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The interaction of gold with the surface of a cylindrical tungsten single crystal
394
Surface Science 169 (1986) 394-404 North-Holland, Amsterdam
THE INTERACTION OF GOLD WITH THE SURFACE OF A CYLINDRICAL TUNGSTEN SINGLE CRYSTAL S. M R O Z * a n d E. B A U E R Physikalisches Institut der Technischen Universiti~t Clausthal, Leibnizstrasse 4, D3392 Clausthal-Zellerfeld, Fed. Rep. of Germany
Received 26 August 1985; accepted for publication 29 November 1985
Gold layers on the surface of a cylindrical tungsten single crystal were investigated by LEED and work function change measurements. Work function changes during gold adsorption on (110), (111), (112), (115) and (001) faces, and structures deduced from LEED patterns for (110), (112), (332) and (001) faces are presented with a common coverage scale. Upon heating the gold-covered crystal up to 1000 K, faceting occurs with faceted regions as large as 20°, 13° and 4° around the [112], [110] and [332] directions, respectively. The diffusion of the gold layer was studied by the movement of the sharp boundary across the cylindrical crystal surface at a temperature of 1000 K. The diffusion rate was found to be much higher in the (112) region than in the (001) region.
1. Introduction The interaction of gold with different microscopic tungsten surfaces has b e e n investigated extensively o n t u n g s t e n tips by field emission microscopy [1-9] and field ion microscopy [7-10]. In these studies the d i s t r i b u t i o n of the deposit is initially i n h o m o g e n e o u s and a n n e a l i n g is necessary to distribute the material across the tip. At the same time material diffuses to the shank, agglomerates if more than the a d s o r b a b l e a m o u n t is deposited, a n d causes faceting. F u r t h e r m o r e , even in the absence of these complications, the distrib u t i o n over the tip is not h o m o g e n e o u s but d e t e r m i n e d by the differences in heat of a d s o r p t i o n a n d to a certain extent - also by differences in the activation energy for surface diffusion between the different planes. These difficulties do not exist o n large flat macroscopic single crystals on which the a d s o r p t i o n of Au was studied by L E E D , AES a n d work f u n c t i o n change (A0) m e t h o d s [11-18]. These studies have p r o d u c e d a considerable a m o u n t of i n f o r m a t i o n on work f u n c t i o n changes a n d the crystalline structures of the adsorbed layers b u t the i n f o r m a t i o n is o b t a i n e d separately for each * On leave from Institute of Experimental Physics. University of Wroc3aw, ul. Cybulskiego 36, PL 50-205 Wrodaw, Poland. 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
s. Mrbz, E. Bauer / Interaction of gold with tungsten
395
particular face. Using a large cylindrical single crystal which is continuously rotated during deposition, it is possible to perform such investigations simultaneously for m a n y faces. Thus, the work function changes and adsorbed layer structures can be obtained on a common coverage scale for all faces investigated. The equilibrium shape of a crystal should be strongly influenced by an adsorbed layer provided that the temperature is high enough so that equilibrium can be reached within acceptable times. This is the case for crystal temperatures T > 0.3Tm, Tm being the melting temperature of the crystal [19]. For tungsten, this is the case for T > 1000 K. Most experimental investigations of the equilibrium shape of crystals were, up to date, performed on very small samples (tips in field emission microscopes and small drops). The results of such investigations have been reviewed by Drechsler [19]. On a macroscopic cylindrical single crystal it is not possible to see if the equilibrium shape induced by the adsorption is directly due to the long diffusion distances necessary. However, it can be expected that terraces parallel to the most stable faces of the crystal will grow during heating of the crystal covered with gold and that this growth can be seen by LEED. Surface diffusion of gold across the cylindrical crystal surface should be easily observed by A~ measurements if the sharp boundary of the adsorbed layer - built up during adsorption with the use of a proper shutter - is maintained during diffusion. Diffusion rates in different surface regions can thus be compared in such experiments. The goal of the present work is to verify and use the possibilities mentioned above.
