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Ion implantation effects in Na β″-alumina crystals
NuclearInstruments and Methods in Physics Research B68 (1992) 422-425 North-Holland
Beam Interactions with Materials&Atoms
Ion implantation effects in Na ß"-alumina crystals F .L. Freiire Jr .
Departamento de Fisica, Pontift'cia Universidade Catôlica do Rio de Janeiro, Cx. Postal 38071, 22453 Rio de Janeiro, RJ, Brazil
Noble-gas ions (Ne, Ar, Kr and Xe) were implanted in single crystals of sodium ß"-alumina with the aim of better understanding the mechanisms of sodium migration within this material under heavy-ion irradiation . The near-surface modifications were determined by using Rutherford backscattering spectrometry and nuclear reaction analysis. The sodium depleted layers were independent of the ion flux and greater than the ion range . The enhanced diffusion of alkali ions is associated with ion damage and is thus related to ion mass. Composition changes generate modifications of the surface dissolving processes . 1. Introduction The ß"-alumina are aluminium oxides characterized by a remarkable value of the room temperature ionic conductivity and by a quite small ionic activation energy . The mobile cations are confined in low-density planar regions (conduction planes) extending perpendicular to the crystallographic c-axis in between two adjacent hexagonal close-packed layers of alumina (spinel block). Due to their chemical and mechanical stabilities, sodium ß"-alumina ceramics have been used for a long time as solid electrolyte in thermoelectric generator devices[1]. Recent investigations have shown that ß"-alumina crystals have a rich ion-exchange chemistry. The rapid ionic transport phenomenon enables sodium ions to be replaced by a number of monovalent, divalent or trivalent ions, in particular the lanthanide elements, producing materials with potential use as laser [2] or optoelectronic materials [3] and in gas sensors [4] . The ion-exchange mechanism and the operation of sodium ß"-alumina electrolytes are critically dependent on the surface properties . So, a better characterization of the factors affecting these properties could be useful for the improvement of these materials. Ion implantation is a technique to modify surface properties widely used in semiconductor and metal surface engineering. Nevertheless, its application to introduce surface modifications in insulating materials is far from being routine. One of the reasons for the difficulty of the application of this technique to insulators is the lack of knowledge, on a microscopic scale, of the physical processes occurring in dielectric materials during ion bombardment. Among the different effects induced by the ion irradiation the enhancement of the ionic mobility is one of the most important features [5]. Thus, ionic conductors, as the ß"-alumina, are natural candidates to investigate this point. In a previous work 0168-583X/92/$05 .00 0 1992 -
Elsevier Science Publishers
B.V.
[6], we showed that the modifications of sodium profiles in ß"-alumina irradiated by argon ions can be described in terms of three main processes : radiation enhanced transport, electric field assisted migration and preferential sputtering of the sodium ions. This last mechanism is strongly dependent on the nuclear stopping power, contrary to some other dielectric materials [7]. The aim of the present study is to obtain more information on the processes which govern the surface modification of sodium ß"-alumina crystals induced by heavy-ion irradiation. As composition changes generate modifications of the surface properties, we also investigated the modifications of the surface dissolving processes . 2. Experimental procedure Single crystals of sodium ß"-alumina were obtained by melting Na 2CO 3, MgO and A1 2 03 with standard procedures [8]. Their composition was Na l.67Mg0 .67 X1033017 and typical dimensions were 10 X 10 x 0.2 mm'. All crystals looked quite good without any macroscopic crackings or extended cleavages . Before the implantations they were annealed at 200°C for 24 h to ensure a nearly complete dehydration and were stored in silica gel to prevent surface contaminations . Ion implantation was performed at room temperature. The beam direction made an angle of 7° with the normal surface direction of the sample, which means that the incidence direction was nearly parallel to the crystallographic c-axis. Some irradiated samples were leached in deionized water (pH ranging from 4 to 5) at 100°C, in static conditions with a surface-to-volume ratio of - 0.02 cm -1. Leaching time was 2 h. All rights reserved
F.L . Freire Jr. / Ion implantation effects in No ß"-alumma crystals
