EXPERIMENTAL AND THEORETICAL STUDY OF CHARGING PROCESSES IN DIELECTRICS

Abstract

In this Thesis, the key parameters affecting the charging processes in a dielectric are studied theoretically and experimentally. When energetic electrons bombard the dielectric, the electrons are able to penetrate into the top few atomic layers of dielectric and can be trapped at the defect sites resulting in the formation of space charge. The amount of space charge and the related potential distribution are studied by the Scanning Electron Microscope (SEM) mirror image technique, where the mirror image is formed by the low energy electrons reflected by the trapped charges. A new method for the determination of charges trapped in a-quartz is proposed to overcome the requirement of very low voltage for a SEM and the assumption of point charge approximation in the image method. It is found that the charging ability decreases in the following order: z-cut, 30°cut, 45°cut, and 60°cut. The charging ability is also reduced after the sample is ?-irradiated. The phenomenon is interpreted by considering permittivity, defects and stresses. In order to study the effect of sample size on the charging processes, the upper surface of the dielectric is covered with a grounded metallic aperture of variable diameter. During the charging, a leakage current towards the aperture which is formed by the charge carriers released from traps is observed when the field created by the trapped charge is sufficiently high. The measurement of this leakage current shows that the time dependence of the trapping rate follows a well-established power law described by Jonscher for the dielectric polarization processes. Polarization and charge diffusion explains our further observation that the space charge field is inversely proportional to the permittivity, where the field is measured by x-ray energy dispersive spectrometry. The activation energy of charge hopping is also estimated to be 0.3 eV from the temperature dependent measurement of leakage current indicating that the charge trapping/detrapping are mainly due to shallow traps. Most importantly, two power relationships are obtained from the investigation of the effect of sample size on transient time, and on space charge field, where transient time is time needed to establish the steady state. The exponent of the second power relationship is termed the size factor, which is affected by permittivity, defect concentration and surface condition. In the theoretical study, a macroscopic approach using Maxwell's and the continuity equations and a microscopic approach using Monte Carlo simulation of hot electron transport are employed to derive the trapped charge distribution and the parameters affecting the distribution. Both approaches show that the trapped charge distribution decreases exponentially from the charging electrode. The fields and size factors obtained compare well with the experimental values. In conclusion, the experimental and theoretical results which show that the space charge distribution and field are dependent on sample size and temperature may have important applications. The charging ability of a dielectric is dependent on different crystalline orientation and r-irradiation. It is therefore important that material characterization of dielectrics should include size factor and its dependence on temperature and crystalline orientation.