2. Experimental Measurements were performed in a metal ultrahigh-vacuum system described in detail elsewhere [20]. The system was pumped with a titanium ion p u m p and a titanium sublimation p u m p cooled with liquid nitrogen. The residual gas pressure during the measurements was in the low 10 - 9 Pa range. The tungsten crystal was a hollow cylinder with a length of 15.8 m m and inner and outer diameters of 7.7 and 8.5 mm, respectively. Inside the crystal was a tungsten filament which could be heated resistively and which was used to heat the crystal by thermal radiation or by electron bombardment. The crystal could be rotated around the cylinder axis which was parallel to the [110] direction at a constant rate of one full rotation per 100 s. To enable unlimited rotation, the crystal was not equipped with a thermocouple and the crystal heating system was calibrated with an optical pyrometer before measurements. One flange of the vacuum system was assigned for a C M A or for a
396
S. Mrbz, E. Bauer / Interaction of gold with tungsten
four-grid L E E D system. Therefore, it was impossible to perform AES analysis and L E E D observations simultaneously (as will be seen below, the intensity of the primary electron beam of the L E E D system was too low to perform the AES analysis with this system). On another flange, an electron gun for the work function change measurements with the retarding field electron beam method was mounted with its axis perpendicular to the crystal axis. The electron beam produced in this gun was magnetically focused with a molybdenum coil wound around the gun and collimated with a slit 6 m m long and 0.13 m m wide placed at a distance of a few millimeters from the crystal surface parallel to the cylinder axis. A~ was measured in the constant current mode which is obtained by automatic adjustment of the voltage between cathode and crystal while the crystal was rotating. The voltage change was recorded as a function of the crystal rotation angle with the help of an X - Y recorder and its difference with respect to the clean surface is the work function change divided by the electron charge: A V = A~/e. The gold source was a ceramic tube with inner and outer diameters of 3 and 4 ram, respectively, and a length of 12 m m which was pressed into a tight 0.3 m m diameter tungsten wire spiral used for resistive heating. The source was surrounded by two concentric tantalum radiation shields. The distance between source and crystal was about 10 cm which ensured homogeneous coverage of the crystal. Two movable shutters were placed between the gold source and the crystal. Shutter I which was close to the source shielded the crystal completely and was used during outgassing of the source. Shutter II was placed very close to the crystal surface (~-0.5 m m distance) and had a sharp edge parallel to the cylinder axis which produced a sharp boundary of the gold layer during deposition on the crystal surface needed for the investigation of the surface diffusion of gold. For the L E E D observations the primary electron beam diameter was reduced by an aperture of 0.2 m m in diameter at the end of the electron gun. This was necessary in order to obtain the high lateral resolution needed for a cylindrical crystal surface. The resulting loss in beam intensity was compensated by viewing the L E E D pattern with a videocamera of especially high sensitivity and displaying it on a TV monitor. The crystal surface was cleaned by heating at 1400 K in oxygen (po2 = 2 x 10 6 Pa) followed by flashing to about 2200 K. The effectiveness of this cleaning procedure was checked at the beginning of the measurements by AES analysis with a Varian CMA system. Subsequently, the C M A head was replaced with the L E E D optics. A more detailed description of the crystal manipulator and of the A~ measuring system is presented in refs. [20,21].
S. Mrbz, E. Bauer / Interaction of goM with tungsten
397
3. Results and discussion
A¢
3.1.
A~ was measured continuously during evaporation of gold onto the rotating crystal. Results for (001), (112), (111), (110) and (115) faces obtained for an electron beam current of 16 nA (at a saturation current of 50-70 nA depending upon the crystallographic direction of the crystal) are presented in fig. 1. The gold coverage is given in number of doses (one dose was obtained during one full rotation of the crystal, i.e. during 100 s) and in number of gold atoms per cm 2. This last scale was obtained on the assumption that the maximum of the work function appears for the (112) face at 6.3 × 10 TM gold atoms per cm 2 as reported by Kolaczkiewicz and Bauer [18].
/~e [ev] 0.8
...-" 11
(110}
-'~.~_ ./ / /
0.6[
(001)/.~/ r.~2X--..T--.~.~ /" ...-'~.~
(112)
i
oi T....
t
/ d~
!0 ....
20 ,e,[~o~'~t~/,~ ~]
10
20
doses
Fig. 1. Work function changes ,A~bas a function of Au coverage 0 of selected low-index faces of the tungsten cylindrical crystal surface. A,# for a clean (001) face is taken as zero.
398
s. Mrbz, E. Bauer / Interaction of gold with tungsten
It was f o u n d that the w o r k function differences b e t w e e n the various faces of the crystal d e p e n d on the electron b e a m c u r r e n t used. This d e p e n d e n c e can result from c h a n g i n g c o n d i t i o n s of electron b e a m focusing a n d f r o m different slopes of the c u r r e n t - voltage characteristic of the system electron gun crystal for different crystal faces. Therefore, the exact A~ values m e a s u r e d on o n e a n d the same surface m a y have to be t a k e n with s o m e c a u t i o n too. This has, however, no influence on the relative coverage d e t e r m i n a t i o n which is b a s e d on the A~ m a x i m a a n d m i n i m a a n d which are o b t a i n e d for the first time s i m u l t a n e o u s l y for different tungsten faces. 3.2. L E E D p a t t e r n s
T h e L E E D p a t t e r n s o b t a i n e d after d e p o s i t i o n of different a m o u n t s of gold a n d heating the crystal for 5 m i n up to a b o u t 1000 K are d e s c r i b e d in fig. 2 for (001), (112), (110) a n d (332) faces. F o r o t h e r faces, L E E D p a t t e r n s clearly c o n n e c t e d to the gold layer have not been found. The results for the (001) a n d (110) faces are in a g r e e m e n t with the d a t a for flat m a c r o s c o p i c single-crystal surfaces [12]. It verifies the usefulness of the c y l i n d r i c a l crystal in e x p e r i m e n t s with a d s o r p t i o n on l o w - i n d e x faces. In view of this it is surprising that, besides
Structure X1×1)c(2x2) p(2xl)
c(2x2)
doublets in [1i0] direction i
p(4x4)
p(6x6)
p(3×1)
p(lxl)
:)(lxl)
Face
p(lxl) i
(001)
(112)
compticated patterns resutting from muttiple scattering
(110)
no structure
(332)
i
no
structure i
0
p(3×1) i
10
p(Bxl) i
i
20
i
8 [_1014at/cm 2]
Fig. 2. Structure of gold layers on (001), (112), (110) and (332) faces as a function of coverage. The LEED patterns were observed after heating the crystal for 5 min up to about 1000 K. The gold coverage was determined by A~ measurements before heating. The substrate unit mesh of the (112) and (332) planes are chosen with the b 1 axis parallel to the [110] direction.
