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Paramagnetic defects induced by ion implantation in oxide glasses
Journal of Non-Crystalline Solids 210 Ž1997. 101–118
Paramagnetic defects induced by ion implantation in oxide glasses L.D. Bogomolova a,) , V.A. Jachkin a , S.A. Prushinsky a , S.A. Dmitriev b, S.V. Stefanovsky b, Yu.G. Teplyakov b, F. Caccavale c , E. Cattaruzza c , R. Bertoncello d , F. Trivillin d a
d
Institute of Nuclear Physics, Moscow State UniÕersity, 119899 Moscow, Russia b State Corporation Radon, RostoÕsky per. 2 r 14, 119121 Moscow, Russia c INFM, Dipartimento di Fisica, Õia Marzolo 8, 35131 Padua, Italy Dipartimento di Chimica Organica, Metallorganica ed Analitica, Õia Loredan 4, 35131 Padua, Italy Received 25 July 1995; revised 24 July 1996
Abstract Radiation defects induced by ion bombardment of multicomponent oxide glasses of five compositions Žphosphates and borosilicates. were investigated by means of electron paramagnetic resonance ŽEPR.. The samples were implanted with Nq, Oq, Arq, Mnq, Cuq and Pbq ions at energy E s 150 keV at three different doses between 3 = 10 15 and 10 17 ionsrcm2. The broad anisotropic EPR spectra with principal g-values answering the relationship g z ) g y ) g x ; g e Ž g e is g-factor of free electron. were observed for the samples of all five compositions. The g-values depend on glass composition. For example, g z ranges from 2.016 to 2.057. Computer simulation shows that the spectra of many samples are superpositions of two spectra with g-values answering the mentioned relationship. These spectra are attributed to molecular Oy 2 ions weakly coupled with glass network. In some samples narrow almost symmetric lines with g s 2.0025 " 0.0005 were observed. The possible radiation defects responsible for this signal are discussed.
1. Introduction Ion implanted glasses have been the object of considerable attention in the past decade w1x. This attention is due to practicable interest connected with modification of physical and chemical properties of near-surface layers of glasses by ion implantation that may have applications in integrated optics w2x. The scientific interest to ion implanted glasses is )
Corresponding author. Fax: q7-095 939 0896; e-mail: [email protected].
associated with the study of the effects of interaction between heavy charged particles and insulators that are a priori amorphous. In addition, heavy ion Žfor example, Pb. implantation is a convenient laboratory technique for producing damage in materials of interest for radioactive waste encapsulation w3x. Among different experimental techniques used for the study of ion-implanted glasses is electron paramagnetic resonance ŽEPR. spectroscopy giving information about structural paramagnetic defects induced by ion implantation. EPR of ion-implanted vitreous silica has been
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 9 1 - 1
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investigated in many works. The EX-center and peroxyradical EPR spectra analogous to those reported for g-irradiated silica glasses w4,5x have been observed for amorphous silica implanted with different ions Žfor example, w6,7x.. The EPR signals from EX-type center, that have not been found in the spectra of g-irradiated glasses w8x, were reported for amorphous silica implanted by some transition metals w9,10x and Siq ions w11x. The EPR spectra of ion-implanted multicomponent glasses have not been extensively studied, to our knowledge. Two types of EPR spectra associated with radiation-induced paramagnetic centers have been observed for Oq-implanted soda–lime silicate ŽCorning 015. glass w12x. One of them which was a broad anisotropic signal has been attributed to silicon–oxygen hole HC1 and HC2 centers earlier observed for g-irradiated silicate glasses w13x. The other which was a rather narrow almost symmetric line with a peak to peak width of D H ( 0.7 mT with g-values ranged between 2.0029 and 2.0064 depending on implantation dose, has been assigned to Oyinterstitials w12x. Approximately similar signals have been observed in commercial Pyrex and multicomponent silicate glasses implanted with different ions at various doses and energies w14x. But these signals have received an other interpretation w14,15x: the almost symmetric signal with g ranging from 2.0018 to 2.0034, denoted by S, has been attributed to the vacancy-type defect with one dangling silicon bond. The difference in the lineshapes, linewidth and spectral parameters of the S-center from the EX-center was interpreted in terms of lack of oxygen in the environment of the silicon atoms with an unpaired electron w15x. The other spectrum, denoted by A, was a broad anisotropic signal and has been attributed to interstitial oxygen species whose structure has not been proposed in w14x. Thus, almost identical EPR spectra observed in reports w12,14,15x have received different interpretations. In order to obtain more information on structural defects induced by ion bombardment in oxide glasses we performed EPR measurements on multicomponent phosphate, borophosphate and borosilicate glasses implanted with different ions. The present paper reports some results of these measurements. Complementary information on the implantation
effects and surface contamination was obtained by secondary ion mass spectrometry ŽSIMS. and X-ray photoelectron spectroscopy ŽXPS..
2. Experimental The molar chemical compositions of glasses used in this work are given in Table 1. The glasses were melted in high-frequency furnace in air at temperatures depending on glass composition. Then they were cast and annealed. The annealing was performed in an electric muffle furnace preheated to an annealing temperature, Ta . At this temperature the samples were held for periods from 30 to 60 min. The Ta value varied from 790 to 850 K depending on glass composition. Then the samples were cooled to room temperature at rate about 0.5 degrees per min. Polished plates were prepared. Sample size was 10 = 20 = 0.5 mm. These plates were irradiated with Nq, Oq, Arq, Mnq, Cuq and Pbq ions at energy E s 150 keV to nominal doses Ž D . equal to D1 s 3 = 10 15 ; D 2 s 2 = 10 16 and D 3 s 10 17 ionsrcm2 . We used 0.5 mArcm2 current density to minimize temperature effects during implantation. Temperatures of substrates during implantation measured by thermocouple were about 350 K. After implantation the slabs were crushed and used for EPR experiments. EPR measurements were performed using a modified spectrometer ŽRE-1306, Soviet model. operating at X-band frequency with 100 kHz modulation. The microwave power and amplitude of RF modulation are given in the text where it is necessary. Most spectra were obtained at 77 K and room temperature. Some measurements were made with variable temperature accessories in the range from 295 to 470 K. Isochronal anneal experiments were Table 1 Glass compositions Žin mol%. Name SiO 2 P2 O5 B 2 O 3 Al 2 O 3 Na 2 O 3 MgO CaO ZnO of glass 1 2 3 4 5
S-2 S-3 P-1 P-13 P-55
70 50 – – –
– – 36 65 60
10 5 – 10 3
– 5 20 10 10
20 20 44 – –
– 5 – 15 12
– 15 – – 10
– – – – 5
L.D. BogomoloÕa et al.r Journal of Non-Crystalline Solids 210 (1997) 101–118
performed with the same accessory. The samples were held at the anneal temperatures for a period of 5 min and returned to room temperature for EPR measurements. The absolute number of defects was determined with reference to hyperfine transition Ž1r2 l 1r2. of Mn2q ions in MgO or CuSO4 P 5H 2 O powder samples previously calibrated at the Soviet Institute of Standards ŽNIFTRI.. The reference samples were pressed in capsules of 1 = 1 = 3 mm and were placed in the tube together with measured powder samples.
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The quoted g-values were referenced to hyperfine lines of the Mn2q:MgO sample. Room temperature Q-band Ž; 37 GHz. EPR data were obtained for several samples ŽRE-1308 Soviet model.. SIMS measurements were performed using an ion microscope ŽCameca ims-4f. equipped with a normal-incidence electron gun used to compensate the charge build-up when profiling insulating samples. The profiles were obtained by 14.5 keV cesium bombardment and by negative ion detection. The
Fig. 1. EPR spectra of the ion-implanted borosilicate S-2 samples Žsee table.; Nq at dose D1 s 3 = 10 15 ionsrcm2 Ža, b. and at D 2 s 2 = 10 16 ionsrcm2 Žc.; Oq at dose D 2 s 2 = 10 16 ionsrcm2 Žd. and Arq at D 2 s 2 = 10 16 ionsrcm2 Že.. The spectra were measured at room temperature with microwave power P s 30 mW and the modulation amplitude A s 0.08 mT for the spectra b–e and with microwave power P s 0.6 mW for spectrum a.
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Fig. 2. EPR spectra of the borosilicate S-3 samples implanted with Nq Ža., Oq Žb. and Arq Žc. at dose D1 s 3=10 15 ionsrcm2 . The data were obtained at room temperature with microwave power P s 40 mW and As 0.08 mT. G are gain coefficients.
cesium beam current was 80 nA over a raster area of 125 = 125 mm2 and secondary ions emitted from a central circular Ž8 mm diameter. area were collected by the spectrometer. The measured ion yield of each species at any instant lies within "5% deviation. The erosion speed was evaluated by measuring the depth of the erosion crater at the end of each analysis by means of a profilometer ŽTencor Alpha Step.. The maximum deviation in measuring a certain depth by this instrument is of the order of 5%. Calibration of the implanted profiles was made by means of the measurement of the total retained doses using Rutherford backscattering spectrometry ŽRBS. analysis with a 2.2 MeV 4 Heq beam ŽQ s 1608.. XPS measurements were achieved with a spectrometer ŽPerkin–Elmer, F 5600ci. using nonmonochromatized MgK a radiation Ž1253.6 eV.. After 2.5 keV Arq sputtering, with an erosion speed of about 3 nmrmin, ‘survey’ spectra, i.e., a large Ž0 % 1150 eV. binding energy ŽBE. region with all the interesting photoelectron peaks, were recorded. The surface charging effect was corrected using as inter-
Fig. 3. The experimental Žsolid curve. and calculated Žcircles. EPR spectra of phosphate P-1 glass implanted with Arq ions at D1 s 3 = 10 15 ionsrcm2 . The spectrum was recorded at room temperature with microwave power P s 30 mW and A s 0.06 mT.
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nal reference the BE of the C 1s peak of hydrocarbon contamination. The atomic compositions were evaluated using sensitivity factors as provided by F V5.44A software.
3. Results Figs. 1–5 show typical EPR spectra observed in ion implanted samples studied in this work. The spectra depend on implantation dose, implants and glass composition. Fig. 1 shows the central part of EPR spectra of S-2 samples implanted with various ions at different doses. Two of them ŽFig. 1a, b. were obtained with separated microwave power. For a S-2 sample implanted with Nq ions at D 1 s 3 = 10 15 ionsrcm2 the spectrum consists of a broad peak A and a narrow almost symmetric signal ŽNS. with g s 2.0006 " 0.0005 and a peak to peak linewidth of D H s 0.15 " 0.05 mT whose intensity decreases with increasing microwave power ŽFig. 1a,b..
