Showing papers on "Amorphous solid published in 1970"

Electrical phenomena in amorphous oxide films

Abstract: Thin layers of insulators, from 100 A to 20 000 A thick, have a number of interesting electrical properties. These properties include forming, which is a profound and essentially permanent change produced by an electrical field, and, after forming, differential negative resistance together with switching and memory phenomena. Electroluminescence and electron emission also occur. These features are shown by most amorphous or microcrystalline insulators of the appropriate thickness, and are particularly clearly shown by the oxides, such as Al2O3, Ta2O5 and SiO. In our review we survey the observed properties of metal-insulator-metal devices involving such insulating layers, and discuss the mechanisms which have been proposed for their operation. Further, since the unusual properties may be technologically important, we outline some of the possible uses of these devices and the relevance of the phenomena to the mechanism of corrosion of certain metals.

A model for the formation of amorphous Si by ion bombardment

Abstract: The effective annealing of ion implantations in Si is aided by the formation of continuous amorphous layer. The amorphous layer regrows epitaxially at 500–600°C and incorporates the dopant in an electrically active, uncompensated form. A phenomenological model is proposed which, with adjustable parameters, accounts for the variation of the critical dose required to produce a continuous amorphous layer by ion bombardment with ion, target, temperature, and, with minor additional assumptions, dose rate.

A model for filament growth and switching in amorphous oxide films

Abstract: The forming process, by which changes are brought about in the electronic conduction of thin films of various binary amorphous oxides and halides, is first described, and experimental evidence is put forward which contradicts an earlier theory of the forming process in gold-silicon monoxide-metal structures. A theory is developed, in terms of the growth and thermal rupture of many conducting filaments through the insulating layer, to explain the observed voltage controlled negative resistance, electron emission, electroluminescence and memory phenomena. The model is presented firstly in its simplest phenomenological form, after which some atomic mechanisms which may be involved are tentatively discussed. It is further shown that there is evidence to suggest that the forming process in important in a wide range of systems. These include metal-oxide-silicon capacitors and transistors, surface-barrier junctions on silicon, oxide-coated thermionic cathodes and anodic oxide layers. Electronic conduction in each of these cases is held to take place through localized filamentary paths. The relationship between this and other filamentary conduction models is briefly discussed, and a general classification of negative-resistance devices into regenerative and non-regenerative systems is made.

Optical and Magnetic Investigations of the Localized States in Semiconducting Glasses

Abstract: We measured optical absorption and magnetic susceptibility of amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ as a function of temperature. An exponential variation of absorption constant with photon energy was found in the range $0.09l\ensuremath{\alpha}l0.5$ ${\mathrm{cm}}^{\ensuremath{-}1}$. A Curie term in the susceptibility was shown to be characteristic of disorder in the vitreous material. A model relating the weak absorption tail to the susceptibility requires highly localized states having an exponential energy distribution in the gap.

Amorphous versus Crystalline GeTe Films. III. Electrical Properties and Band Structure

Abstract: Various transport studies have been carried out on amorphous and crystalline GeTe films of 80 A to 10 μ thickness. Crystalline GeTe has a low resistivity (∼10−4 Ω·cm at 300°K) which increases with temperature slowly and nearly linearly at low temperatures (below ∼300°K) and rapidly at higher temperatures. The hole concentration (N∼1010−1021 cm−3) increases only slightly with temperature. Mobility varies as N−4/3. These results in conjunction with the tunnel‐spectroscopy and optical data show that crystalline GeTe is a degenerate (and thus metallic conduction), p‐type narrow band gap (∼0.1–0.2 eV) semiconductor with Fermi level ∼0.3–0.5 eV inside the valence band. The linear increase of the susceptibility mass with hole concentration, the constancy of the Hall coefficient up toωLτ=0.35 (ωL=Larmor frequency, τ=collision relaxation time), and the monotonic increase of thermopower with temperature indicate that conduction takes place only in a single valence band. Amorphous GeTe films exhibit activated conduction. The dc resistivity varies from 410° to 77°K as ρ0 exp (Eg/2kT), where Eg is about 0.8 eV and ρ0 is the resistivity of crystalline GeTe films. Ac resistivity decreases with frequency (ω) as ω−n, where n lies between 0.5 and 1.0, depending on the temperature. The activation energy for ac conduction decreases rapidly from the dc value to zero with decreasing temperature as well as increasing frequency. The capacitance of amorphous GeTe at 77°K varies as ω−0.2 while the loss factor is independent of the frequency. With increasing dc field, the linear dependence of current on voltage changes to a power relation Vn, where n varies rapidly from ∼3 to 6 or more in a small range of the applied field. At very high fields, I ∝expβF1/2 (β=constant) is observed. These results, together with the tunnel‐spectroscopy, and optical data suggest that amorphous GeTe may be represented as a p‐type semiconductor with band gap ∼0.8 eV with exponential tailing of the bands and a continum of localized states in the vicinity (both sides) of the band edges. Conduction takes place by two parallel processes of intrinsic excitation across the band gap, and thermally and/or field‐assisted hopping from one localized (trapping) state to another. Dc conduction by hopping at low fields is negligible. At higher fields, trap modified space‐charge‐limited current flow and Poole‐Frenkel effect determine the nonlinear field dependence of current.