S. Mrbz, E. Bauer / Interaction of gold with tungsten
399
the structures on the (001), (112) and (110) faces, clear patterns were observed only on the (332) face.
3.3. Faceting in the presence of the gold layer The faceting of the tungsten crystal surface was studied by moving the crystal across the electron beam at a fixed angular position of the crystal corresponding to a particular low-index plane while the LEED pattern was observed. For clean tungsten faces, the LEED pattern moved during the crystal motion due to the continuous change of the angle of incidence of the primary beam. In addition, the spots split due to the influence of the terraces of monatomic height within the coherence length of the primary beam. On the gold-covered surface which was heated for a few minutes up to about 1000 K the situation was in general different: within a certain range of crystal displacements across the primary electron beam the tungsten LEED spots stayed fixed and did not split. Furthermore, the superstructure of the adsorbed layer as seen in the LEED pattern did not change either. The range of crystal positions with unchanged LEED pattern was measured with the micrometer of +4-
20oi Ao~ o - (110) [. . . . o " - q
+ - (112) L
........ (112)
I
~4
A - (332)
15°
i I I
+
o I
(110) 100
o __20/° +! o
50
+
o 10
20
[lo'" otom /om ] 3O
Fig. 3. Width of the faceted regions on the crystal surface (given as an angle Aa) in the vicinity of the [112], [110] and [332] directions after heating the crystal for 5 min at 1000 K as a function of gold coverage.
S. Mrbz, E. Bauer / Interaction of gold with tungsten
400
the sample manipulator and recalculated into the angle on the crystal surface dominated by facets of given low-index orientation. The results of such a determination are presented in fig. 3. Gold coverages are taken from A0 measured during the gold deposition. The results of fig. 3 are semiquantitative because the L E E D pattern deteriorated more and more with increasing displacement of the crystal from the position of the best pattern and it was difficult to determine the exact position at which the pattern changed. Furthermore, the error in the coverage determination could be as large as 2 x 1014 a t o m s / c m 2 for large coverages. It is surprising that no faceting was observed for the (001) face. It was important to ensure that the crystal temperature was high enough to induce faceting and, simultaneously, low enough to prevent gold desorption. This was done by heating a gold-covered crystal (0 = 21 x 1014 a t o m s / c m 2) m a n y times for 5 min to increasingly higher temperature. The sizes of the faceted regions were measured after each heating. It was found that temperatures from 800 to above 1000 K are good for faceting.
eVl 0.81
0.6
~\
B
\ "", ~ t I / \ \ ~ ~ ' ~ / /,~.[ \
/ 0.4
"-.. ml]
'I\"J'~
C
C
\ txxJ'/l" [112] I \ [332] [334] [115]
0.2
[112] I [oo1]
180 o
150 °
120 o
90°
Fig. 4. A,;b profiles; (A) clean tungsten surface; (B) after deposition of gold at room temperature (0 = 22 x 1015 a t o m s / c m 2 ) ; (C) after heating the crystal for 5 min at 1000 K.
S. Mrbz, E. Bauer / Interaction of gold with tungsten
401
Additional information concerning the faceting process can be obtained from changes of the work function profile observed after heating the goldcovered crystal up to 1000 K. An example of such changes is shown in fig. 4. One can see in this figure that after heating new local maxima of the work function appear at the position of the (332), (115) and (334) faces. This can be interpreted as a result of the growth of facets being too small to be seen by LEED.
3.4. Prefiminary observations of gold surface diffusion Although the work function change is caused in general by Au adsorption and faceting, only Au adsorption is responsible for it at low coverages as seen A~ [eV] 0.8
0.6
j I
I
0.4
r,../
,,.
\
I.//B .~.l
0.2 /
[110]
-0.2
~ 180 o
[111]
150 °
[112]
120 °
v
[001] [115]
~. 1 900
[1'12]
60 °
Fig. 5. A~ profiles before and after diffusion of the sharp boundary of the gold layer; (A) before diffusion; (B) after diffusion (after heating the crystal for 55 min at 1000 K); (C) for clean crystal surface (after flash).
