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Cross-sectional TEM investigation of tin-implanted SiO2 glass
Journal of Non-Crystalline Solids 262 (2000) 114±125
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Cross-sectional TEM investigation of tin-implanted SiO2 glass Thomas H oche a,*, Torsten Angermann b a
Otto-Schott-Institut f ur Glaschemie, Friedrich-Schiller-Universit at, Fraunhoferstraûe 6, D-07743, Jena, Germany b Institut f ur Physikalische Chemie, Friedrich-Schiller-Universit at, Lessingstraûe 10, D-07743, Jena, Germany Received 30 June 1999; received in revised form 29 October 1999
Abstract We report on 300 kV Sn -implantation into high purity silica substrates at liquid nitrogen and room temperature. The evolution of the Sn depth pro®les in dependence on subsequent heat treatments was monitored by cross-sectional transmission electron microscopy, energy-dispersive X-ray spectrometry analyses, and Rutherford backscattering spectrometry. Using selected-area electron diraction, the existence of nanocrystalline b-Sn particles could be proved in the as-implanted state independent on the substrate temperature during implantation. After subsequent exposure to 873 K for 1 h, the upper part of the tin distribution becomes oxidised, whereas the deeper fraction of tin nanocrystallites undergoes a coarsening resulting into several 10 nanometer large, spherical b-Sn crystals. If the implantation is followed by a heat treatment at 1373 K for 6 h, tin is entirely oxidised and its distribution is shifted further towards the surface. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.16.B; 68.55.L; 42.70.C; 81.70.J; 61.85
1. Introduction Wide band gap glasses have been super®cially modi®ed for various reasons ranging from locally varying optical properties (e.g., for waveguide applications and photonic switches) to improved fracture resistance and decreased rates of corrosion. For surface modi®cation, a series of experimental techniques including multi-target sputtering [1±3], ion-exchange (see e.g., [4±7]), and surface reactions between a metal±organic complex and the substrate [8] are employed.
* Corresponding author. Tel.: +49-3641 948 538; fax: +493641 948 502. E-mail address: [email protected] (T. HoÈche).
Besides these techniques, ion implantation is a frequently used method to achieve modi®cations of glass surfaces since it allows for the preparation of buried layers including almost all kinds of elements or even compounds when co-implantation is applied and is therefore capable to locally adjust optical properties. Along with the ion implantation, a radiation-induced modi®cation of the hosting substrate structure occurs. Such structural modi®cations are not necessarily disadvantageous since they are under suspicion of provoking nonlinear optical eects in the damaged glass. Moreover, structural defects can be reduced or even entirely annealed by a post-implantation heat treatment. However, such tempering processes not only aect the damaged host but also the depth distribution and microtopology of the implanted species.
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 8 6 - 9
T. H oche, T. Angermann / Journal of Non-Crystalline Solids 262 (2000) 114±125
Ion implantation of silica glass was reported for a variety of ions including Cu [9±13], Ag [14,15], Au [16,17], Ni [13,18], and Pb [19]. Sn ionimplanted silica glass was shown to exhibit nonlinear optical properties after Sn -implantation at 400 kV acceleration voltage and a dose of 2 1017 ions cmÿ2 at room temperature [20,21]. Under the above-mentioned conditions, metallic tin microcrystallites of 4±20 nm diameter are found to be embedded in the silica glass. These nanocrystals are considered to be the major reason for the nonlinear optical eects observed. In this study, we report on Sn -implantation into high purity silica substrates at 300 kV acceleration voltage at two ion doses (1.1 and 1:9 1017 ions cmÿ2 ) and two substrate temperatures during implantation (at liquid-nitrogen and room temperature) [22]. Cross-sectional transmission electron microscopy (XTEM) and energy-dispersive X-ray spectrometry (EDXS) analyses were applied to elucidate the as-implanted state and the in¯uence of various subsequent heat treatments on the Sn depth pro®le. Image-processing routines are used to determine size and number density distributions of tin and tin oxide crystallites, respectively. EDXS depth pro®les are compared with Rutherford backscattering spectrometry (RBS) data. Based on results obtained at as-implanted samples, tin-distribution dierences between samples tempered at 600°C and 1100°C are discussed. Although super®cial tin-enriched layers in glasses are frequently associated with ¯oat glass, the Sn -implanted SiO2 glass investigated here and ¯oat glass have to be carefully distinguished for a couple of reasons including: 1. During ion implantation, the system is far from thermodynamic equilibrium and thus rules of equilibrium thermodynamics (concerning diusion, solubility etc.) must not be applied. 2. Upon ion implantation, local tin concentrations of between 60 and 90 wt% are obtained, whereas in ¯oat glass the maximum tin concentration measured e.g., by Townsend [23] is as low as 0.8 wt% (it is just one of the main intentions of ion implantation to introduce foreign ions at concentrations far beyond values that can be obtained by equilibrium processes). Moreover, the maximum depth of tin pro®les in ¯oat glass-
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es is several 10 lm which is about two orders of magnitude larger than the few 100 nm observed in our Sn -implanted samples. 3. Soda-lime ¯oat glasses are not only chemically very much dierent from high-purity SiO2 glass (and therefore all deduction concerning diusivities must not be made), but the glass network is extremely damaged by the implantation process itself. The resulting radiation-induced structural modi®cation of SiO2 substrate (every substrate atom in the implanted layer is displaced several times during implantation) can be annealed only after an about 1 h lasting heat treatment at 600°C. Thus, also if diusion coef®cients of Sn0 , Sn4 , and O2ÿ were available (they have been determined to our knowledge only for oxygen self diusion, see e.g., [24]), their usage to derive a model that explains the enrichment of tin to the surface would be very questionable. Consequently, tin depth pro®les of Sn -implanted silica glass and ¯oat glass must not be compared.
2. Experimental 2.1. Ion implantation High-purity silica glass substrates of 3.5 mm thickness were polished and subsequently etched in 5% HF to remove grinding and polishing residuals. After removing this way a some 0.5 lm thicksuper®cial layer, the substrates were one-sided implanted with Sn ions at an acceleration voltage of 300 kV. All implanted substrates originated from the same glass batch of synthetic, SQ 1-type silica glass. Although the nominal dose 1 2 was kept constant at 1:5 1017 ions=cm for all implantation, the ®rst series of samples was im2 planted at an eective dose of 1:14 1017 ions=cm at room temperature (samples 1A, 1B, and 1C) 1 In contrast to the nominal dose (which is measured during implantation by counting charges transferred through the SiO2 substrate), the eective dose referred to later on can be directly measured after implantation by means of RBS spectrometry.
