Yuryo Sakurai

Visible photoluminescence from Si clusters in γ-irradiated amorphous SiO2

Visible photoluminescence (PL) bands around 2 eV were studied in 60Co γ‐irradiated (dose<1 MGy) oxygen‐deficient‐type amorphous SiO2 (a‐SiO2) excited by 2–4 eV photons. In addition to the well‐known 1.9 eV PL band due to nonbridging oxygen hole centers, another PL band was observed at 2.2 eV when excited by 3.8 eV photons. The intensity of the 2.2 eV band increases with decreasing oxygen partial pressure during the sample preparation. Electron‐spin‐resonance measurements show that the intensity of the 2.2 eV band is correlated with the concentration of the Eδ′ center, a paramagnetic state of a cluster of silicons. After much higher γ irradiation with a dose up to 10 MGy, a new PL band was induced at 1.75 eV under excitation by 2.5 eV photons, as well as the 1.9 and 2.2 eV PL bands. By comparing its spectral shape and excitation energy with known PL band in Si‐implanted a‐SiO2, it is suggested that the 1.75 eV band is associated with Si nanocrystals formed from Si clusters in a‐SiO2 by the high‐dose γ irra...

The 3.1 eV photoluminescence band in oxygen-deficient silica glass

Abstract Previous studies have reported the existence of a photoluminescence (PL) band near 3.1 eV in silica glass when exposed to ultraviolet (UV) light. The O3SiOOH group, the twofold coordinated silicon lone-pair center ( O  S i  O ) and a germanium impurity were thought to be the origin of this PL band. In this paper, we describe the characteristics of a PL band near 3.1 eV in undoped oxygen-deficient silica glass (low-OH) that has been synthesized with chemical vapor deposition (CVD) soot remelting. At room temperature, we have found two types of PL bands at approximately 3.1 eV in the low-OH glass. One band is observed at 3.08 eV (lifetime, τ∼100 μ s); and the other, at 3.15 eV ( τ∼10 μ s).

Green photoluminescence band in γ-irradiated oxygen-surplus silica glass

Previous studies have reported the existence of a green photoluminescence (PL) band in oxygen-deficient silica glass when exposed to ultraviolet (UV) light. Oxygen deficient defects (Eδ′) were thought to be the origin of this PL band. In this article, we describe the characteristics of a green PL band in oxygen-surplus silica glass (excited by visible and UV light). At room temperature, we observed the full width at half maximum and lifetime of this PL band to be about 0.2 eV and 300 ns, which differed from the previously reported values of 0.4 eV and 20–30 ns, respectively. We propose that the peroxy radical (O3≡Si–O–O↑,↑: unpaired electron) or other oxygen surplus defects are the cause of this green PL band rather than oxygen deficient defects.

Point defects in high purity silica induced by high-dose gamma irradiation

The defects induced by high‐dose (10 MGy) gamma irradiation (60Co) are studied in various types of high‐purity silica glasses [including synthetic crystal (α‐quartz)]. While the defects induced by gamma irradiation of up to 1 MGy have been reported to be generated through the bond breaking of manufacturing‐method‐dependent point defect sites (precursors), such precursor dependency disappears or at least weakens in the defects induced by 10 MGy gamma irradiation. Electron spin resonance, optical absorption, and luminescence investigations suggest that at high‐dose irradiation the defects are created mainly by radiolysis or bond breaking, and associated oxygen diffusion occurred at silicon–oxygen bonds other than at point defect sites. Crystalline α‐quartz shows much higher radiation resistivity than amorphous silica glasses, suggesting that strained silicon–oxygen bonds are the breaking sites.

Characteristic red photoluminescence band in oxygen-deficient silica glass

We studied a red photoluminescence (PL) band at about 1.8 eV with full width at half maximum of 0.2–0.4 eV in a series of oxygen deficient-type silicas before and after γ irradiation. The decay lifetime of the PL was estimated to be about 200 ns. The PL excitation peak was found to be located at 2.1 eV. The intensity of the 1.8 eV band was enhanced by about 100 times after γ irradiation up to a dose of 10 MGy. These results suggest that the 1.8 eV PL is associated with oxygen deficient states in silica glass, which were introduced during manufacture and were enhanced further by the γ irradiation. Comparison of the PL properties was made with other luminescent Si-based materials in terms of the peak energy, lifetime, and temperature dependence.