Esr and optical absorption studies of ion‐implanted silicon

Abstract: The g value, line shape, and linewidth of an ESR signal in Si layers which have been damaged by ion implantation of Si, P, or As at room temperature are found to be identical to those of the electron states observed in amorphous Si films prepared by rf sputtering. Interference phenomena observed in the optical absorption spectra allow a determination of the depth to which the Si has been damaged by the energetic heavy ions. These two techniques together provide a new tool for investigating lattice disorder in ion‐implanted Si layers.

Optical Properties of Amorphous Germanium Films

Abstract: The optical constants of amorphous Ge films formed under well-defined conditions have been determined in the 0.1-25.0-eV spectral range. In the 0.1-1.8-eV range they were determined by analysis of precise reflectance and transmittance data ($\mathrm{RT}$), and in the 0.1-25.0-eV range by a Kramers-Kronig analysis of normal-incidence reflectance data. Both analyses gave the same results in the region of overlap. The absorption edge was found to be quite sharp (0.06 eV) and to occur usually at a photon energy of about 0.6 eV. The position of the edge was normally about 0.2 eV lower than the direct edge in crystalline Ge. No evidence was found for either the spin-orbit split valence band associated with crystalline Ge or a tailing and/or large number of states in the forbidden region. The smallest nonzero value of the absorption measured on the low-energy side of the absorption edge was about 10 ${\mathrm{cm}}^{\ensuremath{-}1}$. There was no evidence for free-carrier absorption further in the infrared. At 0.1 eV, the index of refraction was 3.99 \ifmmode\pm\else\textpm\fi{} 0.04 as determined by the $\mathrm{RT}$ analysis. This value was in excellent agreement with the value of 4.00 derived from the zero-frequency dielectric constant, which has been calculated using the sum rule. This is also the value determined for crystalline Ge in the infrared. Reflectance data for amorphous Ge films deposited and measured in ultra-high vacuum (in situ) are reported for the region 2.0-11.8 eV. The density, determined by weighing films of known thickness, was 12-15% less than the density of crystalline Ge.

Properties of glow-discharge deposited amorphous germanium and silicon

Abstract: Films of silicon and germanium are deposited on glass using the radio-frequency glow-discharge decomposition of silane and germane gases respectively. When grown on a substrate at room temperature the films are amorphous, with a short range order of about 20 A. The resistivities of these films, as deposited, are typically 108Ω cm for silicon and 7 × 103 Ω cm for germanium, measured at 294°K. Thermal activation energies for conduction decrease continuously below the deposition temperature, and at low temperatures germanium follows the relation log ’ = A/T 1 4 , where A is a constant. This would seem to indicate that a hopping process in an impurity band is responsible for conduction at low temperatures. Photoconductivity has been observed in silicon but not in germanium. The threshold energy for this effect decreases with increasing deposition or annealing temperatures. This is also true of the high temperature thermal activation energy. It is suggested that this is due to the de-localisation of states in the valence and conduction bands as the short range order increases. The optical absorption coefficients of germanium and silicon have an exponential dependence on photon energy and the considerable absorption below the fundamental absorption edge of the crystalline form may indicate the presence of localised states in the band gap.

