Pulsed light effects in amorphous As2S3: review

Journal of Materials Science: Materials in Electronics - It is known that amorphous chalcogenide materials exhibit unique optical phenomena, including high nonlinearity, photodarkening, and phase...     

Wide-gap layered oxychalcogenide semiconductors: Materials, electronic structures and optoelectronic properties

Applying the concept of materials design for transparent conductive oxides to layered oxychalcogenides, several p-type and n-type layered oxychalcogenides were proposed as wide-gap semiconductors and their basic optical and electrical properties were examined. The layered oxychalcogenides are composed of ionic oxide layers and covalent chalcogenide layers, which bring wide-gap and conductive properties to these materials, respectively. The electronic structures of the materials were examined by normal/inverse photoemission spectroscopy and energy band calculations. The results of the examinations suggested that these materials possess unique features more than simple wide-gap semiconductors. Namely, the layered oxychalcogenides are considered to be extremely thin quantum wells composed of the oxide and chalcogenide layers or 2D chalcogenide crystals/molecules embedded in an oxide matrix. Observation of step-like absorption edges, large band gap energy and large exciton binding energy demonstrated these features originating from 2D density of states and quantum size effects in these layered materials.     

Pre-exponential factor of Arrhenius equation for the isothermal crystallization of some Se–Ge,Se–In and Se–Te chalcogenide glasses

Compensation law or Meyer–Neldel rule is observed in many activated phenomena, including solid state diffusion in crystals and polymers, dielectric relaxation, conduction and thermally stimulated processes in polymers, and electronic conduction in amorphous semiconductors. In the present paper, we have reported the compensation effect for the isothermal crystallization of some Se–Ge, Se–In and Se–Te chalcogenide glasses. We have observed Meyer–Neldel rule between pre-exponential factor K ₀ and activation energy of crystallization E _c in the present case.     

Binary phase spatial modulation using photoinduced anisotropy in amorphous As(2)S(3) thin film

We present a method for binary phase spatial modulation that uses photoinduced anisotropy in a chalcogenide amorphous As(2)S(3) thin film and its application to binary phase-only filters in a VanderLugt optical correlator. The time-dependent light-transmission properties of the photoilluminated As(2)S(3) thin film are analytically examined by use of third-order nonlinear polarization theory. Experimental results on optical correlation are discussed.     

Phase conjugation in weakly piezoelectric magnetized semiconductor-plasmas

The occurrence of optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in weakly piezoelectric III–V semiconductor plasmas subjected to a large magnetostatic field under off-resonant transition regime has been explored theoretically. The reflectivity of the phase conjugate wave is dependent upon the Brillouin susceptibility and can be significantly enhanced through n-type doping of the crystal and the simultaneous application of a magnetostatic field. Moreover, the threshold pump intensity required for the occurrence of SBS in the crystal with finite optical attenuation can be considerably diminished through proper selection of the doping concentration and magnetostatic field. Consequently, OPC-SBS becomes a possible tool in phase-conjugate optics even under not-too-high power laser excitation by using doped n-type semiconductors kept under the influence of a large magnetostatic field. Numerical estimates made for n-InSb crystal at 77 K duly irradiated by a nanosecond pulsed 10.6 µm CO₂ laser reveal that high OPC-SBS reflectivity can be achieved at excitation intensities below the optical damage threshold if the semiconductor crystal is used as an optical waveguide with an interaction length of a few millimetres.     

Applications of amorphous semiconductor materials in electronics

A brief summary of the present state of the art of amorphous semiconductors and their applications in modern electronics is presented in this paper. The recent developments in chalcogenide glass thin film devices and in amorphous silicon electronic devices are reviewed. A potentially interesting application of a-Si(p⁺-n-i) thin film structures as fast non-volatile electronic memory elements is also discussed.     

