- Email: [email protected]
Nb thin films
Journal
792
of the Less-Common
Metals, 172-l 74 (1991) 792-798
Hydrogen in Nb/Hf/Nb and Nb/Ti/Nb thin films J. Steiger, 0. Boebel, S. Bliisser and A. Weidinger Hahn-Meitner-Znstitut Berlin 1@5 Berlin 39 (F.R.G.)
GmbH, Bereick
Schwerionenphysik,
Glienickerstrasse
155,
Abstract Thin films with layer sequences NbjHf~Nb and NbjTijNbjAu were prepared by electron-beam evaporation in ultrahigh vacuum. The interfaces were characterized by measuring the momentary surface properties by low energy electron diffraction and Auger electron spectroscopy after various evaporation steps. The Nb/Ti films were also investigated by Ruthe~ord backscattering spectroscopy. Hydrogen charging was performed electrolytically or by implantation for NbjHf and Nb/Ti respectively. Hydrogen depth profiles were measured by the 15N nuclear reaction method. In the Nb/Hf films prepared at room temperature we found a strong accumulation of hydrogen in the hafnium layer. Owing to the high substrate temperature during evaporation, the NbjTi films showed some intermixing. In these films, hydrogen was almost equally distributed over the Nb/Ti/Nb layers. The out-diffusion of hydrogen through the gold layer started between 250 and 300 “C. 1. Introduction The study of hydrogen in thin metallic films and multilayers has just started [l-5] and very little is known as yet. It is possible that hydrogen accumulates at the interface of two metals and thereby changes the mechanical (e.g. adhesion) and electrical properties. The accumulation is expected to depend strongly on the quality of the interface (epitaxy, defects, intermixing etc.) and therefore a detailed characterization of the interfaces is necessary. We have started a program to study these phenomena on evaporated Nb/Hf and Nb/Ti thin films. The behaviour of hydrogen in bulk niobium and bulk titanium and hafnium is well known [S] and therefore these metals are well suited to the study of specific changes caused by the thin film structure. A drastic effect is expected because of the different hydride formation enthalpies of these metals which should lead to an accumulation of hydrogen in the hafnium (titanium} layers. However, the fact that the hafnium (titanium) layers are embedded between niobium sheets, which can prevent the lateral lattice expansion, can seriously modify simple extrapolation from the bulk properties. 2. Experimental details The films were prepared by electron-beam evaporation in an ultrahigh vacuum system. The base pressure in the system was p I lo-* Pa when the
6
ElsevierSequoia/Printedin The Netherlands
793
electron beam was off, but increased to lop7 Pa during titanium and hafnium evaporation and to 10m6Pa during niobium evaporation. The main component of the residual gas was always hydrogen. The metal layers (Nb/Hf/Nb or Nb/Ti/Nb/Au) were evaporated one after the other with the same electron gun but from three different crucibles. The surface of the film immediately after each evaporation step was checked by low energy electron diffraction (LEED) and Auger electron spectroscopy ( AES). The Nb/Hf films were evaporated onto annealed niobium foils at room temperature [4,5]. They contained, already without charging, a fair amount of hydrogen (see Fig. 1) which was accumulated in the hafnium layer. Additional hydrogen was introduced by electrolytic charging. Clear LEED spots of the (110) b.c.c. plane of niobium and h.c.p. basal plane of hafnium were observed if the film thicknesses exceeded 20 nm. For thinner films the spots disappeared indicating structural disorder. AES measurements showed no intermixing of niobium and hafnium even on a monolayer scale. In the more recent experiments on Nb/Ti layers, polished (1120) oriented sapphire substrates were used which could be heated during evaporation of the metal films. The intention was to improve the epitaxial growth of the layers. In addition the films were covered with a gold layer to avoid oxidation and hydrogen was introduced by implantation which is easier to
Nb 120
0
100
Nb
Hf
I
300
200 d
400
500
600
(nm)
Fig. 1. Hydrogen depth profile of an Nb/Hf/Nb film before (A) and after (0) hydrogen charging. The solid line is a simulation of the profile assuming constant concentrations within the respective layers. The experimental resolution of the 15N method is folded in. The layer thicknesses assumed in the simulation are 205 and 235 nm for niobium and hafnium respectively; the nominal thicknesses from the quartz monitor during evaporation were 180 and 200 nm respectively.
