission electron microscopy (HRTEM) in conjunction with autocorrelation function (ACF) analysis have been applied to investigate the evolution of structural order in iron ion-implanted amorphous silicon layers. β-FeSi₂ nanocrystallites as small as 5 nm in size were detected in 600 °C annealed for 60 min a-Si layers. The embedded nanocrystalline β-FeSi₂ was found to grow in the interlayer with annealing temperature.     

Memory switching in amorphous Ge---Se---Tl chalcogenide semiconductor films

The switching effects in amorphous GeSe₂, GeSe₄, GeSe₂Tl and GeSe₄Tl thin films have been investigated. The observed switching phenimenon for these compositions was of the memory type. The threshold switching voltage was found to increase linearly with increasing film thickness (80–740 nm), while it decreased exponentially with increasing temperature (T < T_g).

The effect of adding thallium to both amorphous GeSe₂ and amorphous GeSe₄ results in decreasing the values of the threshold electric field, the activation energy of switching, as well as the thermal activation energy of conduction. The results obtained are explained in accordance with the electrothermal model of breakdown.     

Anisotropic phenomena in as-evaporated amorphous chalcogenide thin films

The changes in refractive index and birefringence in as-evaporated As_xS_1−x amorphous films have been measured by means of prism-coupling technique. In particular, the time evolution, annealing and substrate temperature effects, compositional dependencies of the optical anisotropy on the fresh amorphous films are investigated. The analyse of these effects in terms of a proposed microscopic model are discussed. Such a model is shown to qualitatively explain some aspects of this phenomenon.     

Passivating gallium arsenide surface by gallium chalcogenide

We have studied the protective properties of thin films of gallium selenide formed by the method of heterovalent substitution on the surface of GaAs substrates. The data of transmission (Hitachi H-800) and scanning (JEOL JSM-638 OLV) electron microscopy showed that GaAs substrates treated with selenium vapor produced a more pronounced orienting action on the subsequent deposition of GaAs as compared to the substrates covered with a natural oxide. The processing of a GaAs substrate in selenium vapor followed by the removal of the resulting Ga₂Se₃ layer increases the degree of smoothness of the substrate surface on the atomic level.     

Microstructure formation in chalcogenide thin films assisted by thermal dewetting

The spontaneous formation of self-assembled and/or self-organized patterns is a fundamental and technologically significant topic. This process is achieved via a phenomenon called dewetting, should it be thermally induced, or caused by laser exposure. Although dewetting seems to be a well-known phenomenon for metallic and polymeric thin films, no proper investigation regarding glassy thin films seems to have been done. Thus, in the present study we try to elaborate on the process of thermal dewetting applied to glassy thin films of the system Ag_x(As₂₀S₈₀)_100?x.     

Mechanism of photo induced mass transfer in amorphous chalcogenide films

Surface relief gratings produced on a surface of amorphous chalcogenide films As₂₀Se₈₀ are flatten at room temperature under illumination by a near-bandgap polarized light (λ = 650 nm). The rates of the profiles flattening are dependent on the light intensity, polarization direction, and grating period. Two possible flattening mechanisms are selected: viscous flow and volume diffusion, and the flattening rates are calculated for both of them. From the comparison of the theory with the experiments, it is concluded that the process is controlled by anisotropic volume diffusion. The effective photo-induced diffusion coefficients, D_xx, along E-vector of the light polarization, obtained from the flattening kinetics are proportional to the light intensity (D_xx = β_xI) with β_x = 2.5 × 10^− 18 m⁴/J. The diffusion coefficients D_yy along perpendicular direction are four times smaller, independently of the light intensity.     

Direct formation of wafer scale graphene thin layers on insulating substrates by chemical vapor deposition

Direct formation of high-quality and wafer scale graphene thin layers on insulating gate dielectrics such as SiO(2) is emergent for graphene electronics using Si-wafer compatible fabrication. Here, we report that in a chemical vapor deposition process the carbon species dissociated on Cu surfaces not only result in graphene layers on top of the catalytic Cu thin films but also diffuse through Cu grain boundaries to the interface between Cu and underlying dielectrics. Optimization of the process parameters leads to a continuous and large-area graphene thin layers directly formed on top of the dielectrics. The bottom-gated transistor characteristics for the graphene films have shown quite comparable carrier mobility compared to the top-layer graphene. The proposed method allows us to achieve wafer-sized graphene on versatile insulating substrates without the need of graphene transfer.     

