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Retention and re-emission of hydrogen in beryllium studied by the ERD technique
ELSEVIER
Journal of Nuclear Materials 233 237 (1996) 898-901
journalnf nuclear materials
Retention and re-emission of hydrogen in beryllium studied by the ERD technique B.
Tsuchiya ~, K. Morita
Department ~f (Trystalline Materials Science, School ~f Engineerin~,. Nagoya Uniw'rsity, Furo cho. Chiku~a ku, Na~,o3a 464 01. Jalmpl
Abstract
lsochronal and isothermal re-emissions of D atoms retained in Be by room temperature implantation with 3 keV D , ions up to saturation have been studied by combined use of elastic recoil detection (ERD) and Rutherford backscattering (RBS) techniques. In the isochronal re-emission curves, two re-emission stages are observed, a stage at 90°C and a broad stage at 300 + 100°C. It is found from the change in the ERD spectra that the former stage corresponds to re-emission of D retained around the projected range in the bulk and the latter broad stage is composed of two stages, a lower temperature one corresponds to re-emission from the oxidized surface layers and a higher temperature one from the bulk. From the analysis of the isothermal re-emission curves measured at various temperatures, the activation energies at the three re-emission stages are detem~ined and the trapping states for the three stages are discussed.
1. I n t r o d u c t i o n
Beryllium is one of the most interesting materials for plasma facing components and blanket components of a fusion reactor. So far, for evaluation of the recycling, the inventory and the breeding of tritium, retention, re-emission and permeation of hydrogen isotopes in the Be and Be-compounds such as BeO have been extensively studied by various techniques [1-10]. However, there is no reasonable agreement on the diffusivity, solubility, trapping, or surface recombination rate constant for hydrogen in Be. One of the most probable origins for the data scatter is considered to be due to oxidation of the Be surface. The trapping of hydrogen isotopes in Be has been studied by several authors [5,6,10] from measurements of thermal re-emission of their implants by use of the nuclear reaction analysis (NRA) and Rutherford backscattering (RBS) techniques. In an isochronal release experiment [5], two stages were observed, a broad stage at 400 + 100°C
* Corresponding author. Fax: h956106d@ eds.ecip.nagoya- u.ac.ip.
+ 81-52-7894683;
e-mail:
and a stage at 125°C, for a Be sample imphmted with I keV D + ions. The activation energies for detrapping of D were roughly estimated to be ~ 1 eV and ~ 1.8 eV for release stages at 125°C and 400°C, respectively. However, no quantitative analysis was performed yet. In order to predict the transient behavior of tritium in Be, it should be necessary to understand the elementary processes for hydrogen transport and to determine their relevant rate constants. In the paper, we report the experimental results on the themml re-emission of 1.5 keV D ~ implants from Be which were measured by means of the elastic recoil detection (ERD) technique combined with the RBS technique. From close inspection of the change in the ERD spectra, it is found that the isochronal re-emission curves of the D implants consists of three re-emission stages, a stage at 90°C, a broad stage at 350°C for D atoms retained around the projected range of 1.5 keV implants and another broad stage at ~ 250°C for D atoms retained in the oxidized surface layers of l0 nm which have been produced during the implantation. From systematic isothermal re-emission measurements, the actiwttion energies for thermal detrapping of hydrogen from the three traps are determined and the trapping states are discussed.
0022-3115/96/$15.00 Copyright (c) 1996 Elsevier Science B.V. All rights rcserved PII S 0 0 2 2 - 3 1 1 5 ( 9 6 ) 0 0 1 1 6 - X
