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Hydrogen transport in thin nickel films by forward recoil spectroscopy
Scripta
METALLURGICA
Vol. 23, pp. 263-267, 1989 Printed in the U.S.A.
HYDROGEN
Pergamon Press plc All rights r e s e r v e d
TRANSPORT
FORWARD
IN THIN NICKEL FILMS BY RECOIL SPECTROSCOPY
J-P. H i r v o n e n I, J.W. Mayer, and H.H. Johnson Department of Materials Science and Engineering, Cornell University, Ithaca, N.Y. 14853 (Received N o v e m b e r 7, 1988) (Revised D e c e m b e r 12, 1988) Introduction The t r a n s p o r t of h y d r o g e n through thin metal membranes has o f t e n been investigated by p e r m e a t i o n methods (e.g., 1,2). From steady state measurements a p e r m e a t i o n c o e f f i c i e n t w h i c h contains the product of the d i f f u s i v i t y and the lattice s o l u b i l i t y is obtained. With the c u s t o m a r y m e a s u r e m e n t techniques the m i n i m u m t h i c k n e s s of the m e m b r a n e s that can be s t u d i e d is at least a few micrometers. Much thinner films are of interest, however, in modern e l e c t r o n i c s technology. The b e h a v i o r of h y d r o g e n in t h e s e thin films is of c o n s i d e r a b l e interest. Deposition p r o c e s s e s for p r o d u c t i o n of thin films may inadvertently incorporate h y d r o g e n in the films. A m o r p h o u s h y d r o g e n a t e d silicon is perhaps the best known example, but it is b e l i e v e d that n e a r l y all thin films contain hydrogen from the deposition process (3). The structure of thin films is e x p e c t e d to differ s u b s t a n t i a l l y from that of the c o r r e s p o n d i n g b u l k m a t e r i a l . The lattice d e f e c t c o n c e n t r a t i o n s , both dislocations a n d p o i n t defects, can be m u c h h i g h e r in thin films. The c o n c e n t r a t i o n of defects is known to depend on both the d e p o s i t i o n process and the p a r a m e t e r s of the process. Since both the s o l u b i l i t y and d i f f u s i v i t y of hydrogen are known to be influenced by defects, it is a n t i c i p a t e d that hydrogen behavior in thin films will differ from that in conventional bulk material. B e c a u s e of the h i g h d i f f u s i v i t y a n d in m a n y i n s t a n c e s low s o l u b i l i t y of hydrogen, a direct m e a s u r e m e n t of hydrogen transport by concentration profiling is not easy. In fact, there are few if any h y d r o g e n c o n c e n t r a t i o n profiles in high d i f f u s i v i t y materials reported in the l i t e r a t u r e . An e x p e r i m e n t a l arrangement which can be a n t i c i p a t e d to circumvent the known difficulties is a b i l a y e r geometry, w h e r e the top layer is the m a t e r i a l of interest, and the b o t t o m layer serves as a getter to collect the h y d r o g e n which migrates through the top layer. The g e t t e r m a t e r i a l must be a m a t e r i a l of high intrinsic hydrogen solubility. The b u i l d up of h y d r o g e n in the g e t t e r layer can be m e a s u r e d by forward recoil s p e c t r o s c o p y (FRES) (4), which can detect hydrogen concentrations as low as 0.i at. %. In this first report we d e s c r i b e the a p p l i c a t i o n of this a p p r o a c h to the m e a s u r e m e n t of h y d r o g e n transport in nickel. T i t a n i u m is used as the getter material. The b e h a v i o r of h y d r o g e n in bulk n i c k e l is well known, so that nickel is a g o o d m a t e r i a l for t e c h n i q u e development. Forward recoil spectroscopy was used to follow the bu i l d up of h y d r o g e n in the t i t a n i u m layer of a n i c k e l - t i t a n i u m b i l a y e r specimen, as a consequence of h y d r o g e n entry into and transport through the nickel. ipermanent address: Department SF-00170 Helsinki, Finland.
of Physics,
University
263 0 0 3 6 - 9 7 4 8 / 8 9 $3.00 + .00 C o p y r i g h t (c) 1989 Pergamon Press
of Helsinki,
plc
264
HYDROGEN TRANSPORT IN Ni FILMS
Experimental
Vol.
