The origin of the intrinsic 1.9 eV luminescence band in glassy SiO2

IOURNAL

ELSEVIER

OF

Journal of Non-Crystalline Solids i 79 (1994) 51-69

Section 2. Point defects in silica glass: luminescence and optical absorption

The origin of the intrinsic 1.9 eV luminescence band in glassy S i O 2 Linards Skuja * Institute of Solid State Physics, University of Latt'ia, 8 Kengaraga str., Riga LV 1063, Latcia

Abstract The current controversy over the nature of the centers giving rise to the 1.9 eV photoluminescence (PL) band (the R-band), the suggested defect models and the relevant experimental data are briefly reviewed. The luminescence emission, excitation and polarization spectra of neutron-irradiated synthetic silica were studied between 6 and 300 K using site-selective dye-laser and Ar ion laser excitation. Resonant zero-phonon lines (ZPL) were observed below 80 K both in luminescence emission and excitation spectra in the 1.9-2.1 eV region. A vibration line in emission spectra 890 cm-~ below the ZPL energy is attributed to the symmetric stretching vibration of the silicon-non-bridging oxygen bond in the ground electronic state of the non-bridging oxygen hole center. A similar line 860 cm-1 above the ZPL in the excitation spectra corresponds to the same vibration in the excited state. The intensities of the resonant ZPLs are dependent on the excitation energy and show a nearly Gaussian distribution with the peak at 1.935 _+0.01 eV and halfwidth 82 _+ 7 meV. In the zero approximation, this distribution describes the concentration distribution of the PL centers with the respective energies of the excited electronic state. The 4.8 eV excitation band of the 1.9 eV PL is complex, due to different electronic transitions. No ZPLs or vibrational structures are observed under excitation with KrF excimer laser or Xe lamp in this band. The optical absorption in this region is due to overlapping bands of several different defect centers. The low-temperature luminescence bands at 2.05-2.1 eV and 2.35-2.4 eV, excited by the green (2.41 eV) and blue (2.71 eV) Ar ion laser lines, have a nature different from the 1.9 eV band. Several different defects contribute to the 2.0 eV optical absorption band in irradiated glassy SiO 2.

I. Introduction

The luminescence band around 1.9 eV (650 nm) is one of the three presently known emission bands proved to originate from intrinsic defects in glassy SiO 2 (g-SiO2). Despite the number of dedicated studies, its origin remains controversial. There is a general agreement now that the 1.9 eV emission band is due to an oxygen-excess

* Corresponding author. Tel: + 371-2 260-796. Telefax: +371-2 225 039.

defect; however, the basic models for its atomic and electronic structure are still debated. Two structural models are presently prevalently discussed - either a non-bridging oxygen hole center (NBOHC) or an interstitial ozone molecule. The reported peak energies of this red emission range from 1.8 to 1.95 eV, depending on the excitation conditions and the set-up used. The most often reported value is at 1.9 eV (650 nm). Owing to the inhomogeneous broadening caused by the irregular glass structure, the peak energy is generally dependent on the excitation conditions. On the other hand, luminescence centers with

0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00246-J

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L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

peak energies close to this range but of different nature may be also present in irradiated g-SiO 2 (Section 3.3.3). To avoid ambiguity in notation, for brevity and to enlighten the discussion on the center model, we use the structure-unbiased terms 'R-center' and 'R-band' for the (red)'1.9 eV' luminescence center/band throughout this paper. Using only those experimental data which are presently universally agreed, the most accurate definition of the R center is as of a defect which gives rise to an emission band at 1.9 + 0.1 eV, an excitation band at 2.0 + 0.1 eV and has multi-exponential decay with decay constants distributed between 10 and 30 i~s at room temperature. The purposes of this paper are: (i) to review briefly the published experimental data and models. This topic has been recently reviewed in detail by Griscom [1]. In the present paper, we concentrate on auxiliary topics, not covered by that review. A particular emphasis is given to the results obtained on surface centers, which, while being relevant to the current discussion on the R-centers model, have been little accessible to the western reader; (ii) to present and discuss in the model context new spectroscopic data on the R-center, obtained using laser spectroscopy io the liquid helium temperature range and site-selective excitation techniques.

2. History 2.1. The basic optical spectroscopic data of the R-center The optical absorption band at 4.8 eV and Gaussian halfwidth around 1.0 eV, induced by electron irradiation of type-III synthetic silica was first reported by Compton and Arnold [2]. Photoexcitation in this band gives rise to a characteristic red photoluminescence (PL) emissian peak at 1.9 eV (650 nm) with halfwidth 0.17-0.20 eV at 293 K. The emission spectrum was first published by Sigel [3]. Further studies have established that this luminescence band (R-band) is induced in nearly all types of glassy SiO 2 (g-SiO2) exposed to

X-ray [4], gamma [3,5,6], neutron [4], or ultraviolet [7-9] irradiations, ion implantation [10] or to mechanical stress [11]. Streletsky et al. [12] observed the same luminescence centers on the surface of mechanically fractured g-SiO 2. It was subsequently observed that the R-band can also be excited in the weak 2.0 eV (630 nm) optical absorption band of neutron-irradiated gSiO 2 [13]. This emission was first noticed visually from drawing-induced PL centers in g-SiO 2 fibers by Kaiser [14]. The decay of the R-band deviates slightly from exponential, with decay times around 10-15 ~s at 293 K [13,15-18]. It can be estimated from these decay constants that the oscillator strength of the 2.0 eV absorption band is around 10 -4. Similar decay curves are reported for excitation in either 4.8 or 2.0 eV excitation bands [13,17,19]. By contrast with this, Tohmon et al. [18] observed a growth of the 1.9 eV PL intensity during the first 5 ~s after pulsed excitation in the 4.8 eV band. The R-center excitation spectrum in the visible is commonly assumed to coincide with the induced 2.0 eV optical absorption band. However, the exact shape of the excitation spectrum so far has not been reported. The shape of the 2.0 eV absorption band in differently irradiated samples differs noticeably [5]; the peak energies of this band in different ~/-irradiated samples range from to 1.97 to 2.07 eV [6]. Upon annealing, the peak of the band shifts and its halfwidth diminishes [20]. This may indicate that the absorption band is a complex one. The ultraviolet excitation band of the R-center is reported to have a nearly Gaussian shape (maximum at 4.8 eV, half-width 1.05 eV) and to coincide well with the induced 4.8 eV absorption band [3,4,18,19]. A similar 4.8 eV excitation band is ob~erved also for the surface R-centers [21]. However, the excitation spectrum measured for emission at 1.9 eV in ArF excimer laser-irradiated g-SiO 2 was reported to have a rectangular shape, with a 'threshold energy' around 3.9 eV [22]. Since the R-band emission exhibited a nonzero degree of polarization when excited with polarized light in either 4.8 or 2.0 eV excitation bands ( - 1 . 5 % [13] and + 10% [19] respectively),

L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

it was concluded that both excitation bands correspond to different electronic transitions within a single defect [19]. This point, however, is disputed by other investigators [7,9,18,22], who attribute both excitation bands to different defects. Stathis and Kastner [7] reported different growth curves for the vacuum ultraviolet irradiation-induced 4.8 eV absorption band and the 1.9 eV PL bands in Suprasil W silica. The increasing part in the Rband decay curve under 4.8 eV excitation [18] could indicate that energy transfer between the absorbing and emitting sites takes place. The controversy over the 4.8 eV band has been recently reviewed by Griscom [1]. The small Stokes shift between the R-center excitation and emission bands (about 0.1 eV) serves as a hint that vibrational structures could possibly emerge in the optical spectra at liquid helium temperatures. However, the initial studies of the R-band at 6 K, using 4.8 eV excitation, did not reveal any additional structures [17]. Only recently was it understood that this negative result is due to an inhomogeneous broadening. By using site-selective resonance excitation in the low-energy excitation band of the R-center, purely electronic (zero-phonon) line and a vibration 890 c m - i apart was found in the emission spectra at T < 60 K [23,24].

