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Nanoindentation on MgO crystals implanted with lithium ions
Nuclear Instruments and Methods in Physics Research B 191 (2002) 154–157 www.elsevier.com/locate/nimb
Nanoindentation on MgO crystals implanted with lithium ions D. C aceres a, I. Vergara a, R. Gonz alez a
a,*
, Y. Chen b, E. Alves
c
Departamento de Fısica, Escuela Polit ecnica Superior, Universidad Carlos III, Avda. de la Universidad 30, 28911 Legan es, Madrid, Spain b Division of Materials Sciences, Office of Basic Energy Sciences, SC 13, The US Department of Energy, Germantown MD 20874-1290, USA c Instituto Tecnol ogico e Nuclear, EN 10 2686-953 Sacav em, Portugal
Abstract As-grown MgO single crystals, both nominally pure and lithium-doped, were implanted with Li ions with an energy of 175 KeV and a dose of 1017 ions/cm2 . MgO:Li crystals were also implanted after oxidation at 1550 K for 30 min. TRIM calculations yield a range of 610 nm for the implanted ions. Hardness and Young’s modulus were measured in all the samples before and after implantation using a nanoindentation technique. As-grown MgO and MgO:Li crystals show the same hardness value of (9:1 0:2) GPa and Young’s modulus of (290 15) GPa. After oxidation of MgO:Li crystals the hardness is (10:1 0:2) GPa. Implantation of Li ions hardens the near-surface region of all three samples: MgO, MgO:Li and oxidized MgO:Li. Implantation in MgO and MgO:Li showed the same behavior: hardness reaches a maximum value of (16:3 0:2) GPa at a penetration depth of 175 nm, and slowly diminishes with depth. In oxidized MgO:Li crystals the maximum hardness is (17:7 0:2) GPa at a penetration depth of 175 nm. The considerable hardening observed in the implanted regions is attributed to the extraordinarily large concentration of interstitials in this region. Ó 2002 Published by Elsevier Science B.V. Keywords: Nanoindentation; Ion implantation; Defects; MgO
1. Introduction Ion implantation of a crystalline solid alters only the properties of a thin layer of the nearsurface region. In MgO crystals, the defects induced by ion implantation can be monitored by optical absorption bands similar to those observed after neutron irradiation experiments [1–6]. The irradiation of MgO single crystals with energetic neutrons (E > 0:1 MeV) produces stable vacancies
*
Corresponding author. Tel.: +34-91-624-9448; fax: +34-91624-8749. E-mail address: rgonza@fis.uc3m.es (R. Gonzalez).
and interstitials in the anion sublattice. Elastic collisions with energetic particles also produce cation vacancies, but these defects do not survive, because the cation interstitials quickly recombine with the vacancies. Although neutron-irradiation damage is uniform over the entire sample while that due to ion implantation affects only a thin subsurface layer, the vacancy-type defects are expected to be the same for both types of damage. The purpose of this work is to study the hardening induced by Li-implantation in nominally pure MgO crystals and those doped with lithium using the nanoindentation technique. The radiation damage was monitored by optical absorption measurements. Nanoindentation is a very powerful
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 5 3 9 - 6
D. Caceres et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 154–157
155
technique to determine the mechanical properties of the implanted region because it can measure hardness and Young’s modulus of layers as thin as 100 nm and especially the variation of these parameters with the indentation contact depth.
2. Experimental procedure MgO and MgO:Li crystals were grown by an arcfusion technique [7] at the Oak Ridge National Laboratory using high-purity MgO powder from Kanto Chemical Chemistry, Tokyo, Japan. The lithium concentration in MgO:Li crystals was 400 ppm as determined by spectrographic analysis. Single-crystals specimens having {100} faces were cleaved from ingots in the boule. Oxidation of MgO:Li crystals was performed in flowing oxygen gas at 1550 K for 30 min. Optical absorption measurements were made with a Perkin–Elmer Lambda 19 spectrophotometer. Nanonidentation tests were made with a Nanoindenter II’s (Nano Instruments, Inc., Knoxville, TN) mechanical properties microprobe. Each specimen was tested at room temperature using the continuous stiffness measurement technique developed by Oliver and Pethica [8].
