Seebeck coefficient in amorphous chalcogenide films

The Seebeck coefficient determined for a number of amorphous chalcogenide films is linear in 1/T. Slopes were typically 0.1 to 0.2 eV less than the conductivity activation energies. A parameter A in the expression for thermopower appears to be a measure of the disorder, being large for highly disordered materials and small for annealed stoichiometric materials.     

Holographic recording in amorphous chalcogenide thin films

Holographic recording by He–Ne laser (line 632.8 nm) light in amorphous As_0.55Se_0.45 thin films for different film thickness and grating period was studied. A strong dependence of the diffraction efficiency of the gratings on the readout light wavelength (650 nm, 805 nm and 1150 nm) was observed. A decrease in diffraction efficiency for longer wavelengths is explained by a decrease in the photoinduced changes of refractive index. It is shown that high efficiency gratings can be recorded in As_0.55Se_0.45 films with a thickness of ∼1 μm.     

Formation of surface structures on amorphous chalcogenide films

The high spatial resolution of the localized structural transformations induced by different irradiations in amorphous chalcogenides, as well as the possibility of inducing volume expansion, promotes applications of these inorganic resists for optical recording, data storage and makes them attractive for nanolithography. This paper focuses on the fabrication of surface reliefs at submicrometer length scales in a direct, one-step process of recording by light or ion beam on Se layers or Se/As₂S₃ nanolayered films due to induced volume expansion.     

Femtosecond optical Kerr effect study of amorphous chalcogenide films

The non-resonant third-order non-linear optical properties of some amorphous chalcogenide films were studied experimentally by the method of the femtosecond optical heterodyne detection of the optical Kerr effect. The real and imaginary parts of the complex third-order optical non-linearity could be effectively separated and their values and signs could be also determined. Amorphous chalcogenide films showed a very fast response in the range of 200 fs under ultrafast excitation. The ultrafast response and large third-order non-linearity are attributed to the ultrafast distortion of the electron orbitals surrounding the average positions of the nucleus of chalcogen atoms. The high third-order susceptibility and a fast response time of amorphous chalcogenide films make them promising materials for application in advanced techniques especially in optical switching.     

Investigation of anisotropy in as-evaporated amorphous chalcogenide thin films by optical waveguiding

Intrinsic birefringence in as-evaporated As_xS_1−x amorphous films have been measured by means of waveguiding technique. The anisotropy is between the directions in the film plane and the perpendicular ones. Dark relaxation, annealing behavior as well as compositional dependence of the optical anisotropy in fresh amorphous films are used for analysis of the effect in terms of the microscopic model proposed. A simple phenomenological model, based on distinct structural sites in chalcogenides, is shown to qualitatively explain some aspects of this phenomenon.     

A unitary model for reversible vectorial and scalar photo-structural transformations in amorphous chalcogenide films

Experimental results on reversible vectorial photostructural transformations - dichroism and birefringence induced by linearly polarized light - in amorphous AsSe films are reported and explained in the frame of a two-photon model analogous to that formerly proposed for reversible scalar transformations.     

The switching mechanisms in amorphous chalcogenide memory devices

Measurements of the resistance of chalcogenide memory devices switched with pulses which have a long or very short trailing edge, showed no significant difference. This contradicts the switching model as proposed by Cohen et al. Experimental data is presented and a discussion of the complexity of crystallization suggests a modification in which the inner zone of the filament is usually quenched below the crystallization temperature during the set pulse, as a result of current redistribution. Further modifications to the model for the reset event take account of an annulus around the reset filament which is at the optimum temperature for crystallization. Crystallization in that annulus results in shifting of the filament axis. The use of multiple-pulse resetting reduces the chance of crystal growth in the annulus.     

An investigation of the atomic bonding present in amorphous chalcogenide materials by electron spectroscopy

The atomic bonding in amorphous switching materials based on tellurium and germanium has been shown to be largely dependent on the germanium atoms. The results of examination by electron spectroscopy show that germanium atoms can possess five different forms of chemical bonding. These are elemental, oxidised, ‘glassy’, and two states associated with the interatomic compound GeTe. The ‘glassy’ and compound forms of germanium have been found in the amorphous materials. Tellurium, however, shows no significant different in bonding from its elemental state in these materials.     