2.2. Non-bridging oxygen model 2.2.1. Radiochemical data, bulk centers The NBOHC model for the bulk R-centers was originally advanced mainly on the basis of radiochemical arguments [4,25]. It was observed that X-irradiation at 80 K produces 1.9 eV PL centers in numbers proportional to the initial concentration of hydroxyl groups in g-SiO 2 samples [25]. On subsequent warming, about 50% of the induced PL centers disappear between 110 and 130 K, together with the electron paramagnetic resonance (EPR) signal of interstitial hydrogen atoms. This could indicate that the R-centers are NBOHCs, produced in the reaction -Si-O-H

80KX-irrad

Si-O" R-center PL

+ H" ESR

(1)

53

and annihilated in the reve~-se reaction at temperatures above 110 K, whe~ the atomic hydrogen becomes mobile. The remaining X-ray-induced R-centers disappear aro!;nd 230 K [26], i.e., in the temperature region [27,1] where the interstitial molecular hydrogen becomes mobile. The respective tarnishing reaction is -Si-O.

+ H 2

,

-Si-O-H + H-.

(2)

The isochronal annealing curves of the NBOHC EPR signal in type-lll silicas irradiated at 77 K show the same characteristic two-step behavior due to diffusion of atomic and molecular hydrogen [1,27,28]. An explicit mathematical analysis of the annealing processes based on the reactions (1) and (2) has been made by Griscom [27].

2.2.2. Chemical data, surface centers Another independent 'chemical' argument associating the 1.9 eV PL with NBOHCs (and nearly totally overlooked in the western literature) is provided by the work of Streletsky et al. on surface centers [12]. In that study surface Rcenters, along with other surface centers, were obtained by grinding of g-SiO 2 in powder in vacuum or a He atmosphere. The location of the centers thus produced on the surface of glass particles was confirmed by their immediate disappearance upon contact with atmospheric air. In investigations of surface centers, a major advantage over the case of the bulk defects is the ability to study in detail their chemical properties via controlled reactions with active gases, which, unlike the case of reactions with bulk centers, are not diffusion-limited. The number of reacted gas molecules can be determined exactly by monitoring pressure changes and by mass spectrometry (MS); the reaction energies can be determined using microcalorimetry (MC). In parallel, the concentrations and types of paramagnetic surface centers can be monitored by EPR. A pioneering EPR work on SiO2 surface centers and their reactions with gases was made using this techniques by Radtsig et al. (see Refs. [29-35] and references therein). Some of the experimentally observed surface center reactions are listed below (Eqs. (3)-(10)); the techniques employed to monitor reaction products (EPR, MS, MC, PL) are

54

L. Skuja / Journal o f Non-Crystalline Solids 179 (1994) 5 1 - 6 9

indicated below each reaction product• The accuracy of energies are + 10% [34,35]• T=300 K

=-Si" + N20

,

1 (3)

"-centersif--i -~

E'-center EPR

~Si"

--Si-(O-N--N)" + 1.45 eV, EPR

(4) 1

"f

I-"INBOHC'sl

I->Si'(ON2?"I

I-~S;-O•-[

i\ \ /

(3)

MC

T = 470 K

=Si-(O-N-N)"

(7) ]->Si- - -

EPR

- S i - O • + N 2 + 1.61 eV, R-center PL

I

j

MS

(4)

MC

T= 293 K >

-Si-(O-N-N) • + 0 2 EPR

--Si-O-O"

+ N 2 0 + 1.14

POR EPR

eV,

(5)

MS

-=Si-O • + CO

T= 30O K

R-center PL

- S i - O - C . = O, ePR

(6)

- - - S i - O - C • = O T=470 K, - S i " EPR

+ C02,

E'EPR

=Si-O-C" = O

(7)

MS

Fig. 1. Mutual transformations between surface E'-centers, non-bridging oxygen hole centers (NBOHCs) and peroxide radicals in reactions with N20, CO, CO 2, and 02 gases. The numbered arrows correspond to the reaction equation with the same number (see text). The scheme is compiled on the basis of Refs. [12,29-34].

T = 300 K + O 2

EPR

= S i - O - O . + CO2, POR EPR

-Si-O-O"

(8)

MS

+ CO

,

POR EPR

--Si

-Si-O" R-center PL

+ C02,

(9)

MS

+ O 2 T=77-293K ~.

•

E'EPR

---Si-O-O. + 2.59 eV. POR EPR

(10)

MC

By means of reactions (3)-(10) it is possible to interconvert between the surface E'-centers, peroxide radicals (PORs) and NBOHCs. The possible transformation paths are illustrated by the scheme of Fig. 1. The reactions can be repeated ('cycled') along any closed loop in Fig. 1 practically indefinitely without loss of surface center concentrations. The microcalorimetric data [34,35] on reactions (3)-(10) allowed the energy of the siliconnon-bridging oxygen bond to be determined. The dissociation of N20 into N 2 and atomic oxygen requires energy 1.73 eV [36]. Together with the calorimetric data of reactions (3) and (4), this yields the experimental energy of the silicon-

non-bridging oxygen bond as 4.79 eV. Alternatively, this energy can also be determined independently from reactions (6) and (7). The measured net calorimetric effect of reactions (6) and (7) combined is 17 kcal/mol (0.74 eV) [34]. The dissociation reaction CO2 ~ CO + O requires 5.55 eV [34,36]• The difference is 4.81 eV, in good agreement with the value calculated from reactions (3) and (4). Additionally, this value agrees well with the energy of a regular Si-O bond. In the context of the controversy over the R-centers model (see Section 2.3. below), it should be noted that the energy required to dissociate the ozone molecule into 0 2 and O is only 1.11 eV [36]. The energy data of reactions (3)-(10) allow [34,35] the energy of the O - O bonding in peroxide radical to be evaluated as well. It is equal to the energy of the reaction (10) (2.59 eV) plus the energy of O - O bonding in the oxygen molecule (5.11 eV [36]) minus the Si-non-bridging oxygen bonding energy (4.8 eV). This yields 2.87 eV. in Radtsig's works [29-35], the paramagnetic surface non-bridging oxygen radicals (Eqs. (4) and (9)) were not directly detected by EPR; their

L. Skuja /Journal of Non-Crystalline Solids 179 (1994) 51-69 I

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1,6 1,7 1,8 1,9 2,0 2,1 2,2 PHOTON ENERGY

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Fig. 2. Comparison of the time-resolved photoluminescence spectra of the bulk R-centers in neutron-irradiated (1018 n/cm 2) Suprasil W1 silica and of surface R-centers obtained by crushing of g-SiO 2 in an inert atmosphere [21].

presence was logically inferred from the exactly detectable starting and end-products of reactions (3)-(10). The absence of a measurable EPR signal of surface NBOHCs was explained by the orbitally degenerate ground state of these centers

[30,35]. By extending the EPR techniques with photoluminescence methods, Streletsky et al. [12] observed the emission of surface R-centers with intensities proportional to the expected concentrations of surface non-bridging oxygen radicals created or destroyed in reactions (4), (6) and (9). Hence, they concluded that R-centers are due to surface NBOHCs. Subsequent comparison of the luminescence characteristics of the surface Rcenters and of the neutron irradiation-induced bulk R-centers has shown [15] that they are very similar (although not exactly identical). Roomtemperature time-resolved emission spectra of surface R-centers reveal two sub-bands at 1.93 and 1.98 eV [21] (Fig. 2). Both these sub-bands, similar to the 'bulk' R-band, have an excitation peak at 4.8 eV [21]. All of them can be excited by

55

the 2.41 eV Ar laser line [15]. The luminescence decay kinetics of bulk R-centers and both subbands of surface centers are similar, the only difference being that the decay constants of the surface center PL is about 5 p,s larger (15-20 ~s at 293 K) and the decay is less exponential. Measurements of the surface R-center optical absorption spectra using the diffuse reflectance techniques [37] revealed a weak absorption band at 2 eV with halfwidth 0.2 eV and a strong band at 4.7 + 0.2 eV with halfwidth 1.2 eV, similar to the case of bulk R-centers.

