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Ion beam induced luminescence studies of LiAlO2 using negative ions
Radiation Measurements 94 (2016) 49e52
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Ion beam induced luminescence studies of LiAlO2 using negative ions Menglin Qiu a, Yingjie Chu a, Guangfu Wang a, b, *, Mi Xu a, Li Zheng a a b
College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China Beijing Radiation Center, Beijing 100875, China
h i g h l i g h t s Ion beam induced luminescence measurement from LiAlO2 using negative ions is firstly carried out. Fþ, F and F2 centers emission intensity keeps decreasing with the increase of fluence. The intensity of Fn center emission grows obviously in the early stage of irradiation and then gradually decreased to the end. Results obtained verify the possibility of negative ion beam induced luminescence setup for real-time monitoring of irradiation damage.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2016 Received in revised form 13 September 2016 Accepted 15 September 2016 Available online 20 September 2016
Lithium aluminate (LiAlO2) is the candidate material for solid tritium breeder applied in the developing fusion reactors. The research of its defect behavior under ion irradiation was proceeded in the negative ions induced luminescence setup of the GIC4117 Tandem accelerator in Beijing Normal University. The ion beam induced luminescence (IBIL) measurement was performed by 20 keV H ions at room temperature. The luminescence spectra showed seven emission bands: the 4.55 eV may due to a self-trapped exciton (STE), the 4.06 eV and the 1.72 eV may due to impurity or intrinsic defect, the 3.54 eV due to F center, the 3.20 eV due to Fþ center, the 2.93 eV due to F2 center, the 2.30 eV due to F-center aggregates (Fn center), respectively. The intensity evolutions of each band with fluence were presented and the corresponding mechanisms were discussed. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Negative ion irradiation Luminescence LiAlO2 Defect
1. Introduction Luminescence is a sensitive technology to provide information about the defect or impurities in insulator and semiconductor materials, and the detection limit can reach ppb (parts per billion) level. Luminescence induced by ion beam irradiation is usually named Ion beam induced luminescence (IBIL) or ionoluminescence (IL). In addition, compared to other luminescence measurements, the obvious advantage of IBIL is that real-time IBIL measurement can provide in-situ information of damage evolution with ion irradiation dose (Townsend et al, 2007; Townsend, 2016; Townsend and Wang, 2013). However, up to now, most of IBIL measurements (Brooks et al, 2001; Quaranta et al, 2008; Townsend et al, 2012; Crespillo et al, 2016) have been carried out with positive ions. As the samples
* Corresponding author. College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China. E-mail address: [email protected] (G. Wang). http://dx.doi.org/10.1016/j.radmeas.2016.09.005 1350-4487/© 2016 Elsevier Ltd. All rights reserved.
for IBIL work are insulator or semiconductor, it is difficult to avoid the charge accumulation effect on the surface of samples. The charge accumulation effect may influence the intensity of incident ions and the forms of defects, which may have an impact on the luminescence measurement. In case of negative ion irradiation, both charges (incident on material surface by negative ion irradiation and released from material surface as secondary electrons) are negative, making the number of charges incoming to and outgoing from the surface well-balanced (Tsuji et al, 1998; Tsuji et al, 1997). Therefore, luminescence induced by negative ion is quite suitable for studying the irradiation damage in materials. In the practical application, IBIL measurement had been employed in the study of irradiation damage in lithium-based materials because these materials would be interacted with ions as the role of solid tritium breeder for fusion reactors. In the meantime, some structural characterization methods (such as cathodoluminescence, SEM and XRD) had been used on lithiumbased materials to study the structural changes and influence factors during the synthesis and irradiation (Mandal et al, 2010; lez and Correcher, 2014). The main Carrera et al, 2001; Gonza
50
M. Qiu et al. / Radiation Measurements 94 (2016) 49e52
irradiation defects of lithium-based materials were the anion vacancies, obtained by luminescence methods (Asaoka et al, 1992; 1991; Grishmanov et al, 1998; Grishmanov et al, 1997a; Moritani and Moriyama, 1997). Unlike the anion vacancies, the defects connected with basic cations were seldom discussed and studied in existing literatures (Holston et al., 2015a, 2015b). In all of these materials, LiAlO2 has good performances in materials compatibility, stability under irradiation, thermal conductivity and lithium oxide vapor pressures (Johnson et al., 1998; Billone et al., 1993). Nevertheless, the IBIL measurement of LiAlO2 (Asaoka et al., 1992) had been carried out only once, using positive ions. Furthermore, only the defects related to single oxygen vacancy were determined and the luminescence measurement under high fluence had not been carried out yet. Therefore, it's necessary to conduct the more fundamental researches, such as negative ion beam induced luminescence measurements, to study the detailed defect behaviors under ion irradiation which may be conducive to clarify the irradiation damage caused by secondary ions in fusion reactor. In this study, at the negative ion beam induced luminescence setup of GIC4117 tandem accelerator of Beijing Normal University, in-situ luminescence measurements from g-LiAlO2 under 20 keV H irradiation were carried out. The reliable spectra without the charge-up of the samples were obtained to study the damage processes during the negative ion irradiation. 