Atomic structure and short- and medium-range order parameters in amorphous chalcogenide films prepared by different methods

The atomic structure of amorphous As₂Se₃ and As₂S₃ films prepared by thermal evaporation in a vacuum and by RF ion-plasma sputtering has been studied by the methods of X-ray diffraction and Raman spectroscopy. The techniques of film preparation had different conditions of substance vaporization and atom condensation on a substrate. It has been established that films prepared by these methods have significant differences in the dimensions of the medium-range order and in the local atomic structure, which causes considerable differences in their electronic properties.     

An electron diffraction study of amorphous silicon oxide films

Amorphous silicon oxide films have been examined by high energy electron diffraction using the sector-microphotometer method of data collection common to gas phase electron diffraction. This data was analyzed with a least-squares procedure that is designed to minimize extraneous detail in the radial distribution function obtained by the Fourier sine transform of the interference function. The results of this analysis for thin film SiO₂ show that the overall bonding topology of the thin film agress well with that of bulk (vitreous) SiO₂ examined by X-ray diffraction. The experimental short distance parameters for the films whose composition was determined to be ～SiO_1.3, SiO, and SiO_0.8 are found to be consistent with those expected for a mixture of tetrahedrally bonded amorphous Si and SiO₂ phases in which the scale of the Si-like and SiO₂-like regions is of the order of a few basic tetrahedral units. This result is in agreement with previous examinations of SiO powder by X-rays and a previous examination of thin silicon oxide films by electron diffraction.     

Surface pattern recording in amorphous chalcogenide layers

Investigations of light-induced volume expansion and surface pattern recording in amorphous chalcogenide layers and nano-layered structures (NLS) were extended to direct electron-beam recording on Se/As₂S₃ and Sb/As₂S₃ NLS. Light as well as e-beam induced bleaching occurs in all NLS, while volume expansion occurs only in chalcogenide–chalcogenide NLS and in homogeneous Se or As₂S₃ layers. Comparison of these two phenomena revealed the possible role of purely electronic and thermal processes in the interdiffusion and relief formation. The latter is supposed to be connected with radiation-induced defect creation, free volume increase under the increased fluidity conditions as well as with the possible additional influence of electrostatic forces and stress.     

Electron beam poling in amorphous Ge-doped H:SiO2 films

Amorphous Ge-doped H:SiO₂ films on silica, deposited by matrix-distributed electron cyclotron resonance – plasma enhanced chemical vapor deposition, were irradiated with an electron beam while varying the dose. Using the Maker fringe method, second-harmonic generation was measured in the irradiated regions of the films. With a current of 5 nA, and an acceleration voltage of 25 kV for 25 s, a Ge-doped H:SiO₂ film (3.8 at.% Ge) showed a maximum second-order nonlinearity of d₃₃ = 0.0005 pm/V. In contrast, a H:SiO₂ film with a smaller Ge content (1.0 at.% Ge), showed a large SHG: d₃₃ = 0.06 pm/V when irradiated for 15 s. The second-harmonic generation in the films is caused by a frozen-in electric field induced by charge implantation from the electron beam. The strength of the electric field is determined by two conditions: the trapping centers (numbers, depth) and the remaining conductivity under large electric field.     

The preparation and electrical properties of thin chalcogenide semiconductor films

Thin chalcogenide films from the GeTeAsSi system have been prepared using electron beam evaporation and R.F. sputtering techniques. The techniques are described in some details as it is believed that the deposition procedure has a significant on subsequent electrical switching behaviour.     

Thin chalcogenide films prepared by pulsed laser deposition – new amorphous materials applicable in optoelectronics and chemical sensors

Principles, advantages, applications and drawbacks of pulsed laser deposition (PLD) technique for thin films preparation are reviewed. The PLD method is promising for preparation of thin films of complex composition, including rare-earth and Ag-doped chalcogenide films; all components of the target can be evaporated at once. Low volatility and refractory materials can be also deposited. The deposition in vacuum, inert or reactive atmosphere is possible. Results obtained in a study of chalcogenide films are discussed and the current state-of-the-art is reviewed. The composition and structure of PLD films can be different from thermally evaporated films and new materials or materials with new properties applicable in optics and optoelectronics can be prepared. The method can be used for fabrication of different chalcogenide-based sensors and memory materials of complex composition. Photoinduced changes of structure and properties of PLD films and chalcogenides exposed to intense laser pulses are also discussed. The intense laser pulses can change the properties of the materials prepared and can be used for fabrication of chalcogenide-based waveguides, diffractive elements and high-density optical data storage media.     