B. Tsuchiya, K. Morita / Journal of Nuclear Materials 233-237 (1996) 898-901 .
2. E x p e r i m e n t s 0
Be s p e c i m e n s used were disc plates o f 30 m m in diameter and l m m in thickness and of 99 at% in purity, of w h i c h the surface is mirror-likely flat. T h e s p e c i m e n was placed on a m a n i p u l a t o r in contact with a ceramic heater in a U H V c h a m b e r w h i c h was usually evacuated to the base pressure less than 11 × 0 - 7 Pa. The Be s p e c i m e n was implanted with 3 keV D2+ ion b e a m at r o o m temperature up to saturation (5 x 10 ~s D * / c m 2) w h i c h was generated from the sputter ion g u n at a D 2 pressure o f 1.3 x 10 2 Pa in the U H V c h a m b e r into w h i c h the w o r k i n g gas of D : was introduced t h r o u g h a liquid-nitrogen cooled trap. T h e concentration profile of D a t o m s retained in the s p e c i m e n was m e a s u r e d by m e a n s o f the E R D technique, where 1.7 M e V He + ion b e a m was incident at an angle of 80 ° to the surface n o r m a l and recoil D + ions were detected at a forward angle of 80 ° to the surface normal. T h e fluence of He + ions b o m b a r d e d on the s p e c i m e n during the E R D m e a s u r e m e n t was m o n i t o r e d s i m u l t a n e o u s l y by m e a n s of the R B S m e a s u r e m e n t . T h e analysis b e a m fluences used did not affect the a m o u n t o f D retained. Prior to the implantation, the oxidized layers were r e m o v e d by b o m b a r d m e n t with 0.5 keV Ar + ions at an angle of 60 ° to the surface normal. T h e R B S m e a s u r e m e n t s h o w e d that the main impurity was 0.93 at% o x y g e n , that the concentrations of heavier impurities were m u c h less, and that the surface oxide layers o f l0 n m thick, w h i c h is m u c h s m a l l e r than the projected range of 1.5 keV D + implants (35 rim) [11], was produced d u r i n g the saturation implantation o f D, as s h o w n in Fig. 1. T w o kinds of thermal r e - e m i s s i o n e x p e r i m e n t s were done, one is isochronal annealing, in w h i c h the s p e c i m e n was heated for 10 m i n at temperatures from 50°C to 500°C and the other is isothermal annealing, w h i c h was done to determine the activation energies for r e - e m i s s i o n stages
.
.
.
,
.
.
.
,
.
.
.
.
,
, . . . .
,
. . . .
i
. . . .
•
-"G ~
0.4
u.
0.2
100
200
300
.
,
.
.
.
.
400
500
600
('C)
3.1. lsochronal annealing A n isochronal a n n e a l i n g curve o f D retained in Be by r o o m temperature implantation up to saturation is s h o w n as a function o f temperature in Fig. 2. In Fig. 2 the
'
I
'
I
'
I
'
I
surface 0
500 '
I
'
I
ERD spectra 500
saturation Implantation, R . T . isochronal annealing for 1 0 m l n . J ~
. B e , 3 k e V D2*,
tr,
before D imp.
400
• as-implanted
E :
75"c 300
.....
i"~
,~'~ ~ i~,
11o'c -~..
O
o
I
- t'r...~
• 3oo'c . \ " . . . .... .
.~. ~'~ t~;
Z~..~.
aner expose tO
o
lOO 0
400 100
.
3. E x p e r i m e n t a l r e s u l t s
\
0
.
o b s e r v e d at the isochronal a n n e a l i n g experiment. T h e S E M observation in the implanted region after the first isochronal a n n e a l i n g e x p e r i m e n t s h o w e d a n u m b e r of small blisters, possibly due to the formation o f microscopic bubbles and voids during the thermal r e - e m i s s i o n processes [9]. In order to reduce the influence o f the blister on the retention and re-emission, a set o f implantation and thermal annealing e x p e r i m e n t s w a s a l w a y s done with the n o n - i m p l a n t e d virgin part o f the disc s p e c i m e n of 30 m m in diameter.
200
°o~
.
Fig. 2. lsochronal annealing curve of D, retained in a Be sample by room temperature implantation up to saturation, as a function of temperature where the annealing time was 10 rain.
, . . . .
,'~ L~ ,"
,
Annealing Tem~atum
,,
E
.
0.0
RBS spectra o after D imp.
1.7 MeV He + )~Be
.
r,- 0 . 6 o
600 , . . . .
.
_¢ o.8
Depth (A) 2 5 0 0 2000 1500 1000 . . . .
.
Be, 3 keV D2+, mUu ration I m p l a n t a t i o n , R. T. Isochronal annealing for 10 m l n .
••
1.0
.
899
200 300 400 Channel N u m b e r
500
600
Fig. 1. RBS spectra of a 1.7 McV He + ion beam from a Be sample after Ar + sputter cleaning of the surface ( • ) and after saturation implantation with 3 keV D2+ ion beam ( O ) at room temperature. The oxygen peak represents that a BeO layer of 71 ML in thickness was formed.
,~,'~,,-~j,. .
.---.~."7~Y
500
X
xlo
r~-- . . . . . . . . . . .