2. Experiment Based on the original negative-ion implantation system at the GIC4117 tandem accelerator of Beijing Normal University, the negative ion beam induced luminescence setup was built. A schematic diagram of this IBIL set-up was given in Fig. 1. Negative ions were produced by the GIC860A Cs sputtering negative ion source. The ion beam was deflected by two 45 analyzing magnets. For collecting more photons, the angle between ion beam and the surface of sample was 45 and the optical fiber (diameter 600 mm) was perpendicular to the surface of sample. We have used an Ocean Optics spectrometer (QE PRO) for IBIL work. The available wavelength covers range of 197e982 nm with an entrance slit size of 100 mm. Pure single crystals of g-LiAlO2 (10 10 0.5 mm, <100> orientation), polished on the incident face, were purchased from MTI Corporation (KJ Group, China). The sample was irradiated by 20 keV H at room temperature. The beam current was about 900 nA with a diameter of 8 mm and beam current was measured by a Faraday cup before irradiation. The integration time was selected as
Fig. 1. Schematic diagram of the IBIL set-up.
5 s for alone of the spectra and 100 continuous spectra were collected. The total irradiation dose was about 5.6 1015 cm2. During the irradiation, the vacuum in the chamber was kept below 3 106 torr. 3. Result and discussion 3.1. Experimental result Fig. 2 shows the intensity evolution of wavelength spectra with fluence. It is obviously that the intensity around 540 nm increased at first and then decreased tardily. The intensities ranging from 250 nm to 450 nm showed a quick attenuation. To proceed to more quantitative materials science, the spectra data has been transformed into the energy domain (i.e. from wavelength (l) signals of I (l) dl versus l to the energy view of I (E) dE versus E) as only from this view point can make the deconvolution fits of signals. The conversion of the wavelength axis into terms of energy is uncomplicated (E ¼ hc/l). However, a correct transformation for the intensity axis has been made use of the intensity correction from wavelength to energy units of (I(E) ¼ I(l) *hc/E2) (Townsend and Wang, 2013; Crespillo et al., 2016; Wang and Townsend, 2013). Fig. 3 shows five IBIL spectra from LiAlO2 under 20 keV H irradiation at room temperature with the fluences of 5.6 1013,1.7 1014, 5.6 1014, 1.7 1015 and 5.6 1015 cm2, respectively. A broad emission band was observed ranging from about 1.5 eV to 5e V. During the H irradiation, the emission band centered 2.3 eV showed an obvious growth initially and then gradually decreased to the end. The broad emission band ranging from about 3 eV to 5 eV continually decayed to nearly disappear in the end of measurement. In the last stage of the measurement two luminescence peaks centered at about 1.7 and 2.3 eV can be clearly observed with a weak intensity. 3.2. Discussion As the spectra show an obvious overlap, Gauss fitting function has been applied to find the overlapping components. Seven peaks centered at 1.72, 2.30, 2.93, 3.20, 3.54, 4.06 and 4.55 eV are fitted as shown in Fig. 4. Referring to the present studies about the luminescence centers in LiAlO2, Li2O and other lithium based oxides (Asaoka et al, 1992; 1991; Grishmanov et al, 1998; Grishmanov et al, 1997a; Moritani and Moriyama, 1997), the emission band centered at 2.30 eV which existed obviously from start to finish was associated with the F center aggregates (Fn center) (Grishmanov et al., 1998). The Fn center was also reported in the luminescence measurements of Li2O, Li2TiO3, Li2ZrO3 and Li2SnO3 (Moritani and Moriyama, 1997). The 2.93 eV band was due to F2 center (Grishmanov et al., 1997a; Moritani and Moriyama, 1997). The peaks centered at 3.20 eV and 3.54 eV can be related to Fþ type center (oxygen vacancies with one trapped electron) and F type center (oxygen vacancies with two trapped electrons), respectively (Asaoka et al., 1991, 1992). The previous studies about 4.06 eV band were insufficient, suggested to impurity or intrinsic defect (Asaoka et al., 1991; Grishmanov et al., 1998). The 4.55 eV band may be assigned to the recombination of STE as well as several similar observations in other lithium based oxides (Grishmanov et al., 1997b, 1998; Moritani and Moriyama, 1997), but up to now a well defined origin of this band is still to be established. A weak unreported emission band centered at 1.72 eV was observed in this work, which may be due to original impurity Ti3þ (2E/2T) or Cr3þ (2E2/4A2) similar in Al2O3 (Crespillo r et al., 2001). et al., 2016; Molna Fig. 5 shows the evolution of the decomposed emission bands intensities as a function of fluence. We can find that the 2.93, 3.20,
M. Qiu et al. / Radiation Measurements 94 (2016) 49e52
51
Fig. 2. IBIL wavelength spectra from LiAlO2 under 20 keV H irradiation at various fluences.
