Effects of ion implantation on intermediate range order: IR spectra of silica

Journal of Non-Crystalline Solids 120 (1990) 241-249 North-Holland

241

EFFECTS OF ION I M P L A N T A T I O N O N I N T E R M E D I A T E R A N G E ORDER: IR SPECTRA OF SILICA R.H. M A G R U D E R , III a Dept. b Dept. ~'Dept. d Solid

a,

S.H. M O R G A N

b,

R.A. W E E K S c and R.A. Z U H R d

of Physics, Belmont College, Nashville, TN, USA of Physics, Fisk University, Nashville, TN, USA of Materials Science and Engineering, Vanderbilt University, Nashville, TN, USA State Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

The reflectance spectra of ion implanted SiO2 glasses has been measured from 5000 cm-1 to 400 cm 1. The silica was implanted with Ti, Cr, Mn, Fe, Cu and Bi to nominal doses ranging from 1 × 1015 ions/cm2 to 1.2 × 1017 ions/cm2 at an energy of 160 keV and currents of approximately 2.6 ~A/cm2. Changes in the intensity of the 1232 cm -1 and 1015 cm -1 vibrational modes are attributed to changes in the intermediate range order (IRO) and to changes in the concentration of non-bridging oxygen (NBO) defects in the implanted layer. These changes are ion and dose dependent. The differing effects on IRO and NBO are attributed to the chemical interaction of the implanted ions with the substrate.

1. Introduction Ion implantation has become a powerful tool for the modification of surface and near surface regions of insulators [1-3]. Oxide glasses have been modified with greater d o p a n t levels and different chemical states of the implanted specie than obtainable by conventional melting techniques [2]. Modification of the surface of oxide glasses for specific optical and electrical properties m a y become possible [4,5]. To understand the role of the implanted ion in oxide glasses, a better understanding of how the implanted ion affects the structure of the host material is needed. M o r e specifically, understanding changes in the structure of the substrate material will give an insight into how the implanted ion interacts with the host structure and how these interactions m a y then be tailored for specific purposes. Transmission infrared spectroscopy is, in some cases, of limited value for thin layer implanted materials. In our case, the thickness of implanted layers is < 0.2 ixm. Thus, n o r m a l transmission methods are insufficient to resolve alterations of spectra in the implanted layer. The bulk absorption eclipses the effects in the implanted layer. Infrared reflectance techniques [1,2] have been

used to p r o b e the vibrational modes of the implanted layer without excessive interference from the bulk material of the substrate. The infrared reflectance of SiO 2 has been measured b y several researchers [6-8]. Several groups have reported on the changes in the reflectivity spectra due to ion implantation [8-10]. In this research we are concerned with changes in the SiO 2 structure with ion implantation. Specifically, our interests are in the changes in intermediate range order, i.e. in the range 2 to 5nm, as defined b y Galeener [11]. Vibration peaks attributable to longitudinal modes are indicative of a degree of intermediate range order (IRO). T w o infrared b a n d s have been attributed to I R O effects and to local defect structures. A 1200 c m -1 peak in the infrared has been attributed to longitudinal optical (LO) modes [12-14]. H u [12] and H u b n e r et al. [13] have d e m o n s t r a t e d the longitudinal character of this m o d e and have observed its relation to the structures of SiO 2. Changes in the intensity of this m o d e have been observed by Williams and Jeanlog [15], who, using infrared absorption, measured c o o r d i n a t i o n changes in silicate glasses as a function of hydrostatic pressure. T h e y observed a decrease in the intensity of this m o d e with a change in coordination f r o m SiO 4 to SiO 6. O c a n a et al.

