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Low energy and low dose electron irradiation of potassium–lime–silicate glass investigated by XPS. I. Surface composition
Journal of Non-Crystalline Solids 320 (2003) 177–186 www.elsevier.com/locate/jnoncrysol
Low energy and low dose electron irradiation of potassium–lime–silicate glass investigated by XPS. I. Surface composition Ondrej Gedeon b
a,*
, Josef Zemek
b
a Institute of Chemical Technology, Technicka 5, 166 28 Prague 6, Czech Republic Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
Received 20 July 2002; received in revised form 30 September 2002
Abstract X-ray induced photoelectron spectroscopy has been used to study the influence of low-energy electron beam on the pristine potassium–lime–silicate glass surface prepared by fracturing in situ under ultrahigh vacuum. Relatively lowenergy electron beam of 1600 eV with low-electron beam current density of 0.02–0.22 A/m2 and low-electron dose of 29–5200 C/m2 was used. The expected modified near-surface region thickness is in this case comparable with the mean sampling depth of the analytical tool. Therefore, possible changes and modifications due to electron irradiation could be recorded with high sensitivity. The freshly fractured glass surface was found to be significantly enriched with potassium, and slightly with calcium. As a consequence of the lowest electron dose irradiation used, the potassium signal substantially increased by a factor 1.24 relative to the value found for the fresh surface. For higher doses used, the potassium signal continuously increased with the dose to a maximum and decreased thereafter. This variation was accompanied with the qualitative opposite behaviour of calcium signal. The concentrations of the other elements present in the glass, oxygen and silicon, varied only slightly with the electron dose. They can be considered to be constant within experimental uncertainty. In agreement with experimental results, a model assuming mobility of only two most mobile cations, potassium and calcium, was suggested. The models assuming one layer and two layers on the bulk were developed. Their results reproduce well experimental findings: (i) the formation of a potassium-rich surface layer, and (ii) the opposite-like signal variation of calcium in comparison with potassium. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 61.80.)x
1. Introduction
*
Corresponding author. Tel.: +420-2 2431 1918; fax: +420-2 2431 3200. E-mail address: [email protected] (O. Gedeon).
A solid irradiated by high-energy beam undergoes changes in its structure, caused mostly by ionization and ballistic interaction of high-energy particles with solid constituents [1]. This irradiation introduces disorder in the original structure
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00015-2
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causing amorphization of crystalline samples accompanied with volume changes [2]. Phase separation [3] or gas accumulation [4] are reported as examples of many modifications in the irradiated glass. Interaction of an electron beam with a glass containing alkali ions is a point of interest for analysts dealing with electrons as an excitation source. Mobility of alkali ions rapidly increases with electron irradiation and causes distortion of the analytical signal [5]. As a consequence, problems in quantification of recorded spectra are expected. The study of influence of electron bombardment on the alkali glass gives not only an adequate interpretation of spectra but also helps to understand elemental processes and changes in the irradiated glass. It results in the better understanding of glass itself. Interaction of electrons with glass is also important for the radioactive waste deposition in which electrons produced with various energies eventually lead to irreversible changes in glass. One of the most interesting and widely studied phenomena is the alkali ion migration in alkali glass irradiated by electron beams. The phenomenon can be observed even for very low-energy beams due to weakly bonded alkali ions in the glass structure. These changes have been observed as the decay of alkali intensities by many authors in the electron probe microanalysis (EPMA) [5–7] or in Auger electron spectroscopy (AES) [8–11]. If the macroscopic alkali migration is observed by EPMA (i.e., within high-energy region), a decay curve, introduced as the alkali X-ray intensity versus time, can be recorded. The measured decay curves generally display two distinct parts [12]. Instantaneously after the exposure, the recorded X-ray signal shows very slow linear decrease. After some time, denoted as the incubation period, the signal abruptly changes into an exponential-like decay. The incubation time can vary in the range from seconds to hours depending on the exposure conditions and on the type of glass. The alkali migration and oxygen desorption observed has been usually correlated with a mechanism proposed by Lineweaver [13]. The primary electrons produce a negatively charged layer at about the electron penetration depth. The charged layer is
assumed to attract positively charged alkali ions from the surface to bulk. Upon decreasing the energy of the primary electrons, the incubation period first slightly increases. The following variation of the incubation period depends on glass composition but it appears that the incubation period goes to infinity as the energy of primary electrons reaches some threshold value, below which no incubation period is found [14]. This energy threshold is proportional to the mass of alkali ion and it depends on the activation energy of the ion for diffusion in glass. The threshold was found to be around 13 keV for the potassium, and is expected to be around 6 keV for the sodium. To explain the existence of the threshold energy value for the incubation period, it was suggested that alkali ions are released from their original sites with help of elastic scattering [15]. The energy transferred by elastic scattering is inversely proportional to the mass of alkali ion, so that faster primary electron is needed for a release of a heavier ion. However, the energy of primary electrons inside the low-energy region is not high enough to release the alkali ions by this mechanism. The existence of the incubation period sometimes reported in AES (low-energy region), where voltage at/or below 3 kV is commonly used, is strange in framework of this model and other driving forces must be considered to explain transporting mechanism. Primary electrons trapped in a glass create a macroscopic electric field within the exposed volume almost instantly [5,16]. If the overall charge distribution is assumed homogeneous inside the irradiated volume, the macroscopic electric field intensity is a maximum at the surface and decreases linearly, reaching zero at the electron range. An inversion point was suggested by Cazaux [17] to explain the out-migration process from a few uppermost surface layers. However, Miotelo et al. [18] argued based on more general model that the existence of the inversion point should lead to the dielectric breakdown. They suggested Gibbsian segregation of alkali metal species to be a driving force for the out-migration. A formation of neutral alkali atoms under the electron irradiation was also suggested [9,19], but on the other hand, no evidence for this assumption was found by X-ray
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induced photoelectron spectroscopy (XPS) analysis of sodium glass [20], probably due to insufficient chemical shift of the Na 1s line. Brow [21] has shown the formation of metallic sodium on the surface of irradiated binary sodium–silicate glass whereas no metallic sodium was found in a CaOcontaining glass. Using the top-surface sensitive ion scattering spectroscopy (ISS), Then and Pantano [22] observed alkali signal increase at the beginning of electron irradiation and confirmed to-the-surface migration during the initial stages of electron irradiation. As a short summary, for high-energy region towards the bulk migration and incubation period were observed. The elastic scattering assisted hopping in the electric field seems to play an important role in the first stages of migration. In contrary, for low-energy irradiation towards the surface migration was found. The surface relaxation processes and Gibbsian segregation may count for towards the surface migration. In this paper, XPS of potassium–lime–silicate glass irradiated with 1600 eV electron beam is studied. The irradiation time and current were chosen to include low-dose irradiation. The work was inspired by the research conducted for higher electron energy beams and it tries to answer the some of the open questions. What is the role of the electric field for low-energy irradiation? What processes, causing the migration of alkali ions, are common and what are different for both energy regions? What are chemical changes like in the potassium depleted layer? Answers to these questions could bridge these two energy regions. Whereas answers concerning chemical changes will be the subject of the prepared paper, this paper concentrates to the description of quantitative changes of spectral intensities in the course of irradiation. Leaving the assumptions about homogeneity of the analyzed region, layered models are suggested for the interpretation of the most pronounced changes in intensities of potassium and calcium.
