Materials modification by electronic excitation

Excitonic mechanisms of defect formation and of sputtering from surfaces, induced as a consequence of exciton relaxation, are effective in a limited class of wide-gap materials, such as alkali halides, alkaline earth fluorides and fused quartz. In this paper, we point out that modification by electronic excitation can be achieved in a far wider range of materials. First, referring to STM observations of semiconductor surfaces irradiated by laser or intense-electron beams, we show that atomic emissions with electronic origin do take place even in materials in which the bandgap, energy is smaller than the energy to remove an atom from the surface. The yield of this process is linear with the beam intensity for higher incident energies, but superlinear for lower incident energies. Secondly we analyse the phenomena of laser damage and laser ablation in a variety of wide-gap materials, emphasizing the role played by defect creation in the bulk, and atomic emissions from surfaces originating from excitation of existing defects. Finally, we discuss the amorphisation of less-ionic oxides and semiconductors by electron- and ion-beam irradiation in terms of nucleation followed by radiation-induced reactions at the interface between the crystalline and amorphous regions. The similarities and differences of the interface reactions induced by electronic excitation and by hot zones in collision cascades are discussed.