0022-3093/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)

242

R.H. Magruder et al. / IR spectra of silica

[14] confirmed earlier demonstrations of the longitudinal nature of the 1200 cm -1 peak. They use this assignment to investigate the influence of particle shape and size on the infrared powder spectrum of amorphous SiO 2. A band with a peak at - 1015 cm -1, usually observed as a shoulder on the intense 1125 cm -1 band, has been attributed to a non-bridging oxygen (NBO) [1,16]. This type of defect is due to a disruption of the SiO4 tetrahedra and may signal possible changes in the I R O of the structure. This work was performed to investigate the following hypotheses. Intense radiation damage is expected to completely disrupt any I R O at concentration of > 1 × 1016 i o n s / c m 2. Interactions between substrate ions and implanted ions are expected to produce differing structures which may have new vibrational modes. A m o n g defects such as E ' [17,18], peroxy molecule ion [17,18], (NBO) [17,18] and Si-Si homo bonds [17,18], only the NBO has a vibrational mode which has been tentatively identified in the reflectance spectra. Hence the intensity variation of this band may be indicative of chemical interactions between the substrate and the implanted ion. In this work we have used an infrared reflectance technique to measure the effect of Bi, Ti, Cr, Mn, Fe and Cu implantations on the intermediate range order and production of non-bridging oxygen ions caused by ion implantation.

2. Experimental Two types of high purity silica, Spectrosil A and Spectrosil WF, were implanted at room temperature ( - 30°C) with Ti +, Cr +, Mn +, Fe +, Cu + and Bi ++ ions. The sample preparation and chemical impurities of the samples has been previously reported [19]. The samples were discs 2 cm in diameter by 0.1 cm thick and the implantations were into the faces of the discs. Implantation doses ranged from 1 × 1015 i o n s / c m 2 to 1.0 × 1017 i o n s / c m 2 nominal dose. They were implanted at 160 keV and 2.6 ~tA/cm 2 using an Extrion 2001000 ion implantation accelerator. Ion backscattering techniques with He 4 ++ at 2 MeV were used to measure the depth profile. Details of pro-

Table 1 Intensity loss in reflectance of the 1232 cm 1 peak Nominal dose 0.1 0.3

0.5

1.0

3.0

6.0

12.0

0.05 0.01 0.00 0.05 0.07

0.01 0.04 0.01 0.02 0.05 0.05 0.06

( >( 1016

ions/cm2) Ti Cr Mn Fe Cu Bi

0.6 0.01 0.00

0.03 0.00 0.00 0.00 0.01 0.03 0.04 0.01

cedure are reported elsewhere [19]. The implantation and backscattering (RBS) measurements were carried out at Oak Ridge National Laboratory. The doses reported are nominal doses as determined by current integration. These nominal doses are recorded in tables 1 and 2. The RBS measured doses ranged from 98% to 85% of nominal dose. The infrared reflectance spectra were measured using a Bomen Model DA3 Fourier Transform Infrared Spectrometer having a maximum resolution of 0.02 cm-1. This instrument, as configured with a KBr beamsplitter and wide-band MCT detector, has a range of 5000 to 400 cm -1. Each spectrum was obtained using 5 cm -1 resolution and 500 scans were averaged. Measurements were made at several, usually five, positions on the implanted surface to determine the variability of the spectrum as a function of surface position. The instrument was evacuated in order to reduce interference from atmospheric water and CO 2.

Table 2 Intensity increase in reflectance of the 1015 cm-1 band Nominal dose 0.1

0.3

0.5

1.0

3.0

6.0

12.0

( >( 1016 i o n s / c m 2)

Ti Cr Mn Fe Cu Bi

0.08 0.13 0.14

0.12 0.14 0.13 0.14 0.10 0.11 0.12 0.05 0.02

0.09 0.14 0.12 0.13 0.05

0.16 0.11 0.14 0.11 0.13 0.15

R.H. Magruder et al. / IR spectra of silica

Samples were placed on a Barnes Engineering reflection accessory having an angle of incidence of 10 °. The reference was an aluminum mirror. Peak positions were determined by scanning through the data and finding the reciprocal wavelength at which maxima in intensities occur. The error in peak position using this method is _+2 cm -~. The error is 1 part in 1000 (0.001) in reflectance intensity. The scatter in intensities due to sample position was in all cases greater than the uncertainty in a peak intensity as measured at each position. The errors were estimated to be ___0.005 reflectance intensities based on the scatter arising from different positions. The differences of intensities in tables 1 and 2 are given for two significant figures.

