Investigation of optical and radiation properties of oxygen deficient silica glasses1

Journal of Non-Crystalline Solids 248 (1999) 49±64

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Investigation of optical and radiation properties of oxygen de®cient silica glasses 1 A.N. Trukhin a

a,*

, H.-J. Fitting

b

Institute of Solid State Physics, University of Latvia, Kengaraga St. 8, LV-1063, Riga, Latvia b Physics Department, Rostock University, Universit atsplatz 3, D-18051 Rostock, Germany Received 7 August 1998; received in revised form 8 February 1999

Abstract The de®ciency of oxygen in pure silica manifests an absorption band at 5 eV as well as an absorption band of higher intensity at 7.6 eV. The band at 5 eV is associated with lone twofold-coordinated silicon centers. The nature of the main band at 7.6 eV has been studied using silica samples with di€erent levels of oxygen de®ciency. The excitation via the 7.6 eV band produces a photoelectric response as well as inner center and recombination type luminescence. Two main luminescence bands of the twofold-coordinated silicon center appear: a blue band (2.7 eV) and a UV band (4.4 eV). Induced absorption with several bands as well as thermally stimulated luminescence with complex peak structure were observed. Analyzing these data, the nature of the 7.6 eV band cannot be ascribed to a lone point defect, rather, it can be ascribed to the localized states of the disordered structure of silica modi®ed by an oxygen de®cit. Ó 1999 Elsevier Science B.V. All rights reserved.

1. Introduction In spite of the progress in the technology of pure silica production, even in pure silica the low absorption level corresponding to crystalline quartz of the same purity is not obtained. In pure silica a broad absorption band appears at 7.6 eV superposed on the intrinsic absorption tail. It is sometimes accompanied by a band at 5 eV, but sometimes a band at 5 eV is barely detectable. The presence of these absorption bands reduces and * Corresponding author. Tel.: +371-7 260 686; fax: +371-7 112 583; e-mail: [email protected] 1 Presented at OSA topical meeting `Bragg Grating, Photosensitivity, and Poling in Glass Fibers and Waveguides: Applications and Fundamental', Williamsburg, VA, USA, 26± 28 October 1997.

modi®es the application of silica in optics and telecommunication techniques. It now seems clear that the 7.6 eV band is related to the de®cient of oxygen (see for example Refs. [1±5]) and the related oxygen de®cient center (ODC). There are three main approaches to the interpretation of the 7.6 eV band in non-irradiated silica. The ®rst interpretation of this band is as a point defect such as an oxygen vacancy or a relaxed BSi:SiB bond [3,5]. The main argument for that is supported by two facts: ESR E0 -centers appear only after laser irradiation and the heating in H2 (at 800°C) or O2 (at 900°C) atmospheres reduces the absorption band intensities at 5 eV (ODC II) and at 7.6 eV (ODC I) after a certain annealing time. Moreover, after hydrogen treatment the Si±H vibration mode at 2200 cmÿ1 is observed in Raman scattering.

0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 0 8 9 - 7

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A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

The second interpretation is related to the similarity of the silica properties around the 7.6 eV band to the properties observed in the semiconductor chalcogenide glasses [6±8]. Neutron irradiated quartz [6] and Suprasil 1 have been studied and compared. The authors did not ®nd photoluminescence (PL) in non-irradiated samples. (This is in contradiction with data of Ref. [2], where excitation at 7.7 eV always produces luminescence in silica at low temperatures.) Neutron irradiation of 1019 neutrons per cm2 results in the 7.6 eV excitation band of the 4.4 eV UV luminescence. This is explained as an intrinsic point defect because it is observed also in the crystalline quartz structure. In Ref. [7] a 7.9 eV laser excitation at low temperature (25 K) and at room temperature (RT) as well as a 6.4 eV laser excitation at RT were performed. Suprasil samples with a di€erent OH content were investigated. Surprisingly, a very strong red luminescence is observed contrary to other experiments [4]. Therefore, the laser excitation provides a defect creation analogous to cathodoluminescence excitation [9]. In Ref. [8] the lack of defect paramagnetism was explained by negative ÿU defect pairs, with defect states being either double occupied and negatively charged or unoccupied and positively charged. The existence of negative ÿU requires a large electron±phonon coupling. Non-bonding p electrons of oxygen facilitate bond re-arrangements. The third approach is based on studies of the in¯uence of reduced oxidizing melting conditions [4,11] on the optical properties of several pure silicate and germanate glasses, e.g. optical transmission, radiation induced coloration, luminescence, etc. In such a way, the so-called localized states have been determined electronic states of minority structural motifs of the disordered glass network [11]. An analogous state for the crystals is polymorphism. For example, these minority structural motifs correspond, in the case of germanate glasses, to six-coordinated rutile-type structural elements within the main tetrahedral structure. Localized states could be observed as providing the lowest energy states with respect to absorption due to main structural elements. Under excitation the main processes in glasses occur at localized states because of this low state of energy.

