Direct observation of field-injected space charge in a metal-insulator-metal structure

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Abstract
An experimental technique has been developed for measuring space charge in dielectric films, which employs a stress wave that is generated thermoelastically from the absorption of a subnanosecond laser pulse. Spatial resolution in the thickness direction approaching one micron is attainable through a deconvolution procedure, and the sensitivity to areal charge density is about 10 μC/m2. The method is sufficient to resolve the distribution of field-injected charge in films of oriented poly(ethylene terephthalate) which were electrically stressed at room temperature. Concentrations of charge are found within a few microns of either electrode but lesser amounts extend deeply into the dielectric. Enough charge is measured after stressing specimens near breakdown to have reduced the field at the negative electrode by as much as 10% during the period of stress. The gradual decay of injected charge over a period of many days is found to be consistent with a spatially uniform, ohmic conductivity which follows a simple, inverse power law of elapsed time.