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Transmuted isotopes doped in neutron-irradiated ZnO thin films
Nuclear Instruments and Methods in Physics Research B 217 (2004) 429–434 www.elsevier.com/locate/nimb
Transmuted isotopes doped in neutron-irradiated ZnO thin films Hyunsuk Kim a, Kwangsue Park b, Byungdon Min a, Jong Soo Lee a, Kyoungah Cho a, Sangsig Kim a,*, Hyon Soo Han c, Soon Ku Hong d, Takafumi Yao e a Department of Electrical Engineering, Korea University, Seoul 136-701, South Korea NANO scale quantum devices Research Center, Korea Electronics Technology Institute, PyungTaek 451-865, South Korea Radioisotope and Radiation Application Research Team, Korea Atomic Energy Research Institute, Taegeon 305-353, South Korea d Department of Materials Science and Engineering, Chungnam National University, Daejon 305-764, South Korea e Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan b
c
Received 11 April 2003; received in revised form 12 November 2003
Abstract Transmuted isotopes doped in neutron-irradiated ZnO thin films were first identified on the basis of nuclear reactions of ZnO with thermal neutrons, and their existence in the ZnO thin films was then confirmed by photoluminescence (PL). ZnO thin films were irradiated by neutron beams at room temperature. The ZnO films consist of eight constituent (Zn and O) isotopes, of which four isotopes are transmutable by neutron-irradiation; 64 Zn, 68 Zn, 70 Zn and 18 O were expected to be transmuted into 65 Cu, 69 Ga, 71 Ga and 19 F, respectively. The concentrations of these transmuted isotopes were estimated in this study. The neutron-irradiated ZnO thin films were characterized by PL. In the PL spectra, the Cu-related PL peaks were observed, but the Ga- or F-associated PL peaks were absent. This observation confirms the existence of 65 Cu in the ZnO and does not the formation of the other three. The reason for the absence of the Ga- or F-associated PL peaks is discussed in this paper. 2003 Elsevier B.V. All rights reserved. PACS: 61.72.V; 25.40.F; 25.60.D; 32.10.B; 78.55 Keywords: Neutron-transmutation-doping; ZnO; Capture cross section; Isotope; Photoluminescence
1. Introduction Neutron-transmutation-doping (NTD) has been utilized as a means of impurity doping. The NTD is a doping technique based on nuclear
*
Corresponding author. Tel./fax: +82-2-3290-3894. E-mail address: [email protected] (S. Kim).
reactions [1–4]. Constituent isotopes in semiconductors, when they react with thermal neutrons, become other isotopes by obtaining n1 . Some unstable isotopes created by neutron irradiation are transmuted to other atoms after beta and gamma recoils [5]. Impurities can be doped in semiconductors by these nuclear reactions. However, fast neutrons that neutron beams contain damage semiconductors and give no nuclear
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.11.085
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H. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 429–434
reaction because of their very low cross section. Neutron-irradiated semiconductors must be annealed to annihilate damages produced by fast neutrons and to remove unwanted effects induced by thermal neutrons. Dopings in ZnO including p-type ZnO problems have been vigorously studied for possible applications to light emitting devices, surface acoustic wave (SAW) devices and transparent conductor. Thus, it is very important to control doping concentrations related to the electrical and optical properties. Compared with conventional in situ or implantation doping methods, the NTD method can control more accurately concentrations of transmuted impurities. In addition, spatial dispersion of impurity distribution achieved by NTD is very smaller than other doping method. This NTD has been applied to a variety of semiconductors including Si, a-Si:H, Ge, GaAs, InSe, GaP and GaS [5–9]. For NTD-InSe, Sn atoms are transmuted from In. Recently, GaP has attracted considerable attention in the context of epilayers and heterostructures. In neutron-transmuted GaP, Ge and S impurities are transmuted from Ga and P atoms, respectively [10]. And NTD-Si material has already been commercialized for high power devices [11]. Impurities transmuted in ZnO films after neutron irradiation are studied in this work. Transmuted atoms are first identified on the basis of nuclear reactions of ZnO with thermal neutrons and their concentrations are then estimated [12]. The existence of transmuted impurities in ZnO films is confirmed by photoluminescence (PL).
