Space Charge Limited Behavior Research Articles

Critical assessment of the applicability of the Child-Langmuir law to plasma ion source extraction systems

Applicability of the classical one-dimensional and the recently developed multi-dimensional Child-Langmuir (CL) laws to plasma ion source extraction systems is studied. Experiments with a filament-driven multicusp ion source using a simplified extraction system, and comprehensive simulations with two different software packages, suggest that ion beams, which are typically assumed to be space charge or CL limited, are actually divergence-limited. In addition, it was observed that the dynamic plasma meniscus and geometric effects of the extraction system have a greater impact on the ion beam behavior than the effects predicted by the multi-dimensional CL law. Nevertheless, it was found that ion beams produced by the ion source exhibit space charge limited behavior, the beam current scaling with the extraction voltage as , provided that the entire beam is measured and its divergence is kept small and constant for each extraction voltage. This implies that the applicability of the CL law on a specific extraction system cannot be resolved by a straightforward voltage sweep and beam current measurement but instead, the plasma parameters must be adjusted for each extraction voltage. By shifting the focus from the CL limited framework to the divergence-limited framework, a clearer interpretation of the beam current–voltage characteristics of extraction systems can be achieved. This shift in perspective may also lead to the development of new design paradigms that can further enhance the performance of plasma ion source extraction systems.

A temperature dependent study on charge dynamics in organic molecular device: Effect of shallow traps on space charge limited behavior

Shallow traps play a significant role in influencing charge dynamics through organic molecular thin films, such as pentacene. Sandwich cells of pentacene capped by gold electrodes are an excellent specimen to study the nature of underlying charge dynamics. In this paper, self-consistent numerical simulation of I–V characteristics is performed at various temperatures. The results have revealed negative value of Poole Frenkel coefficient. The location of trap energy level is found to be located at 0.24 eV above the highest occupied molecular orbit (HOMO) level of pentacene. Other physical parameters related to trap levels, such as density of states due to traps and effective carrier density due to traps, have also been estimated in this study.

Space Charge Limited Current and Magnetoresistance in Si

Mott and Gurney point out1, for defect-free semiconductors, I-V curve deviates from linear Ohmic type to nonlinear space-charge limited behavior at high electric field. A surprising large magnetoresistance (MR) has been reported in space-charge limited region by Delmo2-4 recently. In present work, I-V and MR curves of silicon samples with different doping concentration are measured. It is observed that I-V curve enters into space charge region at lower voltage in heavily doped samples, however, space-charge limited current is absent in lightly doped samples. Two samples show different types of MR curve. In heavily doped samples, 8% MR is acquired at 3kG and the value of MR increases linearly up to 17%, while MR increases slowly up to 11% in lightly doped samples. It is believed that the dopant and trap in N-type silicon has a strong influence on the space-charge limited current and MR.

Current–voltage characteristics of sol–gel derived SrZrO 3 thin films for resistive memory applications

SrZrO 3 thin films are prepared on Pt/TiO 2/SiO 2/Si substrates by sol–gel process for resistive random access memory applications. The unipolar resistive switching characteristic is achieved in both stoichiometric and Zr-deficient SrZrO 3 thin films. Formation and rupture of filaments composed of oxygen vacancies are proposed to explain the observed resistive switching behaviors. By introducing oxygen vacancies into SrZrO 3 through making the film Zr-deficient, the conduction mechanism of the high resistance state changes from Ohmic to space charge limited behavior. This may be ascribed to more deep traps associated with oxygen vacancies due to Zr-deficiency. Switching endurance is enhanced significantly in Zr-deficient SrZrO 3 due to the reduced Joule heating.

Enhanced charge conduction in discotic liquid crystals

At relatively low electric fields the conduction of thin films (2–6 µm) of the discotic liquid crystal (HAT6–PTP9) sandwiched between ITO electrodes and homeotropically aligned is low and is consistent with space-charge limited behaviour. However when annealed at high temperatures and electric fields the films undergo a transition and the electrical conductivity increases by up to 5–6 orders of magnitude. Studies show that this increase in the conductivity is not due to filament growth or to bulk molecular breakdown but rather to some chemical or physical change at the electrode surface.

Transport of charge and atomic particles in Rydberg state-rich plasmas

New methods make it possible to form considerable flux densities of Rydberg atoms of alkali metals. It is now possible to study the transport processes in regions where the density of Rydberg atoms is large. Examples of such studies have been given by Svensson and coworkers. In the present study, ions and Rydberg states are formed by desorption at 1300-1800 K from an Ir surface covered by a thin graphite layer. Due to the very large cross sections for collision processes involving Rydberg species, the Rydberg state-rich plasma between the Ir emitter and a cold grid electrode is not collision free, even at a pressure of mbar. Electron or emission takes place from the grid at a rate controlled by the flux of and . The transition to penetration of and through the cloud of excited species between the emitter and grid is observed directly by molecular beam and ion sampling to detectors in a separate chamber. There is a space-charge limited behaviour for the positive current through the plasma as well as, in some modes, a clear positive saturation current, which shows that little gas phase ionization takes place. A current larger than expected from the saturation current as well as maxima in the voltage dependences are observed at high Rydberg densities. These effects are probably caused by space charge compensation due to a dielectric phase of condensed excited species, which means, for example, that the effective distance between the emitter and grid is decreased, as observed. The temperature variation of the space charge limited behaviour gives an activation energy of , while the saturation current gives an activation energy of . This agrees well with the electronic excitations at 0.90 eV and at 0.84 eV, which should be important steps in a thermal excitation process from the ground state of Cs.

