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Hydrogen-termination effects on the growth of Ag thin films on Si(111) surfaces
Surface
152
Hydrogen-termination Si( 111) surfaces Masamichi
Received
Naitoh,
Fumiya
23 May 1990; accepted
effects on the growth
Shoji
for publication
and
Kenjiro
Science 242 (1991) 1522156 North-Holland
of Ag thin films on
Oura
16 July 1990
Using elastic recoil detection analysis (ERDA) and low energy electron diffraction (LEED), we have studied the hydrogen termination effects on the growth of Ag thin films on Si(ll1) surfaces. We have found for the first time that adsorbed hydrogen of ahout 1.5 monolayer strongly affects the initial growth process of Ag thin films and promotes the formation of parallel-oriented Ag(ll1) crystallites, Ag(lll)[ll~]~~Si(lll)[ll~]. Absolute coverage of hydrogen measured by ERDA has shown a rather large reduction during the Ag film growth, indicative of the occurrence of the atomic replacement of Ag and hydrogen.
1. Introduction Hydrogen adsorption is one of the most fundamental processes in adsorbate-substrate interactions, and various surface-sensitive techniques have been extensively applied [l]. However, owing to the difficulty in the detection of surface hydrogen by conventional electron-spectroscopic methods, absolute coverage information has usually been lacking in such adsorption studies. Recently, we have successfully applied the elastic recoil detection analysis (ERDA) method by 6 MeV F3+ ion beams to obtain an absolute coverage of hydrogen adsorbed on Si(lOO)-2 X 1 [2] and Si(lll)-7 X 7 [3] surfaces. The ERDA method is simple, reliable and nondestructive and therefore it can be applied to many kinds of more complicated systems involving hydrogen. In a previous paper [4] we reported a new phenomenon of the hydrogen-induced reordering of a Si(lll)fi X fi-Ag surface: room temperature adsorption of atomic hydrogen on the 6 X fi-Ag surface induces a structural transformation from the fi X fi-Ag to a 1 x I-Ag and thermal desorption of hydrogen at higher temperatures causes the recovery of the original 6 x fi-Ag structure. In this paper, we have studied the hydrogen pre-adsorption effect of Si(lll)-7 x 7 surfaces on 003%6028/91/$03.50
fi) 1991
Elsevier Science Publishers
the growth of Ag thin films. Based on experiments involving the combined used of the ERDA method utilizing 6 MeV F’+ ion beams and of the low-energy electron diffraction (LEED) method, we have found out for the first time that 1.5 monolayers (ML) of adsorbed hydrogen strongly affects the initial stage of thin film growth and promotes the growth of epitaxially grown Ag(ll1) crystallites, Ag(lll)[ll~]]~Si(lll)[ll~]. Absolute coverage of hydrogen measured by ERDA has shown a rather large reduction during the Ag film growth, indicative of the occurrence of replacement of Ag and hydrogen atoms on the surface.
2. Experimental Experiments were carried out in an ultra-high vacuum chamber as schematically shown in fig. 1. A 6 MeV F3+ ion beam from a tandem accelerator impinges on the specimen with an angle of 20 o measured from the surface. The beam is collimated to a size of 1 mm diameter and the current is about 3-5 particle nanoamperes. Hydrogen particles recoiled from the specimen can be detected by a silicon surface barrier detector (SSBD) after penetrating an Al filter (thickness of 5.7 pm). Other particles, such as scattered F or recoiled
B.V. (North-Holland)
and Yamada
Science Foundation
M. Nairoh et al. / Hydrogen-termnation
/ FILAMENT
DETECTOR\
\-‘...;1 LEED &"AMBER
Fig. 1. Schematic illustration of the experimental arrangement. A tungsten filament was used to dissociate molecular hydrogen into atomic hydrogen.