2. Experimental procedure ZnO thin films prepared by plasma-assisted molecular-beam epitaxy (PAMBE) were irradiated with neutron beams consisting of thermal and fast neutrons. The flux was 6.73 · 1013 and 1.0 · 1011 cm2 s1 for the thermal and fast neutrons and the irradiation times were 0.5, 24 and 168 h. Four ZnO samples prepared for this study were named samples 1–4 for four different neutron irradiation times (see Table 1). Neutron-irradiated ZnO samples were annealed at 300, 500 and 600 C for
Table 1 Neutron irradiation fluences for four ZnO samples
Sample Sample Sample Sample
1 2 3 4
Irradiation time (h)
Thermal neutron fluence (neutron #/cm2 )
Fast neutron fluence (neutron #/cm2 )
0 0.5 24 168
0 1.21 · 1017 5.81 · 1018 4.07 · 1019
0 1.80 · 1014 8.64 · 1015 6.05 · 1016
30 minutes in an oxygen atmosphere. PL measurements were carried out at 8 K and the light source was the 325 nm line from a He–Cd laser.
3. Results and discussion 3.1. Identification of transmuted atoms in ZnO Six isotopes of Zn and three isotopes of O exist in nature; the nine isotopes are 64 Zn, 65 Zn, 66 Zn, 67 Zn, 68 Zn, 70 Zn, 16 O, 17 O and 18 O. The natural abundances of nine isotopes are listed up in Table 2. Note that the natural abundance of 65 Zn is quite negligible. Accordingly, since the natural abundance of each isotope existing in the ZnO films should be the same as in nature, Zn and O constituents of ZnO films have eight isotopes excluding 65 Zn. When irradiated by neutron beams, some of eight isotopes in ZnO are transmuted by collisions with thermal neutrons. In the collisions, the
Table 2 Natural abundances and cross sections of thermal neutrons for Zn and O isotopes-24 Natural abundance (%)
Cross section (barn)
Zn Zn 66 Zn 67 Zn 68 Zn 70 Zn
48.6 – 27.9 4.1 18.8 0.6
0.74 66 0.9 6.9 0.87 9.11 · 102
16
99.78 0.038 0.205
64 65
O O 18 O 17
1.9 · 104 5.4 · 104 1.6 · 104
H. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 429–434 Table 3 Nuclear reactions for Zn and O isotopes with thermal neutrons
64
Zn
First nuclear reaction, second nuclear reaction
Half life, decay energy
64
244.26 days 1.352 MeV
Zn + n fi 65 Zn + c, Zn + e1 fi 65 Cu + m 66 Zn + n fi 67 Zn + c, 67 Zn stable 67 Zn + n fi 68 Zn + c, 68 Zn stable 68 Zn + n fi 69 Zn + c, 69 Zn fi 69 Ga + b 70 Zn + n fi 71 Zn + c, 71 Zn fi 71 Ga + b 64
66
Zn
67
Zn
68
Zn
70
Zn
16
O
O + n1 fi 17 O + c, O stable 17 O + n1 fi 18 O + c, 18 O stable 18 O + n1 fi 19 O + c, 19 O fi 19 F + b
56.4 min 0.906 MeV 2.45 min 2.813 MeV
16 17
17
O
18
O
26.91 s 4.821 MeV
CðaiÞ ¼ CðcÞ n r f ;
3.2. Estimation of concentrations of transmuted atoms Concentrations of atoms transmuted in ZnO may be briefly estimated here. The concentration CðaiÞ of an isotope formed from an existent isotope after the first nuclear reaction may be first obtained from
ð1Þ
where CðcÞ, n, r and f are concentration of the constituent atom, natural abundance, capture cross section of thermal neutron and fluence of thermal neutron, respectively. And the concentration CðtÞ of the final isotope transmuted after the second nuclear reaction is then estimated from h i CðtÞ ¼ CðaiÞ 1 ð1=2Þelapsed time=half life : ð2Þ Note that the concentration of transmuted isotope is dependent on the elapsed time after the neutron irradiation. For sample 4, the concentration of 65 Cu isotopes transmuted from 64 Zn isotopes existing in ZnO may be estimated. The concentration CðcÞ of the constituent atom may be obtained from CðcÞ ¼
probability of transmutation for each isotope is proportional to the cross section of thermal neutron. The cross sections of nine isotopes for thermal neutrons with room temperature energy (25 meV) are summarized in Table 2; for fast neutrons with energies above more than 0.1 MeV, the cross sections of the nine isotopes are approximately zero. When the isotopes collide with thermal neutrons, nuclear reactions occur and some of the isotopes are transmuted. The nuclear reactions are summarized in Table 3. In Table 3, n, c, e1 , m and b denote thermal neutron, gamma ray, electron, neutrio and beta ray, respectively. After the nuclear reactions, 64 Zn, 68 Zn, 70 Zn and 18 O among eight isotopes present in ZnO are transmuted to 65 Cu, 69 Ga, 71 Ga and 19 F, respectively. Therefore, after neutron irradiation, these four different transmuted impurities are doped in ZnO films.
