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 temperaturedependence of Kramers-Kronig-derived absorption spectra in the range from 8 up to 17.5 eV, togetherwith the features of the Urbach absorption tail and of self-trapped exciton emission. In a paper we recentlypublished [Phys. Rev. Lett. 105, 116401 (2010)] we showed the 10.4-eV resonance in the absorption spectrato feature a close Lorentzian line shape, thus implying a delocalized nature for excitons in a-SiO2. Here weprovide estimations of the main parameters ruling exciton dynamics in SiO2, such as the energy of the meanlattice 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) atT = 10 K in a-SiO2 is determined as well. Our model suggests that a-SiO2 features an indirect gap near 9eV and a direct one near 11 eV, and allows a coherent description of the properties of the intrinsic Urbachabsorption tail. The latter results are satisfactorily explained as arising from the momentary self-trapping of the10.4-eV exciton. As far as near-edge absorption properties are concerned, our model places SiO2 in the widercontext of wide-band-gap solids, such as LiF or NaF, where excitons are weakly scattered, but strongly coupledto phonons. On the whole, the present study shows that exciton dynamics accounts for all optical properties ofa-SiO2 from 8 up to 11 eV.
|Numero di pagine||8|
|Rivista||PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS|
|Stato di pubblicazione||Published - 2011|
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