23,
No.
Procedure
The b i l a y e r samples were p r e p a r e d by e v a p o r a t i o n of t i t a n i u m and nickel layers onto Si02 using an electron gun system. The base pressure was 0.13 Pa and the vacuum during e v a p o r a t i o n was typically 0.66 Pa. The layers of the two metals were evaporated without breaking vacuum between the depositions. The thickness the t i t a n i u m layers was i00 nm; two different thicknesses of nickel, 40 and 175 nm, were i n v e s t i g a t e d . The t h i c k n e s s e s were c o n f i r m e d by the R u t h e r f o K d backscattering (RBS) method. The bilayer samples were e l e c t r o l y t i c a l l y charged with hydrogen in a deaerated 1 N NaOH solution. The voltage and current density were, respectively, 3.3 V and 8 mA/cm 2. During charging the hydrogen m i g r a t e d through the nickel layer and was c o l l e c t e d in the t i t a n i u m layer. The amount of h y d r o g e n in the t i t a n i u m was m e a s u r e d as a f u n c t i o n of c h a r g i n g time, w h i c h was v a r i e d from 2 to 60 seconds. The h y d r o g e n measurements were c a r r i e d out i m m e d i a t e l y after charging, with the time between charging and the b e g i n n i n g of measurement held to less than two minutes. A schematic d i a g r a m of the FRES technique (4) is given in Fig. i. The high energy (2 MeV) alpha p a r t i c l e b e a m is d i r e c t e d to the sample surface with an angle of incidence to the normal of 75 degrees. The r e c o i l e d h y d r o g e n driven from the near surface region by the incident alpha particles is m o n i t o r e d by a surface barrier d e t e c t o r at 150 degrees to the incident beam. To eliminate scattered alpha particles in the outgoing b e a m an absorber foil of I0 ~m Mylar was used.
/
"~oo~.~^
~,~.~ ,~
Fig. i. A s c h e m a t i c picture recoil spectroscopy (FRES). Results
of
forward
and Discussion
Forward recoil spectra for 175 nm thick nickel films are shown in Fig. 2. The small h y d r o g e n peak at a p p r o x i m a t e l y 1 MeV corresponds to the location of the nickel surface. The amount of hydrogen a c c u m u l a t e d in the nickel is less than the m e a s u r e m e n t s e n s i t i v i t y for all c h a r g i n g times, a l t h o u g h some outgassing may have o c c u r r e d b e t w e e n charging and testing. E x a m i n i n g Fig. 2 reveals that the surface c o n t a m i n a t i o n of h y d r o g e n remains constant d u r i n g charging. The FRES s p e c t r u m of the u n c h a r g e d sample (t = 0 s) shows the a c c u m u l a t i o n of hydrogen in the t i t a n i u m layer during deposition. This contamination was taken into account in the analysis of the data. R e l a t i v e yields were converted into hy d r o g e n c o n c e n t r a t i o n s using the RUMP p r o g r a m (5) with a d i f f e r e n t i a l cross section of 0.4x10-2e m2/steradian for the elastic recoil of hydrogen (6). The i n c r e a s e charging time
in h y d r o g e n concentration in the t i t a n i u m is shown in Fig. 3 for both nickel thicknesses
as a f u n c t i o n studied. The
of
Z
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2
HYDROGEN
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IN Ni FILMS
265