794
control than electrolytic charging. A disadvantage of the implantation is that defects are created which can act as traps for hydrogen. The implantation energy (80 keV) was such that the hydrogen atoms stopped predominantly in the niobium film between titanium and the substrate. The substrate temperature was 1473 K during niobium, titanium and niobium evaporation, and the gold layer was evaporated at room temperature. The layer thicknesses were controlled by a quartz monitor and determined later by Rutherford backscattering (approximate thicknesses 150/150/150/75 nm for the layer sequence Nb/Ti/Nb/Au). The LEED spots of the Nb/Ti film were very weak indicating strong structural disturbances. We chose the high substrate temperatures mentioned above since we thought it might finally improve the epitaxial growth. However, it turned out that the cause of the bad epitaxial growth was not the temperature but the substrates; with a new series of substrates we also obtained good films at lower temperatures where the intermixing (see below) is less severe. The hydrogen profiles were measured with the 15N nuclear resonance reaction method [7]. This analysis is based on the fact that the nuclear reaction ‘H( 15N, cry)“C has a sharp resonance at E( 15N) = 6.4 MeV, the reaction cross-section being several orders of magnitude larger in the resonance than off the resonance. Thus changing the energy of the 15N beam and measuring the y-yield gives a direct image of the hydrogen depth profile in the sample. The y-yield was measured with a 6 inch x 6 inch NaI detector. The absolute hydrogen concentration was derived from calibration of the system with a hydrogenated tantalum standard.
3. Experimental
results and discussion
Figure 1 shows the hydrogen depth profile of an Nb/Hf/Nb film before and after hydrogen charging. Before charging, the hydrogen concentration in the hafnium layer is between 6 and 8 at.% and is slightly peaked near the interfaces. After charging, a strong increase in the hydrogen content in the hafnium layer is observed. The solid line in Fig. 1 is a simulation of the spectrum taking into account the experimental resolution. The straggling was calculated from the Bohr formula [8]. Assuming a constant hydrogen concentration within the layers this analysis gives [H] /[Hfl z 1.1 in the hafnium and [H]/[Nb] z 0.1 in the adjacent niobium layers. The strong accumulation of hydrogen in hafnium is expected because of the larger binding enthalpy of hydrogen in hafnium than in niobium. Comparing the free energies of the two systems [6,9], an accumulation of hydrogen in hafnium up to [H]/[Hfl z 1.63 (d-phase) would be expected before any hydrogen in detectable amounts should be dissolved in niobium. The fact that some hydrogen is also found in niobium is a result of either defects or impurities in the niobium layers or the fact that hafnium is embedded in
795
AI,O,
250
Nb
TI
Nb
Au
9 0. 200 G
n
", 150 : E b
100
=
50
4”
0
Energy
I MeVl
Fig. 2. Rutherford backscattering spectrum of the Nb/Ti/Nb/Au film. The dotted simulation of the spectrum assuming the layer sequence as shown in the insert.
line is a
niobium and therefore the lateral expansion necessary for the formation of the hydride is restricted. Figure 2 shows the Rutherford backscattering spectrum of the Nb/Ti/ Nb/Au film. The dotted line is a simulation [lo] of the spectrum assuming the layer sequence Nb/Ti/Nb/Au with thicknesses of 155/130/140/75 nm respectively. The pronounced minimum between the two niobium peaks clearly shows the formation of an intermediate titanium layer but some intermixing is indicated by the fact that the yield does not go to zero between the two niobium peaks and that the fit is rather poor for the superimposed Ti/Nb peak. In future films we will lower the substrate temperature to avoid intermixing.
Au
Nb
Ti
Nb
A ‘ZOJ
400
300 0 -? 3 5
200
s 100
63
7
7.5
Energy
8
8.5
(MeV)
Fig. 3. Hydrogen depth profile for the Nb/Ti/Nb/Au film. The solid curve is a simulation of the spectrum assuming no hydrogen in the gold layer and in the Al,O, substrate and [H]/[M] concentrations of 0.13, 0.12 and 0.1 in the niobium, titanium and niobium layers respectively.
796
Au
6.5
Nb
7
Ti
Nb
A 1,0,
7.5
Energy (MeV) Fig. 4. Hydrogen depth profiles for the Nb/Ti/Nb/Au film for different temperatures. The solid room temperature; n . . .. . 100 “C; fI ~ ~ -, 135 “C; 0-., lines are guides to the eye: A----I-, 180 “C; 17 - ~ -, 210 “C.