2. Experimental Glass was melted using high-purity batch in a Pt crucible and cast on a metal plate to prevent
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crystallization. The plate of glass was annealed at 600 °C for 16 h. The glass was transparent and contained no bubbles or cords. The composition of glass used was: 79.5 wt% SiO2 , 8.5 wt% CaO, 12 wt% K2 O (5.47 at.% of K). A rectangular glass sample (5 mm 8 mm 10 mm) was fixed to a sample holder. Before electron irradiation, a clean glass surface was prepared by fracturing of the glass rod under UHV condition using a standard cleaving device. The clean pristine glass surface was step-by-step irradiated by electron beam with various electron doses. The energy of the electron beam used was 1600 eV and its diameter was set to 0.1 mm. The beam was scanned uniformly over a 10 mm 10 mm area to include the entire glass surface (5 mm 8 mm). The impact angle of the electron beam was perpendicular to the sample surface. The energy of the electron beam was chosen in the way that the expected modified near-surface region thickness was comparable with the mean sampling depth of the analytical tool. Therefore, possible changes and modifications due to electron irradiation could be recorded with high sensitivity. The mean sampling depth of XPS is approximately determined by k cos a [23] where k is the inelastic mean free path of measured photoelectrons and a is the emission angle measured with respect to the surface normal. For the experimental geometry used, the mean sampling depth is equal to k (both the emission angle and the impact angle for primary electron beam are zero). Hence, the mean sampling depths for electrons at specified kinetic energy travelling in SiO2 [24] are as follows: kðSi 2p; 1154 eVÞ ¼ 2:4 nm; kðO 1s; 722 eVÞ ¼ 1:7 nm; kðK 2p; 962 eVÞ ¼ 2:1 nm; kðCa 2p; 909 eVÞ ¼ 2:0 nm; kð1600 eVÞ ¼ 4:2 nm: The compositions of the freshly fractured and electron irradiated glass surfaces (top 5–7 nm, what corresponds to the depth, from which 95% of
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detected signal comes out) were studied by XPS using an angular-resolved photoelectron spectrometer equipped with a twin anode X-ray source, standard Al/Mg anodes, and a hemispherical analyzer. The expected temperature increase in a close vicinity of the beam spot at the glass surface could not exceed 2 °C as estimated from [25] for the highest beam current density used. So, the influence of the temperature increase on atomic transport processes may be neglected. For quantitative analysis, spectra were recorded for 10 min using Mg Ka source, operated at a power of 110 W and at constant pass energy of 100 eV. In addition, a few high-resolution spectra were recorded with pass energy of 20 eV for 120 min. As a result, the total X-ray exposure time could reach 800 min. Although some changes introduced by Xrays in glass are reported [26,27], our exposure conditions were very mild. Non-focused beam, low source power, low-photon density (diameter of the X-ray beam was 10 mm) were used to eliminate the influence of X-ray irradiation during spectra recording. Peak areas were determined following ShirleyÕs inelastic background subtraction method [28]. Atomic concentration of elements found in the analyzed thickness of about 5–7 nm were determined from the O 1s, Si 2p and K 2p lines assuming a simple model of a homogeneous semi-infinite solid [29]. The relative sensitivity factor approach [30] was applied to calculate atomic concentrations. The peak areas were corrected for (i) the measured transmission function of the spectrometer, which comprises all instrumental factors influencing the measured quantity [31], (ii) the photoelectric cross-sections [32], and (iii) the inelastic mean free paths [24] of photoelectrons in question, determining their motion in SiO2 . Experimental uncertainties accompanied with XPS quantitative analysis are estimated to be 7%. The value covers overall uncertainties of the method that are mostly introduced by the background subtraction and correction procedure used
for calculation of concentrations from intensities of spectral lines.
3. Results Atomic concentrations resulted from XPS quantitative analysis described above, calculated from spectra recorded at the emission angle a ¼ 0° with respect to the surface normal, are summarized in Table 1. For the freshly fractured glass surface, the potassium concentration is significantly higher (8.0 at.%) than its mean bulk concentration (5.6 at.%). This increase is accompanied with both a slight increase in the calcium concentration (3.6 at.% compared with its bulk concentration 3.3 at.%) and slight oxygen decrease (59.7 at.% versus 62.8 at.% for bulk). However, the two last values are within the experimental range of uncertainty. Following the first electron irradiation step at 29 C/m2 , potassium concentration increased to 9.9 at.% while calcium significantly decreased to 2.8 at.%. Similar surface enrichment by alkali atoms as potassium, rubidium and caesium studied by ISS was observed for electron irradiated glass surfaces at similar electron dose [22]. As illustrated in Fig. 1, the potassium concentration increases with the dose until the turn-over dose is achieved. It, then, begins to decrease. This behaviour is accompanied with a qualitative opposite curve for calcium. To underline changes caused by electron irradiation, the displayed concentrations in the figure are presented relative to their values corresponding to freshly fractured glass. Although the oxygen curve appears to be similar to the calcium curve, its values fall within the interval of uncertainty of XPS experiment. Therefore, both silicon and oxygen concentration dose dependences can be approximated by a constant values of 28.8 0.8 at.% and 55.6 1.6 at.%, respectively. All results presented in this section up to this point were based on the standard quantification procedure of XPS spectra that assume homogeneous sample in composition, i.e. the intensity Ii of the ith element is related to its concentration chom i by the equation [29]
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Table 1 Atomic concentrations and electron dose used for bulk, freshly fractured and electron irradiated potassium-lime-silicate glass surface No.
Electron dose (C/m2 )
Si (at.%)
O (at.%)
K (at.%)
Ca (at.%)
Bulk 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
– –
28.4 28.7 29.9 29.2 30.6 29.0 29.4 29.5 28.6 29.0 28.5 28.3 28.9 28.6 29.5 28.7 28.3 27.3 27.9
62.8 59.7 57.4 57.4 55.9 56.8 55.8 55.4 55.9 55.2 55.3 54.2 54.4 54.0 52.8 53.7 54.3 56.2 55.8
5.5 8.0 9.9 10.5 10.7 11.3 12.3 12.6 12.9 13.2 13.6 14.7 14.0 14.8 15.1 14.6 14.5 12.8 11.6
3.3 3.6 2.8 2.8 2.8 2.9 2.6 2.6 2.6 2.5 2.6 2.7 2.7 2.6 2.6 2.9 2.9 3.7 4.8
28.8 43.2 57.6 86.4 115.2 158.4 201.6 288.0 374.4 493.2 612.0 730.8 849.6 1087.2 1324.8 2948.4 5179.2
Relative concentration
All presented results are based on the assumption of homogeneity within a sampling depth.
1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0
1000
2000
3000
4000
5000
6000
2
Dose [C/m ]
Fig. 1. Oxygen (j) silicon (r) potassium (N) and calcium () atomic concentration changes due to electron irradiation: atomic concentration of the particular element is normalized with respect to its concentration measured for freshly fractured glass surface. Solid lines represent result of the layer-on-thebulk model and broken line represent two-layer-on-the-bulk model.