2.2. 3. Spectroscopic considerations While the chemical evidence provides strong arguments in favor of the NBOHC model for the R-center, the spectroscopic properties of the center have remained poorly understood. The NBOHCs are well characterized by EPR [1,5,38]. The main problem is the occasionally poor correlation between the intensities of the PL or optical absorption bands, attributed to the R-center and the NBOHC EPR signal. As mentioned above, the EPR signal of surface R-centers was not detected at all. For bulk R-centers, the reported EPR-optical correlations range from reasonable [6] to poor [5,39]. This discrepancy has been discussed by Griscom [1]. It can be explained by assuming that several different defects, e.g., NBOHC and the peroxide radical [1,39], contribute to the 2.0 and 4.8 eV absorption bands. The nature of the electronic transitions corresponding to the 1.9 eV PL and 2.0 eV and 4.8 eV absorption band has remained controversial for a long time. On the one hand, the small Stokes shift between the 1.9 eV PL and 2.0 eV absorption bands was taken as a clear indication that the respective transitions occur between nonbonding states [19]. On the other hand, EPR investigations of non-bridging oxygen centers in alkali silicate glasses (the 'HCl'-center) indicated [40] that the hole is located in an almost pure 2p orbital of the terminal oxygen with the other lone pair 2p orbital split apart by 1.5 eV due to the field of the neighboring alkali ion. By analogy, it was initially suggested [19] that, in the case of the R-center, the optical transitions take place between the 2p non-bonding orbitals of the terminal

56

L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

oxygen, split by about 2 eV because of some unidentified asymmetric perturbation. This 'split oxygen 2p orbitals' interpretation, however, met with two difficulties. First, the EPR work on NBOHCs in pure silica, while confirming that the hole resides in the non-bridging oxygen's pure 2p orbital, gave the value of the splitting between the two 2p orbitals as only ~ 0.3 eV with an inhomogeneous distribution halfwidth of 0.6 eV [38,41]. This was clearly inconsistent with the experimental energy ( ~ 2 eV) and the small (-- 0.2 eV) total (inhomogeneous + homogeneous) halfwidth of the R-band. Second, the degree of polarization of the R-band emission upon linearly polarized excitation in the 2.0 eV absorption band was found to be as low as pH = + 10 to 12% [17,19]. This could indicate that one of the electronic states terminating R-band transitions is twofold degenerate, contradicting the 'split 2p orbital' model (the theoretical value of P for the former case is P = + 1/7; for the case of dipole transitions between non-degenerate states, the theoretical value of P is + 1/2 [42]). An attempt was made to explain the low degree of polarization by large Jahn-Teller (E ® e) splitting of the two oxygen 2p orbitals and hopping of the terminal oxygen between the three potential minima in the configuration coordinate space [17]. Such an assumption required a significant ground state interaction between the terminal oxygen atom and one of the three oxygens from the same SiO 4 tetrahedron, both for HC1 center in alkali silicate glasses and for NBOHC in pure silica. However, in a subsequent EPR study of ~70 enriched silicate glasses, the delocalization of the hole on the two oxygens was not confirmed [43]. Additional clues, which may help to solve that controversy, were provided by the recent study using site-selective luminescence excitation techniques [23,24]. Under laser excitation in the 2.0 eV absorption band at T < 60 K, a resonance zero-phonon line and a satellite line 890 c m apart emerge in the R-band. The satellite line was attributed to the symmetric stretching mode of ~:he silicon-non-bridging oxygen vibration. The zero-phonon line is highly polarized, indicating that the electronk, transition takes place between non-degenerate states. Based on these data, it

was suggested, that the R-band is due to a charge-transfer transition between the half-filled 2p orbital of the non-bridging oxygen atom and the lone-pair 2p-like orbital of one of the three nearest-neighbor oxygen atoms. Further siteselective luminescence studies of the R-center are reported below in Section 3. 2.3. The ozone model

Awazu and Kawazoe [22] have reported that the shape of the ultraviolet excitation spectrum for the 1.9 eV PL in ArF excimer laser pre-irradiated g-SiO 2 is different from the shape of the laser-induced optical absorption band at 4.8 eV and from the previously reported [3,4,18] R-band UV excitation spectra. They observed an excitation band with a flat maximum around 4.2 eV and an abrupt step-like decrease to zero for photon energies < 3.9 eV. This energy almost exactly coincides with the threshold energy for photolysis of ozone molecule in 0 2 and singlet atomic oxygen in excited I O 2 state. Since the formation of interstitial ozone in g-SiO 2 from dissolved oxygen during irradiation with 6.4 eV photons in principle is a likely event, the laser-induced 4.8 eV absorption band was attributed [22] to interstitial ozone molecules. The excitation in this band then produces an oxygen molecule and an excited oxygen atom. Further, Awazu and Kawazoe noticed that the relaxation of the excited 1D 2 atomic oxygen to the ground 3P 2 state for free atoms occurs with emission of a 1.96 eV photon. Again, this is close to the energy of the R-band. Therefore, contrary to the NBOHC model (Section 2.2.), the R-band was attributed to the emission of singlet atomic oxygen [22]. Compared with the NBOHC model (Section 2.2.), in the terms of the ozone model it is easier to explain the experimental observations, disassociating the 4.8 eV absorption from the 1.9 eV PL: the lack of a firm correlation between the intensities of the two bands [7] and the initial increase in the 1.9 eV PL decay curves [18]. However, the reported 1.9 eV PL excitation spectrum [22] which is the cornerstone of the ozone model is very different from the previously reported spectra [3,4,18]. In an attempt tc re-

L. Skuja /Journal of Non-Crystalline Solids 179 (1994) 51-69

solve, this controversy the excitation spectra and decay kinetics of R-band are re-examined in the present work (Sections 3.2.3. and 3.3.4 below).

3. Site-selection photoluminescence studies

3.1. Experimental All of the experiments reported below (except for data of Figs. 14, 15) were performed on type-Ill Corning 7940 synthetic silica sample of dimensions 8 × 8 × 8 mm 3, irradiated with 1020 n e u t r o n s / c m 2. The sample was placed in a helium flow cryostat (LeTbold-Heraeus) equipped with a temperature regulator (6-390 K). The luminescence was registered at right angles to the excitation beam using an AMKO 01-002 LTI 0.2 m monochromator with 1200 l i n e s / m m grating blazed at 300 nm and a Hamamatsu R955 photomultiplier with S20-type spectral response. The dc-rnode emission and excitation spectra were obtained either directly recording the photomultiplier current with a Keithley DAS8-PGA analogto-digital IBM PC interface card or by an Ortec 9302 amplifier-discriminator and an Ortec 994 counter interfaced to a PC. To expand the linear dynamic range of the photon counter, an on-line correction for the dead time of the detection system was included in the data-collecting software. For excitation in the visible region of the spectrum Ar ion laser (Spectra Physics), the lines at 2.41, 2.54 or 2.71 eV were used as well as an Ar laser pumped CW dye laser (Spectra Physics DL200 with Rhodamine 6G dye, continuously tuneable from 1.905 to 2.18 eV, line halfwidth < 0.4 meV). The laser beams were guided to the measurement site over 40 m long fused all-silica optical fiber with fluorinated cladding (Heraeus Fluosil). The power at the sample site was between 5 and 25 mW over the tuning range of the dye laser and in the case of Ar laser excitation it was attenuated so as not to exceed 50 mW. The time-resolved excitation and emission spectra as well as PL kinetics with excitation in the range 1.9-2.7 eV were measured by chopping the laser beam at the output end of the fiber with

