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The influence of neutron-irradiation at low temperatures on the dielectric parameters of 3C-SiC
Physica B 439 (2014) 169–172
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
The influence of neutron-irradiation at low temperatures on the dielectric parameters of 3C-SiC J.A.A. Engelbrecht a,n, G. Deyzel a, E.G. Minnaar a, W.E. Goosen a, I.J. van Rooyen b a b
Physics Department, Nelson Mandela Metropolitan University, P.O. Box 77000, Port Elizabeth 6031, South Africa Fuel Performance and Design Department, Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-6188, USA
art ic l e i nf o
a b s t r a c t
Available online 18 November 2013
3C-SiC wafers were irradiated with neutrons of various fluences and at low (200–400 1C) irradiation temperatures. Fourier transform infrared (FTIR) reflectance spectra were obtained for the samples, and the spectra used to extract the dielectric parameters for each specimen, using statistical curve-fitting procedures. Analysis of all data revealed trends in reflectance peak heights as well as in the dielectric parameters. The surface roughness of the irradiated samples was measured by atomic force spectroscopy (AFM) and certain trends could be ascribed to surface roughness. & 2013 Elsevier B.V. All rights reserved.
Keywords: 3C-SiC Neutron irradiation Infrared reflectance
1. Introduction SiC is used as a containment layer in the triple-coated isotropic (TRISO) layers in the new generation nuclear reactors [1]. As such, the material is expected to be subjected to high fluences of various nuclear fission particles, as well as to various temperatures during the operation of the nuclear reactor. Several investigations into the effects of irradiation on SiC have already been performed [2–13]. This article reports on the analysis of 3C-SiC wafers irradiated at various fluences and at low irradiation temperatures. Infrared spectroscopy, complemented by curve-fitting procedures was used to obtain trends in the observed spectra, as well as in the dielectric parameters extracted from the reflectance spectra.
2. Experimental Samples of SiC in the shape of discs were irradiated at irradiation temperatures of 200 1C, 300 1C and 400 1C, and at fluences ranging from 5 1019 to 1.6 1021 n/cm2. Irradiation of samples was done at the Oak Ridge National Laboratory (ORNL), USA. Information related to specimens analysed in this report is contained in Table 1. A Bruker 80V FTIR/Raman spectrometer, fitted with a Pike 10 Spec specular reflection unit enabling near-normal incidence, was employed to obtain infrared reflectance spectra from samples taking 32 scans at a resolution of 8 cm 1. Surface roughness of the samples was measured using a CSM Instruments Nano-indenter, n
Corresponding author. Tel.: þ 27 41 504 2186; fax: þ 27 41 504 2573. E-mail address: [email protected] (J.A.A. Engelbrecht).
0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.10.059
fitted with an atomic force microscope. Sample areas of size 25 25 μm2 were analysed by AFM. 3. Results and discussion 3.1. Reflectivity In all cases where the irradiation temperature was kept constant and the fluence was increased, it was found that the reflectivity maximum decreased for the higher fluences, as observed in Fig. 1. However, it appears as if the reflectivity reached a limiting value for very high irradiations (Fig. 2). The maximum height of the reflectance peaks of samples irradiated at the same temperatures but with high and low fluences are plotted in Fig. 3. It is clear that the peak heights of the samples irradiated at higher fluences increase with increasing temperature, while the peak heights decrease for the lower fluences. It is not clear exactly what causes these trends, and further work may require transmission electron microscopy to establish whether the morphology of the samples recovered at the higher fluences and temperatures. This hypothesis seems to be supported by the changes in the dielectric parameters of the samples, which has been demonstrated to be linked to surface roughness [14]. 3.2. Dielectric parameters Fig. 4 displays the variation of the high frequency dielectric constants ε1 of the relevant specimens, and reveals a slight decrease in values for low fluences and an increase for high fluences. Variations in the phonon damping constant Γ is shown
170
J.A.A. Engelbrecht et al. / Physica B 439 (2014) 169–172
Table 1 Irradiation temperatures and fluences of specimens analysed. Sample number Irradiation temp. (1C) Fluence ( 1021) Total Becquerel (Bq) 1 2 3 4 5 9 10
Unirradiated 200 200 300 300 400 400
0 0.056 0.097 0.6 1.4 0.092 0.48
0 13 23 182 425 22 114
Fig. 3. Change in the peak height of the reflectivity with change in irradiation temperature and neutron fluence.
