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Photoluminescence and UV–vis studies of pre- and post-irradiated sapphire with 200 MeV Ag8+ ions
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 244 (2006) 187–189 www.elsevier.com/locate/nimb
Photoluminescence and UV–vis studies of pre- and post-irradiated sapphire with 200 MeV Ag8+ ions K.S. Jheeta
a,*
, D.C. Jain a, Fouran Singh b, Ravi Kumar b, K.B. Garg
a
a b
Department of Physics, University of Rajasthan, Jaipur, India Nuclear Science Center, Aruna Asaf Ali Marg, New Delhi, India Available online 20 December 2005
Abstract Photoluminescence (PL) of sapphire irradiated with 200 MeV swift Ag8+ ions with fluence ranging from 1 · 1011 to 1 · 1013 ions/cm2 have been studied at room temperature. The fluence dependent behavior of PL intensities for emission band at 547 nm and a well-known sharp doublet peaks at 693.9 nm and 695.3 nm due to Cr3+ are observed. PL intensity of feature at 547 nm increases with increasing Ag ion fluence till 7 · 1012 ions/cm2 and then show a downward trend with higher fluence. Results are interpreted in terms of defect center production, cluster formation or segregation of the defects. UV–vis spectra for pristine and Ag irradiated sapphire are correlated with PL measurements. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.y; 61.82.Ms; 78.55.m Keywords: Photoluminescence; Swift heavy ion; Sapphire; Color center; Absorption
1. Introduction Natural and synthetic sapphires are among the most important inorganic materials with regards to its interesting characteristics: it is highly refractory and extremely abrasive [1]. Pure sapphire is colorless and does not show any luminescence at all; however, transition metal (TM) impurities (Fe, Ti, Cr) even at a trace level causes a luminescence giving rise to color centers. These color centers can also be induced or modified by ion irradiation [2]. High-energy radiation is required for the generation of primary defects in sapphire; most previous investigations concentrated on irradiation effects of electron, neutron or energetic particles [3,4]. A large amount of work such as Rutherford backscattering analysis, optical absorption
*
Corresponding author. E-mail address: kuldeep_jheeta@rediffmail.com (K.S. Jheeta).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.024
and EPR spectroscopy have been done to characterize the surface morphology, defect production and optical properties of sapphires using low energy ions up to keV range [5,6]. But there is lack of much research work to correlate the surface amorphization, color centers and defects formation in sapphire using swift heavy ions. Optical spectroscopy (e.g. PL, UV–vis, etc.) helps in understanding the presence of defect centers in the solid-state materials. PL technique is a useful tool because of its high resolution and non-destructive nature that gives insight into defects types and their concentration. The aim of this study is to investigate the defect centers in sapphire subjected to very high densities of electronic excitation. PL and UV–vis absorption techniques are used in pristine and irradiated sapphires with different fluence of 200 MeV Ag8+ ions. Luminescence intensity of defect centers and impurity ion in irradiated sapphires are interpreted in terms of concentration of defects centers, cluster formation and annihilation process. UV–vis spectroscopy is used to identify the defect centers and their behavior on irradiation.
188
K.S. Jheeta et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 187–189 35000
The South African sapphire samples of sizes 5 mm · 5 mm · 1 mm were irradiated with 200 MeVAg8+ ion beam at room temperature under high vacuum (6 · 106 Torr) using the 15 UD pelleteron tandem accelerator at Nuclear Science Center, New Delhi, India. The ion flux was 109 ions/cm2 s. The samples were irradiated at fluences 1 · 1011, 5 · 1011, 1 · 1012, 5 · 1012, 7 · 1012 and 1 · 1013 ions/cm2, respectively. One unirradiated sample was kept as pristine. The ion beam fluence was measured by integrating the ion charge on the sample. The pristine and irradiated crystals were characterized with Photoluminescence and UV–vis optical absorption techniques. PL measurements were carried out using Mechelle-900 spectrograph in the range 200–800 nm, using 442 nm He–Cd laser excitation. Optical absorption was taken using a Hitachi 3300 ultraviolet–visible spectrophotometer in the range of 200–800 nm.