600
700
800
900
Channel Number
Fig. 3. ERD spectra of D + recoiled from a Be sample as-implanted up to saturation with 3 keV D2+ ion beam at room temperature ( • ) and subsequently annealed at 75°C (O), 110°C ( X ) and 300°C ( 0 ) for 10 min, respectively. The solid line represents ERD spectrum for the Ar + sputter cleaned Be sample after exposure to D 2 gas at 1.3x 10 2 Pa.
900
B. 7~'uchiya. K. Morita / Journal of Nuclear Materials 233-237 (1996) 898-901 . . . .
i
. . . .
i
. . . .
i
. . . .
= . . . .
Be, 3 keY D2+, saturation Implantation, R. T.
Isothermal annsallng at • o *, v
• ~
_
,> ~v
_
study are consistent with a thermal desorption spectrum with three peaks obtained by Mizusawa at al. [12].
50"C 75"C
95"C
250 "C 300 "C 350 "C _400 "C
3.2. Isothermal annealing
a 10 e. •~
0.1
ee
"a eo
=u 0.01 it.
! .............El..
0.001 0
100
200
Annealing
300 Time
400
500
(mln)
Fig. 4. Isothermal annealing curves of D retained in a Be sample by room temperature implantation up to saturation, as a function of time.
retained number of D in Be was calculated by integration of the ERD peak spectra for example as shown in Fig. 3 which were measured for the specimen as-in]planted ( • ), after annealing for 10 min at 75°C ( O ) , at I10°C ( x ) and at 300°C ( 0 ) . In Fig. 2, two re-emission stages are apparent, a stage at 90°C and a broad stage at 300 4- 100°C. This trend is very consistent with that obtained by Wampler [5], although the temperatures at the two re-emission stages in the present study are a little lower. On the other hand, it is clearly seen from Fig. 3 that as the temperature increases the ERD spectrum at 75°C reduces the peak height a little and shifts the peak channel a little to the high channel number side compared with that for as-implanted, while the ERD spectrum at I I0°C both reduces the peak height considerably and shifts the peak channel higher and that at 300°C also reduces the peak height and shifts back the peak channel a little to the low channel number side compared with the initial one. The change in the ERD spectrum indicates that the isochronal annealing decay curve in Fig. 2 is divided into decay curves with three stages: The ERD spectrum for the as-implanted condition consists of D atoms retained around the projected range of 1.5 keV D + ions in the bulk and in the oxidized surface layer of 10 nm in thickness. At the first stage of 90°C, 60% of the D atoms were re-emitted from the bulk, at the second stage 20% of the D atoms were re-emitted from the oxidized surface layers and at the final stage the remaining D atoms were re-emitted again from the bulk. The three re-emission stages observed in this
In order to determine the activation energies at the three re-emission stages observed above, the isothem]al annealing experiments were performed at lower temperatures of 50, 75 and 95°C and at higher temperatures of 250, 300, 350 and dO0°C. The isothermal re-emission curves at those temperatures are summarized as a function of annealing time in Fig. 4, where the vertical axis represents the retained number normalized by the number of D atoms as-implanted, and the steady state number for deuterium retained at the surface after long annealing times was subtracted from the data points at 350°C and dO0°C. It is clearly seen from Fig. 4 that the re-emission curves at each temperature decrease rapidly in the beginning of the annealing and hereafter more gradually as the annealing time increases. It can be noted from the isochronal annealing data that the initial decay rates in the annealing curves at the lower temperatures reflect the re-emission of D atoms retained rates in the projected range for bulk Be and also that the initial decay rates at the higher temperatures reflect the re-emission of D atoms from the oxidized surface layer and the later decay rates reflect the re-emission from the bulk Be at the final stage.
T
400 I'1 i I '
10"2
1 0 .3
Eo=~,0.14 + 0.06 eV • El= 0 . 2 5 ~
~
~'mj¢~10 "4
+ 0.08 eV
i
k 1
.... 1.0
100 50 ' I isothermal annealing
t \ ~
"-"
lO-e
(°C)
200 ' I
, .... 1.5
7
, .... 2.0 1000/i"
, .... 2.5
, .... 3.0
3.5
( K "~)
Fig. 5. Arrhenius plot of the rate constants ko (O) and k, ( • ) for D atoms to be thermally detrapped from the bulk and kj (zx) from the oxidized surface layers of a Be sample as a function of 1000/T, which were obtained on the assumption of first order reaction kinetics.
