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Defect creation induced by GeV ions in MgO containing Na precipitates
__ __ i!!B
Nuclear Instruments and Methodsin Physics ResearchB 112(19961 112- 115
As3
NOMB
Beam Interactions with Materials & Atoms
ELSBVIER
Defect creation induced by GeV ions in MgO containing Na precipitates M. Beranger ay*, R. Brenier a, B. Canut a, S.M.M. Ramos a, P. Thevenard a, E. Balanzat b, M. Toulemonde b a Dgpartement de Physique des Matt%aux, fJniuersit& Claude Bernard, Lyon I. 69622 Villeurbanne Cedex, France b Centre Interdisciplinaire de Recherches auec les Ions Gourds, 14040 Cuen Cedex, France
Abstract
MgO single crystals containing Na metallic precipitates were irradiated with swift heavy ions. Chemical etching of the samples allows in-depth analysis by optical absorption and channeling Rutherford backscattering spectroscopies. The numerous defects observed in the MgO matrix after irradiation near the surface are shown to arise from ionization processes.
1. Introduction
Implantation of metallic ions in refractory oxides is a good way to obtain, after thermal treatment, metallic precipitates (in the case of alkali ion implantation [ 1.21) or oxidized phases [3]. This procedure allowed us to realize test-systems, composed of MgO single crystals containing Na nanoprecipitates, which were used to study the interactions of highly energetic ions with matter. It has been known for a few years that irradiation of metals or refractory oxides can lead to the creation of defects by electronic processes, provided the excitation level is high enough ([4,5]). Therefore, it appears interesting to study, for high electronic stopping powers, the possible mixing effects occuring in our test-systems between the metallic precipitates and their insulating matrix. In this paper, we report on the effect of high energy (GeV) heavy ions on MgO single crystals containing small Na precipitates. These irradiations lead to a dissolution of the metallic precipitates in their matrix. The defects created during irradiation in the magnesium oxide were studied and correlated to the electronic energy losses of the bombarding ions.
2. Experimental
procedure
Single crystals of magnesium oxide, of (100) orientation, were implanted at room temperature with Na+ ions
l
Correspondingauthor.
0168-583X/96/$15.00 0 19% Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01244-3
using the 200 keV implantor of the Departement de Physique des Mat&iaux. The samples were then subjected to isochronal annealings in order to induce the precipitation of the metallic sodium. The MgO crystals containing Na nanoprecipitates were further irradiated with the following swift heavy ions at GANIL (Caen): - U 803 MeV (experimental conditions: electronic and nuclear energy losses at the entrance of the sample of 41 keV/nm and 0.1 keV/nm; mean projected range N 28 pm; room temperature irradiation) - or Pb 944 MeV (experimental conditions: electronic and nuclear energy losses at the entrance of the sample 38 keV/nm and 0.07 keV/nm; mean projected range - 32 pm; irradiation temperature 17 K). To study the in-depth repartition of the defects created by swift heavy ion irradiation, the surface layer of the MgO crystals was removed by chemical etching with orthophosphoric acid at room temperature. The contamination layer was removed by ion beam milling, and the same procedure (chemical etching and ion beam milling) was successively applied to the samples up to the end of the range of the heavy ions. At each step of the study (i.e. before, after irradiation and during the chemical etching procedure), the MgO-Na samples were characterized by optical absorption spectroscopy, using a CARY 2300 spectrophotometer at room temperature, and by Channeling Rutherford Backscattering Spectroscopy (C-RBS) using a Van de Graaff accelerator (2 MeV 4Hei beam, and beam current of about 15 nA). The etched depths were measured with an “alpha step” profilometer.