The sodium depth profiles were determined using the resonant nuclear reaction "Na(p, a)2° Ne (resonance energy= 591.6 keV) . The proton beams were obtained from the 4 MV Van de Graaff accelerator at the Physics Department of PUC-Rio. Alpha particles were detected using a 450 mm' silicon surface barrier detector (solid angle = 0.11 sr) positioned at 150°. An aluminized mylar foil, 6 Rm thick, was placed in front of the detector to absorb the higher yield of backscattering protons. This technique has the advantage of requiring a low proton flux [9]. Typically, beam currents of the order of 20 nA were used (1= 1 .0 WA/cm2) . No detectable changes in the sodium profiles were induced even when higher proton fluxes were used . The in-depth sodium concentration profile was determined by increasing the proton energy in steps of 2 keV, which corresponds to a depth resolution of about 20 nm . The maximum depth at which sodium could be measured was about 300 nm. Relative errors were of the order of 3%. Rutherford backscattering spectrometry using 2 MeV helium beams was employed to determine the implanted-ions distribution. 3. Results The effects of 100 keV Ar' irradiation as a function of the dose are summarized in fig . 1. We observe a sodium depletion which increases with the irradiation dose. The thickness of the depleted layers was the same for all doses. Steady-state profiles were obtained DEPTH(nm) 100
z O â Z 05
t
W U Z
m z
00
590
300
. UNIRRADIATED 0 4" 10"Ar'/em' 0 8 " 10'~Aricm' A 10"10'~Ai/cm' dF 12 " 10' ° Ar/em' I .1pAlcm'
Rp
O
200
600 610 ENERGY(keV)
620
Fig. 1. Experimental sodium profiles after 1000 keV Ar' irradiation at different doses . The current density was 1 RA/cm2. The dashed and the dashed-dotted lines are the theoretical simulation calculated in ref. [6] for the 4X1016 and 8x 10 16 argon/cm 2 doses.
0
DEPTH(nm) 100 200
423 300
z O
z
O
600 610 ENERGY(keV)
620
Fig. 2 . Experimental sodium profiles after irradiation by 50 and 300 keV Xe' (0 = 2 x 1016 Xe'/cm 2, 1=1 WA/cm2) . The lines connecting the experimental points are only a guide to the eye. for implantation doses higher than 8 x 10 16 ions/cm2. This behaviour can be attributed to the dynamic recovery of irradiation-produced defects which prevent the complete amorphization of the layer just below the surface, as observed for the manganese-implanted ß"alumina [10] . Then, some kind of crystalline order remains and only part of the sodium ions can diffuse. Argon-implanted sapphire also presented a damage layer near the surface in which the crystalline structure remains [11]. On the other hand, a dose greater than 10 16 Ar'/cm 2 is necessary for the observation of modifications of the sodium profile using the (p, a) reaction. In fig. 2 we show the sodium depth profile modifications as a function of the implantation energy for a fixed dose (2 x 10 16 ions/cm2) of xenon ions. The sodium depleted regions present a thickness which increases with the implantation energy. The results obtained for xenon ions confirm the general behaviour first observed in the case of incident argon ions [6,7]. It is important to note that for the sample implanted with 300 keV xenon ions, the thickness of the sodium depleted layer is the same as the one determined in the case of 100 keV argon implantation (see fig. 1). In both cases the projected range, Rp, is the same . In order to compare the radiation effects induced in ß"-alumina crystals by different heavy-ion irradiations, the implantation energies were chosen in order to obtain the same projected range for incident ions with different masses [12] . The doses were nearly the same and the ion current was 1 wA/cm2. The results are shown in fig. 3. It is evident that an increase of the sodium depletion occurs for the heaviest projectiles. From the results presented in fig. 2 (300 keV Xe) and VIII . ION BEAM MODIFICATION
424
F.L . Freire Jr. /Ion implantation effects in Na ß"-alumina crystals
fig. 3 (100 keV Ar and 180 keV Kr), we can conclude that for a fixed RP the thickness of the depleted layer is independent of the ion species, exceeding the RP + ARP depth (projected range plus straggling). The sample implanted with 50 keV Ne ions presents only a slight sodium depletion for depths greater than 100 nm. This result is compatible with the trend observed for heavier ions. The modifications induced by heavy-ion irradiations in the sodium profiles coulJ be described with a simple model, presented in ref. [6] . As the sputtering cross section of the alkali element is related to the nuclear stopping power [7], the increase of sodium depletion shown in fig. 3 can be understood as the combination of the increase of sputtering cross sections of sodium and the enhancement of sodium diffusion, which increases with the damage produced by the heaviest incident ions. The surface sodium distribution of the implanted samples can be modified by leaching them at 100°C, during 2 h. In fig. 4 we show the sodium profiles of an argon-ion implanted crystal (100 keV, 4 x 10'6 Ar +/cm2) before and after leaching. Whereas, for the unimplanted crystal leaching has almost no effect on the sodium profile (not shown in the figure), the implanted one is markedly affected by that treatment . In fact, the broad sodium peak centered at the argon R P position was removed . RBS analysis showed that the implanted-ions distribution is not affected by the leaching process. These results indicate that sodium ions segregate in the region where the maximum energy deposition occurs. These sodium ions are much less