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Table 1 Samples studied, respective implantation temperatures, Sn -implantation doses, thickness of super®cial material loss layer upon ion implantation and heat-treatment schedules Sample
Substrate temperature during implantation (K)
(Nominal)/eective dose (1016 ions cmÿ2 )
Thickness of super®cial material loss layer upon ion implantation (nm)
Subsequent heat treatment
1A 1B 1C 2A 2B 2C
77 77 77 300 300 300
(15.0)/11.4 (15.0)/11.4 (15.0)/11.4 (15.0)/19.2 (15.0)/19.2 (15.0)/19.2
22 22 22 101 101 101
Untreated 873 K, 1 h 1373 K, 6 h Untreated 873 K, 1 h 1373 K, 6 h
whereas in the second series (implanted at 1:92 1017 ions=cm2 , samples 2A, 2B, and 2C), substrates were cooled with liquid nitrogen (77 K). Both series were studied as-implanted and after subsequent heat treatments at 600°C for 1 h and 1100°C for 6 h, respectively. Cooling rates used were larger than 10 K minÿ1 . The notation of all samples is compiled in Table 1. 2.2. XTEM characterisation XTEM foils were prepared as follows. First, implanted substrate pieces of approximately 6 6 mm2 were cut into two equal parts. The latter were glued together face-to-face using epoxy resin in the second step. Third, the resulting sandwiches were cut perpendicular to the interface, planeparallel ground to 100 lm thickness, one-sided polished and dimpled to a residual thickness of 10± 15 lm from the unpolished side. In the fourth step, double-sided Ar ion-beam etching at an acceleration voltage of 2.5 kV and ion-beam currents of 1.0 mA using low incidence angles (6°) was applied until perforation of the foil was just achieved. For the microstructural characterisation of cross-sectional foils, a Hitachi H-8100 II transmission electron microscope operating at 200 kV acceleration voltage was used. EDXS analyses were performed with an attached Link ISIS analyser (Oxford Instruments; Si:Li detector, atmospheric thin window). High-resolution transmission electron microscopy (HRTEM) was performed on a JEOL JEM 4000EX (operation voltage: 400 kV, For the digitipoint-to-point resolution: 1.6 A). sation of plane-®lm TEM negatives, a EIKONIX
99/87 scanner camera was used. Indexing of diffraction patterns was preceded by a rotation averaging of the digitised negative using the image processing environment IDL4.0 (Research Systems, Boulder, CO). 2.3. Rutherford backscattering spectrometry Using Rutherford backscattering spectrometry (RBS), the depth distribution of implanted Sn ions was probed using 1.4 MeV He ions at a backscattering angle of 120°. The RBS detector was calibrated using both an oxidised and a Ge/Au covered silicon wafer. Since the SiO2 substrate is an insulator, excess voltages were avoided by placing an electron source next to the substrate surface in the evacuated scattering chamber ( p 10ÿ3 Pa). 2.4. Surface pro®le measurements After implantation, homogeneous, ¯at depressions were found in the implanted areas. These super®cial substrate material losses were monitored using a surface pro®ler (Dektak 3030 ST, Veco-Instruments) by comparing non-implanted and implanted parts of the substrate. On the basis of these measurements, the depth scale can be calibrated. 3. Results 3.1. As-implanted samples A typical cross-sectional transmission electron microscopy (XTEM) micrograph of the sample implanted at 77 K (1A) is shown in Fig. 1(a).
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Fig. 1. XTEM micrographs recorded at low-temperature implanted samples Timpl: 77 K); the position of the surfaces is indicated by arrows: (a) untreated (1A), (b) heat-treated at 873 K for 1 h (1B) and (c) after heat-treatment at 1373 K for 6 h (1C).
Visual inspection of the micrograph reveals the formation of spherical nanocrystals with an average particle size increasing with depth. The crystals reach their maximum size about 170 nm below the surface. Very small particles exist at greater depths indicating that the range of the implanted Sn goes beyond the maximum deposition layer down to some 300 nm. This observation could be veri®ed by image processing of digitised TEM negatives. The corresponding particle-size distribution is given in Fig. 2(a) along with the number density of tin particles counted in a series of boxes 52 nm wide in the direction perpendicular to the surface (Fig. 2(b)). 2 As proved by electron microdiraction (Fig. 3(a)) and high-resolution TEM (HRTEM) as well (not shown), at both implantation temperatures b-Sn microcrystallites are formed. As shown in the rotation-averaged line scan (Fig. 3(b)) of the electron-diraction pattern indicated in Fig. 3(a), all diraction rings can be attributed to re¯ections of b-Sn. All major re¯ec2 The number density was calculated under the most reasonable assumption that particles counted reside in a wedge-shaped sample (Wedge angle ~12°; wedge parallel to the implanted surface in cross-section). Since the exact measurement of TEMfoil thickness is aicted with considerable errors of about 20%, values of the number density carry large error bars.
tions occur and none of the rings could not be assigned. In the sample implanted with a substrate kept at room temperature (2A), a layer consisting of b-Sn crystallites with a relatively large average size is covering a deeper region hosting particles of only a few nanometer diameter (cf. Fig. 2(c)). Fig. 4(a) reveals that the maximum of the particle distribution is shifted towards the `surface'. In fact, this upper end of the implanted region has little in common with the virgin surface of the substrate prior to implantation since during implantation swelling and self-sputtering are acting simultaneously. The latter process is temperature dependent and therefore depth pro®les of samples 1A and 2A cannot compared without more detailed information on the depth scale as detailed below. Therefore, surface pro®le measurements were applied proving the sample implanted at room temperature to have lost an about 100 nm thick layer of its surface, whereas at 77 K the thickness loss amounts only 22 nm (cf. Table 1). Using elliptical beam spots (main axes 20 and 100 nm), energy-dispersive X-ray spectra were acquired and plotted as a function of depth for samples 1A and 2A (Fig. 5). The resulting pro®le shapes are in excellent agreement with the RBS measurements superimposed. The dierence between absolute values will be discussed below.