heavy elements (Si), are stopped within the Al filter. The SSBD is at 40” to the beam direction and 3 cm away from the specimen. The solid angle of the SSBD is about 3.78 X lo-” sr. The specimen surface can be characterized by a four-grid LEED. A P-doped, 8 Q cm, 8 X 20 X 0.5 mm3 Si(l11) wafer was pre-oxidized and cleaned in situ by direct current heating at 1000°C under a base pressure of 3 X lo-‘” Torr. After the cleaning, a sharp 7 X 7 LEED pattern was observed. A 1800” C tungsten filament, 12 cm from the Si surface, was used to dissociate molecular hydrogen. The exposures were made with the specimen facing the filament and by backfilling the chamber with H, at 5 X lo-’ Torr. Hydrogen coverages at various stages of adsorption and of film growth were obtained at an incident F3+ ion charge of 2 PC, which corresponds an ion dose of 2 x 10” ions cm m2. Ag was evaporated from a tungsten conical basket heater at a deposition rate of roughly 0.5 ML/min, where 1 ML is defined to be 7.8 x lOI atoms cm-2, the ideal Si density in the (111) plane.
3. Results and discussion We have ever studied hydrogen adsorption on a clean Si(lll)-7 x 7 surface kept at RT [3]. We found that the adsorption curve initially reveals a
effects on the growth of Ag on Si(1 II)
153
rather steep increase but it gradually saturates to a value of about 1.5 monolayer (ML) after an exposure of 1000 X lop6 Torr s, i.e., 1000 L (1 L = 1 Langmuir = 1 x lo-” Torr s). LEED patterns taken from the clean surface and the saturated adsorption of hydrogen are shown in figs. 2(a) and (b), respectively. Ag thin film growth has been studied on the latter surface. Shown in fig. 2 are LEED patterns taken at successive stages of Ag deposition at 300°C onto a Si(ll1) substrate carrying adsorbed hydrogen layers saturated at RT prior to Ag deposition. As we can see in fig. 2(c), Ag deposition of 1 ML thickness at 300 o C makes the f order reflection which can be seen in fig. 2(b) around the Si(ll1) integer reflection disappear, and only Si(ll1)) 1 x 1 pattern can be clearly observed. When Ag of 2.5 ML thickness is deposited, however, sharp reflections which can be attributed to an epitaxially grown Ag(ll1) plane start to develop as can be seen in fig. 2(d). With the increase of Ag thickness, such Ag(ll1) reflections become much stronger than the Si(l11). The present observation on hydrogen-adsorbed surfaces is of great importance since it is in sharp contrast with the feature observed on a clean Si(ll1) substrate. It is well known [5] that the growth mode of Ag thin films is layer-plus-islands (Stranski- Krastanov mode) on a clean Si(lll)-7 X 7 surface kept at high temperatures (above 250°C) and that only the fi X fi-Ag reflections due to the two-dimensional adsorption layer can be observed in LEED or RHEED patterns with no noticeable reflections due to Ag islands for low-coverage deposits such as lo-15 MLs. The above described results strongly suggest that 1.5 ML of adsorbed hydrogen affects the initial growth process of Ag and promotes the growth of epitaxial Ag( 111) crystallites, Ag(111)[112]]~Si(111)[112]. In other words, Ag(ll1) crystallites formed on the hydrogenadsorbed Si(ll1) substrate cover a much wider area of the substrate than that on the clean Si(ll1) substrate. A similar hydrogen effect on the Ag film growth has also been observed for the deposition at room temperature (RT). LEED patterns reproduced in figs. 3(a) and (b) were taken from 4g thin films deposited at RT onto Si(l11) substrates with and
154
Fig. 2. LEED patterns
M. Narroh er cd
taken from (a) clean Si(l1 l)-7 X 7 (62 eV), (b) Si(ll I)-7 X 7-H formed after 1000 L exposure (54 eV) taken at successive stages of Ag deposition at 300°C onto the specimen for(c), (d). (e), and (f) are 1, 2.5. 5. and 10 ML. respectively.