100
DEPTH,nm) 200
300
DEPTH(nm) O
100
590
600
200
300
N z
O_
F Q F z W
V z
05
O t) z
0.0
610 ENERGY(keV)
620
Fig. 4. Experimental sodium profiles of argon-implanted ß" alumina crystal before and after leaching in deionised water for2 h at 100°C. bounded than in the underlying undamaged crystal, and can be easily removed during the leaching process. In fact, surface composition changes induced by ion implantation generate modifications of the surface dissolving processes. RBS analyses of the sodium ß"alumina crystal implanted with 50 keV Xe + ions (2 x 10'6 ions/cm=) and leached 2 h at 100°C show the complete absence of the implanted xenon ions. This result could indicate that a layer almost 20 nm thick (R P +AR P of 50 keV Xe +) was removed during the leaching process, contrary to what happened with the 100 keV Ar+ implanted crystal. In the case of low-energy xenon implantation the energy deposited at the near-surface layers of the ß"-alumina was three times greater than that for the argon implantation. So, we can attribute these results to an enhancement of the etching rate for the ion-damaged surfaces of sodium ß"-alumina crystal, which increases with the deposited energy. 4. Discussion and conclusions
" UNIRRADIATED o 50keV-4-ldG Nélcm'
p 100keV-4 " f0 " Ar'/cm' 180 keV-3 .10'° Kr'/ cm' I,IpA/cm'
Fig. 3. Experimental sodium profiles after heavy-ion irradiatiop at different energies selected to give the same theoretical projected range . The ion current was 1 WA/cm2. The lines connecting the experimental points are only a guide to the eye except the dashed-dotted line, which is the theoretical simuLuion forargonions calculated in ref. [6] .
Heavy-ion irradiation of sodium ß"-alumina crystals causes a near-surface sodium depletion . The general trend of the experimental results as a function of implantation parameters is quite similar to that observed in implanted glasses [5,13], with one marked difference: for the same projected range, the thickness of the sodium depleted layer in implanted ß"-alumina does not depend on the ion fluxes or masses, as verified for implanted glasses. For both materials the thickness always exceeds RP + ARP depths values . Some hypotheses have been proposed for explaining this aspect: (i) the defects produced locally are mobile
F.L. Freire Jr. / Ion implantation effects in Na p"-alumina crystals and propagate through the implanted material, (ü) light secondary knock-on atoms, and (iii) relaxation effects produced by secondary electrons (including Auger electrons) . In glasses, this fact has been attributed to possible formation and migration of defects and interstitials [13]. In the case of implanted sodium ß"-alumina, its atomic structure (compact spinel blocks intercalated by the so-called conduction planes) may inhibit this mechanism . In fact, the conduction planes may act as a trap for the vacancies, and the interstitial motion from the damaged regions towards the interior is no longer allowed because of the dense structural arrangement of the spinel blocks . The second hypothesis can be excluded: simulations using the TRIM code [12] show that light secondary knock-on atoms will produce a damaged layer with a thickness dependent on the primary-ion mass. On the other hand, recent results [14] on electron irradiation of sodium ß"alumina indicate that sodium ions transport through the spinel blocks can occur ; however, the electron fluxes and energies required are too high to be obtained with secondary electrons produced during the incident ion stopping process. An alternative explanation based on ion-induced stress fields and defect diffusion, would thus appear more likely; such stress fields would be due to the difference in specific gravity and mechanical properties of the implanted layer and the bulk of the material or due to the formation of gas-filled bubbles [15,16] . These stress fields could partially destroy the layered structure of the sodium ß"alumina, permitting the diffusion of sodium ions (and defects) through the spinel blocks . Below a certain depth the stress vanishes and the remaining conduction planes can inhibit the migration of vacancies and interstitials. Then, sodium ions are restricted in their motion and cannot migrate towards the interior of the crystal . In conclusion, in order to explain the ion implantation induced effects at depths exceeding (RP +AR P ), we may invoke the formation of stress fields which could be responsible by the formation of deformed lattice structure permitting the sodium enhanced transport parallel to the c-axis . The surface composition changes induced by ion implantation were correlated with modifications of the surface dissolving processes. Work is in progress to clarify the correspondence between modification of the surface dissolving processes and the implantation parameters .
