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Radiation-damage recovery in undoped and oxidized Li doped MgO crystals implanted with lithium ions
Nuclear Instruments and Methods in Physics Research B 206 (2003) 148–152 www.elsevier.com/locate/nimb
Radiation-damage recovery in undoped and oxidized Li doped MgO crystals implanted with lithium ions E. Alves
a,*
, R.C. da Silva a, J.V. Pinto a, T. Monteiro b, B. Savoini c, D. C aceres c, R. Gonz alez c, Y. Chen d
a
Dep. Fısica, Instituto Tecnol ogico e Nuclear, E.N. 10, P-2685-953 Sacav em, Portugal Dep. Fısica, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal c Departamento de Fısica, Universidad Carlos III, Legan es, 28911-Madrid, Spain The US Department of Energy, Division of Materials Sciences, Germantown, MD 20874-1290, USA b
d
Abstract Undoped MgO and oxidized Li-doped MgO single crystals were implanted with 1 1017 Liþ /cm2 at 175 keV. The Rutherford backscattering spectrometry (RBS)/channeling data obtained after implantation shows that damage was produced throughout the entire range of the implanted ions. Optical absorption measurements indicate that after implantation the most intense band occurs at 5.0 eV, which has been associated with anion vacancies. After annealing at 450 K the intensity of the oxygen-vacancy band decreases monotonically with temperature and completely disappears at 950 K. A broad extinction band centered at 2.14 eV associated with lithium precipitates emerges gradually and anneals out at 1250 K. RBS/channeling shows that recovery of the implantation damage is completed after annealing the oxidized samples at 1250 K. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: MgO crystals; Li doping; RBS/channeling; Optical absorption
1. Introduction During the last decades ion implantation has been used to improve the surface properties of materials. Particularly, in insulating materials it provides a reliable way to create a broad range of new optical, electric and magnetic properties [1–4]. These new materials reveal a great potential for optical applications, namely, for development of nonlinear optical devices [5,6]. Most of these *
Corresponding author. Tel.: +351-219946086; fax: +351219941525. E-mail address: [email protected] (E. Alves).
properties are due to the formation of nanosized particles (metallic or semiconducting) encapsulated by the insulating host [7]. In MgO the formation and growth of metallic colloids is possible during the thermal treatments necessary to remove the implantation damage [8]. Previous studies followed the formation and annealing of metallic colloids in implanted MgO through the optical absorption due to Mie scattering [9] by these precipitates [10,11]. Despite all the work done there remain open questions concerning the formation of such nanoparticles in MgO which require the study of the interactions between the implanted species, the defects and their annealing behavior.
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00704-3
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 148–152
2. Experimental details Undoped and 400 ppm Li-doped MgO single crystals with (1 0 0) orientation were cleaved from ingots grown by an arc-fusion technique. Some Li doped samples were further oxidized at 1550 K for 1 h. Both types of samples were implanted at room temperature with 175 keV lithium ions with a fluence of 1 1017 /cm2 . Thermal annealings were performed at different temperatures, from 400 to 1550 K for 15 min in flowing argon atmosphere. After implantation and each annealing, RBS/ channeling studies were performed with a 2.0 MeV Heþ beam to study the structural changes. The backscattered particles were detected at 140° and 180° using silicon surface barrier detectors, with resolutions of 13 and 18 keV, respectively. Optical absorption was measured with a Perkin Elmer Lambda-19 spectrophotometer in the UVVIS-IR regions.
3. Results and discussion 3.1. RBS/channeling Implantation of Li ions in oxidized Li-doped MgO produces a large amount of damage as we can see in Fig. 1. The damage starting at the surface gives rise to a small increase of the minimum yield (reciprocal ratio between the RBS yields of the random and aligned spectra taken along the [0 0 1] axis) near the surface region, and progressively increases in depth: the [0 0 1] aligned spec-
2.5
8
2.0
3
O
1.0
7
0.8
6
implanted Li Recoils: Mg O
5 4 3
0.6 0.4
2
0.2
1 0
Recoil/Ion/nm
3.0
oxidized MgO:Li sample random [100] aligned: as-implanted + Tox. = 1250 K unimplanted
Atom/Ion/nm (x10 )
3.5
yield (counts/103)
In our study we combine Rutherford backscattering spectrometry (RBS) with optical absorption to follow the damage recovery and Li precipitation in undoped MgO and oxidized Lidoped MgO single crystals, implanted with lithium ions. RBS/channeling measurements show the presence of extensive damage at depths greater than 400 nm. Almost all the damage produced by the implantation anneals out at 1200 K. Lithium precipitates start to form at 700 K and have a nearly uniform distribution around 1025 K. The precipitates are completely dissolved at 1250 K.
