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Search for implantation-induced “re-solution” of Na precipitates in MgO via optical spectroscopy and in situ TEM
Nuclear Instruments and Methods in Physics Research B12 (1985) 375-381 North-Holland, Amsterdam
SEARCH FOR IMPLANTATION-INDUCED “RESOLUTION” OPTICAL SPECTROSCOPY AND IN SITU TEM
375
OF Na PRECIPITATES
IN MgO VIA
M. TREILLEUX and P. THEVENARD Dkpartement de Physique des Matbriaux, 69622 Villeurbanne C&&x, France
Universit& Claude Bernard Lyon I, 43 Boulevard du 11 Novembre
1918,
M.O. RUAULT, H. BERNAS and J. CI-IAUMONT Centre de Spectromilrie
Nuclbaire et de Spectrom&rie
de Masse, BP no. I, 91406 Orsay Ckdex, France
Received 16 November 1984 and in revised form 29 May 1985
It has previously been shown that high fluence Na implantation into MgO crystals followed by annealing leads to the formation of metallic Na precipitates. We have studied the effect of a subsequent low -fluence Na implantation on such precipitates, i.e., the effect of Na damage cascades whose size is comparable to that of the precipitates. Optical spectroscopy experime.n@and in situ transmission electron microscopy (TEM) experiments performed on-line with the Orsay ion irnplaritorare both discussed. It is shown that simple ballistic ion beam mixing cannot account for the results. The latter indicate that damage-induced mixing at the host matrix-precipitate interface leads to drastic changes in the precipitate composition, presumably due to in-diffusion of oxygen (and possibly Mg) into the precipitate volume.
1. Introduction Ion implantation of ionic crystals causes complex structural changes and chemical effects [l]. The host can incorporate implanted particles in different charge states. The study of the metastable phases produced by a high dose implantation of metallic ions into MgO has been .a subject of interest during the last decade. The main results of these studies are that the nature of the
precipitates in MgO depends on the implanted incident ion species: alkali ions tend to form small metallic precipitates [2-51 while indium or gold ions tend to form binary alloys with magnesium [6,7]. In the case of iron ions, spine1 ferrite particles can be formed [8]. Precipitate formation after implantation and annealing is presumably facilitated by the implantation-induced defects, but chemical interactions are also important since the nature of the precipitate phase depends on the implanted species (see also ref. [9]). We report an attempt to distinguish between the contribution due to simple collisional damage and that due to the more complex interactions of chemical affinity and atomic species mobility. This is performed via optical absorption and in situ TEM experiments on MgO samples in which metallic Na precipitates have previously been formed by implantation and subsequent annealing [2,3] and which are again submitted to Na implantation. Some of our results have already been reported in 0168-583X/85/$03.30 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
prekninary form [lo]. The major r6le of beam induced in-diffusion of host atoms into the solute precipitates is made clear. 2.Optlcal
absorption measurements
2.1. Evolution of MgO crystals implanted with sodium ions
The stable defects created in MgO by ion bombardment lead to optical absorption spectra which exhibit two main absorption bands located at 250 run and 575 nm. These are associated respectively with oxygen (F or F+ centers) and magnesium (V-type centers) vacancies. The optical spectra of as-implanted samples only show the existence of these intrinsic defects; no absorption band due to implanted atoms is observed. After thermal annealing the intrinsic defects (F and V-type centers) progressively disappear: the two absorption bands decrease in amplitude and at 1100 K all the intrinsic point defects are annealed. A concomitant increase in optical absorption is observed in the region of the optical spectra where small metallic particles of pure sodium absorb light when they are embedded in the MgO lattice. Fig. 1 shows the optical absorption spectra of a MgO single crystal implanted with 5 X 1016 sodium ions cm-’ of 150 keV energy. After annealing for 15 min in air at 973 K, partial precipitation of Na atoms is observed via
316
M. Treikx
I
4
3
-Phottn
et al. / Implantation-induced
re-solution
of Na precipitates in MgO
-
energy ($ 5
I
4
Photon energy (eV) I
,
I.5
t DO t
1.5
>, C 2 4, 5 0 ._ z
0 j
0
Wavelength
nm-
Fig. 1. Optical absorption spectra of MgO single crystal implanted with 5 X 1016 sodium ions cmm2 of 150 keV energy: (a) as-implanted (b) annealed for 15 min at 973 K.