(c). (d), (e) and (f) are LEED patterns
1’1g. 3. WEI) patterm ohserved for the RT deposition of Ag (a) with and (b) without r~~t;it~~w;~ldisorder of Ag( I1 1) crystallite& ia relatively small on the hydrogen terminated on the clean Si substrate.
hydrogen termination surface, ib) rotational
at RT (62 eV). (b). Ag deposit
of Si(ll1) surfaces: (a) disorder is remarkable
155
M. Nairoh et al. / Hydrogen-termination effects on the growth of Ag on Si(l I I)
without hydrogen adsorption prior to Ag deposition, respectively. As we can see in fig. 3(b), several azimuthally-rotated Ag(lll) crystallites are formed on clean Si(ll1) substrates, in agreement with previous works [5-71. In contrast, when 1.5 ML of atomic hydrogen are adsorbed prior to Ag deposition, the growth of parallel oriented Ag crystallites i.e., Ag(lll)[ll~]~lSi(lll)[ll2] becomes dominant, as shown in fig. 3(a). From the comparison of figs. 2(f) and 3(a), it can also be seen that Ag deposition temperatures onto hydrogenterminated substrates affect on the Ag(ll1) crystallite formation; rotational disorder still remains slightly in the RT deposit. In fig. 4 absolute hydrogen coverages determined in situ by ERDA during Ag film growth at two substrate temperatures have been plotted against deposition time (dotted lines). Also shown in the figure by solid lines are hydrogen coverage variations with no Ag deposition; at the substrate temperature of 350 o C, hydrogen coverage gradually decreases from the initial value of 1.4 ML to 0.4 ML, probably due to thermal desorption. On the other hand, no noticeable decrease can be seen for the RT clean substrate. When Ag is deposited at 350 o C, hydrogen coverage decreases in a similar way to that observed when no Ag is deposited. In contrast, when Ag is deposited at RT, hydrogen coverage is found to decrease more steeply than that for the clean Si surface with no Ag deposi-
350%
-
FIT
-
-1.5
H/Si(lll) -*- Ag/H/Si( 111)
4
tion. Although the decrease in hydrogen coverage at RT may be apparently interpreted in terms of the screening effect due to overlying Ag layers, this is not the case in our present ERDA method; the depth resolution of the present ERDA method utilizing 6 MeV F’+ ion beams is about 200 A and its probing depth extends more than 3000 A [8]. Therefore, using Ag thin films of lo-15 ML thickness it is hardly possible to screen the underlying hydrogen atoms. Thus, the observed decrease at RT suggests the occurrence of the Ag-induced desorption of hydrogen at the specimen surface. Among several kinds of hydrogenPsilicon bonds, the hydrogen of weaker bonds may be probably replaced by the deposited Ag. As mentioned before, the hydrogen coverage gradually decreases with the increase in Ag deposits at 350 “C and it is to be noted here on an observed correlation between the hydrogen coverage and the appearance of LEED patterns; when the hydrogen coverage decreases to about 0.3 ML, the initially observed sharp spots due to Ag(ll1) crystallites disappear and only the Si(lll)) &x&ptc s o s an be clearly observed, indicative of the dominance of the two-dimensional adsorption layer of Ag. This result indicates that the parallel-oriented Ag(ll1) crystallites which cover a relatively broad area of the substrate are stabilized by hydrogen at the interface and that when hydrogen desorption takes place such broad
H/Si(lll)
+- Ag/H/Si( 111) 1.5 a
2 z
-1.0
1.0
z 5
0 ..
---b_
a 0
2
0 ---b-_-_b__
4
6
TIME
[min.]
8
0 10
a 2
4 0.5 s ”
0.5 ”6 0.0
Y
b 4
8
12 TIME
16
20
24
28
32
[min.]
Fig. 4. Hydrogen concentration (dotted lines) during the Ag film growth on hydrogen terminated Si(ll1) surfaces function of the deposition time for two deposition temperatures: (a) 350°C and (b) RT. The Ag deposition ML/min. The solid lines represent the result obtained when no Ag is deposited.
plotted against as a rate is roughly 0.5
156
M. Nurroh CI 01 / Hvdrogrn-termr,larron
Ag( 111) crystallites are likely thicker crystallites which only stricted area of the Si substrate.