149
0
200
400
600
800
0.0 1000
depth (nm)
1.5
Mg
1.0 0.5 0.0 100
200
300
400
500
600
700
Fig. 1. Random and [1 0 0]-aligned RBS spectra of an oxidized MgO:Li sample. The [1 0 0]-aligned spectrum obtained after annealing at 1250 K is also shown. Inset: Li profile calculated with TRIM for 175 keV energy. The corresponding interstitials (Mg and O) and vacancy distribution profiles are also shown.
trum reveals an enhancement of the dechanneling rate up to at least 400 nm (channel 380). According to TRIM calculations the expected average projected range and depth of maximum concentration of the Li ions are 610 and 660 nm respectively, as we can see in the inset of Fig. 1. The maxima of the damage distributions, interstitials and vacancies, are located at 600 nm, with the damage density increasing from the surface, in agreement with the RBS measurements. After annealing at 1250 K the damage responsible for the observed dechanneling is almost completely removed (cf. Fig. 1). The undoped MgO samples cleaved and polished already present a higher dechanneling rate before the implantation due to the polishing damage at the surface (region I, extending to 200 nm deep, channel 380 in the spectra in Fig. 2(a)). The implantation damage adds to the dechanneling in the deeper region (II), from 200 nm to at least 400 nm (channel 300), as we can see in Fig. 2(a), from the RBS spectrum for the as-implanted state. The normalized yield measured for region II in the [0 0 1] direction falls from 70% in the asimplanted state to 56% after annealing at 875 K. Subsequent annealing at 1025 K reduces it further to 48%. On the contrary, in region I, the recovery seems to stop after the annealing at 875 K. Above this temperature only region II recovers. The
150
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 148–152 3.5 random
(a)
[001] aligned: as-implanted 875 K annealed 1025 K annealed
O
2.5 2.0
II 1.5
Mg
I
3.2. Optical absorption
1.0 0.5 0.0 100
200
300
400
500
3.5
700
800
10
(b)
TA = 1550 K
2.5
O
2.0
PIXE
Mg
X-ray yield (counts)
3.0
10
(implanted face)
Si
10
Cl 10
Ca Fe
10
4 10
1.5
0
channel number
Mg
1.0
Si
0.5 0.0 100
200
300
400
500
(a)
100 200 300 400 500 600
600
700
800
channel number Fig. 2. (a) RBS spectra of a MgO undoped sample implanted with Li ions. For comparison, a random spectrum from an unimplanted MgO crystal is also displayed. The dashed line indicates the two regions referred to in the text: region I, the surface region and region II, the deeper region, starting at 200 nm and extending to at least 400 nm. (b) RBS and PIXE (inset) spectra of a MgO undoped sample implanted with Li ions, after the annealing at 1550 K. The presence of a new, diferent phase of mixed Mg–Si–O composition is clearly detected.
annealing at 1550 K allows further recovery of the implanted region below 200 nm, the normalized yield in the [0 0 1] direction decreases to 35%. However during this annealing we observe the formation of a new structure at the surface. As we can see in Fig. 2(b), a flat profile appears around channel 500 at the theoretical position of the Siedge, and displays no channeling. Proton induced X-ray emission analysis with 2.2 MeV protons
OPTICAL DENSITY
yield (counts/103)
600
Fig. 3(a) shows the optical absorption spectra obtained before and after Li implantation of the oxidized MgO:Li sample, and after annealing at 1250 K. Before implantation a strong absorption is observed at 1.8 eV due to [Li]0 centers [13,14]. After implantation an intense absorption at 5.0 eV is also resolved which has been associated with Fþ centers [15–17]. Other absorptions related with oxygen vacancies [15–17] cannot be resolved due
MgO:Li oxidized at 1550K as-implanted
3
2
1
0
3
annealed 1250 K
before implantation
6
OPTICAL DENSITY
yield (counts/103)
3.0
confirmed this to be Si. These findings, together with simulations with the RUMP code [12] suggest it to be a Mg–Si mixed oxide of the spinel composition Mg4 Si2 O9 . The Si uptake is most probably due to an ion exchange reaction with the tube walls during the annealing.
