Charged Dielectric Films Research Articles

On the quantification of Auger recombination in crystalline silicon

Quantification of Auger recombination in crystalline silicon is usually challenging because it requires distinguishing Auger recombination from extrinsic recombination processes, especially at the surface. In this work we exploit the known injection dependence of surface recombination at surfaces passivated by highly charged dielectric films to assess the value of the ambipolar Auger coefficient Camb in crystalline silicon from carefully calibrated photoconductance decay measurements of high-lifetime Si wafers. A value of Camb = (2.11±0.02) × 10−30 cm6 s−1 is determined for both n- and p-type Si, independent of the dopant concentration for moderately doped samples near 300 K. Furthermore, by exploiting the expected equality of the surface saturation current density J0s for identically processed n- and p-type substrates, we are able to estimate the relative contributions of ehh and eeh Auger processes to Camb under the same doping and injection regime. The resulting ratio Cn/Cp=2.81 is in close agreement with the value of 2.91 derived from the data of Dziewior and Schmid at high dopant concentrations, which we reassess in light of modern mobility models. Based on our own data and literature data at higher concentrations, we offer a revised parameterisation of the Auger recombination rate in c-Si as a function of dopant concentration and injection level which is consistent with these findings. This revised parameterisation is also shown to be consistent with the most recent record lifetimes reported by other authors. Finally, we suggest experimental considerations relevant for future efforts to further refine the Auger parameterisation.

Hybrid Method of FEM and Divergence Theorem to Analyze Ion Flow Field Including Dielectric Film’s Accumulation Charges

Due to the dielectric film with charges from corona discharge of high-voltage direct current (HVDC) overhead lines, the electric field near the greenhouses covered by the dielectric film can be distorted. The electric field at some position on the ground level can be measured under the dielectric film. However, the distribution characteristics of the electric field cannot be obtained. In this article, based on the measurement results of some defined points, an algorithm to calculate the ion flow field with the presence of a dielectric film is proposed. Poisson's equation and the current continuity equation are iteratively solved by the finite-element method (FEM) and the divergence theorem, respectively. The experiment validates the validity of the hybrid method. The research is helpful to understand the influence of charged dielectric film fully.

Advanced modeling of the effective minority carrier lifetime of passivated crystalline silicon wafers

A strong injection level dependence of the effective minority carrier lifetime (τeff) is typically measured at low injection levels for undiffused crystalline silicon (c-Si) wafers symmetrically passivated by a highly charged dielectric film. However, this phenomenon is not yet well understood. In this work, we concentrate on two of those possible physical mechanisms to reproduce measured τeff data of c-Si wafers symmetrically passivated by atomic layer deposited Al2O3. The first assumes the existence of a defective region close to the c-Si surface. The second assumes asymmetric electron and hole lifetimes in the bulk. Both explanations result in an adequate reproduction of the injection dependent τeff found for both n- and p-type c-Si wafers. However, modeling also predicts a distinctly different injection dependence of τeff for the two suggested mechanisms if the polarity of the effective surface charge is inverted. We test this prediction by experimentally inverting the polarity of the effective surface charge using corona charges. From the experiments and simulations, it is concluded that surface damage is the most likely cause of the significant reduction of τeff at low injection levels.

Enhancement of piezoelectricity via electrostatic effects on a textile platform

We have shown the enhanced piezoelectricity by electrostatic effects on a textile based platform. The electrostatic and piezoelectric effects were hybridized by integrating piezoelectric ZnO nanowires and a charged dielectric film on a wearable textile substrate. The hybrid textile nanogenerator produced an output voltage of 8 V and an output current of 2.5 μA. Using a simple AC–DC converter circuit, we operated the green organic light-emitting diode and a liquid crystal display panel using a 100 dB sonic wave.

Nanotribology-based novel characterization techniques for the dielectric charging failure mechanism in electrostatically actuated NEMS/MEMS devices using force–distance curve measurements

The work presents a comprehensive package of novel nanoscale characterization techniques to study dielectric charging in electrostatic nano- and microelectromechanical systems (NEMS and MEMS). The proposed assessment methodologies are based on the force–distance curve (FDC) measurements performed using an atomic force microscope (AFM) to measure, for the first time, the induced surface potential and adhesive force over charged dielectric films. They were employed to study plasma enhanced chemical vapor deposition (PECVD) silicon nitride films for application in electrostatic capacitive RF MEMS switches. Three different techniques were introduced including the application of FDC measurements to study charging in bare SiN x films, metal–insulator–metal (MIM) capacitors, and MEMS switches. The results from the three methods were correlated and compared with the published data from other characterization techniques, mainly charge/discharge current transient (C/DCT) and Kelvin probe force microscopy (KPFM). The unique advantages of the proposed FDC-based characterization techniques are twofold. First, they can measure the multiphysics coupling between the dielectric charging phenomenon and tribological issues at the interface between the switch bridge and the dielectric surface. Second, the FDC-based techniques can measure larger levels of induced surface potential over charged dielectric films which results from the high electric field normally used to actuate MEMS switches. Based on the proposed FDC techniques, the influence of several parameters on dielectric charging/discharging processes was investigated: the dielectric film thickness, deposition conditions, substrate, and electrical stress conditions.