431
¼
ZnO density Avogadro number molecular weight of ZnO 5:66 g=cm3 6:022 1023 81:37 g
¼ 4:19 1022 =cm3 :
ð3Þ
Then, the concentration C(65 Zn) of the 65 Zn isotopes formed from 64 Zn isotopes may be obtained from 65
Cð ZnÞ ¼ CðcÞ n r f 65
¼ Cð ZnÞ r f ¼ ð4:19 1022 =cm3 0:486Þ 0:74 1024 cm2 4:07 1019 =cm2 ¼ 6:14 1017 =cm3 : ð4Þ 65
65
The concentration C( Cu) of Cu transmuted from 65 Zn (with a half life of 244.26 days), when 153 days are elapsed after the neutron irradiation, may be finally obtained from h i 65 65 153 days=244:26 days Cð CuÞ ¼ Cð ZnÞ 1 ð1=2Þ : ð5Þ If the same estimation procedures as above are performed, the concentrations of the transmuted 65 Cu isotopes at elapsed times of 20 and 224 days are obtained to be 3.4 · 1016 /cm3 and
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Table 4 Concentrations of transmuted atoms (cm3 ) Sample 2
Sample 3
Sample 4
Elapsed time (days)
20
20
20
153
224
65
1.0 · 1014 8.3 · 1014 2.8 · 1012 1.6 · 109
4.8 · 1015 4.0 · 1016 1.3 · 1014 7.8 · 1011
3.4 · 1016
2.2 · 1017 2.8 · 1017 9.3 · 1014 5.5 · 1011
3.1 · 1017
Cu 69 Ga 71 Ga 19 F
3.1 · 1017 /cm3 , respectively. Furthermore, the concentrations of 69 Ga, 71 Ga and 19 F isotopes transmuted from 68 Zn, 70 Zn and 18 O isotopes present in ZnO may be also estimated. The concentrations in each sample are summarized in Table 4. 3.3. Photoluminescence The PL spectra of unirradiated ZnO (sample 1) and as-neutron-irradiated ZnO films (samples 2–4) are compared in the wavelength range of 350–700 nm in Fig. 1. Bound exciton peaks and deep level emission peaks are present in each of the four PL spectra [13]. The PL spectra of samples 2–4 seem to be the same as that of sample 1. On the contrary, the inset of Fig. 1 illustrates that the PL spectra of sample 2–4 are a little different in the wavelength range of 450–550 nm from that of
Fig. 1. PL spectra of not annealed ZnO films; (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4. The inset exhibits PL in the wavelength range of 400–650 nm.
sample 1; in the PL spectrum of sample 4, regularly repeated peaks are present over the broad emission bands, but these repeated peaks are absent in the PL spectrum of sample 1. The difference in the PL spectra of unirradiated and as-neutronirradiated ZnO films indicates that the presence of the repeated peaks is associated with the neutronirradiation. Fig. 2 exhibits the PL spectra of sample 4 annealed at 300, 500 and 600 C; for comparison, the PL spectra of sample 1 annealed at the same temperatures are depicted in Fig. 3. A comparison of Figs. 2 and 3 shows that, for the samples 1 and 4, some exciton peaks around 350 nm become stronger in intensity with the increasing annealing temperature. The inset of Fig. 2 shows that, for sample 4, the intensity of the repeated peaks in the
Fig. 2. PL spectra of neutron-irradiated ZnO films (thermal fluence 4 · 1019 /cm2 , sample 4); sample 4 was (a) not annealed, (b) annealed at 300 C, (c) annealed at 500 C and (d) annealed at 600 C. The inset exhibits repeated peaks on the broad emission band.
H. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 429–434
Fig. 3. PL spectra of non-irradiated ZnO films (sample 1); sample 1 was (a) not annealed, (b) annealed at 300 C, (c) annealed at 500 C and (d) annealed at 600 C. The inset exhibits that there are no repeated peaks on the broad emission band.