S. Mrbz, E. Bauer / Interaction of gold with tungsten
402
in fig. 3 by the absence of faceting below about 5 x 1011 Au a t o m s / c m 2. The diffusion front can, therefore, be assumed to be characteristic of the surface diffusion of Au if the coverage at the location of the diffusion front is sufficiently small. If this is not the case then Au diffusion and W surface restructuring may be involved simultaneously. This has to be kept in mind in the following. A sharp boundary of a thick ( 0 - - 3 0 x 1014 a t o m s / c m 2) gold layer approximately parallel to the cylinder axis was obtained using shutter II. Fig. 5 shows the work function profile when the edge of shutter II was placed between the (115) and (001) surface before (A) and after (B) eleven 5 min anneals at 1000 K together with that of the clean surface (C). As clearly seen in fig. 5 the boundary maintains its initial sharpness after all the heatings. The displacement of the boundary after each heating step is plotted as a function of the square root of time of heating at 1000 K for two series of measurements in fig. 6. One can see from this figure that the surface diffusion rate depends strongly on the surface region and that it is maximum near the
d
[mm]
/ 30' ~ [~l
~
/
¢'~ [rainy2] Fig. 6. Position of the sharp boundary of the gold layer as a function of the square root of time of heating the crystal at 1000 K for two series of measurements, d is the distance on the cylinder surface of the sharp boundary measured from the [110] direction.
S. Mrbz, E. Bauer / Interaction of gold with tungsten
403
(112) face. The region is characterized by densely packed W rows parallel to the diffusion direction ([111]) which suggests that diffusion along these rows is particularly easy. This is in agreement with results reported by Bayat and Wassmuth [22,23] for the diffusion of potassium on tungsten (112) and (001) faces. They found that the mean diffusion distance parallel to the rows of atoms on the (112) face was twice as large as that on the (001) face. The present data indicate an even larger anisotropy.
4. Conclusions (1) A cylindrical tungsten crystal not only is ideal for simultaneous collection of data for a large number of surface orientations but is also well suited for the study of low-index faces. (2) Upon heating the gold-covered crystal (112), (110) and (332) facets develop. (3) The rate of diffusion of the gold layer across the cylinder surface normal to the [110] direction is much higher in the (111) and in particular in the (112) region than in the (001) region.
Acknowledgement One of us (SM) would like to thank very much the Deutsche Forschungsgemeinschaft for financial support enabling this work to be performed.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
P.L. Young and R. Gomer, Surface Sci. 44 (1974) 268. J.P. Jones and N.T. Jones, Thin Solid Films 35 (1976) 83. L. Richter and R. Gomer, Surface Sci. 59 (1976) 575. A. Cetronio and J.P. Jones, Thin Solid Films 35 (1976) 113. L Richter and R. Gomer, Phys. Rev. Letters 37 (1976) 763. J.P. Jones and E.W. Roberts, Thin Solid Films 48 (1978) 215. A. Cetronio and J.P. Jones, Surface Sci. 40 (1973) 227. A. Cetronio and J.P. Jones, Surface Sci. 44 (1974) 109. H.M. Montagu-Pollock, T.N. Rhodin and M.J. Southon, Surface Sci. 12 (1968) 1. A. Cavaleru and A. Scortaru, Rev. Roum Phys. 25 (1980) 93. E. Bauer, F. Bonczek, H. Poppa and G. Todd, Surface Sci. 53 (1975) 87. E, Bauer, H. Poppa, G. Todd and P.R. Davis, J. Appl. Phys. 48 (1977) 3773. P.D. Augustus and J.P. Jones, Surface Sci. 64 (1977) 713. J. Polanski, Z. Sidorski and S. Zuber, Proc. 4th Intern. Conf. on Solid Surfaces and 3rd European Conf. on Surface Science, (ICSS-4/ECOSS-3), Cannes, 1980, Vol. 1, Eds. D.A. Degras and M. Costa, p. 233. [15] T, Koshikawa, T. von dem Hagen and E. Bauer, Surface Sci. 109 (1981) 301.
404 [16] [17] [18] [19]
[20] [21] [22] [23]
S. Mrbz, E. Bauer / Interaction of gold with tungsten J. Polanski, Z. Sidorski and S. Zuber, Acta Phys. Polon. A64 (1983) 377. J. Ko/aczkiewicz and E. Bauer, Phys. Rev. Letters 53 (1984) 485. J. Kolaczkiewicz and E. Bauer, Surface Sci. 144 (1984) 477. M. Drechsler, in: Surface Mobilities of Solid Materials. Fundamental Concepts and Applications, NATO ASI Series, Series B: Physics, Vol. 86, Ed. Vu Thien Binh (Plenum, New York, 1983) p. 405. E. Langer, MS Thesis, Technische Universitat Clausthal (1978). T.M. Gardiner, H.M. Kramer and E. Bauer, Surface Sci. 112 (1981) 181. B. Bayat and H.W. Wassmuth, Surface Sci. 133 (1983) 1. B. Bayat and H.W. Wassmuth, Surface Sci. 140 (1984) 511.
