Diffusion and Reactions of Photoinduced Interstitial Oxygen Atoms in

Feb 4, 2014 - However, at the initial stage of the low-temperature F2 laser photolysis, an efficient formation of interstitial ozone molecules (quantu...
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Diffusion and Reactions of Photoinduced Interstitial Oxygen Atoms in Amorphous SiO2 Impregnated with 18O‑Labeled Interstitial Oxygen Molecules Koichi Kajihara,*,† Linards Skuja,‡ and Hideo Hosono§ †

Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 minami-Osawa, Hachioji 192-0397, Japan ‡ Institute of Solid State Physics, University of Latvia, Kengaraga iela 8, LV1063 Riga, Latvia § Materials and Structures Laboratory & Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ABSTRACT: Both diffusion and reactions of interstitial oxygen atoms (O0) in amorphous SiO2 (a-SiO2) were examined using oxygen-excess a-SiO2 containing 18Olabeled interstitial oxygen molecules (O2) and exposed to F2 laser light (hν = 7.9 eV). Both the F2 laser photolysis of interstitial O2 at 77 K and subsequent heat treatment at ≳200 °C give rise to oxygen exchange between residual interstitial O2 and oxygen atoms in the a-SiO2 network, and these temperatures are far lower than the temperature at which conventional thermal network-interstitial oxygen exchange in unirradiated a-SiO2 occurs (≳700 °C). However, at the initial stage of the low-temperature F2 laser photolysis, an efficient formation of interstitial ozone molecules (quantum yield ≳0.06) via nearly exchange-free diffusion of photogenerated interstitial O0 is observed, and this reaction predominates over the network-interstitial oxygen exchange.

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INTRODUCTION Oxygen atoms (O0) are among the most important reactive oxygen species, and amorphous SiO2 (a-SiO2) is a simple oxide useful for studying interactions between O0 and solid oxides. Interstitial O0 is considered to play a crucial role in various reactions in a-SiO2, although its direct detection is difficult because of the absence of distinct spectroscopic signatures. Oxidation of silicon with O0 is faster than that with oxygen molecules (O2) and enables low-temperature synthesis of dielectric a-SiO2 films.1 In contrast to oxidation with O2, oxidation with O0 is accompanied by a significant rearrangement of the Si−O−Si network.2 Vacuum-ultraviolet (VUV) light (hν ≳ 6.2 eV) efficiently decomposes interstitial O2 in aSiO2 and generates interstitial O0.3,4 Theoretical studies have shown that an interstitial O0 in a-SiO2 is readily incorporated into the Si−O−Si network to form a peroxy linkage (POL,  Si−O−O−Si).5−11 The presence of mobile O0 in a-SiO2 is indicated by the creation of interstitial O2 through the radiolytic decomposition of regular Si−O−Si bonds (Frenkel process),5−7,12−17 the formation of interstitial ozone molecules (O3),18 and the interconversion between oxygen dangling bonds (nonbridging oxygen hole center, NBOHC, SiO•) and peroxy radicals (POR, SiOO•).19 Interactions between O0 and the surfaces of a-SiO2 have attracted attention because of their relevance for plasma processing20 and catalysis.21 However, isotope labeling, which is a powerful tool for investigating diffusion and reactions of chemical species, has not been used to study interstitial O0 in a-SiO2. © 2014 American Chemical Society

The purpose of the present study is to provide insight into the properties and reactivities of interstitial O0 in a-SiO2, by combining three techniques: the efficient formation of interstitial O0 via F2 laser (hν = 7.9 eV) photolysis of interstitial O2,3,4 a photoluminescence (PL) technique to measure the 18O fraction of interstitial O2,22,23 and the synthesis of oxygenexcess a-SiO2 containing 18O-labeled interstitial O2.24 It was found that photoexcitation of interstitial O2 causes oxygen exchange with the Si−O−Si network at temperatures far lower than temperatures necessary for conventional thermal networkinterstitial oxygen exchange. Two different diffusion modes of interstitial O0, namely, thermal diffusion of O0 as POL at ≳200 °C and migration of photoexcited O0 without exchange with the Si−O−Si network at 77K, are confirmed. These findings provide insight into the role of O0 in various reactions in and on a-SiO2.