42 5
Acknowledgements
The author would like to thank prof. G. Mariotto who provided him with the sodium ß"-alumina crystals. He is also indebted to prof. S .R. Teixeira for the ion implantation . This work was supported in part by the Secretaria de Ciência e Tecnologia/P .R. and CNPq.
References
[1] M. Sayer, M.F. Bell, B .A . Judd, S. Shervit, K. El Assad and B . Kindl, J . Appl. Phys. 67 (1990) 832 . [2] M. Jansen, A. Alfrey, O.M. Stafsud, B . Dunn, D.L. Yand and G.C . Farrington, Opt. Lett. 10 (1984) 119 . [3] S.C . Adams, B. Dunn and O .M. Stafsud, Opt. Lett. 13 (1988) 1072. [4] J. Lin and W. Weppner, Appl . Phys. A52 (1991) 94. [5] P. Mazzoldi and A. Miotello, Mater. Sci. Eng. A115 (1989) 1, and references therein . [61 G. Mariotto, A . Miotello, G. Delta Mea and F .L . Freire Jr., Nucl. Instr. and Meth . B46 (1990) 107. [7] F.L. Freire Jr ., G . Mariotto and A. Miotello, Radiat . Eff. Def. Sol . (in press) . [8] J.L. Briant and G.C. Farrington, in : Fast Ion Transport in Solids, Electrodes and Electrolytes, eds. P. Vashista, J.N. Munday and G .K. Shenoy (North-Holland, Amsterdam, 1979) p . 395. [9] A . Camera, G . Delta Mea, A.V. Drigo, S. Lo Russo and P . Mazzoldi, J. Non-Cryst. Solids 23 (1977) 123 . [10] W .L . Roth, R.E. Benenson, C. Ji and L. Wielunski, Solid State Ionics 9/10 (1983) 1459. [11] C.W. White, C.J. Mc Hargue, P .S. Sklad, L .A. Boatner and G.C. Farlow, Mater. Sci . Rep . 4 (1989) 41, and references therein. [12] J.P . Biersack, U. Littmark and J. Ziegler, The Stopping and Range of Solids (Pergamon, New York, 1985). [13] G. Battaglin, G. Della Mea, G. de Marchi, P . Mazzoldi, A . Miotello, A . Boscolo Boscoletto and B. Tiveron, Nucl. Instr. and Meth . B19/20 (1987) 948. [14] C .A. Achete, F.L . Freire Jr . and G . Mariotto, J. Phys. D25 (1991) 1009. [15] L. Romana, G. Fuchs, G . Massouras and P. Thevenard, Radiat . Eff. Def. Sol . 115 (1990) 139. [16] M. Grant Norton, E.L. Fleischer, W. Herd, C. Barry Carter, J.W. Mayer and E. Johnson, Phys . Rev . B43 (199i) 9291 .
VIII. ION BEAM MODIFICATION
Beam Interactions with Materials&Atoms
Ion implantation effects in Na ß"-alumina crystals F .L. Freiire Jr .