to coalesce into cover a very re-
4. Summary We have studied hydrogen termination effects on the growth of Ag thin films on Si(lll) surfaces by a combined technique of ERDA and LEED. We have found that adsorbed hydrogen of about 1.5 ML strongly affects the initial growth process of Ag thin films and promotes the formation of parallel oriented Ag( 111) crystallites. For the deposition at RT, hydrogen termination remarkably suppresses the rotational disorder of Ag(ll1) films which is often observed on clean Si(ll1) surfaces On a hydrogen terminated substrate kept at 300 o C, a sharp LEED pattern due to Ag(ll1) epitaxial films can be observed from the initial stage of Ag deposition (2 ML). This result is quite different from the case of clean Si substrate, where LEED spots due to Ag(l11) can be never observed in such an initial growth stage. Absolute hydrogen coverage determined in situ by ERDA has shown a rather large reduction, during the film growth. indicative of the occurrence of atomic replacement of Ag and hydrogen.
effwrs
on the growth of Ag on .%(I I I)
Acknowledgements
The authors would like to acknowledge Emeritus Professor T. Hanawa for his support to the present work. Thanks are also due to Dr K. Umezawa and Mr S. Andoh for their assistance in the experiment. Part of this work was supported by a Grant-inAid for Scientific Research from the Ministry of Education. Science and Culture. Japan.
References [I] For example. see: H. Froitzheim. in: The Chemical Phyvcs of Solid Surfaces and Heterogeneous Catalysis, Vol. 5.Eds. D.A. King and D.P. Woodruff (Elsevier. Amsterdam. 1988). [2] K. Oura. J. Yamane. K. Umezawa. M. Naitoh, F. Shoji and T. Hanawa. Phys. Rev. B 41 (1990) 1200. [3] K. Oura. M. Naitoh, F. Shqi, J. Yamane. K. Umeawa and T. Hanawa. Nucl. lnstrum. Methods B 45 (1990) 199. 141 K. Oura. M. Naitoh. J. Yamane and F. Sho.ji. Surf. SCI. Lett. 230 (1990) L151. [5] For B review. see: G. Le Lay, Surf. Sci. I32 (19X3) 169. [6] M. Saitoh. F. Shqi. K. Oura and T. Hanawa. Surf. Sci. I I? (19X1) 306. [7] E.J. van Loenen. M. lwami, R.M. Tromp and J.F. van Jcr Veen. Surf. Sci. 137 (1984) 1. [X] K. Umezawa, PhD Thesis. Osaka University (19x9). 111 Japanese.
152
Hydrogen-termination Si( 111) surfaces Masamichi
Received
Naitoh,
Fumiya
23 May 1990; accepted
effects on the growth
Shoji
for publication
and
Kenjiro
Science 242 (1991) 1522156 North-Holland
of Ag thin films on
Oura
16 July 1990
Using elastic recoil detection analysis (ERDA) and low energy electron diffraction (LEED), we have studied the hydrogen termination effects on the growth of Ag thin films on Si(ll1) surfaces. We have found for the first time that adsorbed hydrogen of ahout 1.5 monolayer strongly affects the initial growth process of Ag thin films and promotes the formation of parallel-oriented Ag(ll1) crystallites, Ag(lll)[ll~]~~Si(lll)[ll~]. Absolute coverage of hydrogen measured by ERDA has shown a rather large reduction during the Ag film growth, indicative of the occurrence of the atomic replacement of Ag and hydrogen.
1. Introduction Hydrogen adsorption is one of the most fundamental processes in adsorbate-substrate interactions, and various surface-sensitive techniques have been extensively applied [l]. However, owing to the difficulty in the detection of surface hydrogen by conventional electron-spectroscopic methods, absolute coverage information has usually been lacking in such adsorption studies. Recently, we have successfully applied the elastic recoil detection analysis (ERDA) method by 6 MeV F3+ ion beams to obtain an absolute coverage of hydrogen adsorbed on Si(lOO)-2 X 1 [2] and Si(lll)-7 X 7 [3] surfaces. The ERDA method is simple, reliable and nondestructive and therefore it can be applied to many kinds of more complicated systems involving hydrogen. In a previous paper [4] we reported a new phenomenon of the hydrogen-induced reordering of a Si(lll)fi X fi-Ag surface: room temperature adsorption of atomic hydrogen on the 6 X fi-Ag surface induces a structural transformation from the fi X fi-Ag to a 1 x I-Ag and thermal desorption of hydrogen at higher temperatures causes the recovery of the original 6 x fi-Ag structure. In this paper, we have studied the hydrogen pre-adsorption effect of Si(lll)-7 x 7 surfaces on 003%6028/91/$03.50
fi) 1991
Elsevier Science Publishers
the growth of Ag thin films. Based on experiments involving the combined used of the ERDA method utilizing 6 MeV F’+ ion beams and of the low-energy electron diffraction (LEED) method, we have found out for the first time that 1.5 monolayers (ML) of adsorbed hydrogen strongly affects the initial stage of thin film growth and promotes the growth of epitaxially grown Ag(ll1) crystallites, Ag(lll)[ll~]]~Si(lll)[ll~]. Absolute coverage of hydrogen measured by ERDA has shown a rather large reduction during the Ag film growth, indicative of the occurrence of replacement of Ag and hydrogen atoms on the surface.
