Showing papers in "Journal of Non-crystalline Solids in 1972"

States in the gap

Abstract: Experimental evidence about the states in the gap of chalcogenide glasses is discussed. The total concentration of states is estimated from the measurements of the magnetic susceptibility and their density distribution from the optical and photo-emission measurements. Possible models for the interpretation of the experimental facts are considered.

A kinetic treatment of glass formation

Abstract: A kinetic treatment of glass formation is presented. This treatment is based on the construction of time-temperature-transformation curves corresponding to some barely detectable degree of crystallinity. From such curves, the minimum cooling rates required to form glasses of various materials are estimated. The most important factors determining the glass-forming abilities of different materials are suggested to be the magnitude of the viscosity at the melting point and the rate of increase in viscosity with falling temperature below the melting point.

A percolation treatment of dc hopping conduction

Abstract: It is shown that the current paths considered by Miller and Abrahams in their theory of hopping conduction must be modified for large enough samples, such as are met under most experimental conditions. A possible exception is the case of transverse measurements on thin amorphous films. A percolation model for hopping conduction is set up using Miller and Abrahams' impedance representation of the problem. Approximate solutions for the model yield expressions for dc hopping conductivity. It is found that the density of states functions effects the temperature dependence of the conductivity. When the density of states is constant within a certain region around the Fermi level Mott's T − 1 4 law is obtained. For a density appropriate for impurity conduction at small compensations an activated conductivity is obtained. When the density of states increases as the nth power of the distance from the Fermi energy the logarithm of the resistivity is a linear function of Tsu−(n+1)/(n+4). Some comparison of the theory with experiments on impurity conduction and on amorphous semiconductors is made.

Metal contact induced crystallization in films of amorphous silicon and germanium

Abstract: The crystallization and compound formation temperature of vacuum deposited amorphous Si (and Ge) while in contact with various crystalline metals as a thin film sandwich are investigated by electron microscopy and electron diffraction. The results show that in simple eutectic systems the Si crystallizes at 0.72 (and Ge at approximately 0.65) of the eutectic temperature expressed in degrees Kelvin. Compounds are formed generally by the more rapid diffusion of Si into the metal.

Investigation of the localised state distribution in amorphous Si films

Abstract: Field effect techniques have been used to determine the distribution function N (ϵ3) of the localised states in amorphous Si films prepared by glow discharge decomposition of silane. It was found possible to sweep the surface potential through about 0.5 eV and to determine N (e) to within 0.18 eV of the extended states. N (e) curves show a pronounced structure which largely depends on the substrate temperature during deposition of the films (400–630 K). Localised state densities increase with decreasing substrate temperatures. The equilibrium Fermi level generally lies close to a peak in the distribution and N (eF) ∼ 17 cm−3 eV−1. The analysis of the field effect experiments is described in some detail and relevant information from drift mobility and conductivity measurements are discussed. In particular it is found that the distribution of occupied states calculated from the N (e) curves agrees with predictions from the transport experiments. This supports our contention that the N (e) curves represent a volume rather than a surface property of the films.

Electronic transport and state distribution in amorphous Si films

Abstract: The specimens used in this investigation were prepared by the decomposition of silane in an r.f. glow discharge. Substrate temperatures, Td, between 310 K and 670 K were used during deposition. The temperature dependence of both the conductivity and the drift mobility was measured on the same specimens and was studied as a function of Td. An electron beam technique was used in the mobility experiments. In specimens prepared at Td>350 K, current flow above 250 K is associated with electron transport in the extended states near ec. An electron mobility of about 10 cm2 s−1 V−1 is deduced from the experiments. Below 250 K, phonon-assisted hopping of electrons through localized states, about 0.18 eV below ec, becomes the predominant transport mechanism. When specimens are deposited at Td⪝350 K, the Fermi energy satisfies ec−eF⪞0.8 eV and a transition to predominant hopping transport by holes occurs. Based on the transport results and previous field effect measurements, a model for the electronic state distribution is put forward, which also appears to be applicable to the interpretation of transport results in amorphous Si specimens prepared by vacuum evaporation or sputtering.

