Absorption edge in silica glass R. Boscaino, E. Vella and G. Navarra University of Palermo, Department of Physical and Astronomical Sciences Via Archirafi 36 90123 PALERMO, Italy Abstract – Measurements of optical absorption in the v-UV range in a variety of silica glass are used to determine the width of the absorption edge (Urbach energy). Measured values range from 60 meV up to 180 meV. So high a variability over silica types is tentatively ascribed to the different disorder degree, which characterizes different materials. I. INTRODUCTION Transparency of amorphous silica in the vacuum-UV spectral region (E > 6.5 eV, λ < 190 nm) is limited mainly by the intrinsic absorption edge. This can be represented by two separate regions [1]. At higher energy (Tauc region) the absorption coefficient α (E) can be approximated by E E E E g 2 ) ( ) ( - ∝ α (1) where ω h = E is the energy of incident photons and E g is the optical energy gap (≈ 8.5 eV in silica at T=300 K). The experimental investigation of this region allows the measure of the energy gap E g . At lower energies (Urbach region), the absorption coefficient α (E) is well described by the exponential law ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ - = u g o E E E E exp ) ( α α (2) where α o is a parameter characteristic of the material (α o ≈ 5 10 3 cm -1 in amorphous SiO 2 ) and E U is the Urbach energy. The Urbach region covers the range between α(E)=10 cm -1 and α(E)=10 3 cm -1 . In this region the absorption is caused by electronic transitions between extended states and near- edge localized states. The formation of a high-density of electronic localized states in the immediate neighbourhood of the band edges is a peculiarity of the disordered materials with respect to perfectly ordered systems. The Urbach energy E U in (2) (the inverse logarithmic slope of α (E)) determines the extension of the edge tail: low values of E U ensure high transmittance of the material up to E g ; high values of E U degrade its v-UV transparency and limit its use in several application fields such as optics, microelectronics, spectroscopy and F 2 -laser lithography. In crystalline quartz, the absorption tail is rather sharp, corresponding to a low value of E U (50 meV) [2]; in glassy silica values reported in literature range from 60 meV up to 180 meV, for different materials obtained by different preparation routes [2-6]. As a general statement, one expects higher values of E U in those materials featuring higher density of localized electronic states, which are created or stabilized by structural disorder. In this sense E U is often cited as an indicator of the physical disorder degree of the material. It is worth noting that experimental values of E U have been occasionally reported in literature, whereas it is lacking the systematic consideration of its relationship with other quantities related to structural disorder or with the details of the preparation route. This is especially true for materials prepared by the sol-gel technique. This paper is concerned with the Urbach region in glassy silica. We measure the optical absorption spectra of a variety of silica types, we extract the value of the Urbach energy at T = 300°K and we try to single out possible correlations with different typologies (natural or synthetic, wet or dry, sol-gel) of the material. II. MATERIALS AND EXPERIMENTAL METHODS A. The spectral range. Our investigation is limited to the spectral range from 6.2 eV to 8.27 eV (from λ=200 nm down to λ=150 nm). Absorption spectroscopy in this spectral region requires to get free from oxygen absorption, so that vacuum or N 2 - fluxed experimental set-up are required. An addition difficulty is connected with the exponential law (2) characterizing the intrinsic absorption, which imposes severe limits to the bandwidth of the incoming radiation. Due to this restrictions and to the high values of α, the experimental investigation of the Urbach region is usually carried out in films with thickness in the region of μm. However, as the production techniques of thin films are substantially different from bulk materials, it is not obvious that the parameters of the Urbach tail measured in thin films represent reliable values for bulk materials. The experiments described here were carried out in samples obtained by cutting bulk materials grown by conventional procedures. Their thickness is not less than 0.2 mm. This condition limits, for our spectrometer, the detectable range of α(E) to 300 cm -1 and determines the upper limit (∼ 8.3 eV) of the spectral range explored. This range is wide enough to evidence the exponential term of the absorption and measure the Urbach energy E U . When investigating this spectral region, one has to take into account the effectiveness of other contributions to the