Optical motor: Towards the model of life emerge

We show that strong incoherent broad band light in random media pushes an electron in opposite to an electric force direction. The effective electron-electron interaction becomes attractive in this case, electric current flows in opposite to a voltage drop direction and static polarization is induced in opposition to an applied electric field. The induced effective electron-electron attraction results in the electron coagulation and this light driven self-organization might have been the initial pre-biotic stage in life emerge on Earth.     

DC conductivity of silver vanadium tellurite glasses

The mixed electronic-ionic conduction in 0.5[xAg₂O-(1−x)V₂O₅]-0.5TeO₂ glasses with x=0.1-0.8 has been investigated over a wide temperature range (70-425 K). The mechanism of dc conductivity changes from predominantly electronic to ionic within the 30?mol% Ag₂O?40 range; it is correlated with the underlying change in glass structure. The temperature dependence of electronic conductivity has been analyzed quantitatively to determine the applicability of various models of conduction in amorphous semiconducting glasses. At high temperature, T>θ_D/2 (where θ_D is the Debye temperature) the electronic dc conductivity is due to non-adiabatic small polaron hopping of electrons for 0.1?x?0.5. The density of states at Fermi level is estimated to be N(E_F)≈10¹⁹-10²⁰ eV⁻¹ cm⁻³. The carrier density is of the order of 10¹⁹ cm⁻³, with mobility ≈2.3×10⁻⁷-8.6×10⁻⁹ cm² V⁻¹ s⁻¹ at 300 K. The electronic dc conductivity within the whole range of temperature is best described in terms of Triberis-Friedman percolation model. For 0.6?x?0.8, the predominantly ionic dc conductivity is described well by the Anderson-Stuart model.     

Electron traps in wide-gap n-Hg0.3Cd0.7Te characterized by time-resolved photoconductivity

Time-resolved photoconductivity measurements have been used to characterize electron traps in wide-gap n-HgO_0.3Cd_0.7Te for the first time. The characterization was made possible by combining the time-resolved photoconductive data with the analytical method conventionally used in DLTS spectroscopy. Two electron traps were found in the band gap with 61 meV and 79 meV below the conduction band edge, their concentrations are 1.1×10¹³ cm^–3 and 5.8×10¹¹ cm^–3, respectively. Compared with DLTS spectroscopy, this characterization method markedly simplifies sample preparation and experimental procedure.     

Kinetics of photo‐darkening and self‐bleaching in amorphous As2S3 and As2Se3 thin films

The kinetics of photo‐darkening of amorphous As₂S₃ and a‐As₂Se₃ thin films follows a single exponential, but the magnitude and the rate of the process is higher in case of As₂S₃. The kinetics of self‐bleaching (dark relaxation) in advance photo‐darkened state follows a stretched exponential (SRE) with different stretching parameter for a‐As₂S₃ and a‐As₂Se₃. Within the J. C. Phillips approach we suppose that photo‐darkening in amorphous As₂S₃ films is, to some extent, accompanied by changes in short‐range order interactions, while photo‐darkening of amorphous As₂Se₃ is accompanied rather by changes in Coulomb interactions. The self‐bleaching process reduced the magnitude of photo‐darkening up to 45% and 60% for amorphous As₂S₃ and As₂Se₃ films, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Subpicosecond carrier trapping in high-defect-density amorphous Si and GaAs

The relaxation of photoexcited carriers in a-Si and a-GaAs has been studied with subpicosecond time resolution by excite-and-probe experiments. The measured time dependence of the induced reflectivity and absorption reveals that the initial trapping of mobile carriers in high-defect-density materials occurs on a subpicosecond time scale.     