57

a mechanical chopper at 3 kHz. The light-todarkness transition times were better than 10 ixs. The photon counter was gated to collect photons during the first 10-100 ~s after complete closure of the excitation beam. In the resonance PL measurements, this scheme allowed the interference from the elastically scattered excitation light to be suppressed below the detection limits (better than 106 times). However, the signal-to-noise ratio in the time-resolved spectra was much lower as in the dc-mode spectra. The luminescence decay kinetics were measured using mechanically chopped Ar and dye laser beams (as above) or KrF excimer laser (hu = 5.0 eV, Lambda Physik EMG 250) with power density at the sample 20 _+ 10 m J / c m 2. The signal was recorded by a 12-bit multichannel analog waveform recorder with time resolution (channel width) of 1 I~s. Because of a significant pulse-topulse jitter of the excimer laser intensity, the addition of a software trap in the averaging PC program was necessary in order to reject the pulses falling outside the dynamic range of the waveform recorder and ensure correct averaging. The excitation spectra using dye laser were continuously corrected against both temporal and spectral variations of the laser power using an Si photodiode in the reference beam. The reference channel spectral characteristics were calibrated with a Fieldmaster laser power meter (Coherent). Ultraviolet excitation spectra were obtained using a 75 W xenon lamp (Oriel) with Zeiss MQ3 monochromator as an excitation source. The UV excitation spectra were corrected against excitation intensity by means of a sodium salycilate screen. The PL emission spectra were multiplied by (ht,) 2 to correct for monochromator dispersion, but were not corrected against the spectral efficiency of the photomultiplier and grating. An exception are the data for Figs. 6 and 9, which were corrected for the slight variations in emission registration channel spectral sensitivity between 1.9 and 2.2 eV using the spectral characteristic of a standard W lamp and MgO screen. The polarization spectra were obtained by placing dichroic polarization filters in the excitation and emission beams. The extinction ratio (crossed versus parallel transmission) of the ill-

58

L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

ters was better than 1 : 98 in the spectral region of interest. The excitation light was vertically polarized, and the luminescence intensity was measured for both horizontal (lh) and vertical (Iv) positions of the analyzer filter. The polarization spectra were calculated as P(hv) = (1 - Klh/l v) (1 + Klh/Iv) -1, where K(hu)=lv(hv)/lh(hU) is the instrumental correction measured with the sample substituted by a depolarized light source (e.g., milk glass). The optical absorption was measured in a single-beam scheme using a Cary 14 spectrometer.

3.2. Results

1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00

t6 m "O

03 Z ILl I-Z ILl Z ILl

3.2.1. Dye laser excitation The PL emission spectra measured under continuous 1.904 eV dye laser excitation at different temperatures are shown in Fig. 3. The room-temperature PL spectrum is structureless, with an anti-Stokes part. The 'line structure' at 1.904 eV is due to scattered excitation light; the feature to the right of it (denoted 'G') is caused by a minor monochromator grating defect. At lower temperatures, the anti-Stokes part vanishes and two vibrational structures, 50 + 10 cm-1 and 890 + 10 cm -~ (0.11 eV) apart from the excitation line, appear. In the 7 K spectrum, two replicas of each vibration are present. In the time-resolved measurement mode (Fig. 4), the interference from the scattered excitation light is completely suppressed. At temperatures < 90 K, a resonance zero-phonon line (ZPL) appears. Despite the lower signal-to-noise ratio, as compared with the dc excitation case, 890 and 50 cm-1 vibrational lines can be also seen. The apparent width of the ZPL ( = 2 meV) in Fig. 4 is determined by the monochromator resolution; the intrinsic ZPL width is evidently much smaller. Fig. 5 shows time-resolved PL spectra, measured at T - - 7 K at different excitat~.3n energies within the 2.0 eV absorption band. As the excitation energy is increased from 1.92 to 2.1 eV, the relative intensity of the ZPL decreases, the vibrational structures disappear and at 2.1 eV excitation the PL spectrum acquires almost its room temperature, unstructured shape. The dependence of the ZPL intensities on the

03 Lid Z ..I

O I.-O "rfit. 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00

PHOTON ENERGY

/ eV

Fig. 3. The dc-mode photoluminescence spectra of neutronirradiated synthetic SiO 2 glass measured at different temperatures under steady-state dye laser excitation at 1.904 eV. The y-scale is the same for all spectra; the individual baselines are evenly shifted. The off-scale line at 1.904 is due to the scattered excitation light; at low temperatures it is also partially due to resonance luminescence. The feature denoted by G is an artifact not related to the sample, caused by the monochromator diffraction grating. The monochromator spectral slit width is 0.8 nm.

excitation energy is shown in Fig. 6. The excitation energy was stepwise incremented and the emission spectra were measured in the vicinity of each excitation energy. The energy dependence of the integrated intensities of the ZPLs is shown in the inset. To determine the maximum of that distribution more accurately, nine additional spectra were measured with excitation energies in the distribution maximum region and data from them were also included in the inset. A leastsquares Gaussian fit to the ZPL energy dependence yielded a maximum at 1.935 eV with halfwidth 82.5 meV.

L. Skuja / Journal of Non-CrystaUine Solids 179 (1994) 51-69

Room-temperature PL excitation (PLE) spectra measured with dye laser excitation and luminescence emission monitoring energies between 1.80 and 2.03 eV are shown in Fig. 7. The measured PLE peak maxima are between 1.95 and 2.03 eV and their halfwidths are smaller than that of the 2.0 eV absorption band. Fig. 8 shows the evolution of the PLE spectra, measured in the dc mode at emission energy 1.96 eV, with lowering of the temperature. Similar to the case of emission spectra, lines corresponding to two vibrational modes, 50 and 860 cm-1 apart from the excitation lines, appear at T = 6 K. A set of time-resolved excitation spectra, measured at 6.5 K at different excitation energies is shown in Fig. 9. The zero-phonon line intensities exhibit, as expected, the same energy distribution as in Fig. 6. Both vibrational modes are also

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I

•

T=7 K, dye laser excitation I hv__=1.92 eV Time-resolved mode j/" Delay20 rts Gate 80 ,s

!. 1.70

1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00

59

1.80

1.90

2.o0

,. ,.. t 2.10

PHOTON ENERGY / eV Fig. 5. Time-resolved PL spectra of neutron-irradiated silica, measured at T = 7 K at different excitation energies (denoted by arrows) within the 2.0 eV excitation band. All spectra are arbitrarily normalized; the baselines are shifted.

U'J

::3 .D i-

u5 v

Time-resolved mode Excitation at 1.904 eV Delay 20 rLs Gate 80 rLs

50 cm"1

--t--

oo Z uJ IZ uJ O Z uJ O r,o u.I Z =Z n

,--I

O I-O "r" n 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00

PHOTON ENERGY / eV Fig. 4. Time-resolved PL spectra of neutron-irradiated silica measured at different temperatures under dye-laser excitation at 1.904 eV, in the interval between 20 and 100 p.s after the pulse of excitation light. The y-scale is the same for all spectra; the baselines are individually shifted.

present; they disappear for the luminescence monitoring energies above 2.02 eV. For these energies, the PLE spectrum acquires a flat, structureless shape with maximum at energies above the laser tuning range. Figs. 10 and 11 show the polarization spectra, measured at 8 K and at different excitation energies in the ac and dc modes respectively. The degree of polarization exceeds + 0.3 in the ZPL region and decreases significantly (it even changes its sign in the case of the ac-mode spectra) in the i.7-1.8 eV region. With the exception of the immediate ZPL region, the polarization values, measured in the dc mode, were always higher than those acquired in the ac mode. The measurements of polarization time dependences (not shown) confirmed that the polarization is timedependent and decays with time. For example, under excitation at 2.15 eV and luminescence energy 1.96 eV at 8 K, the degree of polarization decays from its steady-state value of +0.15 to + 0.02 in the first 90 p,s after the excitation pulse.