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EXPERIMENTAL PROCEDURES High-purity fluorine-doped a-SiO2 specimens (10 × 6.5 × 0.5 mm3, containing ∼1.4 × 1019 cm−3 of SiF groups and ∼1−2 × 1018 cm−3 of SiOH groups), which have high radiation hardness to F2 laser light, were selected as the host glass. They were sealed in an SiOH-free (SiOH ≲ 1017 cm−3) silica tube filled with 16O2 gas or 18O2 gas (18O isotopic purity of ∼99%). The Received: December 24, 2013 Revised: February 3, 2014 Published: February 4, 2014 4282

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induced absorption at ≳5 eV was stronger than the absorption for the NBOHC component (Figure 1, top panel). This component was attributable to both POR27 and interstitial HO2;28 their presence was confirmed by EPR measurements as described later, but the shapes of the absorption bands have not been determined in a-SiO2 to date. Figure 2 summarizes the variation in the concentrations of interstitial O3 and NBOHC with F2 laser fluence, evaluated

sealed silica tubes were heated for an extended period (1500 h) at a moderate temperature (500 °C) to incorporate interstitial O2 into a-SiO2 while minimizing the lattice-interstitial oxygen exchange.24 These specimens were exposed to F2 laser (LPF210, Lambda Physik; ∼20 ns, ∼10 mJ cm−2 pulse−1) at 2 Hz. To increase the efficiency of the interstitial O2 photolysis, the irradiation was performed at 77 K in a liquid-nitrogen-cooled cryostat.3 The irradiated samples were then removed from the cryostat and subjected to multiple rounds of isochronal pulse annealing for 10 min at temperatures between 100 and 800 °C in increments of 100 °C. Optical absorption spectra were measured by a conventional spectrometer (U-4100, Hitachi) at room temperature. Electron paramagnetic resonance (EPR) spectra were recorded by an X-band EPR spectrometer (Model EMX, Bruker) at 77 K. Both the concentration and the 18O fraction of interstitial O2 were evaluated from the infrared (IR) PL recorded by a Fourier-transform IR Raman spectrometer (Nicolet, Model 960) at room temperature under excitation at 765 nm.22,23

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RESULTS The initial concentration of interstitial O2 was ∼4 × 1017 cm−3. Under the loading condition, the 18O fraction of interstitial O2, f *, was ∼0.9.24 The slight decrease in f * from the 18O isotopic purity of 18O2 gas is due to the oxygen exchange with the Si−16O−Si network during the thermal O2 loading, and SiOH and SiF groups in a-SiO2 do not promote this process.25 Figure 1 shows induced optical absorption spectra of 18O2-loaded

Figure 2. Variations in the concentrations of interstitial O2, interstitial O3, and NBOHC with F2 laser fluence in 18O2-loaded samples irradiated at 77 K. Simultaneous change in the fraction of 18O in the residual interstitial O2 is also shown.

from the peak decomposition of the spectra shown in Figure 1 at ≤5 eV. Figure 2 also shows the concentration of interstitial O2 and f *, separately determined from IR PL measurements. The concentration of interstitial O2 decreased monotonically with F2 laser fluence. The concentration of interstitial O3 attained a maximum at ∼0.3 J cm−2, where conversion to interstitial O3 accounts for ∼80% of the concentration decrease in interstitial O2, and the overall quantum yield for this process was as high as ∼0.06. The initial quantum yield at small fluences should be higher because the photolysis of interstitial O3, which became significant at ≳0.3 J cm−2, was not considered. The concentration of NBOHC increased with an increase in fluence because it is an oxygen-excess species that is stable against F2 laser photolysis. The photolysis of SiO−H bonds also formed NBOHCs.29,30 The 18O fraction of interstitial O2, f *, decreased monotonically with F2 laser fluence, F. The decrease in f * was due to the supply of 16O from the Si−O−Si network22,25

Figure 1. Induced optical absorption spectra of 18O2-loaded samples exposed to F2 laser light at 77 K and measured at room temperature both before annealing and after the isochronal annealing step at 200 °C. Absorption spectra of NBOHC26 and interstitial O318 are also shown.

