Compositional and structural modifications of amorphous SiO 2 by low-energy ion and neutral beam irradiation

Abstract

Compositional and structural modifications induced by irradiating amorphous SiO 2 with low-energy (300–500 eV) ion and neutral beams of inert gases have been studied. The ion beams were extracted from microwave excited plasmas of inert gases (Ne, Ar and Kr), and the neutral beams were generated by subsequent charge-exchange reactions. The SiO 2 layers modified with these beams have been characterized by Auger electron spectroscopy, Rutherford backscattering spectroscopy, reflection high-energy electron diffraction and transmission electron microscopy. It is shown that neutral beam irradiation does not cause preferential sputtering of oxygen from SiO 2, whereas ion beam with the same energy causes significant preferential sputtering. For neutral beam irradiation, densification and crystallization of SiO 2 were observed: nanometer sized α-cristobalite or α-quartz crystals were formed when amorphous SiO 2 was irradiated with more than 10 17 neutrals/cm 2 and the densified SiO 2 layer extended several nanometers beneath these crystals. The reasons for these densification and crystallization phenomena have been discussed in terms of the high temperatures and pressures in local spots formed as a result of bombardment by energetic neutral atoms. For ion beam irradiations, these densification and crystallization phenomena have not been observed.