Ii ¼ T ðEÞN ki chom ; i
ð1Þ
where T ðEÞ is the spectrometer function that includes experimental geometry, electron transmission of the analyzer and sensitivity of the detector. N is the atomic density, and ki is the inelastic mean free path of photoelectron of the ith element in SiO2 . The superscript ÔhomÕ underlines the as-
sumption of homogeneity in composition within the sampling depth. The results based on the assumption of homogeneity of the analyzed sample showed that the most mobile ions are potassium and calcium, while oxygen and silicon can be assumed immobile. It is obvious from above presented results that the assumption of homogeneity is an extensive oversimplification. Although diffusion-like profiles of concentration of mobile elements (Ca, K) can be considered, their introductions could be quite misleading as the existence of two opposite processes (first one, drawing the ion towards the surface and the second one, drawing the ion toward the bulk) can be deduced from Fig. 1. Therefore, simple one-layer-on-the-bulk and two-layer-onthe-bulk models are suggested to explain the measured results. The latter, more complicated model, also serves as a test of stability of the model – two qualitative different results obtained by the above models should signal the instability of the layered model. The Eq. (1) can be easily extended for the layered sample. The intensity of ith element coming from the two layers on the bulk is calculated as follows:
O. Gedeon, J. Zemek / Journal of Non-Crystalline Solids 320 (2003) 177–186
ð2Þ
where tj is a thickness of the jth layer, cji is the concentration of ith element in the jth layer and cbulk is the concentration in bulk. ki (inelastic mean i free path of ith element) was assumed independent of chemical composition and set to value for SiO2 . Hereafter the superscript over a quantity points at the number (1 for the uppermost and 2 for the buried layer) of the layer (or bulk) while the subscript identifies the element. The first term inside braces corresponds to the contribution of the first layer, the second term to the contribution of the buried layer and the last one to the contribution of the bulk. Each layer is assumed to be homogeneous with three free parameters: the thickness of the layer and concentrations of potassium and calcium. Oxygen and silicon are assumed (in consistence with experimental results) to be immobile and their concentrations are set to their bulk values. These assumptions decreased the number of adjustable parameters to three (for the one-layer model) and six (for the two-layer model), respectively. To answer the question, if the results based on the homogeneous semi-infinite sample can be explained by the suggested layered model, we have tried to adjust free parameters in the models so as the layered models reproduced results of the homogeneous semi-infinite sample. This leads to the following set of four equations for the concentration of each element, obtained by the combination of Eqs. (1) and (2) 1 2 t t 1 2 chom ¼ c 1 exp 1 exp þ c i i i k1 k1 1 t1 t t2 exp exp þ cbulk exp : i ki ki ki ð3Þ The set of equation was solved by the least-squares method. The initial estimates of concentrations
were set to their bulk values and the thickness of the upper layer was set to 0.01 nm (near the zero value) for the one-layer model. Both models predict formation of a potassiumrich layer, thickness of which changes with the irradiation dose as is displayed in Fig. 2. The (maximum) thickness of the upper layer is of the order of interatomic distance, what is in an agreement with an idea of segregated particles on the surface [33]. Whereas models yield nearly purely potassium uppermost layer for lower doses (up to approximately 1000 C/m2 ), the decrease of potassium concentration complemented by the increase of calcium concentration can be observed in the layer for higher doses (Fig. 3). The change of the composition of the uppermost layer approximately coincides with the decrease of the thickness of the layer. Both curves indicate saturation of processes after the dose of about 1000 C/m2 that
0.4
Thickness [nm]
1 t Ii ¼ T ðEÞN ki c1i 1 exp ki 2 t t1 2 þ ci 1 exp exp ki ki 1 2 t t bulk exp ; þ ci exp ki ki
0.3 0.2 0.1 0.0 0
1000
2000
3000
4000
5000
6000
2
Dose [C/m ] Fig. 2. Thickness of the upper layer predicted by the one-layeron-the-bulk (solid line) and two-layer-on-the-bulk (broken line) models.
100
Concentration [at %]
182
80
K
60 40 20
Ca
0 0
1000
2000
3000
4000
5000
6000
2
Dose [C/m ]
Fig. 3. Concentrations of Ca and K in the uppermost layer predicted by the one-layer-on-the-bulk (solid line) and twolayer-on-the-bulk (broken line) models.
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leads to the formation of the potassium-rich layer. The increase of calcium concentration with simultaneous decrease of potassium concentration in the upper layer for higher doses does not necessary mean the opposite-like migration of calcium in comparison with potassium. It is rather the consequence of the lower mobility of calcium so that the relative concentration of calcium increases as potassium escapes from the upper layer faster than calcium. Results of the simpler model complemented by the buried 0.5 nm-thick layer of the bulk composition were taken as the initial guess for the two-layer model. However, two-layer models qualitatively converged to the one-layer model, with the uppermost potassium-rich layer and the slightly changed composition (compared with bulk) of the buried layer. The thickness of the formed potassium layer is shown in Fig. 2 and is higher compared with the results of the simpler model. The potassium amount of the upper layer is close to 100 at.%, similarly as in the one-layer model (Fig. 3). In addition, the buried layer is free of potassium with calcium content (2.6 0.9 at.%) slightly lower than in bulk. It can be explained by the migration of calcium from bulk towards the surface as is expected due to its similarity with potassium ion. As the migration of potassium towards the surface is much faster, the calcium increase in the uppermost layer is camouflaged by the faster increase of potassium flow towards the surface. For higher doses, calcium complements the potassium decrease in the upper layer.
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Although Si and O were assumed immobile, their relative concentrations change as the toplayer forms. The oxygen intensity is very sensitive to the upper layer formation due to the high absorption of its spectral line in the layer. Even if not presented, relative intensity of O coming from the model qualitatively follows the experimental data with dose (similarly as for calcium, the oxygen concentration first decreases followed by the increase for higher dosed) so that the difference between experimental and model values are fully within the range of experimental uncertainties (7%). The change of oxygen signal can be therefore fully explained in framework of the suggested layer model without any other assumptions.
4. Discussion The papers dealing with low-energy electron irradiation of alkali glass mostly report the study of sodium glass (Table 2). However, although potassium plays a similar role as a modifier in the silicate network, it differs by its local structure. Because of its larger size, more oxygen anions participate in its nearest neighbourÕs co-ordination shell. Its size also results in its lower mobility due to the relatively high-displacement energy. Therefore, results obtained for sodium ions cannot be simply extended to the potassium glass, although some common qualitative behaviour can be expected. In general, potassium lower mobility results in the longer incubation period, higher doses
Table 2 Selected characteristics reported in papers dealing with low-energy electron irradiation of alkali ion containing glasses (electron energy below 5 keV, except for the EPMA data presented for a comparison) Method/Ref.