Fig. 3. Normalized IBIL energy spectra from LiAlO2 under 20 keV H irradiation at five different fluences.
3.54, 4.06 and 4.55 eV bands have a similar evolution behavior. These luminescence centers kept decreasing to the end with a rapid decay in the early stage (fluence4e7 1014 cm2, approximately). There into, it is worth noting that the 2.93 eV band showed a comparatively slow rate of decay. The intensities of 3.20, 3.54 and 4.06 eV luminescence centers attenuated close to zero after 1.7 1015 cm2 and the 4.55 eV band almost disappeared at a fluence of 7 1014 cm2, but the 2.93 eV emission band maintained a weak intensity after the incipient decrease. In the meantime, the intensity of 2.30 eV emission band increased rapidly at first to a maximum value at the fluence of around 4 1014 cm2, then decayed steadily to the end. Other than the intensity evolutions above, the intensity of 1.72 band seemingly kept constant or decaying with a fairly slower rate. The literatures now available about in-situ luminescence studies (using Heþ ions) of pure LiAlO2 (Asaoka et al., 1992) showed the
Fig. 4. The spectrum induced by H at 20 keV at a fluence of 5.6 1013 cm2. Green lines indicate deconvolution by Gaussian function. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
main emission bands were related to the single oxygen vacancies (F and Fþ) with the formation of point defects. The study about oxygen vacancy aggregation in LiAlO2 caused by high flux was not proceeded yet. However, several similar phenomena concerning the evolution of oxygen vacancy defects were observed in the IBIL measurement of Li2O under high flux radiation (Asaoka et al, 1991; Grishmanov et al, 1998; Grishmanov et al, 1997a). Apart from a small number of defects present before irradiation, the majority of defects resulting in the luminescence were generated by ion bombardment. Due to the ionization and atomic displacement, an increasing number of oxygen vacancies were produced during the irradiation. The reduction of F and Fþ band intensities may be ascribed to a variety of mechanisms. As one F2 center needs two single oxygen vacancy centers (F center or Fþ center) to form, the radiative decay of F and Fþ centers was
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M. Qiu et al. / Radiation Measurements 94 (2016) 49e52
The experiment results show that the setup could provide the high sensitive photon detection and real-time measurements offer the detailed information about the evolution of irradiation damage. It brings us the hope in monitoring the interaction between energetic ions and materials, especially the materials applied under irradiation like present fission reactor and the future nuclear fusion reactors. More IBIL measurements using negative ions and contrastive analysis with the measurements using positive ions are expected in the future.
10
4.55 eV 4.06 eV 3.54 eV 3.20 eV 2.93 eV 2.30 eV 1.72 eV
Normalized yield (a.u.)