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Fig. 2. Depth distribution of average particle diameter (a and c) and number density of tin particles (b and d) for the samples implanted at 77 K (1A) and at room temperature (2A).
3.2. Heat treatment at 873 K for 1 h If the specimen implanted at 77 K substrate temperature is heat treated at 873 K for 1 h (sample
1B), b-Sn spheres with diameters ranging from 65 to 120 nm are formed at depths of some 250 nm (Fig. 1(b)). Super®cial tin crystallites, however, become oxidised and form SnO2 nanocrystals. In addition
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Fig. 3. (a) Selected-area electron diraction pattern of the implantation layer in the as-implanted sample 1A Timpl: 77 K). (b) Radial line scan of the rotation-averaged pattern (a). Positions of b-Sn re¯ections are marked.
to selected-area electron diraction, this ®nding was proved by HRTEM micrographs (cf. Fig. 6). A similar observation (Fig. 4(b)) is made with sample 2B (implanted at 300 K substrate temperature and subsequently heat treated at 873 K for 1 h) where at an average depth of some 150 nm, bSn spheres with smaller diameters (ranging from 25 to 70 nm) are formed from the deeper fraction of tin nanocrystallites. Also here, super®cial tin nanocrystals are transformed into SnO2 crystals. 3.3. Heat treatment at 1373 K for 6 h After exposure to 1373 K for 6 h, an additional shift of the particle distribution towards
the surface of the implanted samples is observed as illustrated by Fig.1(c) (sample 1C) and Fig. 4(c) (sample 2C). Sn-depth distributions detected by EDX spectrometry and RBS (see Fig. 7) parallel this behaviour. The absolute tin concentration obtained by EDXS pro®ling is sensitively in¯uenced by the assumption of a correct thickness of the TEM foil. This is because of absorption processes mainly concerning the oxygen signal which in turn aect the Sn concentration since the total concentration of O, Si, and Sn has to be assumed to be 100%. Due to this considerations, an upper limit of the Sn concentration (thickness zero, i.e., no absorption) and a lower threshold (foil thickness 500 nm, considerable
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Fig. 4. XTEM micrographs recorded at room-temperature implanted samples Timpl: 300 K); the position of the surfaces is indicated by arrows: (a) untreated (2A), (b) heat-treated at 873 K for 1 h (2B) and (c) after heat-treatment at 1373 K for 6 h (2C).
absorption) is also shown in Fig. 7. The actual value (which might be more accurately determined by RBS) should lie somewhere between these limits. A detailed discussion including a comparison of EDXS and RBS spectra will be given below. In contrast to both as-implanted samples and the samples heat treated at 873 K for 1 h as well,
no evidence for the formation of any b-Sn crystallites is given after the heat treatment at 1373 K for 6 h. As proved by the electron microdiraction pattern shown in Fig. 8(a), tin nanocrystals formed upon ion implantation become entirely oxidised instead. In this pattern, owing to the small size to the crystallites, some 20 diraction rings are visible and as proven by the radial
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Fig. 6. HRTEM micrograph recorded at the surface of sample 1B. Lattice-planes and angles between them are indicated for the twinned nanocrystal proving it to be cassiterite.
Fig. 5. EDXS depth pro®les (hollow symbols) of sample 1A Timpl: 77 K) and sample 2A Timpl: 300 K) and respective pro®les from RBS measurements (solid symbols).
section of the rotation averaged pattern (Fig. 8(b)), all rings can be assigned to re¯ections of SnO2 , cassiterite.
4. Discussion A detailed comparison between samples implanted at dierent temperatures is reasonable
Fig. 7. Juxtaposition of EDXS depth pro®le of Sn concentration (assuming three dierent thickness values for the TEM foil) and RBS measurement of the Sn pro®le: (a) sample 1C and (b) sample 2C.
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Fig. 8. (a) Selected-area electron diraction pattern of the super®cial layer of sample 2C Timpl: 300 K, heat treatment at 1373 K for 6 h). (b) Radial line scan of the rotation averaged pattern (a), positions of SnO2 re¯ections are marked.
only if the depth scale is properly calibrated. In this context it is essential to note that the ®gure `thickness of super®cial material loss layer upon ion implantation' given in Table 1, accounts for the sum of two competing eects. On the one hand, ion implantation is accompanied by sputter removal of the substrate material resulting in a (considerable) shift of the depth scale origin. On the other hand, the implanted layer is swelling since implanted Sn ions have to be accommodated and moreover the host material suers from radiation damage. For either samples 1A and 2A, swelling results in a thickness gain of the
implanted layer of about 61 nm. 3 In a ®rst approximation, swelling can be assumed to be independent on the substrate temperature and therefore both series of samples can be compared. Due to the super®cial materials loss upon implantation, sample 1A has lost about 22 nm whereas sample 2A lost as much as 101 nm (Table 1). This eect results in a shift of the depth scales. Returning to Figs. 2 and 5, this obviously corresponds exactly to the shift of the particle size maximum and the maximum tin deposition peak, respectively. The coarsening of crystalline b-Sn spheres of between 65 and 120 nm diameter (1B) and 25±70 nm diameter (2B) after 1 h at 873 K is indicating that oxygen diusion into the tin-implanted substrate must be restricted to the topmost layers since electronegative metal ions do not chemically interact with the SiO2 glass network during the implantation process [25] and, consequently, oxidation can only occur with oxygen from outside the sample. On the contrary, Sn0 diusion, however, must be rather fast since otherwise the coarsening of b-Sn crystals would not be observed. Comparing the estimated depth of occurrence of these spheres (between 200 and 250 nm for sample 1B and about 150 nm for sample 2B) with the tin concentration depth pro®les for samples 1A and 2A (given in Fig. 5) reveals that the large crystalline tin spheres are growing at the deeper tail of the tin distribution. This ®nding is also supported by RBS measurements that exhibit a bimodal tin distributions in samples 1B and 2B. The mobility of Sn0 in the glass network is very likely to be enhanced by structural damage of the implanted glass (glass defect centres within the depth range of the implanted Sn ions). Such enhanced diusion inside the implanted super®cial layer would explain that upon heat treatment the depth pro®le of tin does not considerably propagate into the substrate but is con®ned to the implantation depth.