B. Tsuchiya, K. Morita / Journal of Nuclear Materials 233-237 (1996) 898-901
Since the vertical axis in Fig. 4 is scaled logarithmically, it can also be noted that the slower decay rates at each temperature are approximated by exponential functions, as shown by the solid lines. It was also found from detailed analysis that the whole isothermal annealing curves are approximated by two exponential functions. Thus, the decay constants were determined by best fitting the bi-exponential functions to the isothermal annealing curves in Fig. 4. The decay constants obtained are plotted as a function of 1 0 0 0 / T ( K ) in Fig. 5, where the decay constants for the slower decay rates at the lower temperatures could not be determined because they include two components of D atoms, in the oxidized surface layer and in bulk Be. The activation energies at the three stages were determined from Fig. 5 to be 0.14, 0.26 and 0.89 eV, respectively.
4. Discussion It has been shown in the isothermal annealing experiment that the re-emission curve of D retained at each temperature is approximated by two exponential functions. This fact indicates that the thermal re-emission of D atoms retained in the bulk and in the oxidized surface layers of the Be specimen is described by first order reaction kinetics. Since the re-emission of D from Be takes place through diffusion to the surface after thermal detrapping, the first order reaction kinetics indicates that there is no surface barrier, which is consistent with the recent result obtained by Wampler [10]. Thus, the decay constant determined represents the thermal detrapping rate constant. The activation energies for the re-emisrion from Be at the 90°C stage and the broad ~ 350°C stage are 0.14 and 0.89 eV and that from the oxidized surface layers at the broad ~ 250°C stage is 0.26 eV. The energetic implanted D + ions displace Be atoms from their lattice sites leaving vacancies which trap the implants near the end of their range. The D implants are trapped at lattice defects such as vacancies and voids because the reduced electron density at such sites is an energetically favored environment for D atoms compared with interstitial solute sites [13]. The Be atoms displaced from their lattice sites may also form clusters of B e - D compounds before escaping to the surface. There exist chain-like clusters of BeD 2 with the 4-coordination structure possible for B e - D compounds. It may be speculated that such interstitial B e - D clusters are less stable than D strongly trapped in vacancies or bubbles. In such a case, the low activation energies of 0.14 and 0.26 eV might be ascribed to re-emission due to dissociation of such B e - D
901
interstitial clusters. The activation energy of 0.89 eV also may be ascribed to thermal detrapping from vacancies, voids or bubbles. In order to identify the trapping sites for D implants further, other experiments are needed.
5. Summary Isochronal and isothermal re-emissions of D atoms retained in Be by room temperature implantation with 3 keV D2+ ions up to saturation have been studied by combined use of the ERD and RBS techniques. | n the isochronal re-emission curves, two re-emission stages have been observed, a stage at 90°C and a broad stage at 300_+ 100°C. It has been found from close inspection of the change in the ERD spectra that the former stage is ascribed to thermal re-emission of D implants retained around their projected range in the bulk and the latter broad stage is composed of two stages, a lower temperature one takes place due to re-emission from the oxidized surface layers and a higher temperature one due to re-emission from the bulk. It has been found that the isothermal re-emission curves at each temperature are approximated by two exponential functions. This fact indicates that the thermal re-emission of D from Be is described by first order reaction kinetics. The activation energies at the three stages have been determined, from the analysis of the re-emission curves, to be 0.14, 0.26 and 0.89 eV.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [10] [I 1]
[12] [13]
K.L. Wilson et al., J. Vac. Sci. Technol. A 8 (1990) 1750. E. Abramov et al., J. Nucl. Mater. 175 (1990) 90. J.D. Fowler at al., J. Am. Ceramic Soc. 60 (1977) 155. W.A. Swansiger, J. Vac. Sci. Technol. A 4 (1986) 1216. W.R, Wampler, J. Nucl. Mater. 122-123 (1984) 1598. H. Kawamura et al., J. Nucl. Mater. 176-177 (1990) 66. R.G. Macaulay-Newcombe et al., Fusion Eng. Des. 8 (1991) 419. M.B. Lin et al., J. Nucl. Mater. 79 (1979) 267. R.A. Anderl et al., J. Nucl. Mater. 196-198 (I992) 986. W.R. Wampler, J. Nucl. Mater. 196-198 (1992) 983. H.H. Andersen and .I.F. Ziegler, Hydrogen Stopping Powers and Ranges in All Elements (Plenum Press, New York, 1977). S. Mizusawa, R. Sakamoto, T. Muroga and N. Yoshida, J. Nucl. Mater. (1996), in press. S.M. Mygers, P.M. Richards, W.R. Wampler and F. Besenbacher, J. Nucl. Mater. 165 (1989) 9.
