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Characterisation of defects in LiF implanted with Ar+ using variable energy positron beam
Nuclear Instruments and Methods in Physics Research B 192 (2002) 202–205 www.elsevier.com/locate/nimb
Characterisation of defects in LiF implanted with Arþ using variable energy positron beam E.J. Sendezera a
a,*
, A.T. Davidson a, P.T. Jili a, M.L. Chithambo a, W. Anwand b, G. Brauer b, E.-H. Nicht b
Department of Physics, University of Zululand, P/B X1001 Kwa-Dlangezwa 3886, South Africa b Forschungszentrum Rossendorf, Postfach 510117, 01314 Dresden, Germany
Abstract Slow positron implantation spectroscopy has been used to determine the surface and near surface defect profile in LiF after implantation with 100 keV argon ions for a range of fluences from 1013 to 1016 cm2 . The measured Sparameter is used to characterise the radiation damage as a function of depth. The spatial extent of the defect distributions was estimated by employing the computer program VEPFIT. Maximum lattice damage is shown to occur at incident positron energy of 4.0 keV for low doses and 5.0 keV for the high dose. This energy corresponds to a sample depth of about 200 nm. The positron annihilation spectroscopy results are correlated with optical absorption measurements on the crystals. Ó 2002 Published by Elsevier Science B.V. Keywords: Positron beam; Radiation damage; LiF:Arþ
1. Introduction A high concentration of colour centres, consisting of mainly F and H centres, is produced in a thin layer near the surface of a LiF crystal by ion implantation [1–4]. Optical studies performed by Davenas et al. [5], Afonso et al. [6], Abu-Hassan and Townsend [7] and Davidson et al. [8,9] show that coloration is due to F and F aggregate centres formed in the implanted region. The advent of positron beams [10], which have both depth selectivity and defect sensitivity, enables a non-destructive study of depth profiling measurements on LiF irradiated with 100 keV Ar
*
Corresponding author. E-mail address: [email protected] (E.J. Sendezera).
ions. Upon implantation into the solid the positrons thermalize within few picoseconds and then diffuse until annihilating with electrons. The mean implantation depth, z, is given by z ¼ AEn =q;
ð1Þ
where A and n are constants (taken to be 40 nm g cm3 (keV)n and 1.6, respectively), q is the density (g cm3 ) and E is the energy (keV) [11]. The Doppler broadening of the annihilation line shape, characterised by the S-parameter, which is defined as the area under the central part of the annihilation photopeak divided by the total area [10], is a measure of the electron momentum distribution at the annihilation site. The measured S-parameter is actually a linear superposition of contributing Sparameters from the surface to the bulk and for a multilayer system can be expressed generally as
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 8 6 9 - 8
E.J. Sendezera et al. / Nucl. Instr. and Meth. in Phys. Res. B 192 (2002) 202–205
SðEÞ ¼ Ssurf fsurf ðEÞ þ
X
Si fi ðEÞ;
ð2Þ
i
where Ssurf is the S-parameter at the surface, fsurf ðEÞ is the fraction of positrons annihilating at the surface and i denotes the ith layer. The object of the present work is to study the radiation damage in LiF following the implantation of 100 keV Arþ ions in the fluence range 1013 – 1016 cm2 and to characterise the damage by the slow positron beam technique. The results of the positron annihilation spectroscopy measurements are correlated with the results of optical measurements.