3 A detailed account on how this value was obtained by combining subsequent chemical etching, RBS and infrared re¯ection spectrometry will be given elsewhere.
T. H oche, T. Angermann / Journal of Non-Crystalline Solids 262 (2000) 114±125
Of course, diusion of Sn0 is not restricted to the buried tin layer but those atoms will also be able to reach the surface. Since the heat treatment is performed in air, Sn0 atoms approaching the surface can be oxidised by oxygen penetrating the surface from outside. Taking into account the much dierent melting points of SnO2 and b-Sn, it becomes clear that, due to chemical potentials of tin in metallic tin and cassiterite, once oxidised tin species are hardly capable to leave the tin oxide aggregates again. Thus, tin is trapped at the surface (and subsequently enriched here) and hence we can propose that there are two distinct layers forming at 873 K after 1 h: a super®cial region containing cassiterite nanocrystals and a buried layer which was beyond the diusion range of oxygen and where in order to reduced the overall enthalpy small tin nanocrystals are forming larger aggregates. After exposure to a heat treatment at 1373 K for 6 h, however, large spherical tin microcrystals do not exist. It is subject to speculation whether they were not formed at all or dissolved during later stages of the treatment. Anyway, this ®nding results from enhanced diusion of both Sn0 and oxygen at elevated temperature. Due to the larger mobility, the heat treatment at 1373 K can gradually level out the dierences in the tin depth distribution initially formed by Sn implantation. Particularly from Fig. 7 it is obvious that the width of the tin distribution remains larger after implantation at lower temperature since the maximum of the tin distribution which is shifted towards the surface upon tempering is initially located at greater depth. After 6 h at 1373 K, the tin concentration pro®le is still 1.5 times wider for the low-temperature implanted silica glass as compared with the sample implanted at room temperature. Going back to Figs. 5(b) and 7(b), the question arises as to whether RBS or EDX analyses are more reliable since here the absolute tin concentration determined by either methods diers up to a factor of two. The answer is clear: RBS data are much more reliable. Although some scatter of the data was observed during taking RBS spectra at samples 2A and 2C, the accuracy of the RBS pro®les themselves is within a few percent. On the
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contrary, EDX spectra suer from an unknown variable essentially in¯uencing the results: the thickness of the analysed part of the TEM foil. As indicated in Figs. 7(a) and (b), depending on this parameter, the tin concentration can vary by a factor of about 2.5. In the plot of Fig. 7(a), the absolute Sn concentration determined by RBS and EDXS agrees well for a reasonable TEM-foil thickness of about 100 nm. However, although the TEM foil of sample 2C was thicker than the foil of sample 1C, a thickness of about 500 nm as might be concluded from Fig. 7(b) is not yet reasonable. At this point, a principle dierence between RBS and EDX spectra should be recalled since the latter is responsible for the dierences occurring. RBS data average over an area of between 0.2 and 1 mm2 , whereas EDX spectra were acquired from a few 100 nm wide cross-section of the implanted sample. Besides the systematic error introduced upon quanti®cation of the EDXS data, experimental scatter of the tin distribution is very likely. Finally, it can be concluded that the quanti®cation of the tin concentration depth pro®le is done best with the RBS data and there is very good agreement of the overall shape of the tin distribution when RBS and EDX spectra are compared. Comparing RBS spectra of samples 1A and 2A (Fig. 5) and 1C and 2C (Fig. 7) reveals a signi®cant dierence in the overall tin content since integrating RBS spectra over depth results in areas diering by almost a factor of two. At ®rst glance, this seems to be in contradiction with equal nominal doses. However, the nominal values have to be distinguished from eective doses, since spark discharges (occurring between implanted surface and grounded backside of the sample) or insucient charge conductance distort the nominal dose. In the implantation chamber, no electron source could be installed to balance charges. Therefore, spark discharges as a consequence of excess voltages occur increasing the eective dose in comparison to the nominal dose. On the contrary, the eective dose is reduced by self-sputtering eects and charging of the implanted substrate (the latter eect is responsible for the occurrence of non-Gaussian pro®les observed
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since approaching ions are decelerated or even de¯ected prior to reaching the surface). The rate of self sputtering can be reduced by cooling the substrate upon implantation and consequently at low temperatures, larger doses can be implanted. Moreover, self sputtering does in¯uence the tin distribution depth pro®les. 5. Conclusions After Sn -implantation at room temperature, nanocrystalline b-Sn particles (diameters ranging from 2 to 27 nm) are formed in a 300 nm thick layer of the silica glass. Cooling of the substrate to liquid nitrogen temperature during implantation results in a tin distribution with a deeper maximum since self-sputtering eects are drastically reduced. If the substrate is not cooled, the position of the distribution maximum is shifted towards the surface and the upper tail of the pro®le is even truncated. With all samples a strict correspondence of tin concentration pro®le and particle distribution could be shown. Moreover, the tin distribution determined by EDXS is in good agreement with RBS measurements. Independent on the substrate temperature during implantation, upon tempering at 873 K for 1 h, the tin distribution is becoming wider and bimodal. On the one hand, tin is becoming enriched at the surface (it is trapped by becoming oxidised) and on the other hand, up to 120 nm large b-Sn single crystals are formed in a buried layer at 150± 250 nm depth. The super®cial layer is completely oxidised resulting in nanocrystalline cassiterite (SnO2 ). If the Sn -implantation is follow by an exposure to a heat treatment at 1373 K, a further broadening of the tin distribution can be recognised and the maximum of the tin pro®le is shifted further towards the surface. Tin is found exclusively in cassiterite nanocrystals. Acknowledgements Specimens investigated here are part of T.A.'s PhD thesis. He is grateful to his thesis advisor
Professor H.H. Dunken who gave him the opportunity to work at the Institut f ur Physikalische Chemie of the Friedrich-Schiller-Universitat, Jena. The authors would like to thank Professor W. Wesch (Institut f ur Festk orperphysik, FSU, Jena) and his group for performing the Sn ion implantation as well as RBS measurements. T.H. is indebted to Professor M. R uhle, Max Planck Institute for Metals Research Stuttgart, for the opportunity to use the JEM 4000EX.