Electric field-induced filament formation in AsTeGe glass

Abstract: The Ovshinsky memory effect (electrically reversible bistable resistivity) has been observed in AsTeGe glass (55%-35%-10% respectively by atomic percent). It is generally thought that an electrically induced conducting filament is responsible for the low resistance state. A cinematographic study of filament formation between two point contact probes on the surface of bulk samples of AsTeGe was made. To further characterize these filaments, electron microprobe analysis and X-ray diffraction were also employed. The cinematography results showed that the filament always grew out of the positive voltage terminal and that the formation of a continuous filament coincided with the establishment of a stable low resistance state. The electron microprobe analysis indicated that inside the filament there was a 37.6% (weight percent) increase in Te, a 40.8% decrease in As and a 47% decrease in Ge compared to the composition some distance away from the filament. X-ray diffraction patterns showed that the electric field-induced formation of filaments on the surface of amorphous AsTeGe introduced a definite crystallinity. It was not possible, however, to positively identify the crystalline structure.

Thermochemical studies on amorphous calcium phosphate.

Abstract: Previous studies have indicated that synthetically-precipitated amorphous calcium phosphate, even though not a periodically regular structure, possesses a chemically definable local unit of structure. The present paper is a report of thermochemical studies performed to more clearly define this local chemical unit. Upon ignition in the absence of water, freezedried amorphous calcium phosphate preparations converted into crystalline α-and/or β-tricalcium phosphates. At temperatures just below the crystallization point, the amorphous calcium phosphate became almost completely dehydrated. Additionally, pyrophosphate production at these subcrystallization temperatures was very low. From these data, it appears that water in amorphous calcium phosphate does not exist in combination with the P2O5 component as acid phosphate but retains its molecular identity, suggesting, together with previously reported chemical data, that amorphous calcium phosphate when formed under specific chemical conditions is a hydrated tricalcium phosphate.

Radial distribution studies of amorphous GexTe1−x alloys

Abstract: Radial distribution studies of amorphous Ge χ Te 1−χ alloys have been calculated from X-ray diffraction data for x = 0.11 and 0.54. These show peaks at approximately 2.7 A and 4.2 A. The absence of a peak at the crystalline GeTe first neighbor separation of 3 A is shown to imply that the local coordinations in the amorphous materials are different from those in crystalline GeTe. The areas under the first neighbor peaks indicated that it is very unlikely that the coordination numbers of the Ge and Te atoms are the same. These areas are consistent with models in which the average Ge and Te coordinations are four and two, respectively. This implies that the similarity in the optical absorption edges of crystalline and amorphous GeTe cannot be explained by similarities in short range structures. The similarities in bonding over a wide range of compositions in these amorphous alloys appear to lend support to Mott's picture for the failure of many impurities in many amorphous semiconductors to add significantly to the conductivity. The form of bonding postulated appears to be inconsistent with a microcrystalline picture for these alloys.

A qualitative theory of electrical switching processes in monostable amorphous structures

Abstract: Description of a double-injection space-charge process which can account for the principal features of amorphous threshold switching, and discussion of its applicability to chalcogenide glasses under various operating conditions.

Correlation function approach to the dielectric behaviour of amorphous polymers

Abstract: The equilibrium and dynamic aspects of the dielectric behaviour of polymer systems are considered in macroscopic terms and molecular terms. The role of dipole moment correlations is considered for a model polyether series. It is suggested that studies of certain copolymers reveal that the reorientation term 〈µk(0). µk(t)〉 for a reference dipole in a polymer chain is not given by a simple exponential dependence upon time. A method is given for the evaluation of the macroscopic decay function from experimental dispersion and absorption data, and this is applied to the α relaxation in amorphous polyethyl acrylate.

Steady state and transient photoemission into amorphous insulators

Abstract: Steady state and transient photoemission of carriers from metal electrodes has been used to study the electronic energy level structure and transport properties of two amorphous insulators, the polymer poly-n-vinyl carbazole (PVK) and amorphous selenium. The results for these two materials illustrate different types of information which these combined techniques can give. Fine structure on the steady state spectral response of the photoemission for PVK is identified with vibrational splitting of the valence band which is < 0.1 eV wide. The time resolve transport of holes photoemitted into this band reveals an effective hole mobility which is thermally activated and field dependent, with a value of about 10−7 cm2/V sec at 105 V/ cm and room temperature. For amorphous Se the transient photoemission of holes indicates a transition of the carrier supply from prredominantly internal photogeneration to photoemission. The dependence of the number of transported holes as a function of temperature and applied field shows a change in moving through this transition region. These results are significant with respect to the field dependent photogeneration of carriers in amorphous Se. The observed field dependent supply of photoemitted carriers is attributed to a general metal-insulator interface limitation commonly referred to as an unsaturated blocking contact.