Device model for single carrier organic diodes

We present a unified device model for single layer organic light emitting diodes (LEDs) which includes charge injection, transport, and space charge effects in the organic material. The model can describe both injection limited and space charge limited current flow and the transition between them. We specifically considered cases in which the energy barrier to injection of electrons is much larger than that for holes so that holes dominate the current flow in the device. Charge injection into the organic material occurs by thermionic emission and by tunneling. For Schottky energy barriers less than about 0.3–0.4 eV, for typical organic LED device parameters, the current flow is space charge limited and the electric field in the structure is highly nonuniform. For larger energy barriers the current flow is injection limited. In the injection limited regime, the net injected charge is relatively small, the electric field is nearly uniform, and space charge effects are not important. At smaller bias in the injection limited regime, thermionic emission is the dominant injection mechanism. For this case the thermionic emission injection current and a backward flowing interface recombination current, which is the time reversed process of thermionic emission, combine to establish a quasi-equilibrium carrier density. The quasi-equilibrium density is bias dependent because of image force lowering of the injection barrier. The net device current is determined by the drift of these carriers in the nearly constant electric field. The net device current is much smaller than either the thermionic emission or interface recombination current which nearly cancel. At higher bias, injection is dominated by tunneling. The bias at which tunneling exceeds thermionic emission depends on the size of the Schottky energy barrier. When tunneling is the dominant injection mechanism, a combination of tunneling injection current and the backflowing interface recombination current combine to establish the carrier density. We compare the model results with experimental measurements on devices fabricated using the electroluminescent conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] which by changing the contacts can show either injection limited behavior or space charge limited behavior.

Rapid diffusion of cesium through metal foils: Diffusion limiting at high current densities

The surface ionization of Cs on foil samples of Ir and some other materials was investigated at high temperatures and large atomic Cs beam flux densities. Rapid diffusion through the foil was shown to exist, at current densities up to approximately 5 mA cm -2. The size of the ion current from both sides of the sample was diffusion limited and reached a temperature and flux independent value, for both pulsed (1–10 μs) and DC current measurements. There was no indication of a space charge limited behaviour under the conditions of the experiments. The behaviour observed is not compatible with the equilibrium Saha-Langmuir description of surface ionization. Most of the results, however, agree with our recently published model for coupled surface ionization and diffusion. Diffusion in the metal crystallites and desorption above the vacuum level over a small or vanishing desorption barrier are important processes in the model.

Rapid diffusion of cesium through metal foils: Diffusion limiting at high current densities

The surface ionization of Cs on foil samples of Ir and some other materials was investigated at high temperatures and large atomic Cs beam flux densities. Rapid diffusion through the foil was shown to exist, at current densities up to approximately 5 mA cm −2. The size of the ion current from both sides of the sample was diffusion limited and reached a temperature and flux independent value, for both pulsed (1–10μ) and DC current measurements. There was no indication of a space charge limited behaviour under the conditions of the experiments. The behaviour observed is not compatible with the equilibrium Saha-Langmuir description of surface ionization. Most of the results, however, agree with our recently published model for coupled surface ionization and diffusion. Diffusion in the metal crystallites and desorption above the vacuum level over a small or vanishing desorption barrier are important processes in the model.

Noise in hydrogenated amorphous silicon resistors

Measurements are reported on the noise in hydrogenated amorphous silicon resistors. At zero bias the device can be represented by a resistance R and a leaky capacitor C in parallel. With bias there is excess noise varying as the square of the voltage, as expected for resistance fluctuation noise, but saturating at high voltage. The excess noise is of the form const/ (1 + ω 2 τ 2 1) and can approximately be represented as white noise flowing into the RC parallel circuit. Any 1/ f noise, if present, is completely masked by the latter noise source. The device characteristic shows space charge limited behavior, but the noise at high voltage cannot be fully described by the theory of Zijlstra and Driedonks for such devices.

Classified abstracts

The surface ionization of Cs on foil samples of Ir and some other materials was investigated at high temperatures and large atomic Cs beam flux densities. Rapid diffusion through the foil was shown to exist, at current densities up to approximately 5 mA cm-2. The size of the ion current from both sides of the sample was diffusion limited and reached a temperature and flux independent value, for both pulsed (1–10 μs) and DC current measurements. There was no indication of a space charge limited behaviour under the conditions of the experiments. The behaviour observed is not compatible with the equilibrium Saha-Langmuir description of surface ionization. Most of the results, however, agree with our recently published model for coupled surface ionization and diffusion. Diffusion in the metal crystallites and desorption above the vacuum level over a small or vanishing desorption barrier are important processes in the model.

Deviations from space-charge limited behaviour in CdS crystal diodes

Current-voltage (I-V) characteristics of CdS crystals with indium and gold cathodes have been studied. It has been found that ion bombardment of the crystal before evaporation of the cathode greatly affects the form of the I-V curve. At low fields in the crystals investigated, autocompensation of deep traps by donor impurities appears to reduce the effective trap density to a very low value. At fields in excess of 30 kV/m space charge limited behaviour appears to be succeeded by a double-carrier injection mechanism.