Temperature dependence of reflectivity of amorphous silicon dioxide: Evidence of delocalized excitons weakly scattered by phonons

We studied the reflectivity spectra of amorphous silicon dioxide detected under vacuum UV synchrotron radiation as a function of temperature between 10 and 300 K. Kramers-Kronig dispersion analysis of reflectivity spectra allowed us to determine the absorption coefficient in the range from 8 to 17.5 eV. Spectra show four main peaks, the spectral positions of which are consistent with literature data. An appreciable dependence of the line-shape on temperature is observed for the first two peaks only. We demonstrate the exciton peak at 10.4 eV to have a very good Lorentzian band-shape at all the examined temperatures. Based on existing theoretical models, this allows to argue excitons in SiO2 to be weakly scattered by phonons, thus retaining their mobility properties notwithstanding the effects of exciton-phonon coupling and of intrinsic structural disorder of amorphous SiO2. Moreover, the observed temperature dependence of the peak position together with the features of the Urbach absorption tail and of self-trapped exciton emission allow us to estimate the main parameters ruling exciton dynamics in SiO2. The features of the intrinsic Urbach absorption tail can be satisfactorily explained as a consequence of those of the first excitonic peak, supporting the interpretation of the Urbach tail in SiO2 as a consequence of the momentary self-trapping of the 10.4 eV exciton. Finally, the characteristics of the other energy peaks are discussed and an excitonic origin also for the 11.6 eV peak is put forward. On the whole, our results show that exciton dynamics accounts for all optical properties of pure silicon dioxide from 8 up to 11 eV.