3. Results and discussion Nominally pure MgO, as-grown MgO:Li and oxidized MgO:Li crystals were implanted with 175 keV Li ions at a dose of 1017 ions/cm2 . MgO:Li crystals were also implanted after oxidation at 1550 K for 30 min. According to TRIM calculations the expected range of the implanted ions is 610 nm.
Fig. 1. Optical absorption of a MgO crystal before and after implantation with Li ions.
therefore is positively charged relative to the lattice). The two bands located at 3.49 and 1.27 eV have been associated with anion divacancies, F2 . The band centered at 2.16 eV is due to an unidentified aggregate defect [1–6]. The 2.16 eV band, whose full width at half maximum (FWHM) is 0.3 eV, should not be confused with the 2.3 eV band, which is due to trapped-hole centers [9]. The latter is broader (FWHM ¼ 1:1 eV). Nanoindentation experiments revealed an increase in both hardness and Young’s modulus in the implanted region. Hardness and Young’s modulus in unimplanted crystals have values of (9:1 0:2) and (290 15) GPa, respectively. Fig. 2 shows the variation of hardness and Young’s modulus as a function of contact depth in the implanted region. The hardness increases to (16:3 0:2) GPa at a penetration depth of about 175 nm and slowly diminished thereafter. Young’s modulus reaches a maximum value of (350 15) GPa.
3.1. Undoped MgO crystals
3.2. Lithium-doped MgO crystals
Fig. 1 shows the optical absorption spectrum of a MgO crystal before and after Li-implantation. After implantation several absorption bands appear. As in neutron irradiated crystals [1–6], the most intense absorption occurs at 5.0 eV. This band has been associated with Fþ centers (an oxygen vacancy which has trapped an electron, and
The optical absorption spectra before and after Li-implantation of an as-grown MgO:Li crystal is depicted in Fig. 3 (top). In the as-grown state the band at 4.3 eV is associated with the Fe3þ charge transfer band [10,11]. After Li-implantation the four bands at 5.0, 3.49, 2.16 and 1.27 eV observed in undoped MgO crystals are also apparent.
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D. Caceres et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 154–157
Fig. 2. Hardness and Young’s modulus versus contact depth of a MgO crystal after Li-implantation.
Fig. 3. Optical absorption spectra of as-grown (top) and oxidized (bottom) MgO:Li crystals before and after Li-implantation.
Nanoindentation measurements show that the increase in hardness and Young’s modulus due to the implantation is the same as those for undoped MgO, indicating that doping with lithium does not produce changes in these parameters in agreement with previous findings [12]. The optical absorption spectra of an oxidized MgO:Li crystal before and after Li-implantation are shown in Fig. 3 (bottom). Before implantation two absorption bands at 1.8 and 5.3 eV were observed. The former is due to ½Li0 centers; the linear configuration of this defect is Mg2þ –O2 –Liþ –O , where O refers to an oxygen ion with a trapped hole [13,14]. The structure responsible for the 5.3 eV band has not been identified, but is probably related to some substitutional Li ions with point defect characteristics [15,16]. After implantation, only two additional absorption bands at 5.0 and 3.49 eV were resolved. The bands at 2.16 and 1.27 eV are probably masked by the 1.8 eV band.