h y d r o g e n c o n c e n t r a t i o n in b o t h cases at first i n c r e a s e s in a l i n e a r fashion with time, but w i t h a faster rate for the t h i n n e r n i c k e l foil. This suggests that h y d r o g e n t r a n s p o r t in t h e n i c k e l is c o n t r o l l i n g the b e h a v i o r of the b i l a y e r specimen. At longer c h a r g i n g times the h y d r o g e n c o n c e n t r a t i o n assumes a constant value, w h i c h is the s a m e for b o t h n i c k e l t h i c k n e s s e s . This c o n c e n t r a t i o n c o r r e s p o n d s to TiHI.75. The constant final value indicates that an e q u i l i b r i u m has been e s t a b l i s h e d between the hydrogen c o n c e n t r a t i o n s in the nickel and t i t a n i u m layers, and b e t w e e n the b i l a y e r specimen and the charging solution. A more d e t a i l e d analysis suggests, however, real differences in the behavior of hydrogen in bulk and thin film nickel. It may be a s s u m e d that in the linear region the t r a n s p o r t b e h a v i o r is c o n t r o l l e d by nickel, and that Fick's first law is obeyed. B e c a u s e of the small thicknesses, it may also be assumed that the s t e a d y s t a t e is e s t a b l i s h e d at the b e g i n n i n g of the e l e c t r o c h e m i c a l charging. The h y d r o g e n c o n c e n t r a t i o n at the input surface then corresponds to the s o l u b i l i t y and at the n i c k e l / t i t a n i u m interface it may be taken as zero, assuming that the gettering efficiency of the t i t a n i u m is e s s e n t i a l l y unlimited. With these a s s u m p t i o n s the t r a n s p o r t product, the product of the solubility and the diffusivity, can be obtained as: ox
:
w h e r e D is the d i f f u s i v i t y of h y d r o g e n in nickel, x is the s o l u b i l i t y of h y d r o g e n in nickel, y is the thickness of the Ni layer, N is the atomic density of nickel, n is the c o n c e n t r a t i o n of g e t t e r e d h y d r o g e n in t i t a n i u m (H/cm2), and t is the c h a r g i n g time. When the above formula is applied to the linear parts of Fig. 3 the e x p e r i m e n t a l values of the t r a n s p o r t p r o d u c t Dx p r e s e n t e d in Table 1 are obtained. For comparison, the value of Dx can be c a l c u l a t e d using the s o l u b i l i t y a n d d i f f u s i v i t y of h y d r o g e n in b u l k nickel. The d i f f u s i o n coefficient at room t e m p e r a t u r e is 5x10 -14 m2/s (7). The s o l u b i l i t y depends on the e f f e c t i v e p a r t i a l p r e s s u r e of h y d r o g e n during the e l e c t r o l y t i c charging. The e q u i l i b r i u m value of the hydrogen concentrations in
I
0.4
I
Energy {MeV) I 0 .I8 1.0I
0.6
1.2 I
I./.. I
I
25
,
,~20 >'1o
.~
15 Fig. 2. The forward recoil spectra of hydrogen in a Ni(175 nm)-Ti(100 nm) bilayer sample as a function of time. C o r r e s p o n d i n g charging times are: 1 = 0 s, 2 = 5 s, 3 = i0 s, 4 = 60 s.
o Z
05~
100
I
150 200 Channel
250
1
300
266
HYDROGEN
TRANSPORT
IN Ni FILMS
Vol.
23, No.
60$
600
5O0
"6 l.O0 .~ 3oo
/o Fig.3. The evolution of a hydrogen c o n c e n t r a t i o n in t i t a n i u m in Ni-Ti bilayer s a m p l e s as a f u n c t i o n of c h a r g i n g time. The t h i c k n e s s of a n i c k e l l a y e r is x = 40 nm and o = 175 nm, respectively. The saturation value after a c h a r g i n g of 60 s corresponds to the composition TiHz.75.
~ 200 ~o lOO I
Charging time (s)
TABLE i. The T r a n s p o r t Products, Calculated Solubilities Corresponding E q u i l i b r i u m Hydrogen Pressures.
thickness of Ni film [nm]
transport product [(cm2/s)xat.%]
175 40
5.2xi0 -9 I. 5x10 -9
of
Hydrogen
in
solubility I [at.%]
hydrogen pressure 2 [Pa]
i0 3
7x102° 5.7xi02°
Nickel,
ICalculated u s i n g the e x p e r i m e n t a l values of the t r a n s p o r t p r o d u c t s and diffusion coefficient of hydrogen in bulk nickel at room temperature (7). 2Calculated using the in bulk nickel (I0) .