M. Beranger
e? af./hkl.
fnsfr. and Merh. in Phys. Res. B 112 11996) 112-115
113
3. Results and discussion
Fig. 1 (curve a) presents the optical absorption spectrum of a MgO crystal implanted with 2.7 X lOI Na+ cm-’ at an energy of 53 keV. The projected range is 65 nm for a range straggling of 50 nm. The sample was then annealed up to about 700°C in air. The presence of sodium nanoprecipitates is revealed by the broad absorption band located near 500 nm. After irradiation with lOI* U cm - ’ (curve b), this band disappears, meaning that the Na colloids have been dispersed. The band located near 575 nm can be attributed to some defect creation in MgO. When this sample is further annealed up to 8OO”C,we can observe a re-precipitation of the sodium atoms in the irradiated sample (curve c). The corresponding absorption band is very close to the one of a non-irradiated sample subjected to the same annealing treatment (curve d). This indicates that the Na atoms did not diffuse out from the MgO crystal during irradiation. If we remove - 200 nm from the surface of this irradiated sample, no sodium precipitate can be detected anymore. Therefore, we can conclude that no long range inwards diffusion of the Na atoms occurred during irradiation. F-type centers (oxygen vacancies with trapped electrons) were also created during irradiation, as can be seen on Fig. 2, curve a, in another MgO-Na sample irradiated 0.5
MgO crystal irradiated with 1Ol2 Pb.cm-2
a: as irradiated b: removed depth c: removed depth d: removed depth
0.9 urn 2.7 urn 6.4 urn
i
I-
!O(: 1
300
400
Wavelength Fig. 2. Optical absorption IO” Pbcm-‘.
500
(nm)
spectra of a MgO crystal irradiated
with
--I
with lOI* Pb ionscmm2 (optical absorption band at 250 nm). When we etch the surface of the sample, we observe that the amplitude of the F-type center band decreases progressively up to the end of the Pb range (curves b to d). In order to measure easily the maximum height of this band, the absorption spectrum of a non-irradiated MgO-Na sample was used as a reference. The evolution of the optical density at the maximum value of the F-type band with the removed depth is displayed on Fig. 3. From the optical absorption measurements, we can estimate the number of oxygen vacancies created by irradiation. The optical density OD of the F-type center band is related to the total number of defects NF created per cm2 of irradiated surface by using Smakula’s formula [6,7]:
0.4 C
3 0 3 2 :
4
d
-z 2 a 0.2
0
0.87 X 10” N,= 0.1
0 i
200
400 Wavelength
600 (nm)
Fig. 1. Optical absorption spectra of a MgO crystal containing Na nanoprccipitates, a: sample before U irradiation, b: sample irradiated with IO’* Ucm-*, c: same as a, but after annealings up to 800°C d: same as b, but after annealings up to 8OO’C.
f
n
(n2+2)
2 Y,,V3OD).
where n is the refractive index of the crystal, W,,2 the band half-width in eV and f the oscillator strength. NF corresponds to the integrand of the local defect concentration over the whole range R, of the bombarding ions. The contribution of nuclear processes N,,, to F-type center creation should be related to the integrand of nuclear energy losses over R, by:
V. INSULATING
MATERIALS
M. &ranger
et al./Nucl.
In&. and Merh. in Phys. Res. B I12 119961112-115 50
40
G ; 30 ‘2
600 1000 Energy (keV)
ii? ._ 7
1200
2 cu 20 G : n
10
0
10
Etching
20
depth
0
30
0
5
Fig. 3. Evolution of the optical density at the maximum value of the F-type center band with removed depth, for a MgO crystal irradiated with lOI Pbcmm2.
The contribution of electronic processes iVet, if it does exist, is an unknown function of the electronic energy losses. In a first approach, the following relation may be proposed:
15
10
Etching
(microns)
depth
25
20
(microns)
Fig. 4. 1) RBS spectra of a MgO-Na crystal irradiated with 6X 1Ol2 UcmmZ after removing a depth of 0.41 km, a: random spectrum, b: aligned spectrum. irradiated sample: aligned spectrum, non-irradiated sample. 2) Evolution of the dechanneling yield in a MgO-Na crystal irradiated with 6X lOi Ucme2, a: irradiated sample, b: non-irradiated sample.