the optical absorption band due to sodium precipitates located at 540 nm (fig. 1). The frequency corresponding to absorption of sodium precipitates in MgO is related to the plasma frequency wr, of free electrons in sodium by [ll]: w (Na/MgO)
=
“” (1 + 2#2
’
where E is the dielectric constant of the insulating lattice of MgO (equal to about 3). The mean size of the precipitates can be deduced from the half width of the optical absorption band AE,,, as [ll]: A E,,,
= hu r/R,
where h is the Planck constant, tir the Fermi velocity and R the mean radius of precipitates. In the crystal annealed at 973 K, the mean diameter of sodium aggregates deduced from this formula is about 7.2 nm. This is not inconsistent with the mean diameter deduced from TEM observations. 2.2. Effect of Na + ion implantation into MgO containing sodium precipitates Fig. 2 displays the evolution of the optical spectra of MgO containing sodium precipitates after implantation with 150 keV Na+ ions at fluences of 3 X 1014 and 10” ions cmh2. The amplitude of the absorption band due to sodium precipitates decreases, showing that the fraction of precipitated sodium metal decreases with increasing fluence. This could be interpreted in terms of metallic sodium precipitate dissolution in the MgO lattice under ion beam bombardment. But the position of the Na absorption band shifts towards higher wavelengths and its width decreases which seems to indicate
0 200
500 WaVel8ngth
A “nl_
1000
-
Fig. 2. Optical absorption spectra of MgO crystal implanted with 5 X lOi sodium ions cmm2 of 150 keV energy: (a) annealed for 15 min at 973 K; subsequently implanted with (b) 3X1014 150 keV Na+ ions cme2 and (c) lOI 150 keV Na+ ions cma2. an increase in the mean size of the precipitates (7.8 nm after 3 x 1014 Na+ cmm2 and 8.6 nm after 10” Na+cmW2). Optical absorption observations alone are thus not sufficient to deduce the evolution of the metallic precipitates. In situ TEM observations under implantation provide information on the evolution of sodium precipitates during ion bombardment.
3. Transmission
electron microscopy observatkhs
Previous results [3,12] obtained by TEM on sodium clusters embedded in MgO showed that, though the structures of sodium (bee) and MgO (fee NaCl type) are different, most of the aggregates are in a simple orientation with respect to the matrix ((0Ol)Na (] (0Ol)MgO and [lOO]Na ]] [lOO]MgO) due to the good agreement between their bulk lattice parameters ( aNa = 0.4291 nm and, a,,, = 0.4213 nm). They are partially coherent with the matrix. The contrast of partially coherent precipitates can result from a matrix contrast and a precipitate contrast. From TEM observations of Na aggregates there is no evidence of matrix contrast arising through elastic straining of the lattice, probably because sodium is elastically soft by comparison with MgO. The precipitate contrast can be analyzed in two ways: first, a structure factor contrast when matrix and precipitate are under diffraction conditions and have different structure factors, and, second, an orientation contrast when a certain set of lattice planes in the precipitate is diffracting strongly whereas the matrix is diffracting
M. Treilkux et al. / Implantation - indticed re -solution of Na precipitates
in MgO
371
Fig. 3. MgO single crystal implanted with lOI sodium ions cm- ’ of 750 keV energy, annealed for 40 min at 1073 K, thinned for TEM observations and then implanted with Na2+ Ions accelerated under 60 kV. Dark-field performed in the same crystal area with (110) Na reflection: (a) before implantation. After implantation with (b) 3 X 1Ol3 ions cmW2,(c) 5 X lOI ions cmm2, (d) 5 X 1014 ions cme2
weakly or vice versa. In the case of partially coherent Na precipitates these two kinds of contrast can be observed which, normally, enable us to study the evolution of aggregates. However, bright-field or dark-field images obtained from matrix reflections show an extensive dislocation network [4] which prevents observation of all the Na aggregates. Fortunately dark-field micrographs from precipitate reflections show a strong contrast (orientation contrast) of the aggregates with bright precipitates in a dark matrix. This is the best condition to study the size distribution of precipitates. In the
following experiments dark-field micrographs were performed using (110) Na reflection. In order to follow the size distribution of sodium clusters during Na+ implantation it was necessary to make continuous TEM observations on the same area of the crystal. This was made possible by using the Philips EM 400 electron microscope on-line with the CSNSM ion implantor [13]. The MgO crystal containing sodium precipitates was implanted directly in the microscope chamber with a scanned Na+ ion beam. The conditions of the experiment were the following:
1
_
20
D
nm-
~. 1
20 D nm -
0
Fig. 4. Mg0 sing8e crysti cor~taining sodium precipitates implanted with Na2+ ions accekzited under 6Q kV. Histogmms: concentration C vs diameter D of the visible precipitates observed in dark-field and contained in a small crystal volume (four times the volume imaged in fig. 3). (a) Before implantation. After implantation with (b) 3 X lOI3 ions cm-*, (c} 5 Xl@ ions cm-‘, (d) 1.3 X 1P ions cm-‘, (e) 5 X 1P ions cm- ‘. The point-to-point resoiution of the TEM was better than 10 A.