Optical and bonding model for non-crystalline SiOx and SiOxNy materials

Abstract: Optical data for non-crystalline SiOx materials are presented and analyzed for the energy region 1 to 26 eV. The results indicate that amorphous substances of all intermediate compositions between Si and SiO2 can be formed and that these materials are not simple mixtures of particles of Si and SiO2 but rather the two atom species are blended on an atomic scale. The basic units of this structure are Si tetrahedra (perhaps highly distorted) of the type Si(SiyO4−y) in which the distribution of atoms for all y = 0 to 4 is statistical for any given atom ratio. Further it is found that the optical properties of these layers are determined by the presence and grouping of SiSi and SiO bonds and that clusters of like bonds of the dimension of a Si(Si4) or Si(O4) tetrahedra have optical properties comparable to those exhibited by amorphous silicon or quartz respectively “in bulk”.

A molecular model for the vibrational modes in chalcogenide glasses

Abstract: We demonstrate the use of molecular models for calculating the optic mode frequencies in elemental and compound chalcogenide glasses. Our model gives good agreement with the optic mode frequencies reported for As 2 Se 3 , and for the polymeric fraction in liquid S and amorphous Se and Te. The model is not applicable to amorphous Si and Ge and III–V semiconductors.

A model for an amorphous semiconductor memory device

Abstract: The amorphous semiconductor Ge15Te81X4 may be switched from a high resistance (amorphous) state to a low resistance (crystalline) state and back again. The two different resistive states are stable and can serve as the basis of a memory device. Recent measurements using techniques such as scanning electron micrographs, electromigration, electrical noise, lifetime studies, and metallurgical characteristics have yielded a body of data and ideas from which a model has been synthesized. The essential features of the model incorporate the mechanisms of filament formation and the subsequent resetting of the material into the amorphous state plus residual conducting debris. The conducting debris can serve as the nucleus of subsequent filament formation. The statistical variation in recycling, combined with percolation concepts, will be shown to account for most of the experimental data relating to operation and device lifetime.

Electrothermal switching in amorphous semiconductors

Abstract: Solutions to the complete steady-state heat equations are obtained under the assumption of no significant heating of the electrodes. It has previously been shown that pure thermal effects cannot lead to switching under such conditions. In the present work, several models are investigated in which contact or bulk electronic effects are sufficient to obtain thermally induced differential negative resistance. Current-voltage characteristics, temperature distributions, and current-density distributions are presented.

Frequency-dependence of conductivity in hopping systems

Abstract: A critical review is given of the experimental evidence relating to the frequency dependence of the electrical conductivity, σ (ω), in solids in which the flow of current occurs by hopping of localized carriers, whether electrons or ions. It is pointed out that the same dependence is found in semiconducting amorphous systems and in a wide class of insulators, giving σ α ωn, where 0.5 < n < 1, depending upon temperature. Evidence for a region in which σ α ω2 is considered to be of dubious reliability. It is pointed out that the conventional theoretical approach to the treatment of the frequency dependence of conductivity leaves certain gaps in our understanding of the physical processes taking place. An alternative approach is developed in which a stochastic sequence of hopping current pulses is Fourier-analyzed to give a frequency dependence of the power-law type found experimentally. The analysis leads to the conclusion that no exponent significantly higher than unity is to be expected - in agreement with all available evidence, and the ω2 region is possible as a consequence of the dielectric relaxation in the host matrix in which hopping takes place or of the corresponding hopping process by carriers tightly bound to fixed sites in the matrix. The new theoretical approach enables high-field ac behaviour to be analyzed and experimental results are given to illustrate the discussion. It is also pointed out that the frequency dependence of conductivity is an essential consequence of hopping conduction and not of the disorder of the structure. It is therefore predicted theoretically that similar results should be obtainable in ordered systems in which hopping is known to take place and some experimental results are quoted to support this claim.