Departamento de Fisica, Pontift'cia Universidade Catôlica do Rio de Janeiro, Cx. Postal 38071, 22453 Rio de Janeiro, RJ, Brazil
Noble-gas ions (Ne, Ar, Kr and Xe) were implanted in single crystals of sodium ß"-alumina with the aim of better understanding the mechanisms of sodium migration within this material under heavy-ion irradiation . The near-surface modifications were determined by using Rutherford backscattering spectrometry and nuclear reaction analysis. The sodium depleted layers were independent of the ion flux and greater than the ion range . The enhanced diffusion of alkali ions is associated with ion damage and is thus related to ion mass. Composition changes generate modifications of the surface dissolving processes . 1. Introduction The ß"-alumina are aluminium oxides characterized by a remarkable value of the room temperature ionic conductivity and by a quite small ionic activation energy . The mobile cations are confined in low-density planar regions (conduction planes) extending perpendicular to the crystallographic c-axis in between two adjacent hexagonal close-packed layers of alumina (spinel block). Due to their chemical and mechanical stabilities, sodium ß"-alumina ceramics have been used for a long time as solid electrolyte in thermoelectric generator devices[1]. Recent investigations have shown that ß"-alumina crystals have a rich ion-exchange chemistry. The rapid ionic transport phenomenon enables sodium ions to be replaced by a number of monovalent, divalent or trivalent ions, in particular the lanthanide elements, producing materials with potential use as laser [2] or optoelectronic materials [3] and in gas sensors [4] . The ion-exchange mechanism and the operation of sodium ß"-alumina electrolytes are critically dependent on the surface properties . So, a better characterization of the factors affecting these properties could be useful for the improvement of these materials. Ion implantation is a technique to modify surface properties widely used in semiconductor and metal surface engineering. Nevertheless, its application to introduce surface modifications in insulating materials is far from being routine. One of the reasons for the difficulty of the application of this technique to insulators is the lack of knowledge, on a microscopic scale, of the physical processes occurring in dielectric materials during ion bombardment. Among the different effects induced by the ion irradiation the enhancement of the ionic mobility is one of the most important features [5]. Thus, ionic conductors, as the ß"-alumina, are natural candidates to investigate this point. In a previous work 0168-583X/92/$05 .00 0 1992 -
Elsevier Science Publishers
B.V.
[6], we showed that the modifications of sodium profiles in ß"-alumina irradiated by argon ions can be described in terms of three main processes : radiation enhanced transport, electric field assisted migration and preferential sputtering of the sodium ions. This last mechanism is strongly dependent on the nuclear stopping power, contrary to some other dielectric materials [7]. The aim of the present study is to obtain more information on the processes which govern the surface modification of sodium ß"-alumina crystals induced by heavy-ion irradiation. As composition changes generate modifications of the surface properties, we also investigated the modifications of the surface dissolving processes . 2. Experimental procedure Single crystals of sodium ß"-alumina were obtained by melting Na 2CO 3, MgO and A1 2 03 with standard procedures [8]. Their composition was Na l.67Mg0 .67 X1033017 and typical dimensions were 10 X 10 x 0.2 mm'. All crystals looked quite good without any macroscopic crackings or extended cleavages . Before the implantations they were annealed at 200°C for 24 h to ensure a nearly complete dehydration and were stored in silica gel to prevent surface contaminations . Ion implantation was performed at room temperature. The beam direction made an angle of 7° with the normal surface direction of the sample, which means that the incidence direction was nearly parallel to the crystallographic c-axis. Some irradiated samples were leached in deionized water (pH ranging from 4 to 5) at 100°C, in static conditions with a surface-to-volume ratio of - 0.02 cm -1. Leaching time was 2 h. All rights reserved
F.L . Freire Jr. / Ion implantation effects in No ß"-alumma crystals
The sodium depth profiles were determined using the resonant nuclear reaction "Na(p, a)2° Ne (resonance energy= 591.6 keV) . The proton beams were obtained from the 4 MV Van de Graaff accelerator at the Physics Department of PUC-Rio. Alpha particles were detected using a 450 mm' silicon surface barrier detector (solid angle = 0.11 sr) positioned at 150°. An aluminized mylar foil, 6 Rm thick, was placed in front of the detector to absorb the higher yield of backscattering protons. This technique has the advantage of requiring a low proton flux [9]. Typically, beam currents of the order of 20 nA were used (1= 1 .0 WA/cm2) . No detectable changes in the sodium profiles were induced even when higher proton fluxes were used . The in-depth sodium concentration profile was determined by increasing the proton energy in steps of 2 keV, which corresponds to a depth resolution of about 20 nm . The maximum depth at which sodium could be measured was about 300 nm. Relative errors were of the order of 3%. Rutherford backscattering spectrometry using 2 MeV helium beams was employed to determine the implanted-ions distribution. 3. Results The effects of 100 keV Ar' irradiation as a function of the dose are summarized in fig . 1. We observe a sodium depletion which increases with the irradiation dose. The thickness of the depleted layers was the same for all doses. Steady-state profiles were obtained DEPTH(nm) 100
z O â Z 05
t
W U Z
m z
00
590
300
. UNIRRADIATED 0 4" 10"Ar'/em' 0 8 " 10'~Aricm' A 10"10'~Ai/cm' dF 12 " 10' ° Ar/em' I .1pAlcm'
Rp
O
200
600 610 ENERGY(keV)
620
Fig. 1. Experimental sodium profiles after 1000 keV Ar' irradiation at different doses . The current density was 1 RA/cm2. The dashed and the dashed-dotted lines are the theoretical simulation calculated in ref. [6] for the 4X1016 and 8x 10 16 argon/cm 2 doses.