Investigated type of alkali ion
Energy of primary electrons (keV)
Current density (A/m2 )
Maximum dose (C/m2 )
AES [9] AES [9] AES [39] AES [8] XPS, AES [40] XPS, AES [20] ISS [22] XPS [21] EPMA [41,42] XPS (this work)
Na K Na Na, K Na Na K, Rb, Cs Na K K
1.5, 3 1.5, 3 0.5–4 1.5 2.5, 4.5 2–5 0.5 3 50 1.6
100–1300 500–4000 10 0.1–10 0.012, 0.04 15 0.05 5 1.9 0.02–0.22
3.6 105 2.1 106 900 200–2000 480 10 800 8000 6600 14700 5200
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needed for obtaining comparable results as opposed to sodium glass, and after all in a prolongation of experimental time scale that enables one to study processes that are less time-resolved. The higher near-surface potassium concentration (8.0 at.%) on in situ freshly fractured glass surface in comparison to its bulk content is a result of a surface relaxation, in which the most mobile ions (potassium ions in our case) play the dominant role. The alkali surface concentration increase was also demonstrated experimentally [34] as well as theoretically by molecular dynamics simulation [35]. The suggested layered models confirm the tendency of to-the-surface migration of alkali ion by the prediction of potassium-rich surface layer formation. Consequential irradiation, even in low doses, loosens the glass structure and enables more potassium ions to take part in the relaxation processes. It resulted in the further increase of potassium surface concentration up to 15.1 at.%. This increase is in a consistence with the increase of potassium signal in the uppermost layer observed by Then and Pantano [22] by the top-surface sensitive ISS. The potassium signal increase was also observed by AES [8] in binary potassiumsilicate glass exposed with a low-electron dose. Due to the similarity of calcium ion with potassium one, the participation of calcium at surface relaxation can be expected. The two-layer model prediction of the decrease of calcium concentration in the buried layer may be attributed to the migration of calcium towards the surface. The decrease of the calcium signal is, however, camouflaged by the higher mobility of potassium that enables to form potassium-rich uppermost layer. Both suggested layered models converge to the same qualitative results – formation of the uppermost potassium layer. Its thickness increases with the irradiation dose until the surface relaxation processes are saturated. This corresponds to the turn-over dose, from which a decrease of the potassium signal is observed. This is the point where the electric field stimulated diffusion takes over control. Higher doses lead to the decrease of both potassium concentration and the thickness of the upper layer. The presence of two different stages of migration is also confirmed by the evo-
lution of spectral lines with an electron dose [36]. Therefore, it seems that the dominant driving mechanism of potassium migration at the beginning of electron irradiation is connected with further surface relaxation. This process is very rapid and lower doses should be used to record the earliest stages. As soon as the surface relaxation processes are saturated, the electric field takes its dominant role and draws the alkali ions towards bulk. The reason why the increase of potassium signal (surface relaxation processes) is not observed by analytical methods using higher energies of primary electron beam (EPMA) is evidently connected with their low-surface sensitivity. On the other hand, the electric field is a common driving force count for the decrease of potassium analytical signal, causing final alkali depletion from the exposed glass. The models are also capable to explain a ÔstrangeÕ calcium increase of intensity, already reported by Gossink and Lommen [9], only by the formation of the potassium-rich uppermost layer. Miotelo and Mazzoldi [37] tried to explain this phenomenon by the model of co-operative migration between potassium and calcium. Although the model is able to reproduce the experimental findings very well, the Ôcooperative diffusion coefficientÕ (DCa;K ¼ 5 1015 cm2 /s) used in the model should be surprisingly quite comparable with the calculated Ôstandard diffusion coefficientÕ for calcium (DCa ¼ 5 1015 cm2 /s). It means that potassium escape is assumed to be the driving force for calcium movement into the potassium-depleted region, causing the calcium enrichment proportional to DCa;K o2 nK =ox2 . Instead of co-operation mechanism, results of our layered models suggest that the calcium intensity variation is caused by the higher mobility of potassium ions. The initial decrease of the calcium signal can be explained by the faster migration of potassium towards the surface, resulting in the formation of potassiumrich uppermost layer. The increase of calcium concentration in the upper layer for higher irradiation doses indicates that calcium also takes place in the surface relaxation (as it can be expected due to its similarity with potassium ions). The decrease of calcium in buried layer in framework of the two-layer model also supports this
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idea, as it can be attributed to the migration of calcium towards the surface. Even oxygen outgassing is reported during the irradiation [38] and often used for the interpretation of the formation of alkali depletion layer [13] this experiment cannot confirm it. The current density in experiment reporting oxygen outgassing is significantly higher (approximately of an order) so that the outgassing is probably connected with an extensive temperature increase. Hence, oxygen was taken in the model as immobile. Although oxygen intensity variation can be partly explained by the created top-surface camouflaging, some oxygen mobility can be expected. Especially, the drop of oxygen signal after the fracturing and at the lowest dose irradiation is expected to be accompanied with oxygen transport. However, gain obtained from the assumption of oxygen mobility should be paid by lower stability of models.
5. Summary and conclusions A glass rod fractured in situ in ultrahigh vacuum was used to study electron beam induced modifications of pristine potassium–lime–silicate glass surface. The fresh fractured surface was sequentially irradiated under conditions where thickness of possibly modified surface region is comparable with the mean sampling depth of the analytical tool: by a relatively low-energy electron beam of 1600 eV and a low-current density, with electron doses from 29 to 5200 C/m2 . Moreover, the electron beam scanned over the surface, so that no surface temperature growth due to the irradiation is expected. The freshly fractured glass surface was found slightly enriched by potassium and calcium with respect to the corresponding bulk concentrations. Following the lowest electron dose irradiation, the potassium signal further increased substantially while the calcium signal decreased. For higher electron doses used, the potassium signal continuously increased with dose, accompanied with the calcium signal decrease. However, the opposite dependence was found for higher doses. The simple layered models were suggested. They predict a formation of the potassium up-
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permost layer. The effective potassium concentration increase for lower doses (as well as the new surface creation) can be associated with surface relaxation processes, while the following decrease is suggested to be caused by migration of potassium cations in the electric field created in the irradiated sample. Potassium, as the most mobile element, responds to new forces promptly. The calcium opposite-like qualitative behaviour can be fully explained by the absorption in this uppermost potassium-rich layer and its lower mobility. The above results showed that it is extremely difficult to analyze alkali silicate glass samples by analytical tools using electron beams without severe structural damage and compositional changes. The lowest electron dose, electron beam current density and scanned or defocused electron beam is recommended for lowering the harmful influence of the beam on the analyzed glass.
Acknowledgements This work was financially supported by Grant Agency of the Czech Republic through the grant no. 104/99/1407. It was also part of the research project CEZ: MSM 223100002 Chemistry and technology of materials for technical applications, health and environment protection.
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[9] R.J. Gossink, T.P.A. Lommen, Appl. Phys. Lett. 34 (1979) 444. [10] D.M. Usher, J. Phys. C 14 (1981) 2039. [11] A. Miotelo, J. Phys. C 19 (1986) 445. [12] O. Gedeon, V. Hulinsky, K. Jurek, Mikrochim. Acta (Suppl.) 132 (2000) 505. [13] J.L. Lineweaver, J. Appl. Phys. 38 (1963) 1786. [14] K. Jurek, O. Gedeon, Mikrochim. Acta 139 (2002) 67. [15] O. Gedeon, M. Liska, J. Non-Cryst. Solids 286 (2001) 181. [16] A. Miotelo, Phys. Lett. 103A (1984) 279. [17] J. Cauzax, J. Appl. Phys. 59 (1986) 1418. [18] A. Miotelo, G. Cinque, P. Mazoldi, C.G. Pantano, Phys. Rev. B 43 (5) (1991) 3831. [19] P.G. Whitkop, Surf. Sci. 110 (1981) 261. [20] L.R. Pederson, Surf. Sci. 119 (1982) L307. [21] R.K. Brow, J. Non-Cryst. Solids 175 (1994) 155. [22] A.M. Then, C.G. Pantano, J. Non-Cryst. Solids 120 (1990) 178. [23] J. Zemek, S. Hucek, A. Jablonski, I.S. Tilinin, J. Electron Spectrosc. Relat. Phenom. 76 (1995) 443. [24] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 17 (1991) 927. [25] H.J. Dudek, Z. Angew. Phys. 5&6 (1971) 331. [26] J.E. Shelby, J. Appl. Phys. 51 (5) (1980) 2561. [27] A. Sharma, H. Jain, A.C. Miller, Surf. Interface Anal. 31 (2001) 369.