Introductory talk; Conduction in non-crystalline materials

Abstract: The present position of the theory of conduction in non-crystalline materials is reviewed. Particular attention is given to the behaviour at the mobility edge, the Hall effect, hopping at low temperatures, the metal-non-metal transition in a non-crystalline system and to the effect of correlation and of a random concentration of vacancies on the properties of vanadium monoxide.

A new interpretation of the anomalous properties of vitreous silica

Abstract: This paper deals with the theory of pure silica glass. Earlier theoretical work is reviewed first and then the following topics are treated: The nature of bonding as the basis for the new model; The distribution of oxygen angles and the volume of silicas; New interpretation of acoustic losses, pressure and temperature dependence of the compressibility, thermal expansion and specific heat.

New experiments on the charge-controlled switching effect in amorphous semiconductors

Abstract: Measurements of the charge accumulated in the active glass film up to firing as a function of the delay time in an expanded temperature range confirm our concept of charge control of the reversible switching effect in glass semiconductors. The charge required for switching is directly proportional to the film thickness. This suggests a bulk effect. Experiments with double pulses and a variable time interval between the two firing pulses lead to information on the lifetime of the accumulated charge. All experiments are in agreement with our already published model on the reversible switching effect in glass semiconductors.

Structure and properties of silver phosphate glasses — Infrared and visible spectra

Abstract: The infrared and visible spectra of glasses in the Ag2OP2O5 glass-forming system were obtained. The infrared data were interpreted as indicating the presence of polymeric chains in these glasses. The partial covalent nature of the AgOP bond was discussed. A mixed NaPO3AgPO3 glass showed no unexpected bands in the infrared spectrum, again showing that silver is behaving in a manner similar to alkali metal ions in phosphate glasses. The shift in the absorption edge in the visible spectra of glasses of different Ag/P ratio was shown to arise from either an increase in the concentration of nonbridging oxygens with increasing silver content, or the presence of colloidal silver metal particles.

Ohmic and non-ohmic conduction in some amorphous semiconductors

Abstract: Experimental results concerning ohmic and non-ohmic behaviour in non-annealed and annealed amorphous Ge films are presented and discussed It is shown that a temperature-dependent activation energy can appear as a general consequence of the existence of a tail of states and that a temperature dependence of the form σ −exp (−MT−1/n) may be obtained even if the mobility attributed to various states is nearly temperature-independent, thus accounting for the experimental results obtained on a-Ge films

Basic equations for statistics, recombination processes, and photoconductivity in amorphous insulators and semiconductors

Abstract: The original work of Shockley and Read, which described the statistic of a single trapping level in terms of four simple processes, is applied to insulators and semiconductors containing an arbitrary distribution of trapping levels. These statistics formerly involved solving an arbitrary number of rate equations; however, by the use of a mathematical artifice, a simple formal solution is obtained. The solution is also shown to be generally valid for an arbitrary dependence in energy of the cross sections σ n ( E ) and σ p ( E ) and to be independent of the effects of band to band recombination. Using the statistics, expressions are derived for the rate of recombination and the lifetimes of free carriers in such systems. Also, the ground rules for photoconductivity in amorphous materials are presented; these rules are summarized by two general equations. The first is the steady-state rate equation giving the total net rate of recombination of photoexcited carriers via all the trapping levels in the energy gap. The second is a general statement of charge neutrality, which shows that the excess charge contained in trapping levels above E F 0 (the equilibrium Fermi level) is equal to the charge that has been removed from trapping levels below E F 0.