Hopping conduction and optical properties of amorphous chalcogenide films

Abstract: The frequency dependence of conductivity at room temperature has been measured for amorphous films of As2Te3, Te2AsSi, and a composition containing Te, As, Si, and Ge. Results were similar for the three compositions. The conductivity could be considered to consist of two components, a frequency independent component σ0 and a frequency dependent component σ1; the latter varied as ωs with s usually around 0.8 to 0.9. The component σ1 attained a magnitude comparable to σ0 at 105 to 106 Hz, except in the case of point contacts with light contact pressures, where capacitive coupling increased the magnitude of σ1 relative to σ0. The component σ1 is attributed to a hopping mechanism and σ0 is attributed to intrinsic band conduction. Optical data for the fundamental absorption edge is also included.

Electronic conduction in amorphous semiconductors and the physics of the switching phenomena

Abstract: A three-fold classification of amorphous semiconductors into (i) elemental, (ii) covalent alloys, and (iii) ionic and tightly bound amorphous materials is proposed. The experimental evidence supporting a simple band model for the amorphous covalent alloys is presented. The present understanding of the reversible switching effects and of the switching with memory is discussed.

Photoconductivity of amorphous chalcogenide alloy films

Abstract: Steady-state and time-dependent photoresponse have been studied in a vacuum-deposited thin film of Ge:Si:As:Te alloy. In the steady-state at room temperature the quantum efficiency is essentially independent of photon energy, and the photoresponse linear in photon flux. The photoconductive gain factor exhibits a maximum in the neighborhood of 250°K, and decreases exponentially at lower temperatures. At 77°K, the photoresponse varies approximately as the square root of photon flux; while voltage-current curves in the dark and under illumination give evidence of trap-controlled, space-charge-limited conduction. An analysis of the thermally stimulated current following exposure to pair-producing radiation at low temperatures shows that the dominant trapping centers lie approximately in the middle of the mobility gap, and their density probably exceeds 10 19 /cm 3 . An interpretation of these results is offered in terms of the band model of Cohen, Fritzsche and Ovshinsky.

Optical properties of chalcogenide glasses

Abstract: Glasses based on S, Se, and Te have been under study for a number of years as possible infrared optical materials. Though amorphous in nature, their optical properties are similar to those of crystalline semiconductors. Chalcogenide glasses, as they are called, become transparent on the long wavelength side of an absorption edge while their long wavelength cutoff is determined by a lattice type absorption. In the transparent region, impurity absorption is found. Free carrier absorption is not observed even at elevated temperatures. Other physical properties of chalcogenide glasses related to their use as optical materials, compare favorably with conventional infrared optical materials. Because of their amorphous nature, chalcogenide glasses can be prepared in large, very honogeneous pieces. Optical homogeneity is extremely important when preparing optical components for high resolution optical systems.

Infrared studies of drawn polyethylene part I. Changes in orientation and conformation of highly drawn linear polyethylene

Abstract: Infrared spectroscopy is applied to discuss the orientation, the crystallinity, and the conformation of chain segments in the amorphous regions in drawn high-density polyethylene. The orientation of the crystals as well as the crystallinity are derived from the dichroism and the absorbance, respectively, of the band at 1894 cm-1. The orientation and some aspects about the conformation of the chain segments in the amorphous regions can be obtained from the bands in the 1400-1300 cm-1 region (gauche) and at 1078 cm-1 (gauche and trans). The dichroic studies show a high degree of orientation increasing with draw ratio λ for the chain segments in the crystals, but a low orientation reaching saturation at λ between 5 and 10 for those in the amorphous regions. The experiments indicate a change in crystallinity during the drawing process which depends on the thermal treatment of the undrawn sample. In the amorphous regions the number of CH2 groups in gauche conformations decreases up to λ between 10 and...