solubilities
in this table
and the solubility
and
the
of hydrogen
Fig. 3 corresponds to the 7-phase (TiHx, x=1.5-2) (8). From the dissociation p r e s s u r e of the y-phase the m i n i m u m p a r t i a l p r e s s u r e of h y d r o g e n can be estimated (9) as of the order of i0 -s Pa. A f t e r the two p h a s e region the single ~ p h a s e is o b t a i n e d and the stoichiometry of the ~ p h a s e is changed in an u n k n o w n way with i n c r e a s i n g partial pressure. Thus this e q u i l i b r i u m value cannot be u s e d to d e t e r m i n e the p a r t i a l p r e s s u r e of hydrogen. However, the formula (i) can be u s e d to e s t i m a t e the s o l u b i l i t y of h y d r o g e n in nickel assuming the bulk d i f f u s i v i t y is valid. These values are also shown in Table i: Furthermore, the partial pressures of hydrogen required for these values of the s o l u b i l i t y can be c a l c u l a t e d (i0) and the values p r e s e n t e d in Table 1 obtained. The
values
of
the
solubility
and partial
pressures
deduced
using
the
formula
2
Vol.
23, No.
2
HYDROGEN
TRANSPORT
IN Ni FILMS
267
(i) and shown in Table 1 are of course u n r e a l i s t i c . This means that the diffusion c o e f f i c i e n t of h y d r o g e n in bulk nickel cannot be used and that the h y d r o g e n m i g r a t i o n is e n h a n c e d in the thin film case as c o m p a r e d to the bulk nickel case. The e n h a n c e d t r a n s p o r t of h y d r o g e n is related to the defect structure of the nickel films. An effective d i f f u s i v i t y which exceeds the lattice diffusivity, as in the present case, requires an extended or network defect structure, i.e., a high d i f f u s i v i t y path. Isolated defects which act as hydrogen traps lead to an effective d i f f u s i v i t y which is less than the lattice diffusivity. The m i c r o s t r u c t u r e of deposited thin films is typically one of columnar grains. It may be s p e c u l a t e d that the grain b o u n d a r i e s function as high d i f f u s i v i t y paths for hydrogen. This implies that the effective h y d r o g e n d i f f u s i v i t y is a n i s o t r o p i c in d e p o s i t e d metal films, with the higher value in the direction perpendicular to the plane of the film. Ackowledaements This r e s e a r c h was s u p p o r t e d National Science Foundation.
by
the
Division
of
Materials
Research
of
the
References I. 2.
A.J. Kummick and H.H. Johnson, Met. Trans. 6A, 1087 (1975). J. V61kl and G. Alefeld, in Diffusion in Solids, Recent Developements, ed. by A.S. Nowick and J.J. Burton, p. 231, Academic Press, New York, (1975) 3. J-P. Hirvonen, R. Lappalainen, A. Anttila, and E. Sirvi6, The Proceedings of The First International Conference on Plasma-Surface Engineering, Garmisch-Partenkirchen, FRG, in press (1988). 4. L.C. Feldman and J. W. Mayer, Fundamentals of Surface and Thin Film Analysis, p. 31, N o r t h - H o l l a n d (1986). 5. L.R. Doolittle, Nucl. Inst. Meth.B9, 344 (1985). 6. E. Kennedy, private communication. 7. J. V~ikl and G. Alefeld, in Diffusion in Solids, Recent Developements, ed. by A.S. Nowick and J.J. Burton, p. 249, Academic Press, New York, (1975) 8. E. F r o m m and E. Gebhardt, Gase und Kohlenstoff in Metallen, p. 397, Springer-Verlag, Berlin (1976). 9. E. F r o m m a n d E. Gebhardt, Gase und Kohlenstoff in Metallen, p. 398, Springer-Verlag, Berlin (1976). I0. E. F r o m m and E. Gebhardt, Gase und Kohlenstoff in Metallen, p. 622, Springer-Verlag, Berlin (1976). II. S.M. Myers, P. Nordlander, F. Besenbacher and J.K. N~rskov, Phys. Rev. B, 33, 854 (1986).