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The central part of the spectrum of a S-2 glass implanted with Nq ions at D 2 s 2 = 10 16 ionsrcm2 ŽFig. 1c. also contains the features of A and NS signals but a new line ŽS. appears. The intensity of the latter depends on implants and dose Žsee below.. This signal is well seen in Oq-implanted sample ŽFig. 1d.. The S-signal is also an almost symmetric line with the baseline crossing g-value oscillating between 2.0021 and 2.0032 Ž"0.0005. for different samples and a peak to peak linewidth of D H ranging from 0.3 to 0.4 mT. This component is saturated at higher microwave powers than the NS line. The spectra of the ion-implanted borosilicate S-3 samples are shown in Fig. 2. The features of the shoulder A and the S-signal are seen in the spectrum of Fig. 2b. However, an anisotropic spectrum Žlabeled A. dominates in the spectra of all the S-3 samples except for Oq-implanted glasses where the intensity of the S-signal is large and the A spectrum is a shoulder. The presence of two low-field peaks can be seen in Fig. 2a, c. The relative intensities of these peaks and the positions of their minima were differ-
Fig. 4. The EPR spectra of phosphate P-13 glass implanted with Oq Ža. and Nq Žb. ions at D 2 s 2 = 10 16 ionsrcm2 . The spectra were recorded at room temperature with microwave power P s 40 mW and A s 0.1 mT.
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Fig. 5. The high-field fragments of the experimental Žsolid curves. and calculated EPR spectra of the P-13 glass implanted with Nq at doses D1 s 3 = 10 15 ionsrcm2 Ža.; D 2 s 2 = 10 16 ionsrcm2 Žb. and D 3 s 10 17 ionsrcmy2 Žc..
L.D. BogomoloÕa et al.r Journal of Non-Crystalline Solids 210 (1997) 101–118
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Fig. 5 Žcontinued..
ent for different samples indicating that the A-signal is a superposition of at least two anisotropic spectra. In addition, the broad minima at g ( 1.995 Ža, c. are well resolved in the spectra of the S-3 samples implanted with Nq and Arq ions. The g-values shown in Figs. 1 and 2 are calculated only for the positions of peaks and baseline crossing. The parameters obtained by computer simulation of these spectra will be given below. Fig. 3 shows the EPR spectrum of a phosphate sample P-1 implanted with Arq at D1 s 3 = 10 15 cmy2 . It can be seen that the spectrum contains a shoulder of the A-line and the S-line just as in the case of some borosilicate glasses. Fig. 6 presents the isochronal annealing data for the S-signal and the A-shoulder in the spectrum of the P-1 glass implanted with Nq Ž D 2 s 2 = 10 16 cmy2 .. The temperature dependence of IA and IS was measured Ž IA and IS are shown in Fig. 3.. It is seen in Fig. 6 that this curve for the S-signal differs from the one for the A-shoulder. We assume that they belong to
distinct centers. As follows from Fig. 6 the S-center is thermally more stable than the A-center that begins to decay above 400 K. The relative intensity of
Fig. 6. Isochronal anneal curves: = – the S-center; e – A-centers.
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S- and A-spectra depends on the implantation dose and implants. The shape and linewidth of the S-signal at Q-band and X-band frequency are almost the same within experimental errors. They are independent of the temperature of observation in the range from 77 to 473 K. It should be noted that in the spectra of the S-2 and P-1 samples implanted with Nq ions at doses D 2 and D 3 the weak doublets with a separation of about 13 mT centered at g ( 2.0 were observed. They will be discussed elsewhere together with data for glasses of different compositions implanted with Nq-ions. Fig. 4 shows broadfield scan EPR spectra of a P-13 sample implanted with Nq and Oq ions at dose D 2 s 2 = 10 16 ionsrcmy 2 . These are broad anisotropic spectra Ždenoted by B.. The low field parts of these spectra contain shoulders with small peaks whose positions are approximately identical to those observed for the glasses of the same composition implanted with different ions. The high-field portions of both spectra differ in shape. Fig. 5, that reproduces the high-field portions of the spectra of P-13 samples implanted with Nq ions, illustrates their dependence on the total dose. Fig. 7a shows the dependence of the number of defects responsible for the S-signal in S-2 and P-1 samples on the implantation dose. The relative integrated intensities of S-signals Ž Jr . were estimated using the expression Jr s kIS Ž D H . 2 , where IS is the signal amplitude Žsee Fig. 3., D H is the peak to peak linewidth and k is the parameter determined by lineshape. A P-1 sample implanted with Oq ions at D 2 s 2 = 10 16 cmy2 was used as the reference for other samples. The absolute number of defects Ž N . responsible for the S-signal in this P-1 sample was calculated by comparing the integrated intensity of the S-signal with the one of the standard MgO:Mn2q sample. Values of concentrations of defects in other samples were calculated by multiplying the number N by the ratio JrrJo , where Jo is integrated intensity of S-signal in the mentioned Oq-implanted sample. The accuracy of measurements of relative number of defects is about 20%. The accuracy of measurements of absolute concentrations of defects is not better than "100 %. It can be seen that the concentration of S-defects slowly increases with in-
creasing implantation dose ŽFig. 7a. in comparison with the number of incident ions. As follows from Fig. 7b the number of defects per 1000 incident ions decreases with an increasing dose. Fig. 8a presents the dependence of the number of defects responsible for the S-line in S-2, S-3 and P-1 glasses on atomic mass of incident particles at D 2 s 2 = 10 16 cmy2 . Fig. 8b, c show the number of defects responsible for A- and B-spectra, respectively, as a function of atomic mass of implanted ions at D 2 . The absolute
Fig. 7. Ža. Dose dependence of the relative intensities produced by ion implantation in P-1 and S-2 samples. For the P-1 glass: v – Arq, B – Oq; for S-2 sample: = – Nq, ` – Oq and ^ – Arq. Žb. Dose dependence of the number of S-defects per 1000 implanted ions for the P-1 glass: ` – Arq, B – Oq; for S-2 sample: v – Oq, ^ – Arq and = – Nq. Data for Nq and Arq are given on right scale, for Oq-ions on left scale.
L.D. BogomoloÕa et al.r Journal of Non-Crystalline Solids 210 (1997) 101–118
content of defects of A-type was obtained for a S-3 sample implanted with Nq ions at D 2 . The integrated intensity of the A-spectrum was calculated by double numerical integration of this signal and was compared with the intensity of the signal for the standard CuSO4 P 5H 2 O sample. The procedure of an
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estimation of defect concentrations in the other sample is the same as described above. The reference signal of the B-spectra was the B-spectrum of a P-13 sample implanted with Oq ions D 2 s 2 = 10 16 cmy2 . The integrated intensity of this signal was calculated by double integration of
Fig. 8. Ža. The dependence of the number of defects Žper cmy2 . responsible for S-signals for the P-1 ŽI. and S-2 Ž`. glasses. Žb. The dependence of the number of defects Žper cmy2 . responsible for A-spectra on atomic mass of implanted ions at D 2 s 2 = 10 16 ionsrcm2 for the S-3 Ž`. and S-2 ŽI. glasses. Žc. The dependence of the number of defects Žper cmy2 . responsible for B-spectra for the P-13 ŽB. and P-55 Ž`. glasses at D 2 s 2 = 10 16 ionsrcm2 .
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the spectrum between K 1 and K 2 points ŽFig. 4. and was compared with integrated intensity of the signal for a standard sample CuSO4 P 5H 2 O. The number of defects in other samples was evaluated by means of the procedure described above for the S-defects. The relative concentrations of S- and A-components of EPR spectra in P-1, S-2 and S-3 samples were determined by means of computer lineshape simulation which was performed for all experimental spectra Žsee below.. These concentrations were estimated using the ratios of areas under each spectrum contributing to the good fit calculated spectra. As follows from Fig. 8 the concentration of all defects decreases with increasing atomic mass except for Oq-implanted samples where the number of centers responsible for S-signals is several times larger than in glasses implanted with other ions, i.e., the implantation of oxygen promotes the formation of these centers. Fig. 9a shows the SIMS depth profiles for the P-13 glass implanted with different ions. The depth
profile of Na in a P-1 sample implanted with Mnq is presented in Fig. 9b.
4. Discussion 4.1. Computer simulation of the spectra To calculate EPR spectra we used an IBM compatible PC with Intel 486-DX4-100 processor and Microsoft Excel 7.0 spreadsheet. The synthetic spectra were computed in the form SX Ž H . s
p
p r2
H0 H0
W Ž Q , w . FX Ž H , Q , w .
sin Q dQ d w ,
Ž 1.
where the orientation-dependent transition probability W ŽQ , w . can be expressed w16x by W ŽQ , w . s
1 2
gx gz
2
½ž / 5 g
q g y2
Ž 2.
and SX Ž H . is the first derivative of the EPR absorption spectrum. The corresponding resonance fields are calculated from resonance condition H Ž Q , w . s hnrg b ,
Ž 3.
where g 2 s g z2 cos 2Q q Ž g x2 cos 2w q g y2 sin2w . sin2Q .
Ž 4.
Functions F Ž H, Q , w . are first derivatives of individual absorption lines. The shapes and widths of the latter are due to ‘site to sites’ fluctuations of g-values for paramagnetic species in glasses. We have employed both Gaussian and Lorentzian shapes of individual lines. We used the dependence of widths of individual lines on the Q angle in the form D H Ž Q . s Ž D Hz2 cos 4Q q D H x2, y sin4Q .
Fig. 9. Ža. SIMS concentration depth profiles for the P-13 glass implanted with Nq, Mnq and Pbq at D 2 s 2=10 16 ionsrcm2 . Žb. SIMS depth profile of relative concentration of Na ions for the P-1 glass.
1r2
.
Ž 5.
It has been shown w17,18x that the Eq. Ž5. allows us to obtain good fit parameters of spin-Hamiltonian in the case of weak anisotropy the of g-factor and the absence of correlations between g-components. The summation was made for Q and w angles with steps of 18 and 38, respectively.
L.D. BogomoloÕa et al.r Journal of Non-Crystalline Solids 210 (1997) 101–118
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Fig. 10. Ža. The calculated A 1 and A 2 spectra contributing to A-spectrum of implanted the S-3 glasses. Žb. The experimental Žsolid curve. and calculated Žcircles. spectrum of the S-3 glass implanted with Arq Žat D1 s 3 = 10 15 ionsrcm2 . with ratio of intensities a s A 2 rA 1 s 0.7.
L.D. BogomoloÕa et al.r Journal of Non-Crystalline Solids 210 (1997) 101–118
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Table 2 The g-values and widths of individual lines of spectra of the A- and B-type contributing to EPR spectra of ion-implanted glasses Type of spectra
Name of glass
Implant
D Žionsrcm2 .
gz
gx
gy
D Hz ŽmT.
D Hy ŽmT.
D Hx ŽmT.