The as-prepared Nb/Ti sample contained no hydrogen in detectable amounts. After implantation of hydrogen at 80 keV, profiles as shown in Figs. 3 and 4 were measured with the 15N nuclear reaction method [7]. We present here the raw data, i.e. the y-yield (in counts per microcoulomb) us. the incident energy, since the sudden jump of dE/dx at the Nb-Ti interface creates artificial peaks which could be misleading. This is a technical problem of the present analysis program and could be solved by using an unfolding routine. A simpler way is to simulate the experimental spectrum (see Fig. 3) with a certain hydrogen depth profile. The solid line in Fig. 3 was obtained assuming constant hydrogen concentrations in the different layers and concentration values of [H] / [M] = 0,13,12,10 at.% for the layer sequence Au/Nb/Ti/Nb/substrate. The layer thicknesses obtained from this fit were 80/165/160/175 nm respectively. These values are slightly higher than those obtained from Rutherford backscattering (75/140/130/155 nm) but they are still consistent with them considering the uncertainties of the energy losses of the different projectiles. It should be mentioned that the hydrogen profiles do not necessarily have to coincide with the film layers. An important result derived from Fig. 3 is that the expected selective accumulation of hydrogen in the titanium layer does not occur in this sample. We think that this is due to intermixing, i.e. to the presence of some titanium atoms in niobium and vice versa, although the main layer structure is preserved as can be seen from Fig. 2.
197
The change in the hydrogen profile with increasing temperature is displayed in Fig. 4. In the temperature range shown, the total hydrogen content is preserved but a slight redistribution occurs. The change in the shape is reversible, i.e. the profile at room temperature after heating at 210 “C is the same as before heating. It is interesting to note that the hydrogen content of the first niobium layer increases whereas it decreases in the second layer whilst heating. At even higher temperatures, hydrogen leaves the sample. The out-diffusion starts between 250 and 300 “C.
4. Conclusion The Nb/Hf films prepared at room temperature behaved qualitatively as expected. We found a strong enhancement of the hydrogen concentration in the hafnium layer. Quantitatively, there remain some open questions: why does the hydrogen concentration in hafnium not reach the value of the Hf-H b-phase as expected from free energy considerations? Connected with that, why does the hafnium film not absorb all the hydrogen from the niobium layers? Concerning the film preparation, what is the optimal substrate temperature to obtain good epitaxial growth but avoid intermixing? The Nb/Ti film was prepared at too high a temperature and showed intermixing. Hydrogen was almost equally distributed in the film although the basic layer structure Nb/Ti/Nb was realized as seen from the Rutherford backscattering spectra. Hydrogen can be retained in the film between the gold layer and the substrate up to temperatures of 250 “C and the redistribution of the profile in the Nb/Ti/Nb film can be studied up to this temperature.
Acknowledgment
schaft
The work was supported (SFB 306).
in part by the Deutsche
Forschungsgemein-
References 1 H. Zabel and P. F. Micelli, 2. Phys. Chem. N. F., 163 (1989) 3. P. F. Micelli and H. Zabel, Phys. Reu. Lett., 59 (1987) 1224. 2 J. Ryden, B. Hjorvarsson, T. Ericsson, E. Karlsson, A. Krozer and B. Kasemo, 2. Phys. Chem. N. F., 164 (1989) 1259. 3 W. Wagner, F. Rauch, C. Ottermann and K. Bange, Nucl. In&rum. Methods B, 50 (1990) 27. 4 Y. Q. Sheng, P. Ziegler, E. Recknagel, 0. Boebel, J. Steiger and A. Weidinger, Phys. Rev. B, 41 (1990) 9794. 5 J. Steiger, 0. Boebel, M. Briere, A. Weidinger and P. Ziegler, Nucl. Znstrum. Methods B, 50 (1990) 31.
798 6 T. Schober and H. Wenzl, in G. Alefeld and J. Viilkl (eds.), Hydrogen in Metals II, Topics in Applied Physics, Vol. 29, Springer, Berlin, 1978, p. 36. 7 D. J. Leich and T. A. Tombrello, Nucl. Instrum Methods, 108 (1973) 67. 8 N. Bohr, Kgr. Dan. Vidensk. Selsk. Math. Fys. Medd., 18(4) (1971). 9 G. G. Libowitz, Binary Metal Hydrides, Benjamin, New York, 1965. 10 L. R. Doolittle, Nucl. Znstrum. Methods B, 9 (1985) 344.
