Nanoscopic aspects of electronic aging in dielectrics

This paper presents nanoscopic aspects of electronic aging in dielectrics, as the result of the interaction of excess non-thermalized carriers with the media. Hot electrons are hypothesized as the major source of non-thermal carriers and shown to be a decisive factor in electrical breakdown. The interaction of hot electrons with atomic and molecular solids forming dielectric films is investigated using high-resolution monochromatic electron beam and cryogenic techniques. In these experiments, electrons of a well-defined energy impinge on a dielectric nanometer-thick film grown at cryogenic temperatures in an ultra-high vacuum system. It is possible to observe directly the species created at specific energies by hot (i.e. low-energy) electrons within the range 0 to 20 eV; including highly reactive radicals, positive and negative ions and excited atoms and molecules, as well as the new compounds that result from non-thermal reactions. Energy losses to electronic, vibrational and phonon excitation also can be measured as a function of electron energy. The results of these experiments are reviewed. They serve to explain the creation of local defects, which then can act as inter-and intramolecular (or atomic) traps for electrons and holes in inhomogeneous media. This nanoscale information also allows one to identify the effects of the polaron and that of cumulative material damage and increases in leakage currents, observed on the macroscopic scale. More particularly, it is shown from the results of low-energy electron bombardment of slices of PE HV cables, that simpler molecular solids such as n-alkane films provide insight on the effect of 'hot' electrons in commercial insulating material. Recommendations are given for further research necessary to represent the evolution of the aging process in practical macroscopic materials of interest at the engineering level.