Fig. 1. Variation of reflectivity as function of irradiation temperature and neutron fluence.
Fig. 4. Variation of the high frequency dielectric constant with neutron fluence and irradiation temperature.
Fig. 2. Variation of reflectivity at a constant irradiation temperature, but with increasing neutron irradiation fluence.
in Fig. 5, where changes also follow the previously observed trend, namely an increase in the values for Γ. It has earlier been shown that for rougher sample surfaces ε1 will decrease and Γ will increase [14]. This is clearly the case in particular for the lower fluences in the present study. The changes in the longitudinal (ωL) and transverse (ωT) phonon frequencies are contained in Fig. 6. These are again in agreement with earlier observations; in particular again the (slight) decrease in ωT for the high fluence. It has been noted that ωT is not influenced much by surface morphology [14]. Likewise,
Fig. 5. The effect of irradiation temperature and neutron fluence on the phonon damping constant.
J.A.A. Engelbrecht et al. / Physica B 439 (2014) 169–172
no trends were observed for the free-carrier damping constant γ and the plasma resonance frequency ωp, as expected.
3.3. Surface morphology Three-dimensional images of the surfaces of the various samples analysed are shown in Fig. 7. The root mean square values of the surface roughness as determined by the AFM are plotted in Fig. 8. When comparing Figs. 3 and 8, it is clear that as the surface roughness decreases, the maximum height of the reflectivity
Fig. 6. Longitudinal and transverse phonon frequencies as functions of temperature and neutron fluence.
171
increases, and vice versa (see e.g. the respective trends for high or low fluence samples). At this stage it is not clear what causes the changes in reflectivity with changes in the irradiation temperatures, as depicted in Fig. 8. More so since there seems to be opposite trends between high and low fluence of implantation. It was suggested by Brink et al. that the decrease in maximum reflectivity of irradiated SiC was due to the increase in the values of the phonon damping constants ΓTO and ΓLO [11]. This is confirmed by the present investigation, where an increase in Γ was found with decreasing reflectivity. In addition, earlier research has pointed to the amorphization of SiC during irradiation at lower temperatures [2–4,6], with
Fig.8. Trends in RMS surface roughness as function of irradiation temperature of neutron implanted samples.
Fig. 7. Three-dimensional AFM images of the sample surfaces of the various discs that were irradiated.
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J.A.A. Engelbrecht et al. / Physica B 439 (2014) 169–172
decreasing amorphization as the irradiation temperature increases. Analysis of the defects present in irradiated SiC indicated that at low irradiation temperatures point defects in the form of black spots were formed, which gradually changed into defect clusters and dislocation loops of decreasing numbers as the irradiation temperature increased. The influence of such defects on the reflectivity and dielectric parameters are not known at present, and more work is required. Samples implanted at higher irradiation temperatures should also be evaluated by infrared spectroscopy. In summary, there are detectable trends in both the reflectivity and dielectric parameters of SiC samples implanted at various fluences and low irradiation temperatures. Trends in the dielectric parameters are in agreement with earlier observations for GaAs samples. Acknowledgements Ms. D. Venter and Mr. R. Dix-Peek are thanked for technical assistance with this project. The National Research Foundation of South Africa is gratefully acknowledged for funding provided. Any opinion, findings and conclusions or recommendations expressed in this article are those of the authors, and therefore the NRF does not accept liability in regards thereto.