METALLURGICA
Vol. 23, pp. 263-267, 1989 Printed in the U.S.A.
HYDROGEN
Pergamon Press plc All rights r e s e r v e d
TRANSPORT
FORWARD
IN THIN NICKEL FILMS BY RECOIL SPECTROSCOPY
J-P. H i r v o n e n I, J.W. Mayer, and H.H. Johnson Department of Materials Science and Engineering, Cornell University, Ithaca, N.Y. 14853 (Received N o v e m b e r 7, 1988) (Revised D e c e m b e r 12, 1988) Introduction The t r a n s p o r t of h y d r o g e n through thin metal membranes has o f t e n been investigated by p e r m e a t i o n methods (e.g., 1,2). From steady state measurements a p e r m e a t i o n c o e f f i c i e n t w h i c h contains the product of the d i f f u s i v i t y and the lattice s o l u b i l i t y is obtained. With the c u s t o m a r y m e a s u r e m e n t techniques the m i n i m u m t h i c k n e s s of the m e m b r a n e s that can be s t u d i e d is at least a few micrometers. Much thinner films are of interest, however, in modern e l e c t r o n i c s technology. The b e h a v i o r of h y d r o g e n in t h e s e thin films is of c o n s i d e r a b l e interest. Deposition p r o c e s s e s for p r o d u c t i o n of thin films may inadvertently incorporate h y d r o g e n in the films. A m o r p h o u s h y d r o g e n a t e d silicon is perhaps the best known example, but it is b e l i e v e d that n e a r l y all thin films contain hydrogen from the deposition process (3). The structure of thin films is e x p e c t e d to differ s u b s t a n t i a l l y from that of the c o r r e s p o n d i n g b u l k m a t e r i a l . The lattice d e f e c t c o n c e n t r a t i o n s , both dislocations a n d p o i n t defects, can be m u c h h i g h e r in thin films. The c o n c e n t r a t i o n of defects is known to depend on both the d e p o s i t i o n process and the p a r a m e t e r s of the process. Since both the s o l u b i l i t y and d i f f u s i v i t y of hydrogen are known to be influenced by defects, it is a n t i c i p a t e d that hydrogen behavior in thin films will differ from that in conventional bulk material. B e c a u s e of the h i g h d i f f u s i v i t y a n d in m a n y i n s t a n c e s low s o l u b i l i t y of hydrogen, a direct m e a s u r e m e n t of hydrogen transport by concentration profiling is not easy. In fact, there are few if any h y d r o g e n c o n c e n t r a t i o n profiles in high d i f f u s i v i t y materials reported in the l i t e r a t u r e . An e x p e r i m e n t a l arrangement which can be a n t i c i p a t e d to circumvent the known difficulties is a b i l a y e r geometry, w h e r e the top layer is the m a t e r i a l of interest, and the b o t t o m layer serves as a getter to collect the h y d r o g e n which migrates through the top layer. The g e t t e r m a t e r i a l must be a m a t e r i a l of high intrinsic hydrogen solubility. The b u i l d up of h y d r o g e n in the g e t t e r layer can be m e a s u r e d by forward recoil s p e c t r o s c o p y (FRES) (4), which can detect hydrogen concentrations as low as 0.i at. %. In this first report we d e s c r i b e the a p p l i c a t i o n of this a p p r o a c h to the m e a s u r e m e n t of h y d r o g e n transport in nickel. T i t a n i u m is used as the getter material. The b e h a v i o r of h y d r o g e n in bulk n i c k e l is well known, so that nickel is a g o o d m a t e r i a l for t e c h n i q u e development. Forward recoil spectroscopy was used to follow the bu i l d up of h y d r o g e n in the t i t a n i u m layer of a n i c k e l - t i t a n i u m b i l a y e r specimen, as a consequence of h y d r o g e n entry into and transport through the nickel. ipermanent address: Department SF-00170 Helsinki, Finland.
of Physics,
University
263 0 0 3 6 - 9 7 4 8 / 8 9 $3.00 + .00 C o p y r i g h t (c) 1989 Pergamon Press
of Helsinki,
plc
264
HYDROGEN TRANSPORT IN Ni FILMS
Experimental
Vol.