Raman scattering in amorphous Ge and III–V compounds

Abstract: Raman spectra of amorphous Ge prepared by various methods, and several amorphous III–V compounds (InP, GaP, GaAs, InAs, InSb, AlSb) prepared by flash evaporation or sputtering are reported. The results give a semiquantitative picture of the density of phonon states which preserves the main features of the crystalline material. The Raman spectra of III–V compounds show the presence of V-V bonds whose intensity depends on the preparation of the films.

The structure of amorphous Ge

Abstract: Small angle X-ray scattering experiments, along with radial distribution analysis, have been carried out on amorphous Ge prepared by three methods: evaporation, sputtering and electrodeposition. The small angle scattering shows that the void distributions in the three types of Ge differ considerably, while their radial distribution functions show that the short range order (SRO) is essentially the same. The SRO is shown to be well fitted by the results of recent computer simulations of the deposition process. The accumulated evidence supports the continuous random network model for amorphous Ge.

Structure of amorphous semiconductors

Abstract: The main emphasis of this paper is on the problem of molecular configuration within a single amorphous phase, although the physical properties of amorphous semiconductors are also strongly dependent on the spatial distribution of the constituent phases. Structural and thermal studies indicate a very high degree of local order in amorphous semiconductors; in particular, the coordination to nearest neighbors (NN) is close to that required by the chemical valences and the NN separations differ little from their ideal crystal values. There is still much disagreement on how the elements of local order are assembled in the overall structure. Recent model studies indicate that the relative smallness of the magnitudes of the configurational entropy of glasses is associated with the sharpness of the local order, however it occurs in the structure. Structural models for amorphous solids may be classified as follows: microcrystallite, amorphous cluster and continuous random (including random networks). The nature of these models and their application to the interpretation of experience on tetrahedrally coordinated systems are surveyed. The radial distribution functions and densities of fused silica, a-Si and a-Ge are in excellent accord with the predictions of the random network models as deduced from studies of actual models generated by hand or computer. An important problem for the further development of these models is, how much energy is associated with the bond distortions inherent in the random network. Neither the microcrystallite nor the amorphous cluster type of model have been developed to the point where they have as much predictive capabilities as do the random network models. Actually, in their present states, none of the ordered cluster models account satisfactorily for the diffraction results on a-Si and a-Ge. When amorphous semiconductors are formed by deposition, the driving free energy is usually so high that any of the amorphous structures considered would be permitted thermodynamically; therefore what structure actually appears will be determined primarily by kinetic factors. Recent model studies indicate that when deposition is controlled completely by short range interactions at the surface a continuous random type structure is always formed. In hard sphere packing the structure is of the dense random packed type; when the units bind directionally a random network structure is formed.

Relaxation phenomena in amorphous selenium-tellurium alloys

Abstract: Amorphous specimens of selenium-tellurium alloys containing 0–35 at% Te were held for various times at temperatures ranging from room temperature to 318°K. Changes with time in the microhardness, density, small-angle X-ray scattering and heat capacity were determined. The hardness of crystalline specimens containing 0–100 at% Te and the density of crystalline specimens containing 0–30 at% Te were also measured. The microhardness and density of the amorphous specimens increased with time at room temperature to approximately constant values. With increasing time of annealing, the intensity of small-angle scattering decreased. Peaks developed in the heat capacity versus temperature curves; their height increased with time and decreased with temperature of annealing. The peaks moved to higher temperatures with increasing tellurium concentration. Changes in the enthalpy were determined from the heat capacities. The increases in microhardness, density and height of the heat capacity peaks reflect relaxation towards a more stable structure of smaller molecular mobility. The decrease in small-angle X-ray scattering indicates the disappearance of inhomogeneities in the density during the early stages of annealing. At a later stage, the enthalpy decreased with the most rapid change occurring over a narrow time interval. A spectrum of activation energies centered around 23 kcal (gram-atom) −1 was derived. The effects of composition, temperature and time of annealing on the various properties are explained in terms of structural changes and relaxation kinetics.