0
DEPTH(nm) 100 200
423 300
z O
z
O
600 610 ENERGY(keV)
620
Fig. 2 . Experimental sodium profiles after irradiation by 50 and 300 keV Xe' (0 = 2 x 1016 Xe'/cm 2, 1=1 WA/cm2) . The lines connecting the experimental points are only a guide to the eye. for implantation doses higher than 8 x 10 16 ions/cm2. This behaviour can be attributed to the dynamic recovery of irradiation-produced defects which prevent the complete amorphization of the layer just below the surface, as observed for the manganese-implanted ß"alumina [10] . Then, some kind of crystalline order remains and only part of the sodium ions can diffuse. Argon-implanted sapphire also presented a damage layer near the surface in which the crystalline structure remains [11]. On the other hand, a dose greater than 10 16 Ar'/cm 2 is necessary for the observation of modifications of the sodium profile using the (p, a) reaction. In fig. 2 we show the sodium depth profile modifications as a function of the implantation energy for a fixed dose (2 x 10 16 ions/cm2) of xenon ions. The sodium depleted regions present a thickness which increases with the implantation energy. The results obtained for xenon ions confirm the general behaviour first observed in the case of incident argon ions [6,7]. It is important to note that for the sample implanted with 300 keV xenon ions, the thickness of the sodium depleted layer is the same as the one determined in the case of 100 keV argon implantation (see fig. 1). In both cases the projected range, Rp, is the same . In order to compare the radiation effects induced in ß"-alumina crystals by different heavy-ion irradiations, the implantation energies were chosen in order to obtain the same projected range for incident ions with different masses [12] . The doses were nearly the same and the ion current was 1 wA/cm2. The results are shown in fig. 3. It is evident that an increase of the sodium depletion occurs for the heaviest projectiles. From the results presented in fig. 2 (300 keV Xe) and VIII . ION BEAM MODIFICATION
424
F.L . Freire Jr. /Ion implantation effects in Na ß"-alumina crystals
fig. 3 (100 keV Ar and 180 keV Kr), we can conclude that for a fixed RP the thickness of the depleted layer is independent of the ion species, exceeding the RP + ARP depth (projected range plus straggling). The sample implanted with 50 keV Ne ions presents only a slight sodium depletion for depths greater than 100 nm. This result is compatible with the trend observed for heavier ions. The modifications induced by heavy-ion irradiations in the sodium profiles coulJ be described with a simple model, presented in ref. [6] . As the sputtering cross section of the alkali element is related to the nuclear stopping power [7], the increase of sodium depletion shown in fig. 3 can be understood as the combination of the increase of sputtering cross sections of sodium and the enhancement of sodium diffusion, which increases with the damage produced by the heaviest incident ions. The surface sodium distribution of the implanted samples can be modified by leaching them at 100°C, during 2 h. In fig. 4 we show the sodium profiles of an argon-ion implanted crystal (100 keV, 4 x 10'6 Ar +/cm2) before and after leaching. Whereas, for the unimplanted crystal leaching has almost no effect on the sodium profile (not shown in the figure), the implanted one is markedly affected by that treatment . In fact, the broad sodium peak centered at the argon R P position was removed . RBS analysis showed that the implanted-ions distribution is not affected by the leaching process. These results indicate that sodium ions segregate in the region where the maximum energy deposition occurs. These sodium ions are much less
100
DEPTH,nm) 200
300
DEPTH(nm) O
100
590
600
200
300
N z
O_
F Q F z W
V z
05
O t) z
0.0
610 ENERGY(keV)
620
Fig. 4. Experimental sodium profiles of argon-implanted ß" alumina crystal before and after leaching in deionised water for2 h at 100°C. bounded than in the underlying undamaged crystal, and can be easily removed during the leaching process. In fact, surface composition changes induced by ion implantation generate modifications of the surface dissolving processes. RBS analyses of the sodium ß"alumina crystal implanted with 50 keV Xe + ions (2 x 10'6 ions/cm=) and leached 2 h at 100°C show the complete absence of the implanted xenon ions. This result could indicate that a layer almost 20 nm thick (R P +AR P of 50 keV Xe +) was removed during the leaching process, contrary to what happened with the 100 keV Ar+ implanted crystal. In the case of low-energy xenon implantation the energy deposited at the near-surface layers of the ß"-alumina was three times greater than that for the argon implantation. So, we can attribute these results to an enhancement of the etching rate for the ion-damaged surfaces of sodium ß"-alumina crystal, which increases with the deposited energy. 4. Discussion and conclusions
