Photo-induced Defect Formation and Structural change of Amorphous Silicon Dioxide

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The purpose of this investigation is to clarify the models of (1) luminescence, (2) defect formation and (3) structural change, of amorphous materials induced by electronic excitation. Computer simulation and re-examination of existing literatures have been carried out. It is concluded that all phenomena stated above are due to the relaxation of excitons : (1) the luminescence arises from the radiative recombination of self-trapped excitons, (2) defects are metastable states of excitons created at highly distorted amorphous sites and (3) the structural change is induced as a result of radiative and non-radiative recombinations of excitons. Amorphisation of silicon dioxide by dense electronic excitation along heavy ion paths is also investigated. Further details of the study follow. A.The mechanism of defect formation in amorphous materials by electronic excitationThe structure of self-trapped exitons in crystalline silicon dioxide is reinvestigated. It is concluded that self-trapped excitons formed at highly distorted part of amorphous silicon dioxides are converted to metastable states with lower potential barriers than those in crystalline silicon dioxides. B.The mechanism of structural change of amorphous materials by electronic excitationIt is pointed out that the optical absorption change accompanied with structural changes is much larger than that of defect formation and suggested that the structural change is due to alternation of the ground state potential energy surface by formation of excitons.C.The mechanism of amorphisation by dense electronic excitationThe materials in which irradiation with energetic heavy ions register tracks and those in which excitons are self-trapped have been compared. lt is pointed out that the cause of track registrarion is the dense generation of self-trapped excitons. Results have been published in a few reviews on the effects of electronic excitation and in an original paper.