3. Results

Figure 1 shows the RBS data for the distribution of implanted ions for three doses of Mn. Full width at half m a x i m u m amplitude of the distribution and depth at which the maximums occur are comparable for Ti, Fe, Cr and M n over the range of doses for our samples. The profile for the copper implantation exhibits a bimodal distribution for doses > 3.0 × 1016 i o n s / c m 2 and has been reported previously [19]. The Bi distribution is substantially shallower. The important aspect of these distributions is the depth profile of the implanted species. The depth of the implanted species is _< 0.2 t*m for all ion species and doses. At the peak of the distribution, the concentration of implanted ions for a dose of 6 x 1016 i o n s / c m 2 is 3.6 X 10 21 i o n s / c m -3. The number of S i O 2 molec u l e s / c m - 3 is 22 x 1021. Thus, at the highest dose, the fraction of implanted ions is - 0.18. Typical reflectance spectra from 5000 c m - ~ to 400 cm -1 and are shown in figs. 2(a) and 2(b). The main features are peaks at 3600 cm -~, 1125 cm 1, 800 cm -a, 481 cm -1 and shoulders at - 1 2 5 0 and 1015 cm -1. We have discussed the behavior of the peaks at 3600, 1125, 800 and 481 cm-~ as a function of ion species and dose in a previous paper [9]. Here we concern ourselves with the LO mode in the 1232 cm -~ region and the shoulder at 1015 cm -1.

243

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000

(microns)

Fig. 1. Ion concentration as a function of depth in substrate for 3 nominal doses of Mn.

Figures 3(a) and (b) and 4(a) and (b) show the reflectance spectra from 1400 cm-1 to 400 cm 1 for samples implanted with Ti and Bi to a nominal 6 × 1016 i o n s / c m 2 with a comparison to the spectra of the unimplanted sample. From a visual inspection of these figures there is a notable difference in the spectra for these two ion species in the 1250 cm 1 and 1015 cm 1 shoulder regions. The peak responsible for the shoulder near 1250 cm 2 was established using a Fourier selfdeconvolution procedure [20] and the same procedure was applied here to the 1400-900 cm t region. The deconvolution parameters were varied interactively in order to maximize the resolution enhancement while minimizing the side-lobes. This procedure was used with Bessel function apodization and resulted in a half-width for deconvolution of 39.9 cm 1, a narrowing factor of 1 and a Lorentzian fraction of 0.92 (Gaussian fraction = 1 - 0.92). The deconvoluted spectra were used only to locate the peak position of the LO bond near 1232 cm 1 and to determine if the position of the peak shifted with ion species and dose. A typical spectrum before and after deconvolution is shown in figs. 5(a) and (b). The peak centered at 1232 c m - ] is the LO mode and the one with which we are concerned with in this work. We have noted earlier [9] a shift in peak position of the 1125 c m - 1 peak to be both dose and ion species dependent. These same shifts were observed in the reconstruction spectra. The position of the 1232 c m - a peak remains invariant for all doses and ion

R.H. Magruder et aL / 1R spectra of silica

244 1.00

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i

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2500.

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4250.

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species. However, the intensity, changes with dose and ion species. All intensity changes were measured using the recorded data. The deconvoluted intensities were not used. We must be mindful that the technique of reflectance probes a depth on the order of I ~m for a wavelength in the region of this peak. The depth of the implanted layer from RBS is < 0.2 ~m for all ion species and dose levels. As a result, changes in the spectra are due to approximately - 2 0 % or less of the sample depth. The unperturbed bulk material is the source of the remainder of the reflectance intensity. We attribute changes in reflectance intensity to just the implanted layer. The measured difference of intensities between the implanted and unimplanted substrates are given in table 1. For all cases the intensities of the peak at 1232 cm -1 in implanted samples were equal to or less than that of the peak in an

unimplanted sample, hence the values given are losses in intensity. In a similar fashion, the changes in intensity at shoulder are tabulated in table 2. For all ion species and doses, the 1015 cm -1 intensities of the implanted samples were greater than the intensity in the spectrum of the unimplanted sample. The values tabulated represent increases in intensity in the 1015 cm -1 band of the implanted layer. The intensity at 1232 cm -1 in the spectra of Bi implanted samples decreases for the lowest doses. With doses > 3.0 X 1016 i o n s / c m 2, the decrease in intensity is the largest observed for any ion. Within error, the intensity does not change for doses > 3 x 1016 i o n s / c m 2. The increase in intensity at 1015 cm 1 is the smallest for all doses and ion types. The changes in intensities at 1232 cm -1 in the spectra of the Ti implanted samples for the lowest dose show the largest decrease. This decrease in