18 18

O O + ≡Si−16O−Si≡ ⇄ 16O18O + ≡Si−18O−Si≡ (1)

16 18

O O + ≡Si−16O−Si≡ ⇄ 16O16O + ≡Si−18O−Si≡

samples after irradiation and after the first two annealing steps at 100 and 200 °C. The standard spectra of interstitial O318 and NBOHC26 are also shown. These absorption bands can be distinguished by their different half-widths (∼0.86 eV for O3 and ∼1.1 eV for NBOHC), although their peak positions are nearly identical at 4.8 eV. At the smallest F2 laser fluence (∼0.3 J cm−2), interstitial O3 dominated the optical absorption. This absorption band nearly disappeared after the specimen underwent annealing at 200 °C; its amplitudes after annealing at 100 and 200 °C were ∼0.95 and ∼0.08, respectively, of the amplitude before annealing, and this observation indicates that the decomposition of interstitial O3 mainly occurs between 100 and 200 °C. The optical absorption component of NBOHC increased with an increase in F2 laser fluence. However, the

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This observation confirms the photoassisted oxygen exchange between interstitial O2 and the Si−O−Si network. The reversible second-order exchange rate constant, kF, defined for the forward (eq 1) and backward (eq 2) reactions,22,25 may be expressed by31 df * k = F (N * − f *NT) dF 2 T

(3) 18

where N and N* are the concentrations of the total and Olabeled oxygen atoms, respectively, in the Si−O−Si network. Under the approximation that N* ≪ NT (N*/NT ≲ 10−4 25), eq 3 can be solved as 4283

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=− F=0

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f 0* kFNT 2

(5)

Comparison between eq 5 and the initial decay of f * shown in Figure 2 yielded a kF value of ∼4 × 10−24 cm5 J−1. The exchange rate per second (kFF at F = 20 mJ cm−2) was ∼8 × 10−26 cm3 s−1, which is of approximately equal magnitude as the rate of thermal oxygen exchange at 700 °C in this type of glass (∼1 × 10−25 cm3 s−1 25).

Figure 3. PL spectra of interstitial O2 measured for the 18O2-loaded sample exposed to F2 laser light at ∼0.3 J cm−2. The spectrum at ≤7500 cm−1 was plotted by normalizing the intensity to the peak height of the pure electronic band (PEB). The decrease in the PEB intensity after annealing at ≥400 °C is due to the decrease in f * and the consequent decrease in the PL quantum yield.23

Figure 4. Variations in the concentration of interstitial O2 (a) and in their 18O fraction (b) measured for 18O2-loaded samples exposed to F2 laser light at 77 K, which was followed by isochronal annealing for 10 min at each temperature. In each panel, the data before F2 laser irradiation are represented by data points at the top left. The 18O fraction in the unirradiated sample is shown in panel b for comparison. The error bars represent the experimental uncertainties.

Figure 3 shows PL spectra of interstitial O2 measured for the O2-loaded sample irradiated at F ≃ 0.3 J cm−2. The pure electronic band (PEB) attributed to the a1Δg(v = 0)−X3Σg−(v = 0) transition of O2 was observed at ∼7855 cm−1. The variation in f * was determined by the peak decomposition of the vibrational sideband (VSB) attributed to the a1Δg(v = 0)− X3Σg−(v = 1) transition of O2 located at ∼6200−6500 cm−1. The shape of VSB was nearly unchanged after the F2 laser irradiation and subsequent thermal annealing at ≤200 °C, whereas the components of 16O18O and 16O2 were clearly seen after the annealing at ≥400 °C. The concentration of interstitial O2 was evaluated from the intensity of PEB and f *, while correcting for the decrease in the PL quantum yield in the order of 18O2, 16O18O, and 16O2.23 Figure 4a shows variations in the concentration of interstitial O2 with F2 laser irradiation and subsequent isochronal annealing. The temperature for the thermal restoration of interstitial O2 increased with an increase in F, whereas this process completed at ∼500 °C in all samples. This observation is consistent with results reported previously.3,4 Figure 4b shows the variation in f * for these samples. At F ≃ 9 J cm−2, the decrease in f * was the largest during the irradiation. At smaller F values, in contrast, f * changed most significantly during the thermal annealing between 200 and 500 °C, and the temperature range was lower by ∼400 °C than that of conventional thermal oxygen exchange in an unirradiated sample.24 EPR measurements were performed for a 16O2-loaded sample irradiated at F ≃ 1.5 J cm−2, and the results are