[28] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [29] C.S. Fadley, R.J. Baird, W. Sickhaus, T. Novakov, S.A. Bergstrom, J. Electron Spectrosc. Relat. Phenom. 93 (1974) 4. [30] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, PerkinElmer, Eden Prairie, MN, 1992. [31] P. Jiricek, Czech. J. Phys. 44 (1994) 261. [32] I.M. Band, I.Y. Kharitonov, M.B. Trzhaskovskaya, At. Data Nucl. Data Tables 23 (1979) 443. [33] G. Uebing, Heterogen. Chem. Rev. 3 (1996) 351. [34] G.N. Greaves, W. Smith, E. Giulotto, E. Pantos, J. NonCryst. Solids 222 (1997) 13. [35] J.F. Keslo, C.G. Pantano, S.H. Garofalini, Surf. Sci. 134 (1983) L543. [36] O. Gedeon, J. Zemek, unpublished results. [37] A. Miotelo, P. Mazzoldi, Phys Rev. Lett. 54 (1985) 1675. [38] B.J. Todd, J.L. Lineweaver, J.T. Kerr, J. Appl. Phys. 31 (1960) 51. [39] F. Ohuchi, P.H. Holloway, J. Vac. Sci. Technol. 20 (3) (1982) 863. [40] G. Battaglin, G. Della Mea, G. De Marchi, P. Mazzoldi, O. Puglisi, J. Non-Cryst. Solids 50 (1982) 119. [41] K. Jurek, O. Gedeon, V. Hulinsky, Mikrochim. Acta [Suppl.] 15 (1998) 269. [42] O. Gedeon, K. Jurek, V. Hulinsky, J. Non-Cryst. Solids 246 (1999) 1.
Low energy and low dose electron irradiation of potassium–lime–silicate glass investigated by XPS. I. Surface composition Ondrej Gedeon b
a,*
, Josef Zemek
b
a Institute of Chemical Technology, Technicka 5, 166 28 Prague 6, Czech Republic Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
Received 20 July 2002; received in revised form 30 September 2002
Abstract X-ray induced photoelectron spectroscopy has been used to study the influence of low-energy electron beam on the pristine potassium–lime–silicate glass surface prepared by fracturing in situ under ultrahigh vacuum. Relatively lowenergy electron beam of 1600 eV with low-electron beam current density of 0.02–0.22 A/m2 and low-electron dose of 29–5200 C/m2 was used. The expected modified near-surface region thickness is in this case comparable with the mean sampling depth of the analytical tool. Therefore, possible changes and modifications due to electron irradiation could be recorded with high sensitivity. The freshly fractured glass surface was found to be significantly enriched with potassium, and slightly with calcium. As a consequence of the lowest electron dose irradiation used, the potassium signal substantially increased by a factor 1.24 relative to the value found for the fresh surface. For higher doses used, the potassium signal continuously increased with the dose to a maximum and decreased thereafter. This variation was accompanied with the qualitative opposite behaviour of calcium signal. The concentrations of the other elements present in the glass, oxygen and silicon, varied only slightly with the electron dose. They can be considered to be constant within experimental uncertainty. In agreement with experimental results, a model assuming mobility of only two most mobile cations, potassium and calcium, was suggested. The models assuming one layer and two layers on the bulk were developed. Their results reproduce well experimental findings: (i) the formation of a potassium-rich surface layer, and (ii) the opposite-like signal variation of calcium in comparison with potassium. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 61.80.)x
1. Introduction
*
Corresponding author. Tel.: +420-2 2431 1918; fax: +420-2 2431 3200. E-mail address: [email protected] (O. Gedeon).
A solid irradiated by high-energy beam undergoes changes in its structure, caused mostly by ionization and ballistic interaction of high-energy particles with solid constituents [1]. This irradiation introduces disorder in the original structure
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00015-2
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causing amorphization of crystalline samples accompanied with volume changes [2]. Phase separation [3] or gas accumulation [4] are reported as examples of many modifications in the irradiated glass. Interaction of an electron beam with a glass containing alkali ions is a point of interest for analysts dealing with electrons as an excitation source. Mobility of alkali ions rapidly increases with electron irradiation and causes distortion of the analytical signal [5]. As a consequence, problems in quantification of recorded spectra are expected. The study of influence of electron bombardment on the alkali glass gives not only an adequate interpretation of spectra but also helps to understand elemental processes and changes in the irradiated glass. It results in the better understanding of glass itself. Interaction of electrons with glass is also important for the radioactive waste deposition in which electrons produced with various energies eventually lead to irreversible changes in glass. One of the most interesting and widely studied phenomena is the alkali ion migration in alkali glass irradiated by electron beams. The phenomenon can be observed even for very low-energy beams due to weakly bonded alkali ions in the glass structure. These changes have been observed as the decay of alkali intensities by many authors in the electron probe microanalysis (EPMA) [5–7] or in Auger electron spectroscopy (AES) [8–11]. If the macroscopic alkali migration is observed by EPMA (i.e., within high-energy region), a decay curve, introduced as the alkali X-ray intensity versus time, can be recorded. The measured decay curves generally display two distinct parts [12]. Instantaneously after the exposure, the recorded X-ray signal shows very slow linear decrease. After some time, denoted as the incubation period, the signal abruptly changes into an exponential-like decay. The incubation time can vary in the range from seconds to hours depending on the exposure conditions and on the type of glass. The alkali migration and oxygen desorption observed has been usually correlated with a mechanism proposed by Lineweaver [13]. The primary electrons produce a negatively charged layer at about the electron penetration depth. The charged layer is
assumed to attract positively charged alkali ions from the surface to bulk. Upon decreasing the energy of the primary electrons, the incubation period first slightly increases. The following variation of the incubation period depends on glass composition but it appears that the incubation period goes to infinity as the energy of primary electrons reaches some threshold value, below which no incubation period is found [14]. This energy threshold is proportional to the mass of alkali ion and it depends on the activation energy of the ion for diffusion in glass. The threshold was found to be around 13 keV for the potassium, and is expected to be around 6 keV for the sodium. To explain the existence of the threshold energy value for the incubation period, it was suggested that alkali ions are released from their original sites with help of elastic scattering [15]. The energy transferred by elastic scattering is inversely proportional to the mass of alkali ion, so that faster primary electron is needed for a release of a heavier ion. However, the energy of primary electrons inside the low-energy region is not high enough to release the alkali ions by this mechanism. The existence of the incubation period sometimes reported in AES (low-energy region), where voltage at/or below 3 kV is commonly used, is strange in framework of this model and other driving forces must be considered to explain transporting mechanism. Primary electrons trapped in a glass create a macroscopic electric field within the exposed volume almost instantly [5,16]. If the overall charge distribution is assumed homogeneous inside the irradiated volume, the macroscopic electric field intensity is a maximum at the surface and decreases linearly, reaching zero at the electron range. An inversion point was suggested by Cazaux [17] to explain the out-migration process from a few uppermost surface layers. However, Miotelo et al. [18] argued based on more general model that the existence of the inversion point should lead to the dielectric breakdown. They suggested Gibbsian segregation of alkali metal species to be a driving force for the out-migration. A formation of neutral alkali atoms under the electron irradiation was also suggested [9,19], but on the other hand, no evidence for this assumption was found by X-ray
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induced photoelectron spectroscopy (XPS) analysis of sodium glass [20], probably due to insufficient chemical shift of the Na 1s line. Brow [21] has shown the formation of metallic sodium on the surface of irradiated binary sodium–silicate glass whereas no metallic sodium was found in a CaOcontaining glass. Using the top-surface sensitive ion scattering spectroscopy (ISS), Then and Pantano [22] observed alkali signal increase at the beginning of electron irradiation and confirmed to-the-surface migration during the initial stages of electron irradiation. As a short summary, for high-energy region towards the bulk migration and incubation period were observed. The elastic scattering assisted hopping in the electric field seems to play an important role in the first stages of migration. In contrary, for low-energy irradiation towards the surface migration was found. The surface relaxation processes and Gibbsian segregation may count for towards the surface migration. In this paper, XPS of potassium–lime–silicate glass irradiated with 1600 eV electron beam is studied. The irradiation time and current were chosen to include low-dose irradiation. The work was inspired by the research conducted for higher electron energy beams and it tries to answer the some of the open questions. What is the role of the electric field for low-energy irradiation? What processes, causing the migration of alkali ions, are common and what are different for both energy regions? What are chemical changes like in the potassium depleted layer? Answers to these questions could bridge these two energy regions. Whereas answers concerning chemical changes will be the subject of the prepared paper, this paper concentrates to the description of quantitative changes of spectral intensities in the course of irradiation. Leaving the assumptions about homogeneity of the analyzed region, layered models are suggested for the interpretation of the most pronounced changes in intensities of potassium and calcium.
