Crystallization Of Selenium Research Articles

733. Influence of electric field on the crystallization of amorphous selenium: S A Belan et al, Izv VUZ Fiz, No 6, 1971, 106–109 ( in Russian)

733. Influence of electric field on the crystallization of amorphous selenium: S A Belan et al, Izv VUZ Fiz, No 6, 1971, 106–109 ( in Russian)

Electrical conduction in single crystal trigonal selenium

Experiments are described which provide conclusive evidence for space charge conduction in homogeneous single crystals of trigonal selenium. From measurements of thermal activation energies for both ohmic and space charge limited currents over a broad temperature range localized levels have been calculated to lie at approximately 0.09, 0.21 and 0.47 eV from the valence band. Supporting data on thermally stimulated currents and ac conductivity have further established that conduction processes in this elemental semiconductor must be understood in terms of the normal energy band picture.

599. Crystallization of vitreous selenium and chalcogenide glass films under the influence of organic medium

599. Crystallization of vitreous selenium and chalcogenide glass films under the influence of organic medium

The effect of an electrical field on the crystallization of amorphous selenium

1. Under the action of a field the predominant growth of spherolites is along the lines of force of the field. 2. The direction of the predominant growth depends on the material of the substrate. In the case of a carbon sublayer on mica the degree of extension depends on the thickness of the substrate. 3. The action of a field consists in the fact that it produces an electric current in the Se film: holes on a mica substrate; electrons on a thin carbon layer. 4. The highest growth rate is at those faces of the crystal at which the charges arrive. It is possible that this is associated with the increase in the mobility of the molecules as a result of the release of energy with the annihilation of the opposite charges.

Electroreflectance on Trigonal Selenium

AbstractMeasurements of transverse electroreflectance and of its temperature dependence were performed on trigonal selenium crystals in the energy region between 1.8 and 4.2 eV. The observed structures which show a strong dependence on polarization of light and on orientation of electric field are mainly correlated to exciton transitions. The discussion of the results gives a good accordance with band structure calculations. The direct gap is located at point H of the Brillouin zone and shows a positive temperature coefficient between 5 and 160°K. The first valence band is split into three subbands which lie close together with distances of 42 and 13 meV. Besides the electroreflectance spectrum which is independent of the direction of electric field a signal is observed which depends on its sign. Probably this spectrum is due to lattice deformation created by piezoelectric effect.

284. Electrical conductivity studies on film-like single crystals of hexagonal selenium: N Z Dzhalilov et al, Phys Stat Sol, 40 (1), July 1970, K5– K7

284. Electrical conductivity studies on film-like single crystals of hexagonal selenium: N Z Dzhalilov et al, Phys Stat Sol, 40 (1), July 1970, K5– K7

Dilatometric Study of the Crystallization Behavior of Pure and Doped Selenium

The crystallization behavior of pure vitreous selenium and selenium doped with various concentrations of arsenic, chlorine, bromine, and iodine was investigated by a dilatometric method, with particular emphasis on constancy of the thermal pretreatment of samples. The energy of activation for the crystallization of vitreous selenium was found to be 25 kcal/mole. Arsenic had a very pronounced retarding effect at the 100 ppm level. Chlorine and iodine showed a complex behavior, whereas bromine showed a moderate accelerating effect on the rate of crystallization of selenium.

Polymeric nature of selenium crystallization. II. Crystallization kinetics and secondary crystallization

AbstractThe crystallization of elemental selenium has been studied in light of present concepts of crystallization in organic polymers. Bulk‐crystallization kinetic data as measured by a dynamic density technique and spherulite growth‐rate data as measured by optical microscopy are presented for the temperature range 70°C to 160°C. Plots of extent of isothermal crystallization versus time were sigmoidal in shape. Spherulite growth rates were constant for a given temperature and reached a maximum at approximately 130°C. Evidence is presented for secondary crystallization in selenium, and a model is proposed for destruction of chain folds with interlamellar crystallization during the spherulitic‐to‐“metallic” transformation above 100°C.