8
6
4
References 2
0
0
1
2
3
4 15
5
-2
Fluence (×10 cm ) Fig. 5. Evolutions of the luminescence intensity of the decomposed emission bands as a function of the irradiation fluence during 20 keV H irradiation.
understandable. The decay of F2 centers may due to the formation of higher oxygen vacancy clusters, which can be evidently observed as the incipient increase (fluence around 4 1014 cm2) of Fn center (2.30 eV). The subsequent decrease of Fn center intensity may be due to the further radiation damage like dislocation or amorphization. Furthermore, the growing proportion of nonradiative recombination mechanisms and the effect of stress produced by irradiation were also the causes of attenuation (Crespillo et al 2016; Grishmanov et al, 1997b; Ghamdi and Townsend, 1990). The excepted intensity growths of F, Fþ and F2 centers at the early stage of irradiation (similar to the behavior of Fn center), corresponding to the formation process of each center, were not observed in this work. This phenomenon may be attributed to the high ion flux rate (1.12 1013 cm2 s1). The high ion flux rate can result in the fast formation of vacancy aggregation and lower the critical fluence of amorphization. Moreover, due to the quite similar evolution behavior of the 2.93, 3.20, 3.54, 4.06 and 4.55 eV bands, these luminescence centers may have a similar type of origin (related to the oxygen vacancy). 4. Summary A negative ion beam induced luminescence setup of GIC4117 tandem accelerator of Beijing Normal University was introduced and in-situ luminescence measurement was carried out for the characterization of radiation damage in materials. The IBIL studies of single crystals g-LiAlO2 under high flux radiation, using 20 keV H, were presented. The IBIL spectra show seven emission bands, which can be attributed to the recombination of e-h pairs at oxygen vacancy centers (F, Fþ, F2 and Fn center), STE or impurity. The F, Fþ and F2 emission bands show a similar evolution behavior that the intensities keep decrease to the end with a rapid decay in the early stage. The intensity of Fn center increases to the maximum at first and then decays steadily to the end. These phenomena can be attributed to the formation of oxygen vacancies during the irradiation and several attenuation mechanisms such as: the formation of defect aggregates, the growing proportion of non-radiative recombination mechanisms, or the effect of stress produced by irradiation.
Asaoka, Y., Moriyama, H., Iwasaki, K., Moritani, K., Ito, Y., 1991. In-situ luminescence measurement of irradiation defects in lithium oxide. J. Nucl. Mater. 183 (3), 174e179. Asaoka, Y., Moriyama, H., Iwasaki, K., Moritani, K., Ito, Y., 1992. In-situ luminescence measurement of lithium aluminate under ion beam irradiation. J. Nucl. Mater. (s 191e194), 268e271. Billone, M.C., Dienst, W., Flament, T., Lorenzetto, P., Noda, K., Roux, N., 1993. Iter solid breeder blanket materials database. Breed. Blankets 3e8. Brooks, R.J., Hole, D.E., Townsend, P.D., 2001. Ion beam induced luminescence of materials. Nucl. Instrum. Methods Phys. Res. B 190 (01), 136e140. nez-Becerril, J., Basurto, R., Arenas, J., B. E. L. M., Bulbulian, S., Carrera, L.M., Jime et al., 2001. Tritium recovery from nanostructured LiAlO2. J. Nucl. Mater. 299 (3), 242e249. Crespillo, M.L., Graham, J.T., Zhang, Y., Weber, W.J., 2016. In-situ luminescence monitoring of ion-induced damage evolution in SiO2 and Al2O3. J. Luminescence 172, 208e218. Ghamdi, A.A., Townsend, P.D., 1990. Ion beam excited luminescence of sapphire. Nucl. Instrum. Methods Phys. Res. 46 (1), 133e136. Gonz alez, M., Correcher, V., 2014. On the cathodoluminescence and thermoluminescence emission of lithium titanate ceramics. J. Nucl. Mater. 445 (1e3), 149e153. Grishmanov, V., Tanaka, S., Yoneoka, T., 1997. Influence of irradiation defects and atmosphere on luminescence and tritium release from Li2O. J. Nucl. Mater. 248 (1), 128e131. Grishmanov, V., Tanaka, S., Yoneoka, T., 1997. Defect luminescence in Li2O crystal under g-ray irradiation. Radiat. Eff. Defect. Solids 143 (2), 203e211. Grishmanov, V., Tanaka, S., Ogikubo, K., Hu, Q., 1998. Luminescence in Li2O under ion beam irradiation. Nucl. Instrum. Methods Phys. Res. 141, 392e397. Holston, M.S., Ferguson, I.P., Mcclory, J.W., Giles, N.C., Halliburton, L.E., 2015. Oxygen vacancies in LiAlO2, crystals. Phys. Rev. B 92 (14). Holston, M.S., Mcclory, J.W., Giles, N.C., Halliburton, L.E., 2015. Radiation-induced defects in LiAlO2, crystals: holes trapped by lithium vacancies and their role in thermoluminescence. J. Lumin. 