2. Experimental Glass was melted using high-purity batch in a Pt crucible and cast on a metal plate to prevent
179
crystallization. The plate of glass was annealed at 600 °C for 16 h. The glass was transparent and contained no bubbles or cords. The composition of glass used was: 79.5 wt% SiO2 , 8.5 wt% CaO, 12 wt% K2 O (5.47 at.% of K). A rectangular glass sample (5 mm 8 mm 10 mm) was fixed to a sample holder. Before electron irradiation, a clean glass surface was prepared by fracturing of the glass rod under UHV condition using a standard cleaving device. The clean pristine glass surface was step-by-step irradiated by electron beam with various electron doses. The energy of the electron beam used was 1600 eV and its diameter was set to 0.1 mm. The beam was scanned uniformly over a 10 mm 10 mm area to include the entire glass surface (5 mm 8 mm). The impact angle of the electron beam was perpendicular to the sample surface. The energy of the electron beam was chosen in the way that the expected modified near-surface region thickness was comparable with the mean sampling depth of the analytical tool. Therefore, possible changes and modifications due to electron irradiation could be recorded with high sensitivity. The mean sampling depth of XPS is approximately determined by k cos a [23] where k is the inelastic mean free path of measured photoelectrons and a is the emission angle measured with respect to the surface normal. For the experimental geometry used, the mean sampling depth is equal to k (both the emission angle and the impact angle for primary electron beam are zero). Hence, the mean sampling depths for electrons at specified kinetic energy travelling in SiO2 [24] are as follows: kðSi 2p; 1154 eVÞ ¼ 2:4 nm; kðO 1s; 722 eVÞ ¼ 1:7 nm; kðK 2p; 962 eVÞ ¼ 2:1 nm; kðCa 2p; 909 eVÞ ¼ 2:0 nm; kð1600 eVÞ ¼ 4:2 nm: The compositions of the freshly fractured and electron irradiated glass surfaces (top 5–7 nm, what corresponds to the depth, from which 95% of
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detected signal comes out) were studied by XPS using an angular-resolved photoelectron spectrometer equipped with a twin anode X-ray source, standard Al/Mg anodes, and a hemispherical analyzer. The expected temperature increase in a close vicinity of the beam spot at the glass surface could not exceed 2 °C as estimated from [25] for the highest beam current density used. So, the influence of the temperature increase on atomic transport processes may be neglected. For quantitative analysis, spectra were recorded for 10 min using Mg Ka source, operated at a power of 110 W and at constant pass energy of 100 eV. In addition, a few high-resolution spectra were recorded with pass energy of 20 eV for 120 min. As a result, the total X-ray exposure time could reach 800 min. Although some changes introduced by Xrays in glass are reported [26,27], our exposure conditions were very mild. Non-focused beam, low source power, low-photon density (diameter of the X-ray beam was 10 mm) were used to eliminate the influence of X-ray irradiation during spectra recording. Peak areas were determined following ShirleyÕs inelastic background subtraction method [28]. Atomic concentration of elements found in the analyzed thickness of about 5–7 nm were determined from the O 1s, Si 2p and K 2p lines assuming a simple model of a homogeneous semi-infinite solid [29]. The relative sensitivity factor approach [30] was applied to calculate atomic concentrations. The peak areas were corrected for (i) the measured transmission function of the spectrometer, which comprises all instrumental factors influencing the measured quantity [31], (ii) the photoelectric cross-sections [32], and (iii) the inelastic mean free paths [24] of photoelectrons in question, determining their motion in SiO2 . Experimental uncertainties accompanied with XPS quantitative analysis are estimated to be 7%. The value covers overall uncertainties of the method that are mostly introduced by the background subtraction and correction procedure used
for calculation of concentrations from intensities of spectral lines.
3. Results Atomic concentrations resulted from XPS quantitative analysis described above, calculated from spectra recorded at the emission angle a ¼ 0° with respect to the surface normal, are summarized in Table 1. For the freshly fractured glass surface, the potassium concentration is significantly higher (8.0 at.%) than its mean bulk concentration (5.6 at.%). This increase is accompanied with both a slight increase in the calcium concentration (3.6 at.% compared with its bulk concentration 3.3 at.%) and slight oxygen decrease (59.7 at.% versus 62.8 at.% for bulk). However, the two last values are within the experimental range of uncertainty. Following the first electron irradiation step at 29 C/m2 , potassium concentration increased to 9.9 at.% while calcium significantly decreased to 2.8 at.%. Similar surface enrichment by alkali atoms as potassium, rubidium and caesium studied by ISS was observed for electron irradiated glass surfaces at similar electron dose [22]. As illustrated in Fig. 1, the potassium concentration increases with the dose until the turn-over dose is achieved. It, then, begins to decrease. This behaviour is accompanied with a qualitative opposite curve for calcium. To underline changes caused by electron irradiation, the displayed concentrations in the figure are presented relative to their values corresponding to freshly fractured glass. Although the oxygen curve appears to be similar to the calcium curve, its values fall within the interval of uncertainty of XPS experiment. Therefore, both silicon and oxygen concentration dose dependences can be approximated by a constant values of 28.8 0.8 at.% and 55.6 1.6 at.%, respectively. All results presented in this section up to this point were based on the standard quantification procedure of XPS spectra that assume homogeneous sample in composition, i.e. the intensity Ii of the ith element is related to its concentration chom i by the equation [29]
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181
Table 1 Atomic concentrations and electron dose used for bulk, freshly fractured and electron irradiated potassium-lime-silicate glass surface No.