30000
3. Results and discussion Photoluminescence measurements are done for pristine and six samples of sapphire irradiated with 200 MeV Ag8+ ion beam in the fluence range 1 · 1011– 1 · 1013 ions/cm2. We have observed mainly two features one at 547 nm assigned to a F2þ 2 center (two oxygen vacancies with two electrons) [7] and a well-known sharp doublet at 693.9 nm and 695.3 nm, commonly known as R1 and R2 lines of Cr3+ [8]. We have also observed a peak at 485 nm and this probably corresponds to another defect centers (F+ of F+ centers). These centers grow up to fluence of 7 · 1012 ions/cm2 and then decrease faster similar to those F-center in alkali halides. In order to visualize and discuss these two features, we have divided our PL spectra into two parts. First part concerns with F2þ 2 defect center and is shown in Fig. 1 after magnification. Second part concern with R lines of Cr3+
4000
2+
F 2 defect center
13
1x10 12 7x10 12 5x10 12 1x10 11 5x10 11 1x10 Pristine
Luminescence intensity in a.u.
3500 3000
7x10
12
2500
1x10
13
2000
5x10
1500
1x10
1000
12
12
5x10
11
1x10
500
11
Pristine
0 450
500
550
600
650
Wavelength in nm
Fig. 1. Magnified view of PL spectra of pristine and irradiated sapphires.
Luminescence intensity in a.u.
2. Experimental
12
1x10
25000 20000
13
1x10
15000
12
5x10
10000
12
7x10
11
5x10
5000
pristine
11
1x10
0 688
692
696
700
704
708
Wavelength in nm
Fig. 2. Photoluminescence spectra of R1 and R2 lines of Cr3+ with varying fluence of Ag8+ ion.
ions and is shown in Fig. 2. From Fig. 1, one can see the increase in intensity corresponding to F2þ defect center 2 with fluence and reaches maximum at fluence 7 · 1012 ions/cm2. On further increase of fluence it starts decreasing. This particular behavior can be understood by assuming that the damage induced by ion irradiation increases the point defects concentrations of F+ defects up to fluence 7 · 1012 ions/cm2. The mobility of these defect centers increases as a result of enhanced concentration of these centers. This increases the probability of their aggregation thus forming F2þ 2 defect centers. These defect centers can be considered as aggregate of F+ defect center. Or one can say that F+ defect centers act as precursor for the generation of F2þ defect center [7]. A decrease in the intensity of this 2 defect after 7 · 1012 ion/cm2 can be attributed to an annihilation process of luminescence centers (F+ centers) resulting from a higher disorder induced by irradiation or to the irradiation induced amorphization as a result of cascades quenching [7]. Fig. 2 shows the intensity variation of the well-known sharp doublet (R1 and R2 lines) due to Cr3+ ion impurity, substituting Al site in sapphire. These two R lines are observed at 693.9 nm and 695.3 nm, respectively and assigned to a well-known radiative transition 2E ! 4A2 of the substitutional impurity Cr3+ ions. The intensity of R lines in pristine is extremely small, however, it increases with the fluence and becomes maximum at fluence 1 · 1012 ion/cm2 i.e. 80 times of that in pristine sapphire. At this fluence, contribution of F2þ 2 defect center is either negligible or starts developing. With the further increase of fluence their intensities decrease and once again starts increasing at 7 · 1012 ions/cm2. This gradual decrease in the intensities may be correlated with the generation of F2þ 2 defect center. The decrease in the R line intensities may be understood in terms of Cr ion pair formation as a result of stress induced by irradiation [9]. An increase in intensity of this feature beyond the fluence of 7 · 1012 ions/cm2 is difficult to interpret at this stage. However
K.S. Jheeta et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 187–189
absorption and luminescence bands associated with the transition metal impurities and associated charge transfer phenomenon are left for future.
3.0 2+ F 2 center
Absorption in a.u.
2.8
189
Cr 3+ bands
4. Conclusions
2.6
2.4 12
7x10 2.2
12
1x10
13
2.0
1x10
1.8
5x10 Pristine
11
400
450
500
550
Wavelength in nm
Fig. 3. UV–vis spectra of pristine and irradiated sapphire.