23,
No.
Procedure
The b i l a y e r samples were p r e p a r e d by e v a p o r a t i o n of t i t a n i u m and nickel layers onto Si02 using an electron gun system. The base pressure was 0.13 Pa and the vacuum during e v a p o r a t i o n was typically 0.66 Pa. The layers of the two metals were evaporated without breaking vacuum between the depositions. The thickness the t i t a n i u m layers was i00 nm; two different thicknesses of nickel, 40 and 175 nm, were i n v e s t i g a t e d . The t h i c k n e s s e s were c o n f i r m e d by the R u t h e r f o K d backscattering (RBS) method. The bilayer samples were e l e c t r o l y t i c a l l y charged with hydrogen in a deaerated 1 N NaOH solution. The voltage and current density were, respectively, 3.3 V and 8 mA/cm 2. During charging the hydrogen m i g r a t e d through the nickel layer and was c o l l e c t e d in the t i t a n i u m layer. The amount of h y d r o g e n in the t i t a n i u m was m e a s u r e d as a f u n c t i o n of c h a r g i n g time, w h i c h was v a r i e d from 2 to 60 seconds. The h y d r o g e n measurements were c a r r i e d out i m m e d i a t e l y after charging, with the time between charging and the b e g i n n i n g of measurement held to less than two minutes. A schematic d i a g r a m of the FRES technique (4) is given in Fig. i. The high energy (2 MeV) alpha p a r t i c l e b e a m is d i r e c t e d to the sample surface with an angle of incidence to the normal of 75 degrees. The r e c o i l e d h y d r o g e n driven from the near surface region by the incident alpha particles is m o n i t o r e d by a surface barrier d e t e c t o r at 150 degrees to the incident beam. To eliminate scattered alpha particles in the outgoing b e a m an absorber foil of I0 ~m Mylar was used.
/
"~oo~.~^
~,~.~ ,~
Fig. i. A s c h e m a t i c picture recoil spectroscopy (FRES). Results
of
forward
and Discussion
Forward recoil spectra for 175 nm thick nickel films are shown in Fig. 2. The small h y d r o g e n peak at a p p r o x i m a t e l y 1 MeV corresponds to the location of the nickel surface. The amount of hydrogen a c c u m u l a t e d in the nickel is less than the m e a s u r e m e n t s e n s i t i v i t y for all c h a r g i n g times, a l t h o u g h some outgassing may have o c c u r r e d b e t w e e n charging and testing. E x a m i n i n g Fig. 2 reveals that the surface c o n t a m i n a t i o n of h y d r o g e n remains constant d u r i n g charging. The FRES s p e c t r u m of the u n c h a r g e d sample (t = 0 s) shows the a c c u m u l a t i o n of hydrogen in the t i t a n i u m layer during deposition. This contamination was taken into account in the analysis of the data. R e l a t i v e yields were converted into hy d r o g e n c o n c e n t r a t i o n s using the RUMP p r o g r a m (5) with a d i f f e r e n t i a l cross section of 0.4x10-2e m2/steradian for the elastic recoil of hydrogen (6). The i n c r e a s e charging time
in h y d r o g e n concentration in the t i t a n i u m is shown in Fig. 3 for both nickel thicknesses
as a f u n c t i o n studied. The
of
Z
Vol.
23, No.