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Cross-sectional TEM investigation of tin-implanted SiO2 glass Thomas H oche a,*, Torsten Angermann b a
Otto-Schott-Institut f ur Glaschemie, Friedrich-Schiller-Universit at, Fraunhoferstraûe 6, D-07743, Jena, Germany b Institut f ur Physikalische Chemie, Friedrich-Schiller-Universit at, Lessingstraûe 10, D-07743, Jena, Germany Received 30 June 1999; received in revised form 29 October 1999
Abstract We report on 300 kV Sn -implantation into high purity silica substrates at liquid nitrogen and room temperature. The evolution of the Sn depth pro®les in dependence on subsequent heat treatments was monitored by cross-sectional transmission electron microscopy, energy-dispersive X-ray spectrometry analyses, and Rutherford backscattering spectrometry. Using selected-area electron diraction, the existence of nanocrystalline b-Sn particles could be proved in the as-implanted state independent on the substrate temperature during implantation. After subsequent exposure to 873 K for 1 h, the upper part of the tin distribution becomes oxidised, whereas the deeper fraction of tin nanocrystallites undergoes a coarsening resulting into several 10 nanometer large, spherical b-Sn crystals. If the implantation is followed by a heat treatment at 1373 K for 6 h, tin is entirely oxidised and its distribution is shifted further towards the surface. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.16.B; 68.55.L; 42.70.C; 81.70.J; 61.85
1. Introduction Wide band gap glasses have been super®cially modi®ed for various reasons ranging from locally varying optical properties (e.g., for waveguide applications and photonic switches) to improved fracture resistance and decreased rates of corrosion. For surface modi®cation, a series of experimental techniques including multi-target sputtering [1±3], ion-exchange (see e.g., [4±7]), and surface reactions between a metal±organic complex and the substrate [8] are employed.
* Corresponding author. Tel.: +49-3641 948 538; fax: +493641 948 502. E-mail address: [email protected] (T. HoÈche).
Besides these techniques, ion implantation is a frequently used method to achieve modi®cations of glass surfaces since it allows for the preparation of buried layers including almost all kinds of elements or even compounds when co-implantation is applied and is therefore capable to locally adjust optical properties. Along with the ion implantation, a radiation-induced modi®cation of the hosting substrate structure occurs. Such structural modi®cations are not necessarily disadvantageous since they are under suspicion of provoking nonlinear optical eects in the damaged glass. Moreover, structural defects can be reduced or even entirely annealed by a post-implantation heat treatment. However, such tempering processes not only aect the damaged host but also the depth distribution and microtopology of the implanted species.
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 8 6 - 9
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Ion implantation of silica glass was reported for a variety of ions including Cu [9±13], Ag [14,15], Au [16,17], Ni [13,18], and Pb [19]. Sn ionimplanted silica glass was shown to exhibit nonlinear optical properties after Sn -implantation at 400 kV acceleration voltage and a dose of 2 1017 ions cmÿ2 at room temperature [20,21]. Under the above-mentioned conditions, metallic tin microcrystallites of 4±20 nm diameter are found to be embedded in the silica glass. These nanocrystals are considered to be the major reason for the nonlinear optical eects observed. In this study, we report on Sn -implantation into high purity silica substrates at 300 kV acceleration voltage at two ion doses (1.1 and 1:9 1017 ions cmÿ2 ) and two substrate temperatures during implantation (at liquid-nitrogen and room temperature) [22]. Cross-sectional transmission electron microscopy (XTEM) and energy-dispersive X-ray spectrometry (EDXS) analyses were applied to elucidate the as-implanted state and the in¯uence of various subsequent heat treatments on the Sn depth pro®le. Image-processing routines are used to determine size and number density distributions of tin and tin oxide crystallites, respectively. EDXS depth pro®les are compared with Rutherford backscattering spectrometry (RBS) data. Based on results obtained at as-implanted samples, tin-distribution dierences between samples tempered at 600°C and 1100°C are discussed. Although super®cial tin-enriched layers in glasses are frequently associated with ¯oat glass, the Sn -implanted SiO2 glass investigated here and ¯oat glass have to be carefully distinguished for a couple of reasons including: 1. During ion implantation, the system is far from thermodynamic equilibrium and thus rules of equilibrium thermodynamics (concerning diusion, solubility etc.) must not be applied. 2. Upon ion implantation, local tin concentrations of between 60 and 90 wt% are obtained, whereas in ¯oat glass the maximum tin concentration measured e.g., by Townsend [23] is as low as 0.8 wt% (it is just one of the main intentions of ion implantation to introduce foreign ions at concentrations far beyond values that can be obtained by equilibrium processes). Moreover, the maximum depth of tin pro®les in ¯oat glass-
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es is several 10 lm which is about two orders of magnitude larger than the few 100 nm observed in our Sn -implanted samples. 3. Soda-lime ¯oat glasses are not only chemically very much dierent from high-purity SiO2 glass (and therefore all deduction concerning diusivities must not be made), but the glass network is extremely damaged by the implantation process itself. The resulting radiation-induced structural modi®cation of SiO2 substrate (every substrate atom in the implanted layer is displaced several times during implantation) can be annealed only after an about 1 h lasting heat treatment at 600°C. Thus, also if diusion coef®cients of Sn0 , Sn4 , and O2ÿ were available (they have been determined to our knowledge only for oxygen self diusion, see e.g., [24]), their usage to derive a model that explains the enrichment of tin to the surface would be very questionable. Consequently, tin depth pro®les of Sn -implanted silica glass and ¯oat glass must not be compared.
2. Experimental 2.1. Ion implantation High-purity silica glass substrates of 3.5 mm thickness were polished and subsequently etched in 5% HF to remove grinding and polishing residuals. After removing this way a some 0.5 lm thicksuper®cial layer, the substrates were one-sided implanted with Sn ions at an acceleration voltage of 300 kV. All implanted substrates originated from the same glass batch of synthetic, SQ 1-type silica glass. Although the nominal dose 1 2 was kept constant at 1:5 1017 ions=cm for all implantation, the ®rst series of samples was im2 planted at an eective dose of 1:14 1017 ions=cm at room temperature (samples 1A, 1B, and 1C) 1 In contrast to the nominal dose (which is measured during implantation by counting charges transferred through the SiO2 substrate), the eective dose referred to later on can be directly measured after implantation by means of RBS spectrometry.