The damaged fraction F, of an irradiated crystal can be deduced from its C-RBS spectra by [8]: F = XIR-XNI D
Fitting the optical density spectrum displayed in Fig. 3 by the above mentioned functions yielded n = 4 rf: 1, A keV-’ and B = 7.22 X lo-l6 1.14 X lo-‘* cm-2 cm/keV4. From this relation, we estimate that the nuclear contribution corresponds to only 20% of the total defect
creation. An important part of the F-centers is then induced by electronic processes. The defect concentration due to ionization processes is found to vary as (dE/dx)zr, so it depends strongly on the value of the electronic energy losses. To get informations about the extended defects which may form during irradiation, C-RBS analysis were performed. The RBS spectrum of a MgO-Na crystal irradiated with 6 x lo’* Ucm-* and chemically etched is displayed in Fig. 4.1. The dechanneling yields xN, and x,R of the non-irradiated and irradiated samples were measured behind the damage surface peak. No significant difference between these yields could be measured after a removed depth of about 20 km (Fig. 4.2).
1-XN,
-
This damaged fraction has been plotted on Fig. 5 for this last sample versus the electronic energy losses of the
““n 15 -
. - 10 fi? . G
5-
ot 0
1 ’
..i’llyi
I I
’
I * t
’
10 Electronic
Ao
20 energy
I I
’ * n’
’
’ 8
30 40 loss (keV/nm)
01
Fig. 5. Evolution of the damaged fraction F, of a MgO-Na crystal irradiated stopping power.
with
6X 10”
Ucm-*
versus
the electronic
M. Beranger et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 112-115
incident ions. A threshold of - 20 keV/nm is observed for damage creation in this crystal irradiated with 6 X iO12 U cm -‘; however, more experiments would be required to determine if this threshold is characteristic of the MgO crystal or if it only reflects the sensitivity of the C-RBS analysis. The total damage cross section can be deduced from the C-RBS analysis and is found to be a, = 1.6 X lo- I4 cm- ’ for (dE/dx),, = 38 keV/nm.
4.
Conclusion
In this work, we have shown that the dissolution of Na precipitates in an MgO matrix, under irradiation with swift heavy ions, was not accompanied by any diffusion of the sodium atoms. The defects observed in the oxide were found to arise mostly from electronic processes. The MgO F-type center concentration was found very sensitive on the electronic energy losses and to vary as (d E/dx)zI Numerous defects were also observed by C-RBS above a threshold in the electronic energy losses of - 20 keV/nm.
115
References [l] S. Deying, Y. Saito and S. Suganomata,Jpn. J. Appl. Phys. 33 (1994) L966. [2] P. Thevenard, M. Treilleux, M.O. Ruault. J. Chaumont and H. Bemas, Nucl. Instr. and Meth. B 1 (1984) 235. [3] L.A. Gea, L.A. Boatner, J. Rankin and J.D. Budai, Beam-solid interaction for materials synthesis and characterization, M.R.S. Symp. Proc. 354 (1995). [4] M. Toulemonde and F. Studer, Solid State Phenomena 30/3 1 (1993) 477. [5] D. Lesueur and A. Dunlop, Radiat. Eff. Dcf. Solids 126 (1993) 163. [6] E. Sonder and W.A. Sibley, in: Point Defects in Solids, Vol. 1, eds. J.H. Crawford and L.M. Slitkin (Plenum, New York, London, 1972) p. 269. [7] B.D. Evans, J. Comas and P.R. Malmberg, Phys. Rev. B ti6) (1972) 2453. [8] B. Canut, S.M.M. Ramos, P. Thevenard. N. Moncoffre, A. Benyagoub, G. Marest, A. Meftah, M. Toulemonde and F. Studer, Nucl. Instr. and Meth. B 80/81 (1993) 1114.