2. Experimental Experiments were carried out in an ultra-high vacuum chamber as schematically shown in fig. 1. A 6 MeV F3+ ion beam from a tandem accelerator impinges on the specimen with an angle of 20 o measured from the surface. The beam is collimated to a size of 1 mm diameter and the current is about 3-5 particle nanoamperes. Hydrogen particles recoiled from the specimen can be detected by a silicon surface barrier detector (SSBD) after penetrating an Al filter (thickness of 5.7 pm). Other particles, such as scattered F or recoiled
B.V. (North-Holland)
and Yamada
Science Foundation
M. Nairoh et al. / Hydrogen-termnation
/ FILAMENT
DETECTOR\
\-‘...;1 LEED &"AMBER
Fig. 1. Schematic illustration of the experimental arrangement. A tungsten filament was used to dissociate molecular hydrogen into atomic hydrogen.
heavy elements (Si), are stopped within the Al filter. The SSBD is at 40” to the beam direction and 3 cm away from the specimen. The solid angle of the SSBD is about 3.78 X lo-” sr. The specimen surface can be characterized by a four-grid LEED. A P-doped, 8 Q cm, 8 X 20 X 0.5 mm3 Si(l11) wafer was pre-oxidized and cleaned in situ by direct current heating at 1000°C under a base pressure of 3 X lo-‘” Torr. After the cleaning, a sharp 7 X 7 LEED pattern was observed. A 1800” C tungsten filament, 12 cm from the Si surface, was used to dissociate molecular hydrogen. The exposures were made with the specimen facing the filament and by backfilling the chamber with H, at 5 X lo-’ Torr. Hydrogen coverages at various stages of adsorption and of film growth were obtained at an incident F3+ ion charge of 2 PC, which corresponds an ion dose of 2 x 10” ions cm m2. Ag was evaporated from a tungsten conical basket heater at a deposition rate of roughly 0.5 ML/min, where 1 ML is defined to be 7.8 x lOI atoms cm-2, the ideal Si density in the (111) plane.
3. Results and discussion We have ever studied hydrogen adsorption on a clean Si(lll)-7 x 7 surface kept at RT [3]. We found that the adsorption curve initially reveals a
effects on the growth of Ag on Si(1 II)
153
rather steep increase but it gradually saturates to a value of about 1.5 monolayer (ML) after an exposure of 1000 X lop6 Torr s, i.e., 1000 L (1 L = 1 Langmuir = 1 x lo-” Torr s). LEED patterns taken from the clean surface and the saturated adsorption of hydrogen are shown in figs. 2(a) and (b), respectively. Ag thin film growth has been studied on the latter surface. Shown in fig. 2 are LEED patterns taken at successive stages of Ag deposition at 300°C onto a Si(ll1) substrate carrying adsorbed hydrogen layers saturated at RT prior to Ag deposition. As we can see in fig. 2(c), Ag deposition of 1 ML thickness at 300 o C makes the f order reflection which can be seen in fig. 2(b) around the Si(ll1) integer reflection disappear, and only Si(ll1)) 1 x 1 pattern can be clearly observed. When Ag of 2.5 ML thickness is deposited, however, sharp reflections which can be attributed to an epitaxially grown Ag(ll1) plane start to develop as can be seen in fig. 2(d). With the increase of Ag thickness, such Ag(ll1) reflections become much stronger than the Si(l11). The present observation on hydrogen-adsorbed surfaces is of great importance since it is in sharp contrast with the feature observed on a clean Si(ll1) substrate. It is well known [5] that the growth mode of Ag thin films is layer-plus-islands (Stranski- Krastanov mode) on a clean Si(lll)-7 X 7 surface kept at high temperatures (above 250°C) and that only the fi X fi-Ag reflections due to the two-dimensional adsorption layer can be observed in LEED or RHEED patterns with no noticeable reflections due to Ag islands for low-coverage deposits such as lo-15 MLs. The above described results strongly suggest that 1.5 ML of adsorbed hydrogen affects the initial growth process of Ag and promotes the growth of epitaxial Ag( 111) crystallites, Ag(111)[112]]~Si(111)[112]. In other words, Ag(ll1) crystallites formed on the hydrogenadsorbed Si(ll1) substrate cover a much wider area of the substrate than that on the clean Si(ll1) substrate. A similar hydrogen effect on the Ag film growth has also been observed for the deposition at room temperature (RT). LEED patterns reproduced in figs. 3(a) and (b) were taken from 4g thin films deposited at RT onto Si(l11) substrates with and