(1) The MgO single crystal was first implanted with 750 keV sodium ions accelerated by the 2.5 MeV Van de Graaff accelerator of the “Departement de Physique des Matinal, Lyon”. The fhznce was 10” Naf cm-‘. For these ions the mean penetration depth and the full width at half maximum of the depth distribution de duced from the Dearnaley et al. tables [14] were respectively 720 nm and 190 nm. Then the crystal was annealed for 40 min at 1073 K in order to form Na precipitates in the MgO matrix. (2) A thin foii was prepared for TEM observations from a disc (diameter 3 mm) cut in the crystal. By chemical solution in dilute ~hoph~pho~c acid a thicknesS of about 650 nm was removed from the implanted side, then the splits side was chemically thirmed by using a jet polishing technique until a hole was formed in the implanted zone. (3) After mounting the sample in the Orsay on-line TEM, the Na implantation was performed with Na2’ ions accelerated under 60 kV corresponding to 120 keV Na+ ions. The incidence angle of the ion beam was W,
so that the mean penetration depth and the width at half maximum of the depth distribution of the ions deduced from the Dearnaley et al. tables 1141 were, respectively, 82 nm and 52 nm. The thin foil observations were carried out, at room temperature, after each dose by tilting the sample so that the dark-field images obtained with (110) Na reflection gave the best contrast. The point-to-point resolutiQn of the electron microscope is less than 1 nm. All the micrographs were performed on the same crystal vohnne, so that the size distribution of sodium precipitates cx&d be studied in terms of the ffuence. The micrograph analyses were carried out dire&y on the negative dark-field images using an optic& microscope provided with a micrometric ocular. Fig. Ja shows the initial sodium precipitates in a crystal volume corresponding to a quarter of the total analyzed cryztat volume. The corresponding sodium precipitate size distribution is represented in fig. 4a. The precipitate concentration was about lOI precipitates per cm3; their mean size was about 12.5 nm.
hf. Treilleux et al. / Implantation -induced re -solution of Na precipitates
In situ observations were made for implanted Na fluences up to 1.8 X 1015 ions cmP2. The dose rate was about 10” ions cme2 s-l for low doses (3 x 1013 and 5 X 1013 ions cme2); it was increased up to 1012 ions cmF2 s-l for the highest fluences in order to reduce the implantation time. The first result is that during the implantations a bending of the thin foil is observed. This bending increases with increasing fluence, and occurs in the direction of the incident ions: the rotation angle of the goniometric stage was equal to +6” before implantation, -5’ for 4 X 1014 ions cmm2, - 9’ for 8 x 1014 ions cm -i and -14” for 1.8 X 1015 ions cme2. The sample bends under the strain induced by the accumulation of defects (implanted atoms, interstitials and vacancies) and the corresponding volume increase on the entrance side of the crystal. This effect was also observed on thin MgO crystals (15 X 2.5 X 0.2 mm3) implanted with Ar+ and Ne+ [15,16]. Similar damageinduced warping effects are familiar in semiconductors ]171. The dark-field micrographs of figs. 3a-3d, carried out in the same crystal area, show the evolution of the precipitates during the Na implantation. Up to a fluence of 5 X 1014Na+ cme2 the number of visible precipitates in the dark-field images decreased rapidly. Fig. 4 analyzes the precipitate size in the dark-field images. The total concentration of visible precipitates decreases from 1016 cm- 3 (before implantation) to 2 x lOi cme3 (after 5 X 1014 ion cme2). From the histograms of fig. 4 we can deduce the concentration of the precipitated metallic sodium and the mean diameter of aggregates. Fig. 5 shows the fluence dependence of these two quantities. The concentration of the precipitated sodium metal decreases and the mean diameter of the precipitates increases slightly. These results apparently confirm those deduced from the optical studies. The evolution of the visible precipitate concentration in terms of the fluence is shown in fig. 6 for precipitates whose diameter ranges from 6 to 18 nm. These evolutions are faster for small precipitates than for larger ones. Thus, for these metallic precipitates, the probability of disappearing increases when the diameter of aggregates decreases leading to an increase of the mean diameter of visible precipitates. As opposed to the decrease of the absolute number of visible clusters (fig. 4), their average size is rather weakly affected by Na implanfafion (slight increase due to the preferential vanishing of small aggregates) (fig. 5). This result precludes an interpretation in terms of progressive Na precipitate dissolution under the kinematical effect of incoming Na ions since the latter process would lead to a significant reduction in average precipitate size. Moreover, a careful study of the dark-field micrographs revealed the following important effect:
in MgO
379
nm
Fig. 5. Evolution of the concentration of the precipitated metallic sodium CNa (deduced from the histograms of fig. 4) during the bombardment with a dose D of Na2+ ions accelerated under 60 kV. Part (b) gives evolution of the mean diameter of aggregates 5 vs the dose D.
metallic Na precipitates that had not been affected by Na ion implantation at fluences of some 1014 Na cmb2 (i.e. corresponding to about one displacement per atom on average in the implanted layer, or - alternatively to about a hundredfold cascade overlap) could become invisible, after a further implanted fluence of only lOi Na cme2. This result demonstrates the basically discontinuous nature of the process under study (as opposed to a progressive collisional breakup process).
380
M. Treilleux et al. / Implantation-induced
Fig. 6. Evolution of the aggregate concentration C (deduced from the histograms of fig. 4) vs the dose D of Na2+ ions for precipitates having different diameter +: (a) 6 nm < + < 9 nm;(b) 9 run<+<12 nm;(c) 12 nmcg<15 nm;(d) 15 nm<+
Furthermore, though a precipitate becomes invisible in a dark-field micrograph, a careful study of the corre sponding bright-field micrograph reveals that a contrast remains at the place of the precipitate. We conclude that the Na implantation does not lead to precipitate disruption, nor to a change in its size (as seen in fig. 3), but does lead to a modification of its structure and chemical nature.