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Table 1 Samples studied, respective implantation temperatures, Sn -implantation doses, thickness of super®cial material loss layer upon ion implantation and heat-treatment schedules Sample
Substrate temperature during implantation (K)
(Nominal)/eective dose (1016 ions cmÿ2 )
Thickness of super®cial material loss layer upon ion implantation (nm)
Subsequent heat treatment
1A 1B 1C 2A 2B 2C
77 77 77 300 300 300
(15.0)/11.4 (15.0)/11.4 (15.0)/11.4 (15.0)/19.2 (15.0)/19.2 (15.0)/19.2
22 22 22 101 101 101
Untreated 873 K, 1 h 1373 K, 6 h Untreated 873 K, 1 h 1373 K, 6 h
whereas in the second series (implanted at 1:92 1017 ions=cm2 , samples 2A, 2B, and 2C), substrates were cooled with liquid nitrogen (77 K). Both series were studied as-implanted and after subsequent heat treatments at 600°C for 1 h and 1100°C for 6 h, respectively. Cooling rates used were larger than 10 K minÿ1 . The notation of all samples is compiled in Table 1. 2.2. XTEM characterisation XTEM foils were prepared as follows. First, implanted substrate pieces of approximately 6 6 mm2 were cut into two equal parts. The latter were glued together face-to-face using epoxy resin in the second step. Third, the resulting sandwiches were cut perpendicular to the interface, planeparallel ground to 100 lm thickness, one-sided polished and dimpled to a residual thickness of 10± 15 lm from the unpolished side. In the fourth step, double-sided Ar ion-beam etching at an acceleration voltage of 2.5 kV and ion-beam currents of 1.0 mA using low incidence angles (6°) was applied until perforation of the foil was just achieved. For the microstructural characterisation of cross-sectional foils, a Hitachi H-8100 II transmission electron microscope operating at 200 kV acceleration voltage was used. EDXS analyses were performed with an attached Link ISIS analyser (Oxford Instruments; Si:Li detector, atmospheric thin window). High-resolution transmission electron microscopy (HRTEM) was performed on a JEOL JEM 4000EX (operation voltage: 400 kV, For the digitipoint-to-point resolution: 1.6 A). sation of plane-®lm TEM negatives, a EIKONIX
99/87 scanner camera was used. Indexing of diffraction patterns was preceded by a rotation averaging of the digitised negative using the image processing environment IDL4.0 (Research Systems, Boulder, CO). 2.3. Rutherford backscattering spectrometry Using Rutherford backscattering spectrometry (RBS), the depth distribution of implanted Sn ions was probed using 1.4 MeV He ions at a backscattering angle of 120°. The RBS detector was calibrated using both an oxidised and a Ge/Au covered silicon wafer. Since the SiO2 substrate is an insulator, excess voltages were avoided by placing an electron source next to the substrate surface in the evacuated scattering chamber ( p 10ÿ3 Pa). 2.4. Surface pro®le measurements After implantation, homogeneous, ¯at depressions were found in the implanted areas. These super®cial substrate material losses were monitored using a surface pro®ler (Dektak 3030 ST, Veco-Instruments) by comparing non-implanted and implanted parts of the substrate. On the basis of these measurements, the depth scale can be calibrated. 3. Results 3.1. As-implanted samples A typical cross-sectional transmission electron microscopy (XTEM) micrograph of the sample implanted at 77 K (1A) is shown in Fig. 1(a).
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Fig. 1. XTEM micrographs recorded at low-temperature implanted samples Timpl: 77 K); the position of the surfaces is indicated by arrows: (a) untreated (1A), (b) heat-treated at 873 K for 1 h (1B) and (c) after heat-treatment at 1373 K for 6 h (1C).
Visual inspection of the micrograph reveals the formation of spherical nanocrystals with an average particle size increasing with depth. The crystals reach their maximum size about 170 nm below the surface. Very small particles exist at greater depths indicating that the range of the implanted Sn goes beyond the maximum deposition layer down to some 300 nm. This observation could be veri®ed by image processing of digitised TEM negatives. The corresponding particle-size distribution is given in Fig. 2(a) along with the number density of tin particles counted in a series of boxes 52 nm wide in the direction perpendicular to the surface (Fig. 2(b)). 2 As proved by electron microdiraction (Fig. 3(a)) and high-resolution TEM (HRTEM) as well (not shown), at both implantation temperatures b-Sn microcrystallites are formed. As shown in the rotation-averaged line scan (Fig. 3(b)) of the electron-diraction pattern indicated in Fig. 3(a), all diraction rings can be attributed to re¯ections of b-Sn. All major re¯ec2 The number density was calculated under the most reasonable assumption that particles counted reside in a wedge-shaped sample (Wedge angle ~12°; wedge parallel to the implanted surface in cross-section). Since the exact measurement of TEMfoil thickness is aicted with considerable errors of about 20%, values of the number density carry large error bars.
tions occur and none of the rings could not be assigned. In the sample implanted with a substrate kept at room temperature (2A), a layer consisting of b-Sn crystallites with a relatively large average size is covering a deeper region hosting particles of only a few nanometer diameter (cf. Fig. 2(c)). Fig. 4(a) reveals that the maximum of the particle distribution is shifted towards the `surface'. In fact, this upper end of the implanted region has little in common with the virgin surface of the substrate prior to implantation since during implantation swelling and self-sputtering are acting simultaneously. The latter process is temperature dependent and therefore depth pro®les of samples 1A and 2A cannot compared without more detailed information on the depth scale as detailed below. Therefore, surface pro®le measurements were applied proving the sample implanted at room temperature to have lost an about 100 nm thick layer of its surface, whereas at 77 K the thickness loss amounts only 22 nm (cf. Table 1). Using elliptical beam spots (main axes 20 and 100 nm), energy-dispersive X-ray spectra were acquired and plotted as a function of depth for samples 1A and 2A (Fig. 5). The resulting pro®le shapes are in excellent agreement with the RBS measurements superimposed. The dierence between absolute values will be discussed below.