The localized states are responsible in oxide glasses for a rich quantity of phenomena: photoluminescence, thermally and photostimulated luminescence, radiation coloration properties, etc. The localized states are very sensitive to reducing or oxidizing melting conditions. A de®ciency of oxygen produces some structural changes in glass, including a subnetwork of the localized states. In such a case, as a rule, the absorption threshold is shifted to lower energy or even a separated absorption band appears [11]. However, silica glass is very exclusive in the family of optical glasses, even of silicate glasses, and localized states are not detected optically in silica melts under oxidized conditions [11]. The in¯uence of reduced melt conditions on silica glass leads to the same behavior as in others optical glasses. Thus melting under oxygen de®cient conditions provides a strong absorption band at 7.6 eV in silica. In stoichiometric silica the minority structural motifs, which were responsible for localized state absorption, probably possess optical transitions in the range of those of the majority structural motifs. Therefore, the localized states are not detected at the intrinsic absorption threshold. Reducing melt conditions induce an absorption band below the optical gap like in other glasses [11]. In the present paper, silica with a deliberately high level of oxygen de®ciency and with an increased 7.6 eV band intensity has been studied [12]. We have observed the following phenomena: photoluminescence, thermally and photostimulated luminescence, photo- and X-ray induced absorption. These results are discussed together with earlier data [13,14] of ESR, cathodoluminescence and IR absorption in silica samples with a deliberately high level of oxygen de®ciency. This points to the existence of many defect products that make oxygen de®cient silica analogous to other oxide glasses with well detectable localized states. This approach had been already realized in Ref. [4] for silica samples made in normal or weakly oxygen de®cient conditions. Old and new investigations show similar e€ects induced by oxygen de®ciency. The explanation in the case of low level de®ciency was that a group of complex defects are responsible for the absorption band at 7.6 eV. In this work, we ascribe the 7.6 eV absorption band to

A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

localized states modi®ed by reducing conditions. This modi®cation means that one or several point defects due to oxygen de®ciency appear in the neighborhood of localized states. We will call them reduced localized states (RLS). Under photoexcitation a transformation of RLS could be observed in permanent as well as transient states. Such an explanation is in contradiction with those explaining this band as a single point defect or as a simple oxygen vacancy BSi:SiB [3,5], or as another single point defect of normal tetrahedral structure. The third approach includes the two ®rst explanations given above. The structural area of localized states contains point defects due to oxygen de®ciency and so point defects play an important role in the 7.6 eV band. One of these defects is the twofold-coordinated silicon (Si:). Silicon±silicon bonds cannot be excluded, however, created at high temperatures they are not easily transformable by irradiation. The third explanation also includes the second one. Indeed, the 7.6 eV band is due to localized states as in chalcogenide glasses, with the modi®cation that this band is based on oxygen de®ciency. As received silica glasses made under oxidizing conditions have a hardly detectable 7.6 eV band. 2. Experimental The high purity silica samples studied were silica of KS-4V type IV [2]. The method of preparation was based on electrofusion of synthetic silicon dioxide cristobalite. A high level of oxygen

de®ciency was provided by a reaction of cristobalite with silicon vapor [2]. A de®cit level of about 10ÿ2 wt% has been achieved. The studied series of samples from melt-in-normal to oxidized conditions also includes only samples with low levels of oxygen de®cit. The highest oxygen de®ciency was obtained for three groups of samples-two with a level of 0.015 wt% but di€ering in additional absorption at 6 eV. The third sample group had a de®cit level of 0.007 wt%. The treatment of some samples in hydrogen atmosphere was performed during 2 h in a closed container heated to 800°C. Hydrogen was provided by benzene decomposition. The list of studied samples with their speci®c parameters is presented in Table 1. The samples (size 10 ´ 10 mm2 ) were mounted on the cold ®nger of the cryostat. The photoluminescence (PL) excitation was done via a grating vacuum monochromator (the excitation intensity was detected by use of sodium salicylate light transformation) and a toroidal Al coated mirror by means of hydrogen discharge lamps. One of the lamps was windowless, allowing spectral measurements in the continuous discharge regime (3 A) and another in the spark discharge regime (450 pF, 10 kV) for the PL decay kinetics measurement. Luminescence excitation was also performed with an Ar laser (Spectra Physics) 171-08 and the second harmonic of the 488 nm line. The decay kinetics were recorded with a 127 channel analyzer for the long (ls±ms) time range. Fast photoluminescence (ns) was measured in Irkutsk with a 1024 channel analyzer. An X-ray tube with a tungsten anode (15 mA, 40 kV) and a pulsed electron gun

Table 1 Studied samples and their parameters KS-4V silica

Level of excess silicon

N1 N2 N3 N4 N5 N6 N7 N8 N9

0.015 wt% 0.015 wt% 0.007 wt% No deliberate 0.015 wt% No deliberate No deliberate No deliberate No deliberate

Speci®c properties Additional absorption subband at 6 eV

addition addition addition addition addition

51

Melt after washing in chlorine N1 treated in hydrogen at 800°C Melt after strong washing in oxygen N6 treated in hydrogen at 800°C Melt after washing in oxygen N8 treated in hydrogen at 800°C

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A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

(250 kV, 270 kA, 20 ns) were used to study the in¯uence of ionizing radiation. In that case the decay kinetics were recorded by a fast oscillograph. The luminescence spectra were measured by a grating monochromator MDR-2 with a grating 600 mmÿ1 in the range 1±2.5 eV and 1200 mmÿ1 for the range 2±6 eV. Photoluminescence was recorded via glass ®lters for the excitation spectra and the decay kinetics. The spectral resolution was low and has been determined by low yield of the PL in glasses as well as by low intensity of exciting photons (not exceeding 1012 photons/cm2 ). The resolution was 3 nm for the luminescence spectra and 4±5 nm for the excitation spectra measurements. The luminescence was recorded through a cryostat window and a glass ®lter, via the analyzer and by means of a photoelectron multiplier FEU106 with a photocathode S-20 for the spectral range 1.8±6 eV. The measured curves are presented in ®gures as received; therefore they re¯ect the level of errors. Photoelectric properties of silica were investigated by means of an electrometer. The screening of both the electrometer hot wire and the sample against electrons emitted from metallic construction elements was obtained by means of a te¯on collimator. The photoelectric response was measured as a sample charging current under pulsed excitation. All contacts were made with silver paste. The screening of surface currents was achieved by a silver paste ring electrode connected to ground. 3. Results 3.1. Optical absorption and luminescence In Fig. 1(a),(b) typical optical spectra measured in silica with a di€erent level of oxygen de®cit are presented. The two main bands at 5 and 7.6 eV are observed clearly. In samples N2 the absorption rise at 6 eV is higher than in other samples. Probably this is due to some peculiarities in the glass melt conditions. The intensity of absorption bands at 5 and 7.6 eV are proportional to the oxygen de®ciency.