shown in Figure 5. The signals for NBOHC,32 interstitial HO2,33 and peroxy radicals32,34 were observed, and the total

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Figure 5. EPR spectra (measured at 77 K; microwave power, 2 mW; modulation amplitude, 0.2 mT) of a 16O2-loaded sample after exposure to F2 laser light of ∼1.5 J cm−2 at 77 K and subsequent isochronal annealing at 200 °C. The inset shows variations in concentrations of interstitial HO2, NBOHC, and POR with isochronal annealing for 10 min at each temperature.

concentration was an order of magnitude smaller than the initial concentration of interstitial O2. The signals for silicon dangling bonds (E′ centers, Si•) were undetectable (≲1014cm−3). After annealing at 200 °C, NBOHC and HO2 were largely decomposed, whereas the POR concentration 4284

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increased. The decomposition of POR was observed at ≳300 °C.

The temperature range for this process is relatively wide (∼200−500 °C), and the restoration of interstitial O2 slows with an increase in temperature. This reduced rate results from decreases in the concentration of interstitial O0 and in the probability of dimerization. Thus, the diffusion length of interstitial O0 increases with an increase in temperature. It is noteworthy that interstitial O2 molecules, which survived the initial F2 laser photolysis in the samples irradiated at F ≲ 1.5 J cm−2, undergo the oxygen exchange at ∼200−500 °C. These temperatures are lower by ∼400 °C than those observed in the unirradiated sample (Figure 4b). This phenomenon is explained from the viewpoint that the most probable form of interstitial O0 is POL and its creation and subsequent diffusion signifies the network-interstitial oxygen exchange as

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DISCUSSION Figure 1 shows efficient formation of interstitial O3 in the sample exposed to F2 laser light at F ≃ 0.3 J cm−2. This process involves the cleavage of interstitial O2 into interstitial O0 and its subsequent addition to another interstitial O2. For the samples used in this study, the average separation between interstitial O2 was ∼14 nm, which indicates the high mobility of photogenerated interstitial O0 even at 77 K. It is noteworthy that the thermal decomposition of interstitial O3 at 200 °C maintains f * at a nearly constant value (Figure 4b), which shows that the 18 O fraction of interstitial O3 is as high as the initial f * value.35 This observation suggests that the oxygen exchange with the Si−O−Si network is inconsequential for the formation of interstitial O3 at 77 K. In other words, interstitial O0 reacts readily with interstitial O2 before it can be alternatively incorporated into the Si−O−Si network. Nevertheless, interstitial O0 that failed to meet interstitial O2 may eventually be incorporated into the Si−O−Si network as

≡Si−16O−16O−Si ≡ + 18O18O ⇄ (16O18O2 ) ⇄ ≡Si−16O−18O−Si≡ + 16O18O ≡Si−16O−16O−Si ≡ + 16O18O ⇄ (16O218O) ⇄ ≡Si−16O−18O−Si≡ + 16O18O

→ 2(O0 )* + O2 + ≡Si−O−Si≡ (6)