Journal of Nuclear Materials 233 237 (1996) 898-901
journalnf nuclear materials
Retention and re-emission of hydrogen in beryllium studied by the ERD technique B.
Tsuchiya ~, K. Morita
Department ~f (Trystalline Materials Science, School ~f Engineerin~,. Nagoya Uniw'rsity, Furo cho. Chiku~a ku, Na~,o3a 464 01. Jalmpl
Abstract
lsochronal and isothermal re-emissions of D atoms retained in Be by room temperature implantation with 3 keV D , ions up to saturation have been studied by combined use of elastic recoil detection (ERD) and Rutherford backscattering (RBS) techniques. In the isochronal re-emission curves, two re-emission stages are observed, a stage at 90°C and a broad stage at 300 + 100°C. It is found from the change in the ERD spectra that the former stage corresponds to re-emission of D retained around the projected range in the bulk and the latter broad stage is composed of two stages, a lower temperature one corresponds to re-emission from the oxidized surface layers and a higher temperature one from the bulk. From the analysis of the isothermal re-emission curves measured at various temperatures, the activation energies at the three re-emission stages are detem~ined and the trapping states for the three stages are discussed.
1. I n t r o d u c t i o n
Beryllium is one of the most interesting materials for plasma facing components and blanket components of a fusion reactor. So far, for evaluation of the recycling, the inventory and the breeding of tritium, retention, re-emission and permeation of hydrogen isotopes in the Be and Be-compounds such as BeO have been extensively studied by various techniques [1-10]. However, there is no reasonable agreement on the diffusivity, solubility, trapping, or surface recombination rate constant for hydrogen in Be. One of the most probable origins for the data scatter is considered to be due to oxidation of the Be surface. The trapping of hydrogen isotopes in Be has been studied by several authors [5,6,10] from measurements of thermal re-emission of their implants by use of the nuclear reaction analysis (NRA) and Rutherford backscattering (RBS) techniques. In an isochronal release experiment [5], two stages were observed, a broad stage at 400 + 100°C
* Corresponding author. Fax: h956106d@ eds.ecip.nagoya- u.ac.ip.
+ 81-52-7894683;
e-mail:
and a stage at 125°C, for a Be sample imphmted with I keV D + ions. The activation energies for detrapping of D were roughly estimated to be ~ 1 eV and ~ 1.8 eV for release stages at 125°C and 400°C, respectively. However, no quantitative analysis was performed yet. In order to predict the transient behavior of tritium in Be, it should be necessary to understand the elementary processes for hydrogen transport and to determine their relevant rate constants. In the paper, we report the experimental results on the themml re-emission of 1.5 keV D ~ implants from Be which were measured by means of the elastic recoil detection (ERD) technique combined with the RBS technique. From close inspection of the change in the ERD spectra, it is found that the isochronal re-emission curves of the D implants consists of three re-emission stages, a stage at 90°C, a broad stage at 350°C for D atoms retained around the projected range of 1.5 keV implants and another broad stage at ~ 250°C for D atoms retained in the oxidized surface layers of l0 nm which have been produced during the implantation. From systematic isothermal re-emission measurements, the actiwttion energies for thermal detrapping of hydrogen from the three traps are determined and the trapping states are discussed.
0022-3115/96/$15.00 Copyright (c) 1996 Elsevier Science B.V. All rights rcserved PII S 0 0 2 2 - 3 1 1 5 ( 9 6 ) 0 0 1 1 6 - X