Fig. 1. Optical properties of silica samples (KS-4V) with different levels of oxygen de®cit: (a) 0.015 wt% (Samples N1,N2); (b) 0.007 wt% (Sample N3). 1,2,3,4,5: N1; 10 : N2. Optical spectra ± 1,10 : absorption, 290 K; 2: photoluminescence (PL), 90 K; 3: PL at 4.4 eV excitation.

In Fig. 2 the in¯uence of hydrogen treatment at 800°C on the samples is presented. The treatment temperature was chosen as in Ref. [3]. It is worth to mention that there was a discrepancy in results after treatment in hydrogen at 1200°C, when the intensity of the band at 7.6 eV grew strongly in silica samples melted under normal conditions [4], whereas after treatment at 800°C [3,5] the intensity of this band drops down. In all cases the absorption band at 5 eV has disappeared completely. In samples with a deliberately high level of oxygen de®ciency, the band at 7.6 eV remains with a slight shift to lower energy. For samples treated in hydrogen at 800°C, with a low level de®ciency, the

A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

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Fig. 3. Comparison of the UV band (4.4 eV) excitation spectra with the absorption spectra in silica samples (KS-4V) possessing di€erent levels of oxygen de®cit. 1: N1; 2: N4, 3: N3. Optical spectra ± a: absorption, 290 K; b: PL at 4.4 eV excitation, normalized, 90 K.

Fig. 2. In¯uence of the hydrogen treatment on the photoluminescence (a, dots ± spectra before (Sample N1) and line ± after treatment in hydrogen, (Sample N5)) and optical absorption (b, Samples N1,N5,N8, N9) of KS-4V silica. T ˆ 80 K.

intensity of the band at 7.6 eV diminished, in agreement with the results of [3]. The two luminescence bands at 2.7 (blue) and 4.4 eV (UV), which are proportional to the oxygen de®cit, are excited via the absorption band at 5 eV. The same bands also are excited via the broad band at 7.6 eV (Fig. 1). The excitation spectrum of luminescence does not correspond to the position of the 7.6 eV band and its position is changed with the change of absorption band intensity (Fig. 3) and temperature (Fig. 1). Therefore, the maximum of the excitation band is not a real peak. There is a good correspondence between the absorption at

5 eV and the excitation band for the 4.4 eV UV luminescence as well as a correspondence of the luminescence intensity to the level of oxygen de®cit. The treatment in hydrogen atmosphere leads to the disappearance of the excitation band at 5 eV for both luminescence bands (blue and UV), see Fig. 2. However, both bands can be excited within the range of the 7.6 eV band, and therefore, in spite of the disappearance of the 5 eV band, the corresponding luminescence bands remain, but with a changed and inverted intensity relationship (Fig. 2). The blue band possesses higher intensity at low temperature than the UV band. Therefore, the defects did not disappear but just changed their structure. In Fig. 3 luminescence excitation spectra with normalized intensities are compared for three samples with di€erent absorption intensities of the 7.6 eV band. Evidently the position of the high energy excitation band is dependent on the level of oxygen de®ciency. The lower the level of de®ciency the higher is the energy position of the excitation band maximum. Therefore, some kind of concentration quenching should occur. This correlates with the e€ect of shifting up the right side of this band by cooling (Fig. 1). Therefore, a thermal as well as a concentration quenching of luminescence

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A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

additional to inner center quenching takes place. When changing excitation energy from 6.5 to 7.6 eV, the intensity of the blue band is increased, as the UV band diminishes, see Fig. 1. Cooling to liquid nitrogen temperature (LNT) leads to a signi®cant decrease of the blue band when excited at 5 eV, whereas in Samples N2 an additional excitation band at 6.2 eV appears for the blue emission, Fig. 4. The analogous band appears after treatment in hydrogen (Sample N5, Fig. 2). On the other hand, in Sample N2 there is no trace of hydrogen. In Fig. 5(a) the decay curves of the blue photoluminescence are compared for di€erent excitation energies and temperatures. An exponential decay is obtained for room temperature with a mean decay time s equal to 10.5 ms. These data are similar for non-treated samples. In Sample N2 the decay becomes non-exponential for excitation at 6.2±6.7 eV at low temperature. The non-exponential decay kinetics can be characterized by the duration of the decay and/or by the exponential approximation at the end of the decay. Thus the duration of decay kinetics increases with decrease of the temperature corresponding to the increase of PL intensity for additional excitation at

Fig. 4. Comparison of the PLE and PL spectra in silica samples (KS-4V) at temperatures 80 and 290 K. Sample N2. Luminescence excitation. 1: 5.1 eV, 290 K; 2: 6.2 eV, 80 K; 3: 6.8 eV, 80 K; 4-X-ray, 290 K.

Fig. 5. KS-4V with excess silicon, luminescence decay kinetics (a) and temperature dependence (b) of the 2.7 eV blue band PL. (a) Sample N2 PL decay kinetics curves, excitation energies. 1: 5, 2: 6.7, 3: 7.7 eV, 1,2,3: 290 K, 4: 6.2, 5: 6.7 eV, and 4,5: 80 K. (b) Sample N2, PL at 2.7 eV (6.2 eV excitation): 1 ± intensity; 2 ± decay duration.