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0

Such high mobility of photogenerated interstitial O may play a key role in the rapid low-temperature oxidation of silicon under exposure to VUV light.1 Figure 4a shows that F2 laser irradiation at 77 K efficiently annihilates interstitial O2. However, the concentration of photogenerated species is smaller than that expected from the decrease in the interstitial O2 concentration. For example, the total concentration of oxygen necessary to form the observed quantities of O3, NBOHC, HO2, and POR (∼1.5 × 1017cm−3) was ∼65% of the reduction in O2 in the sample irradiated at F ≃ 1.5 J cm−2. The actual value should be smaller because part of NBOHC is formed by a side reaction: F2 laser photolysis of SiO−H bonds.29,30 The missing excess oxygen is probably incorporated into the a-SiO2 network as POL. This idea is consistent with the observation that f * decreases with F during F2 laser irradiation (Figure 2) because POL mediates the transfer of 16O from the Si−O−Si network to interstitial oxygen species. The thermal decomposition of interstitial O3 mainly occurs between 100 and 200 °C. This process dominates the thermal recovery of interstitial O2 at ≲200 °C in samples irradiated at F ≲ 1.5 J cm−2. However, the concentration of restored interstitial O2 is comparable to that of decomposed interstitial O3, suggesting that interstitial O0 released by this process is mainly used in reactions other than the dimerization and is likely consumed in the formation of POL O3 + ≡Si−O−Si≡ → O2 + ≡Si−O−O−Si≡

CONCLUSIONS Diffusion and reactions of interstitial oxygen atoms (O0) in amorphous SiO2 (a-SiO2) generated by F2 laser photolysis of interstitial oxygen molecules (O2) were studied using oxygenexcess a-SiO2 impregnated with 18O-labeled interstitial O2 and the 16O−18O isotope shift of the infrared photoluminescence of interstitial O2. A large part of photogenerated interstitial O0 turns to peroxy linkages (POL), which mediate the oxygen exchange between residual interstitial O2 and the Si−O−Si network during the F2 laser irradiation. The thermal diffusion of POL observed at ≳200 °C is accompanied by oxygen exchange with residual interstitial O2, and the temperature range is much lower than the temperature at which the conventional thermal oxygen exchange between interstitial O2 and the Si−O−Si network takes place (≳700 °C). When F2 laser fluence is small, interstitial ozone molecules (O3) are formed with a quantum yield as large as ≳0.06. The formation of interstitial O3 and its subsequent thermal decomposition are associated with the concentration change of interstitial O2, whereas the 18O fraction of interstitial O2 remains nearly constant. These observations indicate that interstitial O3 is formed via nearly exchange-free diffusion of photogenerated interstitial O0, which is different from the conventional thermal diffusion of interstitial O0 as POL at ≳200 °C.

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AUTHOR INFORMATION

Corresponding Author

Some small amount of interstitial O0 is used to convert NBOHC into POR,19 as this process also occurs between 100 and 200 °C (Figure 5). The restoration of interstitial O2 at ≳200 °C is attributed to the thermal diffusion of interstitial O0 as POL and its subsequent dimerization3,4 2≡Si−O−O−Si≡ → O2 + 2≡Si−O−Si≡

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At the late stage of the interstitial O2 recovery, the probability of collisions between interstitial O0 and O2 increases because both the diffusion length of interstitial O 0 and the concentration of interstitial O2 increase. The resultant temporal formation of interstitial O3 catalytically decreases f * of interstitial O2.

(O2 )* + O2 + ≡Si−O−Si≡ → O3 + ≡Si−O−O−Si≡

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*(K.K.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the Kurata Memorial Hitachi Science and Technology Foundation, the Collaborative