4. Dicussion and conclusion The area under the optical absorption band at 540 MI is proportional to the quantity of Na in metallic precipitate form. The reduction in band amplitude can be ascribed to either the dispersion (“re-solution”) of Na atoms in the MgO host, presumably due to the progressive breakup of the metallic precipitates under the impact of the incident particles, or to a change in the optical absorption properties of the precipitates (from metallic to insulating). The progressive breakup mechanism is in complete contradiction with our other results. Although the precipitates are no longer all imaged in the dark-field TEM micrographs a contrast remains in the corresponding bright-field micrographs. This, together with the fairly small change in average precipitate size, shows that the main effect of the Na implantation is to induce a structural and/or chemical composition change in the precipitate volume. Struct-
re -solution of Na precipitates
in MgO
ural changes alone in pure Na are insufficient to account for the change in metallic character implied by the optical absorption result, and thus we conclude that the Na implantation produces a compositional change away from metallic Na inside the precipitate. From the TEM results described above, it is clear that this compositional change has a “one-shot” character (see the effect of the 1013 Na cm-’ Implantation), . but that even an incoming Na ion is not efficient in inducing the transition. We infer that collision cascades produced by the incident particles at the host matrix-precipitate interface lead to the incorporation of host (Mg or 0) atoms into the precipitates. This conclusion is reminiscent of “ion-beam mixing” [18] of course, but it contains the ambiguity of that concept. In fact, simple ballistic mixing cannot account for our results (notably the “one-shot” effect and the weak variation of the average precipitate size). Recent work on ion-beam mixing has demonstrated [19,20] that some 60-80% of effect is related to atomic mobility under irradiation and that chemical affinity plays a major role in determining the nature of the ultimate phase thus produced. We suggest that the MgO-Na aggregate system is an extreme case of this mechanism. Because of their small size, the Na precipitates are at very high pressures: diffusion (notably of oxygen) is then considerably enhanced inside the Na metal. Thus the collision cascade at the interface essentially serves to trigger the inward diffusion of the host constituents. The efficiency of this process is somewhat size-dependent because of the diffusion coefficient pressure dependence and also because the surface-to-volume ratio must be maximized in order to enhance the interface interaction. Note that in the present case (as opposed to standard “ion-beam mixing” experiments) there is no need to invoke radiation-enhanced (or induced) diffusion to account for the results, although they are obviously not ruled out. Detailed studies of the precipitate phase modification probability in terms of precipitate size and sample temperature would provide more quantitative information. References [l] A. Perez, M. Treilleux, P. Thevenard, G. Abouchacra, G. Marest, L. Fritsch and J. Serughetti, Metastable Materials Formation by Ion Implantation, eds., S. Picraux and W.J. Choyke (North-Holland, Amsterdam, 1982) p. 159. [2] P. Thevenard, J. Phys. 37 (1976) C7-526. [3] M. Treilleux and G. Chassagne, J. Phys. Lett. 40 (1979) L-161. ]4] M. Treilleux, P. Thevenard, G. Chassagne and L.W. Hobbs, Phys. Stat. Sol. 48 (1978) 425. [5] M. Treilleux and G. Chassagne, J. Phys. Lett. 40 (1979) L-283. [6] A. Perez, M. Treilleux, L. Fritsch and G. Marest, Radiat. Effects 64 (1982) 199.
M. Treilleux et al. / Implantation-induced [7] G. Abouchacra, G. Chassagne and J. Serughetti, Radiat. Effects 64 (1982) 189. [8] A. Perez, M. Treilleux, L. Fritsch, G. Marest, Nucl. Instr. and Meth. 182/183 (1981) 747. [9] M. Guermazi, Thesis, Lyon (1984). [lo] P. Thevenard, M. Treilleux, M. 0. Ruault, J. Chaumont and H. Bemas Nucl. Instr. and Meth. Bl (1984) 235. [ll] W.T. Doyle, Phys. Rev. 111 (1958) 1067. [12] M. Treilleux and J.M. Penisson, 8th European Congress on Electron Microscopy, Budapest (1984). [13] M.O. Ruault, J. Chaumont and H. Bemas, Nucl. Instr. and Meth. 209/210 (1983) 351 and IEEE Trans. Nucl. Sci. NS-30 (1983) 1746.
re -solution of Na precipitates
in MgO
381
[14] G. Deamaley, J.H. Freeman, R.S. Nelson and J.H. Stephen, Ion Implantation (North-Holland, Amsterdam, 1973) p. 766. [15] G.B. Krefft, J. Vat. Sci. TechnoL 14 (1977) 533. 1161 G.B. Krefft, Radiat. Effects 49 (1980) 127. [17] E.P. Eernisse, J. Appl. Phys. 42 (1971) 480. [18] S. Matteson and M.A. Nicolet, Ann. Rev. Mat. Sci. 13 (1983) 339. [19] J. Bottiger, S.K. Nielsen and P.T. Thorsen, Nucl. Instr. and Meth. B7/8 (1985) 707. (201 J.E.E. BagIin, Int. Conf. on Surface Modification of Metals by Ion Beams, Heidelberg (1984).