6.2 eV (Sample N2, Fig. 5(b)). At RT, the duration of PL excited in Sample N2 at 6.2 eV is only few microseconds. At 60±80 K it increases to 4 ms with a probable limit of about 10 ms. Excitation with di€erent photon energies produces a blue band with a similar shape even in di€erent samples, see Fig. 4. The e€ect of the 6.2 eV excitation at LNT was not observed in the Samples N1 and N3, however, it was observed in Sample N4, but with a signi®cantly lower level of luminescence yield. It was found that luminescence centers in Sample N5 are produced by continuous irradiation. Indeed, when excitation was done by short nanosecond pulses of low intensity at any temperatures, we were not able to detect luminescence and therefore to measure decay kinetics in Sample N5 annealed at 330 K. After continuous irradiation at 6 eV of the Sample N5 we were able to measure decay kinetics of PL with the same low

A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

intensity pulses. The decay kinetics have a time constant of about 10 ls at 220 K and about 50 ls at 100 K. The center's production is more e€ective at 60 K than at 290 K (Fig. 6) and the temperature dependence possesses a hysteresis. That means that produced centers can be annealed at higher temperatures. In other words, treatment in hydrogen kills the ability of defects to give luminescence, and phototreatment reveals this ability. However, center production takes place in the range 60±290 K, and therefore, there is a distribution of non-equivalent centers with di€erent thermal stability. The growth kinetics were obtained for the production of centers (Fig. 6, upper insertion). Ceasing of irradiation provides an afterglow in the range of seconds. The kinetics of the afterglow follow a power law (tÿ1:3 ) dependence and are independent of the temperature in our measurement range. As it was mentioned above the annealing of the formed luminescence center requires a higher temperature, the afterglow can show tunneling recombinations of defects pro-

55

duced by irradiation. The treatment in hydrogen demonstrates that luminescence centers can be a€ected by impurities. On the other hand, these centers retain their main properties, e.g. their luminescence band position, however, the process of excitation is changed. The UV band possesses a very fast decay (measured for Sample N1). The measurement was performed with 6 ns pulses on half height. The decay time was estimated by convolution of the measured luminescence decay curve with a shape for the excitation pulse. That determines the level of errors. The mean decay time was equal to 4.5 ‹ 0.5 ns for 5 eV excitation and 3 ‹ 1 ns for 7.7 eV excitation. 3.2. Radiation e€ects The end of X-ray irradiation of silica is followed by a long term afterglow, see Fig. 7. This points to recombination processes. The afterglow was investigated at di€erent temperatures. A

Fig. 6. In¯uence of the hydrogen treatment on kinetics of photoluminescence of KS-4V silica (Sample N5; a non-irradiated sample does not provide luminescence under short ns pulses excitation).

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A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

Fig. 7. X-ray excited afterglow kinetics in silica samples (KS4V) with a high level of oxygen de®cit (double logarithmic plot). The remaining decay after cooling in curve 2 points at the existence of tunneling recombination processes. Sample N1, 1: 80 K, 2: 240 K, then cooling to 80 K. Straight line tÿ0:7 approximation.

special experiment was done during the regime of afterglow: the sample was cooled down, Fig. 7, curve 2. Then the intensity of afterglow strongly decreases but not to zero. Moreover, the afterglow continues with kinetics according to a power law tÿ0:7 . This, together with data in Fig. 7, indicate the existence of tunneling recombination processes. In Fig. 8(a) thermally stimulated luminescence (TSL) curves are presented. Obviously the TSL curves are practically similar for all samples of KS4V silica. The intensity of TSL is proportional to the oxygen de®ciency. In spite of the rich quantity of TSL peaks the luminescence bands are the same in TSL as in X-ray excited luminescence, Fig. 8 (insert). There is the blue band at 2.7 eV as well as an additional band at 3.4 eV and a broad UV band at 4.4 eV. The treatment in hydrogen a€ects the TSL peaks in the way shown in Fig. 8(b). The low temperature peaks at 125 and 170 K remain whereas the 245 and 400 K peaks are strongly diminished. The existence of TSL, together with afterglow, indicate the existence of photostimulated luminescence (PSL) for the blue band. The corresponding excitation spectrum is presented in Fig. 9. This spectrum also corresponds well to the induced absorption spectrum, induced by means of both X-ray and UV (7.7 eV) irradiation. Gener-

Fig. 8. (a) Comparison of di€erent kinds of luminescence; TSL of silica (KS-4V) with di€erent levels of oxygen de®cit samples N1±N4 (1±4) after X-ray excitation at 80 K; insert: X-ray excited luminescence, TSL of 100 K and PL at 290 K. (b) TSL curves after photoexcitation and comparison of hydrogen treatment on TSL.

ally, bands at 4.5, 5.2 and 5.7 eV are observed. In the induced absorption spectra there are broad bands in the low energy range. Furthermore, the induced absorption bands are similar for low temperatures (80 K) and RT. Radiation processes do not occur only under Xray irradiation but also under photo illumination. However, there are some di€erences in the TSL peak position at low temperatures. The peaks at 125 and 170 K, the main peaks after X-ray irradiation, are not developed after 7.7 eV UV irradiation. Instead a broad peak at 200 K is observed. TSL peaks at 200 K are composed of many peaks, see Fig. 10. The peak at 400 K has the same position after both kinds of excitations, X-ray and UV. The excitation spectrum for TSL is presented in Fig. 11, curve 2. The kinetics of the TSL intensity

A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

Fig. 9. X-ray induced optical absorption (1, 2, 3) and the blue luminescence excitation spectrum in a photostimulated bleaching process in silica samples KS-4V with a high level of oxygen de®cit. Sample N1. 1, 2, 4: X-ray induced absorption spectra. 1: 80 K, 2: absorption after heating to 250 K, 3: excitation of photostimulated luminescence, 2.7 eV band, 80 K, 4: 290 K.