V. INSULATING MATERIALS
Nuclear Instruments and Methodsin Physics ResearchB 112(19961 112- 115
As3
NOMB
Beam Interactions with Materials & Atoms
ELSBVIER
Defect creation induced by GeV ions in MgO containing Na precipitates M. Beranger ay*, R. Brenier a, B. Canut a, S.M.M. Ramos a, P. Thevenard a, E. Balanzat b, M. Toulemonde b a Dgpartement de Physique des Matt%aux, fJniuersit& Claude Bernard, Lyon I. 69622 Villeurbanne Cedex, France b Centre Interdisciplinaire de Recherches auec les Ions Gourds, 14040 Cuen Cedex, France
Abstract
MgO single crystals containing Na metallic precipitates were irradiated with swift heavy ions. Chemical etching of the samples allows in-depth analysis by optical absorption and channeling Rutherford backscattering spectroscopies. The numerous defects observed in the MgO matrix after irradiation near the surface are shown to arise from ionization processes.
1. Introduction
Implantation of metallic ions in refractory oxides is a good way to obtain, after thermal treatment, metallic precipitates (in the case of alkali ion implantation [ 1.21) or oxidized phases [3]. This procedure allowed us to realize test-systems, composed of MgO single crystals containing Na nanoprecipitates, which were used to study the interactions of highly energetic ions with matter. It has been known for a few years that irradiation of metals or refractory oxides can lead to the creation of defects by electronic processes, provided the excitation level is high enough ([4,5]). Therefore, it appears interesting to study, for high electronic stopping powers, the possible mixing effects occuring in our test-systems between the metallic precipitates and their insulating matrix. In this paper, we report on the effect of high energy (GeV) heavy ions on MgO single crystals containing small Na precipitates. These irradiations lead to a dissolution of the metallic precipitates in their matrix. The defects created during irradiation in the magnesium oxide were studied and correlated to the electronic energy losses of the bombarding ions.
2. Experimental
procedure
Single crystals of magnesium oxide, of (100) orientation, were implanted at room temperature with Na+ ions
l
Correspondingauthor.
0168-583X/96/$15.00 0 19% Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01244-3
using the 200 keV implantor of the Departement de Physique des Mat&iaux. The samples were then subjected to isochronal annealings in order to induce the precipitation of the metallic sodium. The MgO crystals containing Na nanoprecipitates were further irradiated with the following swift heavy ions at GANIL (Caen): - U 803 MeV (experimental conditions: electronic and nuclear energy losses at the entrance of the sample of 41 keV/nm and 0.1 keV/nm; mean projected range N 28 pm; room temperature irradiation) - or Pb 944 MeV (experimental conditions: electronic and nuclear energy losses at the entrance of the sample 38 keV/nm and 0.07 keV/nm; mean projected range - 32 pm; irradiation temperature 17 K). To study the in-depth repartition of the defects created by swift heavy ion irradiation, the surface layer of the MgO crystals was removed by chemical etching with orthophosphoric acid at room temperature. The contamination layer was removed by ion beam milling, and the same procedure (chemical etching and ion beam milling) was successively applied to the samples up to the end of the range of the heavy ions. At each step of the study (i.e. before, after irradiation and during the chemical etching procedure), the MgO-Na samples were characterized by optical absorption spectroscopy, using a CARY 2300 spectrophotometer at room temperature, and by Channeling Rutherford Backscattering Spectroscopy (C-RBS) using a Van de Graaff accelerator (2 MeV 4Hei beam, and beam current of about 15 nA). The etched depths were measured with an “alpha step” profilometer.
