J. Kakalios

Hydrogen diffusion in amorphous silicon

Abstract Hydrogen diffusion in doped and compensated a-Si:H has been measured by secondary-ion mass spectrometry profiling in the temperature range 155-300°C. Doping reduces the activation energy and enhances the diffusion coefficient by up to three orders of magnitude at 200°C, and a correlation between the diffusion coefficient and the dangling-bond density is found. An analysis of three different diffusion models indicates that the breaking of weak Si – Si bonds by hydrogen may be an important process. The relation between the diffusion results and the thermal equilibration of the electronic structure is discussed. The hydrogen diffusion coefficient in a-Si:H has been measured over the temperature range 155-300°C, with particular emphasis on the effects of doping and compensation. In all cases D H is thermally activated with an energy 1[sdot]2-1[sdot]5 eV. The diffusion coefficient decreases slowly with time which is attributed to the disorder-induced variation in site energies. We find that D H is gre...

Properties of amorphous semiconducting multilayer films

Abstract The structural, optical, and electrical properties of amorphous multilayer films containing up to 180 double layers of a-Si:H/a-SiNx and a-Si:H/a-SiOx were studied. For a-Si:H layer thickness d >100 A the transport properties are dominated by space-charge doping with a-SiNx positive and a-SiOx negatively charged. For d A quantum-well effects increase the optical and electrical gaps. A d =12 A multilayer film shows no evidence for the predicted loss of extended states in two-dimensional disordered systems.

Transient striatal γ local field potentials signal movement initiation in rats

Transient coherent neural oscillations, as indicated by local field potentials, are thought to underlie key perceptual and cognitive events. We report a transient, state-dependent 50 Hz oscillation recorded from electrodes placed in the striatum of awake, behaving rats. These coherent oscillations, which we term &ggr;50, occurred in brief (150 ms) events co-incident with the initiation of movement. On navigation tasks, the animals speed increased dramatically at the precise moment of the &ggr;50 event. This synchronous oscillation may provide a key to understanding striatal function, as well as basal ganglia pathology, which often impairs the control of voluntary movements.

Measuring fundamental frequencies in local field potentials

Neural processes display rhythmic oscillations in local field potentials; identification of their characteristic frequencies is complicated due to their highly non-stationary nature. A simple technique, combining Fourier transforms and correlation coefficients yields unambiguous determinations of the frequencies without a priori filtering. This procedure also provides quantitative information concerning interactions between frequencies. Fundamental frequencies in local field potential data acquired from the hippocampus, cortex, and striatum from awake, behaving rats were calculated using this technique. Characteristic frequencies identified using this technique from hippocampus and cortex agreed with known oscillations. Application to dorsal striatal local field potentials identified a low-frequency theta component as well as a narrow gamma band oscillation at 50-55 Hz.

The role of dangling bonds in the transport and recombination of a-Si:Ge: H alloys

Abstract Measurements are reported of the luminescence, time-of-flight photoconductivity and electron spin resonance (ESR) in a-Si: Ge: H alloys. In the alloys it is found that the luminescence intensity varies with defect density according to a tunnelling model similar to that in a-Si: H. However, the tunnelling distance decreases with Ge concentration up to about 30% and then increases. This nonmonotonic behaviour is attributed to the additional chemical disorder of the alloys. Time-of-flight experiments confirm that the dangling bonds are the dominant deep trap in the alloys. We find that the capture cross-sections for electrons and holes are very similar to those measured in a-Si: H, and are independent of whether the trap is a Ge or Si dangling bond.

Structural and electronic properties of dual plasma codeposited mixed-phase amorphous/nanocrystalline thin films

A dual-plasma codeposition system capable of synthesizing thin films of mixed-phase materials consisting of nanoparticles of one type of material embedded within a thin film semiconductor or insulator matrix is described. This codeposition process is illustrated by the growth of hydrogenated amorphous silicon (a-Si:H) films containing silicon nanocrystalline inclusions (a/nc-Si:H). A capacitively coupled flow-through plasma reactor is used to generate silicon nanocrystallites of diameter 5 nm, which are entrained by a carrier gas and introduced into a capacitively coupled plasma enhanced chemical vapor deposition reactor with parallel plate electrodes, in which a-Si:H is synthesized. The structural and electronic properties of these mixed-phase a/nc-Si:H films are investigated as a function of the silicon nanocrystal concentration. At a moderate concentration (crystalline fraction 0.02–0.04) of silicon nanocrystallites, the dark conductivity is enhanced by up to several orders of magnitude compared to mixed-phase films with either lower or higher densities of nanoparticle inclusions. These results are interpreted in terms of a model whereby in films with a low nanocrystal concentration, conduction is influenced by charges donated into the a-Si:H film by the inclusions, while at high nanocrystal densities electronic transport is affected by increased disorder introduced by the nanoparticles.

Light-induced changes of the non-Gaussian 1/f noise statistics in doped hydrogenated amorphous silicon

Abstract The influence of metastable photo-induced conductance changes (the Staebler-Wronski effect) on the 1/f noise power spectra of co-planar current fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) is studied. The average magnitude and spectral slope of the 1/f noise are not altered significantly due to illumination, when measured with identical d.c. currents passing through the film before and after light exposure. The conductance noise in the annealed state is strongly non-Gaussian, which is reflected in large noise power correlation coefficients between different octaves, a noise power distribution which is much broader than the expected x 2 distribution and a 1/f-like second spectrum. Following light exposure, the correlation coefficients decrease, the noise power distribution is well described by a x 2 distribution and the second spectrum becomes frequency independent. Annealing the a-Si:H film to 450K restores the non-Gaussian statistics of the 1/f noise. The non-Gaussian noi...

Excitation and temperature dependence of the photo-induced excess conductivity in doping-modulated amorphous silicon

Abstract The dependence of the photo-induced excess conductivity in doping-modulated amorphous silicon on exposure time, light intensity, exposure temperature and boron-doping concentration is investigated. Our results indicate that the excess-conductivity effect arises from charge storage in a novel defect centre. The growth of the excess conductivity is thermally activated and obeys a power-law dependence on exposure time and light intensity, where the exposure-time exponent increases with light intensity.

Bias annealing of doped amorphous silicon

Abstract Experiments are described which explore the changes in the densities of electrons or holes in doped amorphous silicon when it is annealed with an applied bias. Such bias annealing produces a metastable enhancement of the doping efficiency by altering the distribution of localized states. A model is proposed in which the effects are directly related to the doping mechanism of amorphous silicon.

Resource Letter GP-1: Granular physics or nonlinear dynamics in a sandbox

This Resource Letter provides a guide to the literature on the statics and dynamics of granular media. Journal article and book references are provided for the following topics: Packing, Angle of Repose, Avalanches and Granular Flow, Hoppers and Jamming, Vertically Vibrated Induced Phenomena, Avalanche Stratification, and Axial Segregation.