160, 43e49. Johnson, C.E., Noda, K., Roux, N., 1998. Ceramic breeder materials: status and needs. J. Nucl. Mater. (s 258e263), 140e148. Mandal, D., Shenoi, M.R.K., Ghosh, S.K., 2010. Synthesis & fabrication of lithiumtitanate pebbles for iter breeding blanket by solid state reaction & spherodization. Fusion Eng. Des. 85 (5), 819e823. r, G., Benabdesselam, M., Borossay, J., Lapraz, D., Iacconi, P., Kortov, V.S., et al., Molna 2001. Photoluminescence and thermoluminescence of titanium ions in sapphire crystals. Radiat. Meas. 33 (5), 663e667. Moritani, K., Moriyama, H., 1997. In situ luminescence measurement of irradiation defects in ternary lithium ceramics under ion beam irradiation. J. Nucl. Mater. 248 (4), 132e139. Quaranta, A., Dran, J.C., Salomon, J., Tonezzer, M., Scian, C., Beck, L., et al., 2008. Ion beam induced luminescence on white inorganic pigments for paintings. Nucl. Instrum. Methods Phys. Res. 266 (10), 2301e2305. Townsend, P.D., 2012. Variations on the use of ion beam luminescence. Nucl. Instrum. Methods Phys. Res. 286 (9), 35e39. Townsend, P.D., 2016. Unexploited information from luminescence spectra, 3 (1), 15e24. Townsend, P.D., Wang, Y., 2013. Defect studies using advances with ion beam excited luminescence. Energy Proced. 41, 64e79. Townsend, P.D., Khanlary, M., Hole, D.E., 2007. Information obtainable from ion beam luminescence. Surf. Coat. Technol. 201 (19), 8160e8164. Tsuji, H., Ishikawa, J., Ikeda, S., Gotoh, Y., 1997. Slightly negative surface potential and charging model of insulator in the negative-ion implantation. Nucl. Instrum. Methods Phys. Res. 127, 278e281. Tsuji, H., Gotoh, Y., Ishikawa, J., 1998. Secondary electron emission and surface potential of sio 2, film surface by negative ion bombardment. Nucl. Instrum. Methods Phys. Res. 141 (1e4), 645e651. Wang, Y., Townsend, P.D., 2013. Potential problems in collection and data processing of luminescence signals. J. Luminescence 142 (142), 202e211.
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Ion beam induced luminescence studies of LiAlO2 using negative ions Menglin Qiu a, Yingjie Chu a, Guangfu Wang a, b, *, Mi Xu a, Li Zheng a a b
College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China Beijing Radiation Center, Beijing 100875, China
h i g h l i g h t s Ion beam induced luminescence measurement from LiAlO2 using negative ions is firstly carried out. Fþ, F and F2 centers emission intensity keeps decreasing with the increase of fluence. The intensity of Fn center emission grows obviously in the early stage of irradiation and then gradually decreased to the end. Results obtained verify the possibility of negative ion beam induced luminescence setup for real-time monitoring of irradiation damage.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2016 Received in revised form 13 September 2016 Accepted 15 September 2016 Available online 20 September 2016
Lithium aluminate (LiAlO2) is the candidate material for solid tritium breeder applied in the developing fusion reactors. The research of its defect behavior under ion irradiation was proceeded in the negative ions induced luminescence setup of the GIC4117 Tandem accelerator in Beijing Normal University. The ion beam induced luminescence (IBIL) measurement was performed by 20 keV H ions at room temperature. The luminescence spectra showed seven emission bands: the 4.55 eV may due to a self-trapped exciton (STE), the 4.06 eV and the 1.72 eV may due to impurity or intrinsic defect, the 3.54 eV due to F center, the 3.20 eV due to Fþ center, the 2.93 eV due to F2 center, the 2.30 eV due to F-center aggregates (Fn center), respectively. The intensity evolutions of each band with fluence were presented and the corresponding mechanisms were discussed. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Negative ion irradiation Luminescence LiAlO2 Defect
1. Introduction Luminescence is a sensitive technology to provide information about the defect or impurities in insulator and semiconductor materials, and the detection limit can reach ppb (parts per billion) level. Luminescence induced by ion beam irradiation is usually named Ion beam induced luminescence (IBIL) or ionoluminescence (IL). In addition, compared to other luminescence measurements, the obvious advantage of IBIL is that real-time IBIL measurement can provide in-situ information of damage evolution with ion irradiation dose (Townsend et al, 2007; Townsend, 2016; Townsend and Wang, 2013). However, up to now, most of IBIL measurements (Brooks et al, 2001; Quaranta et al, 2008; Townsend et al, 2012; Crespillo et al, 2016) have been carried out with positive ions. As the samples
* Corresponding author. College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China. E-mail address: [email protected] (G. Wang). http://dx.doi.org/10.1016/j.radmeas.2016.09.005 1350-4487/© 2016 Elsevier Ltd. All rights reserved.