5
4
3
(b)
2
1
undoped MgO as-implanted
2 875 K 1025 K
1 1250 K
0 6
5
4
3
2
1
ENERGY (eV) Fig. 3. Optical absorption spectra of (a) oxidized MgO:Li, before and after implantation, and after annealing at 1250 K. (b) Undoped MgO sample implanted with Li ions, isochronally annealed in an argon atmosphere at several selected temperatures.
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 148–152
to the high intensity of these two bands. The annealing at 1250 K not only eliminates completely the damage responsible for the band at 5.0 eV – the intensity decrease of 97% is in perfect agreement with the recovery and residual damage level recorded by RBS/channeling – but it also strongly affects the intensity of the [Li]0 band at 1.8 eV. Fig. 3(b) shows the absorption spectra after implantation and at some selected annealing temperatures of an undoped MgO sample. Again, the 5.0 eV band associated with anion vacancies appears after implantation. The absorption in its high energy side is probably induced by the same defects responsible for the 5.7 eV absorption band observed in deformed MgO crystals [18]. The 1.8 eV band is absent, indicating that no [Li]0 centers were formed. In addition, two small bands at 3.49 and 2.16 eV are observed in the as-implanted sample. The former has been assigned to F2 centers, and the latter to an unidentified aggregated defect [15–17]. The recovery of the 5.0 eV band starts at 500 K and is completed at 900 K. The intensity of the 5.7 eV band diminishes at a rate lower than that of the 5.0 eV band, and completely vanishes at 1250 K. The annealing behavior of the implantation defects follows the kinetics previously found in neutron-irradiated MgO single crystals [15]. This recovery correlates well with the RBS results, indicating a continuous decrease of the defects produced by the implanted Li. After annealing at 875 K a broad band covering the visible spectrum and extended into the infrared and ultraviolet regions emerges (Fig. 3(b)). Its peak moves towards lower energies with increasing temperature, and the bandwidth decreases. At 1025 K it peaks at 2.14 eV. This behavior has already been observed in MgO single crystals and correlated to Mie scattering from lithium nanocolloids [10,11]. Thermal treatment at 1250 K anneals out this extinction band, and the sample becomes transparent.
4. Conclusions Lithium implantation in undoped MgO single crystals produces an intense broad absorption band centered at 5.0 eV, due to implantation
151
damage. No evidence for the formation of [Li]0 centers was found after implantation. Intrinsic defects start to anneal at 500 K and completely vanish at 1250 K. The extinction bands observed after thermal annealings indicate the formation and growth of Li colloids. These colloids dissolve at 1250 K and the MgO crystals turn transparent. In oxidized MgO:Li samples, both the 5.0 and 1.8 eV absorption bands are present after implantation. The damage recovery is complete after annealing at 1250 K, and is accompanied by a decrease of the [Li]0 absorption band.
Acknowledgements We acknowledge with thanks the Portuguese– Spanish Bilateral Research Collaboration Grant. Research at the University Carlos III was supported by the CICYT of Spain. The research of Y.C. is an outgrowth of past investigations performed at the Solid State Division of the Oak Ridge National Laboratory. Research at ITN was partially supported by CRUP under contract AE43/01.