References [1] W. Wacholz, International Working Group on Gas-Cooled Reactors (IWGGCAR), vol. 19, 1998, p. 61. [2] L.L. Snead, S.J. Zinkle, J.C. Hay, M.C. Osborne, Nucl. Instrum. Methods B 141 (1998) 123. [3] L.L. Snead, M.C. Osborne, R.A. Lowden, J. Strizak, R.J. Shinavski, K.L. More, W.S. Eatherly, J Bailey, A.M. Williams, J. Nucl. Mater. 253 (1998) 20. [4] L.L. Snead, J.C. Hay, J. Nucl. Mater. 273 (1999) 213. [5] A.I. Ryazanov, A.V. Klaptsov, A. Kohyama, H. Kishimoto, J. Nucl. Mater. 307-311 (2002) 1107. [6] G. Newsome, L.L. Snead, T. Hinoki, Y. Katho, D. Peters, J. Nucl. Mater. 371 (2007) 76. [7] K. Katoh, N. Hashimoto, S. Kondo, L.L. Snead, A. Kohyama, J. Nucl. Mater. 351 (2006) 228. [8] S. Kondo, Y. Katoh, L.L. Snead, J. Nucl. Mater. 382 (2008) 160. [9] Y. Katoh, S. Kondo, L.L. Snead, J. Nucl. Mater. 382 (2008) 170. [10] S. Kondo, Y. Katoh, L.L. Snead, Appl. Phys. Lett. 93 (2008) 163110. [11] D.J. Brink, J.B. Malherbe, J. Camassel, Nucl. Instrum. Methods Phys. B 267 (2009) 2716. [12] Y. Katoh, T. Nozawa, L.L. Snead, K. Ozawa, H. Tanigawa, J. Nucl. Mater. 417 (2011) 400. [13] A. Janse van Vuuren, Radiation Damage in GaAs and SiC, M.Sc. Dissertation, Nelson Mandela Metropolitan University, South Africa, 2011. [14] J.A.A. Engelbrecht, N.G. Hashe, K.T. Hillie, C.H. Claassens, Physica B 401-402 (2007) 238.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
The influence of neutron-irradiation at low temperatures on the dielectric parameters of 3C-SiC J.A.A. Engelbrecht a,n, G. Deyzel a, E.G. Minnaar a, W.E. Goosen a, I.J. van Rooyen b a b
Physics Department, Nelson Mandela Metropolitan University, P.O. Box 77000, Port Elizabeth 6031, South Africa Fuel Performance and Design Department, Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-6188, USA
art ic l e i nf o
a b s t r a c t
Available online 18 November 2013
3C-SiC wafers were irradiated with neutrons of various fluences and at low (200–400 1C) irradiation temperatures. Fourier transform infrared (FTIR) reflectance spectra were obtained for the samples, and the spectra used to extract the dielectric parameters for each specimen, using statistical curve-fitting procedures. Analysis of all data revealed trends in reflectance peak heights as well as in the dielectric parameters. The surface roughness of the irradiated samples was measured by atomic force spectroscopy (AFM) and certain trends could be ascribed to surface roughness. & 2013 Elsevier B.V. All rights reserved.
Keywords: 3C-SiC Neutron irradiation Infrared reflectance
1. Introduction SiC is used as a containment layer in the triple-coated isotropic (TRISO) layers in the new generation nuclear reactors [1]. As such, the material is expected to be subjected to high fluences of various nuclear fission particles, as well as to various temperatures during the operation of the nuclear reactor. Several investigations into the effects of irradiation on SiC have already been performed [2–13]. This article reports on the analysis of 3C-SiC wafers irradiated at various fluences and at low irradiation temperatures. Infrared spectroscopy, complemented by curve-fitting procedures was used to obtain trends in the observed spectra, as well as in the dielectric parameters extracted from the reflectance spectra.