R.H. Magruder et aL / IR spectra of silica 1 °00

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Wavelength ( l / c m ) Fig. 3. Reflectance spectra of silica from 1400-400 cm 1: (a) implanted with Ti to a nominal dose of 6x1016 ions/cm2; (b) unimplanted.

intensity recovers, returning to the unimplanted intensity within error for the 6.0 x 1016 i o n / c m 2 dose. The intensity at 1015 cm -1 increases with increasing implantation dose, and is the largest increase for all ion species and doses. There is no change at 1232 cm -1 for Cr implantation doses < 1 x 1016 i o n s / c m 2. With larger doses, there is a decrease in intensity at 1232 cm -1. At 1015 cm -1 a large increase in intensity occurs at dose of 3 x 1015 i o n s / c m 2. The rate of increase as a function of dose decreases with increasing dose. In the case of Mn implantation, no decrease at 1232 cm -1 is detected, within error, for all doses. There is an increase in intensity at 1015 cm -1 for dose levels < 1 x 1016 i o n s / c m 2. There is no additional increase for all larger doses.For the largest Fe implantation doses, a small decrease in intensity is observed at 1232 cm -1. The intensity at

1015 cm -1 increases for a dose of l x l 0 1 6 i o n s / c m 2 and is marginally greater for higher dose levels.For Cu implantation for doses > 3 x 1015 i o n s / c m 2 there is an initial decrease at 1232 cm-1. The decrease continues to a dose of 3 x 1016 i o n s / c m 2 and does not change for higher doses. At 1015 cm -1 an initial large increase in intensity is followed by smaller increases with increasing dose level.

4. D i s c u s s i o n

Implantation of these ions has two general effects. The first is radiation damage producing electronic as well as atomic defects which have been previously described for neutron irradiation. Bates et al. [21] have reported losses in the intensity of the LO mode at 1232 c m - 1 in the infrared

246

R.H. Magruder et al. / 1R spectra of silica i

t .00

=

c

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~ (b)

0.80

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(l/cm)

Fig. 4. Reflectance spectra of silica from 1400-400 c m - l : (a) implanted with Bi to a nominal dose of 6×1016 ions/cm2; (b) unimplanted.

absorption spectra with neutron irradiation. They summarize their R a m a n measurements to indicate that neutron irradiation enhances the disorder by increasing the number of defects in the glass. We interpret their decrease in the intensity of the LO mode as a disruption of the I R O of the host material. The second general effect is chemical interactions of implanted ions with the host substrate. It m a y not be possible to separate these two general effects. However, for the intermediate range order in the case of oxide glasses, we can to a degree envision that until a sufficient number of ions are implanted, their concentration must be that they are incapable of creating new chemical phases that extend throughout the implanted layer and hence changes in the host structure will be small. The possibility of an implanted ion interacting with an existing defect by implantation to create extended defects does exist at higher con-

centration of implanted ions. These extended defects may then produce additional changes. Radiation damage in the form of network rupture and broken bonds may result in increased ion motility [22] and some rearrangement, changing the I R O of the host material by changing the average ring size [11]. In the following discussion we propose two hypotheses. First, the intensity of 1232 cm-1 band is directly proportional to the number of rings occupying a volume whose radius is in the range of 2 to 5 nm and serves as a measure of the I R O in unirradiated silica. A decrease in the intensity of the 1232 cm -1 peak is proportional to a decrease in the n u m b e r of tings in the I R O volume. The second hypothesis is that the intensity of the 1015 cm -1 shoulder is inversely proportional to the degree of connectivity of the network. An increase in intensity is indicative of increasing