A1
S-3
Arq
3 = 10 15
1
Arq
3 = 10 15
0.9
1.1
1.1
B1
P-13
Naq
3 = 10 15 % 10 17
3
0.7
0.7
B2
P-13
Naq
3 = 10 15 % 10 17
3
0.9
0.9
S
P-1
Naq
3 = 10 15 % 10 17
2.0007 "0.0005 1.993 "0.005 2.0005 "0.0005 2.0020 "0.0005 2.0025 "0.0005
1
S-3
2.009 "0.001 2.004 "0.001 2.002 "0.001 2.011 "0.01 2.0025 "0.0005
1
A2
2.0165 "0.0010 2.023 "0.001 2.057 "0.005 2.036 "0.005 2.0025 "0.0005
0.6
0.6
0.6
As mentioned above, the anneal data ŽFig. 6. indicate that the S-signal and A-spectrum belong to distinctive centers. Only the A-spectrum is observed in the S-3 glass implanted with different ions, except Oq, at D1 and D 2 . As follows from Fig. 2a, c the
A-spectrum has a complicated shape which we assume is due to at least two distinct centers. Fig. 10 illustrates the result of computer simulation of the A-spectra. We assumed that it consists of two spectra ŽA 1 and A 2 .. The principal g-values
Fig. 11. The calculated A 1 - and S-spectra contributing to the EPR spectra of the P-1 glass. The spectrum of Fig. 3 was obtained with ratio of intensity b s SrA 1 s 0.1.
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were varied as well as the widths of individual lines Ž D Hi ., their shape ŽLorentzian and Gaussian. and the ratio of intensities of A 1 and A 2-spectra Ž a .. Fig. 10a presents the computer-simulated spectra A 1 and A 2 . They have three principal g-values and a Lorentzian shape of individual lines of width D Hi . The g-values and widths of individual lines contributing to A 1- and A 2-spectra of S-3 glass implanted with Arq ions at dose D 1 are given in Table 2. It should be noted that the variations of g-values are accompanied by the change of D Hi and a . We estimate the errors in computer simulated g-values given in Table 2 as the highest possible spread of g-factors at which an average deviation of points of a calculated spectrum from an experimental one is within "5% limits when D Hi and a vary. Fig. 10b reproduces the experimental spectrum Žsolid curve. for the S-3 sample implanted with Arq Žat D1 . and computer-simulated spectrum Žcircles.. The best fit occurs when the ratio of the intensities
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of A 1- and A 2-spectra a s A 2rA 1 s 0.7. The a value varies depending on dose and implanted ion. The A-spectra of the S-2 samples can be also constructed from A 1 and A 2 spectra. As follows from Figs. 1–3 in some cases the observed spectra contain the S-line. Fig. 3 shows the experimental spectrum of a P-1 sample implanted with Arq at D 1 s 3 = 10 15 cmy2 Žsolid curve. and computer-simulated spectrum Žcircles.. The latter was obtained by addition of two spectra Žshown in Fig. 11. with ratio b s SrA 1 s 0.1. The spectrum A 1 has the same g-values as the A 1-spectrum for a S-3 glass and larger linewidths. The spectrum S was fitted to a Lorentzian function with g s 2.0025 " 0.0005 and D H s 0.6 mT. The B-spectra for P-13 glasses presented in Fig. 4 and the spectra of P-55 glasses Žnot shown. have the diffuse shape of low-field portions Žwith weak maxima. which indicates almost continuous distribution of g z-values. The singularities that are manifested in
Fig. 12. The calculated B1 - and B 2 -spectra contributing to the EPR spectra of the P-13 and P-55 glass ŽFig. 4Fig. 5.. The calculated spectra shown in Fig. 5 were obtained with ratios of intensities of g s B1 rB 2 equal to 0.0025 Ža., 0.1 Žb. and 0.7 Žc..
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high-field parts of the spectra help to expand these spectra into components. They were simulated assuming that they consist of two spectra ŽB 1 and B 2 . reproduced in Fig. 12. These spectra were calculated by varying g x , g y , g z-values and D Hi assuming that the individual lines have Lorentzian shape. The best fit parameters for B 1- and B 2-spectra are given in Table 2. The shape of the spectra observed for P-13 and P-55 glasses depends on the contributions of B 1- and B 2-spectra. For example, the best fit ratios g s B 1rB 2 for the spectrum of P-13 glass implanted Nq ions ŽFig. 5. are 0.0025; 0.1 and 0.7 for D 1 , D 2 and D 3 , respectively, i.e., the intensity of B 1-spectrum increases with dose. This spectrum has high intensity in the case of Oq-implanted glasses. For example, for the P-13 glass implanted with Oq ions at D 2 the g-value is equal to 0.8 whereas for the same glass implanted with Nq at D 2 g s 0.1. Therefore, the shape of high-field portions of the spectra presented in Fig. 4 differ. Thus, in the ion-implanted samples studied in the present work two main spectra are observed. One of them is the narrow symmetric line of Lorentzian shape with g ( g e Žwhere g e is g-factor of free
electron. named the S-signal. The other is a broad anisotropic spectrum with g z ) g y ) g x ( ge .
Ž 6.
The principal g-values of this spectrum are a function of sample compositions ŽA 1-, A 2-, B 1-, B 2-spectra.. In addition, in the spectra of the S-2 glass the narrow EPR signal with g s 2.0006 is observed. We assume that the defect responsible for this signal is an EX-type center. Our assumption is based on gvalue, linewidth and saturation behavior w6x. The EX-center has been observed in the spectra of some g-irradiated borosilicate glasses Žfor example, Pyrex w19x which is phase separated glass w3x.. We shall not discuss this center in the present work as well as the doublets observed for some Nq-implanted samples. The possible origins of S-, A- and B-spectra will be considered. 4.2. A- and B-spectra The broad anisotropic spectra Žlabeled A or B depending on glass composition. were observed for
Table 3 The g-values for some oxygen-associated defects Center HC1 ŽSi–OHC. HC2 ŽSi–OHC. SHC1 ŽSi–OHC. Al–OHC Oy Oy Oy 2 Oy 2 Oy 2 Oy 2 Oy 2 Oy 2 Oy 2 Oy 2 Ž . Oy 3 ozonid Ž . Oy 3 ozonid ‘So called’ Oy 3 ‘So called’ Oy 3 COq COy COy 2 COy 2 y CO 2
Matrix alkali-silicate glasses CaO–Al 2 O 3 –SiO 2 glasses KBr NaOHP H 2 O MgO Žsurface. ZnO Žsurface. MoO 3 Žsurface. Al 2 O 3 Žsurface. silica-gel SiO 2rAl 2 O 3 CaX-zeolite Ca 5 ŽPO4 . 3 OH KClO 3 SrŽClO 3 . 2 SiO 2 TiO 2 CO:zeolite CO:MgOŽsurface. KO 2 ŽCH 2 .COH CaCO 3 Oyq COrMgO
gz 2.0213 2.0158 2.0173 2.0186 1.987 2.002 2.062 2.042 2.016 2.039 2.025 2.024 2.057 2.058 2.011 2.0095 2.0080 2.0070 2.0045 2.0021 2.0047 2.0032 2.0043
gx 2.0023 2.0118 2.0026 2.0025 2.226 2.007 2.0018 2.0030 2.0042 2.0040 2.0031 2.0022 2.000 2.003 2.0026 2.0020 2.0030 2.0010 2.0005 2.0055 1.9957 1.9978 1.9977
gy
Ref.
2.0088 2.0127 2.0109 2.010 2.226 2.007 2.0089 2.007 2.0098 2.009 2.0095 2.0068 2.007 2.003 2.018 2.016 2.0045 2.0010 2.0005 2.0055 2.0021 2.0017 2.0022
w13x w13x w21x w21x w22x w23x w23x w23x w23x w23x w23x w23x w22x w24x w23x w23x w23x w23x w25x w26x w27x w28x w29x
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all five glasses examined in this work as well as for other ion-implanted glasses w12,14,15x. As mentioned above the similar spectrum observed in Oq-implanted silicate glass w12x has been attributed to silicon–oxygen hole centers ŽOHC. HC1 and HC2 w13x. The principal g-values of the spectra for these centers answer Eq. Ž6.. The OHC are commonly formed in g-irradiated multicomponent glasses. They involve a hole trapped on one or two oxygenŽs. bonded with a high charge network-forming cation ŽM.. For convenience these defects will be referred to as ‘M–OHC’, where M s B, Si, Ge, Al. The principal g-values for some M–OHC are given in Table 3. If the nucleus of M has non-zero spin, hyperfine structure should be present in EPRspectrum of M–OHC w20,21x. The spectra of the A-type ŽA 1 and A 2 . observed in borosilicate samples S-2 and S-3 can be, in principle, assigned to various silicon–oxygen hole centers. However, such an attribution of the A 1-spectrum in the case of P-1 samples becomes problematic since all cations Ž31 P, 27Al, 23 Na. entering into this glass have non-zero nuclear spins. The assumption that the A 1-spectrum of the P-1 samples belongs to M–OHC with unresolved hyperfine structure, for example due to 27Al, is contradicted by the small width of the A 1-spectrum ŽF 2 mT ŽFig. 11.. whereas the total width of the spectrum induced by Al–OHC is in the order of 10 mT w21x. At the same time the g x- and g y-values of A 2-spectrum are not typical of M–OHC w13,20,21x. Therefore, we assume that all the broad anisotropic spectra observed in various ion-implanted oxide glasses belong to identical defects. Only oxygen is constituent common to these glasses, i.e., the A- and B-spectra should be attributed to oxygen-related electron states. The absence of the resolved hyperfine structure in the A 1- and B-spectra of implanted phosphate glasses indicates that the defects responsible for these spectra are not bonded with network-forming cations. Since implantation involves extensive vacancy-interstitial formation it is reasonable to consider interstitial oxygen species are likely candidates for A- and B-spectra in ion-implanted oxide glasses. Several kinds of charged oxygen species have been reported q y y 3y y including Oq and O4y 2 , O , O , O2 , O2 , O3 w23,30x. On the basic of Eq. Ž6. which follows from
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the theory of Kanzig and Cohen w31x, numerous experimental data w22x and reviews of oxygen species on oxide surface w23,30x, we propose molecular Oy 2 ions as the most likely candidates for the A- and B-spectra. As follows from Refs. w23,31x the g z-component is sensitive to the environment of the Oy 2 ions. This value depends on the oxidation state of the nearest cation ŽM nq . and decreases from 2.11 " 0.05 at n s 1 to 2.015 " 0.005 at n s 6 in the case of Oy 2 ions adsorbed on oxide surfaces w23x. The authors of w23x concluded that g z gives a measure of the cation charge at the adsorption site. The appearance of several peaks corresponding to various g z indicates the existence of several adsorption sites in the system although various g z-values can be observed for one type of cations if adsorption sites have different local environments w23x. The diffuse shape of the low-field parts of the A- and B-spectra which is evident in the large D Hz necessary for computer simulation of these spectra, especially in the case of the phosphate glasses, can be due to the existence of many sites with various degrees of distortions of local environments of the Oy 2 ions. Since all the EPR spectra observed in the present work were constructed from the mentioned four spectra we assume that the majority of Oy 2 ions is located in some defined positions in ion implanted glasses. However, the identification of these positions is a difficult problem since the stabilization of Oy 2 ions depends on a large number of parameters. As follows from Ref. w23x, where empirical dependence of g z-components of Oy 2 ion spectra on cation charge of adsorption site has been found, the g z-value can be used to establish the nature of the adsorption site on an oxide surface. This dependence cannot be applied to bulk samples automatically since the gtensor for Oy ion absorbed on surfaces may be 2 calculated based on the ionic model of this defect whereas in many bulk samples the g-values do not fit the predictions of this model w23x. Nevertheless, based on a comparison of g z obtained for the A 2-, B 1- and B 2-spectra with literature data w22–24x we assume that in ion-implanted sam4q ples the Oy , Ca2q ŽMg 2q . 2 ions are located near Si 3q Ž 2q . and Al Zn sites, respectively Žsee Table 3.. However, such an attribution can be considered only as speculative. For example, g z f 2.017 which is