for IBIL work are insulator or semiconductor, it is difficult to avoid the charge accumulation effect on the surface of samples. The charge accumulation effect may influence the intensity of incident ions and the forms of defects, which may have an impact on the luminescence measurement. In case of negative ion irradiation, both charges (incident on material surface by negative ion irradiation and released from material surface as secondary electrons) are negative, making the number of charges incoming to and outgoing from the surface well-balanced (Tsuji et al, 1998; Tsuji et al, 1997). Therefore, luminescence induced by negative ion is quite suitable for studying the irradiation damage in materials. In the practical application, IBIL measurement had been employed in the study of irradiation damage in lithium-based materials because these materials would be interacted with ions as the role of solid tritium breeder for fusion reactors. In the meantime, some structural characterization methods (such as cathodoluminescence, SEM and XRD) had been used on lithiumbased materials to study the structural changes and influence factors during the synthesis and irradiation (Mandal et al, 2010; lez and Correcher, 2014). The main Carrera et al, 2001; Gonza
50
M. Qiu et al. / Radiation Measurements 94 (2016) 49e52
irradiation defects of lithium-based materials were the anion vacancies, obtained by luminescence methods (Asaoka et al, 1992; 1991; Grishmanov et al, 1998; Grishmanov et al, 1997a; Moritani and Moriyama, 1997). Unlike the anion vacancies, the defects connected with basic cations were seldom discussed and studied in existing literatures (Holston et al., 2015a, 2015b). In all of these materials, LiAlO2 has good performances in materials compatibility, stability under irradiation, thermal conductivity and lithium oxide vapor pressures (Johnson et al., 1998; Billone et al., 1993). Nevertheless, the IBIL measurement of LiAlO2 (Asaoka et al., 1992) had been carried out only once, using positive ions. Furthermore, only the defects related to single oxygen vacancy were determined and the luminescence measurement under high fluence had not been carried out yet. Therefore, it's necessary to conduct the more fundamental researches, such as negative ion beam induced luminescence measurements, to study the detailed defect behaviors under ion irradiation which may be conducive to clarify the irradiation damage caused by secondary ions in fusion reactor. In this study, at the negative ion beam induced luminescence setup of GIC4117 tandem accelerator of Beijing Normal University, in-situ luminescence measurements from g-LiAlO2 under 20 keV H irradiation were carried out. The reliable spectra without the charge-up of the samples were obtained to study the damage processes during the negative ion irradiation. 2. Experiment Based on the original negative-ion implantation system at the GIC4117 tandem accelerator of Beijing Normal University, the negative ion beam induced luminescence setup was built. A schematic diagram of this IBIL set-up was given in Fig. 1. Negative ions were produced by the GIC860A Cs sputtering negative ion source. The ion beam was deflected by two 45 analyzing magnets. For collecting more photons, the angle between ion beam and the surface of sample was 45 and the optical fiber (diameter 600 mm) was perpendicular to the surface of sample. We have used an Ocean Optics spectrometer (QE PRO) for IBIL work. The available wavelength covers range of 197e982 nm with an entrance slit size of 100 mm. Pure single crystals of g-LiAlO2 (10 10 0.5 mm, <100> orientation), polished on the incident face, were purchased from MTI Corporation (KJ Group, China). The sample was irradiated by 20 keV H at room temperature. The beam current was about 900 nA with a diameter of 8 mm and beam current was measured by a Faraday cup before irradiation. The integration time was selected as
Fig. 1. Schematic diagram of the IBIL set-up.