2. Experiment Pure LiF plates of thickness 1 mm were obtained from BDH (Merck) in the United Kingdom. Samples of 8 8 mm2 were cut from the plates using diamond saw. The samples were implanted with 100 keV Arþ ions in the fluence range 1013 –1016 ions cm2 at the Schonland Research Centre for Nuclear Sciences at the University of the Witwatersrand. Optical absorption spectra were measured at wavelengths between 200 and 600 nm using a CARY 400 spectrophotometer in order to optically characterise the defects in the ion-implanted LiF. Positron beam measurements were performed with a variable energy slow positron beam at Research Centre Rossendorf, Germany. The beam has a diameter of 3 mm and a maximum implantation energy of 50 keV. Positrons from 22 Na source (30 mCi) were moderated by a tungsten foil of about 6 mm diameter and a thickness of 5 lm in order to produce a monoenergetic beam. After moderation the positrons are pulled off by a 30 V beam formation section and then transported and focused by magnetic field and accelerated to achieve the required positron implantation energies [12]. The implanted positrons rapidly thermalize within a few picoseconds and then diffuse (in the absence of an electric field) until they annihilate with electrons, producing pairs of gamma ray (511 keV) photons that are detected by a high-
203
purity germanium (HPGe) detector with a resolution of 1.26 keV (FWHM) at 511 keV. The positron annihilation spectra (5 105 counts) were accumulated at positron beam energies ranging from 0.03 to 25 keV, which corresponds to a maximum mean implantation depth of 2.61 lm. The vacancy-type defects act as trapping sites for positrons and annihilation with low energy valence electrons at these defects results in a narrowing of the photopeak corresponding to an increase in the S-parameter.
3. Results and discussion Optical absorption spectra of LiF following implantation with Arþ ions are shown in Fig. 1. The F band at 245 nm (5.06 eV) is present at fluence range of 1013 –1016 cm2 . The F2 band at 444 nm (2.79 eV) is only present at fluences 1014 –1016 cm2 and is absent at the lowest dose of 1013 cm2 . With increasing fluence the F and F2 bands grow progressively to their highest sizes at the fluence of 1016 cm2 . F band absorption is characteristic of ion-implanted alkali halides and comprises an anion vacancy which has trapped an electron. The positron annihilation data was analysed using the computer program VEPFIT [13]. In this analysis the program was used to fit the experimental data by analysing the energy dependence of the S-parameter. In Fig. 2 we show the S-parameter (normalised to the bulk value) versus incident
Fig. 1. Absorbance versus photon wavelength between 200 and 600 nm for LiF after implantation with Arþ ions.
204
E.J. Sendezera et al. / Nucl. Instr. and Meth. in Phys. Res. B 192 (2002) 202–205
Fig. 2. S-parameter versus positron implantation energy. Data are normalised to the bulk. The solid lines indicate the best fit to experimental data.
positron energy at various fluences. Since the highest beam energy used was 25 keV, corresponding to a maximum mean implantation depth of 2.61 lm, which is deeper than the penetration depth of the 100-keV Arþ , the entire implanted region can be considered to be probed by annihilated positrons. The presence of vacancy-like defects, acting as positron traps, is clearly visible through its effect on the dependence of the S-parameter on the incident positron energy E in Fig. 2. The common high energy asymptote represents the characteristic value for undamaged bulk material (Sb ) and indicates that above 25 keV, the positrons are implanted deep enough to annihilate beyond the disordered layer produced by ion implantation. At E ¼ 0, the S-parameter approaches the surface value (Ss ), which is slightly different for each fluence investigated. The difference is not important in the present context, as it merely reflects different surface conditions of the samples and the important differences are in the shape of the curves. The curve for the unimplanted sample decreases monotonically between the surface and the bulk Sparameters but, on the contrary, well-defined peaks appear around 5 keV for the 1016 cm2 implant and at 4 keV for the 1015 cm2 implant. Smaller peaks are barely visible for the low dose samples. The existence of peaks show that the positrons implanted with the peak energy come to