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characteristic of the A 1-spectrum corresponds to the cations with high charge w23x which are absent in glasses examined in the present work and Refs. w12,14x. At the same time, g z f 2.017 has been w x observed for Oy 2 in the natural CaF2 crystal 32 and a-Al 2 O 3 w33x. Such low g z-values can be due to a decrease in a distance between cations and Oy 2 ions imposed by the structure of bulk samples. According to Ref. w23x the decrease in distance cation-Oy 2 ions leads to a decrease in g z-value. It should be noted that the errors given in Table 2 for the g-values of the A- and B-spectra are essentially larger than the ones obtained for g-factors at computer simulations of M–OHC Žfor example, Ref. w x w21x. and Oy 2 ions 34 in g-irradiated glasses. This is due mainly to wide distribution of g-values in implanted samples because of larger distortions of local environments of Oy ions associated with 2 changes in glass structure in the implantation layer. The summation of greater number of spectra with g-components answering Eq. Ž6. leads in many cases to a better coincidence of calculated and experimental spectra but such artificial procedures do not give new information for understanding the nature of the formation of point defects in ion implanted glasses. We do not exclude also that oxygen species other than Oy ions contribute partially to the broad 2 anisotropic spectra observed in ion-implanted glasses. However, we have now no direct data indicating their presence. As follows from Fig. 8b, c the number of Oy 2 ions responsible for A- and B-spectra in Oq-implanted samples is not greater than in glasses implanted with Nq ions that have a mass near the mass of oxygen. It suggests that the formation of interstiions is mainly due to displaced but not tial Oy 2 implanted oxygen atoms. The displacement of oxygen atoms occurs presumably in the process of elastic collisions. It can be seen in Fig. 8b, c that the concentration of Oy 2 ions depends on glass compositions and decreases with an increasing mass of incident ions. It can be seen in Fig. 9a that the width of depth profiles of implanted ions decreases and their maxima shift towards the surface of the substrate with an increasing mass of incident particles. Based on this result together with the data of Ref. w14x, indicating the increase in the number of Oy 2 ions with increasing energy of incident ion, we assume
that an increase in a projected range of implanted ions promotes the formation of the interstitial molecular Oy 2 ions. 4.3. S-signal It is obvious from Figs. 1–3 that the isotropic lines of width 0.3 F D H F 0.4 mT, labeled S-signal, were observed in the spectra of both borosilicate glasses ŽS-2 and S-3. and phosphate P-1 glass. As mentioned above a similar line has been observed in the spectra of silicate glasses w12,14x. Their baseline crossing is g G g e . The differences in g-values estimated for the S-lines in different samples is presumably due to the presence of an underlying A-component. The computer simulation of S-line gives g s 2.0025 " 0.0005. The S-signals have similar behavior as functions of dose and implants in S-2, S-3 and P-1 glasses ŽFigs. 7 and 8a. and belong presumably to identical defects. A similar signal has been attributed to a vacancy-type defect with one dangling bond localized on a silicon atom in Ref. w14x. However, based on the results obtained in the present work we eliminate the silicon–oxygen unit as possible origin of the S-spectrum since the phosphate P-1 glass contains no silicon. The similarity of S-spectra observed for glasses of different compositions showing the existence of their common source suggests that there should be a constituent common to these glasses. As follows from Table 1, only sodium and oxygen are the constituents to fit this description. Based on the fact that the intensity of the S-spectrum is several times greater in Oq-implanted samples than in samples implanted with other ions we assign this to a defect involving oxygen. At the same time, as follows from Table 1 sodium is present only in S-2, S-3 and P-1 glasses in which the S-signal was observed and is absent in P-13 and P-55 glasses where this signal was not found. The greatest number of S-defects was detected in the P-1 glass in which the concentration of Na 2 O is maximum. These data indirectly indicate that the Naq ions play some role in the formation of defects of the S-type. It should be noted that the identification of the narrow symmetric line with g ( g e is difficult. As mentioned above the signal of the S-type observed in
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Ref. w12x for Oq-implanted silicate glasses has been attributed to Oy-interstitials. However, such assignment seems to be in disagreement with the theory, which predicts g H ) g 5 ( g e w30x and the experimental data for various matrices Žsee, for example, Table 3.. The analysis data presented in Refs. w22,23x show that the spectra of such oxygen species as Oy, Oy 2, q Oy and O are essentially anisotropic. The 3 2 Lorentzian shape of the S-signal indicates exchange interactions between defects, responsible for this signal. However, the average g-value for all the above mentioned defects is more than 2.01 whereas g ( g e for the S-spectrum. As noted in Ref. w23x there are several cases where the nature of the oxygen species is not clear. The almost isotropic spectra were observed for ‘soy called’ Oy form and 3 ions which has an O 2 P O differs from classic ozonide ions. The g-values attributed to the ‘so-called’ Oy 3 ions are given in Table 3 together with those for ozonide. Although a number of reports have been published on the existence of the O4y ion, there is little direct evidence for its identification. The O4y species is a 25-electron radical. A small departure of the principal g-values from g e is expected w23x. It is predicted for this ion that g 5 ; g H ; 2.0023 w23x. The line with g s 2.0030 and width D H s 0.3 mT has been reported for oxygen species adsorbed on ZnO and TiO 2 w23x. Recently w35x the single line with g s 2.00247 and D H ( 0.1 mT ŽEX-center. has been observed in the spectrum of SiO 2 thermally grown on silicon. It has been shown w36x that this line can be attributed to hole trapped on the site of the Si vacancy where an unpaired electron is postulated to be localized over bordering oxygen atoms. The study of parameters controlling the generation of intrinsic EX defects led the authors of Ref. w37x to the conclusion that these defects are located in a region of the SiO 2 film supersaturated with oxygen and that they can be considered as excess-oxygen centers rather than Sibased defects. Thus, both large molecular ions ŽO4y, O 2 P Oy . and EX-type centers can be responsible for the Sspectrum in ion-implanted oxide glasses. Both groups of species are related to aggregation of oxygens in some regions of the implanted layer. Such regions can be formed as a result of sodium depletion which
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occurs near the surface of implanted substrate ŽFig. 9b., mainly due to the sputtering process w1,38x. Finally, we cannot exclude complete centers associated with carbon impurity, which can penetrate into the surface layer from the pump oil during the implantation process, as a possible source of the S-spectrum. As follows from Table 3 some carbon– . w23,25–29x exhibit oxygen ions ŽCOq, COy, COy 2 almost isotropic EPR spectra with g ( g e . In accordance with XPS data, all the implanted samples studied in the present work have a very high Žfrom ; 20 to 70 at.%. surface concentration of carbon which penetrates into the glass matrices and contaminates a region of about 20 nm below the surface. At higher depths carbon concentration reaches a steady value of ; 1 at.%. Thus, in our opinion the S-line can be due to three types of defects: Ži. large oxygen molecular ions ŽO4y, O 2 P Oy .; Žii. a hole localized on several oxygens ŽEX-type center w35–37x. and Žiii. the carbon– oxygen ions. The more detailed experiments are necessary for clearing this problem.
5. Conclusion From the EPR study of ion-implanted oxide glasses of five compositions and compared with data in literature we found specific features which are characteristic for defects induced by implantation in comparison with those produced by g-irradiation. The principal ones are electron states weakly coupled to the glass-network. The EPR spectra in ionimplanted glasses are attributed to molecular oxygen ions which accumulate in interstitials and voids of implanted layer. Such defects are formed due to displacement of atoms induced by elastic collisions.
References w1x G.W. Arnold and P. Mazzoldi, in: Ion Beam Modification of Insulators, ed. P. Mazzoldi and G.W. Arnold ŽElsevier, Amsterdam, 1987. ch. 5, p. 195. w2x H. Hosono and R.A. Zuhr, J. Non-Cryst. Solids 178 Ž1994. 160. w3x G.W. Arnold, Radiat. Eff. 98 Ž1986. 55. w4x R.A. Weeks, J. Appl. Phys. 27 Ž1956. 1376. w5x E.J. Friebel, D.L. Griscom, M. Stapelbrook and R.A. Weeks, Phys. Rev. Lett. 42 Ž1979. 1346.