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Fig. 2. Depth distribution of average particle diameter (a and c) and number density of tin particles (b and d) for the samples implanted at 77 K (1A) and at room temperature (2A).
3.2. Heat treatment at 873 K for 1 h If the specimen implanted at 77 K substrate temperature is heat treated at 873 K for 1 h (sample
1B), b-Sn spheres with diameters ranging from 65 to 120 nm are formed at depths of some 250 nm (Fig. 1(b)). Super®cial tin crystallites, however, become oxidised and form SnO2 nanocrystals. In addition
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Fig. 3. (a) Selected-area electron diraction pattern of the implantation layer in the as-implanted sample 1A Timpl: 77 K). (b) Radial line scan of the rotation-averaged pattern (a). Positions of b-Sn re¯ections are marked.
to selected-area electron diraction, this ®nding was proved by HRTEM micrographs (cf. Fig. 6). A similar observation (Fig. 4(b)) is made with sample 2B (implanted at 300 K substrate temperature and subsequently heat treated at 873 K for 1 h) where at an average depth of some 150 nm, bSn spheres with smaller diameters (ranging from 25 to 70 nm) are formed from the deeper fraction of tin nanocrystallites. Also here, super®cial tin nanocrystals are transformed into SnO2 crystals. 3.3. Heat treatment at 1373 K for 6 h After exposure to 1373 K for 6 h, an additional shift of the particle distribution towards
the surface of the implanted samples is observed as illustrated by Fig.1(c) (sample 1C) and Fig. 4(c) (sample 2C). Sn-depth distributions detected by EDX spectrometry and RBS (see Fig. 7) parallel this behaviour. The absolute tin concentration obtained by EDXS pro®ling is sensitively in¯uenced by the assumption of a correct thickness of the TEM foil. This is because of absorption processes mainly concerning the oxygen signal which in turn aect the Sn concentration since the total concentration of O, Si, and Sn has to be assumed to be 100%. Due to this considerations, an upper limit of the Sn concentration (thickness zero, i.e., no absorption) and a lower threshold (foil thickness 500 nm, considerable
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Fig. 4. XTEM micrographs recorded at room-temperature implanted samples Timpl: 300 K); the position of the surfaces is indicated by arrows: (a) untreated (2A), (b) heat-treated at 873 K for 1 h (2B) and (c) after heat-treatment at 1373 K for 6 h (2C).
absorption) is also shown in Fig. 7. The actual value (which might be more accurately determined by RBS) should lie somewhere between these limits. A detailed discussion including a comparison of EDXS and RBS spectra will be given below. In contrast to both as-implanted samples and the samples heat treated at 873 K for 1 h as well,
no evidence for the formation of any b-Sn crystallites is given after the heat treatment at 1373 K for 6 h. As proved by the electron microdiraction pattern shown in Fig. 8(a), tin nanocrystals formed upon ion implantation become entirely oxidised instead. In this pattern, owing to the small size to the crystallites, some 20 diraction rings are visible and as proven by the radial
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Fig. 6. HRTEM micrograph recorded at the surface of sample 1B. Lattice-planes and angles between them are indicated for the twinned nanocrystal proving it to be cassiterite.
Fig. 5. EDXS depth pro®les (hollow symbols) of sample 1A Timpl: 77 K) and sample 2A Timpl: 300 K) and respective pro®les from RBS measurements (solid symbols).
section of the rotation averaged pattern (Fig. 8(b)), all rings can be assigned to re¯ections of SnO2 , cassiterite.
4. Discussion A detailed comparison between samples implanted at dierent temperatures is reasonable
Fig. 7. Juxtaposition of EDXS depth pro®le of Sn concentration (assuming three dierent thickness values for the TEM foil) and RBS measurement of the Sn pro®le: (a) sample 1C and (b) sample 2C.
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Fig. 8. (a) Selected-area electron diraction pattern of the super®cial layer of sample 2C Timpl: 300 K, heat treatment at 1373 K for 6 h). (b) Radial line scan of the rotation averaged pattern (a), positions of SnO2 re¯ections are marked.
only if the depth scale is properly calibrated. In this context it is essential to note that the ®gure `thickness of super®cial material loss layer upon ion implantation' given in Table 1, accounts for the sum of two competing eects. On the one hand, ion implantation is accompanied by sputter removal of the substrate material resulting in a (considerable) shift of the depth scale origin. On the other hand, the implanted layer is swelling since implanted Sn ions have to be accommodated and moreover the host material suers from radiation damage. For either samples 1A and 2A, swelling results in a thickness gain of the
implanted layer of about 61 nm. 3 In a ®rst approximation, swelling can be assumed to be independent on the substrate temperature and therefore both series of samples can be compared. Due to the super®cial materials loss upon implantation, sample 1A has lost about 22 nm whereas sample 2A lost as much as 101 nm (Table 1). This eect results in a shift of the depth scales. Returning to Figs. 2 and 5, this obviously corresponds exactly to the shift of the particle size maximum and the maximum tin deposition peak, respectively. The coarsening of crystalline b-Sn spheres of between 65 and 120 nm diameter (1B) and 25±70 nm diameter (2B) after 1 h at 873 K is indicating that oxygen diusion into the tin-implanted substrate must be restricted to the topmost layers since electronegative metal ions do not chemically interact with the SiO2 glass network during the implantation process [25] and, consequently, oxidation can only occur with oxygen from outside the sample. On the contrary, Sn0 diusion, however, must be rather fast since otherwise the coarsening of b-Sn crystals would not be observed. Comparing the estimated depth of occurrence of these spheres (between 200 and 250 nm for sample 1B and about 150 nm for sample 2B) with the tin concentration depth pro®les for samples 1A and 2A (given in Fig. 5) reveals that the large crystalline tin spheres are growing at the deeper tail of the tin distribution. This ®nding is also supported by RBS measurements that exhibit a bimodal tin distributions in samples 1B and 2B. The mobility of Sn0 in the glass network is very likely to be enhanced by structural damage of the implanted glass (glass defect centres within the depth range of the implanted Sn ions). Such enhanced diusion inside the implanted super®cial layer would explain that upon heat treatment the depth pro®le of tin does not considerably propagate into the substrate but is con®ned to the implantation depth.