R.H. Magruder et al. / IR spectra of silica 1.00

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numbers of NBOs and consequently of loss of connectivity in the host material. This creation of broken bonds may increase ion mobility and result in changes in the IRO. The relative effect of the implanted ions at lowest dose on the I R O as measured by loss of I(1232 cm - t ) is in the order of Mn < Cr < Fe < Bi < Cu < Ti and at the highest dose Mn < Ti < Fe < Cr < Cu < Bi. Throughout the range of dose Mn implantations decrease the I R O least. The largest decrease is produced by Cu and Bi at the highest doses. Ti implantation causes the largest effect at low dose but is similar to Mn at the highest dose. At the lowest doses, the relative effect of implantation on the 1015 cm -1 shoulder is Bi < Ti < Fe < Cu < Mn < Cr while at the highest doses the relative effect is in the order of Bi < Cr < Mn < Fe < Cu < Ti.

The large loss of intensity in the 1232 cm -1 peak with increasing Bi implantation is indicative of a decrease in the ring structure of the host material. This decrease is accompanied by the smallest increase in the S i - O dangling bonds as indicated by the small increases in the 1015 c m - t band. Our interpretation of these results is that the Bi interacts chemically with the SiO network upon implantation to create a new Bi-Si phase in the implanted layer, destroying the ring structure but because of chemical interactions not creating dangling S i - O bonds. Additional evidence for this interpretation is found in the optical spectra for Bi implanted samples [23]. For low doses the optical spectra show no evidence for a B 2 center (neutral oxygen vacancy). An optical band due to these centers has been shown to be present in samples implanted with Cr, M n and Fe [24]. We conclude from these observations that the Bi interacts with

248

R.H. Magruder et al. / IR spectra of silica

these oxygen vacancies or dangling Si type structures to form new phases. This interaction destroys Si dangling bonds. The spectra of Ti implanted samples show a large loss in the 1232 cm -1 band only at the lowest dose level. We attribute this large deviance to radiation damage similar to the changes observed in neutron irradiated silica. With increased dose level, the loss in the 1232 cm -~ bond is reduced. The intensity returns to almost the unimplanted value, but with large increases in the number of S i - O dangling bonds. We suggest that the Ti ions are incorporated into the SiO network as a network former maintaining the overall ring structure of the material while giving rise to a large number of defect centers due to the radiation damage. The spectra of Mn and Fe samples, e.g. NBOs, exhibit only small changes in the 1232 cm -1 band for all doses while showing an increase, with initial dose, of the 1015 cm -1 band. This 1015 cm -1 band increases slightly for subsequent doses. Perez et al. [25] have reported the formation of FeO, Fe203 and Fe ° precipitates with ion implantation at 1.2 x 1017 i o n / c m 2 dose levels in SiO 2 We interpret our results to indicate that both Mn and Fe precipitate to form these particles. Both Fe and Mn have approximately half filled 3d shells, and it is not surprising that the behave similarly. We suggest that these precipitates are small, i.e. less than 3 nm in size, and cause only a small disruption in the network, insufficient to alter basic ring structure of the host material. The spectra of Cr implantated samples show no change at the 1232 cm -a from the unimplanted value for doses < 1.0 x 1016 i o n s / c m 2. However, large changes at the 1015 cm -1 at these dose levels are observed indicating an increase in the S i - O dangling bond concentration. For these lower doses, the effects of Cr are similar to those of Fe and Mn. Cr, Mn and Fe all have partially filled 3d shell configurations and behave chemically in a similar manner. However, at higher dose levels > 3.0 × 1016 i o n s / c m 2, the effect on the NBOs differs from that of Mn and Fe. There are increased losses at the 1232 cm -1 and smaller increases at the 1015 c m - 1 band than for the lower implantation doses. We attribute this behavior to

a chemical interaction similar to that of the Bi implanted sample. Cu implantations show increasing loss at the 1232 cm -1, saturating for doses >_3.0x1016 i o n s / c m 2. There are also increases at 1015 cm -1 with increasing dose levels. Magruder et al. [9] attribute absorption bands in the spectra of Cu implanted samples to Cu colloids in both spherical and spheroidal shapes. These colloids may be of a size to disrupt the ring structure as their size increases from those formed at the lower doses. The formation of Cu ° colloids removes the Cu from chemical interactions with NBOs. Consequently the largest numbers of NBOs are observed in the Cu implanted sample.