Surface Science 169 (1986) 394-404 North-Holland, Amsterdam
THE INTERACTION OF GOLD WITH THE SURFACE OF A CYLINDRICAL TUNGSTEN SINGLE CRYSTAL S. M R O Z * a n d E. B A U E R Physikalisches Institut der Technischen Universiti~t Clausthal, Leibnizstrasse 4, D3392 Clausthal-Zellerfeld, Fed. Rep. of Germany
Received 26 August 1985; accepted for publication 29 November 1985
Gold layers on the surface of a cylindrical tungsten single crystal were investigated by LEED and work function change measurements. Work function changes during gold adsorption on (110), (111), (112), (115) and (001) faces, and structures deduced from LEED patterns for (110), (112), (332) and (001) faces are presented with a common coverage scale. Upon heating the gold-covered crystal up to 1000 K, faceting occurs with faceted regions as large as 20°, 13° and 4° around the [112], [110] and [332] directions, respectively. The diffusion of the gold layer was studied by the movement of the sharp boundary across the cylindrical crystal surface at a temperature of 1000 K. The diffusion rate was found to be much higher in the (112) region than in the (001) region.
1. Introduction The interaction of gold with different microscopic tungsten surfaces has b e e n investigated extensively o n t u n g s t e n tips by field emission microscopy [1-9] and field ion microscopy [7-10]. In these studies the d i s t r i b u t i o n of the deposit is initially i n h o m o g e n e o u s and a n n e a l i n g is necessary to distribute the material across the tip. At the same time material diffuses to the shank, agglomerates if more than the a d s o r b a b l e a m o u n t is deposited, a n d causes faceting. F u r t h e r m o r e , even in the absence of these complications, the distrib u t i o n over the tip is not h o m o g e n e o u s but d e t e r m i n e d by the differences in heat of a d s o r p t i o n a n d to a certain extent - also by differences in the activation energy for surface diffusion between the different planes. These difficulties do not exist o n large flat macroscopic single crystals on which the a d s o r p t i o n of Au was studied by L E E D , AES a n d work f u n c t i o n change (A0) m e t h o d s [11-18]. These studies have p r o d u c e d a considerable a m o u n t of i n f o r m a t i o n on work f u n c t i o n changes a n d the crystalline structures of the adsorbed layers b u t the i n f o r m a t i o n is o b t a i n e d separately for each * On leave from Institute of Experimental Physics. University of Wroc3aw, ul. Cybulskiego 36, PL 50-205 Wrodaw, Poland. 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
s. Mrbz, E. Bauer / Interaction of gold with tungsten
395
particular face. Using a large cylindrical single crystal which is continuously rotated during deposition, it is possible to perform such investigations simultaneously for m a n y faces. Thus, the work function changes and adsorbed layer structures can be obtained on a common coverage scale for all faces investigated. The equilibrium shape of a crystal should be strongly influenced by an adsorbed layer provided that the temperature is high enough so that equilibrium can be reached within acceptable times. This is the case for crystal temperatures T > 0.3Tm, Tm being the melting temperature of the crystal [19]. For tungsten, this is the case for T > 1000 K. Most experimental investigations of the equilibrium shape of crystals were, up to date, performed on very small samples (tips in field emission microscopes and small drops). The results of such investigations have been reviewed by Drechsler [19]. On a macroscopic cylindrical single crystal it is not possible to see if the equilibrium shape induced by the adsorption is directly due to the long diffusion distances necessary. However, it can be expected that terraces parallel to the most stable faces of the crystal will grow during heating of the crystal covered with gold and that this growth can be seen by LEED. Surface diffusion of gold across the cylindrical crystal surface should be easily observed by A~ measurements if the sharp boundary of the adsorbed layer - built up during adsorption with the use of a proper shutter - is maintained during diffusion. Diffusion rates in different surface regions can thus be compared in such experiments. The goal of the present work is to verify and use the possibilities mentioned above.