3 A detailed account on how this value was obtained by combining subsequent chemical etching, RBS and infrared re¯ection spectrometry will be given elsewhere.
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Of course, diusion of Sn0 is not restricted to the buried tin layer but those atoms will also be able to reach the surface. Since the heat treatment is performed in air, Sn0 atoms approaching the surface can be oxidised by oxygen penetrating the surface from outside. Taking into account the much dierent melting points of SnO2 and b-Sn, it becomes clear that, due to chemical potentials of tin in metallic tin and cassiterite, once oxidised tin species are hardly capable to leave the tin oxide aggregates again. Thus, tin is trapped at the surface (and subsequently enriched here) and hence we can propose that there are two distinct layers forming at 873 K after 1 h: a super®cial region containing cassiterite nanocrystals and a buried layer which was beyond the diusion range of oxygen and where in order to reduced the overall enthalpy small tin nanocrystals are forming larger aggregates. After exposure to a heat treatment at 1373 K for 6 h, however, large spherical tin microcrystals do not exist. It is subject to speculation whether they were not formed at all or dissolved during later stages of the treatment. Anyway, this ®nding results from enhanced diusion of both Sn0 and oxygen at elevated temperature. Due to the larger mobility, the heat treatment at 1373 K can gradually level out the dierences in the tin depth distribution initially formed by Sn implantation. Particularly from Fig. 7 it is obvious that the width of the tin distribution remains larger after implantation at lower temperature since the maximum of the tin distribution which is shifted towards the surface upon tempering is initially located at greater depth. After 6 h at 1373 K, the tin concentration pro®le is still 1.5 times wider for the low-temperature implanted silica glass as compared with the sample implanted at room temperature. Going back to Figs. 5(b) and 7(b), the question arises as to whether RBS or EDX analyses are more reliable since here the absolute tin concentration determined by either methods diers up to a factor of two. The answer is clear: RBS data are much more reliable. Although some scatter of the data was observed during taking RBS spectra at samples 2A and 2C, the accuracy of the RBS pro®les themselves is within a few percent. On the
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contrary, EDX spectra suer from an unknown variable essentially in¯uencing the results: the thickness of the analysed part of the TEM foil. As indicated in Figs. 7(a) and (b), depending on this parameter, the tin concentration can vary by a factor of about 2.5. In the plot of Fig. 7(a), the absolute Sn concentration determined by RBS and EDXS agrees well for a reasonable TEM-foil thickness of about 100 nm. However, although the TEM foil of sample 2C was thicker than the foil of sample 1C, a thickness of about 500 nm as might be concluded from Fig. 7(b) is not yet reasonable. At this point, a principle dierence between RBS and EDX spectra should be recalled since the latter is responsible for the dierences occurring. RBS data average over an area of between 0.2 and 1 mm2 , whereas EDX spectra were acquired from a few 100 nm wide cross-section of the implanted sample. Besides the systematic error introduced upon quanti®cation of the EDXS data, experimental scatter of the tin distribution is very likely. Finally, it can be concluded that the quanti®cation of the tin concentration depth pro®le is done best with the RBS data and there is very good agreement of the overall shape of the tin distribution when RBS and EDX spectra are compared. Comparing RBS spectra of samples 1A and 2A (Fig. 5) and 1C and 2C (Fig. 7) reveals a signi®cant dierence in the overall tin content since integrating RBS spectra over depth results in areas diering by almost a factor of two. At ®rst glance, this seems to be in contradiction with equal nominal doses. However, the nominal values have to be distinguished from eective doses, since spark discharges (occurring between implanted surface and grounded backside of the sample) or insucient charge conductance distort the nominal dose. In the implantation chamber, no electron source could be installed to balance charges. Therefore, spark discharges as a consequence of excess voltages occur increasing the eective dose in comparison to the nominal dose. On the contrary, the eective dose is reduced by self-sputtering eects and charging of the implanted substrate (the latter eect is responsible for the occurrence of non-Gaussian pro®les observed
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since approaching ions are decelerated or even de¯ected prior to reaching the surface). The rate of self sputtering can be reduced by cooling the substrate upon implantation and consequently at low temperatures, larger doses can be implanted. Moreover, self sputtering does in¯uence the tin distribution depth pro®les. 5. Conclusions After Sn -implantation at room temperature, nanocrystalline b-Sn particles (diameters ranging from 2 to 27 nm) are formed in a 300 nm thick layer of the silica glass. Cooling of the substrate to liquid nitrogen temperature during implantation results in a tin distribution with a deeper maximum since self-sputtering eects are drastically reduced. If the substrate is not cooled, the position of the distribution maximum is shifted towards the surface and the upper tail of the pro®le is even truncated. With all samples a strict correspondence of tin concentration pro®le and particle distribution could be shown. Moreover, the tin distribution determined by EDXS is in good agreement with RBS measurements. Independent on the substrate temperature during implantation, upon tempering at 873 K for 1 h, the tin distribution is becoming wider and bimodal. On the one hand, tin is becoming enriched at the surface (it is trapped by becoming oxidised) and on the other hand, up to 120 nm large b-Sn single crystals are formed in a buried layer at 150± 250 nm depth. The super®cial layer is completely oxidised resulting in nanocrystalline cassiterite (SnO2 ). If the Sn -implantation is follow by an exposure to a heat treatment at 1373 K, a further broadening of the tin distribution can be recognised and the maximum of the tin pro®le is shifted further towards the surface. Tin is found exclusively in cassiterite nanocrystals. Acknowledgements Specimens investigated here are part of T.A.'s PhD thesis. He is grateful to his thesis advisor
Professor H.H. Dunken who gave him the opportunity to work at the Institut f ur Physikalische Chemie of the Friedrich-Schiller-Universitat, Jena. The authors would like to thank Professor W. Wesch (Institut f ur Festk orperphysik, FSU, Jena) and his group for performing the Sn ion implantation as well as RBS measurements. T.H. is indebted to Professor M. R uhle, Max Planck Institute for Metals Research Stuttgart, for the opportunity to use the JEM 4000EX.
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