5. Conclusions (1) Implantation significantly alters the vibration spectra due to modes arising from intermediate (2-5 nm) order. (2) This alteration is due to disruption of the ring structure. (3) The effects of this disruption are ion specific. (4) The chemical interactions between substrate ions and implanted ions are ion specific with Ti and Bi having the highest reactivity.

References [1] G.W. Arnold and P.M. Mazzoldi, in: Ion Beam Modification of Insulators, eds. P. M. Mazzoldi and G. W. Arnold (Elsevier, Amsterdam, 1987). [2] P.D. Townsend, Pep. Prog. Phys. 50 (1987) 501. [3] A. Perez, J. Bert, G. Marest, B. Sawicka and J. Sawicki, Nucl. Instr. and Meth. 209/210 (1983) 281. [4] J.E. Gall, R.D. Standley and W.M. Gibson, Appl. Phys. Lett. 21 (1972) 72. [5] J.P. Kurmer and C.L. Tang, Appl. Phys. Lett. 42 (1983) 146. [6] P.H. Gaskell and D.W. Johnson, J. Non-Cryst. Solids 20 (1976) 155. [7] I. Simon, in: Modern Aspects of the Vitreous State, ed. J.D. Mackenzie (Butterworth, London, 1960). [8] G.W. Arnold, Radiat. Eff. 65 (1982) 17. [9] R.H. Magruder, S.H. Morgan, R.A. Weeks and R. Zuhr, S.P.I.E. 970 (1988) 10. [10] G.W. Arnold and J.A. Borders, J. Appl. Phys. 48 (1977) 1488.

R.H. Magruder et al. / IR spectra of silica

[11] F.L. Galeener, in: The Physics and Technology of Amorphous SIO2, ed. R.A.B. Devine (Plenum Press, New York, 1988) p. 1. [12] S.M. Hu, J. Appl. Phys. 51 (1980) 5945. [13] K. Hubner, L. Schumann, A. Lehmann, H.H. Vajen and G. Zuther, Phys. Status Solidi B104 (1981) K1. [14] M. Ocana, V. Fornes and C.J. Serna, J. Non-Cryst. Solids 107 (1989) 187. [15] Q. Williams and R. Jeanlog, Science, 239 (1988) 902. [16] G.W. Arnold, in: The Physics of MOS Insulators, eds. G. Lucovsky, S.T. Pantelides and F.L. Galeener (Pergamon Press, New York 1980) p. 112. [17] E.J. Friebele and D.L. Griscom, in: Treatise on Materials Science and Technology, Vol. 17, eds. M. Tomozawa and R.H. Doremus (Academic Press, New York, 1979) p. 257. [18] D.L. Griscom, in: The Physics and Technology of

[19] [20] [21] [22] [23] [24] [25]

249

Amorphous SiO2, ed. R.A.B. Devine (Plenum Press, New York, 1988) p. 125. R.A. Weeks, H. Hosono, R. Zuhr, R.H. Magruder and H. Mogul, Mater. Res. Soc. Proc. 152 (1989) 115. J.K. Kauppinen, D.J. Moffatt, H.H. Mantsch and D.G. Cameron. Appl. Spectrosc. 35 (1981) 271. J.B. Bates, R.W. Hendricks and L.B. Shaffer, J. Chem. Phys. 61 (1974) 4163. W. Primak, The Compacted States of Vitreous Silica (Gordon and Breach, New York, 1975). R.H. Magruder III and R.A. Zuhr, to be pubhshed. J.D. Stark, R.A. Weeks, G. Whichard, D.L. Kinser and R.A. Zuhr, J. Non-Cryst. Solids, 95&96 (1987) 685. A. Perez, M. Treilleux, T. Capra and D.L. Griscom, J. Mater. Res. 2 (1987) 910.