rest in a region where there is an important concentration of positron traps, thus forming a localized state characterised by an S-parameter at a defect. The S-parameter gradually decreases beyond positron beam energy corresponding to its peak value and reaches the bulk value beyond 25 keV. The peak damage shifts towards the surface with decreasing ion dose from 1016 to 1013 cm2 . The VEPFIT analysis of the data identifies four layers in the irradiated samples, where the undamaged part is included as one layer. These layers and their respective positron diffusion lengths, normalised S-parameters and ion doses are shown in Table 1. In general there appears to be a surface layer about 50 nm thick whose properties have been changed by the passage of the implanted ions into the sample. This layer is characterized by a shorter positron diffusion length (7 nm) than that of the bulk (79 nm). A layer with a high Sparameter and a short diffusion length (1 nm) follows the surface layer. The high S-parameter may be the result of positrons annihilating with electrons with very low momentum associated with F centres. We have observed the existence of these
Table 1 ‘VEPFIT’ fitted layers, ion doses, normalized S-parameters (S=Sbulk ) and diffusion lengths Ion dose (cm2 )
Layer position from the surface (nm)
Normalized S-parameter (S=Sbulk )
Diffusion length (nm)
1013
0–59 59–229 229–587 587–bulk
1.069 1.044 1.022 1.000
7 1 5 79
1014
0–45 45–166 166–441 441–bulk
1.088 1.069 1.041 1.000
7 1 5 79
1015
0–21 21–281 281–548 548–bulk
1.030 1.065 1.029 1.000
7 1 2 79
1016
0–101 101–360 360–1143 1143-bulk
1.032 1.088 1.013 1.000
7 1 2 79
E.J. Sendezera et al. / Nucl. Instr. and Meth. in Phys. Res. B 192 (2002) 202–205
centres in our optical absorption measurements shown in Fig. 1. The spatial extent of the defect distribution is generally larger in high dose samples as compared to that for low dose samples. Also, at high fluences the region of damage extends beyond the penetration depth of the implanted ions as calculated by the simulation code TRIM. A possible cause of this deep damage is a knock-on effect, which is well known in metals [4,14].
4. Conclusion Positron beam results of depth profiling of defects in LiF implanted with 100 keV Arþ ions at fluences in the range 1013 –1016 cm2 are presented. These results reveal the existence of different layers of damage below the implanted surface with varying positron diffusion lengths. The layer with the most damage has the smallest diffusion length (1 nm) and the bulk (undamaged layer) has the largest diffusion length (79 nm). Optical absorption measurements suggest that positrons annihilate with electrons at F and Faggregate centres. Our results show that damage produced by ion implantation is detected beyond the ion implantation depth as determined by TRIM calculations. Such an effect is known in certain oxide materials [14] and in Alþ -implanted LiF [4] and is now observed also in LiF implanted with Arþ ions.
205
Acknowledgements The authors wish to thank the Schonland Research Centre for Nuclear Sciences of the University of Witwatersrand for the ion implantation of our samples, the University of Zululand and NRF for their financial support.
References [1] L.W. Hobbs, Surface and Defect Properties of Solids, Vol. 4, Chemical Society, 1975. [2] P.D. Townsend, Rep. Prog. Phys. 50 (1987) 501. [3] J.D. Comins, A.T. Davidson, T.E. Derry, Defect Diffus. Forum 57/58 (1988) 409. [4] E.J. Sendezera, A.T. Davidson, W. Anwand, G. Brauer, E.-M. Nitch, Appl. Surf. Sci. 149 (1999) 125. [5] J. Davenas, A. Perez, P. Thavenard, C.H.S. Dupuy, Phys. Status Solidi a 19 (1973) 679. [6] C.N. Afonso, C. Ortiz, G.J. Clark, Phys. Status Solidi b 131 (1985) 87. [7] L.H. Abu-Hassan, P.D. Townsend, J. Phys. C: Solid State Phys. 19 (1986) 19. [8] A.T. Davidson, J.D. Comins, T.E. Derry, Radiat. Eff. 90 (1985) 213. [9] A.T. Davidson, J.D. Comins, T.E. Derry, F.S. Khumalo, Radiat. Eff. 98 (1986) 305. [10] P.J. Schultz, K.G. Lynn, Rev. Mod. Phys. 60 (1988) 701. [11] A. Vehanen, K. Saarinen, P. Hautojarvi, H. Huomo, Phys. Rev. B 35 (1987) 4606. [12] W. Anwand, H.R. Kissener, G. Brauer, Acta Phys. Pol. A 88 (1) (1995) 100. [13] A. Van Veen, H. Schut, J. de Vries, R.A. Hakvoort, M.R. Ijpma, AIP Conf. Proc. 218 (1990) 171. [14] E. Friedland, Nucl. Instr. and Meth. B 116 (1996) 136.