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(21) Radzig, V. A. Defects on Activated Silica Surface. In Defects in SiO2 and Related Dielectrics: Science and Technology; Pacchioni, G., Skuja, L., Griscom, D. L., Eds.; NATO Science Series II: Mathematics, Physics and and Chemistry; Kluwer Academic Publishers: Dordrecht, Netherlands, 2000; pp 339−370. (22) Kajihara, K.; Miura, T.; Kamioka, H.; Hirano, M.; Skuja, L.; Hosono, H. Oxygen Exchange at the Internal Surface of Amorphous SiO2 Studied by Photoluminescence of Isotopically Labeled Oxygen Molecules. Phys. Rev. Lett. 2009, 102, 175502. (23) Kajihara, K.; Miura, T.; Kamioka, H.; Hirano, M.; Skuja, L.; Hosono, H. Isotope Effect on the Infrared Photoluminescence Decay of Interstitial Oxygen Molecules in Amorphous SiO2. Appl. Phys. Express 2009, 2, 056502. (24) Kajihara, K.; Hirano, M.; Skuja, L.; Hosono, H. Oxygen-Excess Amorphous SiO2 with 18O-Labeled Interstitial Oxygen Molecules. J. Non-Cryst. Solids 2011, 357, 1842−1845. (25) Kajihara, K.; Miura, T.; Kamioka, H.; Hirano, M.; Skuja, L.; Hosono, H. Exchange between Interstitial Oxygen Molecules and Network Oxygen Atoms in Amorphous SiO2 Studied by 18O Isotope Labeling and Infrared Photoluminescence Spectroscopy. Phys. Rev. B 2011, 83, 064202. (26) Skuja, L.; Kajihara, K.; Hirano, M.; Hosono, H. Visible to Vacuum-UV Range Optical Absorption of Oxygen Dangling Bonds in Amorphous SiO2. Phys. Rev. B 2011, 84, 205206. (27) Skuja, L.; Hirano, M.; Hosono, H.; Kajihara, K.; Silin, A. UVInduced Effects in Glassy Silica: Transformation of Peroxy Radical to Oxygen Dangling Bonds. Glass Sci. Technol. 2002, 75C, 24−29. (28) Lightfoot, P. D.; Cox, R. A.; Crowley, J. N.; Destriau, M.; Hayman, G. D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. Organic Peroxy Radicals: Kinetics, Spectroscopy and Tropospheric Chemistry. Atmos. Environ. 1992, 26A, 1805−1961. (29) Kajihara, K.; Skuja, L.; Hirano, M.; Hosono, H. Diffusion and Reactions of Hydrogen in F2-Laser-Irradiated SiO2 Glass. Phys. Rev. Lett. 2002, 89, 135507. (30) Kajihara, K.; Skuja, L.; Hirano, M.; Hosono, H. In Situ Observation of the Formation, Diffusion and Reactions of Hydrogenous Species in F2-Laser-Irradiated SiO2 Glass Using a Pump-andProbe Technique. Phys. Rev. B 2006, 74, 094202. (31) Equation 13 in ref 25 becomes d(2C** + C*)/dt = k[CTN* − (2C** + C*)NT/2] by eliminating the diffusion term and the distribution in k. Equation 3 is obtained by inserting the relation f * = (C* + 2C**)/(2CT) and replacing k with kF. (32) Stapelbroek, M.; Griscom, D. L.; Friebele, E. J.; Sigel, G. H., Jr. Oxygen-Associated Trapped-Hole Centers in High-Purity Fused Silicas. J. Non-Cryst. Solids 1979, 32, 313−326. (33) Kajihara, K.; Hirano, M.; Skuja, L.; Hosono, H. Role of Interstitial Voids in Oxides on Formation and Stabilization of Reactive Radicals: Interstitial HO2 Radicals in F2-Laser-Irradiated Amorphous SiO2. J. Am. Chem. Soc. 2006, 128, 5371−5374. (34) Friebele, E. J.; Griscom, D. L.; Stapelbroek, M.; Weeks, R. A. Fundamental Defect Centers in Glass: The Peroxy Radical in Irradiated, High-Purity, Fused Silica. Phys. Rev. Lett. 1979, 42, 1346−1349. (35) The average f * value of O2 generated by decomposition of 16 18 O O2 is much less than 1; it is 2/3 under the assumption that the isotopomers are equilibriated and that the kinetic isotope effects can be neglected.

Research Project of Materials and Structures Laboratory, Tokyo Institute of Technology, and the Advanced Photon Science Alliance Project of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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