L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

60

The degree of polarization values in the ZPL region reported here are slightly (7-10%) lower than previously reported for 1.96 eV excitation energy [23]. A careful check revealed that the earlier measured values are inaccurate due to a bug in the correction software. The present values should be considered as more accurate. Photoluminescence decay kinetics measurements (not shown) were performed at 8 K for different excitation (1.92-2.15 eV) and emission (1.7-2 eV) energies. In all cases the usual, slightly distorted exponential decay was observed with average decay constant around 18-20 I~s. The smallest deviation of the decay kinetics f~om the monoexponential law was observed when they were measured on zero-phonon lines; however, a significant non-exponentiality still remained even in these cases.

120

,

640 ,

WAVELENGTH / nm 620 600 , , , j ~

580

100 ,¢_.

~

~

Optical absorption

:3

80 t..) O

¢-

4,--

X tO :,m

60 ¢J

8 8 .¢_

3 ~=o

1 80 eV

40

<

E

2

-.I

20

i

1.9

1.95

2.0

2.05

2.1

2.15

Photon energy / eV

Fig. 7. Comparison of the bandshapes of the 2.0 eV optical absorption band (O, the right y-scale) and luminescence excitation spectra recorded using a dye laser at emission energies from 1.80 to 2.03 eV. The respective emission energies are shown on each spectrum. The figure shows the correct relative intensities of the excitation spectra obtained at different emission energies. T = 293 K. Recorded in dc mode for vertical luminescence polarization. The spectra, taken at 1.94, 2.00 and 2.03 eV are extral:alated over the interfering laser line (not shown).

100

-II80-

t1.4,1J f I I l II ,~'l~l IT I It

"' 1 ~:40 " I

-

I

F

Sample: .

Corning7940 irrad'l~)i°n/cm2

HH

_

20

0 I

.9

,

I

1.92

1

I

I

,

I

,

I

1.94 1.96 1 . 9 8 2.0 PHOTON ENERGY / eV

,

I

2.02

,

I

2.04

Fig. 6. A set of 11 emission spectra, measured at T = 8 K in time-resolved mode in the zero-phonon line region at 11 different excitation energies from 1.91 to 2.01 eV with step 0.01 eV. All spectra are corrected for spectral variations of the excitation intensity and emission channel sensitivity. The inset shows the measured distribution of the zeroophonon line integrated intensities (O) and its Gaussian fit.

3.2.2 Ar laser excitation Figs. 12 and 13 show emission spectra measured with Ar laser line excitation at 2.41 and 2.71 eV, respectively. Comparison of the spectra obtained at 293 and at 8 K, as well as de-mode and time-resolved spectra in Fig. 12, exposes, apart from the 'usual' 1.9 eV PL band, two additional bands located at 2.05 and 232 eV in case of 2.41 eV excitation (Fig. 12) and at 2.1 and 2.38 eV in the case of 2.71 eV excitation (Fig. 13). Polarization spectra with the 2.71 eV excitation were also measured (Fig. 13); however, they were obtained using non-polarized excitation; therefore, the absolute degree of polarization values in Fig. 13 are lower than the true values and can serve only for relative comparison.

L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

3.2.3. Excitation in the 4.8 ~'V band Ultraviolet excitation spectra of the R-band in neutron irradiated Suprasil W1 silica are shown in Fig. 14. The spectra measured at 280 and 180 K can be fitted very well with a single Gaussian with a peak at 4.88 eV, halfwidths 1.15 and 1.08 eV and amplitudes 18.9 and 30.8 a.u., respectively. Below 180 K, the shape of the spectrum changes and at 8 K it splits into two poorly-resolved peaks with maxima around 4.3 and 5.1 eV. The PL emission spectra excited in different parts of these U V excitation bands are almost identical; however, a slight increase in the 2.0-2.1 eV region of the emission spectra is observed for the excitation energies >/4.4 eV (inset of Fig. 14). Decay kinetics measurements at T = 8 K, emission energy 1.91 eV and under KrF excimer laser (5.0 eV) excitation (not shown) gave decay curves similar to those reported previously

50

WAVELENGTH / nm 620 600

640 ,

,

580

,

,

45 I -"1~ 40 6K as

~ 25

.fi

~ 2o

.3 10

o

,

1.9

I

.,,tt 1.94 eV

r"!

J

80

60 50

1.g6 eV

2.02 eV

i , 'l !~, .,.1. I

TMlev

E w 20

I

I .933 eV

Energy / eV

2.00 eV

.o_

!

lO o 1.85 1.9 1.95 2.0 2.05

1.98 eV

7- 4o

,/~

t

. 4o

~ 60 c

,i

i

Sample: Coming7949 irrad 1020n/cm2

T=6.5K

Delay=10/as

J i

2.05 2.1 2.15 .95 2.0 Excitation photon energy / eV Fig. 9. A set of photoluminescence excitation spectra, measured at 6.5 K with dye laser in time-resolved mode at seven emission energies ranging from 1.92 to 2.04 eV. The emission energies are marked by arrows. All spectra are drawn to a common y-sca'e and are corrected against spectral variations in excitation intensity and emission registration channel response. The inset shows the distribution of zero-phonon line intensities (C)) and their Gaussian least-squares fit (line).

-'I~-

"~

0

i

100 -1.12

61

I

1.95

,

I

2.0

,

I,,

2.05

,

,

2.1

2.15

Excitation photon energy / eV Fig. 8. Temperature dependence of the photoluminescence excitation spectra, measured in dc-mode at luminescence monitoring energy 1.96 eV and dye laser excitation. The chopped-off line at 1.96 eV is due to scattered excitation light; the small line to the left of it is an artifact, caused by the monochromator. The monochromator resolution is show.~, by vertical bars. All spectra are individually normalized; the shifted baselines are indicated on the right-hand side.

[13,15-17]. Using a non-linear multi exponential curve fit, the decay data for the first 250 lXS could be fitted well by the sum of three exponents with lifetimes 11, 18.7 and 45 ixs and statistical weights 50, 85 and 12, respectively. However, this fit was not unique, and other fit parameter sets with average lifetime around 15-20 IXS gave almost the same quality fits. Evidently, a continuous distribution of decay constants (and non-radiative transition probabilities) is a more adequate model for this case. The rise time of the luminescence intensity after the laser pulse was shorter than the channel width (1 ixs) of the waveform recorder. Attempts were made to bleach the 4.8 eV absorption band using 5.0 eV pulses from the KrF excimer laser. Fig. 15 shows the absorption spectra of neutron-irradiated Suprasil W1 measured at 7 K after irradiation with up to 6100 pulses with energy density 60 m J / c m 2, delivered at 2 Hz. Both the 4.8 eV absorption band and the

L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

62

intensity of the 1.9 eV PL, excited by 5.0 eV photons, did not decrease by more than 5-7% as a consequence of bleaching.

WAVELENGTH / nm 700 670 640

730

0.4

I

I

I

610

I

I

1.96 eV 1

i 2.00 eV

0.35

3.3. Discussion

T=8K DC mode I

3.3.1. Vibrational structure and distribution of zero-phonon lines The initial helium temperature investigation of the R-band, using laser excitation (He-Ne, 1.96 eV) in the 2.0 eV absorption b~md [23] has shown that, below 80 K, zero-phonon lines (ZPLs) exist underneath the inhomogeneously broadened contour of the R-band and that they can be 'picked out' by narrow band excitation, matching the ZPL energy. This situation is well known from the studies of rare-earth ion doped glasses; the basic theory of site selective fluorescence line narrowing is well understood [44,45]. In the simplest approximation it is assumed that site-to-site variations in the glass matrix causes mainly parallel energy shifts of the PL spectra originating from

1.91 eVl

0.3 w IT

t

to

I

,

~ 0.25 z O 0.2

i 0.15 0.1

0.0

i

,

I

.