Hardness and Young’s modulus were measured in an oxidized crystal before implantation. The resulting values are (10:1 0:2) and (290 15) 0 GPa, respectively, suggesting that ½Li centers and/ or the defects absorbing at 5.3 eV are responsible for the observed 10% increase in hardness, in agreement with earlier results [12]. Fig. 4 shows hardness and Young’s modulus versus contact depth after implantation. At about 175 nm both parameters reach their maximum values of (17:7 0:2) GPa and (370 20) GPa, respectively. Again, Li-implantation induces an important hard-ening of the implanted region. The maximum of the hardness and Young’s modulus occurs at a tip-contact depth of about 175 nm. However, TRIM calculations gave a range of about 610 nm for both the implanted ions and the vacancy distribution. This discrepancy is due to the fact that the plastic region produced by indentation is about three times deeper than the tip contact depth [17].
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order of 1021 cm3 compared to 1018 cm3 for the neutron-irradiated crystal [2]. We conclude that the considerable hardening observed in the implanted region is due to the extraordinarily large concentration of interstitials.
Acknowledgements Research at the University Carlos III was supported by the CICYT of Spain. We also acknowledge the support of CRUP (action no. E-43/ 01). The research of Y.C. is an outgrowth of past investigations performed at the Solid State Division of the Oak Ridge National Laboratory.
References [1] [2] [3] [4] [5] Fig. 4. Hardness and Young’s modulus versus contact depth of an oxidized MgO:Li crystal after Li-implantation.
[6] [7]
4. Conclusion The incremental increase in hardness, DH , which is the difference in hardness between an implanted and an unimplanted region can be as high as 7.6 GPa. On the other hand, DH for a neutron-irradiated crystal is only 3.2 GPa; this crystal was irradiated to a dose of 6:9 1018 n cm2 , with resulting anion vacancies of the order of 3:4 1018 cm3 . This hardening had been attributed to interstitials [18] much more than vacancies. The much larger DH in the implanted region is not surprising; the concentration of defects, both interstitials and vacancies, is much higher in the implanted region than in the neutronirradiated crystal. In the implanted crystals, the implanted Li ions and the damage are confined to a region of 610 nm. Their concentrations are of the
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
B. Henderson, R.D. King, Philos. Mag. A 13 (1966) 1149. W.A. Sibley, Y. Chen, Phys. Rev. 160 (1967) 712. B. Henderson, J.E. Wertz, Adv. Phys. 17 (1968) 749. Y. Chen, J.L. Kolopus, W.A. Sibley, Phys. Rev. 182 (1969) 960. L.A. Kappers, R.L. Kroes, E.B. Hensley, Phys. Rev. B 1 (1970) 4151. M.A. Monge, A.I. Popov, C. Ballesteros, R. Gonzalez, Y. Chen, E.A. Kotomin, Phys. Rev. B 62 (2000) 9299. M.M. Abraham, C.T. Butler, Y. Chen, J. Chem. Phys 55 (1971) 3752. J.B. Pethica, W.C. Oliver, Mater. Res. Soc. Symp. Proc. 130 (1989) 13. Y. Chen, M.M. Abraham, J. Phys. Chem. Solids 51 (1990) 747. R.W. Soshea, A.J. Dekker, J.P. Sturtz, J. Phys. Chem. Solids 5 (1958) 23. W.A. Sibley, J.L. Kolopus, W.C. Mallard, Phys. Status Solidi 31 (1969) 223. D. Caceres, I. Vergara, R. Gonzalez, Y. Chen, Philos. Mag., in press. J. Narayan, M.M. Abraham, Y. Chen, H.T. Tohver, Philos. Mag. A 37 (1978) 909. M.M. Abraham, Y. Chen, L.A. Boatner, R.W. Reynolds, Phys. Rev. Lett. 37 (1976) 849. J.B. Lacy, M.M. Abraham, J.L. Bold u, Y. Chen, J. Narayan, H.T. Tohver, Phys. Rev. B 18 (1978) 4136. V.M. Orera, Y. Chen, M.M. Abraham, Philos. Mag. A 41 (1980) 431. G.M. Pharr, W.C. Oliver, MRS Bull. 17 (1992) 28. D. Caceres, I. Vergara, R. Gonzalez, Y Chen, Nucl. Instr. and Meth. B 191 (2002) 178.