792
of the Less-Common
Metals, 172-l 74 (1991) 792-798
Hydrogen in Nb/Hf/Nb and Nb/Ti/Nb thin films J. Steiger, 0. Boebel, S. Bliisser and A. Weidinger Hahn-Meitner-Znstitut Berlin 1@5 Berlin 39 (F.R.G.)
GmbH, Bereick
Schwerionenphysik,
Glienickerstrasse
155,
Abstract Thin films with layer sequences NbjHf~Nb and NbjTijNbjAu were prepared by electron-beam evaporation in ultrahigh vacuum. The interfaces were characterized by measuring the momentary surface properties by low energy electron diffraction and Auger electron spectroscopy after various evaporation steps. The Nb/Ti films were also investigated by Ruthe~ord backscattering spectroscopy. Hydrogen charging was performed electrolytically or by implantation for NbjHf and Nb/Ti respectively. Hydrogen depth profiles were measured by the 15N nuclear reaction method. In the Nb/Hf films prepared at room temperature we found a strong accumulation of hydrogen in the hafnium layer. Owing to the high substrate temperature during evaporation, the NbjTi films showed some intermixing. In these films, hydrogen was almost equally distributed over the Nb/Ti/Nb layers. The out-diffusion of hydrogen through the gold layer started between 250 and 300 “C. 1. Introduction The study of hydrogen in thin metallic films and multilayers has just started [l-5] and very little is known as yet. It is possible that hydrogen accumulates at the interface of two metals and thereby changes the mechanical (e.g. adhesion) and electrical properties. The accumulation is expected to depend strongly on the quality of the interface (epitaxy, defects, intermixing etc.) and therefore a detailed characterization of the interfaces is necessary. We have started a program to study these phenomena on evaporated Nb/Hf and Nb/Ti thin films. The behaviour of hydrogen in bulk niobium and bulk titanium and hafnium is well known [S] and therefore these metals are well suited to the study of specific changes caused by the thin film structure. A drastic effect is expected because of the different hydride formation enthalpies of these metals which should lead to an accumulation of hydrogen in the hafnium (titanium} layers. However, the fact that the hafnium (titanium) layers are embedded between niobium sheets, which can prevent the lateral lattice expansion, can seriously modify simple extrapolation from the bulk properties. 2. Experimental details The films were prepared by electron-beam evaporation in an ultrahigh vacuum system. The base pressure in the system was p I lo-* Pa when the
6
ElsevierSequoia/Printedin The Netherlands
793
electron beam was off, but increased to lop7 Pa during titanium and hafnium evaporation and to 10m6Pa during niobium evaporation. The main component of the residual gas was always hydrogen. The metal layers (Nb/Hf/Nb or Nb/Ti/Nb/Au) were evaporated one after the other with the same electron gun but from three different crucibles. The surface of the film immediately after each evaporation step was checked by low energy electron diffraction (LEED) and Auger electron spectroscopy ( AES). The Nb/Hf films were evaporated onto annealed niobium foils at room temperature [4,5]. They contained, already without charging, a fair amount of hydrogen (see Fig. 1) which was accumulated in the hafnium layer. Additional hydrogen was introduced by electrolytic charging. Clear LEED spots of the (110) b.c.c. plane of niobium and h.c.p. basal plane of hafnium were observed if the film thicknesses exceeded 20 nm. For thinner films the spots disappeared indicating structural disorder. AES measurements showed no intermixing of niobium and hafnium even on a monolayer scale. In the more recent experiments on Nb/Ti layers, polished (1120) oriented sapphire substrates were used which could be heated during evaporation of the metal films. The intention was to improve the epitaxial growth of the layers. In addition the films were covered with a gold layer to avoid oxidation and hydrogen was introduced by implantation which is easier to
Nb 120
0
100
Nb
Hf
I
300
200 d
400
500
600
(nm)
Fig. 1. Hydrogen depth profile of an Nb/Hf/Nb film before (A) and after (0) hydrogen charging. The solid line is a simulation of the profile assuming constant concentrations within the respective layers. The experimental resolution of the 15N method is folded in. The layer thicknesses assumed in the simulation are 205 and 235 nm for niobium and hafnium respectively; the nominal thicknesses from the quartz monitor during evaporation were 180 and 200 nm respectively.