I

.

I

,

I

,

t

,

I

,

I

,

I

1.65 1.7 1.75 1.8 1.85 1.9 1.95 2.0 2.05 2.1 PHOTON ENERGY / eV Fig. 11. The de-mode luminescence emission polarization spectrum, measured at 8 K under dye laser excitation at 1.91, 1.96, 2.00 and 2.15 eV. The excitation energies are marked by

arrows.

730

0.4

I

WAVELENGTH / nm 700 670 640 I

0.35

I

each slightly different site, but other parameters of the PL center - the shape of the spectrum, transition probabilities, phonon energies, etc. remain relatively constant. Under these assumptions, the intensity of the luminescence, I, as the function of excitation, Eex¢, and emission, Eem, energies is given by

I

196.v I 1.2.oo.v

,,,°v.l

0.3

610

I

/

Delay=15 #s Gate= 80 ~

LU LU 0.25 li:

.n

,,o,

Ci 0.2

I ( Eem , Eexc) +oo

z

_o m

n-

0.15

a

1

- fo

/

0.1

5

O 0.05 o.

(11)

0.0 -0.05 ti

~ / t i i l" l t ~ l" "t t ~ t

I

I

I

I

I

.

I

-0.1 1.65 1.7 1.75 1.8 1.65 1.9 1.95 2.0 2.05 2.1 PHOTON ENERGY I eV

Fig. 10. Time-resolved luminescence emission polarization spectrum, measured at 8 K under dye laser excitation at 1.91, 1.96, 2.00 and 2.15 eV. The excitation energies are marked by

arrows.

W ( E ) K ( E e x c - e ) L ( E e m - e ) de,

where K(E) and L(E) are the homogeneous contours of the absorption and emission spectra, respectively, defined in coordinates where E = 0 corresponds to the zero-phonon line. The integration parameter, E, can be conveniently viewed as the zero-phonon energy. W(E) is the probability distribution for sites with ZPL energy e. As follows from Eq. (11), in order to maximize the site selection effect, it is advantageous to choose the excitation energy, Eex¢, as low as

L. Skuja /Journal of Non-Crystalline Solids 179 (1994) 51-69

700 I

WAVELENGTH /nm 650 600 560 I

I

530

I

I

100 T=8 K, DC

mode

i

80

~2 ~r ID

60

¢-

~ 4o if) ID

.c_ E 20

1.7

1.8

1.9

2.0

2.1

2.2

2.3

PHOTON ENERGY / eV Fig. 12. Photoluminescence emission spectra measured under excitation with 2.41 eV (514.5 nm) A r laser line. The de-mode spectra are drawn to the same y-scale. The time-resolved spectra ( O ) were measured within time window I35 I~s delayed 10 ~s after the excitation pulse.

ally bonded to proton, i.e., forms a bound hydroxyl group. Since the formation of NBOHC by removing of the proton from the OH group in the reaction of Eq. (1), apart from slightly changing the effective mass, should also have some effect on the Si-O bond force constant, 890 cm-t for the Si-O stretch mode in NBOHC seems a plausible value. The origin of the low-frequency mode at about 50 cm-~ remains currently unexplained. A similar low-frequency mode has been observed in the Raman spectra of both dry and wet silica [46], with a little higher intensity in the O-H-containing glasses. The same two vibration modes are observed also in the low-temperature excitation spectra (Figs. 8 and 9). The energy of the high-frequency mode in this case is 860 cm -~, indicating the usually observed lowering of the force constants in the excited state of the center, compared with the ground state. This confirms that the 890 cmline is due to the ground state vibration, contrary

700

possible in the low-energy wing of the distribution W(E). In that case, the non-selective excitation in the phonon side-wings of the centers with ZPL energies smaller than Eex¢ is minimized. Indeed, as seen from Fig. 5, the relative intensities of the zero-phonon lines in the PL emission spectra gradually decrease as the excitation energy is increased. The ZPLs and vibrational structures in the emission spectra obtained at the lowest available laser excitation energy, Eexc = 1.904 eV, (Figs. 3, 4) are much better resolved compared with the previously reported [23] H e Ne laser (1.96 eV) excited spectra. The ZPL is observed even at liquid nitrogen temperatures (Fig. 4). The local vibration with energy 890 cm- ~ (Figs. 3,4) is attributed here to the symmetric stretching mode of the silicon-non-bridging oxygen bond in the ground state of the center. As shown by Raman investigations of 'wet' fused silica [46,47], a similar Si-O stretching mode has an energy of 970 cm-~ if the non-bridging oxygen is addition-

63

/

'

100~

=I ~-

/

Polarization ' L__.6 ~_~ Intensity

f\"

1 ""~

+

+

\

.'"C" /

"o; I/

500

'1o'

'

f~

~=80 L

~ 60

WAVELENGTH /nm 650 600 550

\

/

"0.15=~ k: " " ' °

T=.K

. "~

~

0.05 , 0~

.

t 1.8

T =2 2.0

2.2

2.4

2.6

0.0

PHOTON ENERGY / eV Fig. 13. The dc mode photoluminescence emission and polarization (right-hand scale) spectra measured at 8 and 293 K with 2.71 eV (457.9 nm) Ar laser line excitation. The excitation light was not fully polarized; therefore, the measured polarization values may be lower than the true values.

L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

64

to our initial guess [23] that it could be a 'pseudoline' due to the ZPL of the centers, selectively excited to the first vibrational level of their excited state. Such pseudolines can appear with significant intensity if the halfwidth of W(E) is larger than the vibration energy. In the framework of the model assumptions of Eq. (11), the integral intensity of the resonantly excited ZPL is proportional to the number of the centers having this transition energy, i.e., to the distribution function, W(E). The measured intensity distribution of the ZPL can be fitted by a Gaussian with maximum at 1.935 + 0.01 eV and halfwidth 82 + 7 meV (Figs. 6 and 9). It is inter-

WAVELENGTH / nm 300 270 250

330

6

I

3

2--

•r. 6 E o

o=5 O

~4 ttl ~.3 O

0 ~

I

#~

3.0

1 - beforelaser irradiation _ 2 after1100shots (60 mJ/cmz) 3 - after6100shots

3.5

4.0

4.5

5.0

5.5

6.0

Photon energy / eV

o~ 5 04

E=4

P, =='3

1

230

I

le K / 12~ K

'

03

I

Sample SW707,. Suprasil Wl, 10 "~ n°/cm2 T=7 K

Fig. 15. Effects of KrF laser (5.0 eV) irradiation at T = 7 K on the 4.8 eV absorption band in neutron-irradiated Suprasil W1 silica.

oO

oo°

,

~

~

.

--

-~

U.l 0.

0~2

oooo°°

lie0 K / 12~ K

,9o ol nO IO0

I

'

I

'

I

'

I

,

I

'

I

,

I

]I/T=SK\\ B,O

2o ,.i G,.