wavelength range of 450–550 nm becomes stronger with the increasing annealing temperature; for sample 1, there are still no repeated peaks in the PL spectra in the inset of Fig. 3. Fig. 4 represents the net intensity of the repeated peaks obtained by subtracting the PL spectra of sample 1 from the PL spectra of sample 4. The repeated peaks shown in Fig. 4 are attributed to only the transmuted
433
atoms formed in the neutron-irradiated ZnO films. It is observed here that annealing improves emission efficiency of the transmuted atoms. The repeated PL peaks in the wavelength range of 450–550 nm are attributed to one of the four transmuted atoms 65 Cu, 69 Ga, 71 Ga and 19 F, as expected in Section 3.1. It has been known that any peaks related to 69 Ga, 71 Ga and 19 F do not appear in this wavelength range [14,15]. Interestingly, the PL peaks responsible for Cu atoms for Cu-doped ZnO reported in [16,17] have the same peak position and lineshapes as those of the repeated PL peaks observed in this study. This indicates that the repeated peaks are attributed to Cu atoms and that the transmuted 65 Cu atoms exist in the neutron-irradiated ZnO films. PL spectra were taken for sample 4 when 20, 153 and 244 days passed after the neutron irradiation to see that the 65 Cu concentration in ZnO increases with elapsed time as calculated in Table 4. Fig. 5 shows how the intensity of the repeated peaks depends on the elapsed time. The 65 Curelated peaks in the range 450–550 nm get distinctively stronger in intensity for the PL taken for sample 4 at longer elapsed time, as expected in Section 3.2; according to Table 4, the concentration of 65 Cu is 3.4 · 1016 , 2.2 · 1017 and 3.1 · 1017 /cm3 at 20, 153 and 244 days after
(d) (d)
Intensity (a.u.)
Intensity (a.u.)
(c)
(b)
(c) (b) (a)
(a)
450
480
510
540
Wavelength [nm]
Fig. 4. Net intensity of the repeated PL peaks for sample 4: (a) not annealed, (b) annealed at 300 C, (c) annealed at 500 C and (d) annealed at 600 C.
460
480
500
520
540
Wavelength [nm]
Fig. 5. Repeated peaks on the broad emission band; for sample 1 (a) after 20 elapsed days and for sample 4, (b) after 20 elapsed days, (c) after 153 elapsed days and (d) after 244 elapsed days.
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neutron irradiation, respectively. This observation implies that the concentration of the 65 Cu impurities in the neutron-irradiated ZnO films increases with elapsed time. Now we turn our attention to the transmuted 69 Ga, 71 Ga and 19 F atoms. On the basis of the reports in [14,15], the Ga- and F-related peaks should be seen at 369 and 510 nm, respectively, in the PL spectra of the neutron-irradiated ZnO films. Unfortunately, the Ga- and F-related peaks could not be observed in this study. It was reported in [14] that the Ga-related peaks are present in ZnO having higher Ga concentrations than 1018 / cm3 . The absence of the Ga-related peaks is thereby due to the too low Ga concentration (2.8 · 1017 /cm3 ) for our neutron-irradiated ZnO films. And the absence of the F-related peaks is also due to the same reason; the estimated concentration of F impurity in our the neutron-irradiated ZnO films is 5.5 · 1011 /cm3 .
4. Conclusions ZnO thin films grown by PAMBE were irradiated by neutron beams and some constituent isotopes in ZnO were transmuted. Transmuted atoms are expected to be 65 Cu, 69 Ga, 71 Ga and 19 F, and the concentrations of these transmuted atoms are estimated. The presence of 65 Cu formed in ZnO was confirmed by PL measurement, but the presence of 69 Ga, 71 Ga and 19 F could not be confirmed in the PL study. The reason for the absence of the Ga- and F-associated PL peaks is the too low concentrations of the transmuted Ga and F atoms.
Acknowledgements This work was supported by the Õ03 Nuclear R&D Program and National R&D project for Nano Science and Technology.