" UNIRRADIATED o 50keV-4-ldG Nélcm'
p 100keV-4 " f0 " Ar'/cm' 180 keV-3 .10'° Kr'/ cm' I,IpA/cm'
Fig. 3. Experimental sodium profiles after heavy-ion irradiatiop at different energies selected to give the same theoretical projected range . The ion current was 1 WA/cm2. The lines connecting the experimental points are only a guide to the eye except the dashed-dotted line, which is the theoretical simuLuion forargonions calculated in ref. [6] .
Heavy-ion irradiation of sodium ß"-alumina crystals causes a near-surface sodium depletion . The general trend of the experimental results as a function of implantation parameters is quite similar to that observed in implanted glasses [5,13], with one marked difference: for the same projected range, the thickness of the sodium depleted layer in implanted ß"-alumina does not depend on the ion fluxes or masses, as verified for implanted glasses. For both materials the thickness always exceeds RP + ARP depths values . Some hypotheses have been proposed for explaining this aspect: (i) the defects produced locally are mobile
F.L. Freire Jr. / Ion implantation effects in Na p"-alumina crystals and propagate through the implanted material, (ü) light secondary knock-on atoms, and (iii) relaxation effects produced by secondary electrons (including Auger electrons) . In glasses, this fact has been attributed to possible formation and migration of defects and interstitials [13]. In the case of implanted sodium ß"-alumina, its atomic structure (compact spinel blocks intercalated by the so-called conduction planes) may inhibit this mechanism . In fact, the conduction planes may act as a trap for the vacancies, and the interstitial motion from the damaged regions towards the interior is no longer allowed because of the dense structural arrangement of the spinel blocks . The second hypothesis can be excluded: simulations using the TRIM code [12] show that light secondary knock-on atoms will produce a damaged layer with a thickness dependent on the primary-ion mass. On the other hand, recent results [14] on electron irradiation of sodium ß"alumina indicate that sodium ions transport through the spinel blocks can occur ; however, the electron fluxes and energies required are too high to be obtained with secondary electrons produced during the incident ion stopping process. An alternative explanation based on ion-induced stress fields and defect diffusion, would thus appear more likely; such stress fields would be due to the difference in specific gravity and mechanical properties of the implanted layer and the bulk of the material or due to the formation of gas-filled bubbles [15,16] . These stress fields could partially destroy the layered structure of the sodium ß"alumina, permitting the diffusion of sodium ions (and defects) through the spinel blocks . Below a certain depth the stress vanishes and the remaining conduction planes can inhibit the migration of vacancies and interstitials. Then, sodium ions are restricted in their motion and cannot migrate towards the interior of the crystal . In conclusion, in order to explain the ion implantation induced effects at depths exceeding (RP +AR P ), we may invoke the formation of stress fields which could be responsible by the formation of deformed lattice structure permitting the sodium enhanced transport parallel to the c-axis . The surface composition changes induced by ion implantation were correlated with modifications of the surface dissolving processes. Work is in progress to clarify the correspondence between modification of the surface dissolving processes and the implantation parameters .
42 5
Acknowledgements
The author would like to thank prof. G. Mariotto who provided him with the sodium ß"-alumina crystals. He is also indebted to prof. S .R. Teixeira for the ion implantation . This work was supported in part by the Secretaria de Ciência e Tecnologia/P .R. and CNPq.
References
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VIII. ION BEAM MODIFICATION