Characterisation of defects in LiF implanted with Arþ using variable energy positron beam E.J. Sendezera a
a,*
, A.T. Davidson a, P.T. Jili a, M.L. Chithambo a, W. Anwand b, G. Brauer b, E.-H. Nicht b
Department of Physics, University of Zululand, P/B X1001 Kwa-Dlangezwa 3886, South Africa b Forschungszentrum Rossendorf, Postfach 510117, 01314 Dresden, Germany
Abstract Slow positron implantation spectroscopy has been used to determine the surface and near surface defect profile in LiF after implantation with 100 keV argon ions for a range of fluences from 1013 to 1016 cm2 . The measured Sparameter is used to characterise the radiation damage as a function of depth. The spatial extent of the defect distributions was estimated by employing the computer program VEPFIT. Maximum lattice damage is shown to occur at incident positron energy of 4.0 keV for low doses and 5.0 keV for the high dose. This energy corresponds to a sample depth of about 200 nm. The positron annihilation spectroscopy results are correlated with optical absorption measurements on the crystals. Ó 2002 Published by Elsevier Science B.V. Keywords: Positron beam; Radiation damage; LiF:Arþ
1. Introduction A high concentration of colour centres, consisting of mainly F and H centres, is produced in a thin layer near the surface of a LiF crystal by ion implantation [1–4]. Optical studies performed by Davenas et al. [5], Afonso et al. [6], Abu-Hassan and Townsend [7] and Davidson et al. [8,9] show that coloration is due to F and F aggregate centres formed in the implanted region. The advent of positron beams [10], which have both depth selectivity and defect sensitivity, enables a non-destructive study of depth profiling measurements on LiF irradiated with 100 keV Ar
*
Corresponding author. E-mail address: [email protected] (E.J. Sendezera).
ions. Upon implantation into the solid the positrons thermalize within few picoseconds and then diffuse until annihilating with electrons. The mean implantation depth, z, is given by z ¼ AEn =q;
ð1Þ
where A and n are constants (taken to be 40 nm g cm3 (keV)n and 1.6, respectively), q is the density (g cm3 ) and E is the energy (keV) [11]. The Doppler broadening of the annihilation line shape, characterised by the S-parameter, which is defined as the area under the central part of the annihilation photopeak divided by the total area [10], is a measure of the electron momentum distribution at the annihilation site. The measured S-parameter is actually a linear superposition of contributing Sparameters from the surface to the bulk and for a multilayer system can be expressed generally as
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 8 6 9 - 8
E.J. Sendezera et al. / Nucl. Instr. and Meth. in Phys. Res. B 192 (2002) 202–205
SðEÞ ¼ Ssurf fsurf ðEÞ þ
X
Si fi ðEÞ;
ð2Þ
i
where Ssurf is the S-parameter at the surface, fsurf ðEÞ is the fraction of positrons annihilating at the surface and i denotes the ith layer. The object of the present work is to study the radiation damage in LiF following the implantation of 100 keV Arþ ions in the fluence range 1013 – 1016 cm2 and to characterise the damage by the slow positron beam technique. The results of the positron annihilation spectroscopy measurements are correlated with the results of optical measurements.