Electrical switching studies on As<Subscript>40</Subscript>Te<Subscript>60-x</Subscript>Se<Subscript>x</Subscript> and As<Subscript>35</Subscript>Te<Subscript>65-x</Subscript>Se<Subscript>x</Subscript> glasses

The I–V characteristics of bulk As₄₀Te_60-xSe_x and As₃₅Te_65-xSe_x glasses have been studied with a current sweep of 0–18 mA-0, over a wide range of compositions (4≤x≤22). All the glasses studied showed a threshold electrical switching behaviour. The number of switching cycles withstood by the samples has been found to depend on the ON-state current. It is seen that the switching voltages increase with increase in selenium content. Further, the switching voltages are found to be almost independent of the thickness of the sample (d), in the range 0.18–0.3 mm. Also, the switching voltages and the number of switching cycles withstood by the samples are found to decrease with temperature. Received: 6 November 2002 / Accepted: 8 November 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +91-80/360-0135, E-mail: sasokan@isu.iisc.ernet.in     

Light-intensity dependence of slow-relaxation phenomena in semi-insulating GaAs

Quenching-only and quenching-enhancement phenomena in low-temperature photoconductivity (PC) of SI GaAs have been studied as a function of light intensity for photons in 1.0–1.2 eV energy range. Quenching-only of PC occurs only at high light intensities (above 10¹⁴ photons/cm² s) and reflects well-known bleaching of EL2 defects. On the contrary, the quenching-enhancement effect can be observed only for several orders of magnitude lower light intensities and neither the quenching nor the enhancement part of low-temperature evolution of PC is directly connected with EL2 defects, but reflects the time evolution of the occupancy of deep traps other than EL2. It was also found that bleaching of EL2 is quite an unefficient process.     

Field dependence of hole mobility in TPD-doped polystyrene

Field and temperature dependence of hole mobility in N,N^′-diphenyl-N,N^′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) doped in polystyrene (PS) is studied using the transient photoconductivity technique. We observe both the positive and negative field dependence of mobility with increasing field and temperature. The field and temperature at which negative field dependence begins is low compared with earlier reports on similar systems (with 20 wt% dopant concentration). Results are discussed on the basis of the Gaussian disorder model (GDM), which predicts that the interplay of both the energetic and positional disorder of dopant molecules in the sample decides the slope of the logμ versus E^1/2 plot. The observed mobility dependence is rationalized on the basis of low energetic disorder in the sample. The reason for low energetic disorder is purely due to the film morphology of the sample. Even for a dopant concentration of 20 wt%, we observe clustering and chaining of TPD molecules, which may provide low energetic and positional disorder.     

Study of hydrogenated amorphous silicon nitride films prepared by RF magnetron sputtering

Hydrogenated amorphous silicon nitride films have been deposited by the rf magnetron sputtering method with non-stoichiometric and stoichiometric compositions using a poly-Si target and a mixture of Ar, H₂ and N₂ as the sputtering gas. Data on optical and infrared absorption, electrical conductivity, breakdown voltage, capacitance measurements and thermal evolution of hydrogen have been presented as a function of nitrogen concentration in the films, especially in the stoichiometric region of composition. Attempts have been made to identify the roles of hydrogen and nitrogen in determining the electrical and optical properties and thermal stability exhibited by the films. Properties relevant for device application of the material have been shown to be comparable to those obtained by glow discharge or electron cyclotron resonance plasma chemical vapour deposition methods of deposition. RF magnetron sputtering has therefore been suggested as a viable alternative to the more widely adopted CVD methods for device applications of silicon nitride, where the use of hazardous process gases can be avoided.     

Sol-gel preparation and characterization of CoFe2O4-SiO2 nanocomposites

Magnetic nanocomposites formed by cobalt ferrite particles dispersed in a silica matrix were prepared by a sol-gel process. The effects of the thermal treatment temperature and the salt concentration on the structural and magnetic properties of the composites were investigated. By controlling these parameters, CoFe₂O₄/SiO₂ nanocomposites with different crystallite size and magnetic properties were obtained. By increasing the annealing temperature and salt concentration, composites with a progressive increase in the coercive field and of the density of magnetization were produced. In particular, a nanocomposite, with a Fe/Si molar concentration of 21%, obtained by drying the gel at 150 °C and further annealing at 800 °C, has a coercivity of 2000 Oe, which is more than twice higher than the coercivity of bulk cobalt ferrite.     