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w6x R.A. Devine and A. Golansky, J. Appl. Phys. 54 Ž1983. 3833. w7x S.L. Derryberry, R.A. Weeks, R.A. Weller and M. Mendenhall, Nucl. Instrum. Meth. B59&60 Ž1991. 1320. w8x R.A. Weeks, J. Non-Cryst. Solids 179 Ž1994. 1. w9x G. Whichard, H. Hosono, R.A. Weeks, R.A. Zuhr and R.H. Magruder, J. Appl. Phys. 67 Ž1990. 7526. w10x H. Hosono and R.A. Weeks, Phys. Rev. B40 Ž1989. 10543. w11x H. Hosono, Y. Abe, K. Oyoshi and S. Tanaka, Phys. Rev. B43 Ž1992. 11966. w12x A.I. MacGregor and R.K. MacCrone, J. Non-Cryst. Solids 102 Ž1988. 30. w13x D.L. Griscom, J. Non-Cryst. Solids 64 Ž1984. 229. w14x L.D. Bogomolova, A.A. Deshkovskaya, N.A. Krasil’nikova, G. Battaglin and F. Caccavale, J. Non-Cryst. Solids 151 Ž1992. 23. w15x L.D. Bogomolova and N.A. Krasil’nikova, Proc. of XV Intern. Congress on Glass, Vol. 2a, ed. O.V. Mazurin ŽLeningrad, 1989. p. 13. w16x B. Bleany, Proc. Phys. Soc. London, Sect. A, 75 Ž1960. 621. w17x J. Kliava, EPR Spectroscopy of Non-Ordered Solids ŽZinatne, Riga, 1980. p. 320. w18x L.D. Bogomolova, E.G. Grechko, V.A. Jachkin, N.A. Krasil’nikova, V.V. Sakharov and T.V. Semenova, Fiz. Khim. Stekla 13 Ž1987. 202. w19x G. Brown, J. Mater. Sci. 10 Ž1975. 1841. w20x D.L. Griscom, J. Non-Cryst. Solids 13 Ž1973–1974. 251. w21x D.A. Dutt, P.L. Higby, C.I. Merzbacher and D.L. Griscom, J. Non-Cryst. Solids 135 Ž1991. 122. w22x Landolt–Bornstein, Numerical Data and Functional Relation-
w23x w24x w25x w26x w27x w28x w29x w30x w31x w32x w33x w34x w35x w36x w37x w38x
ships in Science and Technology, New Series, Magnetic Properties of Free Radicals, ed. H. Fisher and K.-H. Hellwege, Group II, Vol. 9, Part A ŽSpringer, Berlin, 1977. pp. 81–119. M. Che and A.J. Tench, Adv. Catal. 32 Ž1983. 1. C. Rey, J. Trombe and J. Dugas, C. R. Acad. Sci. 283 Ž1976. 277. J.C. Vedrine and C. Naccache, Chem. Phys. Lett. 18 Ž1978. 190. J.H. Lundsdorf and J.P. Jayne, J. Chem. Phys. 44 Ž1960. 1492. K. Toriyama, M. Iwasaki, S. Noda and B. Eda, J. Am. Chem. Soc. 93 Ž1971. 6415. R.S. Eachus and M.C.R. Symons, J. Chem. Soc. A Ž1968. 2433. C. Naccache, Chem. Phys. Lett. 11 Ž1973. 323. M. Che and A.J. Tench, Adv. Catal. 31 Ž1982. 77. W. Kanzig and M.H. Cohen, Phys. Rev. Lett. 3 Ž1959. 509. H. Bill, Solid State Commun. 15 Ž1974. 911. D.D. Eley and M.A. Zammit, J. Catal. 21 Ž1971. 366. D.L. Griscom, P. Hart, J.M. Jewell, J.T. Kohli and J.E. Shelby, J. Non-Cryst. Solids 103 Ž1988. 300. A. Stesmans, Phys. Rev. B45 Ž1992. 9501. A. Stesmans and F. Scheerlink, Phys. Rev. B5 Ž1994. 5204. A. Stesmans and F. Scheerlink, J. Non-Cryst. Solids 187 Ž1995. 119. G. Battaglin, A. Bascoletto, G. Della Mea, G.D. Marchi, P. Mazzoldi, A. Miotello and B. Tiveron, Radiat. Eff. 98 Ž1986. 101.
Paramagnetic defects induced by ion implantation in oxide glasses L.D. Bogomolova a,) , V.A. Jachkin a , S.A. Prushinsky a , S.A. Dmitriev b, S.V. Stefanovsky b, Yu.G. Teplyakov b, F. Caccavale c , E. Cattaruzza c , R. Bertoncello d , F. Trivillin d a
d
Institute of Nuclear Physics, Moscow State UniÕersity, 119899 Moscow, Russia b State Corporation Radon, RostoÕsky per. 2 r 14, 119121 Moscow, Russia c INFM, Dipartimento di Fisica, Õia Marzolo 8, 35131 Padua, Italy Dipartimento di Chimica Organica, Metallorganica ed Analitica, Õia Loredan 4, 35131 Padua, Italy Received 25 July 1995; revised 24 July 1996
Abstract Radiation defects induced by ion bombardment of multicomponent oxide glasses of five compositions Žphosphates and borosilicates. were investigated by means of electron paramagnetic resonance ŽEPR.. The samples were implanted with Nq, Oq, Arq, Mnq, Cuq and Pbq ions at energy E s 150 keV at three different doses between 3 = 10 15 and 10 17 ionsrcm2. The broad anisotropic EPR spectra with principal g-values answering the relationship g z ) g y ) g x ; g e Ž g e is g-factor of free electron. were observed for the samples of all five compositions. The g-values depend on glass composition. For example, g z ranges from 2.016 to 2.057. Computer simulation shows that the spectra of many samples are superpositions of two spectra with g-values answering the mentioned relationship. These spectra are attributed to molecular Oy 2 ions weakly coupled with glass network. In some samples narrow almost symmetric lines with g s 2.0025 " 0.0005 were observed. The possible radiation defects responsible for this signal are discussed.
1. Introduction Ion implanted glasses have been the object of considerable attention in the past decade w1x. This attention is due to practicable interest connected with modification of physical and chemical properties of near-surface layers of glasses by ion implantation that may have applications in integrated optics w2x. The scientific interest to ion implanted glasses is )
Corresponding author. Fax: q7-095 939 0896; e-mail: [email protected].
associated with the study of the effects of interaction between heavy charged particles and insulators that are a priori amorphous. In addition, heavy ion Žfor example, Pb. implantation is a convenient laboratory technique for producing damage in materials of interest for radioactive waste encapsulation w3x. Among different experimental techniques used for the study of ion-implanted glasses is electron paramagnetic resonance ŽEPR. spectroscopy giving information about structural paramagnetic defects induced by ion implantation. EPR of ion-implanted vitreous silica has been
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 9 1 - 1
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investigated in many works. The EX-center and peroxyradical EPR spectra analogous to those reported for g-irradiated silica glasses w4,5x have been observed for amorphous silica implanted with different ions Žfor example, w6,7x.. The EPR signals from EX-type center, that have not been found in the spectra of g-irradiated glasses w8x, were reported for amorphous silica implanted by some transition metals w9,10x and Siq ions w11x. The EPR spectra of ion-implanted multicomponent glasses have not been extensively studied, to our knowledge. Two types of EPR spectra associated with radiation-induced paramagnetic centers have been observed for Oq-implanted soda–lime silicate ŽCorning 015. glass w12x. One of them which was a broad anisotropic signal has been attributed to silicon–oxygen hole HC1 and HC2 centers earlier observed for g-irradiated silicate glasses w13x. The other which was a rather narrow almost symmetric line with a peak to peak width of D H ( 0.7 mT with g-values ranged between 2.0029 and 2.0064 depending on implantation dose, has been assigned to Oyinterstitials w12x. Approximately similar signals have been observed in commercial Pyrex and multicomponent silicate glasses implanted with different ions at various doses and energies w14x. But these signals have received an other interpretation w14,15x: the almost symmetric signal with g ranging from 2.0018 to 2.0034, denoted by S, has been attributed to the vacancy-type defect with one dangling silicon bond. The difference in the lineshapes, linewidth and spectral parameters of the S-center from the EX-center was interpreted in terms of lack of oxygen in the environment of the silicon atoms with an unpaired electron w15x. The other spectrum, denoted by A, was a broad anisotropic signal and has been attributed to interstitial oxygen species whose structure has not been proposed in w14x. Thus, almost identical EPR spectra observed in reports w12,14,15x have received different interpretations. In order to obtain more information on structural defects induced by ion bombardment in oxide glasses we performed EPR measurements on multicomponent phosphate, borophosphate and borosilicate glasses implanted with different ions. The present paper reports some results of these measurements. Complementary information on the implantation
effects and surface contamination was obtained by secondary ion mass spectrometry ŽSIMS. and X-ray photoelectron spectroscopy ŽXPS..
2. Experimental The molar chemical compositions of glasses used in this work are given in Table 1. The glasses were melted in high-frequency furnace in air at temperatures depending on glass composition. Then they were cast and annealed. The annealing was performed in an electric muffle furnace preheated to an annealing temperature, Ta . At this temperature the samples were held for periods from 30 to 60 min. The Ta value varied from 790 to 850 K depending on glass composition. Then the samples were cooled to room temperature at rate about 0.5 degrees per min. Polished plates were prepared. Sample size was 10 = 20 = 0.5 mm. These plates were irradiated with Nq, Oq, Arq, Mnq, Cuq and Pbq ions at energy E s 150 keV to nominal doses Ž D . equal to D1 s 3 = 10 15 ; D 2 s 2 = 10 16 and D 3 s 10 17 ionsrcm2 . We used 0.5 mArcm2 current density to minimize temperature effects during implantation. Temperatures of substrates during implantation measured by thermocouple were about 350 K. After implantation the slabs were crushed and used for EPR experiments. EPR measurements were performed using a modified spectrometer ŽRE-1306, Soviet model. operating at X-band frequency with 100 kHz modulation. The microwave power and amplitude of RF modulation are given in the text where it is necessary. Most spectra were obtained at 77 K and room temperature. Some measurements were made with variable temperature accessories in the range from 295 to 470 K. Isochronal anneal experiments were Table 1 Glass compositions Žin mol%. Name SiO 2 P2 O5 B 2 O 3 Al 2 O 3 Na 2 O 3 MgO CaO ZnO of glass 1 2 3 4 5
S-2 S-3 P-1 P-13 P-55
70 50 – – –
– – 36 65 60
10 5 – 10 3
– 5 20 10 10
20 20 44 – –
– 5 – 15 12
– 15 – – 10
– – – – 5
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performed with the same accessory. The samples were held at the anneal temperatures for a period of 5 min and returned to room temperature for EPR measurements. The absolute number of defects was determined with reference to hyperfine transition Ž1r2 l 1r2. of Mn2q ions in MgO or CuSO4 P 5H 2 O powder samples previously calibrated at the Soviet Institute of Standards ŽNIFTRI.. The reference samples were pressed in capsules of 1 = 1 = 3 mm and were placed in the tube together with measured powder samples.
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The quoted g-values were referenced to hyperfine lines of the Mn2q:MgO sample. Room temperature Q-band Ž; 37 GHz. EPR data were obtained for several samples ŽRE-1308 Soviet model.. SIMS measurements were performed using an ion microscope ŽCameca ims-4f. equipped with a normal-incidence electron gun used to compensate the charge build-up when profiling insulating samples. The profiles were obtained by 14.5 keV cesium bombardment and by negative ion detection. The
Fig. 1. EPR spectra of the ion-implanted borosilicate S-2 samples Žsee table.; Nq at dose D1 s 3 = 10 15 ionsrcm2 Ža, b. and at D 2 s 2 = 10 16 ionsrcm2 Žc.; Oq at dose D 2 s 2 = 10 16 ionsrcm2 Žd. and Arq at D 2 s 2 = 10 16 ionsrcm2 Že.. The spectra were measured at room temperature with microwave power P s 30 mW and the modulation amplitude A s 0.08 mT for the spectra b–e and with microwave power P s 0.6 mW for spectrum a.