Effects of sample history and an electrostatic field on selenium crystallization

Effects of sample history and an electrostatic field on selenium crystallization

The polymeric nature of selenium crystallization. I. Morphology and thermodynamic considerations

AbstractThe morphology of selenium has been studied in light of present concepts of crystallinity in crystalline organic polymers. Spherulitic crystallization and resultant morphology were examined as a function of temperature by transmission and replication electron microscopy, electron diffraction, and optical microscopy. Evidence is given for polymer chain folding during crystallization of trigonal selenium. Theoretical thermodynamic calculations presented yield several thermodynamic values including an equilibrium melting point of 219.2°C, a fold surface energy of 337 ergs/cm2 and a lateral surface energy of 9.8 ergs/cm2.

Charge transport through α-monoclinic selenium

The existence of surface barriers on α-monoclinic selenium crystals has been demonstrated. Photometric measurements indicate electron barrier energies of 1·05 and 1·3 e V, respectively, for Ga and Au contacts. The mobilities of holes and electrons have been measured by a time-of-flight technique to be about 0·2cm 2/Vsec and l·6cm 2/Vsec, respectively, at room temperature. The hole mobility was found to be limited by traps 0·23 ± 0.01 eV above the valence levels, while the electron mobility is an intrinsic mobility limited by scattering. It was found that in the region of low carrier density (i.e. no space charge effects) the collection efficiency was limited by diffusion of carriers into the metal contact.

Conductivity changes associated with the crystallization of amorphous selenium

When a sample of amorphous selenium is annealed at a constant temperaturem the conductivity remains at first approximately constant, but after a certain time there is an abrupt increases of several orders of magnitude to a maximum followed very often by a slight decrease to a constant value. The maximum point has been ascribed by some workers to complete crystallization of the amorphous phase and the subsequent decrease to growth in the size of the crystallization grains bringing about a reduction in their total surface area along which the current is purported to flow. However, studies of spherulite growth in the present work indicate that transformation is not always complete at the conductivity maximum. Selenium samples were annealed at given temperatures for various lengths of time, quenched in water and then measured for electrical conductivity and spherulite size at room temperature. The results suggest that the rapid rise of conductivity is due to the establishement of a conducting path through the selenium by the touching of spherulites in regions of high nucleation. It was found that samples cooled to room temperature and then annealed at a higher temperature showed much shorter times to conductivity maximum than ones colled directly from the melting point to the same annealing temperature. This was found to be due to the fact that the nucleation rate dependes on the thermal history of the sample while the radial growth rate of the spherulites does not.In crystallized samples a sudden drop of conductance was observed when the applied voltage exceeded a certain value. On reducing the voltage, the lower conductance persisted for a time. Recovery to the higher conductance state took place over a period of several days or seconds depending on the temperature and the presence of chlorine in the sample. This switching effect would appear to be associated with a transition between the amorphous and crystalline states of selenium.

Chain folding during selenium crystallization

Journal of Polymer Science Part B: Polymer LettersVolume 8, Issue 1 p. 37-40 Article Chain folding during selenium crystallization Richard G. Crystal, Richard G. Crystal Research Laboratories Xerox Corporation Rochester, N.Y. 14603Search for more papers by this author Richard G. Crystal, Richard G. Crystal Research Laboratories Xerox Corporation Rochester, N.Y. 14603Search for more papers by this author First published: January 1970 https://doi.org/10.1002/pol.1970.110080107Citations: 13AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Citing Literature Volume8, Issue1January 1970Pages 37-40 RelatedInformation

Raman spectrum and force constants of S 42−

Raman spectroscopy using a He-Ne laser as excitation source has been used to identify a new species of molecular sulfur in concentrated aqueous solutions of sulfur in sodium sulfide. A normal coordinate vibrational analysis has been performed based on a non-linear four-atom model having SS bond lengths = 2.04 A ̊ , SSS bond angles = 105°, and the SSSS dihedral angle = 90°. The potential function used was the internal-rotation-excluded version of the Mizushima-Morino potential (1) and the force constants were the generalized valence forces derived by Venkateswarlu and Thirugnanasambandam (2) from a normal coordinate analysis of the S 8 molecule. Excellent agreement between observed and predicted Raman shifts and polarization characteristics was obtained. The four-atom species is presumably present in aqueous solution as the S 4 2− dianion. The mechanism of crystallization of rhombic sulfur and trigonal selenium from solutions in sodium sulfide is discussed in terms of this tetra-atomic species.

