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Potassium adsorption on β-FeSi2 thin films
Surface Science Letters 248 (1991) L255-L258 North-Holland
L255
Surface Science Letters
Potassium adsorption on fl-FeSi 2 thin films S. K e n n o u a a n d T.A. N g u y e n T a n b a Department of Physics, University of loannina, PO Box 1186, GR 451 10 Ioannina, Greece b LEPES-CNRS, BP 166, 38042 Grenoble, France Received 16 October 1990; accepted for publication 13 February 1991
Potassium adsorption on thin fl-FeSi 2 epitaxiaUy grown on Si(111) was studied with ultra-violet photoemission spectroscopy (UPS), Auger electron spectroscopy (AES) and work function (WF) measurements. A value of (4.65 + 0.03) eV was measured for the absolute W F of the silicide surface. A combination of the experimental results u p o n K deposition indicates a plausible mode of K adsorption on fl-FeSi 2. Initially K adsorbs on Si triangular sites with a Fe atom underneath up to a coverage of - 2 x 1014 atoms cm - 2 and then on Si triangular sites with no Fe underneath up to a saturation coverage of - 2 x 1014 atoms cm -2.
1. Introduction Recently, there exists a great experimental and theoretical interest in the alkali behaviour on semiconductor surfaces [1-7] and especially on semiconductors of technological importance such as Si, GaAs and InP [4-7]. In this Letter we report for the first time on the adsorption of K on a semiconducting fl-FeSi 2 overlayer epitaxially grown on Si(111). In a forthcoming paper we study further the effect of K on the interaction of the silicide surface with oxygen
[8]• 2. Experimental The experiments were performed in an U H V chamber equipped with LEED, AES and UPS, and with a base pressure of 5 x 10 -l° mbar during Fe and K deposition. The apparatus has been described earlier [9]. Potassium was evaporated from a commercial dispenser (SAES Getter) and Fe from a laboratory-made source where pure iron metal was heated by electron bombardment. The atomic K flux during deposition was not measured but was approximately constant. The W F changes were measured from the low energy cut-off of the
UPS spectra with the sample negatively polarized, in order to obtain a sharp secondary electron edge.
3. Results An epitaxial fl-FeSi 2thin film was formed after deposition of Fe (40 A) on Si(111) at RT and annealing at 550 o C, as described previously [10]. It was characterized by UPS, L E E D and AES. F r o m previously reported T E M observations on similarly prepared specimens, the epitaxial relationship is Si(111) parallel to (101) or (110) of fl-FeSi 2 [10,11], which was also concluded from our L E E D observations. The absolute WF value of both fl-FeSi 2 and the Si(111)(7 x 7) was measured from the UPS spectra. It was found that the respective values were (4.50 _ 0.03) eV for Si(111) (7 x 7) and (4.65 +_ 0.03) eV for fl-FeSi 2 and the former value is in agreement with that given by S6benne [12]. In fl-FeSi 2 the Auger peak-to-peak height ratio of Fe MVV (47 eV) to Si LVV (92 eV) was 0.35 and it was always the same for every new epitaxial film. The UPS spectrum showed the characteristic features of the semiconducting iron disilicide [13]. Fig. 1 shows the variation of the Auger peak-
0039-6028/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
S. Kennou, TA. Nguyen Tan / Potassium adsorption on fl-FeSi 2 thin films
L256
i i
i
•
!
K
( 252
eV }
I
•
A Si ( 9 2 eV)
Fe (47 eV) c. 13 ]_ ix. EL
j.¢
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~2,/J
0
,
40
,
h
80
i
,
,
120
K
i
160
I
i
I
i
200
L
240
DEPOSITION
i
b
280
Y
i ~f
k
320
450
TIME //s
Fig. 1. Auger peak-to-peak height (APPH) of K (252 eV), Si (92 eV) and Fe (47 eV) versus K deposition time.