Chemical aspects of glass formation in telluride systems

Abstract: The compositional dependence of the electrical activation energy, the optical absorption and the glass transition temperature in typical binary, pseudo-binary and ternary glass forming telluride systems has been measured in an effort to correlate chemical binding with physical properties. A modification of the valency satisfaction model to include chemical ordering effects is required to account for the property singularities observed in the GeTe system at the composition GeTe2. The ordering effects in Te-based alloys containing Ge are further verified by an endothermic disordering reaction in these liquids peaking between 400–475°C depending on composition.

Diffraction studies of glass structure. I. Theory and quasi-crystalline model

Abstract: A unified outline of the theory of X-ray and neutron scattering from glasses is presented, starting from the formalism of Van Hove. The nature of the structural information obtainable from scattering experiments is discussed and it is emphasized that the experimental data are usually inadequate for a full determination of the structure. It is necessary to use some kind of model in attempting to overcome this problem and a simple quasi-crystalline model is described. The use of this model is illustrated by considering its application to a range of simple disordered structures which are known exactly, namely computer-generated random sphere packings.

Crystallization processes in a-Ge thin films

Abstract: The authors studied the nucleation rate and the rate of crystal growth in a-Ge films, paying particular attention to the effects of residual gases. Activation energies for the relevant processes were obtained.

Diffraction studies of glass structure

Abstract: The use of X-ray and neutron diffraction techniques to investigate both short and intermediate range order in glasses is briefly reviewed with an emphasis on the correct methods of extracting structural parameters from experimental data. The important role played by complementary modelling studies and the need for accurate quantitative comparisons between the predictions of models and experiment are discussed. It is concluded that modern diffraction experiments are capable of yielding data of high quality which, when combined with the latest computer modelling techniques, should considerably enhance the present state of knowledge concerning the structure of the vitreous state.

Is there an intimate relation between amorphous and crystalline semiconductors

Abstract: Pressure measurements have often been used to clarify the band structure and the fundamental properties of crystalline semiconductors. An account is given of such measurements on the optical absorption edge and the refractive index of amorphous Ge. Less complete data on Si, GaP and GaAs are also reported. The implication of the results for present models of the amorphous band structure and the optical and transport properties is examined. Finally, the value of the Penn gap in the amorphous phase is estimated from the calculated heat of crystallization.

Diffraction studies of glass structure: II. The structure of vitreous germania

Abstract: The effects of pile irradiation on the polymorphs of germania are described and it is shown that the structural effects are small. For the glass, density changes on irradiation are smaller than those resulting from different methods of preparation. The X-ray diffraction pattern of vitreous germania has been measured for Q between 0.8 and 16 A −1 . The experimental method and data reduction procedures are described. The structure of vitreous GeO 2 is discussed, using the correlation function D X ( r ) obtained by fourier transformation of the X-ray intensity data. This is compared with correlation functions derived from neutron diffraction experiments and from quasi-crystalline models based on the various crystalline polymorphs of GeO 2 . The structure is shown to be built of GeO 4 tetrahedra: the coordination number of oxygen about germanium is 4.0 to within one or two per cent, the GeO distance is 1.74 A and the average GeOGe angle is 133°. The distribution of GeOGe angles is fairly narrow but cannot be determined quantitatively with data of present resolution. The average structure of the glass closely resembles that of the quartz modification as smeared out in the quasi-crystalline model. The remaining differences between model and experiment may be accounted for in terms of an unwinding (and disordering) of the crystal structure by relative rotation of tetrahedra, keeping the average intertetrahedral angle of 133°.

Electrical conduction in evaporated amorphous silicon films

Abstract: We report the results of measurement and analysis of the electrical conductivity as a function of temperature and anneal for amorphous silicon films. The resistivity of the films between 77 and 300°K increasesc with annealing. Refractory electrodes were used. The extrapolated portions of T − 1 4 fits to the log of the conductivity give physically unreasonable parameters for the T − 1 4 formula.