Electron dose (C/m2 )
Si (at.%)
O (at.%)
K (at.%)
Ca (at.%)
Bulk 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
– –
28.4 28.7 29.9 29.2 30.6 29.0 29.4 29.5 28.6 29.0 28.5 28.3 28.9 28.6 29.5 28.7 28.3 27.3 27.9
62.8 59.7 57.4 57.4 55.9 56.8 55.8 55.4 55.9 55.2 55.3 54.2 54.4 54.0 52.8 53.7 54.3 56.2 55.8
5.5 8.0 9.9 10.5 10.7 11.3 12.3 12.6 12.9 13.2 13.6 14.7 14.0 14.8 15.1 14.6 14.5 12.8 11.6
3.3 3.6 2.8 2.8 2.8 2.9 2.6 2.6 2.6 2.5 2.6 2.7 2.7 2.6 2.6 2.9 2.9 3.7 4.8
28.8 43.2 57.6 86.4 115.2 158.4 201.6 288.0 374.4 493.2 612.0 730.8 849.6 1087.2 1324.8 2948.4 5179.2
Relative concentration
All presented results are based on the assumption of homogeneity within a sampling depth.
1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0
1000
2000
3000
4000
5000
6000
2
Dose [C/m ]
Fig. 1. Oxygen (j) silicon (r) potassium (N) and calcium () atomic concentration changes due to electron irradiation: atomic concentration of the particular element is normalized with respect to its concentration measured for freshly fractured glass surface. Solid lines represent result of the layer-on-thebulk model and broken line represent two-layer-on-the-bulk model.
Ii ¼ T ðEÞN ki chom ; i
ð1Þ
where T ðEÞ is the spectrometer function that includes experimental geometry, electron transmission of the analyzer and sensitivity of the detector. N is the atomic density, and ki is the inelastic mean free path of photoelectron of the ith element in SiO2 . The superscript ÔhomÕ underlines the as-
sumption of homogeneity in composition within the sampling depth. The results based on the assumption of homogeneity of the analyzed sample showed that the most mobile ions are potassium and calcium, while oxygen and silicon can be assumed immobile. It is obvious from above presented results that the assumption of homogeneity is an extensive oversimplification. Although diffusion-like profiles of concentration of mobile elements (Ca, K) can be considered, their introductions could be quite misleading as the existence of two opposite processes (first one, drawing the ion towards the surface and the second one, drawing the ion toward the bulk) can be deduced from Fig. 1. Therefore, simple one-layer-on-the-bulk and two-layer-onthe-bulk models are suggested to explain the measured results. The latter, more complicated model, also serves as a test of stability of the model – two qualitative different results obtained by the above models should signal the instability of the layered model. The Eq. (1) can be easily extended for the layered sample. The intensity of ith element coming from the two layers on the bulk is calculated as follows:
O. Gedeon, J. Zemek / Journal of Non-Crystalline Solids 320 (2003) 177–186
ð2Þ
where tj is a thickness of the jth layer, cji is the concentration of ith element in the jth layer and cbulk is the concentration in bulk. ki (inelastic mean i free path of ith element) was assumed independent of chemical composition and set to value for SiO2 . Hereafter the superscript over a quantity points at the number (1 for the uppermost and 2 for the buried layer) of the layer (or bulk) while the subscript identifies the element. The first term inside braces corresponds to the contribution of the first layer, the second term to the contribution of the buried layer and the last one to the contribution of the bulk. Each layer is assumed to be homogeneous with three free parameters: the thickness of the layer and concentrations of potassium and calcium. Oxygen and silicon are assumed (in consistence with experimental results) to be immobile and their concentrations are set to their bulk values. These assumptions decreased the number of adjustable parameters to three (for the one-layer model) and six (for the two-layer model), respectively. To answer the question, if the results based on the homogeneous semi-infinite sample can be explained by the suggested layered model, we have tried to adjust free parameters in the models so as the layered models reproduced results of the homogeneous semi-infinite sample. This leads to the following set of four equations for the concentration of each element, obtained by the combination of Eqs. (1) and (2) 1 2 t t 1 2 chom ¼ c 1 exp 1 exp þ c i i i k1 k1 1 t1 t t2 exp exp þ cbulk exp : i ki ki ki ð3Þ The set of equation was solved by the least-squares method. The initial estimates of concentrations
were set to their bulk values and the thickness of the upper layer was set to 0.01 nm (near the zero value) for the one-layer model. Both models predict formation of a potassiumrich layer, thickness of which changes with the irradiation dose as is displayed in Fig. 2. The (maximum) thickness of the upper layer is of the order of interatomic distance, what is in an agreement with an idea of segregated particles on the surface [33]. Whereas models yield nearly purely potassium uppermost layer for lower doses (up to approximately 1000 C/m2 ), the decrease of potassium concentration complemented by the increase of calcium concentration can be observed in the layer for higher doses (Fig. 3). The change of the composition of the uppermost layer approximately coincides with the decrease of the thickness of the layer. Both curves indicate saturation of processes after the dose of about 1000 C/m2 that
0.4
Thickness [nm]
1 t Ii ¼ T ðEÞN ki c1i 1 exp ki 2 t t1 2 þ ci 1 exp exp ki ki 1 2 t t bulk exp ; þ ci exp ki ki
0.3 0.2 0.1 0.0 0
1000
2000
3000
4000
5000
6000
2
Dose [C/m ] Fig. 2. Thickness of the upper layer predicted by the one-layeron-the-bulk (solid line) and two-layer-on-the-bulk (broken line) models.
100
Concentration [at %]
182
80
K
60 40 20
Ca
0 0
1000
2000
3000
4000
5000
6000
2
Dose [C/m ]
Fig. 3. Concentrations of Ca and K in the uppermost layer predicted by the one-layer-on-the-bulk (solid line) and twolayer-on-the-bulk (broken line) models.
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leads to the formation of the potassium-rich layer. The increase of calcium concentration with simultaneous decrease of potassium concentration in the upper layer for higher doses does not necessary mean the opposite-like migration of calcium in comparison with potassium. It is rather the consequence of the lower mobility of calcium so that the relative concentration of calcium increases as potassium escapes from the upper layer faster than calcium. Results of the simpler model complemented by the buried 0.5 nm-thick layer of the bulk composition were taken as the initial guess for the two-layer model. However, two-layer models qualitatively converged to the one-layer model, with the uppermost potassium-rich layer and the slightly changed composition (compared with bulk) of the buried layer. The thickness of the formed potassium layer is shown in Fig. 2 and is higher compared with the results of the simpler model. The potassium amount of the upper layer is close to 100 at.%, similarly as in the one-layer model (Fig. 3). In addition, the buried layer is free of potassium with calcium content (2.6 0.9 at.%) slightly lower than in bulk. It can be explained by the migration of calcium from bulk towards the surface as is expected due to its similarity with potassium ion. As the migration of potassium towards the surface is much faster, the calcium increase in the uppermost layer is camouflaged by the faster increase of potassium flow towards the surface. For higher doses, calcium complements the potassium decrease in the upper layer.
183
Although Si and O were assumed immobile, their relative concentrations change as the toplayer forms. The oxygen intensity is very sensitive to the upper layer formation due to the high absorption of its spectral line in the layer. Even if not presented, relative intensity of O coming from the model qualitatively follows the experimental data with dose (similarly as for calcium, the oxygen concentration first decreases followed by the increase for higher dosed) so that the difference between experimental and model values are fully within the range of experimental uncertainties (7%). The change of oxygen signal can be therefore fully explained in framework of the suggested layer model without any other assumptions.