to-peak height for K (252 eV), Si (92 eV) and Fe (47 eV) as a function of K deposition time at room temperature. Initially, the K peak increases almost linearly up to - 1 6 0 s indicating a constant sticking coefficient. Beyond 160 s it continues to increase at a slower pace up to saturation (t >_ 420 s). During deposition the substrate peaks decrease following the growth of the K peak up to 160 s of K deposition where the total attenuation of both Fe and Si is - 30%. Upon further K deposition, the peak of Fe (47 eV) remains unchanged up to saturation, whereas the Si (92 eV) continues to decrease. The total attenuation of Si (92 eV) at saturation is - 56%. Fig. 2 shows the WF change versus K deposition time. The WF initially decreases linearly up to - 80 s and then begins to level-off up to about 120 s of K deposition. At - 160 s deposition time there is a further slight decrease of the WF, lying almost within the limits of experimental error, until a plateau is reached beyond - 240 s, without a minimum. The total decrease of the W F value at saturation is 3.2 eV. In the same figure the variation of the Auger peak-to-peak height ratio of Fe(47 eV)/Si(92 eV) is also traced versus K deposition time. This ratio remains constant at its initial value of 0.35 up to - 1 6 0 s and then
increases gradually up to - 0.55 after - 320 s of K deposition. Fig. 3 shows the photoemission spectra from the valence band region of clean #-FeSi 2 and after K deposition up to saturation. The clean surface shows in particular two peaks at - 0 . 8 and - 2 eV below E v corresponding to "nonbonding 3dFe states" and "bonding F e d - S i s p
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~
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-
-
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Fig. 2. Variation of the work function (a) and the Auger peak-to-peak height ratio of Fe(47 eV)/Si(92 eV), (b) with K deposition time.
S. Kennou, T.A. Nguyen Tan / Potassium adsorption on fl-FeSi 2 thin films
L257
pattern of the fl-FeSi 2 at any stage of K deposition.
ia )
LK = 0
Ib)
tl~
I¢~
tK : 350s
=30s
4. Discussion (c)
113)
W
~
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,
Z
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I
I
I
i
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I
I
I
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I
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ENERGY
teV )
Fig. 3. UV photoernission spectra of clean and K-covered fl-FeSi 2-
states" respectively [10]. The broad peak centered around 4 eV arises from F e 3 d - S i 3 p hybridized states [17]. Upon K deposition the shape of the spectrum suffers little change with a somewhat greater attenuation of the iron related features. No extra spots were observed in the L E E D
•" - -
7.83
A*
-~,
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[]
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q . . . . ,c
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,
,, zx"/,F,,
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Fig. 4. Atomic arrangement of the fl-FeSi 2 (110) surface and suggested sites for K adsorption. ( o ) outermost Si atoms (level 0), ([]) outermost Fe atoms (level - 0 . 6 ,~), ( ~ ) initial K,d s sites, (A) subsequent Ka0 s sites.
The correlation of the experimental results presents some evidence for the mode of K adsorption on fl-FeSi 2. In the early stages of adsorption and up to a deposition time of - 1 6 0 s adsorbed K most probably occupies sites involving both silicon and iron atoms, whereas upon further adsorption the adatoms occupy sites involving predominantly silicon. This is evidenced by the AES resuits (fig. 1 and curve b in fig. 2). Up to - 160 s deposition time both iron and silicon substrate peaks decrease in parallel and the K peak increases at a constant rate, whereas upon further deposition only the silicon substrate peak decreases while the Fe peak remains nearly constant. This picture is also supported by the rapid decrease of the Fe derived peaks in the UPS spectra in the initial stage of K deposition. An adsorption site assignment for potassium on fl-FeSi 2 which is consistent with the above observation, can be proposed in connection with fig. 4, showing the atomic arrangement of the fl-FeSi2(ll0 ) surface. This picture was obtained from data in ref. [14] assuming that the outermost layer of surface atoms consists of silicon atoms. We propose that initially potassium occupies the triangular silicon sites with an iron atom underneath. Such sites are marked by an ( X ) or ( + ) in fig. 4 and are of two types, iron I ( x ) or II ( + ) , each arranged in alternate parallel rows. Since the distance between two adjacent Fe(I) and Fe(II) atoms is - 2.5 .~, roughly equal to the diameter of a potassium ion, the first potassium adatoms, even if they are completely ionized, must occupy only one type of the above sites. We can now postulate that the saturation of all equivalent sites (either + or x ) corresponds to a K deposition time of - 1 6 0 s, thus obtaining a calibration of the absolute coverage of potassium. Taking into account the dimensions of the fl-FeSi 2 unit cell (7.8 x 12.5 ,~2) and the fact that there are two equivalent triangular Si sites with Fe underneath per unit cell we obtain a K coverage of 2.0 x 1014 atoms cm -2 at - 160 s deposition time.
