Switching in amorphous selenium

Threshold and memory switching effects have been observed in bulk samples of amorphous selenium, using platinum electrodes, with interelectrode spacing of 100 μm. I-V characteristics are compared with microscopic observations of the intelectrode material. This reveals that the switching process seems to be dominated by a diffusion of Pt from the electrodes into the interelectrode Se. However, there are many similarities between switching in Se and those of the chalcogenide alloys.     

Molecular dynamic computer experiments on the structure of amorphous selenium

This work demonstrates the feasibility of deriving intramolecular atomic interaction potentials by fitting the results of molecular dynamic model calculations to experimental data - specifically, to atomic radial pair distribution functions (diffraction data) and vibrational frequencies (optical data). An intermolecular atomic distribution function is also derived for amorphous selenium, on the basis of the model calculations and the presumption of predominantly ring-type molecular units.     

Recombination processes in amorphous selenium

In order to determine the recombination lifetime τ_r we have studied the photoconduction under pulsed light in amorphous selenium. Experimental results, relative to the variation of τ_r with temperature, enable us to calculate the parameters which characterized the recombination centers: capture cross sections S_n and S_p (and their variations with T), localization of the level E_r, density of states N_r.     

Photodarkening of amorphous selenium

Various parameters associated with photodarkening of amorphous selenium have been measured at 100 K. The spectral response of photodarkening shows a step at the absorption edge indicating that electron-hole pair creation is a basic feature of the process. It is shown that the slope of the Urbach edge decreases during photodarkening, and it is found that the change in the optical joint-density-of-states peaks in the region of photon energy between 2.0 and 2.5 eV. A film of a-Se subjected to ion bombardment at 100 K does not then show photodarkening, and this and the other observations are discussed in terms of a model of local change in atomic arrangement.     

Acoustic behaviour of amorphous selenium

The attenuation of longitudinal sound waves in amorphous selenium has been measured at frequencies of 15 and 25 MHz and in the range of temperature between 10 and 310 K. It is shown that absorption exhibits a minimum around Tτ150 K, while a pronounced maximum takes place around 30 K. Such a behaviour is briefly discussed in the frame of acoustic properties of amorphous material below the glass temperature. It is shown that the experimental results can be described, almost qualitatively, in terms of a recently proposed theoretical model involving a stochastic resonance that seems to be characteristic of the disordered systems.     

Substitutional doping of chemically vapor-deposited amorphous silicon

Dc conductivity, absorption coefficient, photoconductivity, and magnetoresistance of phosphorus-doped amorphous Si films prepared by chemical vapor deposition (CVD) have been measured as a function of doping ratio. These results indicate that phosphorus doping reduces localized states in the mobility gap, narrows the tailing width below the extended states, and that phosphorus donors form the impurity band at 0.15 eV below E_c at a doping ratio of about 1×10^-2. It is also found that electronic properties of CVD amorphous Si can be controlled in a wide range by substitutional doping of phosphorus atoms.     

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.     

Defect levels in the band gap of amorphous selenium

Energy locations in the band gap have been determined for the thermally accessible levels of the negative-U defect centers in amorphous selenium. Both the temperature dependence of the steady-state photocurrents, and an analysis of emission currents in the post-transit regime of a time-of-flight transient photoconductivity experiment on the same samples, agree on the presence of defect levels at (0.42 ± 0.04) eV above the valence band mobility edge and (0.53 ± 0.06) eV below the conduction band. Both measured current levels and the resolved energy positions of the defects are subject to the Poole-Frenkel effect.     

The effect of nitrogen doping on amorphous germanium

The effects of nitrogen doping on the electrical and optical properties of amorphous germanium are investigated. It is found that within the low nitrogen concentrations that cause no appreciable change in the optical energy gap, the room temperature conductivity and the B coefficient in the optical absorption show a maximum at the same nitrogen concentration. This behavior is interpreted by a delocalization of the electronic states in the conduction band due to the nitrogen incorporation.     

2p Photoabsorption in amorphous,trigonal and monoclinic selenium

The 2p photoabsorption spectra of amorphous and polycrystalline trigonal and α-monoclinic selenium have been studied. Our experimental results, accounting for the transition probabilities and inner level broadening, are in qualitative agreement with Kramer's pseudo-potential formalism calculations of the density of unoccupied states. The amorphous selenium spectrum seems intermediate between the two crystalline form spectra and we deduce that both trigonal chains and monoclinic rings are randomly distributed in the amorphous phase.     

Preparation and characterization of amorphous cadmium–selenium ribbons

Chalcogenide glasses based on the cadmium–selenium system, with the selenium composition varying from 0 to 7.5 wt% have been prepared using melt-quenching method i.e., single-roller quenching technique. The X-ray diffraction (XRD) and selected area electron diffraction (SAD) patterns of the CdSe ribbons indicate that the ribbons are amorphous. The transmission electron microscopy (TEM) studies carried out on these ribbons reveal that the constituents are inhomogeneously distributed in these ribbons. The temperature dependence of the electrical resistivity, ρ and thermoelectric power (TEP) of these ribbons has been studied in the temperature range 30–350 °C. The sudden jump in the values of electrical resistivity at a specific temperature for each case in these ribbons has been correlated with the phase transition i.e., the onset of crystallization in these materials during heating. The crystallization temperature, T_c has been found to be a function of Se content of these ribbons. The phase change in these ribbons as a result of heating does not seem to affect the variation of TEP with temperature. However, the slope of TEP versus temperature curves depends on Se content in these ribbons. The differential scanning calorimetry (DSC) of these ribbons indicates that the supercooled region in these ribbons extends from 50 to 70 °C. The composition CdSe ribbon with 0.5 wt% Se has the highest value of T_c and glass forming ability, K_g = 0.7.     

Molecular dynamics study of amorphous titanium silicate

The molecular dynamics computer simulation was used to study the local structure of titanium in silica glass. The empirical potential function used in previous simulations of other glasses performed in our lab was extended to study titanium in silica. The results indicate that at low concentrations (<10 mol% TiO₂), titanium is mainly tetrahedral in bulk silica. At higher TiO₂ concentrations, titanium coordination increases. Slower quench rates enhance these trends observed in the changes in titanium coordination. A cluster of overcoordinated species similar to rutile was found in TiO₂: 9SiO₂ and is discussed within the constraints of the MD experiment. Overall, the MD result are consistent with EXAFS data of bond lengths and coordination numbers of TiO₂ in titanium silicate glasses.     

Chemical shifts of the K-absorption discontinuities of germanium and selenium in some amorphous systems

In the present investigation the chemical shifts of the K-absorption discontinuities of germanium and selenium have been studied in their amorphous chemical compounds Ge₁₀Se₉₀, Ge₃₀Se₇₀, Ge₁₀Se₈₀Te₁₀ and Ge_33.3Se₄₀Te_26.7 using a 1 m Cauchois type bent crystal (mica) X-ray spectrograph. A graph of the chemical shift ΔE in binary compounds against the effective charge q on the absorbing atoms has been plotted. This plot is helpful in determining the effective charges in ternary compounds in which they cannot be calculated theoretically.