References [1] P.D. Townsend, in: F. Agull o-L opez (Ed.), Insulating Materials for Optoelectronics, World Scientific Publishing, 1995. [2] E. Alves, C. McHargue, R.C. da Silva, C. Jesus, O. Conde, M.F. da Silva, J.C. Soares, Surf. Coat. Technol. 128–129 (2000) 434. [3] J.C. McCallum, L.D. Morphet, Nucl. Instr. and Meth. B 148 (1999) 726. [4] G.N. van den Hoven, E. Snoeks, A. Polman, J.W.M. van Uffelen, Y.S. Oei, M.K. Smit, Appl. Phys. Lett. 62 (1993) 3065. [5] A. Polman, J. Appl. Phys. 82 (1997) 1. [6] P. Chakraborty, J. Mater. Sci. 33 (1998) 2235. [7] C.W. White, J.D. Budai, S.P. Withrow, J.G. Zhu, E. Sonder, R.A. Zhur, A. Meldrum, D.M. Hembree, D.O. Henderson, S. Prawer, Nucl. Instr. and Meth. B 141 (1998) 228. [8] R.L. Zimmerman, D. Ila, E.K. Williams, D.B. Poker, D.K. Hensley, C. Klatt, S. Kalbitzer, Nucl. Instr. and Meth. B 148 (1999) 1064. [9] A.E. Hughes, S.C. Jain, Adv. Phys. 28 (1979) 717. [10] G. Marichy, G. Chassagne, D. Duran, Phys. Status Solidi (b) 92 (1979) 221.
152
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 148–152
[11] M.A. van Huis, A.V. Fedorov, A. Van Veen, F. Labohm, H. Schut, P.E. Mijnarends, B.J. Kooi, J.Th.M. De Hosson, Mater. Sci. Forum 363–365 (2001) 448. [12] L.R. Doolitle, Nucl. Instr. and Meth. B 9 (1985) 344. [13] J. Narayan, M.M. Abraham, Y. Chen, H.T. Tohver, Philos. Mag. A 37 (1978) 909. [14] M.M. Abraham, Y. Chen, L.A. Boatner, R.W. Reynolds, Phys. Rev. Lett. 37 (1976) 849.
[15] Y. Chen, J.L. Kolopus, W.A. Sibley, Phys. Rev. 186 (1969) 865. [16] R. Gonzalez, Y. Chen, R.M. Sebeck, G.P. Williams Jr., R.T. Williams, W. Gellermann, Phys. Rev. B 43 (1991) 5228. [17] M.A. Monge, A.I. Popov, C. Ballesteros, R. Gonzalez, Y. Chen, E.A. Kotomin, Phys. Rev. B 62 (2000) 9299. [18] Y. Chen, M.M. Abraham, T.J. Turner, C.M. Nelson, Philos. Mag. 32 (1975) 99.
Radiation-damage recovery in undoped and oxidized Li doped MgO crystals implanted with lithium ions E. Alves
a,*
, R.C. da Silva a, J.V. Pinto a, T. Monteiro b, B. Savoini c, D. C aceres c, R. Gonz alez c, Y. Chen d
a
Dep. Fısica, Instituto Tecnol ogico e Nuclear, E.N. 10, P-2685-953 Sacav em, Portugal Dep. Fısica, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal c Departamento de Fısica, Universidad Carlos III, Legan es, 28911-Madrid, Spain The US Department of Energy, Division of Materials Sciences, Germantown, MD 20874-1290, USA b
d
Abstract Undoped MgO and oxidized Li-doped MgO single crystals were implanted with 1 1017 Liþ /cm2 at 175 keV. The Rutherford backscattering spectrometry (RBS)/channeling data obtained after implantation shows that damage was produced throughout the entire range of the implanted ions. Optical absorption measurements indicate that after implantation the most intense band occurs at 5.0 eV, which has been associated with anion vacancies. After annealing at 450 K the intensity of the oxygen-vacancy band decreases monotonically with temperature and completely disappears at 950 K. A broad extinction band centered at 2.14 eV associated with lithium precipitates emerges gradually and anneals out at 1250 K. RBS/channeling shows that recovery of the implantation damage is completed after annealing the oxidized samples at 1250 K. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: MgO crystals; Li doping; RBS/channeling; Optical absorption
1. Introduction During the last decades ion implantation has been used to improve the surface properties of materials. Particularly, in insulating materials it provides a reliable way to create a broad range of new optical, electric and magnetic properties [1–4]. These new materials reveal a great potential for optical applications, namely, for development of nonlinear optical devices [5,6]. Most of these *
Corresponding author. Tel.: +351-219946086; fax: +351219941525. E-mail address: [email protected] (E. Alves).