SEARCH FOR IMPLANTATION-INDUCED “RESOLUTION” OPTICAL SPECTROSCOPY AND IN SITU TEM
375
OF Na PRECIPITATES
IN MgO VIA
M. TREILLEUX and P. THEVENARD Dkpartement de Physique des Matbriaux, 69622 Villeurbanne C&&x, France
Universit& Claude Bernard Lyon I, 43 Boulevard du 11 Novembre
1918,
M.O. RUAULT, H. BERNAS and J. CI-IAUMONT Centre de Spectromilrie
Nuclbaire et de Spectrom&rie
de Masse, BP no. I, 91406 Orsay Ckdex, France
Received 16 November 1984 and in revised form 29 May 1985
It has previously been shown that high fluence Na implantation into MgO crystals followed by annealing leads to the formation of metallic Na precipitates. We have studied the effect of a subsequent low -fluence Na implantation on such precipitates, i.e., the effect of Na damage cascades whose size is comparable to that of the precipitates. Optical spectroscopy experime.n@and in situ transmission electron microscopy (TEM) experiments performed on-line with the Orsay ion irnplaritorare both discussed. It is shown that simple ballistic ion beam mixing cannot account for the results. The latter indicate that damage-induced mixing at the host matrix-precipitate interface leads to drastic changes in the precipitate composition, presumably due to in-diffusion of oxygen (and possibly Mg) into the precipitate volume.
1. Introduction Ion implantation of ionic crystals causes complex structural changes and chemical effects [l]. The host can incorporate implanted particles in different charge states. The study of the metastable phases produced by a high dose implantation of metallic ions into MgO has been .a subject of interest during the last decade. The main results of these studies are that the nature of the
precipitates in MgO depends on the implanted incident ion species: alkali ions tend to form small metallic precipitates [2-51 while indium or gold ions tend to form binary alloys with magnesium [6,7]. In the case of iron ions, spine1 ferrite particles can be formed [8]. Precipitate formation after implantation and annealing is presumably facilitated by the implantation-induced defects, but chemical interactions are also important since the nature of the precipitate phase depends on the implanted species (see also ref. [9]). We report an attempt to distinguish between the contribution due to simple collisional damage and that due to the more complex interactions of chemical affinity and atomic species mobility. This is performed via optical absorption and in situ TEM experiments on MgO samples in which metallic Na precipitates have previously been formed by implantation and subsequent annealing [2,3] and which are again submitted to Na implantation. Some of our results have already been reported in 0168-583X/85/$03.30 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
prekninary form [lo]. The major r6le of beam induced in-diffusion of host atoms into the solute precipitates is made clear. 2.Optlcal
absorption measurements
2.1. Evolution of MgO crystals implanted with sodium ions
The stable defects created in MgO by ion bombardment lead to optical absorption spectra which exhibit two main absorption bands located at 250 run and 575 nm. These are associated respectively with oxygen (F or F+ centers) and magnesium (V-type centers) vacancies. The optical spectra of as-implanted samples only show the existence of these intrinsic defects; no absorption band due to implanted atoms is observed. After thermal annealing the intrinsic defects (F and V-type centers) progressively disappear: the two absorption bands decrease in amplitude and at 1100 K all the intrinsic point defects are annealed. A concomitant increase in optical absorption is observed in the region of the optical spectra where small metallic particles of pure sodium absorb light when they are embedded in the MgO lattice. Fig. 1 shows the optical absorption spectra of a MgO single crystal implanted with 5 X 1016 sodium ions cm-’ of 150 keV energy. After annealing for 15 min in air at 973 K, partial precipitation of Na atoms is observed via
316
M. Treikx
I
4
3
-Phottn
et al. / Implantation-induced
re-solution
of Na precipitates in MgO
-
energy ($ 5
I
4
Photon energy (eV) I
,
I.5
t DO t
1.5
>, C 2 4, 5 0 ._ z
0 j
0
Wavelength
nm-
Fig. 1. Optical absorption spectra of MgO single crystal implanted with 5 X 1016 sodium ions cmm2 of 150 keV energy: (a) as-implanted (b) annealed for 15 min at 973 K.