5 s for alone of the spectra and 100 continuous spectra were collected. The total irradiation dose was about 5.6 1015 cm2. During the irradiation, the vacuum in the chamber was kept below 3 106 torr. 3. Result and discussion 3.1. Experimental result Fig. 2 shows the intensity evolution of wavelength spectra with fluence. It is obviously that the intensity around 540 nm increased at first and then decreased tardily. The intensities ranging from 250 nm to 450 nm showed a quick attenuation. To proceed to more quantitative materials science, the spectra data has been transformed into the energy domain (i.e. from wavelength (l) signals of I (l) dl versus l to the energy view of I (E) dE versus E) as only from this view point can make the deconvolution fits of signals. The conversion of the wavelength axis into terms of energy is uncomplicated (E ¼ hc/l). However, a correct transformation for the intensity axis has been made use of the intensity correction from wavelength to energy units of (I(E) ¼ I(l) *hc/E2) (Townsend and Wang, 2013; Crespillo et al., 2016; Wang and Townsend, 2013). Fig. 3 shows five IBIL spectra from LiAlO2 under 20 keV H irradiation at room temperature with the fluences of 5.6 1013,1.7 1014, 5.6 1014, 1.7 1015 and 5.6 1015 cm2, respectively. A broad emission band was observed ranging from about 1.5 eV to 5e V. During the H irradiation, the emission band centered 2.3 eV showed an obvious growth initially and then gradually decreased to the end. The broad emission band ranging from about 3 eV to 5 eV continually decayed to nearly disappear in the end of measurement. In the last stage of the measurement two luminescence peaks centered at about 1.7 and 2.3 eV can be clearly observed with a weak intensity. 3.2. Discussion As the spectra show an obvious overlap, Gauss fitting function has been applied to find the overlapping components. Seven peaks centered at 1.72, 2.30, 2.93, 3.20, 3.54, 4.06 and 4.55 eV are fitted as shown in Fig. 4. Referring to the present studies about the luminescence centers in LiAlO2, Li2O and other lithium based oxides (Asaoka et al, 1992; 1991; Grishmanov et al, 1998; Grishmanov et al, 1997a; Moritani and Moriyama, 1997), the emission band centered at 2.30 eV which existed obviously from start to finish was associated with the F center aggregates (Fn center) (Grishmanov et al., 1998). The Fn center was also reported in the luminescence measurements of Li2O, Li2TiO3, Li2ZrO3 and Li2SnO3 (Moritani and Moriyama, 1997). The 2.93 eV band was due to F2 center (Grishmanov et al., 1997a; Moritani and Moriyama, 1997). The peaks centered at 3.20 eV and 3.54 eV can be related to Fþ type center (oxygen vacancies with one trapped electron) and F type center (oxygen vacancies with two trapped electrons), respectively (Asaoka et al., 1991, 1992). The previous studies about 4.06 eV band were insufficient, suggested to impurity or intrinsic defect (Asaoka et al., 1991; Grishmanov et al., 1998). The 4.55 eV band may be assigned to the recombination of STE as well as several similar observations in other lithium based oxides (Grishmanov et al., 1997b, 1998; Moritani and Moriyama, 1997), but up to now a well defined origin of this band is still to be established. A weak unreported emission band centered at 1.72 eV was observed in this work, which may be due to original impurity Ti3þ (2E/2T) or Cr3þ (2E2/4A2) similar in Al2O3 (Crespillo r et al., 2001). et al., 2016; Molna Fig. 5 shows the evolution of the decomposed emission bands intensities as a function of fluence. We can find that the 2.93, 3.20,
M. Qiu et al. / Radiation Measurements 94 (2016) 49e52
51
Fig. 2. IBIL wavelength spectra from LiAlO2 under 20 keV H irradiation at various fluences.
Fig. 3. Normalized IBIL energy spectra from LiAlO2 under 20 keV H irradiation at five different fluences.