Electrostatic Defect Mapping of Xerographic Photoreceptors with a Capacitive Probe

We have recently developed a non-contact technique capable of detecting microscopic variations in the surface potential of charged dielectric films such as xerographic photoreceptors. The technique is based on measuring the charge induced on a small capacitive probe held at a constant distance from a charged sample surface. Distance control is achieved by aerodynamic floating, which is an inexpensive and simple passive feedback system capable of maintaining a constant probe-sample separation despite minor variations in sample morphology. We have used the technique to detect the presence of microscopic electrostatic defects in organic photoreceptors, such as charge deficient spots (CDSs), which are a source of image degradation in xerographic copiers and printers.

Controlling probe-sample separation with aerodynamic floating

We have recently developed a noncontact technique capable of detecting microscopic variations in the surface potential of charged dielectric films. The technique is based on measuring the charge induced on a small capacitive probe held at a constant distance from a charged sample surface. Distance control is achieved by aerodynamic floating, which is an inexpensive and simple passive feedback system capable of maintaining a constant probe-sample separation despite minor variations in sample morphology. We have used the technique to detect the presence of microscopic electrostatic defects in organic photoreceptors, such as charge deficient spots, which are responsible for image degradation in xerographic copiers and printers. However, aerodynamic floating could be implemented in other applications requiring accurate control over probe-sample separation.

Theoretical Characteristics of Surface Electric Potential Sensor with Vibrating Shutter and Its Use for Measuring

A device termed the surface electric potential sensor is described. It is used for detecting the surface electric potential of a charged dielectric film and consists of a vibrating shutter, a sensing electrode, and a preamplifier. The theoretical analysis shows that the sensor has a constant current characteristic, that its sensitivity becomes greater as its vibrating frequency becomes higher, and that the preamplifier has an input resistance which makes the sensitivity maximum. Although the calibrating method for measuring the electric potential using the sensor is not the standard one, it is now in use due to its low cost and the absence of electrical danger. Hence, the method has been compared with the compensation method known as the standard and most accurate one. It is found that more accurate measurement by the calibrating method can be performed using the concept of “correcting distance”.

Electrically Conductive Polymers

Polymeric materials are typically considered as insulators, and in fact important applications do rely on their poor conductivity— e.g., electrical cable insulation and charged dielectric films (electrets, electrical analogs of magnets), the latter finding use in microphones. Research in the last decade, however, has lead to the discovery of polymeric materials with extremely high conductivity, approaching that of copper. This brief article will highlight recent work in the synthesis, processing and applications of these novel materials.Typical polymers, the oxidant (or “dopant”) used to create carriers, and room temperature conductivities are given in Table I. A key feature shared by these materials is delocalized (at least over a few repeat units) π-electron density. Such unsaturated polymers facilitate carrier generation because of the ability for resonance derealization of the resulting radical ions, which can also offer good intramolecular carrier mobility. In addition, the geometry of π-orbitals allows for good orbital overlap and encouragesintermolecular carrier transport.That the polymer chains are considerably shorter than typical sample dimensions indicates that intermolecular transport is dominant, especially in view of the disorder observed in most conducting polymer systems. However, as the number of defects (crosslinks, “twists” which inhibit conjugation) decreases, it might be anticipated that carriers can travel greater distances along a chain before a (presumably) higher activation energy, inter molecular electron transfer becomes necessary, thus affording higher conductivity. Indeed, it has been recently reported that samples of very high quality, oriented polyacetylene, when treated with iodine,exhibit conductivities approaching that of copper at room temperature.

Production and Charge Decay of Film Electrets

The object of this work has been to obtain charged dielectric films, electrets, that retain their charge over periods of years when left open circuited. Such films are receiving increasing attention because of their potential use in practical devices such as condenser microphones, electrostatic recorders, air filters, etc. Procedures have been devised to form highly charged thin films and to measure their decay rates. Charge decay comparisons are made among different materials at various temperatures, charged in different ways. A procedure is developed that enables long lifetimes of electrets at room temperature to be predicted by extrapolating short charge decay times at elevated temperatures. A number of materials are identified as suitable for very longlived electrets.