2. Experiment Pure LiF plates of thickness 1 mm were obtained from BDH (Merck) in the United Kingdom. Samples of 8 8 mm2 were cut from the plates using diamond saw. The samples were implanted with 100 keV Arþ ions in the fluence range 1013 –1016 ions cm2 at the Schonland Research Centre for Nuclear Sciences at the University of the Witwatersrand. Optical absorption spectra were measured at wavelengths between 200 and 600 nm using a CARY 400 spectrophotometer in order to optically characterise the defects in the ion-implanted LiF. Positron beam measurements were performed with a variable energy slow positron beam at Research Centre Rossendorf, Germany. The beam has a diameter of 3 mm and a maximum implantation energy of 50 keV. Positrons from 22 Na source (30 mCi) were moderated by a tungsten foil of about 6 mm diameter and a thickness of 5 lm in order to produce a monoenergetic beam. After moderation the positrons are pulled off by a 30 V beam formation section and then transported and focused by magnetic field and accelerated to achieve the required positron implantation energies [12]. The implanted positrons rapidly thermalize within a few picoseconds and then diffuse (in the absence of an electric field) until they annihilate with electrons, producing pairs of gamma ray (511 keV) photons that are detected by a high-
203
purity germanium (HPGe) detector with a resolution of 1.26 keV (FWHM) at 511 keV. The positron annihilation spectra (5 105 counts) were accumulated at positron beam energies ranging from 0.03 to 25 keV, which corresponds to a maximum mean implantation depth of 2.61 lm. The vacancy-type defects act as trapping sites for positrons and annihilation with low energy valence electrons at these defects results in a narrowing of the photopeak corresponding to an increase in the S-parameter.
3. Results and discussion Optical absorption spectra of LiF following implantation with Arþ ions are shown in Fig. 1. The F band at 245 nm (5.06 eV) is present at fluence range of 1013 –1016 cm2 . The F2 band at 444 nm (2.79 eV) is only present at fluences 1014 –1016 cm2 and is absent at the lowest dose of 1013 cm2 . With increasing fluence the F and F2 bands grow progressively to their highest sizes at the fluence of 1016 cm2 . F band absorption is characteristic of ion-implanted alkali halides and comprises an anion vacancy which has trapped an electron. The positron annihilation data was analysed using the computer program VEPFIT [13]. In this analysis the program was used to fit the experimental data by analysing the energy dependence of the S-parameter. In Fig. 2 we show the S-parameter (normalised to the bulk value) versus incident
Fig. 1. Absorbance versus photon wavelength between 200 and 600 nm for LiF after implantation with Arþ ions.
204
E.J. Sendezera et al. / Nucl. Instr. and Meth. in Phys. Res. B 192 (2002) 202–205
Fig. 2. S-parameter versus positron implantation energy. Data are normalised to the bulk. The solid lines indicate the best fit to experimental data.
positron energy at various fluences. Since the highest beam energy used was 25 keV, corresponding to a maximum mean implantation depth of 2.61 lm, which is deeper than the penetration depth of the 100-keV Arþ , the entire implanted region can be considered to be probed by annihilated positrons. The presence of vacancy-like defects, acting as positron traps, is clearly visible through its effect on the dependence of the S-parameter on the incident positron energy E in Fig. 2. The common high energy asymptote represents the characteristic value for undamaged bulk material (Sb ) and indicates that above 25 keV, the positrons are implanted deep enough to annihilate beyond the disordered layer produced by ion implantation. At E ¼ 0, the S-parameter approaches the surface value (Ss ), which is slightly different for each fluence investigated. The difference is not important in the present context, as it merely reflects different surface conditions of the samples and the important differences are in the shape of the curves. The curve for the unimplanted sample decreases monotonically between the surface and the bulk Sparameters but, on the contrary, well-defined peaks appear around 5 keV for the 1016 cm2 implant and at 4 keV for the 1015 cm2 implant. Smaller peaks are barely visible for the low dose samples. The existence of peaks show that the positrons implanted with the peak energy come to