a detailed and more work is required to comment on this issue. Optical absorption spectra for pristine and irradiated sapphires are shown in Fig. 3. The spectrum of pristine sapphire is quite similar to the earlier reported one [8]. On irradiation the general shape of the spectra did not change much. However, features related to various defect centers can be seen. A feature appearing at 443 nm in irradiated samples, almost absent in pristine sample, may be assigned to F2þ 2 defect center. The intensity of this feature initially increased with fluence up to 7 · 1012 ions/cm2 and then starts decreasing with further increase of fluence. The similar behavior was seen in PL measurements. The absorption band corresponding to impurity ion Cr3+(in traces) were observed at 393 nm (4A2 ! 4F1) and 564 nm (4A2 ! 4F1) [10]. The intensities of these two bands behaved in the similar manner as observed in PL spectra. Other
Single crystals of sapphire were irradiated with varying fluence of 200 MeV Ag8+ ion beam and luminescence induced by these ions was measured. Mainly two luminescence features were observed such as F2þ 2 defect center and R lines of Cr3+. The fluence dependent behavior of these features showed that concentration of defect centers increased up to fluence 7 · 1012 ions/cm2, giving rise to cumulative response. On the contrary, a decrease in intensity is observed for still higher fluence, which could be due to higher degradation of the material and consequently annihilation of luminescence centers. References [1] J. Garcia-Guinea, J. Rubio, V. Correcher, F.J. Valle-Fuentes, Rad. Measur. 33 (2001) 653. [2] P. Jonnard, C. Bonnelle, G. Blaise, G. Remond, R.C. Carmes, J. Appl. Phys. 88 (11) (2000) 6413. [3] G.W. Arnold, W.D. Compton, Phys. Rev. Lett. 4 (1960) 66. [4] K.H. Lee, G.E. Holmberg, J.H. Crawford, Solid State Commun. 20 (1976) 183. [5] S. Furuno, N. Sasajina, K. Hojou, K. Izui, T. Muromura, T. Matsui, Nucl. Instr. and Meth. B 127–128 (1997) 181. [6] N. Sasajina, T. Matsui, S. Furuno, K. Hojou, H. Otsu, Nucl. Instr. and Meth. B 148 (1999) 745. [7] T. Mohanty, N.C. Mishra, F. Singh, U. Tiwari, D. Kanjilal, Nucl. Instr. and Meth. B 212 (2003) 179. [8] T.H. Maiman, R.H. Hoskins, I.J. D’Haenens, C.K. Asawa, V. Evthov, Phys. Rev. B 123 (1961) 1151. [9] R.C. Powell, Phys. Rev. 115 (1967) 265. [10] M. Ghamnia, C. Jardin, M. Bouslama, J. Electron. Spectrosc. Relat. Phenom. 133 (2003) 55.
Nuclear Instruments and Methods in Physics Research B 244 (2006) 187–189 www.elsevier.com/locate/nimb
Photoluminescence and UV–vis studies of pre- and post-irradiated sapphire with 200 MeV Ag8+ ions K.S. Jheeta
a,*
, D.C. Jain a, Fouran Singh b, Ravi Kumar b, K.B. Garg
a
a b
Department of Physics, University of Rajasthan, Jaipur, India Nuclear Science Center, Aruna Asaf Ali Marg, New Delhi, India Available online 20 December 2005
Abstract Photoluminescence (PL) of sapphire irradiated with 200 MeV swift Ag8+ ions with fluence ranging from 1 · 1011 to 1 · 1013 ions/cm2 have been studied at room temperature. The fluence dependent behavior of PL intensities for emission band at 547 nm and a well-known sharp doublet peaks at 693.9 nm and 695.3 nm due to Cr3+ are observed. PL intensity of feature at 547 nm increases with increasing Ag ion fluence till 7 · 1012 ions/cm2 and then show a downward trend with higher fluence. Results are interpreted in terms of defect center production, cluster formation or segregation of the defects. UV–vis spectra for pristine and Ag irradiated sapphire are correlated with PL measurements. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.y; 61.82.Ms; 78.55.m Keywords: Photoluminescence; Swift heavy ion; Sapphire; Color center; Absorption
1. Introduction Natural and synthetic sapphires are among the most important inorganic materials with regards to its interesting characteristics: it is highly refractory and extremely abrasive [1]. Pure sapphire is colorless and does not show any luminescence at all; however, transition metal (TM) impurities (Fe, Ti, Cr) even at a trace level causes a luminescence giving rise to color centers. These color centers can also be induced or modified by ion irradiation [2]. High-energy radiation is required for the generation of primary defects in sapphire; most previous investigations concentrated on irradiation effects of electron, neutron or energetic particles [3,4]. A large amount of work such as Rutherford backscattering analysis, optical absorption
*
Corresponding author. E-mail address: kuldeep_jheeta@rediffmail.com (K.S. Jheeta).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.024
and EPR spectroscopy have been done to characterize the surface morphology, defect production and optical properties of sapphires using low energy ions up to keV range [5,6]. But there is lack of much research work to correlate the surface amorphization, color centers and defects formation in sapphire using swift heavy ions. Optical spectroscopy (e.g. PL, UV–vis, etc.) helps in understanding the presence of defect centers in the solid-state materials. PL technique is a useful tool because of its high resolution and non-destructive nature that gives insight into defects types and their concentration. The aim of this study is to investigate the defect centers in sapphire subjected to very high densities of electronic excitation. PL and UV–vis absorption techniques are used in pristine and irradiated sapphires with different fluence of 200 MeV Ag8+ ions. Luminescence intensity of defect centers and impurity ion in irradiated sapphires are interpreted in terms of concentration of defects centers, cluster formation and annihilation process. UV–vis spectroscopy is used to identify the defect centers and their behavior on irradiation.