2
HYDROGEN
TRANSPORT
IN Ni FILMS
265
h y d r o g e n c o n c e n t r a t i o n in b o t h cases at first i n c r e a s e s in a l i n e a r fashion with time, but w i t h a faster rate for the t h i n n e r n i c k e l foil. This suggests that h y d r o g e n t r a n s p o r t in t h e n i c k e l is c o n t r o l l i n g the b e h a v i o r of the b i l a y e r specimen. At longer c h a r g i n g times the h y d r o g e n c o n c e n t r a t i o n assumes a constant value, w h i c h is the s a m e for b o t h n i c k e l t h i c k n e s s e s . This c o n c e n t r a t i o n c o r r e s p o n d s to TiHI.75. The constant final value indicates that an e q u i l i b r i u m has been e s t a b l i s h e d between the hydrogen c o n c e n t r a t i o n s in the nickel and t i t a n i u m layers, and b e t w e e n the b i l a y e r specimen and the charging solution. A more d e t a i l e d analysis suggests, however, real differences in the behavior of hydrogen in bulk and thin film nickel. It may be a s s u m e d that in the linear region the t r a n s p o r t b e h a v i o r is c o n t r o l l e d by nickel, and that Fick's first law is obeyed. B e c a u s e of the small thicknesses, it may also be assumed that the s t e a d y s t a t e is e s t a b l i s h e d at the b e g i n n i n g of the e l e c t r o c h e m i c a l charging. The h y d r o g e n c o n c e n t r a t i o n at the input surface then corresponds to the s o l u b i l i t y and at the n i c k e l / t i t a n i u m interface it may be taken as zero, assuming that the gettering efficiency of the t i t a n i u m is e s s e n t i a l l y unlimited. With these a s s u m p t i o n s the t r a n s p o r t product, the product of the solubility and the diffusivity, can be obtained as: ox
:
w h e r e D is the d i f f u s i v i t y of h y d r o g e n in nickel, x is the s o l u b i l i t y of h y d r o g e n in nickel, y is the thickness of the Ni layer, N is the atomic density of nickel, n is the c o n c e n t r a t i o n of g e t t e r e d h y d r o g e n in t i t a n i u m (H/cm2), and t is the c h a r g i n g time. When the above formula is applied to the linear parts of Fig. 3 the e x p e r i m e n t a l values of the t r a n s p o r t p r o d u c t Dx p r e s e n t e d in Table 1 are obtained. For comparison, the value of Dx can be c a l c u l a t e d using the s o l u b i l i t y a n d d i f f u s i v i t y of h y d r o g e n in b u l k nickel. The d i f f u s i o n coefficient at room t e m p e r a t u r e is 5x10 -14 m2/s (7). The s o l u b i l i t y depends on the e f f e c t i v e p a r t i a l p r e s s u r e of h y d r o g e n during the e l e c t r o l y t i c charging. The e q u i l i b r i u m value of the hydrogen concentrations in
I
0.4
I
Energy {MeV) I 0 .I8 1.0I
0.6
1.2 I
I./.. I
I
25
,
,~20 >'1o
.~
15 Fig. 2. The forward recoil spectra of hydrogen in a Ni(175 nm)-Ti(100 nm) bilayer sample as a function of time. C o r r e s p o n d i n g charging times are: 1 = 0 s, 2 = 5 s, 3 = i0 s, 4 = 60 s.
o Z
05~
100
I
150 200 Channel
250
1
300
266
HYDROGEN
TRANSPORT
IN Ni FILMS
Vol.
23, No.
60$
600
5O0
"6 l.O0 .~ 3oo
/o Fig.3. The evolution of a hydrogen c o n c e n t r a t i o n in t i t a n i u m in Ni-Ti bilayer s a m p l e s as a f u n c t i o n of c h a r g i n g time. The t h i c k n e s s of a n i c k e l l a y e r is x = 40 nm and o = 175 nm, respectively. The saturation value after a c h a r g i n g of 60 s corresponds to the composition TiHz.75.
~ 200 ~o lOO I
Charging time (s)
TABLE i. The T r a n s p o r t Products, Calculated Solubilities Corresponding E q u i l i b r i u m Hydrogen Pressures.
thickness of Ni film [nm]
transport product [(cm2/s)xat.%]
175 40
5.2xi0 -9 I. 5x10 -9
of
Hydrogen
in
solubility I [at.%]
hydrogen pressure 2 [Pa]
i0 3
7x102° 5.7xi02°
Nickel,
ICalculated u s i n g the e x p e r i m e n t a l values of the t r a n s p o r t p r o d u c t s and diffusion coefficient of hydrogen in bulk nickel at room temperature (7). 2Calculated using the in bulk nickel (I0) .