A review of the specific role of oxygen in irreversible photo- and thermally induced changes of the optical properties of thin film amorphous chalcogenides

Recent results concerning the influence of oxygen on the photo- and thermally induced changes of optical properties of thin amorphous chalcogenides are summarized. It is shown that interaction of the surface of thin films with oxygen considerably affects the shift of the optical gap. The influence of oxygen is perhaps twofold. It behaves probably like a “catalyzer in the process of oxygen-assisted bond reconstruction” as proposed by Spence and Elliott [Phys. Rev. B 39 (1989) 5452], but it also enters directly into the film network forming strong covalent bonds with germanium and chalcogen atoms, respectively. Hence, the presence of oxygen during illumination or annealing of thin amorphous chalcogenide films most probably affects (i) the density of dangling and homopolar bonds, respectively, (ii) and actual chemical composition of the surface layer making this one oxygen rich (in normally deposited thin films) but also, chalcogen poor (in some obliquely deposited thin films). The role of oxygen in the case of illumination or annealing of amorphous chalcogenide films is of considerable importance and should not be neglected.     

Material science aspects of phase change optical recording

Chalcogenide thin films are used as the recording medium for phase change-type optical memory discs. The films are switched between amorphous and crystalline states using the heat of a focussed laser beam. Large reflectivity differences between amorphous and crystalline states are then used to store and retrieve the information. An active chalcogenide layer for this purpose should have a high optical absorption coefficient (α), and good structural and thermal stability. It should be possible to switch the chalcogenide layer between amorphous and crystalline states repeatedly within a short duration, the optical contrast should be high, and the material must have large cycling capability. Keeping the above requirements in mind, we have carried out systematic investigation of structural, optical and crystallization behaviour of thin films of various compositions of GaGeTe, Sb₂Te₃ and BiSe. These studies have shown that these materials can be good candidates for use as recording media in erasable phase-change optical recording.     

Reversible photoinduced changes in the properties of chalcogenide vitreous semiconductors

Recent studies of the reversible photoinduced changes in the optical, physicochemical and contact properties of the chalcogenide vitreous semiconductors are rewiewed. A conclusion is reached that these changes result from photostructural transformations.     

Immersion holography based on amorphous chalcogenide films

A solid immersion holographic method for the recording of refractive index and surface-relief modulated gratings with a period of 0.1–1 μm in amorphous films of chalcogenide semiconductors As₂S₃ and As–S–Se has been developed and studied. The laser immersion interference lithography can be used as a low-cost method for the exposure of large surfaces with regular patterns like subwavelength-gratings and microsieves. The polarization sensitive properties of the subwavelength refractive-index modulated transmission gratings were studied. The possibility to use the amorphous chalcogenide films as a media for holographic recording and storage of information with high density is discussed.     

Manipulation and observation of Ag photodoping patterns in GeS2 amorphous films by a dual functional laser scanning microfabrication/microscope system

The photodoping phenomenon of Ag is one of the light-induced phenomena in GeS(2) amorphous chalcogenide films. It has potential as a process for fabricating photonic structures, such as waveguides and micro-optics, but its fabrication method is still under research. A dual functional laser scanning system integrating microfabrication and microscope systems was developed. In situ nanoscale fabrication by a UV laser effective for photodoping, and observation by a VIS wavelength laser, which does not affect the material, were demonstrated under same setup in one system. Several fine doped patterns were fabricated and the optical performances were evaluated. These results give the feasibility of forming various photonic structures.     

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.     

Emission and absorption spectroscopy of magnetic materials

In recent years, optical methods have been accepted as a useful means for studying the magnetic properties in solids. In this paper it will be shown, from an introductory point of view, what kind of information optical studies can provide. The most numerous investigations have been performed with insulating materials. The observed phenomena can be divided into four types: first those where spectra can be described by a simple molecular field picture; second those where one incorporates small quantities of magnetic ions into a diamagnetic host and studies the spectra of various neighbour configurations; third those where one deals with the simultaneous excitation of states in the electronic system (excitons) and the spin system (magnons) which gives rise to the so-called magnon sidebands in absorption and emission spectra; and fourth those which deal with the exchange of energy between various magnetic sublattices by the magnetic interactions. Much less information is available on magnetic semiconductors. Yet here a very striking and perhaps technologically important effect has been observed, namely a considerable red shift of the absorption edge in going through the magnetic transition temperature, e.g., in EuO and EuS. In metals, so far only a few experiments have been performed, and only those on Ni and MnAg are mentioned here. In addition, the question of short-range correlations, which can be observed via optical spectra will be discussed. Finally, recent reviews in this field will be compiled to facilitate a deeper understanding.     