794
control than electrolytic charging. A disadvantage of the implantation is that defects are created which can act as traps for hydrogen. The implantation energy (80 keV) was such that the hydrogen atoms stopped predominantly in the niobium film between titanium and the substrate. The substrate temperature was 1473 K during niobium, titanium and niobium evaporation, and the gold layer was evaporated at room temperature. The layer thicknesses were controlled by a quartz monitor and determined later by Rutherford backscattering (approximate thicknesses 150/150/150/75 nm for the layer sequence Nb/Ti/Nb/Au). The LEED spots of the Nb/Ti film were very weak indicating strong structural disturbances. We chose the high substrate temperatures mentioned above since we thought it might finally improve the epitaxial growth. However, it turned out that the cause of the bad epitaxial growth was not the temperature but the substrates; with a new series of substrates we also obtained good films at lower temperatures where the intermixing (see below) is less severe. The hydrogen profiles were measured with the 15N nuclear resonance reaction method [7]. This analysis is based on the fact that the nuclear reaction ‘H( 15N, cry)“C has a sharp resonance at E( 15N) = 6.4 MeV, the reaction cross-section being several orders of magnitude larger in the resonance than off the resonance. Thus changing the energy of the 15N beam and measuring the y-yield gives a direct image of the hydrogen depth profile in the sample. The y-yield was measured with a 6 inch x 6 inch NaI detector. The absolute hydrogen concentration was derived from calibration of the system with a hydrogenated tantalum standard.
3. Experimental
results and discussion
Figure 1 shows the hydrogen depth profile of an Nb/Hf/Nb film before and after hydrogen charging. Before charging, the hydrogen concentration in the hafnium layer is between 6 and 8 at.% and is slightly peaked near the interfaces. After charging, a strong increase in the hydrogen content in the hafnium layer is observed. The solid line in Fig. 1 is a simulation of the spectrum taking into account the experimental resolution. The straggling was calculated from the Bohr formula [8]. Assuming a constant hydrogen concentration within the layers this analysis gives [H] /[Hfl z 1.1 in the hafnium and [H]/[Nb] z 0.1 in the adjacent niobium layers. The strong accumulation of hydrogen in hafnium is expected because of the larger binding enthalpy of hydrogen in hafnium than in niobium. Comparing the free energies of the two systems [6,9], an accumulation of hydrogen in hafnium up to [H]/[Hfl z 1.63 (d-phase) would be expected before any hydrogen in detectable amounts should be dissolved in niobium. The fact that some hydrogen is also found in niobium is a result of either defects or impurities in the niobium layers or the fact that hafnium is embedded in
795
AI,O,
250
Nb
TI
Nb
Au
9 0. 200 G
n
", 150 : E b
100
=
50
4”
0
Energy
I MeVl
Fig. 2. Rutherford backscattering spectrum of the Nb/Ti/Nb/Au film. The dotted simulation of the spectrum assuming the layer sequence as shown in the insert.
line is a
niobium and therefore the lateral expansion necessary for the formation of the hydride is restricted. Figure 2 shows the Rutherford backscattering spectrum of the Nb/Ti/ Nb/Au film. The dotted line is a simulation [lo] of the spectrum assuming the layer sequence Nb/Ti/Nb/Au with thicknesses of 155/130/140/75 nm respectively. The pronounced minimum between the two niobium peaks clearly shows the formation of an intermediate titanium layer but some intermixing is indicated by the fact that the yield does not go to zero between the two niobium peaks and that the fit is rather poor for the superimposed Ti/Nb peak. In future films we will lower the substrate temperature to avoid intermixing.
Au
Nb
Ti
Nb
A ‘ZOJ
400
300 0 -? 3 5
200
s 100
63
7
7.5
Energy
8
8.5
(MeV)
Fig. 3. Hydrogen depth profile for the Nb/Ti/Nb/Au film. The solid curve is a simulation of the spectrum assuming no hydrogen in the gold layer and in the Al,O, substrate and [H]/[M] concentrations of 0.13, 0.12 and 0.1 in the niobium, titanium and niobium layers respectively.
796
Au
6.5
Nb
7
Ti
Nb
A 1,0,
7.5
Energy (MeV) Fig. 4. Hydrogen depth profiles for the Nb/Ti/Nb/Au film for different temperatures. The solid room temperature; n . . .. . 100 “C; fI ~ ~ -, 135 “C; 0-., lines are guides to the eye: A----I-, 180 “C; 17 - ~ -, 210 “C.