References [1] E.D. Wheeler, J.L. Boone, J.W. Farmer, H.R. Chandrasekha, Phys. Rev. B 53 (1996) 15617. [2] J. Liu, P.-X. Wang, J. Appl. Phys. 86 (1999) 764. [3] S.C. Creamer, P.C. Rice-Evans, G.A. Gledhill, J.D. Collins, Philos. Mag. B 63 (1991) 923. [4] J.V. Patal, J.G. Williams, G.E. Stillman, J. Appl. Phys. 73 (1993) 3734. [5] M.A. Vesaghi, Phys. Rev. B 25 (1982) 5436. [6] H. Hamanaka, K. Kuriyama, M. Yahagi, M. Satoh, K. Iwamura, C. Kim, F. Shiraishi, K. Tsuji, S. Minomura, Appl. Phys. Lett. 45 (1984) 786. [7] K. Kuriyama, M. Yahagi, K. Iwamura, C. Kim, Y. Kim, J. Appl. Phys 54 (1984) 673. [8] A.A. Homs, B. Mari, J. Appl. Phys. 88 (2000) 4654. [9] J.E. Mueler, W. Kellner, H. Kniepkamp, E.W. Hass, G. Fishcher, J. Appl. Phys. 51 (1980) 3178. [10] K. Kuriyama, K. Ohbora, M. Okada, Solid State Commun. 113 (2000) 415. [11] M. Coeck, N. Balcaen, T. Van Hoecke, B. Van Waeyenberge, D. Segers, C. Dauwe, C. Laermans, J. Appl. Phys. 87 (2000) 3674. [12] Firestone, B. Richard, Table of Isotopes, 8th ed., WileyInterscience, New York, 1996. [13] C. Boemare, T. Monteiro, M.J. Soares, J.G. Guilherme, E. Alves, Physica B 308–310 (2001) 985. [14] H.J. Ko, Y.F. Chen, S.K. Hong, H. Wenisch, T. Yao, Appl. Phys. Lett. 77 (2000) 3761. [15] W.F. Wei, Phys. Rev. B 15 (1977) 2250. [16] P.J. Dean, D.J. Robbins, S.G. Bishop, J.A. Savage, P. Porteous, J. Phys. C 14 (1981) 2847. [17] D.J. Robbins, D.C. Herbert, P.J. Dean, J. Phys. C 14 (1981) 2859.
Transmuted isotopes doped in neutron-irradiated ZnO thin films Hyunsuk Kim a, Kwangsue Park b, Byungdon Min a, Jong Soo Lee a, Kyoungah Cho a, Sangsig Kim a,*, Hyon Soo Han c, Soon Ku Hong d, Takafumi Yao e a Department of Electrical Engineering, Korea University, Seoul 136-701, South Korea NANO scale quantum devices Research Center, Korea Electronics Technology Institute, PyungTaek 451-865, South Korea Radioisotope and Radiation Application Research Team, Korea Atomic Energy Research Institute, Taegeon 305-353, South Korea d Department of Materials Science and Engineering, Chungnam National University, Daejon 305-764, South Korea e Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan b
c
Received 11 April 2003; received in revised form 12 November 2003
Abstract Transmuted isotopes doped in neutron-irradiated ZnO thin films were first identified on the basis of nuclear reactions of ZnO with thermal neutrons, and their existence in the ZnO thin films was then confirmed by photoluminescence (PL). ZnO thin films were irradiated by neutron beams at room temperature. The ZnO films consist of eight constituent (Zn and O) isotopes, of which four isotopes are transmutable by neutron-irradiation; 64 Zn, 68 Zn, 70 Zn and 18 O were expected to be transmuted into 65 Cu, 69 Ga, 71 Ga and 19 F, respectively. The concentrations of these transmuted isotopes were estimated in this study. The neutron-irradiated ZnO thin films were characterized by PL. In the PL spectra, the Cu-related PL peaks were observed, but the Ga- or F-associated PL peaks were absent. This observation confirms the existence of 65 Cu in the ZnO and does not the formation of the other three. The reason for the absence of the Ga- or F-associated PL peaks is discussed in this paper. 2003 Elsevier B.V. All rights reserved. PACS: 61.72.V; 25.40.F; 25.60.D; 32.10.B; 78.55 Keywords: Neutron-transmutation-doping; ZnO; Capture cross section; Isotope; Photoluminescence
1. Introduction Neutron-transmutation-doping (NTD) has been utilized as a means of impurity doping. The NTD is a doping technique based on nuclear
*
Corresponding author. Tel./fax: +82-2-3290-3894. E-mail address: [email protected] (S. Kim).
reactions [1–4]. Constituent isotopes in semiconductors, when they react with thermal neutrons, become other isotopes by obtaining n1 . Some unstable isotopes created by neutron irradiation are transmuted to other atoms after beta and gamma recoils [5]. Impurities can be doped in semiconductors by these nuclear reactions. However, fast neutrons that neutron beams contain damage semiconductors and give no nuclear
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.11.085
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H. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 429–434
reaction because of their very low cross section. Neutron-irradiated semiconductors must be annealed to annihilate damages produced by fast neutrons and to remove unwanted effects induced by thermal neutrons. Dopings in ZnO including p-type ZnO problems have been vigorously studied for possible applications to light emitting devices, surface acoustic wave (SAW) devices and transparent conductor. Thus, it is very important to control doping concentrations related to the electrical and optical properties. Compared with conventional in situ or implantation doping methods, the NTD method can control more accurately concentrations of transmuted impurities. In addition, spatial dispersion of impurity distribution achieved by NTD is very smaller than other doping method. This NTD has been applied to a variety of semiconductors including Si, a-Si:H, Ge, GaAs, InSe, GaP and GaS [5–9]. For NTD-InSe, Sn atoms are transmuted from In. Recently, GaP has attracted considerable attention in the context of epilayers and heterostructures. In neutron-transmuted GaP, Ge and S impurities are transmuted from Ga and P atoms, respectively [10]. And NTD-Si material has already been commercialized for high power devices [11]. Impurities transmuted in ZnO films after neutron irradiation are studied in this work. Transmuted atoms are first identified on the basis of nuclear reactions of ZnO with thermal neutrons and their concentrations are then estimated [12]. The existence of transmuted impurities in ZnO films is confirmed by photoluminescence (PL).