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Fig. 2. EPR spectra of the borosilicate S-3 samples implanted with Nq Ža., Oq Žb. and Arq Žc. at dose D1 s 3=10 15 ionsrcm2 . The data were obtained at room temperature with microwave power P s 40 mW and As 0.08 mT. G are gain coefficients.
cesium beam current was 80 nA over a raster area of 125 = 125 mm2 and secondary ions emitted from a central circular Ž8 mm diameter. area were collected by the spectrometer. The measured ion yield of each species at any instant lies within "5% deviation. The erosion speed was evaluated by measuring the depth of the erosion crater at the end of each analysis by means of a profilometer ŽTencor Alpha Step.. The maximum deviation in measuring a certain depth by this instrument is of the order of 5%. Calibration of the implanted profiles was made by means of the measurement of the total retained doses using Rutherford backscattering spectrometry ŽRBS. analysis with a 2.2 MeV 4 Heq beam ŽQ s 1608.. XPS measurements were achieved with a spectrometer ŽPerkin–Elmer, F 5600ci. using nonmonochromatized MgK a radiation Ž1253.6 eV.. After 2.5 keV Arq sputtering, with an erosion speed of about 3 nmrmin, ‘survey’ spectra, i.e., a large Ž0 % 1150 eV. binding energy ŽBE. region with all the interesting photoelectron peaks, were recorded. The surface charging effect was corrected using as inter-
Fig. 3. The experimental Žsolid curve. and calculated Žcircles. EPR spectra of phosphate P-1 glass implanted with Arq ions at D1 s 3 = 10 15 ionsrcm2 . The spectrum was recorded at room temperature with microwave power P s 30 mW and A s 0.06 mT.
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nal reference the BE of the C 1s peak of hydrocarbon contamination. The atomic compositions were evaluated using sensitivity factors as provided by F V5.44A software.
3. Results Figs. 1–5 show typical EPR spectra observed in ion implanted samples studied in this work. The spectra depend on implantation dose, implants and glass composition. Fig. 1 shows the central part of EPR spectra of S-2 samples implanted with various ions at different doses. Two of them ŽFig. 1a, b. were obtained with separated microwave power. For a S-2 sample implanted with Nq ions at D 1 s 3 = 10 15 ionsrcm2 the spectrum consists of a broad peak A and a narrow almost symmetric signal ŽNS. with g s 2.0006 " 0.0005 and a peak to peak linewidth of D H s 0.15 " 0.05 mT whose intensity decreases with increasing microwave power ŽFig. 1a,b..
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The central part of the spectrum of a S-2 glass implanted with Nq ions at D 2 s 2 = 10 16 ionsrcm2 ŽFig. 1c. also contains the features of A and NS signals but a new line ŽS. appears. The intensity of the latter depends on implants and dose Žsee below.. This signal is well seen in Oq-implanted sample ŽFig. 1d.. The S-signal is also an almost symmetric line with the baseline crossing g-value oscillating between 2.0021 and 2.0032 Ž"0.0005. for different samples and a peak to peak linewidth of D H ranging from 0.3 to 0.4 mT. This component is saturated at higher microwave powers than the NS line. The spectra of the ion-implanted borosilicate S-3 samples are shown in Fig. 2. The features of the shoulder A and the S-signal are seen in the spectrum of Fig. 2b. However, an anisotropic spectrum Žlabeled A. dominates in the spectra of all the S-3 samples except for Oq-implanted glasses where the intensity of the S-signal is large and the A spectrum is a shoulder. The presence of two low-field peaks can be seen in Fig. 2a, c. The relative intensities of these peaks and the positions of their minima were differ-
Fig. 4. The EPR spectra of phosphate P-13 glass implanted with Oq Ža. and Nq Žb. ions at D 2 s 2 = 10 16 ionsrcm2 . The spectra were recorded at room temperature with microwave power P s 40 mW and A s 0.1 mT.
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Fig. 5. The high-field fragments of the experimental Žsolid curves. and calculated EPR spectra of the P-13 glass implanted with Nq at doses D1 s 3 = 10 15 ionsrcm2 Ža.; D 2 s 2 = 10 16 ionsrcm2 Žb. and D 3 s 10 17 ionsrcmy2 Žc..
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Fig. 5 Žcontinued..
ent for different samples indicating that the A-signal is a superposition of at least two anisotropic spectra. In addition, the broad minima at g ( 1.995 Ža, c. are well resolved in the spectra of the S-3 samples implanted with Nq and Arq ions. The g-values shown in Figs. 1 and 2 are calculated only for the positions of peaks and baseline crossing. The parameters obtained by computer simulation of these spectra will be given below. Fig. 3 shows the EPR spectrum of a phosphate sample P-1 implanted with Arq at D1 s 3 = 10 15 cmy2 . It can be seen that the spectrum contains a shoulder of the A-line and the S-line just as in the case of some borosilicate glasses. Fig. 6 presents the isochronal annealing data for the S-signal and the A-shoulder in the spectrum of the P-1 glass implanted with Nq Ž D 2 s 2 = 10 16 cmy2 .. The temperature dependence of IA and IS was measured Ž IA and IS are shown in Fig. 3.. It is seen in Fig. 6 that this curve for the S-signal differs from the one for the A-shoulder. We assume that they belong to
distinct centers. As follows from Fig. 6 the S-center is thermally more stable than the A-center that begins to decay above 400 K. The relative intensity of
Fig. 6. Isochronal anneal curves: = – the S-center; e – A-centers.
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S- and A-spectra depends on the implantation dose and implants. The shape and linewidth of the S-signal at Q-band and X-band frequency are almost the same within experimental errors. They are independent of the temperature of observation in the range from 77 to 473 K. It should be noted that in the spectra of the S-2 and P-1 samples implanted with Nq ions at doses D 2 and D 3 the weak doublets with a separation of about 13 mT centered at g ( 2.0 were observed. They will be discussed elsewhere together with data for glasses of different compositions implanted with Nq-ions. Fig. 4 shows broadfield scan EPR spectra of a P-13 sample implanted with Nq and Oq ions at dose D 2 s 2 = 10 16 ionsrcmy 2 . These are broad anisotropic spectra Ždenoted by B.. The low field parts of these spectra contain shoulders with small peaks whose positions are approximately identical to those observed for the glasses of the same composition implanted with different ions. The high-field portions of both spectra differ in shape. Fig. 5, that reproduces the high-field portions of the spectra of P-13 samples implanted with Nq ions, illustrates their dependence on the total dose. Fig. 7a shows the dependence of the number of defects responsible for the S-signal in S-2 and P-1 samples on the implantation dose. The relative integrated intensities of S-signals Ž Jr . were estimated using the expression Jr s kIS Ž D H . 2 , where IS is the signal amplitude Žsee Fig. 3., D H is the peak to peak linewidth and k is the parameter determined by lineshape. A P-1 sample implanted with Oq ions at D 2 s 2 = 10 16 cmy2 was used as the reference for other samples. The absolute number of defects Ž N . responsible for the S-signal in this P-1 sample was calculated by comparing the integrated intensity of the S-signal with the one of the standard MgO:Mn2q sample. Values of concentrations of defects in other samples were calculated by multiplying the number N by the ratio JrrJo , where Jo is integrated intensity of S-signal in the mentioned Oq-implanted sample. The accuracy of measurements of relative number of defects is about 20%. The accuracy of measurements of absolute concentrations of defects is not better than "100 %. It can be seen that the concentration of S-defects slowly increases with in-
creasing implantation dose ŽFig. 7a. in comparison with the number of incident ions. As follows from Fig. 7b the number of defects per 1000 incident ions decreases with an increasing dose. Fig. 8a presents the dependence of the number of defects responsible for the S-line in S-2, S-3 and P-1 glasses on atomic mass of incident particles at D 2 s 2 = 10 16 cmy2 . Fig. 8b, c show the number of defects responsible for A- and B-spectra, respectively, as a function of atomic mass of implanted ions at D 2 . The absolute
Fig. 7. Ža. Dose dependence of the relative intensities produced by ion implantation in P-1 and S-2 samples. For the P-1 glass: v – Arq, B – Oq; for S-2 sample: = – Nq, ` – Oq and ^ – Arq. Žb. Dose dependence of the number of S-defects per 1000 implanted ions for the P-1 glass: ` – Arq, B – Oq; for S-2 sample: v – Oq, ^ – Arq and = – Nq. Data for Nq and Arq are given on right scale, for Oq-ions on left scale.
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content of defects of A-type was obtained for a S-3 sample implanted with Nq ions at D 2 . The integrated intensity of the A-spectrum was calculated by double numerical integration of this signal and was compared with the intensity of the signal for the standard CuSO4 P 5H 2 O sample. The procedure of an
109
estimation of defect concentrations in the other sample is the same as described above. The reference signal of the B-spectra was the B-spectrum of a P-13 sample implanted with Oq ions D 2 s 2 = 10 16 cmy2 . The integrated intensity of this signal was calculated by double integration of
Fig. 8. Ža. The dependence of the number of defects Žper cmy2 . responsible for S-signals for the P-1 ŽI. and S-2 Ž`. glasses. Žb. The dependence of the number of defects Žper cmy2 . responsible for A-spectra on atomic mass of implanted ions at D 2 s 2 = 10 16 ionsrcm2 for the S-3 Ž`. and S-2 ŽI. glasses. Žc. The dependence of the number of defects Žper cmy2 . responsible for B-spectra for the P-13 ŽB. and P-55 Ž`. glasses at D 2 s 2 = 10 16 ionsrcm2 .
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the spectrum between K 1 and K 2 points ŽFig. 4. and was compared with integrated intensity of the signal for a standard sample CuSO4 P 5H 2 O. The number of defects in other samples was evaluated by means of the procedure described above for the S-defects. The relative concentrations of S- and A-components of EPR spectra in P-1, S-2 and S-3 samples were determined by means of computer lineshape simulation which was performed for all experimental spectra Žsee below.. These concentrations were estimated using the ratios of areas under each spectrum contributing to the good fit calculated spectra. As follows from Fig. 8 the concentration of all defects decreases with increasing atomic mass except for Oq-implanted samples where the number of centers responsible for S-signals is several times larger than in glasses implanted with other ions, i.e., the implantation of oxygen promotes the formation of these centers. Fig. 9a shows the SIMS depth profiles for the P-13 glass implanted with different ions. The depth
profile of Na in a P-1 sample implanted with Mnq is presented in Fig. 9b.
4. Discussion 4.1. Computer simulation of the spectra To calculate EPR spectra we used an IBM compatible PC with Intel 486-DX4-100 processor and Microsoft Excel 7.0 spreadsheet. The synthetic spectra were computed in the form SX Ž H . s
p
p r2
H0 H0
W Ž Q , w . FX Ž H , Q , w .
sin Q dQ d w ,
Ž 1.
where the orientation-dependent transition probability W ŽQ , w . can be expressed w16x by W ŽQ , w . s
1 2
gx gz
2
½ž / 5 g
q g y2
Ž 2.
and SX Ž H . is the first derivative of the EPR absorption spectrum. The corresponding resonance fields are calculated from resonance condition H Ž Q , w . s hnrg b ,
Ž 3.
where g 2 s g z2 cos 2Q q Ž g x2 cos 2w q g y2 sin2w . sin2Q .