The as-prepared Nb/Ti sample contained no hydrogen in detectable amounts. After implantation of hydrogen at 80 keV, profiles as shown in Figs. 3 and 4 were measured with the 15N nuclear reaction method [7]. We present here the raw data, i.e. the y-yield (in counts per microcoulomb) us. the incident energy, since the sudden jump of dE/dx at the Nb-Ti interface creates artificial peaks which could be misleading. This is a technical problem of the present analysis program and could be solved by using an unfolding routine. A simpler way is to simulate the experimental spectrum (see Fig. 3) with a certain hydrogen depth profile. The solid line in Fig. 3 was obtained assuming constant hydrogen concentrations in the different layers and concentration values of [H] / [M] = 0,13,12,10 at.% for the layer sequence Au/Nb/Ti/Nb/substrate. The layer thicknesses obtained from this fit were 80/165/160/175 nm respectively. These values are slightly higher than those obtained from Rutherford backscattering (75/140/130/155 nm) but they are still consistent with them considering the uncertainties of the energy losses of the different projectiles. It should be mentioned that the hydrogen profiles do not necessarily have to coincide with the film layers. An important result derived from Fig. 3 is that the expected selective accumulation of hydrogen in the titanium layer does not occur in this sample. We think that this is due to intermixing, i.e. to the presence of some titanium atoms in niobium and vice versa, although the main layer structure is preserved as can be seen from Fig. 2.
197
The change in the hydrogen profile with increasing temperature is displayed in Fig. 4. In the temperature range shown, the total hydrogen content is preserved but a slight redistribution occurs. The change in the shape is reversible, i.e. the profile at room temperature after heating at 210 “C is the same as before heating. It is interesting to note that the hydrogen content of the first niobium layer increases whereas it decreases in the second layer whilst heating. At even higher temperatures, hydrogen leaves the sample. The out-diffusion starts between 250 and 300 “C.
4. Conclusion The Nb/Hf films prepared at room temperature behaved qualitatively as expected. We found a strong enhancement of the hydrogen concentration in the hafnium layer. Quantitatively, there remain some open questions: why does the hydrogen concentration in hafnium not reach the value of the Hf-H b-phase as expected from free energy considerations? Connected with that, why does the hafnium film not absorb all the hydrogen from the niobium layers? Concerning the film preparation, what is the optimal substrate temperature to obtain good epitaxial growth but avoid intermixing? The Nb/Ti film was prepared at too high a temperature and showed intermixing. Hydrogen was almost equally distributed in the film although the basic layer structure Nb/Ti/Nb was realized as seen from the Rutherford backscattering spectra. Hydrogen can be retained in the film between the gold layer and the substrate up to temperatures of 250 “C and the redistribution of the profile in the Nb/Ti/Nb film can be studied up to this temperature.
Acknowledgment
schaft
The work was supported (SFB 306).
in part by the Deutsche
Forschungsgemein-
References 1 H. Zabel and P. F. Micelli, 2. Phys. Chem. N. F., 163 (1989) 3. P. F. Micelli and H. Zabel, Phys. Reu. Lett., 59 (1987) 1224. 2 J. Ryden, B. Hjorvarsson, T. Ericsson, E. Karlsson, A. Krozer and B. Kasemo, 2. Phys. Chem. N. F., 164 (1989) 1259. 3 W. Wagner, F. Rauch, C. Ottermann and K. Bange, Nucl. In&rum. Methods B, 50 (1990) 27. 4 Y. Q. Sheng, P. Ziegler, E. Recknagel, 0. Boebel, J. Steiger and A. Weidinger, Phys. Rev. B, 41 (1990) 9794. 5 J. Steiger, 0. Boebel, M. Briere, A. Weidinger and P. Ziegler, Nucl. Znstrum. Methods B, 50 (1990) 31.
798 6 T. Schober and H. Wenzl, in G. Alefeld and J. Viilkl (eds.), Hydrogen in Metals II, Topics in Applied Physics, Vol. 29, Springer, Berlin, 1978, p. 36. 7 D. J. Leich and T. A. Tombrello, Nucl. Instrum Methods, 108 (1973) 67. 8 N. Bohr, Kgr. Dan. Vidensk. Selsk. Math. Fys. Medd., 18(4) (1971). 9 G. G. Libowitz, Binary Metal Hydrides, Benjamin, New York, 1965. 10 L. R. Doolittle, Nucl. Znstrum. Methods B, 9 (1985) 344.