Nanoindentation on MgO crystals implanted with lithium ions D. C aceres a, I. Vergara a, R. Gonz alez a
a,*
, Y. Chen b, E. Alves
c
Departamento de Fısica, Escuela Polit ecnica Superior, Universidad Carlos III, Avda. de la Universidad 30, 28911 Legan es, Madrid, Spain b Division of Materials Sciences, Office of Basic Energy Sciences, SC 13, The US Department of Energy, Germantown MD 20874-1290, USA c Instituto Tecnol ogico e Nuclear, EN 10 2686-953 Sacav em, Portugal
Abstract As-grown MgO single crystals, both nominally pure and lithium-doped, were implanted with Li ions with an energy of 175 KeV and a dose of 1017 ions/cm2 . MgO:Li crystals were also implanted after oxidation at 1550 K for 30 min. TRIM calculations yield a range of 610 nm for the implanted ions. Hardness and Young’s modulus were measured in all the samples before and after implantation using a nanoindentation technique. As-grown MgO and MgO:Li crystals show the same hardness value of (9:1 0:2) GPa and Young’s modulus of (290 15) GPa. After oxidation of MgO:Li crystals the hardness is (10:1 0:2) GPa. Implantation of Li ions hardens the near-surface region of all three samples: MgO, MgO:Li and oxidized MgO:Li. Implantation in MgO and MgO:Li showed the same behavior: hardness reaches a maximum value of (16:3 0:2) GPa at a penetration depth of 175 nm, and slowly diminishes with depth. In oxidized MgO:Li crystals the maximum hardness is (17:7 0:2) GPa at a penetration depth of 175 nm. The considerable hardening observed in the implanted regions is attributed to the extraordinarily large concentration of interstitials in this region. Ó 2002 Published by Elsevier Science B.V. Keywords: Nanoindentation; Ion implantation; Defects; MgO
1. Introduction Ion implantation of a crystalline solid alters only the properties of a thin layer of the nearsurface region. In MgO crystals, the defects induced by ion implantation can be monitored by optical absorption bands similar to those observed after neutron irradiation experiments [1–6]. The irradiation of MgO single crystals with energetic neutrons (E > 0:1 MeV) produces stable vacancies
*
Corresponding author. Tel.: +34-91-624-9448; fax: +34-91624-8749. E-mail address: rgonza@fis.uc3m.es (R. Gonzalez).
and interstitials in the anion sublattice. Elastic collisions with energetic particles also produce cation vacancies, but these defects do not survive, because the cation interstitials quickly recombine with the vacancies. Although neutron-irradiation damage is uniform over the entire sample while that due to ion implantation affects only a thin subsurface layer, the vacancy-type defects are expected to be the same for both types of damage. The purpose of this work is to study the hardening induced by Li-implantation in nominally pure MgO crystals and those doped with lithium using the nanoindentation technique. The radiation damage was monitored by optical absorption measurements. Nanoindentation is a very powerful
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 5 3 9 - 6
D. Caceres et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 154–157
155
technique to determine the mechanical properties of the implanted region because it can measure hardness and Young’s modulus of layers as thin as 100 nm and especially the variation of these parameters with the indentation contact depth.
2. Experimental procedure MgO and MgO:Li crystals were grown by an arcfusion technique [7] at the Oak Ridge National Laboratory using high-purity MgO powder from Kanto Chemical Chemistry, Tokyo, Japan. The lithium concentration in MgO:Li crystals was 400 ppm as determined by spectrographic analysis. Single-crystals specimens having {100} faces were cleaved from ingots in the boule. Oxidation of MgO:Li crystals was performed in flowing oxygen gas at 1550 K for 30 min. Optical absorption measurements were made with a Perkin–Elmer Lambda 19 spectrophotometer. Nanonidentation tests were made with a Nanoindenter II’s (Nano Instruments, Inc., Knoxville, TN) mechanical properties microprobe. Each specimen was tested at room temperature using the continuous stiffness measurement technique developed by Oliver and Pethica [8].