the optical absorption band due to sodium precipitates located at 540 nm (fig. 1). The frequency corresponding to absorption of sodium precipitates in MgO is related to the plasma frequency wr, of free electrons in sodium by [ll]: w (Na/MgO)
=
“” (1 + 2#2
’
where E is the dielectric constant of the insulating lattice of MgO (equal to about 3). The mean size of the precipitates can be deduced from the half width of the optical absorption band AE,,, as [ll]: A E,,,
= hu r/R,
where h is the Planck constant, tir the Fermi velocity and R the mean radius of precipitates. In the crystal annealed at 973 K, the mean diameter of sodium aggregates deduced from this formula is about 7.2 nm. This is not inconsistent with the mean diameter deduced from TEM observations. 2.2. Effect of Na + ion implantation into MgO containing sodium precipitates Fig. 2 displays the evolution of the optical spectra of MgO containing sodium precipitates after implantation with 150 keV Na+ ions at fluences of 3 X 1014 and 10” ions cmh2. The amplitude of the absorption band due to sodium precipitates decreases, showing that the fraction of precipitated sodium metal decreases with increasing fluence. This could be interpreted in terms of metallic sodium precipitate dissolution in the MgO lattice under ion beam bombardment. But the position of the Na absorption band shifts towards higher wavelengths and its width decreases which seems to indicate
0 200
500 WaVel8ngth
A “nl_
1000
-
Fig. 2. Optical absorption spectra of MgO crystal implanted with 5 X lOi sodium ions cmm2 of 150 keV energy: (a) annealed for 15 min at 973 K; subsequently implanted with (b) 3X1014 150 keV Na+ ions cme2 and (c) lOI 150 keV Na+ ions cma2. an increase in the mean size of the precipitates (7.8 nm after 3 x 1014 Na+ cmm2 and 8.6 nm after 10” Na+cmW2). Optical absorption observations alone are thus not sufficient to deduce the evolution of the metallic precipitates. In situ TEM observations under implantation provide information on the evolution of sodium precipitates during ion bombardment.
3. Transmission
electron microscopy observatkhs
Previous results [3,12] obtained by TEM on sodium clusters embedded in MgO showed that, though the structures of sodium (bee) and MgO (fee NaCl type) are different, most of the aggregates are in a simple orientation with respect to the matrix ((0Ol)Na (] (0Ol)MgO and [lOO]Na ]] [lOO]MgO) due to the good agreement between their bulk lattice parameters ( aNa = 0.4291 nm and, a,,, = 0.4213 nm). They are partially coherent with the matrix. The contrast of partially coherent precipitates can result from a matrix contrast and a precipitate contrast. From TEM observations of Na aggregates there is no evidence of matrix contrast arising through elastic straining of the lattice, probably because sodium is elastically soft by comparison with MgO. The precipitate contrast can be analyzed in two ways: first, a structure factor contrast when matrix and precipitate are under diffraction conditions and have different structure factors, and, second, an orientation contrast when a certain set of lattice planes in the precipitate is diffracting strongly whereas the matrix is diffracting
M. Treilkux et al. / Implantation - indticed re -solution of Na precipitates
in MgO
371
Fig. 3. MgO single crystal implanted with lOI sodium ions cm- ’ of 750 keV energy, annealed for 40 min at 1073 K, thinned for TEM observations and then implanted with Na2+ Ions accelerated under 60 kV. Dark-field performed in the same crystal area with (110) Na reflection: (a) before implantation. After implantation with (b) 3 X 1Ol3 ions cmW2,(c) 5 X lOI ions cmm2, (d) 5 X 1014 ions cme2
weakly or vice versa. In the case of partially coherent Na precipitates these two kinds of contrast can be observed which, normally, enable us to study the evolution of aggregates. However, bright-field or dark-field images obtained from matrix reflections show an extensive dislocation network [4] which prevents observation of all the Na aggregates. Fortunately dark-field micrographs from precipitate reflections show a strong contrast (orientation contrast) of the aggregates with bright precipitates in a dark matrix. This is the best condition to study the size distribution of precipitates. In the
following experiments dark-field micrographs were performed using (110) Na reflection. In order to follow the size distribution of sodium clusters during Na+ implantation it was necessary to make continuous TEM observations on the same area of the crystal. This was made possible by using the Philips EM 400 electron microscope on-line with the CSNSM ion implantor [13]. The MgO crystal containing sodium precipitates was implanted directly in the microscope chamber with a scanned Na+ ion beam. The conditions of the experiment were the following:
1
_
20
D
nm-
~. 1
20 D nm -
0
Fig. 4. Mg0 sing8e crysti cor~taining sodium precipitates implanted with Na2+ ions accekzited under 6Q kV. Histogmms: concentration C vs diameter D of the visible precipitates observed in dark-field and contained in a small crystal volume (four times the volume imaged in fig. 3). (a) Before implantation. After implantation with (b) 3 X lOI3 ions cm-*, (c} 5 Xl@ ions cm-‘, (d) 1.3 X 1P ions cm-‘, (e) 5 X 1P ions cm- ‘. The point-to-point resoiution of the TEM was better than 10 A.