2. Experimental Measurements were performed in a metal ultrahigh-vacuum system described in detail elsewhere [20]. The system was pumped with a titanium ion p u m p and a titanium sublimation p u m p cooled with liquid nitrogen. The residual gas pressure during the measurements was in the low 10 - 9 Pa range. The tungsten crystal was a hollow cylinder with a length of 15.8 m m and inner and outer diameters of 7.7 and 8.5 mm, respectively. Inside the crystal was a tungsten filament which could be heated resistively and which was used to heat the crystal by thermal radiation or by electron bombardment. The crystal could be rotated around the cylinder axis which was parallel to the [110] direction at a constant rate of one full rotation per 100 s. To enable unlimited rotation, the crystal was not equipped with a thermocouple and the crystal heating system was calibrated with an optical pyrometer before measurements. One flange of the vacuum system was assigned for a C M A or for a
396
S. Mrbz, E. Bauer / Interaction of gold with tungsten
four-grid L E E D system. Therefore, it was impossible to perform AES analysis and L E E D observations simultaneously (as will be seen below, the intensity of the primary electron beam of the L E E D system was too low to perform the AES analysis with this system). On another flange, an electron gun for the work function change measurements with the retarding field electron beam method was mounted with its axis perpendicular to the crystal axis. The electron beam produced in this gun was magnetically focused with a molybdenum coil wound around the gun and collimated with a slit 6 m m long and 0.13 m m wide placed at a distance of a few millimeters from the crystal surface parallel to the cylinder axis. A~ was measured in the constant current mode which is obtained by automatic adjustment of the voltage between cathode and crystal while the crystal was rotating. The voltage change was recorded as a function of the crystal rotation angle with the help of an X - Y recorder and its difference with respect to the clean surface is the work function change divided by the electron charge: A V = A~/e. The gold source was a ceramic tube with inner and outer diameters of 3 and 4 ram, respectively, and a length of 12 m m which was pressed into a tight 0.3 m m diameter tungsten wire spiral used for resistive heating. The source was surrounded by two concentric tantalum radiation shields. The distance between source and crystal was about 10 cm which ensured homogeneous coverage of the crystal. Two movable shutters were placed between the gold source and the crystal. Shutter I which was close to the source shielded the crystal completely and was used during outgassing of the source. Shutter II was placed very close to the crystal surface (~-0.5 m m distance) and had a sharp edge parallel to the cylinder axis which produced a sharp boundary of the gold layer during deposition on the crystal surface needed for the investigation of the surface diffusion of gold. For the L E E D observations the primary electron beam diameter was reduced by an aperture of 0.2 m m in diameter at the end of the electron gun. This was necessary in order to obtain the high lateral resolution needed for a cylindrical crystal surface. The resulting loss in beam intensity was compensated by viewing the L E E D pattern with a videocamera of especially high sensitivity and displaying it on a TV monitor. The crystal surface was cleaned by heating at 1400 K in oxygen (po2 = 2 x 10 6 Pa) followed by flashing to about 2200 K. The effectiveness of this cleaning procedure was checked at the beginning of the measurements by AES analysis with a Varian CMA system. Subsequently, the C M A head was replaced with the L E E D optics. A more detailed description of the crystal manipulator and of the A~ measuring system is presented in refs. [20,21].
S. Mrbz, E. Bauer / Interaction of goM with tungsten
397
3. Results and discussion
A¢
3.1.
A~ was measured continuously during evaporation of gold onto the rotating crystal. Results for (001), (112), (111), (110) and (115) faces obtained for an electron beam current of 16 nA (at a saturation current of 50-70 nA depending upon the crystallographic direction of the crystal) are presented in fig. 1. The gold coverage is given in number of doses (one dose was obtained during one full rotation of the crystal, i.e. during 100 s) and in number of gold atoms per cm 2. This last scale was obtained on the assumption that the maximum of the work function appears for the (112) face at 6.3 × 10 TM gold atoms per cm 2 as reported by Kolaczkiewicz and Bauer [18].
/~e [ev] 0.8
...-" 11
(110}
-'~.~_ ./ / /
0.6[
(001)/.~/ r.~2X--..T--.~.~ /" ...-'~.~
(112)
i
oi T....
t
/ d~
!0 ....
20 ,e,[~o~'~t~/,~ ~]
10
20
doses
Fig. 1. Work function changes ,A~bas a function of Au coverage 0 of selected low-index faces of the tungsten cylindrical crystal surface. A,# for a clean (001) face is taken as zero.
398
s. Mrbz, E. Bauer / Interaction of gold with tungsten
It was f o u n d that the w o r k function differences b e t w e e n the various faces of the crystal d e p e n d on the electron b e a m c u r r e n t used. This d e p e n d e n c e can result from c h a n g i n g c o n d i t i o n s of electron b e a m focusing a n d f r o m different slopes of the c u r r e n t - voltage characteristic of the system electron gun crystal for different crystal faces. Therefore, the exact A~ values m e a s u r e d on o n e a n d the same surface m a y have to be t a k e n with s o m e c a u t i o n too. This has, however, no influence on the relative coverage d e t e r m i n a t i o n which is b a s e d on the A~ m a x i m a a n d m i n i m a a n d which are o b t a i n e d for the first time s i m u l t a n e o u s l y for different tungsten faces. 3.2. L E E D p a t t e r n s
T h e L E E D p a t t e r n s o b t a i n e d after d e p o s i t i o n of different a m o u n t s of gold a n d heating the crystal for 5 m i n up to a b o u t 1000 K are d e s c r i b e d in fig. 2 for (001), (112), (110) a n d (332) faces. F o r o t h e r faces, L E E D p a t t e r n s clearly c o n n e c t e d to the gold layer have not been found. The results for the (001) a n d (110) faces are in a g r e e m e n t with the d a t a for flat m a c r o s c o p i c single-crystal surfaces [12]. It verifies the usefulness of the c y l i n d r i c a l crystal in e x p e r i m e n t s with a d s o r p t i o n on l o w - i n d e x faces. In view of this it is surprising that, besides
Structure X1×1)c(2x2) p(2xl)
c(2x2)
doublets in [1i0] direction i
p(4x4)
p(6x6)
p(3×1)
p(lxl)
:)(lxl)
Face
p(lxl) i
(001)
(112)
compticated patterns resutting from muttiple scattering
(110)
no structure
(332)
i
no
structure i
0
p(3×1) i
10
p(Bxl) i
i
20
i
8 [_1014at/cm 2]
Fig. 2. Structure of gold layers on (001), (112), (110) and (332) faces as a function of coverage. The LEED patterns were observed after heating the crystal for 5 min up to about 1000 K. The gold coverage was determined by A~ measurements before heating. The substrate unit mesh of the (112) and (332) planes are chosen with the b 1 axis parallel to the [110] direction.