solubilities
in this table
and the solubility
and
the
of hydrogen
Fig. 3 corresponds to the 7-phase (TiHx, x=1.5-2) (8). From the dissociation p r e s s u r e of the y-phase the m i n i m u m p a r t i a l p r e s s u r e of h y d r o g e n can be estimated (9) as of the order of i0 -s Pa. A f t e r the two p h a s e region the single ~ p h a s e is o b t a i n e d and the stoichiometry of the ~ p h a s e is changed in an u n k n o w n way with i n c r e a s i n g partial pressure. Thus this e q u i l i b r i u m value cannot be u s e d to d e t e r m i n e the p a r t i a l p r e s s u r e of hydrogen. However, the formula (i) can be u s e d to e s t i m a t e the s o l u b i l i t y of h y d r o g e n in nickel assuming the bulk d i f f u s i v i t y is valid. These values are also shown in Table i: Furthermore, the partial pressures of hydrogen required for these values of the s o l u b i l i t y can be c a l c u l a t e d (i0) and the values p r e s e n t e d in Table 1 obtained. The
values
of
the
solubility
and partial
pressures
deduced
using
the
formula
2
Vol.
23, No.
2
HYDROGEN
TRANSPORT
IN Ni FILMS
267
(i) and shown in Table 1 are of course u n r e a l i s t i c . This means that the diffusion c o e f f i c i e n t of h y d r o g e n in bulk nickel cannot be used and that the h y d r o g e n m i g r a t i o n is e n h a n c e d in the thin film case as c o m p a r e d to the bulk nickel case. The e n h a n c e d t r a n s p o r t of h y d r o g e n is related to the defect structure of the nickel films. An effective d i f f u s i v i t y which exceeds the lattice diffusivity, as in the present case, requires an extended or network defect structure, i.e., a high d i f f u s i v i t y path. Isolated defects which act as hydrogen traps lead to an effective d i f f u s i v i t y which is less than the lattice diffusivity. The m i c r o s t r u c t u r e of deposited thin films is typically one of columnar grains. It may be s p e c u l a t e d that the grain b o u n d a r i e s function as high d i f f u s i v i t y paths for hydrogen. This implies that the effective h y d r o g e n d i f f u s i v i t y is a n i s o t r o p i c in d e p o s i t e d metal films, with the higher value in the direction perpendicular to the plane of the film. Ackowledaements This r e s e a r c h was s u p p o r t e d National Science Foundation.
by
the
Division
of
Materials
Research
of
the
References I. 2.
A.J. Kummick and H.H. Johnson, Met. Trans. 6A, 1087 (1975). J. V61kl and G. Alefeld, in Diffusion in Solids, Recent Developements, ed. by A.S. Nowick and J.J. Burton, p. 231, Academic Press, New York, (1975) 3. J-P. Hirvonen, R. Lappalainen, A. Anttila, and E. Sirvi6, The Proceedings of The First International Conference on Plasma-Surface Engineering, Garmisch-Partenkirchen, FRG, in press (1988). 4. L.C. Feldman and J. W. Mayer, Fundamentals of Surface and Thin Film Analysis, p. 31, N o r t h - H o l l a n d (1986). 5. L.R. Doolittle, Nucl. Inst. Meth.B9, 344 (1985). 6. E. Kennedy, private communication. 7. J. V~ikl and G. Alefeld, in Diffusion in Solids, Recent Developements, ed. by A.S. Nowick and J.J. Burton, p. 249, Academic Press, New York, (1975) 8. E. F r o m m and E. Gebhardt, Gase und Kohlenstoff in Metallen, p. 397, Springer-Verlag, Berlin (1976). 9. E. F r o m m a n d E. Gebhardt, Gase und Kohlenstoff in Metallen, p. 398, Springer-Verlag, Berlin (1976). I0. E. F r o m m and E. Gebhardt, Gase und Kohlenstoff in Metallen, p. 622, Springer-Verlag, Berlin (1976). II. S.M. Myers, P. Nordlander, F. Besenbacher and J.K. N~rskov, Phys. Rev. B, 33, 854 (1986).