SPECTRAL NATURE OF LUMINOSITY ASSOCIATED WITH THE SURFACE FLASHOVER PROCESS

Investigations of the luminous events associated with the break-down a long the surfaces of large band gap insulators such as polycrystalline alumina and monocrystalline quartz in vacuum were made by employing a phototube and a very sensitive photomultiplier tube (PMT). The onset of luminous signals, in general, was found to start much before the on set of voltage collapse during breakdown. The early onset of luminosit y may be due to the presence of various defect or trapping centers wit hin the forbidden gap of the insulator. The spectral nature of the lig ht emitted during breakdown was resolved (in terms of the wavelength) by interfacing a monochromator with the PMT. In the case of the monocr ystalline quartz specimen, the breakdown luminosity was observed to be associated with the defects located at 2.76 and 1.91 eV below the con duction band edge. The breakdown luminosity of polycrystalline alumina was found to be associated with the defects corresponding to energy l evels at approximately 1.91, 2.25, 2.45 and 2.76 eV. The formation of these defects is discussed in terms of the nonstoichiometric nature of the lattice structure at the surface. Besides, the samples were found to emit light when no breakdown occurs. This no-breakdown light emiss ion was found to be attributed to the deep level defects. The observed results suggest that the surface flashover process is primarily contr olled by the defect or trapping centers located within the forbidden g ap of the insulators.