S. Mrbz, E. Bauer / Interaction of gold with tungsten
399
the structures on the (001), (112) and (110) faces, clear patterns were observed only on the (332) face.
3.3. Faceting in the presence of the gold layer The faceting of the tungsten crystal surface was studied by moving the crystal across the electron beam at a fixed angular position of the crystal corresponding to a particular low-index plane while the LEED pattern was observed. For clean tungsten faces, the LEED pattern moved during the crystal motion due to the continuous change of the angle of incidence of the primary beam. In addition, the spots split due to the influence of the terraces of monatomic height within the coherence length of the primary beam. On the gold-covered surface which was heated for a few minutes up to about 1000 K the situation was in general different: within a certain range of crystal displacements across the primary electron beam the tungsten LEED spots stayed fixed and did not split. Furthermore, the superstructure of the adsorbed layer as seen in the LEED pattern did not change either. The range of crystal positions with unchanged LEED pattern was measured with the micrometer of +4-
20oi Ao~ o - (110) [. . . . o " - q
+ - (112) L
........ (112)
I
~4
A - (332)
15°
i I I
+
o I
(110) 100
o __20/° +! o
50
+
o 10
20
[lo'" otom /om ] 3O
Fig. 3. Width of the faceted regions on the crystal surface (given as an angle Aa) in the vicinity of the [112], [110] and [332] directions after heating the crystal for 5 min at 1000 K as a function of gold coverage.
S. Mrbz, E. Bauer / Interaction of gold with tungsten
400
the sample manipulator and recalculated into the angle on the crystal surface dominated by facets of given low-index orientation. The results of such a determination are presented in fig. 3. Gold coverages are taken from A0 measured during the gold deposition. The results of fig. 3 are semiquantitative because the L E E D pattern deteriorated more and more with increasing displacement of the crystal from the position of the best pattern and it was difficult to determine the exact position at which the pattern changed. Furthermore, the error in the coverage determination could be as large as 2 x 1014 a t o m s / c m 2 for large coverages. It is surprising that no faceting was observed for the (001) face. It was important to ensure that the crystal temperature was high enough to induce faceting and, simultaneously, low enough to prevent gold desorption. This was done by heating a gold-covered crystal (0 = 21 x 1014 a t o m s / c m 2) m a n y times for 5 min to increasingly higher temperature. The sizes of the faceted regions were measured after each heating. It was found that temperatures from 800 to above 1000 K are good for faceting.
eVl 0.81
0.6
~\
B
\ "", ~ t I / \ \ ~ ~ ' ~ / /,~.[ \
/ 0.4
"-.. ml]
'I\"J'~
C
C
\ txxJ'/l" [112] I \ [332] [334] [115]
0.2
[112] I [oo1]
180 o
150 °
120 o
90°
Fig. 4. A,;b profiles; (A) clean tungsten surface; (B) after deposition of gold at room temperature (0 = 22 x 1015 a t o m s / c m 2 ) ; (C) after heating the crystal for 5 min at 1000 K.
S. Mrbz, E. Bauer / Interaction of gold with tungsten
401
Additional information concerning the faceting process can be obtained from changes of the work function profile observed after heating the goldcovered crystal up to 1000 K. An example of such changes is shown in fig. 4. One can see in this figure that after heating new local maxima of the work function appear at the position of the (332), (115) and (334) faces. This can be interpreted as a result of the growth of facets being too small to be seen by LEED.
3.4. Prefiminary observations of gold surface diffusion Although the work function change is caused in general by Au adsorption and faceting, only Au adsorption is responsible for it at low coverages as seen A~ [eV] 0.8
0.6
j I
I
0.4
r,../
,,.
\
I.//B .~.l
0.2 /
[110]
-0.2
~ 180 o
[111]
150 °
[112]
120 °
v
[001] [115]
~. 1 900
[1'12]
60 °
Fig. 5. A~ profiles before and after diffusion of the sharp boundary of the gold layer; (A) before diffusion; (B) after diffusion (after heating the crystal for 55 min at 1000 K); (C) for clean crystal surface (after flash).
