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Abstract
The knowledge advancement in the physics of silicon dioxide has promoted ground-breaking progress, from microelectronics to fibre optics. However, the SiO2 exciton decay mechanism is still mostly unrevealed. Here, we analyse the temperature dependence of interband-excited luminescence and the reflectivity by means of synchrotron radiation on a wide selection of SiO2 materials. This enables us to decouple the band-to-band recombination steps from non-radiative decay pathways that typically mask the relaxation mechanisms. We show that band-to-band excitations decay into two competitive correlated channels leading to green and red luminescence so far ascribed to independent transitions. Here we discuss the assignment to a dual relaxation route involving either ‘free’ or ‘interacting’ non-bridging-oxygen sites. Such an interpretation suggests an explanation for the elusive non-bridging-oxygen centres in quartz. The reflectivity spectra finally demonstrates a general relationship between exciton spectral position and bandwidth in SiO2 and clarifies the role of disorder in exciton localization.
Highlights
The knowledge advancement in the physics of silicon dioxide has promoted ground-breaking progress, from microelectronics to fibre optics
We investigated a set of silica and quartz specimens rationally selected so as to represent the largest variety of SiO2-based materials currently known
Such comparative approach is strategic for two main reasons: First, it enables us to distinguish the spectral signatures of the self-trapped exciton (STE) decay from different sample-dependent photoluminescence contributions
Summary
The knowledge advancement in the physics of silicon dioxide has promoted ground-breaking progress, from microelectronics to fibre optics. Despite the fundamental role of elementary NBO point defects in bond-breaking events in SiO2, their red photoluminescence has never been observed in quartz, except for locally amorphized crystals by neutron irradiation[37,38] For this reason, early interpretations of the band-to-band excited photoluminescence spectra did not explicitly consider the NBO emission as a possible contribution to the radiative decay of STEs. Aim of the present work is to obtain a definite spectral insight into the relationship between NBO and STE, so as to face an unresolved critical point. By using synchrotron light band-to-band excited photoluminescence experiments conducted over a wide range of temperatures on a comprehensive set of silica specimens obtained through different synthetic methods and treatment conditions, as well as on pristine and neutron-irradiated quartz, we reveal that the band-to-band excited green and red luminescence in silica are not independent transitions, but instead arise from the bifurcation of the decay process of intrinsic excitations into a dual relaxation route involving either ‘free’ or ‘interacting’ NBO configurations. Our data suggest an explanation for the absence of the red luminescence in untreated quartz and a general relationship between exciton trapping and structural disorder in SiO2
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