Investigation of the formation of external surface layers in epoxy composites

We investigate time intervals of the formation of external surface layers around disperse filler particles during structure formation of epoxy composites. A model of change in the potential of external surface layers during structure formation is constructed, and time intervals of change in their size are established.     

Submicron surface relief fabrication technology for epitaxial GaAs structures with thin AlGaAs stop layers

Basic technology for the fabrication of submicron surface reliefs in epitaxial GaAs structures with thin AlGaAs stop layers has been developed using electron-beam lithography. High sensitivity of the characteristics of vertical-cavity surface-emitting lasers (VCSELs) with respect to the level of optical losses allows this technology to be used for controlling the mode composition and stabilizing the polarization of VCSEL radiation.     

Anomalous formation of micrometer-thick amorphous oxide surficial layers during high-temperature oxidation of ZrAl2

The thermal oxidation of ZrAl₂ in the temperature range of 550–750 °C in pure oxygen has been investigated by a combinational experimental approach using X-ray diffraction, scanning electron microscopy/energy dispersive spectrometer, Auger electron spectroscopy and cross-sectional transmission electron microscopy. The thermal oxidation leads to the growth of anomalously thick (up to 4.5 μm) amorphous (Zr_0.33Al_0.67)O_1.66 surficial layers at temperatures as high as 750 °C. The oxidation kinetics obeys a parabolic law with an activation energy of 143 kJ/mol. The underlying mechanism for the formation of such micrometer-thick amorphous oxide surficial layers has been discussed on the basis of interface thermodynamics and the occurrence of high interface stability associated with a synchronous oxidation of Al and Zr elements.     

Anomaly of the thickness dependence of photodarkening in amorphous chalcogenide films

The reversible photodarkening phenomenon in evaporated films of amorphous As-S and As-Se was investigated as a function of film thickness. For stoichiometric films irradiated by band gap illumination at room temperature, the photodarkening disappears when the films are thinner than 50 nm. This anomaly implies that the structure of the films is dependent on the film thickness and /or that the photodarkening exhibits a surface behaviour that is different from bulk bahaviour.     

Limiting of photo induced changes in amorphous chalcogenide/alumino-silicate nanomultilayers

Photo induced changes in amorphous As₂₀Se₈₀/alumino-silicate nanomultilayers (NML) produced by pulsed laser deposition (PLD) method have been studied in this work. The aim was to investigate the photo induced optical and surface relief changes due to the band gap illumination under the size- and hard cover limited conditions. It was observed that the hard cover layer on the surface of the uniform film or alumino-silicate sub-layers in the NML structure influences the photo darkening and restricts surface relief formations in As₂₀Se₈₀ film or in the related NML compared with this effect in a pure chalcogenide layer. The influence of hard layers is supposed to be connected with limiting the free volume formation at the initial stage of the transformation process, which in turn limits the atomic movement and so the surface relief formation.     

AFM evidence of surface relief formation and models of fatigue crack nucleation

Atomic force microscopy and high resolution scanning electron microscopy were used to find the true surface relief corresponding to persistent slip markings emerging from persistent slip bands on fatigued polycrystalline austenitic and ferritic stainless steels. The persistent slip markings are formed by extrusions and intrusions. The shape of extrusions and intrusions was documented and the kinetics of the extrusion growth was studied in both steels. The experimental data were discussed and compared with the predictions of the recent models of fatigue crack nucleation.     

Thermal fringing at turn-over in an amorphous chalcogenide thin film sandwich structure

Experimental evidence has been obtained which demonstrates the importance of the thermal properties of the substrate with regard to the onset of negative differential resistance or threshold switching effects in thin film amorphous chalcogenide sandwich structure devices. Because the turn-over voltage V_t depends on the substrate thermal conductance, device geometries which enhance lateral heat flow in the substrate will have higher values of V_t than geometries which do not. Experiments have been performed on thin film devices using a-As₂SeTe₂ as the active material. The circular devices were prepared with various radii on substrates in which lateral heat flow could take place. It was observed that V_t increased in a well-behaved manner with decreasing radius, thus showing that Joule heating plays a strong role in determining V_t for the chosen chalcogenide. Additional evidence for thermal spreading was provided by thin film thermometry.