L258
S. Kennou, T.A. Nguyen Tan /Potassium adsorption on fl-FeSi 2 thin films
Upon further K deposition new sites are occupied as evidenced by the break in the AES and WF curves for K deposition (figs. 1 and 2) and saturation is obtained at a K coverage of approximately 4.0 x 1014 atoms cm -2. Since the new sites must involve only silicon atoms in the outermost surface layer (only the SiLVV signals decreases with the deposition time) a natural choice for the position of adsorbing K atoms beyond the coverage of 2 x 1014 atoms c m - 2 is on top of the triangular silicon sites marked with a (zx) in fig. 4. This type of sites lie near the center of the squares formed by the Fe(II) atoms marked by ( + ) in fig. 4 and have no iron atoms underneath. In contrast, the centers of the squares formed by the Fe(I) atoms marked by ( × ) in fig. 4 are on top of the first layer Si atoms. Assuming K adatoms always prefer hollow triangular adsorption sites it appears as the most likely arrangement that the K adatoms initially occupy the ( + ) sites up to a coverage of 2 x 1014 atoms cm -z and then continue to adsorb on (zx) sites. The average distance between (zx) marked sites and ( + ) marked sites is about 5 #,, that is slightlYolarger than the K bulk metal diameter of - 4.6 A. As there are two (zx) sites per unit cell, the coverage after complete saturation of these sites becomes 4 x 1014 atoms cm -2 in agreement with the intensity of the K Auger signal. The above arrangement of adsorbed K is consistent with the absence of extra spots in the LEED pattern of fl-FeSi 2. Using the above coverage calibration and the constancy of the sticking coefficient in the early stages of adsorption one can obtain from the initial slope of the W F curve (fig. 2) an initial dipole moment of - 5 . 4 D for adsorbed potassium. This value can be compared with the initial dipole moments for K on Fe(100) 4.4 D [15], on Fe(ll0) 6.0 D [16], on F e ( l l l ) 3.9 D [15] and on S i ( l l l ) 3.9 D [4]. It is in better agreement with the
initial dipole m o m e n t of K on Fe than that of K on Si. On the other hand, the final W F change at saturation, - 3 . 2 eV, is the same as that observed for K on S i ( l l l ) [2]. These two observations are also consistent with adsorbed K at low coverage interacting with iron lying slightly below the outermost layer of silicon atoms. As the K coverage increases the interaction with the substrate is mainly limited to the outermost layer of silicon atoms.