(1) The MgO single crystal was first implanted with 750 keV sodium ions accelerated by the 2.5 MeV Van de Graaff accelerator of the “Departement de Physique des Matinal, Lyon”. The fhznce was 10” Naf cm-‘. For these ions the mean penetration depth and the full width at half maximum of the depth distribution de duced from the Dearnaley et al. tables [14] were respectively 720 nm and 190 nm. Then the crystal was annealed for 40 min at 1073 K in order to form Na precipitates in the MgO matrix. (2) A thin foii was prepared for TEM observations from a disc (diameter 3 mm) cut in the crystal. By chemical solution in dilute ~hoph~pho~c acid a thicknesS of about 650 nm was removed from the implanted side, then the splits side was chemically thirmed by using a jet polishing technique until a hole was formed in the implanted zone. (3) After mounting the sample in the Orsay on-line TEM, the Na implantation was performed with Na2’ ions accelerated under 60 kV corresponding to 120 keV Na+ ions. The incidence angle of the ion beam was W,
so that the mean penetration depth and the width at half maximum of the depth distribution of the ions deduced from the Dearnaley et al. tables 1141 were, respectively, 82 nm and 52 nm. The thin foil observations were carried out, at room temperature, after each dose by tilting the sample so that the dark-field images obtained with (110) Na reflection gave the best contrast. The point-to-point resolutiQn of the electron microscope is less than 1 nm. All the micrographs were performed on the same crystal vohnne, so that the size distribution of sodium precipitates cx&d be studied in terms of the ffuence. The micrograph analyses were carried out dire&y on the negative dark-field images using an optic& microscope provided with a micrometric ocular. Fig. Ja shows the initial sodium precipitates in a crystal volume corresponding to a quarter of the total analyzed cryztat volume. The corresponding sodium precipitate size distribution is represented in fig. 4a. The precipitate concentration was about lOI precipitates per cm3; their mean size was about 12.5 nm.
hf. Treilleux et al. / Implantation -induced re -solution of Na precipitates
In situ observations were made for implanted Na fluences up to 1.8 X 1015 ions cmP2. The dose rate was about 10” ions cme2 s-l for low doses (3 x 1013 and 5 X 1013 ions cme2); it was increased up to 1012 ions cmF2 s-l for the highest fluences in order to reduce the implantation time. The first result is that during the implantations a bending of the thin foil is observed. This bending increases with increasing fluence, and occurs in the direction of the incident ions: the rotation angle of the goniometric stage was equal to +6” before implantation, -5’ for 4 X 1014 ions cmm2, - 9’ for 8 x 1014 ions cm -i and -14” for 1.8 X 1015 ions cme2. The sample bends under the strain induced by the accumulation of defects (implanted atoms, interstitials and vacancies) and the corresponding volume increase on the entrance side of the crystal. This effect was also observed on thin MgO crystals (15 X 2.5 X 0.2 mm3) implanted with Ar+ and Ne+ [15,16]. Similar damageinduced warping effects are familiar in semiconductors ]171. The dark-field micrographs of figs. 3a-3d, carried out in the same crystal area, show the evolution of the precipitates during the Na implantation. Up to a fluence of 5 X 1014Na+ cme2 the number of visible precipitates in the dark-field images decreased rapidly. Fig. 4 analyzes the precipitate size in the dark-field images. The total concentration of visible precipitates decreases from 1016 cm- 3 (before implantation) to 2 x lOi cme3 (after 5 X 1014 ion cme2). From the histograms of fig. 4 we can deduce the concentration of the precipitated metallic sodium and the mean diameter of aggregates. Fig. 5 shows the fluence dependence of these two quantities. The concentration of the precipitated sodium metal decreases and the mean diameter of the precipitates increases slightly. These results apparently confirm those deduced from the optical studies. The evolution of the visible precipitate concentration in terms of the fluence is shown in fig. 6 for precipitates whose diameter ranges from 6 to 18 nm. These evolutions are faster for small precipitates than for larger ones. Thus, for these metallic precipitates, the probability of disappearing increases when the diameter of aggregates decreases leading to an increase of the mean diameter of visible precipitates. As opposed to the decrease of the absolute number of visible clusters (fig. 4), their average size is rather weakly affected by Na implanfafion (slight increase due to the preferential vanishing of small aggregates) (fig. 5). This result precludes an interpretation in terms of progressive Na precipitate dissolution under the kinematical effect of incoming Na ions since the latter process would lead to a significant reduction in average precipitate size. Moreover, a careful study of the dark-field micrographs revealed the following important effect:
in MgO
379
nm
Fig. 5. Evolution of the concentration of the precipitated metallic sodium CNa (deduced from the histograms of fig. 4) during the bombardment with a dose D of Na2+ ions accelerated under 60 kV. Part (b) gives evolution of the mean diameter of aggregates 5 vs the dose D.
metallic Na precipitates that had not been affected by Na ion implantation at fluences of some 1014 Na cmb2 (i.e. corresponding to about one displacement per atom on average in the implanted layer, or - alternatively to about a hundredfold cascade overlap) could become invisible, after a further implanted fluence of only lOi Na cme2. This result demonstrates the basically discontinuous nature of the process under study (as opposed to a progressive collisional breakup process).
380
M. Treilleux et al. / Implantation-induced
Fig. 6. Evolution of the aggregate concentration C (deduced from the histograms of fig. 4) vs the dose D of Na2+ ions for precipitates having different diameter +: (a) 6 nm < + < 9 nm;(b) 9 run<+<12 nm;(c) 12 nmcg<15 nm;(d) 15 nm<+
Furthermore, though a precipitate becomes invisible in a dark-field micrograph, a careful study of the corre sponding bright-field micrograph reveals that a contrast remains at the place of the precipitate. We conclude that the Na implantation does not lead to precipitate disruption, nor to a change in its size (as seen in fig. 3), but does lead to a modification of its structure and chemical nature.