2. Experimental procedure ZnO thin films prepared by plasma-assisted molecular-beam epitaxy (PAMBE) were irradiated with neutron beams consisting of thermal and fast neutrons. The flux was 6.73 · 1013 and 1.0 · 1011 cm2 s1 for the thermal and fast neutrons and the irradiation times were 0.5, 24 and 168 h. Four ZnO samples prepared for this study were named samples 1–4 for four different neutron irradiation times (see Table 1). Neutron-irradiated ZnO samples were annealed at 300, 500 and 600 C for
Table 1 Neutron irradiation fluences for four ZnO samples
Sample Sample Sample Sample
1 2 3 4
Irradiation time (h)
Thermal neutron fluence (neutron #/cm2 )
Fast neutron fluence (neutron #/cm2 )
0 0.5 24 168
0 1.21 · 1017 5.81 · 1018 4.07 · 1019
0 1.80 · 1014 8.64 · 1015 6.05 · 1016
30 minutes in an oxygen atmosphere. PL measurements were carried out at 8 K and the light source was the 325 nm line from a He–Cd laser.
3. Results and discussion 3.1. Identification of transmuted atoms in ZnO Six isotopes of Zn and three isotopes of O exist in nature; the nine isotopes are 64 Zn, 65 Zn, 66 Zn, 67 Zn, 68 Zn, 70 Zn, 16 O, 17 O and 18 O. The natural abundances of nine isotopes are listed up in Table 2. Note that the natural abundance of 65 Zn is quite negligible. Accordingly, since the natural abundance of each isotope existing in the ZnO films should be the same as in nature, Zn and O constituents of ZnO films have eight isotopes excluding 65 Zn. When irradiated by neutron beams, some of eight isotopes in ZnO are transmuted by collisions with thermal neutrons. In the collisions, the
Table 2 Natural abundances and cross sections of thermal neutrons for Zn and O isotopes-24 Natural abundance (%)
Cross section (barn)
Zn Zn 66 Zn 67 Zn 68 Zn 70 Zn
48.6 – 27.9 4.1 18.8 0.6
0.74 66 0.9 6.9 0.87 9.11 · 102
16
99.78 0.038 0.205
64 65
O O 18 O 17
1.9 · 104 5.4 · 104 1.6 · 104
H. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 429–434 Table 3 Nuclear reactions for Zn and O isotopes with thermal neutrons
64
Zn
First nuclear reaction, second nuclear reaction
Half life, decay energy
64
244.26 days 1.352 MeV
Zn + n fi 65 Zn + c, Zn + e1 fi 65 Cu + m 66 Zn + n fi 67 Zn + c, 67 Zn stable 67 Zn + n fi 68 Zn + c, 68 Zn stable 68 Zn + n fi 69 Zn + c, 69 Zn fi 69 Ga + b 70 Zn + n fi 71 Zn + c, 71 Zn fi 71 Ga + b 64
66
Zn
67
Zn
68
Zn
70
Zn
16
O
O + n1 fi 17 O + c, O stable 17 O + n1 fi 18 O + c, 18 O stable 18 O + n1 fi 19 O + c, 19 O fi 19 F + b
56.4 min 0.906 MeV 2.45 min 2.813 MeV
16 17
17
O
18
O
26.91 s 4.821 MeV
CðaiÞ ¼ CðcÞ n r f ;
3.2. Estimation of concentrations of transmuted atoms Concentrations of atoms transmuted in ZnO may be briefly estimated here. The concentration CðaiÞ of an isotope formed from an existent isotope after the first nuclear reaction may be first obtained from
ð1Þ
where CðcÞ, n, r and f are concentration of the constituent atom, natural abundance, capture cross section of thermal neutron and fluence of thermal neutron, respectively. And the concentration CðtÞ of the final isotope transmuted after the second nuclear reaction is then estimated from h i CðtÞ ¼ CðaiÞ 1 ð1=2Þelapsed time=half life : ð2Þ Note that the concentration of transmuted isotope is dependent on the elapsed time after the neutron irradiation. For sample 4, the concentration of 65 Cu isotopes transmuted from 64 Zn isotopes existing in ZnO may be estimated. The concentration CðcÞ of the constituent atom may be obtained from CðcÞ ¼
probability of transmutation for each isotope is proportional to the cross section of thermal neutron. The cross sections of nine isotopes for thermal neutrons with room temperature energy (25 meV) are summarized in Table 2; for fast neutrons with energies above more than 0.1 MeV, the cross sections of the nine isotopes are approximately zero. When the isotopes collide with thermal neutrons, nuclear reactions occur and some of the isotopes are transmuted. The nuclear reactions are summarized in Table 3. In Table 3, n, c, e1 , m and b denote thermal neutron, gamma ray, electron, neutrio and beta ray, respectively. After the nuclear reactions, 64 Zn, 68 Zn, 70 Zn and 18 O among eight isotopes present in ZnO are transmuted to 65 Cu, 69 Ga, 71 Ga and 19 F, respectively. Therefore, after neutron irradiation, these four different transmuted impurities are doped in ZnO films.