Characterization of amorphous silicon films by contactless transient photoconductivity measurements

Contactless transient photoconductivity measurements of intrinsic a-Si: H films in the microwave frequency range are presented. The measurements are evaluated quantitatively and the electron drift mobility is determined. It is shown that the influence of the surface on the observed decay behaviour can be neglected. A relation between the long time decay behaviour and the position of the Fermi level is observed.     

Magnetic response of dendritic structures at infrared frequencies

We propose a quasi-periodic dendritic resonant magnetic model that can be used for the realization of the negative permeability at infrared frequencies. The numerical simulation exhibits a magnetic response when the incident light is perpendicular to the plane of the model. The copper dendritic structures are prepared by the electro-deposition method. And the transmission spectrum of this dendritic structure shows a magnetic response at infrared frequencies with a maximum transmission −7.95 dB. Further studies demonstrate that the magnetic response relate to the fractal dimension of the dendritic structure to a certain extent.     

Trap levels in layered semiconductor Ga2SeS

Trap levels in nominally undoped Ga₂SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga₂SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7×10⁻²³, 1.8×10⁻²³ and 2.8×10⁻²² cm² with concentrations of 1.3×10¹², 5.4×10¹² and 4.2×10¹² cm⁻³, respectively.     

Trapping centers and their distribution in Tl2Ga2Se3S layered single crystals

Thermally stimulated current (TSC) measurements with current flowing perpendicular to the layers were carried out on Tl₂Ga₂Se₃S layered single crystals in the temperature range of 10-260 K. The experimental data were analyzed by using different methods, such as curve fitting, initial rise and isothermal decay methods. The analysis revealed that there were three trapping centers with activation energies of 12, 76 and 177 meV. It was concluded that retrapping in these centers was negligible, which was confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping. The capture cross section and the concentration of the traps have been also determined. An exponential distribution of electron traps was revealed from the analysis of the TSC data obtained at different light illumination temperatures. This experimental technique provided values of 10 and 88 meV/decade for the traps distribution related to two different trapping centers.     

Optical and electronic properties of hydrogenated amorphous silicon films deposited by square-wave modulated RF discharges of silane-he mixtures

The optoelectronic properties of a-Si: H films deposited under very different plasma conditions have been studied as functions of the square-wave modulation frequency of a 13.56 MHz rf discharge. Results for two extreme deposition conditions are presented. At both high and low deposition rates, the modulation of the rf discharge results in a negligible effect or a deterioration of the film properties, particularly the photoconductivity which we have found to be the most sensitive parameter. At high deposition rates, modulation of the discharge at 10 Hz results in the incorporation of particles into the film which produce enhanced absorption in the film. Therefore, for our reactor configuration, the modulation of the discharge does not produce an improvement but rather a degradation of the film properties.     

Electrical activity and precipitation behavior of copper in gallium arsenide

The electrical properties and preferred lattice site of Cu in GaAs were investigated combining electrical and optical measurements with ion beam and structural analysis. From this comprehensive study it was determined that Cu introduces two levels in the band gap, that the concentration of electrically active centers introduced by Cu diffusion is considerably smaller than the total Cu concentration, that this ratio of electrically active to total Cu concentration depends strongly on the cooling speed after diffusion, and that the portion of Cu that remains electrically inactive forms Cu-Ga precipitates.     