4. Dicussion and conclusion The area under the optical absorption band at 540 MI is proportional to the quantity of Na in metallic precipitate form. The reduction in band amplitude can be ascribed to either the dispersion (“re-solution”) of Na atoms in the MgO host, presumably due to the progressive breakup of the metallic precipitates under the impact of the incident particles, or to a change in the optical absorption properties of the precipitates (from metallic to insulating). The progressive breakup mechanism is in complete contradiction with our other results. Although the precipitates are no longer all imaged in the dark-field TEM micrographs a contrast remains in the corresponding bright-field micrographs. This, together with the fairly small change in average precipitate size, shows that the main effect of the Na implantation is to induce a structural and/or chemical composition change in the precipitate volume. Struct-
re -solution of Na precipitates
in MgO
ural changes alone in pure Na are insufficient to account for the change in metallic character implied by the optical absorption result, and thus we conclude that the Na implantation produces a compositional change away from metallic Na inside the precipitate. From the TEM results described above, it is clear that this compositional change has a “one-shot” character (see the effect of the 1013 Na cm-’ Implantation), . but that even an incoming Na ion is not efficient in inducing the transition. We infer that collision cascades produced by the incident particles at the host matrix-precipitate interface lead to the incorporation of host (Mg or 0) atoms into the precipitates. This conclusion is reminiscent of “ion-beam mixing” [18] of course, but it contains the ambiguity of that concept. In fact, simple ballistic mixing cannot account for our results (notably the “one-shot” effect and the weak variation of the average precipitate size). Recent work on ion-beam mixing has demonstrated [19,20] that some 60-80% of effect is related to atomic mobility under irradiation and that chemical affinity plays a major role in determining the nature of the ultimate phase thus produced. We suggest that the MgO-Na aggregate system is an extreme case of this mechanism. Because of their small size, the Na precipitates are at very high pressures: diffusion (notably of oxygen) is then considerably enhanced inside the Na metal. Thus the collision cascade at the interface essentially serves to trigger the inward diffusion of the host constituents. The efficiency of this process is somewhat size-dependent because of the diffusion coefficient pressure dependence and also because the surface-to-volume ratio must be maximized in order to enhance the interface interaction. Note that in the present case (as opposed to standard “ion-beam mixing” experiments) there is no need to invoke radiation-enhanced (or induced) diffusion to account for the results, although they are obviously not ruled out. Detailed studies of the precipitate phase modification probability in terms of precipitate size and sample temperature would provide more quantitative information. References [l] A. Perez, M. Treilleux, P. Thevenard, G. Abouchacra, G. Marest, L. Fritsch and J. Serughetti, Metastable Materials Formation by Ion Implantation, eds., S. Picraux and W.J. Choyke (North-Holland, Amsterdam, 1982) p. 159. [2] P. Thevenard, J. Phys. 37 (1976) C7-526. [3] M. Treilleux and G. Chassagne, J. Phys. Lett. 40 (1979) L-161. ]4] M. Treilleux, P. Thevenard, G. Chassagne and L.W. Hobbs, Phys. Stat. Sol. 48 (1978) 425. [5] M. Treilleux and G. Chassagne, J. Phys. Lett. 40 (1979) L-283. [6] A. Perez, M. Treilleux, L. Fritsch and G. Marest, Radiat. Effects 64 (1982) 199.
M. Treilleux et al. / Implantation-induced [7] G. Abouchacra, G. Chassagne and J. Serughetti, Radiat. Effects 64 (1982) 189. [8] A. Perez, M. Treilleux, L. Fritsch, G. Marest, Nucl. Instr. and Meth. 182/183 (1981) 747. [9] M. Guermazi, Thesis, Lyon (1984). [lo] P. Thevenard, M. Treilleux, M. 0. Ruault, J. Chaumont and H. Bemas Nucl. Instr. and Meth. Bl (1984) 235. [ll] W.T. Doyle, Phys. Rev. 111 (1958) 1067. [12] M. Treilleux and J.M. Penisson, 8th European Congress on Electron Microscopy, Budapest (1984). [13] M.O. Ruault, J. Chaumont and H. Bemas, Nucl. Instr. and Meth. 209/210 (1983) 351 and IEEE Trans. Nucl. Sci. NS-30 (1983) 1746.
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[14] G. Deamaley, J.H. Freeman, R.S. Nelson and J.H. Stephen, Ion Implantation (North-Holland, Amsterdam, 1973) p. 766. [15] G.B. Krefft, J. Vat. Sci. TechnoL 14 (1977) 533. 1161 G.B. Krefft, Radiat. Effects 49 (1980) 127. [17] E.P. Eernisse, J. Appl. Phys. 42 (1971) 480. [18] S. Matteson and M.A. Nicolet, Ann. Rev. Mat. Sci. 13 (1983) 339. [19] J. Bottiger, S.K. Nielsen and P.T. Thorsen, Nucl. Instr. and Meth. B7/8 (1985) 707. (201 J.E.E. BagIin, Int. Conf. on Surface Modification of Metals by Ion Beams, Heidelberg (1984).