Radiative decay of vacuum-ultraviolet excitation of silica synthesized by molecular precursors of Si- Si sites: An indicator of intracenter relaxation of neutral oxygen vacancies A. Paleari,* N. Chiodini, D. Di Martino, and F. Meinardi Istituto Nazionale Fisica della Materia, Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 53, I-20125 Milano, Italy sReceived 21 May 2004; revised manuscript received 27 September 2004; published 1 February 2005d A well defined type of oxygen deficiency has been induced in the silica network by introducing Si- Si groups through a molecular doping based on the sol-gel synthesis. In this material, we have investigated the visible and UV photoluminescence sPLd, up to 7 eV, arising from localized states excited in the energy range 3 – 12 eV by synchrotron radiation. The PL excitation spectrum is dominated by one main sub-band-gap excitation band, peaked at 7.1 with a full width of half-maximum of 0.8 eV. No PL arises exciting at 5 eV, specifically no PL band at 4.4 and 2.7 eV are observed, ruling out the formation of twofold coordinated -Si- sites. These data confirm all previous theoretical and experimental assignments of transitions at about 7 – 8 eV to localized excitations of neutral-oxygen-vacancy sNOVd sites. Exciting in the vacuum-UV sVUVd above 7 eV, the investigated material does not show the typical 4.4 and 2.7 eV PL observed in fused silica. The peculiar NOV configuration appears to inhibit the photoconversion process responsible in fused silica for the VUV excitation of the twofold coordinated silicon emission. The main emission is peaked at about 3.7 eV, with a PL lifetime of about 1 ms. A minor component with a much slower lifetime s800 msd has been detected at 2.9 eV, with a further minor excitation channel at about 6 eV. All emissions excited at 7.1 eV show an intensity decrease with the temperature, but a negligible thermal change of the lifetime. The results give an indication of the possible energy level structure of NOV and evidence an efficient nonradiative decay mecha- nism of the excited state, caused by a strong electron-phonon interaction during the VUV excitation of the defect. The analysis of the data suggests a large Si- Si bond relaxation of about 0.1 nm, giving definite experimental confirmation of previous theoretical calculations. DOI: 10.1103/PhysRevB.71.075101 PACS numberssd: 78.55.Qr, 81.20.Fw I. INTRODUCTION Silicon-silicon covalent bonds in the 4:2 coordination structure of silicon dioxide are among the key defect con- figurations responsible for defect-related processes in SiO 2 , many of them with a demonstrated technological relevance. Point defects play an important role in several optical appli- cations of silica, such as in photorefractive writing of optical waveguides and in vacuum-ultraviolet sVUVd photo- lithography. 1–3 Nevertheless, the attribution of the optical features arising from point defects are still debated. 4,5 Quite surprisingly, a major uncertainty concerns the identification of transitions pertaining to the neutral oxygen vacancy sNOVd in the form of wSi-Siw sites, which is expected to be the basic and more abundant intrinsic defect in the silica network. Optical absorption at 7.6 eV in oxygen deficient or neutron irradiated silica is almost consensually 5 attributed to excitation of this site from its ground-state S 0 singlet to an excited S 1 state. This attribution is based on experimental and theoretical evidence. 4 Alternative assignments were pro- posed earlier 5 and more recently. 6,7 Further complications come from controversial assignments of the emission bands observed by excitation at 7.6 eV: a fast 4.4 eV sfew ns decay timed and a slow sabout 10 ms of decay timed 2.7 eV photo- luminescence sPLd bands. These emissions are excited within PL excitation sPLEd bands at 5 and 6.9 eV, and also in agree- ment with a four-level intracenter scheme comprising three singlet states S 0 , S 1 , and S 2 , and a triplet T 1 level. The S 0 → S 1 and S 0 → S 2 transitions are responsible for excitation bands at 5 and 6.9 eV PL, respectively, while PL at 4.4 and 2.7 eV are assigned to S 1 → S 0 and T 1 → S 0 transitions, re- spectively. In this scheme, the 7.6 eV band was initially in- terpreted as caused by transitions to a further excited state of the same defect, with a wSi-Siw configuration. 8 However, successive studies, with detailed consideration of the kinetics of the PL decay processes excited at 5, 6.9, and 7.6 eV, resulted in a new scenario with two kinds of oxygen- deficient centers sODCsd, named ODCsId and ODCsIId, ab- sorbing at 7.6 and 5 eV, respectively. 9 A few studies inves- tigated the possible attribution of ODCsId and ODCsIId to relaxed wSi-Siw and unrelaxed wSi ¯ Siw sites, respectively, 10 also by means of theoretical calculations. 11–14 More recently, during the last decade, Skuja 15 proposed a completely different ODCsIId model, now widely accepted, where the phenomenological scheme of the 5 and 6.9 eV excited PL, with all implications on lifetimes, temperature dependence of relative intensities, and cation-substitution effects on transition rates, is well accounted for by a twofold-coordinated -Si- configuration, 4,16 whose C 2v local symmetry is also consistent with polarization-sensitive PL measurements. 17 Ab initio calculations have recently con- firmed this proposal. 18,19 However, the upgraded interpreta- tion of ODCsIId further complicated the assignment of the ODCsId optical activity. In fact the problem arose of explain- ing the coincident PL features excited at 7.6 and 5 eV. A model of defect conversion upon 7.6 eV excitation from wSi-Siw sites to -Si- configuration, with a possible nonra- diative decay towards two E8 centers si.e., unpaired sp 3 elec- PHYSICAL REVIEW B 71, 075101 s2005d 1098-0121/2005/71s7d/075101s7d/$23.00 ©2005 The American Physical Society 075101-1