3.54, 4.06 and 4.55 eV bands have a similar evolution behavior. These luminescence centers kept decreasing to the end with a rapid decay in the early stage (fluence4e7 1014 cm2, approximately). There into, it is worth noting that the 2.93 eV band showed a comparatively slow rate of decay. The intensities of 3.20, 3.54 and 4.06 eV luminescence centers attenuated close to zero after 1.7 1015 cm2 and the 4.55 eV band almost disappeared at a fluence of 7 1014 cm2, but the 2.93 eV emission band maintained a weak intensity after the incipient decrease. In the meantime, the intensity of 2.30 eV emission band increased rapidly at first to a maximum value at the fluence of around 4 1014 cm2, then decayed steadily to the end. Other than the intensity evolutions above, the intensity of 1.72 band seemingly kept constant or decaying with a fairly slower rate. The literatures now available about in-situ luminescence studies (using Heþ ions) of pure LiAlO2 (Asaoka et al., 1992) showed the
Fig. 4. The spectrum induced by H at 20 keV at a fluence of 5.6 1013 cm2. Green lines indicate deconvolution by Gaussian function. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
main emission bands were related to the single oxygen vacancies (F and Fþ) with the formation of point defects. The study about oxygen vacancy aggregation in LiAlO2 caused by high flux was not proceeded yet. However, several similar phenomena concerning the evolution of oxygen vacancy defects were observed in the IBIL measurement of Li2O under high flux radiation (Asaoka et al, 1991; Grishmanov et al, 1998; Grishmanov et al, 1997a). Apart from a small number of defects present before irradiation, the majority of defects resulting in the luminescence were generated by ion bombardment. Due to the ionization and atomic displacement, an increasing number of oxygen vacancies were produced during the irradiation. The reduction of F and Fþ band intensities may be ascribed to a variety of mechanisms. As one F2 center needs two single oxygen vacancy centers (F center or Fþ center) to form, the radiative decay of F and Fþ centers was
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The experiment results show that the setup could provide the high sensitive photon detection and real-time measurements offer the detailed information about the evolution of irradiation damage. It brings us the hope in monitoring the interaction between energetic ions and materials, especially the materials applied under irradiation like present fission reactor and the future nuclear fusion reactors. More IBIL measurements using negative ions and contrastive analysis with the measurements using positive ions are expected in the future.
10
4.55 eV 4.06 eV 3.54 eV 3.20 eV 2.93 eV 2.30 eV 1.72 eV
Normalized yield (a.u.)
8
6
4
References 2
0
0
1
2
3
4 15
5
-2
Fluence (×10 cm ) Fig. 5. Evolutions of the luminescence intensity of the decomposed emission bands as a function of the irradiation fluence during 20 keV H irradiation.
understandable. The decay of F2 centers may due to the formation of higher oxygen vacancy clusters, which can be evidently observed as the incipient increase (fluence around 4 1014 cm2) of Fn center (2.30 eV). The subsequent decrease of Fn center intensity may be due to the further radiation damage like dislocation or amorphization. Furthermore, the growing proportion of nonradiative recombination mechanisms and the effect of stress produced by irradiation were also the causes of attenuation (Crespillo et al 2016; Grishmanov et al, 1997b; Ghamdi and Townsend, 1990). The excepted intensity growths of F, Fþ and F2 centers at the early stage of irradiation (similar to the behavior of Fn center), corresponding to the formation process of each center, were not observed in this work. This phenomenon may be attributed to the high ion flux rate (1.12 1013 cm2 s1). The high ion flux rate can result in the fast formation of vacancy aggregation and lower the critical fluence of amorphization. Moreover, due to the quite similar evolution behavior of the 2.93, 3.20, 3.54, 4.06 and 4.55 eV bands, these luminescence centers may have a similar type of origin (related to the oxygen vacancy). 4. Summary A negative ion beam induced luminescence setup of GIC4117 tandem accelerator of Beijing Normal University was introduced and in-situ luminescence measurement was carried out for the characterization of radiation damage in materials. The IBIL studies of single crystals g-LiAlO2 under high flux radiation, using 20 keV H, were presented. The IBIL spectra show seven emission bands, which can be attributed to the recombination of e-h pairs at oxygen vacancy centers (F, Fþ, F2 and Fn center), STE or impurity. The F, Fþ and F2 emission bands show a similar evolution behavior that the intensities keep decrease to the end with a rapid decay in the early stage. The intensity of Fn center increases to the maximum at first and then decays steadily to the end. These phenomena can be attributed to the formation of oxygen vacancies during the irradiation and several attenuation mechanisms such as: the formation of defect aggregates, the growing proportion of non-radiative recombination mechanisms, or the effect of stress produced by irradiation.
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