188
K.S. Jheeta et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 187–189 35000
The South African sapphire samples of sizes 5 mm · 5 mm · 1 mm were irradiated with 200 MeVAg8+ ion beam at room temperature under high vacuum (6 · 106 Torr) using the 15 UD pelleteron tandem accelerator at Nuclear Science Center, New Delhi, India. The ion flux was 109 ions/cm2 s. The samples were irradiated at fluences 1 · 1011, 5 · 1011, 1 · 1012, 5 · 1012, 7 · 1012 and 1 · 1013 ions/cm2, respectively. One unirradiated sample was kept as pristine. The ion beam fluence was measured by integrating the ion charge on the sample. The pristine and irradiated crystals were characterized with Photoluminescence and UV–vis optical absorption techniques. PL measurements were carried out using Mechelle-900 spectrograph in the range 200–800 nm, using 442 nm He–Cd laser excitation. Optical absorption was taken using a Hitachi 3300 ultraviolet–visible spectrophotometer in the range of 200–800 nm.
30000
3. Results and discussion Photoluminescence measurements are done for pristine and six samples of sapphire irradiated with 200 MeV Ag8+ ion beam in the fluence range 1 · 1011– 1 · 1013 ions/cm2. We have observed mainly two features one at 547 nm assigned to a F2þ 2 center (two oxygen vacancies with two electrons) [7] and a well-known sharp doublet at 693.9 nm and 695.3 nm, commonly known as R1 and R2 lines of Cr3+ [8]. We have also observed a peak at 485 nm and this probably corresponds to another defect centers (F+ of F+ centers). These centers grow up to fluence of 7 · 1012 ions/cm2 and then decrease faster similar to those F-center in alkali halides. In order to visualize and discuss these two features, we have divided our PL spectra into two parts. First part concerns with F2þ 2 defect center and is shown in Fig. 1 after magnification. Second part concern with R lines of Cr3+
4000
2+
F 2 defect center
13
1x10 12 7x10 12 5x10 12 1x10 11 5x10 11 1x10 Pristine
Luminescence intensity in a.u.
3500 3000
7x10
12
2500
1x10
13
2000
5x10
1500
1x10
1000
12
12
5x10
11
1x10
500
11
Pristine
0 450
500
550
600
650
Wavelength in nm
Fig. 1. Magnified view of PL spectra of pristine and irradiated sapphires.
Luminescence intensity in a.u.
2. Experimental
12
1x10
25000 20000
13
1x10
15000
12
5x10
10000
12
7x10
11
5x10
5000
pristine
11
1x10
0 688
692
696
700
704
708
Wavelength in nm
Fig. 2. Photoluminescence spectra of R1 and R2 lines of Cr3+ with varying fluence of Ag8+ ion.
