Electronic conduction in silicon-rich thin films

Silicon-rich silicon nitride (SRN) films were grown by low pressure chemical vapour deposition (LPCVD) with excess silicon concentrations varying from 8.8% to 12.8% by varying the reactant gas phase ratio from R(_n) = 4.0 to R(_n) = 0.25. Dichlorsilane and ammonia were the reactant gases and nitrogen was used as the carrier gas. All films were found to be predominantly ε-Si(_3)N(_9) with free silicon crystallites being found in the films with the greatest silicon content. The conduction mechanism at high temperatures and electric fields is due to Poole-Frenkel emission of trapped electrons (holes) from relatively deep defect levels to the conduction (valence) band tails. From a steady-state analysis the effective trap depth, Ф(_t). was found to be approximately 1.1 eV and it decreased slightly with increasing silicon content of the films. Low values of calculated dielectric constant were found and attributed to the build up of space charge near the injecting contact. For thin films ((^.)1000 Å) steady state analysis cannot be considered accurate unless the effects of trapped space charge are taken into account. For positive (negative) applied bias voltages, the flat band shift is in a positive (negative) direction implying a net increase in negative (positive) charge within the SRN film. A logarithmic time dependence is found for the transient flat band shift while for long periods of time (i.e. t , 1s) the current transient was inversely proportional to time. A charge trapping model is presented which predicts a logarithmic increase in the flat band shift with time. The model is based on the assumption that charge carriers, holes for negative bias and electrons for positive bias, tunnel from the silicon valence and conduction bands into the deep defect levelsin the SRN film. Very good agreement was found between the data and the model for low electric fields. At high electric fields, the situation becomes complicated by Poole-Frenkel 're-emission' from the traps which leads to a 'saturation of the flat band shift with time. At high electric fields, the current transient also becomes dominated by Poole-Frenkel emission of trapped carriers. It was also found from the current transients that the calculated dynamic dielectric constant decreases with increasing time reflecting the increase in trapped charge. From the Schottky curves for times of one second. E(_d) was found to vary from 5.09 to 5.26 for excess silicon content 8.8% to 12.3%. For these SRN films the density of trapping centres near the SRN-silicon interface was found to be of the order of 3x10 19om-3.Silicon-rich silicon oxide (SRO) films were grown in a atmospheric pressure CVD reactor using nitrous oxide and silane as the reactants and nitrogen as the main carrier gas. The conduction in silicon-rich oxide (SRO) was investigated using standard I-V techniques. Three models of conduction were investigated. These were a symmetrical Schottky barrier model similar to that used for polycrystalline silicon, a model based on a type of Fowler-Nordheim tunnelling between silicon crystallites in the SRO film and a model based on Poole-Frenkel emission from the silicon crystallites into the conduction band or band tails of the film. It was extremely difficult to assess the true nature of the conduction in the silicon-rich oxide (SRO) films. However, the evidence seems to point towards the Poole-Frenkel mechanism as being responsible for conduction

Development of a 6×6 Element Air-Coupled Multiple Moving Membrane Capacitive Micromachined Ultrasonic Transducer Array, M3-CMUT, for High Resolution Detection Applications

AbstractA 2-D multiple moving membrane capacitive micromachined ultrasonic transducer (M3-CMUT) array has been developed for air-coupled detection purposes in the megahertz frequency range. The transducer includes an array of 6×6 M3-CMUT elements. This transducer benefits from a novel configuration where a stack of two vibrating membranes suspended over a fixed grounded electrode are involved in the signal transmission and detection. Using this configuration, the amplitude of membrane vibration is increased and that enhances the transducer power output and sensitivity. Electrical and acoustic characterization of this transducer array is presented in this paper

Capacitive Micromachined Ultrasonic Transducer Array with Pencil Beam Shape and Wide Range Beam Steering

AbstractA capacitive micromachined ultrasonic transducer (CMUT) array is designed as an alternative to conventional piezoelectric transducers. A thin silicon nitride membrane is suspended over a bottom electrode on a silicon wafer. In the immersion mode, the transducer cell shape and dimensions are optimized for an operating frequency of 10MHz. We show that the proposed imager array can generate a pencil shape beam with a ∼1.5° half beam width, enhancing the detector resolution. A phased array technique is employed to excite multiple cells using time-delayed signals to steer the acoustic beam toward the object. This eliminates the need to mechanically move the detector, simplifying the transducer driving system. Moreover, unlike conventional transducers, the pencil beam can be effectively steered over a wide range of angles without producing grating lobes, which minimizes power loss in undesired directions. This can also improve the signal to noise ratio of the imager CMUT array

Seizure first aid training for people with Epilepsy (SAFE) frequently attending emergency departments and their significant others : results of a UK multi-centre randomised controlled pilot trial