2. Experimental Samples of SiC in the shape of discs were irradiated at irradiation temperatures of 200 1C, 300 1C and 400 1C, and at fluences ranging from 5 1019 to 1.6 1021 n/cm2. Irradiation of samples was done at the Oak Ridge National Laboratory (ORNL), USA. Information related to specimens analysed in this report is contained in Table 1. A Bruker 80V FTIR/Raman spectrometer, fitted with a Pike 10 Spec specular reflection unit enabling near-normal incidence, was employed to obtain infrared reflectance spectra from samples taking 32 scans at a resolution of 8 cm 1. Surface roughness of the samples was measured using a CSM Instruments Nano-indenter, n
Corresponding author. Tel.: þ 27 41 504 2186; fax: þ 27 41 504 2573. E-mail address: [email protected] (J.A.A. Engelbrecht).
0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.10.059
fitted with an atomic force microscope. Sample areas of size 25 25 μm2 were analysed by AFM. 3. Results and discussion 3.1. Reflectivity In all cases where the irradiation temperature was kept constant and the fluence was increased, it was found that the reflectivity maximum decreased for the higher fluences, as observed in Fig. 1. However, it appears as if the reflectivity reached a limiting value for very high irradiations (Fig. 2). The maximum height of the reflectance peaks of samples irradiated at the same temperatures but with high and low fluences are plotted in Fig. 3. It is clear that the peak heights of the samples irradiated at higher fluences increase with increasing temperature, while the peak heights decrease for the lower fluences. It is not clear exactly what causes these trends, and further work may require transmission electron microscopy to establish whether the morphology of the samples recovered at the higher fluences and temperatures. This hypothesis seems to be supported by the changes in the dielectric parameters of the samples, which has been demonstrated to be linked to surface roughness [14]. 3.2. Dielectric parameters Fig. 4 displays the variation of the high frequency dielectric constants ε1 of the relevant specimens, and reveals a slight decrease in values for low fluences and an increase for high fluences. Variations in the phonon damping constant Γ is shown
170
J.A.A. Engelbrecht et al. / Physica B 439 (2014) 169–172
Table 1 Irradiation temperatures and fluences of specimens analysed. Sample number Irradiation temp. (1C) Fluence ( 1021) Total Becquerel (Bq) 1 2 3 4 5 9 10
Unirradiated 200 200 300 300 400 400
0 0.056 0.097 0.6 1.4 0.092 0.48
0 13 23 182 425 22 114
Fig. 3. Change in the peak height of the reflectivity with change in irradiation temperature and neutron fluence.
Fig. 1. Variation of reflectivity as function of irradiation temperature and neutron fluence.
Fig. 4. Variation of the high frequency dielectric constant with neutron fluence and irradiation temperature.
Fig. 2. Variation of reflectivity at a constant irradiation temperature, but with increasing neutron irradiation fluence.
in Fig. 5, where changes also follow the previously observed trend, namely an increase in the values for Γ. It has earlier been shown that for rougher sample surfaces ε1 will decrease and Γ will increase [14]. This is clearly the case in particular for the lower fluences in the present study. The changes in the longitudinal (ωL) and transverse (ωT) phonon frequencies are contained in Fig. 6. These are again in agreement with earlier observations; in particular again the (slight) decrease in ωT for the high fluence. It has been noted that ωT is not influenced much by surface morphology [14]. Likewise,
Fig. 5. The effect of irradiation temperature and neutron fluence on the phonon damping constant.
J.A.A. Engelbrecht et al. / Physica B 439 (2014) 169–172
no trends were observed for the free-carrier damping constant γ and the plasma resonance frequency ωp, as expected.
3.3. Surface morphology Three-dimensional images of the surfaces of the various samples analysed are shown in Fig. 7. The root mean square values of the surface roughness as determined by the AFM are plotted in Fig. 8. When comparing Figs. 3 and 8, it is clear that as the surface roughness decreases, the maximum height of the reflectivity
Fig. 6. Longitudinal and transverse phonon frequencies as functions of temperature and neutron fluence.