Optical properties of chalcogenide glasses with ion-synthesized copper nanoparticles

Substrates of chalcogenide glassy semiconductors As₂S₃ and Ge_15.8As₂₁S_63.2 are implanted with Cu⁺ ions (energy 40 keV, radiation dose 1.5 × 10¹⁷ ion/cm², fixed current density in the ion beam 1 μA/cm²). The composite layers are analyzed by measuring linear optical transmittance and recording nonlinear optical absorption using the Z-scan technique at 780 nm (probe laser radiation with 150-fs pulses; intensity of 25–100 mW). It is ascertained for the irradiated materials that (1) the linear transmission characteristic of the optical surface plasmon resonance (SPR) band, which indicates the formation of copper nanoparticles in the near-surface region, has emerged and (2) there are simultaneously saturated and two-photon nonlinear absorption types; the latter prevails as the intensity of laser irradiation is increased.     

Controlling optical response of Opal/Ge2Sb2Te5 hybrid film structures

Hybrid film structures of the photonic crystal (opal)/glassy chalcogenide semiconductor (Ge₂Sb₂Te₅) type, which exhibit strong variations in the intensity of reflected light due to the resonant excitation of anomalous diffraction (Wood anomaly), have been synthesized. A method of controlling the resonant optical response of the obtained structures is proposed that is based on a thermoinduced phase transition from the amorphous to crystalline state in the Ge₂Sb₂Te₅ film.     

Long-distance indirect excitation of nanoplasmonic resonances

In nanoscopic systems, size, geometry, and arrangement are the crucial determinants of the light-matter interaction and resulting nanoparticles excitation. At optical frequencies, one of the most prominent examples is the excitation of localized surface plasmon polaritons, where the electromagnetic radiation is coupled to the confined charge density oscillations. Here, we show that beyond direct near- and far-field excitation, a long-range, indirect mode of particle excitation is available in nanoplasmonic systems. In particular, in amorphous arrays of plasmonic nanodiscs we find strong collective and coherent influence on each particle from its entire active neighborhood. This dependency of the local field response on excitation conditions at distant areas brings exciting possibilities to engineer enhanced electromagnetic fields through controlled, spatially configured illumination.     

On the light absorption in amorphous semiconductors

Recent measurements of optical absorption in doped amorphous semiconductors (a-Si : H and a-Ge : H) have revealed that long-range potential fluctuations play a decisive role in the absorption mechanism. The theory of such effects was initially developed for crystalline semiconductors, and it is usually not applied in the appropriate form for the interpretation of experimental data in amorphous materials. We use this theory in the analysis of optical-absorption data for amorphous semiconductors, and show its quantitative agreement with experimental results.     

Dynamics of the Optical Stark Effect in Semiconductors

We present in this paper a comprehensive study of the optical Stark effect of excitons in semiconductors. After a review of our experimental results, and of some theoretical interpretations, we develop a new treatment of the optical Stark effect in the framework of nonlinear optics. This allows one to take into account transient phenomena such as spectral and temporal oscillations and leads to a proper interpretation of the very surprising dynamics experimentally observed.     

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.     

Pulsed laser deposited alumino-silicate thin films and amorphous chalcogenide/alumino-silicate structures

Alumino-silicate coatings and structures formed from alumino-silicate and amorphous chalcogenide submicrometer layers were prepared by pulsed laser deposition. Fabricated thin films were characterized in terms of their structure, morphology, topography, chemical composition, optical properties, and basic anticorrosive functionality. Prepared coatings are amorphous, smooth, without micrometer-sized droplets, with chemical composition close to parent targets. Spectral dependencies of refractive indices and extinction coefficients were derived from variable angle spectroscopic ellipsometry data. Amorphous chalcogenide/alumino-silicate structures present large refractive index differences of individual layers (Δn ~ 1.2 at 1550 nm) which could be useful for optical systems working at infrared telecommunication band wavelengths. Basic anticorrosion data of alumino-silicate layers show promising anticorrosion behavior.