B. Tsuchiya, K. Morita / Journal of Nuclear Materials 233-237 (1996) 898-901 .
2. E x p e r i m e n t s 0
Be s p e c i m e n s used were disc plates o f 30 m m in diameter and l m m in thickness and of 99 at% in purity, of w h i c h the surface is mirror-likely flat. T h e s p e c i m e n was placed on a m a n i p u l a t o r in contact with a ceramic heater in a U H V c h a m b e r w h i c h was usually evacuated to the base pressure less than 11 × 0 - 7 Pa. The Be s p e c i m e n was implanted with 3 keV D2+ ion b e a m at r o o m temperature up to saturation (5 x 10 ~s D * / c m 2) w h i c h was generated from the sputter ion g u n at a D 2 pressure o f 1.3 x 10 2 Pa in the U H V c h a m b e r into w h i c h the w o r k i n g gas of D : was introduced t h r o u g h a liquid-nitrogen cooled trap. T h e concentration profile of D a t o m s retained in the s p e c i m e n was m e a s u r e d by m e a n s o f the E R D technique, where 1.7 M e V He + ion b e a m was incident at an angle of 80 ° to the surface n o r m a l and recoil D + ions were detected at a forward angle of 80 ° to the surface normal. T h e fluence of He + ions b o m b a r d e d on the s p e c i m e n during the E R D m e a s u r e m e n t was m o n i t o r e d s i m u l t a n e o u s l y by m e a n s of the R B S m e a s u r e m e n t . T h e analysis b e a m fluences used did not affect the a m o u n t o f D retained. Prior to the implantation, the oxidized layers were r e m o v e d by b o m b a r d m e n t with 0.5 keV Ar + ions at an angle of 60 ° to the surface normal. T h e R B S m e a s u r e m e n t s h o w e d that the main impurity was 0.93 at% o x y g e n , that the concentrations of heavier impurities were m u c h less, and that the surface oxide layers o f l0 n m thick, w h i c h is m u c h s m a l l e r than the projected range of 1.5 keV D + implants (35 rim) [11], was produced d u r i n g the saturation implantation o f D, as s h o w n in Fig. 1. T w o kinds of thermal r e - e m i s s i o n e x p e r i m e n t s were done, one is isochronal annealing, in w h i c h the s p e c i m e n was heated for 10 m i n at temperatures from 50°C to 500°C and the other is isothermal annealing, w h i c h was done to determine the activation energies for r e - e m i s s i o n stages
.
.
.
,
.
.
.
,
.
.
.
.
,
, . . . .
,
. . . .
i
. . . .
•
-"G ~
0.4
u.
0.2
100
200
300
.
,
.
.
.
.
400
500
600
('C)
3.1. lsochronal annealing A n isochronal a n n e a l i n g curve o f D retained in Be by r o o m temperature implantation up to saturation is s h o w n as a function o f temperature in Fig. 2. In Fig. 2 the
'
I
'
I
'
I
'
I
surface 0
500 '
I
'
I
ERD spectra 500
saturation Implantation, R . T . isochronal annealing for 1 0 m l n . J ~
. B e , 3 k e V D2*,
tr,
before D imp.
400
• as-implanted
E :
75"c 300
.....
i"~
,~'~ ~ i~,
11o'c -~..
O
o
I
- t'r...~
• 3oo'c . \ " . . . .... .
.~. ~'~ t~;
Z~..~.
aner expose tO
o
lOO 0
400 100
.
3. E x p e r i m e n t a l r e s u l t s
\
0
.
o b s e r v e d at the isochronal a n n e a l i n g experiment. T h e S E M observation in the implanted region after the first isochronal a n n e a l i n g e x p e r i m e n t s h o w e d a n u m b e r of small blisters, possibly due to the formation o f microscopic bubbles and voids during the thermal r e - e m i s s i o n processes [9]. In order to reduce the influence o f the blister on the retention and re-emission, a set o f implantation and thermal annealing e x p e r i m e n t s w a s a l w a y s done with the n o n - i m p l a n t e d virgin part o f the disc s p e c i m e n of 30 m m in diameter.
200
°o~
.
Fig. 2. lsochronal annealing curve of D, retained in a Be sample by room temperature implantation up to saturation, as a function of temperature where the annealing time was 10 rain.
, . . . .
,'~ L~ ,"
,
Annealing Tem~atum
,,
E
.
0.0
RBS spectra o after D imp.
1.7 MeV He + )~Be
.
r,- 0 . 6 o
600 , . . . .
.
_¢ o.8
Depth (A) 2 5 0 0 2000 1500 1000 . . . .
.
Be, 3 keV D2+, mUu ration I m p l a n t a t i o n , R. T. Isochronal annealing for 10 m l n .
••
1.0
.
899
200 300 400 Channel N u m b e r
500
600
Fig. 1. RBS spectra of a 1.7 McV He + ion beam from a Be sample after Ar + sputter cleaning of the surface ( • ) and after saturation implantation with 3 keV D2+ ion beam ( O ) at room temperature. The oxygen peak represents that a BeO layer of 71 ML in thickness was formed.
,~,'~,,-~j,. .
.---.~."7~Y
500
X
xlo
r~-- . . . . . . . . . . .