57

Fig. 11. KS-4V with excess Si (Sample N1): 1 ± absorption spectrum; 2 ± excitation spectrum of TSL peak at 400 K; 3 ± excitation spectrum of photoelectric response.

Fig. 12. Photo excitation kinetics of the TSL peak at 420 K and of the ESR (E0 ) intensity, in KS-4V silica (N1) at T ˆ 290 K. Fig. 10. KS-4V with excess Si (Sample N1). TSL excited by 7.7 eV photons at di€erent temperatures during 45 min. Excitation temperature corresponds to the beginning of each curve. Different symbols are chosen that to separate the curves.

growth is almost linear with the excitation dose used in this work (excitation through a vacuum monochromator); it is a little bit sublinear for the lowest excitation photon energies. The eciency of the TSL 420 K peak creation is increasing with excitation temperature, see Fig. 10. X-ray and photo irradiation are followed by ESR signal appearance, probably, of the E0 center. The intensity of the TSL peak at 400 K, excited at 290 K with white light of a deuterium lamp in

vacuum, grows sublinear over an excitation time of 2 h, Fig. 12. Under the same conditions we get an ESR signal in the E0 center range, see insert of Fig. 12. The intensity of this ESR signal saturates in a few minutes of excitation with VUV light. Therefore, there is no correlation to the TSL peak at 400 K whose intensity is still growing, even after several hours. The ESR signal disappears after heating. 3.3. Photoelectric properties Illumination of the samples in the range of the 7.6 eV absorption band yields a photoelectric

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A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

response. The illumination without an external ®eld leads to a negative charge current pulse. Therefore, a photoelectric polarization takes place, and the Dember e€ect due to the better mobility of electrons is observed. The following pulses of light produce currents of smaller intensities. Therefore, we may conclude that space charge limited currents due to charge trapping are measured. Low energy photons provide pulses of long duration. By contrast, higher energy photons rapidly create a space charge, Fig. 13. The external ®eld ampli®es the photoelectric response intensity. An e€ect of changing the photoelectric response polarity was observed with 7.5 eV excitation. That can be explained as an in¯uence not only of photoelectric polarization but of photoconductivity too. Indeed the e€ect of switching on±o€ the electric ®eld leads to pulses of di€erent polarities in the outer circuit. If the illumination under ®eld is started with hm > 7.6 eV, the corresponding pulses possess a polarity opposite to the ®eld similar to removing the ®eld. However, illumination with hm < 7.6 eV provides pulses corresponding to the ®eld direction, Fig. 13. An explanation of this e€ect can be given by the di€erent penetration depths of the excitation light. High energy photons have small penetration depths with dense charge release at the front electrode. The ®eld between the electrode and the sample holder is diminished for a short time, resembling the e€ect of taking o€ the ®eld.

On the other hand, low energy photons penetrate deeper and the charge density is smaller. The electrometer registers a current in the direction of the ®eld. The intensity of the photoelectric response is asymmetric with respect to the ®eld direction. That should be due to the Dember e€ect and the di€erent ability of charge injection for electrons and holes. The photoelectric response threshold has been found at 6.2 eV, Fig. 11, curve 3. 4. Discussion 4.1. Interpretation of photoconductivity and TSL excitation spectra We observed photoconductivity in the range of the 7.6 eV absorption band in silica samples with high levels of oxygen de®ciency. It is in a good agreement with TSL excitation in the same energy range (Fig. 11). In previous investigations the photoconductivity in the range of 7.6 eV band was not detected in samples with low levels of oxygen de®ciency [4]. The photoconductivity signal was, possibly, too low to be detected. Photoconductivity and TSL excitation are started with the beginning of the absorption band (6.4 eV, Fig. 11). Therefore, a subband of electronic states appears in oxygen de®cient samples. The lack of photoconductivity in samples with low de®ciency supports this explanation, because the intensity of the photoelectric response is proportional to the level of de®ciency. This opinion is in agreement with the data of complicated processes of luminescence excitation in this absorption band. 4.2. Excitation of the twofold-coordinated silicon center luminescence in the 7.6 eV absorption band

Fig. 13. Shape and sign of photoelectric response of a KS-4V silica sample at 290 K.

Two luminescence bands are excited via the 7.6 eV band, the blue one at 2.7 eV and the UV band at 4.4 eV. The photoluminescence duration is in the range of ms for the blue band and of ns for the UV band when the excitation is performed over the 7.6 eV band with short (few ns) light pulses. Therefore, the nature of these bands is based on the twofold-coordinated silicon center, well stud-

A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

ied in previous papers [15,16]. However, the process of excitation is di€erent from that of inner center excitation. Indeed in pure innercenter processes, when the excitation was done in the range of the 5 eV band (a singlet±singlet transition in the twofold-coordinated silicon center [16]), the luminescence intensity is proportional to the level of de®ciency and its decay kinetics are exponential with a mean decay time of 10.5 ms (Fig. 5(a), curve 1) for the blue band and 4.5 ns for the UV band in our samples with a deliberately high level of oxygen de®ciency. That is in good agreement with the data [15±18] for normal silica samples made under usual conditions. For the case of excitation in the range of the 7.6 eV band the excitation spectra are di€erent for di€erent samples (Figs. 1 and 3) showing complicated radiation and non-radiation processes. The duration of the blue luminescence increases to 13 ms for excitation at 7.6 eV and RT (Fig. 5(a), curve 3) and becomes non-exponential at low temperatures (Fig. 5(a), curves 4, 5). The decay of the UV band is about 3 ns [17,18] and is even faster than for 5 eV excitation. These data show an interaction of the excited states with their surroundings. Therefore, in spite of similar luminescence band positions, the emitting centers should be di€erent from those of lone twofold-coordinated silicon centers. Such a di€erence can be understood by analyzing speci®c properties of Samples N2 and N5, where the blue luminescence is more a€ected by a special sample preparation (N2) and in Sample N5 by treatment in hydrogen. In both these cases there is an excitation band at 6 eV where the intensity increases with cooling. The di€erence is that in N2 permanent centers exist (Fig. 5) but in N5 only a transient center forms (Fig. 6). 4.3. Instability of luminescence intensity under continuous excitation Under laser excitation with the second harmonic (244 nm) of an argon laser (90 mW), the hydrogen treated Sample N5 shows a decrease in blue luminescence intensity during a 30 min irradiation period by a factor of 2, see Fig. 14(b), curve 2. In Sample N1 (Fig. 14 (b), curve 1) such a laser excitation gives a sinusoidal variation of