0

'

3.5

~

,

4.0

4.5

5.0

I

5.5

PHOTON ENERGY / eV Fig. 14. Excitation spectra of the 1.9 eV PL band in neutronirradiated silica (Suprasil Wl, 1019 neutrons/cm 2) measured at temperatures between 293 and 8 K. The emission was monitored at 1.905 eV with a bandpass of 0.05 eV. The upper part shows the ratio of the low-T excitation spectra to the room-temperature spectrum. The inset shows the PL emission spectra, measured at T = 8 K and excitation energies 3.75 eV (A), 4.4 eV (B) and 5.0 eV (C). The spectral resolution is shown by vertical bars.

esting to note that this distribution is much narrower than the distribution of energy splitting between the two oxygen 2p ar-orbitals in NBOHC" peak at 0.3 eV, halfwidth 0.5 eV, used to simulate the NBOHC EPR spectrum [38,41]. However, the site energy distribution function, W(E), obtained experimentally here should be viewed with some caution. The main limitation of the model leading to Eq. (1~) is the assumption that the ZPL energy is the only parameter of the center that is affected by site-to-site variations. In reality, other parameters, affecting the PL intensity, like the radiative and non-radiative transition probabilities, are also distributed. This is proved most obviously by the non-exponential luminescence decay kinetics. As shown by the still remaining non-exponentiality in the PL decay kinetics measured directly on ZPLs, the distributions of different parameters are not necessarily connected through a common parameter - the ZPL energy. The single-peaked distribution of the ZPL intensities (Figs. 6 and 9) indicate that ZPLs originate from a single type of NBOHCs in neutron-

65

L. Skuja / J o u r n a l o f Non-Crystalline Solids 179 (1994) 5 1 - 6 9

irradiated type-IIl silica. However, several types of NBOHCs, as suggested by Nagasawa et al. [6] from the peak shifts of the 2.0 eV absorption band, can still exist, if one assumes that they undergo stronger atomic relaxations and do not exhibit ZPLs.

ny

ny nx

3.3.2. Polarization spectra and the R-center model

The relatively high values of the PL degree of polarization in the ZPL region ( + 3 0 to +35%, Figs. 10 and 11) indicate, that the electronic transition occurs between non-degenerate states. For randomly oriented centers in the glass, the upper theoretical limits of the luminescence polarization values are P = + 0.5 for a dipole transition between non-degenerate states and + 1 / 7 for transitions to a doubly degenerate state (e.g., A ~ E transitions) [42]. This conclusion is contrary to the earlier work [17,19]. The previously reported low polarization values (10-12% [17,19]), which were then taken as an indication that a degenerate state takes part, were measured with a 700 nm cut-off filter under 1.96 eV excitation. These values are in agreement with the low-energy part of the presently measured spectra (Fig. 11). However, as presently evident, they cannot be extrapolated to the whole R-band. The sharp lines seen in the polarization spectra (Figs. 10, 11) at 1.8 and 1.85 eV are due to the transfer of the intensity of the highly polarized ZPL to lower energies by the 890 cm -~ full symmetric vibration. The gradual decrease of the degree of polarization towards the low-energy side from the ZPL (Figs. 10, 11) shows that the angle, a, between the electric dipole momenta of the absorption and emission transitions increases at lower emission energies. The dependence of the luminescence polarization, P, on t~ is [42] p ( a ) = (3cos2 o t - 1 ) / ( c o s 2 ot + 3). (12) A trivial explanation of the polarization spectra of Figs. 10 and 11 would be that the R-band is actually a superposition of two bands of entirely different PL centers. However, this would require an accidental near coincidence of the PL and PLE spectra and luminescence kinetics, which is an unlikely event. The variation in the angles, re, within a single luminescence band can occur, if

Fig. 16. An energy level scheme, suggested to explain the optical properties of the non-bridging oxygen hole center [231. The e-bonding in SiO4 is depicted by straight lines; the non-bonding oxygen ~ orbitals are shown, n L is the lone pair orbital one of the three ligand oxygen atoms. The 1.9 eV emission/2 eV absorption transition is n L ~ ny charge transfer transition polarized approximately along the line connecting both oxygen atoms, a is the angle between the transition dipole momenta of 4.8 eV absorption and 1.9 eV emission.

the band is due to several closely spaced electronic transitions with different values of a, i.e., another electronic term in close vicinity to the one taking part in the zero-phonon transition exists. In addition, both terms can be mixed by low-symmetry vibrational modes. The polarization data of the present work agree well with the recently suggested model [23,24] of electronic transitions in NI3OHC (Fig. 16). The 1.9 eV PL and 2.0 eV absorption transition occurs due to the charge transfer transition in NBOHC between the half-filled non-bonding 2p "rr orbital of the non-bridging oxygen atom and the lone-pair 2p orbital of one of the ligand oxygens (denoted ny and n L in Fig. 16). The dipole momentum of such a transition is oriented approximately along the edge of an SiO 4 tetrahedron. Since only non-bonding states are involved, the Stokes shift is small and ZPLs can be observed at low temperatures. The relatively long PL lifetime (about 20 i~s) is consistent with a charge-transfer transition between weakly overlapping states. The splitting between the oxygen ny and n x orbitals variates from site to site due to the glassy disorder. In the excited state of the center, with both ny and n x orbitals doubly occupied, a finite

66

L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

probability exists either for the luminescence transition of hole from nL orbital back to ny or to somewhat lower-lying nx. Along with the vibrational mixing, this can explain the decrease of the PL degree of polarization at lower emission energies (Figs. 10 and 11). The lower values of the polarization in time-resolved spectra (Fig. 10) as compared with the de-mode spectra (Fig. 11) indicate that the emission transitions, whose dipole momenta are oriented at wider angles away from the absorption transition, generally have lower transition rates. The decrease of polarization at energies below the ZPL energy could be alternatively explained by the excited state transfer of the hole to one of the other two n L orbitals. Or it can, as in Griscom's self-trapped hole (STH 2) model [48], delocalize over the lone pair orbitals of two adjacent ligand oxygens. The currently available optical data do not allow one to select between this or the former alternatives. From the luminescence point of view, there should be no difficulties detecting NBOHCs with nearly degenerate oxygen 2p non-bonding orbitals. With both orbitals approaching degeneracy, transitions to and from them cannot be modelled by a single fully-anisotropic electric dipole [42]. This is illustrated by the less than theoretical values (P = 0.5) of the degree of polarization found in the zero-phonon lines (Fig. 10). On the other hand, the subset of the NBOHCs with nearly degenerate 2p orbitals could be particularly hard to observe with EPR because of the relatively strong spin-orbit coupling and, as a consequence, very fast spin-lattice relaxation. As described above in Section 2.2.2., the EPR signal of surface NBOHCs was not detected at all, despite all the chemical evidence of their presence. The subset of NBOHCs with big 2p orbital splittings, which is easily detectable by EPR, in its turn can be harder to detect with PL techniques, since any asymmetric perturbation (like the proton of an adjacent hydroxyl group, as suggested for the 'wet' NBOHC [38]) should lead to increased atomic relaxations in the excited state and hence to an increase in the non-radiative transition probabilities. For example, the structurally similar HCI center [40] in alkali silicates,

with O 2p orbital splitting of 1.5 eV due to the neighboring alkali ion, exhibits no luminescence at all [25]. Thus, in addition to the possible reasons, listed in Ref. [1], the long-standing controversy over the correlation between the EPR signal of the wet NBOHC and R-band luminescence (Section 2.2.3.) could also be caused by both investigation methods sampling different and only partially overlapping subsets of the inhomogeneously broadened population of the NBOHCs in glass. 3.3.3. Emission bands under Ar laser excitation Photoluminescence spectra measured at 8 K with Ar laser line excitation at 2.41 and 2.71 eV reveal two previously unreported bands with maxima at 2.05 and 2.32 eV (Fig. 12) and at 2.10 and 2.38 eV (Fig. 13), respectively. It is likely that the same two PL bands are seen both under 2.41 and 2.71 eV excitation and that the peak shifts at about 0.07 eV between the two cases are due tn the inhomogeneous broadening. The 2.35 eV band is completely, but the 2.07 eV band only partially, quenched at room temperature. Comparison of the de-mode and time-resolved spectra (Fig° 12) reveals that the 2.35 eV band has lifetimes < 3 I~s, but the 2.07 eV band has lifetime comparable with the R-center (about 10 I~s). Several spectral characteristics of the 2.07 eV band (lifetime, temperature dependence (Fig. 12), degree of polarization (Fig. 13) and the relatively small Stokes shift) are quite similar to those of the R-center. The main differences between the two PL bands, apart from the 0.15 eV peak shift are the complete absence of ZPL and vibrational structures in the 2.07 eV band (Figs. 9, 12 and 13) and that it is less efficiently excited in the ultraviolet (Fig. 14, inset). If the 2.07 eV band is indeed due to some 'perturbed' version of the R-center, it should have an easily detectable NBOHC EPR signal, as discussed in Section 3.3.2. The 1.98 eV band, seen in the PL spectra of surface R-centers (Fig. 2) has a nature different from the bulk 2.07 eV band, since the former band is excited at 3.68 eV (compare Figs. 2 and 14). 3.3.4. The nature of the 4.8 eV band The UV excitation spectrum of the R-band measured at 293 K (Fig. 14) has the usual Gauss-