M. Beranger
e? af./hkl.
fnsfr. and Merh. in Phys. Res. B 112 11996) 112-115
113
3. Results and discussion
Fig. 1 (curve a) presents the optical absorption spectrum of a MgO crystal implanted with 2.7 X lOI Na+ cm-’ at an energy of 53 keV. The projected range is 65 nm for a range straggling of 50 nm. The sample was then annealed up to about 700°C in air. The presence of sodium nanoprecipitates is revealed by the broad absorption band located near 500 nm. After irradiation with lOI* U cm - ’ (curve b), this band disappears, meaning that the Na colloids have been dispersed. The band located near 575 nm can be attributed to some defect creation in MgO. When this sample is further annealed up to 8OO”C,we can observe a re-precipitation of the sodium atoms in the irradiated sample (curve c). The corresponding absorption band is very close to the one of a non-irradiated sample subjected to the same annealing treatment (curve d). This indicates that the Na atoms did not diffuse out from the MgO crystal during irradiation. If we remove - 200 nm from the surface of this irradiated sample, no sodium precipitate can be detected anymore. Therefore, we can conclude that no long range inwards diffusion of the Na atoms occurred during irradiation. F-type centers (oxygen vacancies with trapped electrons) were also created during irradiation, as can be seen on Fig. 2, curve a, in another MgO-Na sample irradiated 0.5
MgO crystal irradiated with 1Ol2 Pb.cm-2
a: as irradiated b: removed depth c: removed depth d: removed depth
0.9 urn 2.7 urn 6.4 urn
i
I-
!O(: 1
300
400
Wavelength Fig. 2. Optical absorption IO” Pbcm-‘.
500
(nm)
spectra of a MgO crystal irradiated
with
--I
with lOI* Pb ionscmm2 (optical absorption band at 250 nm). When we etch the surface of the sample, we observe that the amplitude of the F-type center band decreases progressively up to the end of the Pb range (curves b to d). In order to measure easily the maximum height of this band, the absorption spectrum of a non-irradiated MgO-Na sample was used as a reference. The evolution of the optical density at the maximum value of the F-type band with the removed depth is displayed on Fig. 3. From the optical absorption measurements, we can estimate the number of oxygen vacancies created by irradiation. The optical density OD of the F-type center band is related to the total number of defects NF created per cm2 of irradiated surface by using Smakula’s formula [6,7]:
0.4 C
3 0 3 2 :
4
d
-z 2 a 0.2
0
0.87 X 10” N,= 0.1
0 i
200
400 Wavelength
600 (nm)
Fig. 1. Optical absorption spectra of a MgO crystal containing Na nanoprccipitates, a: sample before U irradiation, b: sample irradiated with IO’* Ucm-*, c: same as a, but after annealings up to 800°C d: same as b, but after annealings up to 8OO’C.
f
n
(n2+2)
2 Y,,V3OD).
where n is the refractive index of the crystal, W,,2 the band half-width in eV and f the oscillator strength. NF corresponds to the integrand of the local defect concentration over the whole range R, of the bombarding ions. The contribution of nuclear processes N,,, to F-type center creation should be related to the integrand of nuclear energy losses over R, by:
V. INSULATING
MATERIALS
M. &ranger
et al./Nucl.
In&. and Merh. in Phys. Res. B I12 119961112-115 50
40
G ; 30 ‘2
600 1000 Energy (keV)
ii? ._ 7
1200
2 cu 20 G : n
10
0
10
Etching
20
depth
0
30
0
5
Fig. 3. Evolution of the optical density at the maximum value of the F-type center band with removed depth, for a MgO crystal irradiated with lOI Pbcmm2.
The contribution of electronic processes iVet, if it does exist, is an unknown function of the electronic energy losses. In a first approach, the following relation may be proposed:
15
10
Etching
(microns)
depth
25
20
(microns)
Fig. 4. 1) RBS spectra of a MgO-Na crystal irradiated with 6X 1Ol2 UcmmZ after removing a depth of 0.41 km, a: random spectrum, b: aligned spectrum. irradiated sample: aligned spectrum, non-irradiated sample. 2) Evolution of the dechanneling yield in a MgO-Na crystal irradiated with 6X lOi Ucme2, a: irradiated sample, b: non-irradiated sample.