S. Mrbz, E. Bauer / Interaction of gold with tungsten
402
in fig. 3 by the absence of faceting below about 5 x 1011 Au a t o m s / c m 2. The diffusion front can, therefore, be assumed to be characteristic of the surface diffusion of Au if the coverage at the location of the diffusion front is sufficiently small. If this is not the case then Au diffusion and W surface restructuring may be involved simultaneously. This has to be kept in mind in the following. A sharp boundary of a thick ( 0 - - 3 0 x 1014 a t o m s / c m 2) gold layer approximately parallel to the cylinder axis was obtained using shutter II. Fig. 5 shows the work function profile when the edge of shutter II was placed between the (115) and (001) surface before (A) and after (B) eleven 5 min anneals at 1000 K together with that of the clean surface (C). As clearly seen in fig. 5 the boundary maintains its initial sharpness after all the heatings. The displacement of the boundary after each heating step is plotted as a function of the square root of time of heating at 1000 K for two series of measurements in fig. 6. One can see from this figure that the surface diffusion rate depends strongly on the surface region and that it is maximum near the
d
[mm]
/ 30' ~ [~l
~
/
¢'~ [rainy2] Fig. 6. Position of the sharp boundary of the gold layer as a function of the square root of time of heating the crystal at 1000 K for two series of measurements, d is the distance on the cylinder surface of the sharp boundary measured from the [110] direction.
S. Mrbz, E. Bauer / Interaction of gold with tungsten
403
(112) face. The region is characterized by densely packed W rows parallel to the diffusion direction ([111]) which suggests that diffusion along these rows is particularly easy. This is in agreement with results reported by Bayat and Wassmuth [22,23] for the diffusion of potassium on tungsten (112) and (001) faces. They found that the mean diffusion distance parallel to the rows of atoms on the (112) face was twice as large as that on the (001) face. The present data indicate an even larger anisotropy.
4. Conclusions (1) A cylindrical tungsten crystal not only is ideal for simultaneous collection of data for a large number of surface orientations but is also well suited for the study of low-index faces. (2) Upon heating the gold-covered crystal (112), (110) and (332) facets develop. (3) The rate of diffusion of the gold layer across the cylinder surface normal to the [110] direction is much higher in the (111) and in particular in the (112) region than in the (001) region.
Acknowledgement One of us (SM) would like to thank very much the Deutsche Forschungsgemeinschaft for financial support enabling this work to be performed.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
P.L. Young and R. Gomer, Surface Sci. 44 (1974) 268. J.P. Jones and N.T. Jones, Thin Solid Films 35 (1976) 83. L. Richter and R. Gomer, Surface Sci. 59 (1976) 575. A. Cetronio and J.P. Jones, Thin Solid Films 35 (1976) 113. L Richter and R. Gomer, Phys. Rev. Letters 37 (1976) 763. J.P. Jones and E.W. Roberts, Thin Solid Films 48 (1978) 215. A. Cetronio and J.P. Jones, Surface Sci. 40 (1973) 227. A. Cetronio and J.P. Jones, Surface Sci. 44 (1974) 109. H.M. Montagu-Pollock, T.N. Rhodin and M.J. Southon, Surface Sci. 12 (1968) 1. A. Cavaleru and A. Scortaru, Rev. Roum Phys. 25 (1980) 93. E. Bauer, F. Bonczek, H. Poppa and G. Todd, Surface Sci. 53 (1975) 87. E, Bauer, H. Poppa, G. Todd and P.R. Davis, J. Appl. Phys. 48 (1977) 3773. P.D. Augustus and J.P. Jones, Surface Sci. 64 (1977) 713. J. Polanski, Z. Sidorski and S. Zuber, Proc. 4th Intern. Conf. on Solid Surfaces and 3rd European Conf. on Surface Science, (ICSS-4/ECOSS-3), Cannes, 1980, Vol. 1, Eds. D.A. Degras and M. Costa, p. 233. [15] T, Koshikawa, T. von dem Hagen and E. Bauer, Surface Sci. 109 (1981) 301.
404 [16] [17] [18] [19]
[20] [21] [22] [23]
S. Mrbz, E. Bauer / Interaction of gold with tungsten J. Polanski, Z. Sidorski and S. Zuber, Acta Phys. Polon. A64 (1983) 377. J. Ko/aczkiewicz and E. Bauer, Phys. Rev. Letters 53 (1984) 485. J. Kolaczkiewicz and E. Bauer, Surface Sci. 144 (1984) 477. M. Drechsler, in: Surface Mobilities of Solid Materials. Fundamental Concepts and Applications, NATO ASI Series, Series B: Physics, Vol. 86, Ed. Vu Thien Binh (Plenum, New York, 1983) p. 405. E. Langer, MS Thesis, Technische Universitat Clausthal (1978). T.M. Gardiner, H.M. Kramer and E. Bauer, Surface Sci. 112 (1981) 181. B. Bayat and H.W. Wassmuth, Surface Sci. 133 (1983) 1. B. Bayat and H.W. Wassmuth, Surface Sci. 140 (1984) 511.