4. Discussion The papers dealing with low-energy electron irradiation of alkali glass mostly report the study of sodium glass (Table 2). However, although potassium plays a similar role as a modifier in the silicate network, it differs by its local structure. Because of its larger size, more oxygen anions participate in its nearest neighbourÕs co-ordination shell. Its size also results in its lower mobility due to the relatively high-displacement energy. Therefore, results obtained for sodium ions cannot be simply extended to the potassium glass, although some common qualitative behaviour can be expected. In general, potassium lower mobility results in the longer incubation period, higher doses
Table 2 Selected characteristics reported in papers dealing with low-energy electron irradiation of alkali ion containing glasses (electron energy below 5 keV, except for the EPMA data presented for a comparison) Method/Ref.
Investigated type of alkali ion
Energy of primary electrons (keV)
Current density (A/m2 )
Maximum dose (C/m2 )
AES [9] AES [9] AES [39] AES [8] XPS, AES [40] XPS, AES [20] ISS [22] XPS [21] EPMA [41,42] XPS (this work)
Na K Na Na, K Na Na K, Rb, Cs Na K K
1.5, 3 1.5, 3 0.5–4 1.5 2.5, 4.5 2–5 0.5 3 50 1.6
100–1300 500–4000 10 0.1–10 0.012, 0.04 15 0.05 5 1.9 0.02–0.22
3.6 105 2.1 106 900 200–2000 480 10 800 8000 6600 14700 5200
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needed for obtaining comparable results as opposed to sodium glass, and after all in a prolongation of experimental time scale that enables one to study processes that are less time-resolved. The higher near-surface potassium concentration (8.0 at.%) on in situ freshly fractured glass surface in comparison to its bulk content is a result of a surface relaxation, in which the most mobile ions (potassium ions in our case) play the dominant role. The alkali surface concentration increase was also demonstrated experimentally [34] as well as theoretically by molecular dynamics simulation [35]. The suggested layered models confirm the tendency of to-the-surface migration of alkali ion by the prediction of potassium-rich surface layer formation. Consequential irradiation, even in low doses, loosens the glass structure and enables more potassium ions to take part in the relaxation processes. It resulted in the further increase of potassium surface concentration up to 15.1 at.%. This increase is in a consistence with the increase of potassium signal in the uppermost layer observed by Then and Pantano [22] by the top-surface sensitive ISS. The potassium signal increase was also observed by AES [8] in binary potassiumsilicate glass exposed with a low-electron dose. Due to the similarity of calcium ion with potassium one, the participation of calcium at surface relaxation can be expected. The two-layer model prediction of the decrease of calcium concentration in the buried layer may be attributed to the migration of calcium towards the surface. The decrease of the calcium signal is, however, camouflaged by the higher mobility of potassium that enables to form potassium-rich uppermost layer. Both suggested layered models converge to the same qualitative results – formation of the uppermost potassium layer. Its thickness increases with the irradiation dose until the surface relaxation processes are saturated. This corresponds to the turn-over dose, from which a decrease of the potassium signal is observed. This is the point where the electric field stimulated diffusion takes over control. Higher doses lead to the decrease of both potassium concentration and the thickness of the upper layer. The presence of two different stages of migration is also confirmed by the evo-
lution of spectral lines with an electron dose [36]. Therefore, it seems that the dominant driving mechanism of potassium migration at the beginning of electron irradiation is connected with further surface relaxation. This process is very rapid and lower doses should be used to record the earliest stages. As soon as the surface relaxation processes are saturated, the electric field takes its dominant role and draws the alkali ions towards bulk. The reason why the increase of potassium signal (surface relaxation processes) is not observed by analytical methods using higher energies of primary electron beam (EPMA) is evidently connected with their low-surface sensitivity. On the other hand, the electric field is a common driving force count for the decrease of potassium analytical signal, causing final alkali depletion from the exposed glass. The models are also capable to explain a ÔstrangeÕ calcium increase of intensity, already reported by Gossink and Lommen [9], only by the formation of the potassium-rich uppermost layer. Miotelo and Mazzoldi [37] tried to explain this phenomenon by the model of co-operative migration between potassium and calcium. Although the model is able to reproduce the experimental findings very well, the Ôcooperative diffusion coefficientÕ (DCa;K ¼ 5 1015 cm2 /s) used in the model should be surprisingly quite comparable with the calculated Ôstandard diffusion coefficientÕ for calcium (DCa ¼ 5 1015 cm2 /s). It means that potassium escape is assumed to be the driving force for calcium movement into the potassium-depleted region, causing the calcium enrichment proportional to DCa;K o2 nK =ox2 . Instead of co-operation mechanism, results of our layered models suggest that the calcium intensity variation is caused by the higher mobility of potassium ions. The initial decrease of the calcium signal can be explained by the faster migration of potassium towards the surface, resulting in the formation of potassiumrich uppermost layer. The increase of calcium concentration in the upper layer for higher irradiation doses indicates that calcium also takes place in the surface relaxation (as it can be expected due to its similarity with potassium ions). The decrease of calcium in buried layer in framework of the two-layer model also supports this
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idea, as it can be attributed to the migration of calcium towards the surface. Even oxygen outgassing is reported during the irradiation [38] and often used for the interpretation of the formation of alkali depletion layer [13] this experiment cannot confirm it. The current density in experiment reporting oxygen outgassing is significantly higher (approximately of an order) so that the outgassing is probably connected with an extensive temperature increase. Hence, oxygen was taken in the model as immobile. Although oxygen intensity variation can be partly explained by the created top-surface camouflaging, some oxygen mobility can be expected. Especially, the drop of oxygen signal after the fracturing and at the lowest dose irradiation is expected to be accompanied with oxygen transport. However, gain obtained from the assumption of oxygen mobility should be paid by lower stability of models.
5. Summary and conclusions A glass rod fractured in situ in ultrahigh vacuum was used to study electron beam induced modifications of pristine potassium–lime–silicate glass surface. The fresh fractured surface was sequentially irradiated under conditions where thickness of possibly modified surface region is comparable with the mean sampling depth of the analytical tool: by a relatively low-energy electron beam of 1600 eV and a low-current density, with electron doses from 29 to 5200 C/m2 . Moreover, the electron beam scanned over the surface, so that no surface temperature growth due to the irradiation is expected. The freshly fractured glass surface was found slightly enriched by potassium and calcium with respect to the corresponding bulk concentrations. Following the lowest electron dose irradiation, the potassium signal further increased substantially while the calcium signal decreased. For higher electron doses used, the potassium signal continuously increased with dose, accompanied with the calcium signal decrease. However, the opposite dependence was found for higher doses. The simple layered models were suggested. They predict a formation of the potassium up-
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permost layer. The effective potassium concentration increase for lower doses (as well as the new surface creation) can be associated with surface relaxation processes, while the following decrease is suggested to be caused by migration of potassium cations in the electric field created in the irradiated sample. Potassium, as the most mobile element, responds to new forces promptly. The calcium opposite-like qualitative behaviour can be fully explained by the absorption in this uppermost potassium-rich layer and its lower mobility. The above results showed that it is extremely difficult to analyze alkali silicate glass samples by analytical tools using electron beams without severe structural damage and compositional changes. The lowest electron dose, electron beam current density and scanned or defocused electron beam is recommended for lowering the harmful influence of the beam on the analyzed glass.
Acknowledgements This work was financially supported by Grant Agency of the Czech Republic through the grant no. 104/99/1407. It was also part of the research project CEZ: MSM 223100002 Chemistry and technology of materials for technical applications, health and environment protection.
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