Objective To determine the feasibility and optimal design of a randomised controlled trial (RCT) of Seizure First Aid Training For Epilepsy (SAFE). Design Pilot RCT with embedded microcosting. Setting Three English hospital emergency departments (EDs). Participants Patients aged ≥16 with established epilepsy reporting ≥2 ED visits in the prior 12 months and their significant others (SOs). Interventions Patients (and their SOs) were randomly allocated (1:1) to SAFE plus treatment-as-usual (TAU) or TAU alone. SAFE is a 4-hour group course. Main outcome measures Two criteria evaluated a definitive RCT’s feasibility: (1) ≥20% of eligible patients needed to be consented into the pilot trial; (2) routine data on use of ED over the 12 months postrandomisation needed securing for ≥75%. Other measures included eligibility, ease of obtaining routine data, availability of self-report ED data and comparability, SAFE’s effect and intervention cost. Results Of ED attendees with a suspected seizure, 424 (10.6%) patients were eligible; 53 (12.5%) patients and 38 SOs consented. Fifty-one patients (and 37 SOs) were randomised. Routine data on ED use at 12 months were secured for 94.1% patients. Self-report ED data were available for 66.7% patients. Patients reported more visits compared with routine data. Most (76.9%) patients randomised to SAFE received it and no related serious adverse events occurred. ED use at 12 months was lower in the SAFE+TAU arm compared with TAU alone, but not significantly (rate ratio=0.62, 95% CI 0.33 to 1.17). A definitive trial would need ~674 patient participants and ~39 recruitment sites. Obtaining routine data was challenging, taking ~8.5 months. Conclusions In satisfying only one predetermined ‘stop/go’ criterion, a definitive RCT is not feasible. The low consent rate in the pilot trial raises concerns about a definitive trial’s finding’s external validity and means it would be expensive to conduct. Research is required into how to optimise recruitment from the target population

Mental models of high reliability systems

Reliable performance in complex systems is determined in part by the ade quacy with which mental models of the system capture accurately the dimen sions of system coupling and system complexity. Failure to register coupling and complexity leads the observer to intervene into an imagined technology that does not exist and to convert opportunities for error into actual errors. To decrease the frequency with which this conversion occurs, people can make their models more complex or the systems they monitor less complex. Neither type of change is as daunting as it may appear, and this is illustrated by an analysis of the mental model and system design associated with the invasion of Grenada.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68652/2/10.1177_108602668900300203.pd

Magnetic Order in YBa2_2Cu3_3O6+x_{6+x} Superconductors

Polarized and unpolarized neutron diffraction has been used to search for magnetic order in YBa$_2$Cu$_3$O$_{6+x}$ superconductors. Most of the measurements were made on a high quality crystal of YBa$_2$Cu$_3$O$_{6.6}$. It is shown that this crystal has highly ordered ortho-II chain order, and a sharp superconducting transition. Inelastic scattering measurements display a very clean spin-gap and pseudogap with any intensity at 10 meV being 50 times smaller than the resonance intensity. The crystal shows a complicated magnetic order that appears to have three components. A magnetic phase is found at high temperatures that seems to stem from an impurity with a moment that is in the $a$-$b$ plane, but disordered on the crystal lattice. A second ordering occurs near the pseudogap temperature that has a shorter correlation length than the high temperature phase and a moment direction that is at least partly along the c-axis of the crystal. Its moment direction, temperature dependence, and Bragg intensities suggest that it may stem from orbital ordering of the $d$-density wave (DDW) type. An additional intensity increase occurs below the superconducting transition. The magnetic intensity in these phases does not change noticeably in a 7 Tesla magnetic field aligned approximately along the c-axis. Searches for magnetic order in YBa$_2$Cu$_3$O$_{7}$ show no signal while a small magnetic intensity is found in YBa$_2$Cu$_3$O$_{6.45}$ that is consistent with c-axis directed magnetic order. The results are contrasted with other recent neutron measurements.Comment: 11 pages with 10 figure

Do Neuro-Muscular Adaptations Occur in Endurance-Trained Boys and Men?

Most research on the effects of endurance training has focused on endurance training's health-related benefits and metabolic effects in both children and adults. The purpose of this study was to examine the neuromuscular effects of endurance training and to investigate whether they differ in children (9.0-12.9 years) and adults (18.4-35.6 years). Maximal isometric torque, rate of torque development (RTD), rate of muscle activation (Q30), electromechanical delay (EMD), and time to peak torque and peak RTD were determined by isokinetic dynamometry and surface electromyography (EMG) in elbow and knee flexion and extension. The subjects were 12 endurance-trained and 16 untrained boys, and 15 endurance-trained and 20 untrained men. The adults displayed consistently higher peak torque, RTD, and Q30, in both absolute and normalized values, whereas the boys had longer EMD (64.7+/-17.1 vs. 56.6+/-15.4 ms) and time to peak RTD (98.5+/-32.1 vs. 80.4+/-15.0 ms for boys and men, respectively). Q30, normalized for peak EMG amplitude, was the only observed training effect (1.95+/-1.16 vs. 1.10+/-0.67 ms for trained and untrained men, respectively). This effect could not be shown in the boys. The findings show normalized muscle strength and rate of activation to be lower in children compared with adults, regardless of training status. Because the observed higher Q30 values were not matched by corresponding higher performance measures in the trained men, the functional and discriminatory significance of Q30 remains unclear. Endurance training does not appear to affect muscle strength or rate of force development in either men or boys