431
¼
ZnO density Avogadro number molecular weight of ZnO 5:66 g=cm3 6:022 1023 81:37 g
¼ 4:19 1022 =cm3 :
ð3Þ
Then, the concentration C(65 Zn) of the 65 Zn isotopes formed from 64 Zn isotopes may be obtained from 65
Cð ZnÞ ¼ CðcÞ n r f 65
¼ Cð ZnÞ r f ¼ ð4:19 1022 =cm3 0:486Þ 0:74 1024 cm2 4:07 1019 =cm2 ¼ 6:14 1017 =cm3 : ð4Þ 65
65
The concentration C( Cu) of Cu transmuted from 65 Zn (with a half life of 244.26 days), when 153 days are elapsed after the neutron irradiation, may be finally obtained from h i 65 65 153 days=244:26 days Cð CuÞ ¼ Cð ZnÞ 1 ð1=2Þ : ð5Þ If the same estimation procedures as above are performed, the concentrations of the transmuted 65 Cu isotopes at elapsed times of 20 and 224 days are obtained to be 3.4 · 1016 /cm3 and
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Table 4 Concentrations of transmuted atoms (cm3 ) Sample 2
Sample 3
Sample 4
Elapsed time (days)
20
20
20
153
224
65
1.0 · 1014 8.3 · 1014 2.8 · 1012 1.6 · 109
4.8 · 1015 4.0 · 1016 1.3 · 1014 7.8 · 1011
3.4 · 1016
2.2 · 1017 2.8 · 1017 9.3 · 1014 5.5 · 1011
3.1 · 1017
Cu 69 Ga 71 Ga 19 F
3.1 · 1017 /cm3 , respectively. Furthermore, the concentrations of 69 Ga, 71 Ga and 19 F isotopes transmuted from 68 Zn, 70 Zn and 18 O isotopes present in ZnO may be also estimated. The concentrations in each sample are summarized in Table 4. 3.3. Photoluminescence The PL spectra of unirradiated ZnO (sample 1) and as-neutron-irradiated ZnO films (samples 2–4) are compared in the wavelength range of 350–700 nm in Fig. 1. Bound exciton peaks and deep level emission peaks are present in each of the four PL spectra [13]. The PL spectra of samples 2–4 seem to be the same as that of sample 1. On the contrary, the inset of Fig. 1 illustrates that the PL spectra of sample 2–4 are a little different in the wavelength range of 450–550 nm from that of
Fig. 1. PL spectra of not annealed ZnO films; (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4. The inset exhibits PL in the wavelength range of 400–650 nm.
sample 1; in the PL spectrum of sample 4, regularly repeated peaks are present over the broad emission bands, but these repeated peaks are absent in the PL spectrum of sample 1. The difference in the PL spectra of unirradiated and as-neutronirradiated ZnO films indicates that the presence of the repeated peaks is associated with the neutronirradiation. Fig. 2 exhibits the PL spectra of sample 4 annealed at 300, 500 and 600 C; for comparison, the PL spectra of sample 1 annealed at the same temperatures are depicted in Fig. 3. A comparison of Figs. 2 and 3 shows that, for the samples 1 and 4, some exciton peaks around 350 nm become stronger in intensity with the increasing annealing temperature. The inset of Fig. 2 shows that, for sample 4, the intensity of the repeated peaks in the
Fig. 2. PL spectra of neutron-irradiated ZnO films (thermal fluence 4 · 1019 /cm2 , sample 4); sample 4 was (a) not annealed, (b) annealed at 300 C, (c) annealed at 500 C and (d) annealed at 600 C. The inset exhibits repeated peaks on the broad emission band.
H. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 217 (2004) 429–434
Fig. 3. PL spectra of non-irradiated ZnO films (sample 1); sample 1 was (a) not annealed, (b) annealed at 300 C, (c) annealed at 500 C and (d) annealed at 600 C. The inset exhibits that there are no repeated peaks on the broad emission band.
wavelength range of 450–550 nm becomes stronger with the increasing annealing temperature; for sample 1, there are still no repeated peaks in the PL spectra in the inset of Fig. 3. Fig. 4 represents the net intensity of the repeated peaks obtained by subtracting the PL spectra of sample 1 from the PL spectra of sample 4. The repeated peaks shown in Fig. 4 are attributed to only the transmuted
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atoms formed in the neutron-irradiated ZnO films. It is observed here that annealing improves emission efficiency of the transmuted atoms. The repeated PL peaks in the wavelength range of 450–550 nm are attributed to one of the four transmuted atoms 65 Cu, 69 Ga, 71 Ga and 19 F, as expected in Section 3.1. It has been known that any peaks related to 69 Ga, 71 Ga and 19 F do not appear in this wavelength range [14,15]. Interestingly, the PL peaks responsible for Cu atoms for Cu-doped ZnO reported in [16,17] have the same peak position and lineshapes as those of the repeated PL peaks observed in this study. This indicates that the repeated peaks are attributed to Cu atoms and that the transmuted 65 Cu atoms exist in the neutron-irradiated ZnO films. PL spectra were taken for sample 4 when 20, 153 and 244 days passed after the neutron irradiation to see that the 65 Cu concentration in ZnO increases with elapsed time as calculated in Table 4. Fig. 5 shows how the intensity of the repeated peaks depends on the elapsed time. The 65 Curelated peaks in the range 450–550 nm get distinctively stronger in intensity for the PL taken for sample 4 at longer elapsed time, as expected in Section 3.2; according to Table 4, the concentration of 65 Cu is 3.4 · 1016 , 2.2 · 1017 and 3.1 · 1017 /cm3 at 20, 153 and 244 days after
(d) (d)
Intensity (a.u.)
Intensity (a.u.)
(c)
(b)
(c) (b) (a)
(a)
450
480
510
540
Wavelength [nm]
Fig. 4. Net intensity of the repeated PL peaks for sample 4: (a) not annealed, (b) annealed at 300 C, (c) annealed at 500 C and (d) annealed at 600 C.
460
480
500
520
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Wavelength [nm]
Fig. 5. Repeated peaks on the broad emission band; for sample 1 (a) after 20 elapsed days and for sample 4, (b) after 20 elapsed days, (c) after 153 elapsed days and (d) after 244 elapsed days.
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neutron irradiation, respectively. This observation implies that the concentration of the 65 Cu impurities in the neutron-irradiated ZnO films increases with elapsed time. Now we turn our attention to the transmuted 69 Ga, 71 Ga and 19 F atoms. On the basis of the reports in [14,15], the Ga- and F-related peaks should be seen at 369 and 510 nm, respectively, in the PL spectra of the neutron-irradiated ZnO films. Unfortunately, the Ga- and F-related peaks could not be observed in this study. It was reported in [14] that the Ga-related peaks are present in ZnO having higher Ga concentrations than 1018 / cm3 . The absence of the Ga-related peaks is thereby due to the too low Ga concentration (2.8 · 1017 /cm3 ) for our neutron-irradiated ZnO films. And the absence of the F-related peaks is also due to the same reason; the estimated concentration of F impurity in our the neutron-irradiated ZnO films is 5.5 · 1011 /cm3 .
4. Conclusions ZnO thin films grown by PAMBE were irradiated by neutron beams and some constituent isotopes in ZnO were transmuted. Transmuted atoms are expected to be 65 Cu, 69 Ga, 71 Ga and 19 F, and the concentrations of these transmuted atoms are estimated. The presence of 65 Cu formed in ZnO was confirmed by PL measurement, but the presence of 69 Ga, 71 Ga and 19 F could not be confirmed in the PL study. The reason for the absence of the Ga- and F-associated PL peaks is the too low concentrations of the transmuted Ga and F atoms.
Acknowledgements This work was supported by the Õ03 Nuclear R&D Program and National R&D project for Nano Science and Technology.
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