properties are due to the formation of nanosized particles (metallic or semiconducting) encapsulated by the insulating host [7]. In MgO the formation and growth of metallic colloids is possible during the thermal treatments necessary to remove the implantation damage [8]. Previous studies followed the formation and annealing of metallic colloids in implanted MgO through the optical absorption due to Mie scattering [9] by these precipitates [10,11]. Despite all the work done there remain open questions concerning the formation of such nanoparticles in MgO which require the study of the interactions between the implanted species, the defects and their annealing behavior.
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00704-3
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 148–152
2. Experimental details Undoped and 400 ppm Li-doped MgO single crystals with (1 0 0) orientation were cleaved from ingots grown by an arc-fusion technique. Some Li doped samples were further oxidized at 1550 K for 1 h. Both types of samples were implanted at room temperature with 175 keV lithium ions with a fluence of 1 1017 /cm2 . Thermal annealings were performed at different temperatures, from 400 to 1550 K for 15 min in flowing argon atmosphere. After implantation and each annealing, RBS/ channeling studies were performed with a 2.0 MeV Heþ beam to study the structural changes. The backscattered particles were detected at 140° and 180° using silicon surface barrier detectors, with resolutions of 13 and 18 keV, respectively. Optical absorption was measured with a Perkin Elmer Lambda-19 spectrophotometer in the UVVIS-IR regions.
3. Results and discussion 3.1. RBS/channeling Implantation of Li ions in oxidized Li-doped MgO produces a large amount of damage as we can see in Fig. 1. The damage starting at the surface gives rise to a small increase of the minimum yield (reciprocal ratio between the RBS yields of the random and aligned spectra taken along the [0 0 1] axis) near the surface region, and progressively increases in depth: the [0 0 1] aligned spec-
2.5
8
2.0
3
O
1.0
7
0.8
6
implanted Li Recoils: Mg O
5 4 3
0.6 0.4
2
0.2
1 0
Recoil/Ion/nm
3.0
oxidized MgO:Li sample random [100] aligned: as-implanted + Tox. = 1250 K unimplanted
Atom/Ion/nm (x10 )
3.5
yield (counts/103)
In our study we combine Rutherford backscattering spectrometry (RBS) with optical absorption to follow the damage recovery and Li precipitation in undoped MgO and oxidized Lidoped MgO single crystals, implanted with lithium ions. RBS/channeling measurements show the presence of extensive damage at depths greater than 400 nm. Almost all the damage produced by the implantation anneals out at 1200 K. Lithium precipitates start to form at 700 K and have a nearly uniform distribution around 1025 K. The precipitates are completely dissolved at 1250 K.
149
0
200
400
600
800
0.0 1000
depth (nm)
1.5
Mg
1.0 0.5 0.0 100
200
300
400
500
600
700
Fig. 1. Random and [1 0 0]-aligned RBS spectra of an oxidized MgO:Li sample. The [1 0 0]-aligned spectrum obtained after annealing at 1250 K is also shown. Inset: Li profile calculated with TRIM for 175 keV energy. The corresponding interstitials (Mg and O) and vacancy distribution profiles are also shown.