3. Results and discussion Nominally pure MgO, as-grown MgO:Li and oxidized MgO:Li crystals were implanted with 175 keV Li ions at a dose of 1017 ions/cm2 . MgO:Li crystals were also implanted after oxidation at 1550 K for 30 min. According to TRIM calculations the expected range of the implanted ions is 610 nm.
Fig. 1. Optical absorption of a MgO crystal before and after implantation with Li ions.
therefore is positively charged relative to the lattice). The two bands located at 3.49 and 1.27 eV have been associated with anion divacancies, F2 . The band centered at 2.16 eV is due to an unidentified aggregate defect [1–6]. The 2.16 eV band, whose full width at half maximum (FWHM) is 0.3 eV, should not be confused with the 2.3 eV band, which is due to trapped-hole centers [9]. The latter is broader (FWHM ¼ 1:1 eV). Nanoindentation experiments revealed an increase in both hardness and Young’s modulus in the implanted region. Hardness and Young’s modulus in unimplanted crystals have values of (9:1 0:2) and (290 15) GPa, respectively. Fig. 2 shows the variation of hardness and Young’s modulus as a function of contact depth in the implanted region. The hardness increases to (16:3 0:2) GPa at a penetration depth of about 175 nm and slowly diminished thereafter. Young’s modulus reaches a maximum value of (350 15) GPa.
3.1. Undoped MgO crystals
3.2. Lithium-doped MgO crystals
Fig. 1 shows the optical absorption spectrum of a MgO crystal before and after Li-implantation. After implantation several absorption bands appear. As in neutron irradiated crystals [1–6], the most intense absorption occurs at 5.0 eV. This band has been associated with Fþ centers (an oxygen vacancy which has trapped an electron, and
The optical absorption spectra before and after Li-implantation of an as-grown MgO:Li crystal is depicted in Fig. 3 (top). In the as-grown state the band at 4.3 eV is associated with the Fe3þ charge transfer band [10,11]. After Li-implantation the four bands at 5.0, 3.49, 2.16 and 1.27 eV observed in undoped MgO crystals are also apparent.
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D. Caceres et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 154–157
Fig. 2. Hardness and Young’s modulus versus contact depth of a MgO crystal after Li-implantation.
Fig. 3. Optical absorption spectra of as-grown (top) and oxidized (bottom) MgO:Li crystals before and after Li-implantation.
Nanoindentation measurements show that the increase in hardness and Young’s modulus due to the implantation is the same as those for undoped MgO, indicating that doping with lithium does not produce changes in these parameters in agreement with previous findings [12]. The optical absorption spectra of an oxidized MgO:Li crystal before and after Li-implantation are shown in Fig. 3 (bottom). Before implantation two absorption bands at 1.8 and 5.3 eV were observed. The former is due to ½Li0 centers; the linear configuration of this defect is Mg2þ –O2 –Liþ –O , where O refers to an oxygen ion with a trapped hole [13,14]. The structure responsible for the 5.3 eV band has not been identified, but is probably related to some substitutional Li ions with point defect characteristics [15,16]. After implantation, only two additional absorption bands at 5.0 and 3.49 eV were resolved. The bands at 2.16 and 1.27 eV are probably masked by the 1.8 eV band.
Hardness and Young’s modulus were measured in an oxidized crystal before implantation. The resulting values are (10:1 0:2) and (290 15) 0 GPa, respectively, suggesting that ½Li centers and/ or the defects absorbing at 5.3 eV are responsible for the observed 10% increase in hardness, in agreement with earlier results [12]. Fig. 4 shows hardness and Young’s modulus versus contact depth after implantation. At about 175 nm both parameters reach their maximum values of (17:7 0:2) GPa and (370 20) GPa, respectively. Again, Li-implantation induces an important hard-ening of the implanted region. The maximum of the hardness and Young’s modulus occurs at a tip-contact depth of about 175 nm. However, TRIM calculations gave a range of about 610 nm for both the implanted ions and the vacancy distribution. This discrepancy is due to the fact that the plastic region produced by indentation is about three times deeper than the tip contact depth [17].
