Unraveling exciton dynamics in amorphous silicon dioxide: Interpretation of the optical features from 8 to 11 eV.

Abstract

We introduce a model comprehensively describing the optical features of amorphous silicon dioxide (a-SiO2) in the spectral range from 8 up to 11 eV. Our model is grounded on the critical analysis of the temperature dependence of Kramers-Kronig-derived absorption spectra in the range from 8 up to 17.5 eV, together with the features of the Urbach absorption tail and of self-trapped exciton emission. In a paper we recently published [Phys. Rev. Lett. 105, 116401 (2010)] we showed the 10.4-eV resonance in the absorption spectra to feature a close Lorentzian line shape, thus implying a delocalized nature for excitons in a-SiO2. Here we provide estimations of the main parameters ruling exciton dynamics in SiO2, such as the energy of the mean lattice vibrational mode coupled to excitons (¯hω0 = 0.083 eV), the half width of the excitonic energy band (B = 2 eV), the root-mean-square amplitude of site-to-site energy fluctuations of exciton energy (D = 0.7 eV), and the exciton-phonon coupling constant (g = 2.1). The quantum yield of excitonic emission (η = 10−3) at T = 10 K in a-SiO2 is determined as well. Our model suggests that a-SiO2 features an indirect gap near 9 eV and a direct one near 11 eV, and allows a coherent description of the properties of the intrinsic Urbach absorption tail. The latter results are satisfactorily explained as arising from the momentary self-trapping of the 10.4-eV exciton. As far as near-edge absorption properties are concerned, our model places SiO2 in the wider context of wide-band-gap solids, such as LiF or NaF, where excitons are weakly scattered, but strongly coupled to phonons. On the whole, the present study shows that exciton dynamics accounts for all optical properties of a-SiO2 from 8 up to 11 eV.