trum reveals an enhancement of the dechanneling rate up to at least 400 nm (channel 380). According to TRIM calculations the expected average projected range and depth of maximum concentration of the Li ions are 610 and 660 nm respectively, as we can see in the inset of Fig. 1. The maxima of the damage distributions, interstitials and vacancies, are located at 600 nm, with the damage density increasing from the surface, in agreement with the RBS measurements. After annealing at 1250 K the damage responsible for the observed dechanneling is almost completely removed (cf. Fig. 1). The undoped MgO samples cleaved and polished already present a higher dechanneling rate before the implantation due to the polishing damage at the surface (region I, extending to 200 nm deep, channel 380 in the spectra in Fig. 2(a)). The implantation damage adds to the dechanneling in the deeper region (II), from 200 nm to at least 400 nm (channel 300), as we can see in Fig. 2(a), from the RBS spectrum for the as-implanted state. The normalized yield measured for region II in the [0 0 1] direction falls from 70% in the asimplanted state to 56% after annealing at 875 K. Subsequent annealing at 1025 K reduces it further to 48%. On the contrary, in region I, the recovery seems to stop after the annealing at 875 K. Above this temperature only region II recovers. The
150
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 148–152 3.5 random
(a)
[001] aligned: as-implanted 875 K annealed 1025 K annealed
O
2.5 2.0
II 1.5
Mg
I
3.2. Optical absorption
1.0 0.5 0.0 100
200
300
400
500
3.5
700
800
10
(b)
TA = 1550 K
2.5
O
2.0
PIXE
Mg
X-ray yield (counts)
3.0
10
(implanted face)
Si
10
Cl 10
Ca Fe
10
4 10
1.5
0
channel number
Mg
1.0
Si
0.5 0.0 100
200
300
400
500
(a)
100 200 300 400 500 600
600
700
800
channel number Fig. 2. (a) RBS spectra of a MgO undoped sample implanted with Li ions. For comparison, a random spectrum from an unimplanted MgO crystal is also displayed. The dashed line indicates the two regions referred to in the text: region I, the surface region and region II, the deeper region, starting at 200 nm and extending to at least 400 nm. (b) RBS and PIXE (inset) spectra of a MgO undoped sample implanted with Li ions, after the annealing at 1550 K. The presence of a new, diferent phase of mixed Mg–Si–O composition is clearly detected.
annealing at 1550 K allows further recovery of the implanted region below 200 nm, the normalized yield in the [0 0 1] direction decreases to 35%. However during this annealing we observe the formation of a new structure at the surface. As we can see in Fig. 2(b), a flat profile appears around channel 500 at the theoretical position of the Siedge, and displays no channeling. Proton induced X-ray emission analysis with 2.2 MeV protons
OPTICAL DENSITY
yield (counts/103)
600
Fig. 3(a) shows the optical absorption spectra obtained before and after Li implantation of the oxidized MgO:Li sample, and after annealing at 1250 K. Before implantation a strong absorption is observed at 1.8 eV due to [Li]0 centers [13,14]. After implantation an intense absorption at 5.0 eV is also resolved which has been associated with Fþ centers [15–17]. Other absorptions related with oxygen vacancies [15–17] cannot be resolved due
MgO:Li oxidized at 1550K as-implanted
3
2
1
0
3
annealed 1250 K
before implantation
6
OPTICAL DENSITY
yield (counts/103)
3.0
confirmed this to be Si. These findings, together with simulations with the RUMP code [12] suggest it to be a Mg–Si mixed oxide of the spinel composition Mg4 Si2 O9 . The Si uptake is most probably due to an ion exchange reaction with the tube walls during the annealing.
5
4
3
(b)
2
1
undoped MgO as-implanted
2 875 K 1025 K
1 1250 K
0 6
5
4
3
2
1
ENERGY (eV) Fig. 3. Optical absorption spectra of (a) oxidized MgO:Li, before and after implantation, and after annealing at 1250 K. (b) Undoped MgO sample implanted with Li ions, isochronally annealed in an argon atmosphere at several selected temperatures.