Correlation between the green photoluminescence band and the peroxy radical in γ-irradiated silica glass

We studied the origin of a green photoluminescence (PL) in silica glass at about 2.25 eV using electron spin resonance. The 2.25 eV PL band (lifetime: ∼300 ns, full width at half maximum: ∼0.2 eV) appears strong in γ-irradiated high-oxygen content silica glass suggesting that this PL is associated with a large surplus of oxygen states in silica glass. We investigated the correlation between the 2.25 eV PL band and the peroxy radical by the isochronal annealing method. Peroxy radicals are formed by the O3≡Si–O–O• bond (•: unpaired electron) and the small “peroxy radical” (SPR) bond in silica glass. Based on the annealing characteristics of the 2.25 eV PL band and the peroxy radical, we suggest that the PL band is associated with a surplus of oxygen defects formed by the SPR rather than the O3≡Si–O–O• bond.

Photoluminescence of oxygen-deficient-type defects in a-SiO2

Abstract Oxygen-deficient-type defects in a-SiO 2 were studied by means of photoluminescence (PL) measurements. Various properties of the 4.4-eV PL such as the decay lifetime and the temperature dependence in oxygen-deficient-type a-SiO 2 can be explained in terms of an energy diagram involving two configurations of the oxygen-deficient-type defect. The 4.4-eV PL observed from the ion-implanted thermal oxides and the oxides prepared by the plasma-enhanced CVD method, has a stretched-exponential decay, suggesting a large structural distribution in the local network structures. A PL band at ∼ 1.8 eV associated with highly oxygen-deficit states is also observed in oxygen-deficient-type a-SiO 2 after high-dose γ-irradiation (dose: 10 MGy).

Excitation energy dependence of the photoluminescence band at 2.7 and 4.3 eV in silica glass at low temperature

In this paper, we report the excitation energy dependence of the 2.7 and 4.3 eV photoluminescence (PL) bands in oxygen deficient silica glass at low temperature (∼20 K). The increase or decrease of the PL intensity at low temperatures is different for different exciting light wavelengths. The PL intensity tended to decrease with low temperatures when the excitation was near the upper and lower end of the excited level. The peak energy of the excitation spectrum increases with cooling. These results indicate that the change in excitation level with cooling is associated with the low-temperature dependence of light emission. Thermal motion is suppressed, when the sample temperature is lowered, and the energy-width of the excited level decreases, i.e., the light emission probability decreases (the emission intensity decreases), when near the upper and lower end of the excitation level. These phenomena were observed in the low-temperature dependence of the 4.3 eV emission intensity.

Radial distribution of some defect-related optical absorption and PL bands in silica glasses

Abstract In order to investigate the origins of defect-related optical absorption and photoluminescence (PL) bands in silica glasses, we measured the radial distribution of paramagnetic defect centers, optical absorption band, and PL band in various silica glass rods. We studied the radial distribution of the following: the E ′ center and the 5.8 eV absorption band, non-bridging oxygen hole center and the 1.9 eV PL band and 4.8 eV absorption band, and 5.0 eV absorption band and the 2.7 and 4.3 eV PL bands. As a result of this study, we confirmed the correlation of both the radial distributions. These results suggest the following: the E ′ center is the origin of the 5.8 eV absorption band, the non-bridging oxygen hole center is the origin of the 1.9 eV PL band and 4.8 eV absorption band, and the oxygen vacancy is the origin of the 5.0 eV absorption band, the 2.7 and 4.3 eV PL bands.

Low temperature dependence of photoluminescence band near 2.0 eV in silica glass

Previous studies have reported the existence of a 1.9 eV photoluminescence (PL) band in silica glass exposed to visible and ultraviolet light. Nonbridging oxygen hole centers (O3≡Si–O↑,↑: unpaired electron) were thought to be the origin of this PL band. In this article, we describe the characteristics of a 1.9 eV PL band in silica glass (excited by visible and ultraviolet light). At low temperature (∼20 K), we observed broadening of the full width at half maximum and a peak shift for this PL band. We propose that the change of peak energy and full width at half maximum for the 1.9 eV PL band at low temperature was really caused by the existence of another peak rather than by structural changes in the local network structure.