Preparation and characteristics of?a?BaO–Al2O3–B2O3–SiO2–La2O3 glass ceramic via spray pyrolysis

The characteristics of a BaO–Al₂O₃–B₂O₃–SiO₂–La₂O₃ glass ceramic prepared by spray pyrolysis were studied. Glass powders with spherical shape and amorphous phase were prepared by complete melting at a preparation temperature of 1 500°C. The mean size and geometric standard deviation of the powders prepared at the temperature of 1 500°C were 0.6 μm and 1.3. The glass powders had similar composition to that of the spray solution. The glass transition temperature (T _g) of the glass powders was 600.3°C. Two crystallization exothermic peaks were observed at 769.3 and 837.8°C. Densification of the specimen started at a sintering temperature of 600°C, in which Ba₄La₆O(SiO₄)₆ as main crystal structure was observed. Complete densification of the specimen occurred at a sintering temperature of 800°C. The specimens sintered at temperatures above 800°C had main crystal structure of BaAl₂Si₂O₈.     

High Fe2+/3+ trap concentration in heavily compensated implanted InP

High Fe concentrations (up to 2×10¹⁹ cm^-3) have been implanted in n-doped InP to compensate the substrate donors. The resulting semi-insulating layers have been investigated by current–voltage (I-V) measurements and photo-induced current transient spectroscopy (PICTS) analyses to characterise the Fe activation process and to study the Fe related deep levels. The activation of the Fe^2+/3+ trap has been assessed by the identification of the deep level located at E_C-0.64 eV. The outcomes of the PICTS measurements have been correlated with the electrically active Fe concentration calculated from a numerical simulation of the I-V characteristics. We observe an increasing linear relation between the electrically active Fe concentration and the substrate doping density, with a maximum active Fe concentration as high as 2×10¹⁸ cm^-3, i.e. more than an order of magnitude above the equilibrium Fe solid solubility. These data are presented and their implications discussed. Received: 4 September 2000 / Accepted: 7 February 2001 / Published online: 23 May 2001     

Thermally stimulated currents in semi-insulating GaAs Schottky diodes and their simulation

We report the investigation of the non-irradiated and irradiated-with-pions Schottky diodes made on semi-insulating GaAs. Thermally stimulated currents have been measured experimentally and modeled numerically. To reveal the influence of the single levels, we used the thermal emptying of the traps by fractional heating. Attention is paid to the comparative analysis of the distribution of the parameters of different samples produced and processed by the same technique, contrary to the usual approach of the analysis of a few different samples. The following main conclusions are drawn. First of all, many different levels (from 8 to 12) have been found in the temperature range from 90 K to 300 K in all samples. Their activation energies range from 0.07 up to 0.55 eV, their capture cross-sections are 10^-22–10^-14 cm², and initial occupation is 2×10¹¹–5×10¹⁴ cm^-3. The irradiation with pions does not influence the density of most levels significantly. On the other hand, levels with activation energies of about 0.07–0.11 eV, 0.33–0.36 eV, 0.4–0.42 eV, and 0.48–0.55 eV have been found only in the irradiated samples. Irradiation also increases the inhomogeneity of the crystals, which causes the scattering of the activation energies obtained by fractional heating technique. Received: 13 November 1998 / Accepted: 16 April 1999 / Published online: 4 August 1999     

Optical decoherence times and spectral diffusion in an Er-doped optical fiber measured by two-pulse echoes, stimulated photon echoes, and spectral hole burning

Two-pulse and stimulated photon echoes and spectral hole burning were measured on the transition from the lowest component of the ⁴I_15/2 manifold to the lowest component of ⁴I_13/2 of Er³⁺ in a silicate optical fiber at 1.6 K. The two-pulse echo decays gave decoherence times as long as 230 ns for magnetic fields above 2 T. A large field dependent contribution to the homogeneous line width of >2 MHz was found and interpreted in terms of coupling to magnetic tunneling modes (TLS) in the glass. The stimulated echoes measured at 2 T showed spectral diffusion of 0.8 MHz/decade of time between 0.4 and 500 μs. Spectral diffusion in this high field region is attributed to coupling to elastic TLS modes which have a distribution of flip rates in glasses. Time-resolved spectral hole burning at very low field showed stronger spectral diffusion of 5.7 MHz/decade of time, attributed to coupling to magnetic spin-elastic TLS modes.