Ž 4.
Functions F Ž H, Q , w . are first derivatives of individual absorption lines. The shapes and widths of the latter are due to ‘site to sites’ fluctuations of g-values for paramagnetic species in glasses. We have employed both Gaussian and Lorentzian shapes of individual lines. We used the dependence of widths of individual lines on the Q angle in the form D H Ž Q . s Ž D Hz2 cos 4Q q D H x2, y sin4Q .
Fig. 9. Ža. SIMS concentration depth profiles for the P-13 glass implanted with Nq, Mnq and Pbq at D 2 s 2=10 16 ionsrcm2 . Žb. SIMS depth profile of relative concentration of Na ions for the P-1 glass.
1r2
.
Ž 5.
It has been shown w17,18x that the Eq. Ž5. allows us to obtain good fit parameters of spin-Hamiltonian in the case of weak anisotropy the of g-factor and the absence of correlations between g-components. The summation was made for Q and w angles with steps of 18 and 38, respectively.
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Fig. 10. Ža. The calculated A 1 and A 2 spectra contributing to A-spectrum of implanted the S-3 glasses. Žb. The experimental Žsolid curve. and calculated Žcircles. spectrum of the S-3 glass implanted with Arq Žat D1 s 3 = 10 15 ionsrcm2 . with ratio of intensities a s A 2 rA 1 s 0.7.
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Table 2 The g-values and widths of individual lines of spectra of the A- and B-type contributing to EPR spectra of ion-implanted glasses Type of spectra
Name of glass
Implant
D Žionsrcm2 .
gz
gx
gy
D Hz ŽmT.
D Hy ŽmT.
D Hx ŽmT.
A1
S-3
Arq
3 = 10 15
1
Arq
3 = 10 15
0.9
1.1
1.1
B1
P-13
Naq
3 = 10 15 % 10 17
3
0.7
0.7
B2
P-13
Naq
3 = 10 15 % 10 17
3
0.9
0.9
S
P-1
Naq
3 = 10 15 % 10 17
2.0007 "0.0005 1.993 "0.005 2.0005 "0.0005 2.0020 "0.0005 2.0025 "0.0005
1
S-3
2.009 "0.001 2.004 "0.001 2.002 "0.001 2.011 "0.01 2.0025 "0.0005
1
A2
2.0165 "0.0010 2.023 "0.001 2.057 "0.005 2.036 "0.005 2.0025 "0.0005
0.6
0.6
0.6
As mentioned above, the anneal data ŽFig. 6. indicate that the S-signal and A-spectrum belong to distinctive centers. Only the A-spectrum is observed in the S-3 glass implanted with different ions, except Oq, at D1 and D 2 . As follows from Fig. 2a, c the
A-spectrum has a complicated shape which we assume is due to at least two distinct centers. Fig. 10 illustrates the result of computer simulation of the A-spectra. We assumed that it consists of two spectra ŽA 1 and A 2 .. The principal g-values
Fig. 11. The calculated A 1 - and S-spectra contributing to the EPR spectra of the P-1 glass. The spectrum of Fig. 3 was obtained with ratio of intensity b s SrA 1 s 0.1.
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were varied as well as the widths of individual lines Ž D Hi ., their shape ŽLorentzian and Gaussian. and the ratio of intensities of A 1 and A 2-spectra Ž a .. Fig. 10a presents the computer-simulated spectra A 1 and A 2 . They have three principal g-values and a Lorentzian shape of individual lines of width D Hi . The g-values and widths of individual lines contributing to A 1- and A 2-spectra of S-3 glass implanted with Arq ions at dose D 1 are given in Table 2. It should be noted that the variations of g-values are accompanied by the change of D Hi and a . We estimate the errors in computer simulated g-values given in Table 2 as the highest possible spread of g-factors at which an average deviation of points of a calculated spectrum from an experimental one is within "5% limits when D Hi and a vary. Fig. 10b reproduces the experimental spectrum Žsolid curve. for the S-3 sample implanted with Arq Žat D1 . and computer-simulated spectrum Žcircles.. The best fit occurs when the ratio of the intensities
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of A 1- and A 2-spectra a s A 2rA 1 s 0.7. The a value varies depending on dose and implanted ion. The A-spectra of the S-2 samples can be also constructed from A 1 and A 2 spectra. As follows from Figs. 1–3 in some cases the observed spectra contain the S-line. Fig. 3 shows the experimental spectrum of a P-1 sample implanted with Arq at D 1 s 3 = 10 15 cmy2 Žsolid curve. and computer-simulated spectrum Žcircles.. The latter was obtained by addition of two spectra Žshown in Fig. 11. with ratio b s SrA 1 s 0.1. The spectrum A 1 has the same g-values as the A 1-spectrum for a S-3 glass and larger linewidths. The spectrum S was fitted to a Lorentzian function with g s 2.0025 " 0.0005 and D H s 0.6 mT. The B-spectra for P-13 glasses presented in Fig. 4 and the spectra of P-55 glasses Žnot shown. have the diffuse shape of low-field portions Žwith weak maxima. which indicates almost continuous distribution of g z-values. The singularities that are manifested in
Fig. 12. The calculated B1 - and B 2 -spectra contributing to the EPR spectra of the P-13 and P-55 glass ŽFig. 4Fig. 5.. The calculated spectra shown in Fig. 5 were obtained with ratios of intensities of g s B1 rB 2 equal to 0.0025 Ža., 0.1 Žb. and 0.7 Žc..
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high-field parts of the spectra help to expand these spectra into components. They were simulated assuming that they consist of two spectra ŽB 1 and B 2 . reproduced in Fig. 12. These spectra were calculated by varying g x , g y , g z-values and D Hi assuming that the individual lines have Lorentzian shape. The best fit parameters for B 1- and B 2-spectra are given in Table 2. The shape of the spectra observed for P-13 and P-55 glasses depends on the contributions of B 1- and B 2-spectra. For example, the best fit ratios g s B 1rB 2 for the spectrum of P-13 glass implanted Nq ions ŽFig. 5. are 0.0025; 0.1 and 0.7 for D 1 , D 2 and D 3 , respectively, i.e., the intensity of B 1-spectrum increases with dose. This spectrum has high intensity in the case of Oq-implanted glasses. For example, for the P-13 glass implanted with Oq ions at D 2 the g-value is equal to 0.8 whereas for the same glass implanted with Nq at D 2 g s 0.1. Therefore, the shape of high-field portions of the spectra presented in Fig. 4 differ. Thus, in the ion-implanted samples studied in the present work two main spectra are observed. One of them is the narrow symmetric line of Lorentzian shape with g ( g e Žwhere g e is g-factor of free
electron. named the S-signal. The other is a broad anisotropic spectrum with g z ) g y ) g x ( ge .
Ž 6.
The principal g-values of this spectrum are a function of sample compositions ŽA 1-, A 2-, B 1-, B 2-spectra.. In addition, in the spectra of the S-2 glass the narrow EPR signal with g s 2.0006 is observed. We assume that the defect responsible for this signal is an EX-type center. Our assumption is based on gvalue, linewidth and saturation behavior w6x. The EX-center has been observed in the spectra of some g-irradiated borosilicate glasses Žfor example, Pyrex w19x which is phase separated glass w3x.. We shall not discuss this center in the present work as well as the doublets observed for some Nq-implanted samples. The possible origins of S-, A- and B-spectra will be considered. 4.2. A- and B-spectra The broad anisotropic spectra Žlabeled A or B depending on glass composition. were observed for
Table 3 The g-values for some oxygen-associated defects Center HC1 ŽSi–OHC. HC2 ŽSi–OHC. SHC1 ŽSi–OHC. Al–OHC Oy Oy Oy 2 Oy 2 Oy 2 Oy 2 Oy 2 Oy 2 Oy 2 Oy 2 Ž . Oy 3 ozonid Ž . Oy 3 ozonid ‘So called’ Oy 3 ‘So called’ Oy 3 COq COy COy 2 COy 2 y CO 2
Matrix alkali-silicate glasses CaO–Al 2 O 3 –SiO 2 glasses KBr NaOHP H 2 O MgO Žsurface. ZnO Žsurface. MoO 3 Žsurface. Al 2 O 3 Žsurface. silica-gel SiO 2rAl 2 O 3 CaX-zeolite Ca 5 ŽPO4 . 3 OH KClO 3 SrŽClO 3 . 2 SiO 2 TiO 2 CO:zeolite CO:MgOŽsurface. KO 2 ŽCH 2 .COH CaCO 3 Oyq COrMgO
gz 2.0213 2.0158 2.0173 2.0186 1.987 2.002 2.062 2.042 2.016 2.039 2.025 2.024 2.057 2.058 2.011 2.0095 2.0080 2.0070 2.0045 2.0021 2.0047 2.0032 2.0043
gx 2.0023 2.0118 2.0026 2.0025 2.226 2.007 2.0018 2.0030 2.0042 2.0040 2.0031 2.0022 2.000 2.003 2.0026 2.0020 2.0030 2.0010 2.0005 2.0055 1.9957 1.9978 1.9977
gy
Ref.