154
Fig. 2. LEED patterns
M. Narroh er cd
taken from (a) clean Si(l1 l)-7 X 7 (62 eV), (b) Si(ll I)-7 X 7-H formed after 1000 L exposure (54 eV) taken at successive stages of Ag deposition at 300°C onto the specimen for(c), (d). (e), and (f) are 1, 2.5. 5. and 10 ML. respectively.
(c). (d), (e) and (f) are LEED patterns
1’1g. 3. WEI) patterm ohserved for the RT deposition of Ag (a) with and (b) without r~~t;it~~w;~ldisorder of Ag( I1 1) crystallite& ia relatively small on the hydrogen terminated on the clean Si substrate.
hydrogen termination surface, ib) rotational
at RT (62 eV). (b). Ag deposit
of Si(ll1) surfaces: (a) disorder is remarkable
155
M. Nairoh et al. / Hydrogen-termination effects on the growth of Ag on Si(l I I)
without hydrogen adsorption prior to Ag deposition, respectively. As we can see in fig. 3(b), several azimuthally-rotated Ag(lll) crystallites are formed on clean Si(ll1) substrates, in agreement with previous works [5-71. In contrast, when 1.5 ML of atomic hydrogen are adsorbed prior to Ag deposition, the growth of parallel oriented Ag crystallites i.e., Ag(lll)[ll~]~lSi(lll)[ll2] becomes dominant, as shown in fig. 3(a). From the comparison of figs. 2(f) and 3(a), it can also be seen that Ag deposition temperatures onto hydrogenterminated substrates affect on the Ag(ll1) crystallite formation; rotational disorder still remains slightly in the RT deposit. In fig. 4 absolute hydrogen coverages determined in situ by ERDA during Ag film growth at two substrate temperatures have been plotted against deposition time (dotted lines). Also shown in the figure by solid lines are hydrogen coverage variations with no Ag deposition; at the substrate temperature of 350 o C, hydrogen coverage gradually decreases from the initial value of 1.4 ML to 0.4 ML, probably due to thermal desorption. On the other hand, no noticeable decrease can be seen for the RT clean substrate. When Ag is deposited at 350 o C, hydrogen coverage decreases in a similar way to that observed when no Ag is deposited. In contrast, when Ag is deposited at RT, hydrogen coverage is found to decrease more steeply than that for the clean Si surface with no Ag deposi-
350%
-
FIT
-
-1.5
H/Si(lll) -*- Ag/H/Si( 111)
4
tion. Although the decrease in hydrogen coverage at RT may be apparently interpreted in terms of the screening effect due to overlying Ag layers, this is not the case in our present ERDA method; the depth resolution of the present ERDA method utilizing 6 MeV F’+ ion beams is about 200 A and its probing depth extends more than 3000 A [8]. Therefore, using Ag thin films of lo-15 ML thickness it is hardly possible to screen the underlying hydrogen atoms. Thus, the observed decrease at RT suggests the occurrence of the Ag-induced desorption of hydrogen at the specimen surface. Among several kinds of hydrogenPsilicon bonds, the hydrogen of weaker bonds may be probably replaced by the deposited Ag. As mentioned before, the hydrogen coverage gradually decreases with the increase in Ag deposits at 350 “C and it is to be noted here on an observed correlation between the hydrogen coverage and the appearance of LEED patterns; when the hydrogen coverage decreases to about 0.3 ML, the initially observed sharp spots due to Ag(ll1) crystallites disappear and only the Si(lll)) &x&ptc s o s an be clearly observed, indicative of the dominance of the two-dimensional adsorption layer of Ag. This result indicates that the parallel-oriented Ag(ll1) crystallites which cover a relatively broad area of the substrate are stabilized by hydrogen at the interface and that when hydrogen desorption takes place such broad
H/Si(lll)
+- Ag/H/Si( 111) 1.5 a
2 z
-1.0
1.0
z 5
0 ..