The damaged fraction F, of an irradiated crystal can be deduced from its C-RBS spectra by [8]: F = XIR-XNI D
Fitting the optical density spectrum displayed in Fig. 3 by the above mentioned functions yielded n = 4 rf: 1, A keV-’ and B = 7.22 X lo-l6 1.14 X lo-‘* cm-2 cm/keV4. From this relation, we estimate that the nuclear contribution corresponds to only 20% of the total defect
creation. An important part of the F-centers is then induced by electronic processes. The defect concentration due to ionization processes is found to vary as (dE/dx)zr, so it depends strongly on the value of the electronic energy losses. To get informations about the extended defects which may form during irradiation, C-RBS analysis were performed. The RBS spectrum of a MgO-Na crystal irradiated with 6 x lo’* Ucm-* and chemically etched is displayed in Fig. 4.1. The dechanneling yields xN, and x,R of the non-irradiated and irradiated samples were measured behind the damage surface peak. No significant difference between these yields could be measured after a removed depth of about 20 km (Fig. 4.2).
1-XN,
-
This damaged fraction has been plotted on Fig. 5 for this last sample versus the electronic energy losses of the
““n 15 -
. - 10 fi? . G
5-
ot 0
1 ’
..i’llyi
I I
’
I * t
’
10 Electronic
Ao
20 energy
I I
’ * n’
’
’ 8
30 40 loss (keV/nm)
01
Fig. 5. Evolution of the damaged fraction F, of a MgO-Na crystal irradiated stopping power.
with
6X 10”
Ucm-*
versus
the electronic
M. Beranger et al./Nucl.
Instr. and Meth. in Phys. Res. B 112 (1996) 112-115
incident ions. A threshold of - 20 keV/nm is observed for damage creation in this crystal irradiated with 6 X iO12 U cm -‘; however, more experiments would be required to determine if this threshold is characteristic of the MgO crystal or if it only reflects the sensitivity of the C-RBS analysis. The total damage cross section can be deduced from the C-RBS analysis and is found to be a, = 1.6 X lo- I4 cm- ’ for (dE/dx),, = 38 keV/nm.
4.
Conclusion
In this work, we have shown that the dissolution of Na precipitates in an MgO matrix, under irradiation with swift heavy ions, was not accompanied by any diffusion of the sodium atoms. The defects observed in the oxide were found to arise mostly from electronic processes. The MgO F-type center concentration was found very sensitive on the electronic energy losses and to vary as (d E/dx)zI Numerous defects were also observed by C-RBS above a threshold in the electronic energy losses of - 20 keV/nm.
115
References [l] S. Deying, Y. Saito and S. Suganomata,Jpn. J. Appl. Phys. 33 (1994) L966. [2] P. Thevenard, M. Treilleux, M.O. Ruault. J. Chaumont and H. Bemas, Nucl. Instr. and Meth. B 1 (1984) 235. [3] L.A. Gea, L.A. Boatner, J. Rankin and J.D. Budai, Beam-solid interaction for materials synthesis and characterization, M.R.S. Symp. Proc. 354 (1995). [4] M. Toulemonde and F. Studer, Solid State Phenomena 30/3 1 (1993) 477. [5] D. Lesueur and A. Dunlop, Radiat. Eff. Dcf. Solids 126 (1993) 163. [6] E. Sonder and W.A. Sibley, in: Point Defects in Solids, Vol. 1, eds. J.H. Crawford and L.M. Slitkin (Plenum, New York, London, 1972) p. 269. [7] B.D. Evans, J. Comas and P.R. Malmberg, Phys. Rev. B ti6) (1972) 2453. [8] B. Canut, S.M.M. Ramos, P. Thevenard. N. Moncoffre, A. Benyagoub, G. Marest, A. Meftah, M. Toulemonde and F. Studer, Nucl. Instr. and Meth. B 80/81 (1993) 1114.
V. INSULATING MATERIALS