2.0088 2.0127 2.0109 2.010 2.226 2.007 2.0089 2.007 2.0098 2.009 2.0095 2.0068 2.007 2.003 2.018 2.016 2.0045 2.0010 2.0005 2.0055 2.0021 2.0017 2.0022
w13x w13x w21x w21x w22x w23x w23x w23x w23x w23x w23x w23x w22x w24x w23x w23x w23x w23x w25x w26x w27x w28x w29x
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all five glasses examined in this work as well as for other ion-implanted glasses w12,14,15x. As mentioned above the similar spectrum observed in Oq-implanted silicate glass w12x has been attributed to silicon–oxygen hole centers ŽOHC. HC1 and HC2 w13x. The principal g-values of the spectra for these centers answer Eq. Ž6.. The OHC are commonly formed in g-irradiated multicomponent glasses. They involve a hole trapped on one or two oxygenŽs. bonded with a high charge network-forming cation ŽM.. For convenience these defects will be referred to as ‘M–OHC’, where M s B, Si, Ge, Al. The principal g-values for some M–OHC are given in Table 3. If the nucleus of M has non-zero spin, hyperfine structure should be present in EPRspectrum of M–OHC w20,21x. The spectra of the A-type ŽA 1 and A 2 . observed in borosilicate samples S-2 and S-3 can be, in principle, assigned to various silicon–oxygen hole centers. However, such an attribution of the A 1-spectrum in the case of P-1 samples becomes problematic since all cations Ž31 P, 27Al, 23 Na. entering into this glass have non-zero nuclear spins. The assumption that the A 1-spectrum of the P-1 samples belongs to M–OHC with unresolved hyperfine structure, for example due to 27Al, is contradicted by the small width of the A 1-spectrum ŽF 2 mT ŽFig. 11.. whereas the total width of the spectrum induced by Al–OHC is in the order of 10 mT w21x. At the same time the g x- and g y-values of A 2-spectrum are not typical of M–OHC w13,20,21x. Therefore, we assume that all the broad anisotropic spectra observed in various ion-implanted oxide glasses belong to identical defects. Only oxygen is constituent common to these glasses, i.e., the A- and B-spectra should be attributed to oxygen-related electron states. The absence of the resolved hyperfine structure in the A 1- and B-spectra of implanted phosphate glasses indicates that the defects responsible for these spectra are not bonded with network-forming cations. Since implantation involves extensive vacancy-interstitial formation it is reasonable to consider interstitial oxygen species are likely candidates for A- and B-spectra in ion-implanted oxide glasses. Several kinds of charged oxygen species have been reported q y y 3y y including Oq and O4y 2 , O , O , O2 , O2 , O3 w23,30x. On the basic of Eq. Ž6. which follows from
115
the theory of Kanzig and Cohen w31x, numerous experimental data w22x and reviews of oxygen species on oxide surface w23,30x, we propose molecular Oy 2 ions as the most likely candidates for the A- and B-spectra. As follows from Refs. w23,31x the g z-component is sensitive to the environment of the Oy 2 ions. This value depends on the oxidation state of the nearest cation ŽM nq . and decreases from 2.11 " 0.05 at n s 1 to 2.015 " 0.005 at n s 6 in the case of Oy 2 ions adsorbed on oxide surfaces w23x. The authors of w23x concluded that g z gives a measure of the cation charge at the adsorption site. The appearance of several peaks corresponding to various g z indicates the existence of several adsorption sites in the system although various g z-values can be observed for one type of cations if adsorption sites have different local environments w23x. The diffuse shape of the low-field parts of the A- and B-spectra which is evident in the large D Hz necessary for computer simulation of these spectra, especially in the case of the phosphate glasses, can be due to the existence of many sites with various degrees of distortions of local environments of the Oy 2 ions. Since all the EPR spectra observed in the present work were constructed from the mentioned four spectra we assume that the majority of Oy 2 ions is located in some defined positions in ion implanted glasses. However, the identification of these positions is a difficult problem since the stabilization of Oy 2 ions depends on a large number of parameters. As follows from Ref. w23x, where empirical dependence of g z-components of Oy 2 ion spectra on cation charge of adsorption site has been found, the g z-value can be used to establish the nature of the adsorption site on an oxide surface. This dependence cannot be applied to bulk samples automatically since the gtensor for Oy ion absorbed on surfaces may be 2 calculated based on the ionic model of this defect whereas in many bulk samples the g-values do not fit the predictions of this model w23x. Nevertheless, based on a comparison of g z obtained for the A 2-, B 1- and B 2-spectra with literature data w22–24x we assume that in ion-implanted sam4q ples the Oy , Ca2q ŽMg 2q . 2 ions are located near Si 3q Ž 2q . and Al Zn sites, respectively Žsee Table 3.. However, such an attribution can be considered only as speculative. For example, g z f 2.017 which is
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characteristic of the A 1-spectrum corresponds to the cations with high charge w23x which are absent in glasses examined in the present work and Refs. w12,14x. At the same time, g z f 2.017 has been w x observed for Oy 2 in the natural CaF2 crystal 32 and a-Al 2 O 3 w33x. Such low g z-values can be due to a decrease in a distance between cations and Oy 2 ions imposed by the structure of bulk samples. According to Ref. w23x the decrease in distance cation-Oy 2 ions leads to a decrease in g z-value. It should be noted that the errors given in Table 2 for the g-values of the A- and B-spectra are essentially larger than the ones obtained for g-factors at computer simulations of M–OHC Žfor example, Ref. w x w21x. and Oy 2 ions 34 in g-irradiated glasses. This is due mainly to wide distribution of g-values in implanted samples because of larger distortions of local environments of Oy ions associated with 2 changes in glass structure in the implantation layer. The summation of greater number of spectra with g-components answering Eq. Ž6. leads in many cases to a better coincidence of calculated and experimental spectra but such artificial procedures do not give new information for understanding the nature of the formation of point defects in ion implanted glasses. We do not exclude also that oxygen species other than Oy ions contribute partially to the broad 2 anisotropic spectra observed in ion-implanted glasses. However, we have now no direct data indicating their presence. As follows from Fig. 8b, c the number of Oy 2 ions responsible for A- and B-spectra in Oq-implanted samples is not greater than in glasses implanted with Nq ions that have a mass near the mass of oxygen. It suggests that the formation of interstiions is mainly due to displaced but not tial Oy 2 implanted oxygen atoms. The displacement of oxygen atoms occurs presumably in the process of elastic collisions. It can be seen in Fig. 8b, c that the concentration of Oy 2 ions depends on glass compositions and decreases with an increasing mass of incident ions. It can be seen in Fig. 9a that the width of depth profiles of implanted ions decreases and their maxima shift towards the surface of the substrate with an increasing mass of incident particles. Based on this result together with the data of Ref. w14x, indicating the increase in the number of Oy 2 ions with increasing energy of incident ion, we assume
that an increase in a projected range of implanted ions promotes the formation of the interstitial molecular Oy 2 ions. 4.3. S-signal It is obvious from Figs. 1–3 that the isotropic lines of width 0.3 F D H F 0.4 mT, labeled S-signal, were observed in the spectra of both borosilicate glasses ŽS-2 and S-3. and phosphate P-1 glass. As mentioned above a similar line has been observed in the spectra of silicate glasses w12,14x. Their baseline crossing is g G g e . The differences in g-values estimated for the S-lines in different samples is presumably due to the presence of an underlying A-component. The computer simulation of S-line gives g s 2.0025 " 0.0005. The S-signals have similar behavior as functions of dose and implants in S-2, S-3 and P-1 glasses ŽFigs. 7 and 8a. and belong presumably to identical defects. A similar signal has been attributed to a vacancy-type defect with one dangling bond localized on a silicon atom in Ref. w14x. However, based on the results obtained in the present work we eliminate the silicon–oxygen unit as possible origin of the S-spectrum since the phosphate P-1 glass contains no silicon. The similarity of S-spectra observed for glasses of different compositions showing the existence of their common source suggests that there should be a constituent common to these glasses. As follows from Table 1, only sodium and oxygen are the constituents to fit this description. Based on the fact that the intensity of the S-spectrum is several times greater in Oq-implanted samples than in samples implanted with other ions we assign this to a defect involving oxygen. At the same time, as follows from Table 1 sodium is present only in S-2, S-3 and P-1 glasses in which the S-signal was observed and is absent in P-13 and P-55 glasses where this signal was not found. The greatest number of S-defects was detected in the P-1 glass in which the concentration of Na 2 O is maximum. These data indirectly indicate that the Naq ions play some role in the formation of defects of the S-type. It should be noted that the identification of the narrow symmetric line with g ( g e is difficult. As mentioned above the signal of the S-type observed in
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Ref. w12x for Oq-implanted silicate glasses has been attributed to Oy-interstitials. However, such assignment seems to be in disagreement with the theory, which predicts g H ) g 5 ( g e w30x and the experimental data for various matrices Žsee, for example, Table 3.. The analysis data presented in Refs. w22,23x show that the spectra of such oxygen species as Oy, Oy 2, q Oy and O are essentially anisotropic. The 3 2 Lorentzian shape of the S-signal indicates exchange interactions between defects, responsible for this signal. However, the average g-value for all the above mentioned defects is more than 2.01 whereas g ( g e for the S-spectrum. As noted in Ref. w23x there are several cases where the nature of the oxygen species is not clear. The almost isotropic spectra were observed for ‘soy called’ Oy form and 3 ions which has an O 2 P O differs from classic ozonide ions. The g-values attributed to the ‘so-called’ Oy 3 ions are given in Table 3 together with those for ozonide. Although a number of reports have been published on the existence of the O4y ion, there is little direct evidence for its identification. The O4y species is a 25-electron radical. A small departure of the principal g-values from g e is expected w23x. It is predicted for this ion that g 5 ; g H ; 2.0023 w23x. The line with g s 2.0030 and width D H s 0.3 mT has been reported for oxygen species adsorbed on ZnO and TiO 2 w23x. Recently w35x the single line with g s 2.00247 and D H ( 0.1 mT ŽEX-center. has been observed in the spectrum of SiO 2 thermally grown on silicon. It has been shown w36x that this line can be attributed to hole trapped on the site of the Si vacancy where an unpaired electron is postulated to be localized over bordering oxygen atoms. The study of parameters controlling the generation of intrinsic EX defects led the authors of Ref. w37x to the conclusion that these defects are located in a region of the SiO 2 film supersaturated with oxygen and that they can be considered as excess-oxygen centers rather than Sibased defects. Thus, both large molecular ions ŽO4y, O 2 P Oy . and EX-type centers can be responsible for the Sspectrum in ion-implanted oxide glasses. Both groups of species are related to aggregation of oxygens in some regions of the implanted layer. Such regions can be formed as a result of sodium depletion which
117
occurs near the surface of implanted substrate ŽFig. 9b., mainly due to the sputtering process w1,38x. Finally, we cannot exclude complete centers associated with carbon impurity, which can penetrate into the surface layer from the pump oil during the implantation process, as a possible source of the S-spectrum. As follows from Table 3 some carbon– . w23,25–29x exhibit oxygen ions ŽCOq, COy, COy 2 almost isotropic EPR spectra with g ( g e . In accordance with XPS data, all the implanted samples studied in the present work have a very high Žfrom ; 20 to 70 at.%. surface concentration of carbon which penetrates into the glass matrices and contaminates a region of about 20 nm below the surface. At higher depths carbon concentration reaches a steady value of ; 1 at.%. Thus, in our opinion the S-line can be due to three types of defects: Ži. large oxygen molecular ions ŽO4y, O 2 P Oy .; Žii. a hole localized on several oxygens ŽEX-type center w35–37x. and Žiii. the carbon– oxygen ions. The more detailed experiments are necessary for clearing this problem.
5. Conclusion From the EPR study of ion-implanted oxide glasses of five compositions and compared with data in literature we found specific features which are characteristic for defects induced by implantation in comparison with those produced by g-irradiation. The principal ones are electron states weakly coupled to the glass-network. The EPR spectra in ionimplanted glasses are attributed to molecular oxygen ions which accumulate in interstitials and voids of implanted layer. Such defects are formed due to displacement of atoms induced by elastic collisions.
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