ions and is shown in Fig. 2. From Fig. 1, one can see the increase in intensity corresponding to F2þ defect center 2 with fluence and reaches maximum at fluence 7 · 1012 ions/cm2. On further increase of fluence it starts decreasing. This particular behavior can be understood by assuming that the damage induced by ion irradiation increases the point defects concentrations of F+ defects up to fluence 7 · 1012 ions/cm2. The mobility of these defect centers increases as a result of enhanced concentration of these centers. This increases the probability of their aggregation thus forming F2þ 2 defect centers. These defect centers can be considered as aggregate of F+ defect center. Or one can say that F+ defect centers act as precursor for the generation of F2þ defect center [7]. A decrease in the intensity of this 2 defect after 7 · 1012 ion/cm2 can be attributed to an annihilation process of luminescence centers (F+ centers) resulting from a higher disorder induced by irradiation or to the irradiation induced amorphization as a result of cascades quenching [7]. Fig. 2 shows the intensity variation of the well-known sharp doublet (R1 and R2 lines) due to Cr3+ ion impurity, substituting Al site in sapphire. These two R lines are observed at 693.9 nm and 695.3 nm, respectively and assigned to a well-known radiative transition 2E ! 4A2 of the substitutional impurity Cr3+ ions. The intensity of R lines in pristine is extremely small, however, it increases with the fluence and becomes maximum at fluence 1 · 1012 ion/cm2 i.e. 80 times of that in pristine sapphire. At this fluence, contribution of F2þ 2 defect center is either negligible or starts developing. With the further increase of fluence their intensities decrease and once again starts increasing at 7 · 1012 ions/cm2. This gradual decrease in the intensities may be correlated with the generation of F2þ 2 defect center. The decrease in the R line intensities may be understood in terms of Cr ion pair formation as a result of stress induced by irradiation [9]. An increase in intensity of this feature beyond the fluence of 7 · 1012 ions/cm2 is difficult to interpret at this stage. However
K.S. Jheeta et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 187–189
absorption and luminescence bands associated with the transition metal impurities and associated charge transfer phenomenon are left for future.
3.0 2+ F 2 center
Absorption in a.u.
2.8
189
Cr 3+ bands
4. Conclusions
2.6
2.4 12
7x10 2.2
12
1x10
13
2.0
1x10
1.8
5x10 Pristine
11
400
450
500
550
Wavelength in nm
Fig. 3. UV–vis spectra of pristine and irradiated sapphire.
a detailed and more work is required to comment on this issue. Optical absorption spectra for pristine and irradiated sapphires are shown in Fig. 3. The spectrum of pristine sapphire is quite similar to the earlier reported one [8]. On irradiation the general shape of the spectra did not change much. However, features related to various defect centers can be seen. A feature appearing at 443 nm in irradiated samples, almost absent in pristine sample, may be assigned to F2þ 2 defect center. The intensity of this feature initially increased with fluence up to 7 · 1012 ions/cm2 and then starts decreasing with further increase of fluence. The similar behavior was seen in PL measurements. The absorption band corresponding to impurity ion Cr3+(in traces) were observed at 393 nm (4A2 ! 4F1) and 564 nm (4A2 ! 4F1) [10]. The intensities of these two bands behaved in the similar manner as observed in PL spectra. Other
Single crystals of sapphire were irradiated with varying fluence of 200 MeV Ag8+ ion beam and luminescence induced by these ions was measured. Mainly two luminescence features were observed such as F2þ 2 defect center and R lines of Cr3+. The fluence dependent behavior of these features showed that concentration of defect centers increased up to fluence 7 · 1012 ions/cm2, giving rise to cumulative response. On the contrary, a decrease in intensity is observed for still higher fluence, which could be due to higher degradation of the material and consequently annihilation of luminescence centers. References [1] J. Garcia-Guinea, J. Rubio, V. Correcher, F.J. Valle-Fuentes, Rad. Measur. 33 (2001) 653. [2] P. Jonnard, C. Bonnelle, G. Blaise, G. Remond, R.C. Carmes, J. Appl. Phys. 88 (11) (2000) 6413. [3] G.W. Arnold, W.D. Compton, Phys. Rev. Lett. 4 (1960) 66. [4] K.H. Lee, G.E. Holmberg, J.H. Crawford, Solid State Commun. 20 (1976) 183. [5] S. Furuno, N. Sasajina, K. Hojou, K. Izui, T. Muromura, T. Matsui, Nucl. Instr. and Meth. B 127–128 (1997) 181. [6] N. Sasajina, T. Matsui, S. Furuno, K. Hojou, H. Otsu, Nucl. Instr. and Meth. B 148 (1999) 745. [7] T. Mohanty, N.C. Mishra, F. Singh, U. Tiwari, D. Kanjilal, Nucl. Instr. and Meth. B 212 (2003) 179. [8] T.H. Maiman, R.H. Hoskins, I.J. D’Haenens, C.K. Asawa, V. Evthov, Phys. Rev. B 123 (1961) 1151. [9] R.C. Powell, Phys. Rev. 115 (1967) 265. [10] M. Ghamnia, C. Jardin, M. Bouslama, J. Electron. Spectrosc. Relat. Phenom. 133 (2003) 55.