L. Skuja /Journal of Non-Crystalline Solids 179 (1994) 51-69

ian shape in agreement with the previous reports [3,4,18]. The 'step-like' excitation spectrum with a threshold at 3.9 eV, which is reported for the ArF laser-irradiated sample [22], is not observed in our neutron-irradiated samples. Therefore, at least in the neutron-irradiated samples, the 4.8 eV excitation band of the R-c~nter is different from that [22] attributed to t.,nterstmal ozone molecules. Further, in the UV bleachi~g experiment (Fig. 15), after 6100 excimer lasdt pulses the total energy absorbed by the 4.8 eV band was 366 J / c m 2 or 4.6 × 1020 5 eV photons/cm 2. The estimated upper concentration limit of the absorbing centers is about 1018 centers/cm 7 (in a 2 mm thick sample). This means that each center was excited on average more than 400 times. Since only about 5% of the centers were bleached, the probability of bleaching the center by a single 5 eV photon is about 1.3 × 10 -4. On the other hand, the quantum yield of the UV-excited 1.9 eV PL is definitely higher than 0.1. Therefore the dominant mechanism for the 4.8 eV UV excitation of the R-band cannot involve photodestruction of ozone (as suggested by the ozone model [22], Section 2.3.) or of any other absorbing species. It should be noted at this point, however, that the 4.8 eV excitation and 4.8 eV absorption bands may not be identical: only part of the centers contributing to the 4.8 eV absorption band may contribute also to R-band PL. A different 4.8 eV absorption band, which was easily bleachable by 5.0 eV light and correlated with the intensity of the peroxide radical E P R signal, was observed in gamma-irradiated type IV silica by Hosono and Weeks [39]. The R-band UV excitation spectra measured at 280 and 180 K (Fig. 14) have Gaussian shape, a peak at 4.88 eV and halfwidths 1.15 and 1.08 eV, respectively. On the other hand, the reported halfwidth of the UV-induced (or bleached) 4.8 eV absorption band(s) is smaller 0.8 eV [1,7,22,39] or 0.60 eV [49]. This could indicate that absorption bands of different defect centers coexist at this energy, as previously suggested [1]. Apart from the results of Refs. [39,49], there is other evidence that the peroxide radical should I

.

.

67

give an absorption band in the 5 eV region. Studies of the diffuse reflection spectra of surface peroxide radicals indicate that they have an absorption band at 5.4 eV with halfwidth 1.2 eV [37]. Peroxide radicals in C H 3 - O - O and in H O-O" have absorption bands between 5 and 5.4 and between 5.2 and 5.9 eV, respectively [50]. However, it is unlikely that peroxy radicals could be responsible for the R band, since the chemical properties of the R-center and peroxy radicals are entirely different (Sections 2.2.1. and 2.2.2.). Despite the finding that the two strongest clues for associating the R-center with ozone - the threshold in the UV cxcitation spectrum [22] and the increasing part in the R-band decay kinetics [18] - are not intrinsic features of the R-center, the interstitial ozone still remains a strong candidate [22,1] to be one of the several species contributing to the 4.8 eV absorption band, since its formation under far-UV irradiation in O2-containing silica is a probable process. At 8 K, the shape of the 4.8 eV excitation band changes, with dominant growth of the intensity in the 4.4 and 5.1 eV regions. This suggests that the wide '4.8 eV' excitation band is actually a superposition of at least two electronic transitions. Upon cooling of the sample to 8 K, the intensity of the 1.9 eV PL increases similarly both under 2.41 and 4.8 eV excitation (compare Figs. 12 and 14). Along with the observation of an instantaneous onset of PL emission intensity after pulsed excitation at 5 eV (Section 3.2.3.) and previously reported non-zero PL polarization values upon excitation in the 4.8 eV band [13], this seems to uphold the attribution of the 4.8 eV excitation band to an electronic transittion of the R-center. In the NBOHC model [19,23], the 4.8 eV band is attributed to a transition from the it-bonding orbital to the 2p non-bonding orbital (cr~ ~ ny in Fig. 16). The dipole moment of the trz ~ n y transition is oriented perpendicular to the direction of the Si-O band (the z-direction in Fig. 16). The hole in the or-orbital is transferred non-radiatively to one of the nl. orbitaIs. The charge transfer emission transition is polarized approximately parallel to the O - O direction and in this way forms an

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L. Skuja / Journal of Non-Crystalline Solids 179 (1994) 51-69

angle, a, with the absorption transition. For all possible orientations of the absorption transition dipole in the plane, normal to the z-direction, the values of a are near to the 'magic angle' of arccos(3 -t/2) = 55° at which, according to Eq. (12), the dependence P = P ( a ) crosses zero. This agrees well with the ex~-efimentally observed small value ( - 1 . 5 % ) of the degree of polarization [13] upon excitation in the 4.8 eV band. The energy of the 4.8 eV transition is compatible with the observed shift of g2 from the freeelectron value (g2 = 2.0095) of the 'wet' OHC EPR signal [38]. The 2.0 eV transition in Fig. 16 should have no effect on the g-factor since the orbital angular moment matrix elements between n L and ny are zero (see, for example Ref. [40]).

4. Conclusion

The spectroscopic and chemical properties of R-centers discussed here are in a reasonable agreement with the non-bridging oxygen hole center (NBOHC) model. The spectroscopic characteristics of the R-center in neutro.n-irradiated silica are different from those associated with interstitial ozone molecules. The significantly improved understanding of the EPR-active defects in glassy SiP 2 which has been achieved in the last decades is to a large extent due to taking into account the inhomogeneous broadening effects in glass. This aspect has so far been nearly totally neglected in the optical spectroscopy studies of silica. The results of the present work show that inhomogeneous broadening is important also in the interpretation of optical spectra of silica glasses. The correlation between any two investigation methods applied to the same type of defects in glass may become anomalous if these methods sample different subsets from the total population of inequivalent defects of this type. For example, the luminescence intensity may cease to be proportional to the intensity of the absorption band of the same center during bleaching or annealing experiments if the fraction of centers with lower q~antum yield bleaches more easily. "Internal correlations between the distributions of transition energies

and singlet-to-triplet transition rates have been shown to be responsible for 'anomalous' peak shifts of the triplet luminescence of the B2-center in oxygen-deficient silica [51]. Similar effects, as well as accidental overlapping of different optical bands in the same spectral region, may be the cause of the long-standing dispute on the corrCation between the R-center optical manifestations and NBOHC EPR signal. This conelation must be proved in order to settle finally the controversy over the origin of the R-band. This work was performed at Miinster University with support from the Alexander von Humboldt Foundation (Born, Germany). During the preparation of the manuscript, the work was supported by a Latvian Science Council grant No 93-656. The author is grateful to colleagues at Miinster University, particularly to Dr A. Naber and Dr R. Basfeld, for generous help with the experiments and Professor F. Fischer for hosting this project. Dr D.L. Griscom is thanked for several enlightening discussions. This paper was presented at the R.A. Weeks symposium with travel support from the International Science Foundation.

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