600
700
800
900
Channel Number
Fig. 3. ERD spectra of D + recoiled from a Be sample as-implanted up to saturation with 3 keV D2+ ion beam at room temperature ( • ) and subsequently annealed at 75°C (O), 110°C ( X ) and 300°C ( 0 ) for 10 min, respectively. The solid line represents ERD spectrum for the Ar + sputter cleaned Be sample after exposure to D 2 gas at 1.3x 10 2 Pa.
900
B. 7~'uchiya. K. Morita / Journal of Nuclear Materials 233-237 (1996) 898-901 . . . .
i
. . . .
i
. . . .
i
. . . .
= . . . .
Be, 3 keY D2+, saturation Implantation, R. T.
Isothermal annsallng at • o *, v
• ~
_
,> ~v
_
study are consistent with a thermal desorption spectrum with three peaks obtained by Mizusawa at al. [12].
50"C 75"C
95"C
250 "C 300 "C 350 "C _400 "C
3.2. Isothermal annealing
a 10 e. •~
0.1
ee
"a eo
=u 0.01 it.
! .............El..
0.001 0
100
200
Annealing
300 Time
400
500
(mln)
Fig. 4. Isothermal annealing curves of D retained in a Be sample by room temperature implantation up to saturation, as a function of time.
retained number of D in Be was calculated by integration of the ERD peak spectra for example as shown in Fig. 3 which were measured for the specimen as-in]planted ( • ), after annealing for 10 min at 75°C ( O ) , at I10°C ( x ) and at 300°C ( 0 ) . In Fig. 2, two re-emission stages are apparent, a stage at 90°C and a broad stage at 300 4- 100°C. This trend is very consistent with that obtained by Wampler [5], although the temperatures at the two re-emission stages in the present study are a little lower. On the other hand, it is clearly seen from Fig. 3 that as the temperature increases the ERD spectrum at 75°C reduces the peak height a little and shifts the peak channel a little to the high channel number side compared with that for as-implanted, while the ERD spectrum at I I0°C both reduces the peak height considerably and shifts the peak channel higher and that at 300°C also reduces the peak height and shifts back the peak channel a little to the low channel number side compared with the initial one. The change in the ERD spectrum indicates that the isochronal annealing decay curve in Fig. 2 is divided into decay curves with three stages: The ERD spectrum for the as-implanted condition consists of D atoms retained around the projected range of 1.5 keV D + ions in the bulk and in the oxidized surface layer of 10 nm in thickness. At the first stage of 90°C, 60% of the D atoms were re-emitted from the bulk, at the second stage 20% of the D atoms were re-emitted from the oxidized surface layers and at the final stage the remaining D atoms were re-emitted again from the bulk. The three re-emission stages observed in this
In order to determine the activation energies at the three re-emission stages observed above, the isothem]al annealing experiments were performed at lower temperatures of 50, 75 and 95°C and at higher temperatures of 250, 300, 350 and dO0°C. The isothermal re-emission curves at those temperatures are summarized as a function of annealing time in Fig. 4, where the vertical axis represents the retained number normalized by the number of D atoms as-implanted, and the steady state number for deuterium retained at the surface after long annealing times was subtracted from the data points at 350°C and dO0°C. It is clearly seen from Fig. 4 that the re-emission curves at each temperature decrease rapidly in the beginning of the annealing and hereafter more gradually as the annealing time increases. It can be noted from the isochronal annealing data that the initial decay rates in the annealing curves at the lower temperatures reflect the re-emission of D atoms retained rates in the projected range for bulk Be and also that the initial decay rates at the higher temperatures reflect the re-emission of D atoms from the oxidized surface layer and the later decay rates reflect the re-emission from the bulk Be at the final stage.
T
400 I'1 i I '
10"2
1 0 .3
Eo=~,0.14 + 0.06 eV • El= 0 . 2 5 ~
~
~'mj¢~10 "4
+ 0.08 eV
i
k 1
.... 1.0
100 50 ' I isothermal annealing
t \ ~
"-"
lO-e
(°C)
200 ' I
, .... 1.5
7
, .... 2.0 1000/i"
, .... 2.5
, .... 3.0
3.5
( K "~)
Fig. 5. Arrhenius plot of the rate constants ko (O) and k, ( • ) for D atoms to be thermally detrapped from the bulk and kj (zx) from the oxidized surface layers of a Be sample as a function of 1000/T, which were obtained on the assumption of first order reaction kinetics.