59

Fig. 14. PL kinetics during continuous excitation of KS-4V with excess silicon: (a) PL at 4.4 eV changes with time under hydrogen light source excitation at 90 K. Samples ± 1: N3; 2: N1; 3: N4. (b) 2.7 eV PL kinetics under laser excitation (90 mW, 244 nm) and 293 K. Samples ± 1: N1; 2: N5.

intensity. We did not get such an e€ect under low intensity excitation in the range of 5 eV, but we did get analogous intensity variations under low intensity excitation in the range of the 7.6 eV band. The explanation of these experiments is based on the properties of the long living triplet state of the twofold-coordinated silicon center. In the case of hydrogen-free samples excited by a laser, the centers excited to the triplet state interact with some nearest interstitial atoms and molecules. The latter ones move to the excited center and form some complexes and bonds with them, diminishing the emitting centers concentration (decreasing part of the kinetics). But these absorbed species can be removed from the excited states thus increasing the luminescence intensity. In such a way the transient nature of luminescence in hydrogen treated samples can be explained. When hydrogen is removed

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from the defect the luminescence can be detected, but the center still feels hydrogen in the nearest position, and therefore, the blue luminescence properties di€er from the lone center case. Similarly to the hydrogen treated sample, the excitation via the 7.6 eV band in hydrogen-free samples reveals complex geometric structure and energy levels due to the glass manufacturing process. Some part of the complex can be removed in the excited state. In Ref. [4], during continuous excitation at 7.6 eV, even an increase of concentration of the lone twofold-coordinated silicon centers was measured by means of the UV band intensity increase excited at 5 eV (80 K). Therefore, the range of the 7.6 eV band cannot be explained just as highest excited states of a single luminescence center. Indeed, during continuous excitation in the range of the 7.6 eV band by means of a hydrogen discharge source the UV band (also the blue band) shows a certain instability of luminescence intensity response. This is observed for actually studied samples with high oxygen de®ciency (Fig. 14(a), curves 1, 2) as well as for samples studied earlier and published in Refs. [2,4] (Fig. 14(a), curve 3). For samples with a low level of oxygen de®ciency complicated UV band kinetics were obtained, sometimes with a decrease at the beginning followed by an increase. Sometimes a decrease was obtained after a long time of growth. For studied samples an analogous turnaround kinetics is observed (Fig. 14). Analogous kinetics were observed when studying photoelectron emission from interface states in system SiO2 ±Si [21]. The photoelectron emission yield spectrum possesses a band at 7.6 eV. The changes in photoelectron emission yield under continuous excitation at 7.6 eV were explained as thermally reversible transformations of emitting states. Therefore, we may conclude that under 7.6 eV excitation a complicated process of transient and permanent changes of the luminescence center concentration occurs due to photochemical dissociation and recombination in the structural elements responsible for the 7.6 eV absorption band. Under photons in the range of 7.6 eV the fragments of photochemical reconstruction may be removed to di€erent distances and stabilized in potential wells of di€erent depths. On the other

hand, these fragments can be released from these wells and di€use back to the initial complex structure. In this way, the concentration of luminescence centers can be changed and we observe the periodic variation in intensity during continuous excitation. We have to assume that adhesive species at the structural centers like hydrogen, chlorine or something other will give preference to non-radiative transitions versus radiative ones. Therefore, we may state that the absorption band at 7.6 eV belongs to a complex quasi-molecular system in the silica network. 4.4. Relation between the blue (2.7 eV) band and the UV (4.4 eV) band The 7.6 eV excitation not only creates light emitting centers but also permanent radiation defects. These are detected by measuring TSL curves (Figs. 8 and 10) and ESR (Fig. 12). The resulting blue luminescence in TSL spectra shows that radiative recombination in all these cases is associated with the twofold-coordinated silicon triplet± singlet transition. The relation between the blue 2.7 eV band and the 4.4 eV UV band depends on the recombination processes (Fig. 4, curve 4) on the one hand and on the inner center relaxation on the other hand (Fig. 8(a), insert). It has long been known, that the UV band is almost non-existent when the exciting photon energy is higher than 9 eV, [2,20,22]. The UV band in recombination processes is not well developed. We can understand its causes on example of Samples N2 and N4. The analogous relation between these two bands is observed in PL at low temperatures with an excitation at 6 eV for the samples N2 and N4. This e€ect can be explained as a complex of twofold-coordinated silicon probably with chlorine in KS-4V samples. The UV band in complexes is suppressed and, therefore, we can conclude that the recombination process takes place eciently with complexes and less eciently with lone twofold-coordinated silicon. That may be deduced from the relation between the blue and the UV band under di€erent ionizing excitation. If the interaction would occur only with lone centers, then the UV band should be the most intense one. For example, under high energy electron beam pulses