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 148–152
to the high intensity of these two bands. The annealing at 1250 K not only eliminates completely the damage responsible for the band at 5.0 eV – the intensity decrease of 97% is in perfect agreement with the recovery and residual damage level recorded by RBS/channeling – but it also strongly affects the intensity of the [Li]0 band at 1.8 eV. Fig. 3(b) shows the absorption spectra after implantation and at some selected annealing temperatures of an undoped MgO sample. Again, the 5.0 eV band associated with anion vacancies appears after implantation. The absorption in its high energy side is probably induced by the same defects responsible for the 5.7 eV absorption band observed in deformed MgO crystals [18]. The 1.8 eV band is absent, indicating that no [Li]0 centers were formed. In addition, two small bands at 3.49 and 2.16 eV are observed in the as-implanted sample. The former has been assigned to F2 centers, and the latter to an unidentified aggregated defect [15–17]. The recovery of the 5.0 eV band starts at 500 K and is completed at 900 K. The intensity of the 5.7 eV band diminishes at a rate lower than that of the 5.0 eV band, and completely vanishes at 1250 K. The annealing behavior of the implantation defects follows the kinetics previously found in neutron-irradiated MgO single crystals [15]. This recovery correlates well with the RBS results, indicating a continuous decrease of the defects produced by the implanted Li. After annealing at 875 K a broad band covering the visible spectrum and extended into the infrared and ultraviolet regions emerges (Fig. 3(b)). Its peak moves towards lower energies with increasing temperature, and the bandwidth decreases. At 1025 K it peaks at 2.14 eV. This behavior has already been observed in MgO single crystals and correlated to Mie scattering from lithium nanocolloids [10,11]. Thermal treatment at 1250 K anneals out this extinction band, and the sample becomes transparent.
4. Conclusions Lithium implantation in undoped MgO single crystals produces an intense broad absorption band centered at 5.0 eV, due to implantation
151
damage. No evidence for the formation of [Li]0 centers was found after implantation. Intrinsic defects start to anneal at 500 K and completely vanish at 1250 K. The extinction bands observed after thermal annealings indicate the formation and growth of Li colloids. These colloids dissolve at 1250 K and the MgO crystals turn transparent. In oxidized MgO:Li samples, both the 5.0 and 1.8 eV absorption bands are present after implantation. The damage recovery is complete after annealing at 1250 K, and is accompanied by a decrease of the [Li]0 absorption band.
Acknowledgements We acknowledge with thanks the Portuguese– Spanish Bilateral Research Collaboration Grant. Research at the University Carlos III was supported by the CICYT of Spain. The research of Y.C. is an outgrowth of past investigations performed at the Solid State Division of the Oak Ridge National Laboratory. Research at ITN was partially supported by CRUP under contract AE43/01.
References [1] P.D. Townsend, in: F. Agull o-L opez (Ed.), Insulating Materials for Optoelectronics, World Scientific Publishing, 1995. [2] E. Alves, C. McHargue, R.C. da Silva, C. Jesus, O. Conde, M.F. da Silva, J.C. Soares, Surf. Coat. Technol. 128–129 (2000) 434. [3] J.C. McCallum, L.D. Morphet, Nucl. Instr. and Meth. B 148 (1999) 726. [4] G.N. van den Hoven, E. Snoeks, A. Polman, J.W.M. van Uffelen, Y.S. Oei, M.K. Smit, Appl. Phys. Lett. 62 (1993) 3065. [5] A. Polman, J. Appl. Phys. 82 (1997) 1. [6] P. Chakraborty, J. Mater. Sci. 33 (1998) 2235. [7] C.W. White, J.D. Budai, S.P. Withrow, J.G. Zhu, E. Sonder, R.A. Zhur, A. Meldrum, D.M. Hembree, D.O. Henderson, S. Prawer, Nucl. Instr. and Meth. B 141 (1998) 228. [8] R.L. Zimmerman, D. Ila, E.K. Williams, D.B. Poker, D.K. Hensley, C. Klatt, S. Kalbitzer, Nucl. Instr. and Meth. B 148 (1999) 1064. [9] A.E. Hughes, S.C. Jain, Adv. Phys. 28 (1979) 717. [10] G. Marichy, G. Chassagne, D. Duran, Phys. Status Solidi (b) 92 (1979) 221.
152
E. Alves et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 148–152
[11] M.A. van Huis, A.V. Fedorov, A. Van Veen, F. Labohm, H. Schut, P.E. Mijnarends, B.J. Kooi, J.Th.M. De Hosson, Mater. Sci. Forum 363–365 (2001) 448. [12] L.R. Doolitle, Nucl. Instr. and Meth. B 9 (1985) 344. [13] J. Narayan, M.M. Abraham, Y. Chen, H.T. Tohver, Philos. Mag. A 37 (1978) 909. [14] M.M. Abraham, Y. Chen, L.A. Boatner, R.W. Reynolds, Phys. Rev. Lett. 37 (1976) 849.
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