On the reliability of amorphous chalcogenide switching devices

Switching devices consisting of a thin amorphous AsTeGe film sandwiched between two molybdenum electrodes were prepared by electron-beam evaporation and investigated by subsequent switching. The number of switching events from the beginning of the test until a prefixed change in switching voltage appeared, was defined as the “lifetime”. Samples which were subjected to an ageing process showed relatively stable operation, and longer lifetime than unaged samples. In addition, the lifetime was found to depend strongly on device geometry and to be limited by the formation of crystalline regions in the amorphous film. This was explained to be due to heating effects during the preswitching region, during the transition from off-state to on-state, and during the on-state, although the underlying switching model is assumed to be non-thermal.     

The crystallization of amorphous germanium films

Germanium thin films were prepared in an amorphous form by vacuum deposition onto room temperature fused silica substrates. The amorphous—crystalline transition was studied as a function of time and temperature by measuring the optical transmission near 0.65 μm, where the absorption constant is most sensitive to the phase transformation. At a fixed temperature, the time for half the volume of the sample to become crystallized was found to be consistent with the relation t_c = τ exp(E₀/kT), with an activation energy E₀ = 2.96 eV.     

Local order and microstructure of germanium chalcogenide films

Amorphous films of nominal composition GeTe₂ have been observed by energy filtered electron diffraction and by electron microscopy. The diffracted intensity curves may vary from region to region within a film and changes may be brought about by electron beam heating, giving rise to observable changes in the radial distribution functions. Gross features are not apparent in the electron micrographs and glassy phase separation if it exists in these films must be restricted to a very fine scale (20 Å or less). Similar effects have been observed in a variety of other telluride and also selenide films.     

Properties of filaments in amorphous chalcogenide semiconducting threshold switches

The temperature dependence of the conductivity has been measured for Si Te As Ge amorphous thin-film threshold switches in the unformed, formed and heat-treated states. The results indicate that there are two distinct values of activation energy after forming, each considerably less than the value for the unformed device. It is inferred that each activation energy of the formed device corresponds to a different glass structure; one value corresponds to a filament estimated to be a few microns in diameter, and the other is a consequence of less localised structural changes extending to much larger distances radially. This latter region of structural change has been confirmed experimentally by a radial conductivity probing technique, and has been found to extend out to 100 μm from the filament. The effect of heat treating the glass and the effect that circuit parameters have on the devices will also be discussed.     

Electrical and optical properties of chalcogenide amorphous semiconductors modified with Ni

The electrical and optical properties of the chalcogenide semiconductor (Se₃₂Te₃₂As₄Ge₃₂)_100?xNixitx have been studied. As the Ni concentration is increased the electrical dc conductivity is drastically increased and variable range hopping conduction becomes dominant even above room temperature. The optical energy gap decreases with the Ni concentration from 1.18–0.95 eV. Ni-atoms in the chalcogenide semiconductor donate free electrons which occupy the gap state. This occupation causes the shift of the Fermi level toward the conduction band. It is an effect of this shift that the thermal activation energy is decreased. The decrease in optical energy gap is independent of the shift of the Fermi level and is ascribable to the appearance of the additional level located at 0.95 eV above the top of the valence band. This level originates from the 3d-level of the Ni-atom.     

The photo-crystallization of amorphous selenium thin films

The photocrystallization of amorphous selenium under the influence of light or electrons used to produce hole-electron pairs has been studied. Illumination increases the growth rate of crystallites and modifies their morphology. Conversely, electron irradiation alters the structure of the amorphous material and induces a decrease of the nucleation rate. An explanation is proposed, which takes into account recent publications on the ‘band structure’ and the nature of bonds in amorphous selenium.