A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

(20 ns) the UV band is the main one in the luminescence spectrum of oxygen de®cient samples. This was explained in Ref. [9,10] as an impact excitation which corresponds to resonance secondary electron excitation via the transition band at 5 eV of the lone twofold-coordinated silicon centers. When thermalized electron±holes recombine on complexes, the triplet excited states of the twofold-coordinated silicon correspond to the antibonding states in these complexes and we observe mainly blue luminescence. 4.5. Radiation processes and induced defect nature Recombination of permanent induced defects can be done not only by thermal stimulation but by photobleaching of radiation induced color centers. The observed photostimulated luminescence is of the same nature as in TSL and XRL spectra. Many peaks produced by photostimulation indicate a number of induced color centers. Their photobleaching yields in the same blue luminescence of the twofold-coordinated silicon. Therefore, we may conclude that a complex radiation reconstruction of oxygen de®cient lattice imperfections of silica takes place. The necessity of long term excitation for the photocreation of TSL peaks and the higher eciency of that with increasing temperatures, Fig. 10, as well as the periodic variation of intensity under continuous 7.6 eV irradiation points to some di€usion processes that involve separated parts from the complex. Oxygen de®ciency provides not only hole traps, but also electron traps, leading to an increase of the self-trapped hole centers in regions of the `normal' glass network [14]. Also a charge trapping of both polarities at di€erent parts of structural elements responsible for the 7.6 eV absorption takes place. Tunneling recombination occurs for some separated charge pairs not placed far away from each other, Fig. 7. The rich quantity of TSL peaks, on one hand, and the same luminescence content in the peak structure, on the other hand, demonstrate that many types of defect separations should exist. The intensity of TSL is proportional to the relative concentration of these defects. Thus, the photoreconstruction related to the 7.6 eV absorption

61

band is very complicated as it was also obtained previously for silica with low level de®ciency [4]. At least four TSL peaks, a complex photoinduced absorption spectrum and additionally an ESR signal, which does not correlate in growth with TSL (Fig. 12), do underline that. The nature for all photoinduced defects is still unknown, however, recombination processes are always involved in the blue luminescence of the twofold-coordinated silicon center. Another well-known defect is the E0 -center. Its signal appears under photoreconstruction in accordance with previous studies [3,6]. However, the E0 -center concentration is saturated much faster than the TSL intensity. Therefore, E0 -center precursors may be related only partly to the defects studied, and so, we cannot agree with the assumption in Refs. [3,6] that the band at 7.6 eV is related to the Si±Si bond only. Indeed, IR measurements of hydrogen treated silica samples, performed in Ref. [13], show that the concentrations of Si±H and Si±O±H are approximately equal, demonstrating that the main reaction is Si± O bond breaking BSi±O±SiB ‡ H2 ! BSi±H ‡ BSi±O±H:

…1†

The intensity of the 7.6 eV band was not changed strongly after hydrogen treatment of the samples with a high level of oxygen de®ciency (Fig. 2). Therefore, we cannot con®rm the proposition that the 7.6 eV band is due to point defects like the Si±Si bond in a normal tetrahedral network of silica. It was found in Ref. [14] that the de®cit of oxygen apparently increases the number of electron traps, stabilizing a larger number of self-trapped holes centers (STH) in the continuous defect-free silica network, more than is observed in stoichiometric glass. Therefore, X-ray excitation produces TSL peaks at 125 and 175 K corresponding to the annealing temperatures of two STH types detected and distinguished by electron spin resonance (ESR) [14]. Photoexcitation of oxygen-de®cient glass produces other TSL peaks at 105 and 200 K, presently attributed to hole trapping at perturbed sites in silicon-rich regions (Fig. 8(b), curve 3). Therefore, activation energies corresponding to peaks at 105 and 200 K are related to

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subband electronic states caused by complex defects and are not direct related to the valence band of silica. Then the released hole cannot reach Si± O±Si sites because they recombine with nearby trapped electrons. The complex induced absorption also indicates a creation of di€erent products as a consequence of defect reconstruction responsible for the 7.6 eV absorption band. The fact that we did not observe red luminescence either after X-ray irradiation or in PL of the KS-4V samples in pure state or with excess silicon, may allow two principal conclusions. First, the complex defect related to the 7.6 eV band cannot be transformed to non-bridging oxygen hole centers (NBOHC). Secondly, the studied samples are suciently free of hydrogen and alkali ions. Therefore, the induced absorption band at 4.5 eV is related to another center, not to the NBOHC (Fig. 9). The induced band at 5.7 eV, very probably, can be related to the E0 center whereas the induced band at 5 eV may be associated with the lone twofold-coordinated silicon center as a part transformed by light to localized states. All these bands can be photobleached easily at low temperatures (LNT) with the appearance of photostimulated luminescence where the main band is the triplet±singlet transition blue band of the twofold-coordinated silicon. At RT the photobleaching with corresponding photostimulated luminescence could not be observed; however, the positions of induced bands are very similar to those at low temperatures. That may be explained in terms of metastable (low temperatures) and stable (higher temperatures) modi®cations of the same radiation induced defect. 4.6. Model for the 7.6 eV absorption band nature Let us estimate the concentration of structural elements responsible for the 7.6 eV absorption band. In a ®rst step we determine the concentration of lone twofold-coordinated silicon centers in very high oxygen de®cient samples with about 0.015 wt% excess silicon. The absorption band at 5 eV shows an intensity of about 2 cmÿ1 in such samples (Fig. 1(a), curves 1, 10 ). This so-called B2 band is due to a singlet±singlet allowed transition [15,16]. Taking into account the 4.4 eV band with

a lifetime of 4 ns [17,18] we can estimate the transition probability or oscillator strength and then use the Smakula formula [19] to formulate the center concentration. The lifetime s of an excited state is related to the oscillator strength f by the following expression given in Ref. [19]: fs ˆ