Morphology of Spherulites and Single Crystals of Trigonal Selenium

The crystallization of amorphous selenium is shown to occur by the formation of lamellas in which the molecular axis lies across the wide face and perpendicular to the growth direction. Below about 200°C the lamellas grow radially from a common nucleus branching noncrystallographically to produce a spherical crystalline mass. The theoretical selenium chain length in the quenched amorphous phase is much longer than the average lamella widths observed at large supercoolings. This indicates that at the edges the chains must fold back into the lamella if stress accumulation due to interconnection of the two phases of different density is to be avoided. It is suggested that such an array of chain folds introduces a dilation of the lattice which is accommodated if the lamella twists. Where it is cooperative, such twisting within the radial array of lamellas is shown to be responsible for the annular ring structure of selenium spherulites. In crystallization above 200°C the radial symmetry begins to breakdown and the lamella width approaches the mean chain length. Some fractionation then appears to take place to produce sets of lamellae of different widths which presumably contain fully extended chains. Clearer evidence of such fractionation was shown in a crystal grown under high pressure where the melt viscosity is lower. These smaller supercoolings also led to the growth of leaf-shaped single crystals having the same internal structure as the lamellas but with no set width. It is surmised that these are produced by a bundle-like accretion of chains. Such leaf crystals were observed within the bulk of `single' crystals grown by the traveling solvent or the Czochralski method. It is pointed out that the breakdown of periodicity in the selenium covalent bonding at impurity terminated chain ends, estimated at about 1016 to 1017/cc, or at chain folds, must have an appreciable influence on the charge transport properties of single crystals of trigonal selenium.

Transient Photoconductivity in Trigonal Selenium Single Crystals

Measurements of recombination lifetimes in single crystals of trigonal selenium, which is a p-type extrinsic semiconductor, have been made by studying the decay of photoconductivity induced by a 1.5 μsec light pulse. The addition of sufficiently intense dc illumination allows traps to be filled and a simple exponential decay obtained, which is interpreted to be a direct measure of the recombination lifetime. Lifetimes were found to range between 10 μsec and 34 μsec. From a knowledge of these lifetimes and absolute dc photoconductive gain measurements, photoconductivity mobilities were found to be 40±8 cm2·V−1·sec−1 parallel to the c axis and 17±4 cm2·V−1·sec−1 perpendicular to the c axis. The temperature dependence of the majority-carrier drift mobility can be obtained by measuring the relative pulsed photoconductive gain as a function of temperature. These experiments are in good accord with studies of acoustoelectric current saturation. No evidence is found for a barrier-limited mobility. Other experiments indicate that electrons are mobile in trigonal selenium, allow an estimate of their drift mobility, and give evidence of multiple trapping of minority carriers in deep traps.

Seeded Growth of Selenium Crystals under High Pressure

Two improvements in the technique of growing selenium single crystals under high pressure have been made: (1) the use of seed crystals of predetermined orientation, and (2) the annealing of the newly grown crystals during depressurization. The method of seed preparation and the characteristics of the resulting crystals are described. Odd growth patterns were observed at the seed-new growth interface.

Charge Accumulation Effect in Selenium Crystals under Applied Electric Field

Charge Accumulation Effect in Selenium Crystals under Applied Electric Field

Crystallization of Vitrious Selenium containing Sulfur

Crystallization of Vitrious Selenium containing Sulfur

Growth of single crystals of selenium in gels at ambient temperatures

The growth of selenium single crystals in organic gel media has been accomplished at near ambient temperatures. At least two different crystallographic modifications of selenium can be obtained depending upon the specifics of the growth system. The preparative techniques developed typically involve the diffusion of selenium solutions, e.g. carbon disulfide, methylene iodide etc. into toluene gels of styrenemaleic anhydride copolymers. The resulting single crystals whose size was in the order of 1.5–2 mm, were characterized by x-ray diffraction studies and suitable chemical analyses. The effects of different experimental system variables including selenium solvent, type of gel medium etc. on crystal characteristics are considered.