---b_
a 0
2
0 ---b-_-_b__
4
6
TIME
[min.]
8
0 10
a 2
4 0.5 s ”
0.5 ”6 0.0
Y
b 4
8
12 TIME
16
20
24
28
32
[min.]
Fig. 4. Hydrogen concentration (dotted lines) during the Ag film growth on hydrogen terminated Si(ll1) surfaces function of the deposition time for two deposition temperatures: (a) 350°C and (b) RT. The Ag deposition ML/min. The solid lines represent the result obtained when no Ag is deposited.
plotted against as a rate is roughly 0.5
156
M. Nurroh CI 01 / Hvdrogrn-termr,larron
Ag( 111) crystallites are likely thicker crystallites which only stricted area of the Si substrate.
to coalesce into cover a very re-
4. Summary We have studied hydrogen termination effects on the growth of Ag thin films on Si(lll) surfaces by a combined technique of ERDA and LEED. We have found that adsorbed hydrogen of about 1.5 ML strongly affects the initial growth process of Ag thin films and promotes the formation of parallel oriented Ag( 111) crystallites. For the deposition at RT, hydrogen termination remarkably suppresses the rotational disorder of Ag(ll1) films which is often observed on clean Si(ll1) surfaces On a hydrogen terminated substrate kept at 300 o C, a sharp LEED pattern due to Ag(ll1) epitaxial films can be observed from the initial stage of Ag deposition (2 ML). This result is quite different from the case of clean Si substrate, where LEED spots due to Ag(l11) can be never observed in such an initial growth stage. Absolute hydrogen coverage determined in situ by ERDA has shown a rather large reduction, during the film growth. indicative of the occurrence of atomic replacement of Ag and hydrogen.
effwrs
on the growth of Ag on .%(I I I)
Acknowledgements
The authors would like to acknowledge Emeritus Professor T. Hanawa for his support to the present work. Thanks are also due to Dr K. Umezawa and Mr S. Andoh for their assistance in the experiment. Part of this work was supported by a Grant-inAid for Scientific Research from the Ministry of Education. Science and Culture. Japan.
References [I] For example. see: H. Froitzheim. in: The Chemical Phyvcs of Solid Surfaces and Heterogeneous Catalysis, Vol. 5.Eds. D.A. King and D.P. Woodruff (Elsevier. Amsterdam. 1988). [2] K. Oura. J. Yamane. K. Umezawa. M. Naitoh, F. Shoji and T. Hanawa. Phys. Rev. B 41 (1990) 1200. [3] K. Oura. M. Naitoh, F. Shqi, J. Yamane. K. Umeawa and T. Hanawa. Nucl. lnstrum. Methods B 45 (1990) 199. 141 K. Oura. M. Naitoh. J. Yamane and F. Sho.ji. Surf. SCI. Lett. 230 (1990) L151. [5] For B review. see: G. Le Lay, Surf. Sci. I32 (19X3) 169. [6] M. Saitoh. F. Shqi. K. Oura and T. Hanawa. Surf. Sci. I I? (19X1) 306. [7] E.J. van Loenen. M. lwami, R.M. Tromp and J.F. van Jcr Veen. Surf. Sci. 137 (1984) 1. [X] K. Umezawa, PhD Thesis. Osaka University (19x9). 111 Japanese.