171
increases, and vice versa (see e.g. the respective trends for high or low fluence samples). At this stage it is not clear what causes the changes in reflectivity with changes in the irradiation temperatures, as depicted in Fig. 8. More so since there seems to be opposite trends between high and low fluence of implantation. It was suggested by Brink et al. that the decrease in maximum reflectivity of irradiated SiC was due to the increase in the values of the phonon damping constants ΓTO and ΓLO [11]. This is confirmed by the present investigation, where an increase in Γ was found with decreasing reflectivity. In addition, earlier research has pointed to the amorphization of SiC during irradiation at lower temperatures [2–4,6], with
Fig.8. Trends in RMS surface roughness as function of irradiation temperature of neutron implanted samples.
Fig. 7. Three-dimensional AFM images of the sample surfaces of the various discs that were irradiated.
172
J.A.A. Engelbrecht et al. / Physica B 439 (2014) 169–172
decreasing amorphization as the irradiation temperature increases. Analysis of the defects present in irradiated SiC indicated that at low irradiation temperatures point defects in the form of black spots were formed, which gradually changed into defect clusters and dislocation loops of decreasing numbers as the irradiation temperature increased. The influence of such defects on the reflectivity and dielectric parameters are not known at present, and more work is required. Samples implanted at higher irradiation temperatures should also be evaluated by infrared spectroscopy. In summary, there are detectable trends in both the reflectivity and dielectric parameters of SiC samples implanted at various fluences and low irradiation temperatures. Trends in the dielectric parameters are in agreement with earlier observations for GaAs samples. Acknowledgements Ms. D. Venter and Mr. R. Dix-Peek are thanked for technical assistance with this project. The National Research Foundation of South Africa is gratefully acknowledged for funding provided. Any opinion, findings and conclusions or recommendations expressed in this article are those of the authors, and therefore the NRF does not accept liability in regards thereto.
References [1] W. Wacholz, International Working Group on Gas-Cooled Reactors (IWGGCAR), vol. 19, 1998, p. 61. [2] L.L. Snead, S.J. Zinkle, J.C. Hay, M.C. Osborne, Nucl. Instrum. Methods B 141 (1998) 123. [3] L.L. Snead, M.C. Osborne, R.A. Lowden, J. Strizak, R.J. Shinavski, K.L. More, W.S. Eatherly, J Bailey, A.M. Williams, J. Nucl. Mater. 253 (1998) 20. [4] L.L. Snead, J.C. Hay, J. Nucl. Mater. 273 (1999) 213. [5] A.I. Ryazanov, A.V. Klaptsov, A. Kohyama, H. Kishimoto, J. Nucl. Mater. 307-311 (2002) 1107. [6] G. Newsome, L.L. Snead, T. Hinoki, Y. Katho, D. Peters, J. Nucl. Mater. 371 (2007) 76. [7] K. Katoh, N. Hashimoto, S. Kondo, L.L. Snead, A. Kohyama, J. Nucl. Mater. 351 (2006) 228. [8] S. Kondo, Y. Katoh, L.L. Snead, J. Nucl. Mater. 382 (2008) 160. [9] Y. Katoh, S. Kondo, L.L. Snead, J. Nucl. Mater. 382 (2008) 170. [10] S. Kondo, Y. Katoh, L.L. Snead, Appl. Phys. Lett. 93 (2008) 163110. [11] D.J. Brink, J.B. Malherbe, J. Camassel, Nucl. Instrum. Methods Phys. B 267 (2009) 2716. [12] Y. Katoh, T. Nozawa, L.L. Snead, K. Ozawa, H. Tanigawa, J. Nucl. Mater. 417 (2011) 400. [13] A. Janse van Vuuren, Radiation Damage in GaAs and SiC, M.Sc. Dissertation, Nelson Mandela Metropolitan University, South Africa, 2011. [14] J.A.A. Engelbrecht, N.G. Hashe, K.T. Hillie, C.H. Claassens, Physica B 401-402 (2007) 238.