D. Caceres et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 154–157
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order of 1021 cm3 compared to 1018 cm3 for the neutron-irradiated crystal [2]. We conclude that the considerable hardening observed in the implanted region is due to the extraordinarily large concentration of interstitials.
Acknowledgements Research at the University Carlos III was supported by the CICYT of Spain. We also acknowledge the support of CRUP (action no. E-43/ 01). The research of Y.C. is an outgrowth of past investigations performed at the Solid State Division of the Oak Ridge National Laboratory.
References [1] [2] [3] [4] [5] Fig. 4. Hardness and Young’s modulus versus contact depth of an oxidized MgO:Li crystal after Li-implantation.
[6] [7]
4. Conclusion The incremental increase in hardness, DH , which is the difference in hardness between an implanted and an unimplanted region can be as high as 7.6 GPa. On the other hand, DH for a neutron-irradiated crystal is only 3.2 GPa; this crystal was irradiated to a dose of 6:9 1018 n cm2 , with resulting anion vacancies of the order of 3:4 1018 cm3 . This hardening had been attributed to interstitials [18] much more than vacancies. The much larger DH in the implanted region is not surprising; the concentration of defects, both interstitials and vacancies, is much higher in the implanted region than in the neutronirradiated crystal. In the implanted crystals, the implanted Li ions and the damage are confined to a region of 610 nm. Their concentrations are of the
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
B. Henderson, R.D. King, Philos. Mag. A 13 (1966) 1149. W.A. Sibley, Y. Chen, Phys. Rev. 160 (1967) 712. B. Henderson, J.E. Wertz, Adv. Phys. 17 (1968) 749. Y. Chen, J.L. Kolopus, W.A. Sibley, Phys. Rev. 182 (1969) 960. L.A. Kappers, R.L. Kroes, E.B. Hensley, Phys. Rev. B 1 (1970) 4151. M.A. Monge, A.I. Popov, C. Ballesteros, R. Gonzalez, Y. Chen, E.A. Kotomin, Phys. Rev. B 62 (2000) 9299. M.M. Abraham, C.T. Butler, Y. Chen, J. Chem. Phys 55 (1971) 3752. J.B. Pethica, W.C. Oliver, Mater. Res. Soc. Symp. Proc. 130 (1989) 13. Y. Chen, M.M. Abraham, J. Phys. Chem. Solids 51 (1990) 747. R.W. Soshea, A.J. Dekker, J.P. Sturtz, J. Phys. Chem. Solids 5 (1958) 23. W.A. Sibley, J.L. Kolopus, W.C. Mallard, Phys. Status Solidi 31 (1969) 223. D. Caceres, I. Vergara, R. Gonzalez, Y. Chen, Philos. Mag., in press. J. Narayan, M.M. Abraham, Y. Chen, H.T. Tohver, Philos. Mag. A 37 (1978) 909. M.M. Abraham, Y. Chen, L.A. Boatner, R.W. Reynolds, Phys. Rev. Lett. 37 (1976) 849. J.B. Lacy, M.M. Abraham, J.L. Bold u, Y. Chen, J. Narayan, H.T. Tohver, Phys. Rev. B 18 (1978) 4136. V.M. Orera, Y. Chen, M.M. Abraham, Philos. Mag. A 41 (1980) 431. G.M. Pharr, W.C. Oliver, MRS Bull. 17 (1992) 28. D. Caceres, I. Vergara, R. Gonzalez, Y Chen, Nucl. Instr. and Meth. B 191 (2002) 178.