ˆ

c 1 k2 2e2 …2p†2 n…Eeff =E†2  2 …4  10ÿ9 s† k=5000A n…Eeff =E†

2

;

…2†

 and n is the where the wavelength k is given in A refraction index. The energy relation is approximated by the Onsager expression Eeff /E0 ˆ 3n2 / (2n2 + 1). Then Smakula's formula for an absorption band can be written [19]  2 Eeff f ˆ Abakamax D; …3† N E where N is the number of absorbing centers per cm3 , the factor A ˆ 0.82 ´ 1017 /cm3 , a is a factor determined by the lineshape (usually 1 6 a 6 p=2), kamax is the maximum absorption coecient, and D is the full width at half maximum intensity (FWHM) of the band. With an oscillator strength estimated at about 0.14 [23], the concentration of the corresponding centers is N < 1018 cmÿ3 . This approximately corresponds to 0.01 wt% excess silicon. The 7.6 eV band intensity is more than 100 cmÿ1 for the same concentration 0.01 wt% of excess silicon. For this band we can make again an estimation of the center concentration by means of Smakula's formula. With the same value of oscillator strength, because the UV band at 4.4 eV is also excited, we get a very high center concentration of about 1020 cmÿ3 . However, the mean decay time of the 4.4 eV UV band excited at 7.6 eV is only about 2 ns, changing somewhat in di€erent samples [17,18] and temperature and even being non-exponential [24]. We think that this is due to peculiarities of the center energetic structure. Even using this value of mean lifetime, the conclusion does not change the situation. The estimated concentration of structural elements responsible for the band at 7.6 eV strongly exceeds the

A.N. Trukhin, H.-J. Fitting / Journal of Non-Crystalline Solids 248 (1999) 49±64

deliberately made oxygen de®cit. The high center concentration corresponds to the situation when excess silicon perturbs states of the host material. Therefore, this band covers the bands of highest electronic transitions of the twofold-coordinated silicon. So, the 7.6 eV band may be ascribed to structural elements a€ecting many atoms in the surrounding as was predicted already in Ref. [4]. This situation is analogous to that observed in germanium dioxide, as well as in alkali silicates, alkali germanate and phosphate glasses. There, all the phenomena mentioned above are ascribed to the behavior of localized states [11]. The di€erence is that all the phenomena observed there were obtained not only in reduced glasses but in glasses melted under normal and even oxidizing conditions. In silica, the band at 7.6 eV appears only in oxygen de®cient samples with the properties of localized states. The ability of a glass to exhibit photoinduced transformations is based on its relaxation properties. They are related to each other as wood ¯oating in a river, when, from time to time, a jam appears. Normal ¯oating corresponds to the crystallization process. Analogous to ¯oating in a river the jam is related, as a rule, to one log arresting the whole system. This log, in the language of glass specialists, corresponds to the so-called strained bonds. This jam is a localized state; but strained bonds may correspond ®nally also to point defects. In such a way we imagine the structure related to the 7.6 eV absorption band. The redox conditions change the quantity of strained bonds and increase the concentration of localized states and the absorption level of the exciting light. They work as conditions generally increasing the photosensitivity. Photoexcitation with di€erent energies acts on these strained bonds and induces changes within the localized states. Multiple step reactions allow low energy photon induced reconstruction. Probably, excited states related to spread wavefunctions and involving many point defects play the main role re¯ecting the complex and controversial phenomena observed in experiment. But the use of high intensity laser excitation with high photon energy such as 7.9 eV makes the phenomena still more complex.

63

5. Conclusions The nature of the 7.6 eV absorption band cannot be explained as a single point defect in as-received non-irradiated silica with oxygen de®ciency. Its nature is explained as a manifestation of localized states modi®ed by oxygen de®cient defects. Many experimental data and their analysis indicate: 1. The intensity of the 7.6 eV band corresponds to the concentration of defects strongly exceeding that of deliberately created oxygen de®cit. This band covers the highest excited states of the twofold-coordinated silicon center appearing in silica under oxygen de®ciency. 2. Radiation processes on the atomic structure responsible for the 7.6 eV absorption band lead to the photoelectric response, to many peaks of the thermostimulated luminescence (TSL), the ESR signal of the E0 center as well as to many bands of irradiation induced absorption. This indicates that the atomic structure responsible for the band at 7.6 eV possesses a sub-band of electronic states in addition to the conduction and valence bands of the silica structure, and their excitation leads to a photoinduced transformation of the primary atomic structure induced by oxygen de®ciency. The intensities of all TSL peaks are proportional to the oxygen de®cit. Many TSL peaks as well as many bands of induced absorption point to di€erent kinds of defects appearing as a consequence of the phototransformation. The intensity of TSL re¯ects the recombining defect concentration. However, this recombination leads to the appearance of luminescence bands (blue and UV) of the twofold-coordinated silicon. Also this center can be excited via the 7.6 eV absorption band in a fast process of photochemical transformation and that causes modi®cations of the kinetic parameters with respect to those of the lone twofold-coordinated silicon center. Therefore, the twofold-coordinated silicon is an essential part, maybe only transient, of the atomic structure responsible for the 7.6 eV band. 3. The treatment in hydrogen leads to increase in complexity of oxygen de®cient related defects.

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Hydrogen kills the ability of defects to give luminescence and phototreatment reveals this ability.

Acknowledgements This work was supported by the grants 96.0665 of the Scienti®c Society of Latvia and the German DAAD support 322-OP Rostock-Riga-Universities, subtitle `Dielectric Layers'. The authors are indebted to Professor E. Radzhabov for decay kinetics measurements in ns range and to Professors V. Radzig and L. Skuja for fruitful discussion.

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