rest in a region where there is an important concentration of positron traps, thus forming a localized state characterised by an S-parameter at a defect. The S-parameter gradually decreases beyond positron beam energy corresponding to its peak value and reaches the bulk value beyond 25 keV. The peak damage shifts towards the surface with decreasing ion dose from 1016 to 1013 cm2 . The VEPFIT analysis of the data identifies four layers in the irradiated samples, where the undamaged part is included as one layer. These layers and their respective positron diffusion lengths, normalised S-parameters and ion doses are shown in Table 1. In general there appears to be a surface layer about 50 nm thick whose properties have been changed by the passage of the implanted ions into the sample. This layer is characterized by a shorter positron diffusion length (7 nm) than that of the bulk (79 nm). A layer with a high Sparameter and a short diffusion length (1 nm) follows the surface layer. The high S-parameter may be the result of positrons annihilating with electrons with very low momentum associated with F centres. We have observed the existence of these
Table 1 ‘VEPFIT’ fitted layers, ion doses, normalized S-parameters (S=Sbulk ) and diffusion lengths Ion dose (cm2 )
Layer position from the surface (nm)
Normalized S-parameter (S=Sbulk )
Diffusion length (nm)
1013
0–59 59–229 229–587 587–bulk
1.069 1.044 1.022 1.000
7 1 5 79
1014
0–45 45–166 166–441 441–bulk
1.088 1.069 1.041 1.000
7 1 5 79
1015
0–21 21–281 281–548 548–bulk
1.030 1.065 1.029 1.000
7 1 2 79
1016
0–101 101–360 360–1143 1143-bulk
1.032 1.088 1.013 1.000
7 1 2 79
E.J. Sendezera et al. / Nucl. Instr. and Meth. in Phys. Res. B 192 (2002) 202–205
centres in our optical absorption measurements shown in Fig. 1. The spatial extent of the defect distribution is generally larger in high dose samples as compared to that for low dose samples. Also, at high fluences the region of damage extends beyond the penetration depth of the implanted ions as calculated by the simulation code TRIM. A possible cause of this deep damage is a knock-on effect, which is well known in metals [4,14].
4. Conclusion Positron beam results of depth profiling of defects in LiF implanted with 100 keV Arþ ions at fluences in the range 1013 –1016 cm2 are presented. These results reveal the existence of different layers of damage below the implanted surface with varying positron diffusion lengths. The layer with the most damage has the smallest diffusion length (1 nm) and the bulk (undamaged layer) has the largest diffusion length (79 nm). Optical absorption measurements suggest that positrons annihilate with electrons at F and Faggregate centres. Our results show that damage produced by ion implantation is detected beyond the ion implantation depth as determined by TRIM calculations. Such an effect is known in certain oxide materials [14] and in Alþ -implanted LiF [4] and is now observed also in LiF implanted with Arþ ions.
205
Acknowledgements The authors wish to thank the Schonland Research Centre for Nuclear Sciences of the University of Witwatersrand for the ion implantation of our samples, the University of Zululand and NRF for their financial support.
References [1] L.W. Hobbs, Surface and Defect Properties of Solids, Vol. 4, Chemical Society, 1975. [2] P.D. Townsend, Rep. Prog. Phys. 50 (1987) 501. [3] J.D. Comins, A.T. Davidson, T.E. Derry, Defect Diffus. Forum 57/58 (1988) 409. [4] E.J. Sendezera, A.T. Davidson, W. Anwand, G. Brauer, E.-M. Nitch, Appl. Surf. Sci. 149 (1999) 125. [5] J. Davenas, A. Perez, P. Thavenard, C.H.S. Dupuy, Phys. Status Solidi a 19 (1973) 679. [6] C.N. Afonso, C. Ortiz, G.J. Clark, Phys. Status Solidi b 131 (1985) 87. [7] L.H. Abu-Hassan, P.D. Townsend, J. Phys. C: Solid State Phys. 19 (1986) 19. [8] A.T. Davidson, J.D. Comins, T.E. Derry, Radiat. Eff. 90 (1985) 213. [9] A.T. Davidson, J.D. Comins, T.E. Derry, F.S. Khumalo, Radiat. Eff. 98 (1986) 305. [10] P.J. Schultz, K.G. Lynn, Rev. Mod. Phys. 60 (1988) 701. [11] A. Vehanen, K. Saarinen, P. Hautojarvi, H. Huomo, Phys. Rev. B 35 (1987) 4606. [12] W. Anwand, H.R. Kissener, G. Brauer, Acta Phys. Pol. A 88 (1) (1995) 100. [13] A. Van Veen, H. Schut, J. de Vries, R.A. Hakvoort, M.R. Ijpma, AIP Conf. Proc. 218 (1990) 171. [14] E. Friedland, Nucl. Instr. and Meth. B 116 (1996) 136.