B. Tsuchiya, K. Morita / Journal of Nuclear Materials 233-237 (1996) 898-901
Since the vertical axis in Fig. 4 is scaled logarithmically, it can also be noted that the slower decay rates at each temperature are approximated by exponential functions, as shown by the solid lines. It was also found from detailed analysis that the whole isothermal annealing curves are approximated by two exponential functions. Thus, the decay constants were determined by best fitting the bi-exponential functions to the isothermal annealing curves in Fig. 4. The decay constants obtained are plotted as a function of 1 0 0 0 / T ( K ) in Fig. 5, where the decay constants for the slower decay rates at the lower temperatures could not be determined because they include two components of D atoms, in the oxidized surface layer and in bulk Be. The activation energies at the three stages were determined from Fig. 5 to be 0.14, 0.26 and 0.89 eV, respectively.
4. Discussion It has been shown in the isothermal annealing experiment that the re-emission curve of D retained at each temperature is approximated by two exponential functions. This fact indicates that the thermal re-emission of D atoms retained in the bulk and in the oxidized surface layers of the Be specimen is described by first order reaction kinetics. Since the re-emission of D from Be takes place through diffusion to the surface after thermal detrapping, the first order reaction kinetics indicates that there is no surface barrier, which is consistent with the recent result obtained by Wampler [10]. Thus, the decay constant determined represents the thermal detrapping rate constant. The activation energies for the re-emisrion from Be at the 90°C stage and the broad ~ 350°C stage are 0.14 and 0.89 eV and that from the oxidized surface layers at the broad ~ 250°C stage is 0.26 eV. The energetic implanted D + ions displace Be atoms from their lattice sites leaving vacancies which trap the implants near the end of their range. The D implants are trapped at lattice defects such as vacancies and voids because the reduced electron density at such sites is an energetically favored environment for D atoms compared with interstitial solute sites [13]. The Be atoms displaced from their lattice sites may also form clusters of B e - D compounds before escaping to the surface. There exist chain-like clusters of BeD 2 with the 4-coordination structure possible for B e - D compounds. It may be speculated that such interstitial B e - D clusters are less stable than D strongly trapped in vacancies or bubbles. In such a case, the low activation energies of 0.14 and 0.26 eV might be ascribed to re-emission due to dissociation of such B e - D
901
interstitial clusters. The activation energy of 0.89 eV also may be ascribed to thermal detrapping from vacancies, voids or bubbles. In order to identify the trapping sites for D implants further, other experiments are needed.
5. Summary Isochronal and isothermal re-emissions of D atoms retained in Be by room temperature implantation with 3 keV D2+ ions up to saturation have been studied by combined use of the ERD and RBS techniques. | n the isochronal re-emission curves, two re-emission stages have been observed, a stage at 90°C and a broad stage at 300_+ 100°C. It has been found from close inspection of the change in the ERD spectra that the former stage is ascribed to thermal re-emission of D implants retained around their projected range in the bulk and the latter broad stage is composed of two stages, a lower temperature one takes place due to re-emission from the oxidized surface layers and a higher temperature one due to re-emission from the bulk. It has been found that the isothermal re-emission curves at each temperature are approximated by two exponential functions. This fact indicates that the thermal re-emission of D from Be is described by first order reaction kinetics. The activation energies at the three stages have been determined, from the analysis of the re-emission curves, to be 0.14, 0.26 and 0.89 eV.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [10] [I 1]
[12] [13]
K.L. Wilson et al., J. Vac. Sci. Technol. A 8 (1990) 1750. E. Abramov et al., J. Nucl. Mater. 175 (1990) 90. J.D. Fowler at al., J. Am. Ceramic Soc. 60 (1977) 155. W.A. Swansiger, J. Vac. Sci. Technol. A 4 (1986) 1216. W.R, Wampler, J. Nucl. Mater. 122-123 (1984) 1598. H. Kawamura et al., J. Nucl. Mater. 176-177 (1990) 66. R.G. Macaulay-Newcombe et al., Fusion Eng. Des. 8 (1991) 419. M.B. Lin et al., J. Nucl. Mater. 79 (1979) 267. R.A. Anderl et al., J. Nucl. Mater. 196-198 (I992) 986. W.R. Wampler, J. Nucl. Mater. 196-198 (1992) 983. H.H. Andersen and .I.F. Ziegler, Hydrogen Stopping Powers and Ranges in All Elements (Plenum Press, New York, 1977). S. Mizusawa, R. Sakamoto, T. Muroga and N. Yoshida, J. Nucl. Mater. (1996), in press. S.M. Mygers, P.M. Richards, W.R. Wampler and F. Besenbacher, J. Nucl. Mater. 165 (1989) 9.