References [1] A. Franciosi, P. Soukiassian, P. Philhp, S. Chang, A. Wall, A. Raisanen and N. Troulier, Phys. Rev. B 35 (1987) 910. [2] Y. Enta, T. Kinoshita, S. Suzuki and S. Kono, Phys. Rev. B 36 (1987) 9801. [3] U.A. Ditzinger, Ch. Lunau, B. Schieweck, St. Tosch, H. Neddermeyer and M. Hanbiicken, Surf. Sci. 211/212 (1989) 707. [4] E.M. Oeilig and R. Miranda, Surf. Sci. 1977 (1986) L974. [5] S. Kennou, S. Ladas, M. Kamaratos and C.A. Papageorgopoulos, Surf. Sci. 216 (1989) 462. [6] M.G. Beni, U. Del Pennino, C. Mariani, S. Valeri and J.A. Schaefer, Surf. Sci. 211/212 (1989) 659. [7] H. Starnberg, P. Soukiassian, M.H. Bakshi and Z. Hurych, Surf. Sci. 224 (1989) 13. [8] S. Kennou, T.A. Nguyen Tan and R.C. Cinti, ECOSS-11, Surf. Sci., in press. [9] R.C. Cinti, T.A. Nguyen Tan, Y. Capiomont and S. Kennou, Surf. Sci. 134 (1983) 755. [10] N. Cherief, R.C. Cinti, M. De Crescenzi, J. Derrien, T.A. Nguyen Tan and J.Y. Veuillen, Appl. Surf. Sci. 41/42 (1989) 241. [11] N. Cherief, C. D'Anterroches, R.C. Cinti, T.A. Nguyen Tan and J. Derrien, Appl. Phys. Lett. 55 (1989) 1671. [12] C.A. S~benne, Nuovo Cimento 39 (1977) 768. [13] B. Egert and G. Panzer, Phys. Rev. B 29 (1984) 3293. [14] Y. Dusansoy, J. Protas, R. Nandji and B. Roques, Acta Cryst. B 27 (1971) 1209. [15] S.B. Lee, M. Weiss and G. Ertl, Surf. Sci. 108 (1981) 357. [16] G. Brod~n and H.P. Bonzel, Surf. Sci. 84 (1979) 106. [17] N. CherieL Doctoral Thesis, Grenoble, 1989.
L255
Surface Science Letters
Potassium adsorption on fl-FeSi 2 thin films S. K e n n o u a a n d T.A. N g u y e n T a n b a Department of Physics, University of loannina, PO Box 1186, GR 451 10 Ioannina, Greece b LEPES-CNRS, BP 166, 38042 Grenoble, France Received 16 October 1990; accepted for publication 13 February 1991
Potassium adsorption on thin fl-FeSi 2 epitaxiaUy grown on Si(111) was studied with ultra-violet photoemission spectroscopy (UPS), Auger electron spectroscopy (AES) and work function (WF) measurements. A value of (4.65 + 0.03) eV was measured for the absolute W F of the silicide surface. A combination of the experimental results u p o n K deposition indicates a plausible mode of K adsorption on fl-FeSi 2. Initially K adsorbs on Si triangular sites with a Fe atom underneath up to a coverage of - 2 x 1014 atoms cm - 2 and then on Si triangular sites with no Fe underneath up to a saturation coverage of - 2 x 1014 atoms cm -2.
1. Introduction Recently, there exists a great experimental and theoretical interest in the alkali behaviour on semiconductor surfaces [1-7] and especially on semiconductors of technological importance such as Si, GaAs and InP [4-7]. In this Letter we report for the first time on the adsorption of K on a semiconducting fl-FeSi 2 overlayer epitaxially grown on Si(111). In a forthcoming paper we study further the effect of K on the interaction of the silicide surface with oxygen
[8]• 2. Experimental The experiments were performed in an U H V chamber equipped with LEED, AES and UPS, and with a base pressure of 5 x 10 -l° mbar during Fe and K deposition. The apparatus has been described earlier [9]. Potassium was evaporated from a commercial dispenser (SAES Getter) and Fe from a laboratory-made source where pure iron metal was heated by electron bombardment. The atomic K flux during deposition was not measured but was approximately constant. The W F changes were measured from the low energy cut-off of the
UPS spectra with the sample negatively polarized, in order to obtain a sharp secondary electron edge.
3. Results An epitaxial fl-FeSi 2thin film was formed after deposition of Fe (40 A) on Si(111) at RT and annealing at 550 o C, as described previously [10]. It was characterized by UPS, L E E D and AES. F r o m previously reported T E M observations on similarly prepared specimens, the epitaxial relationship is Si(111) parallel to (101) or (110) of fl-FeSi 2 [10,11], which was also concluded from our L E E D observations. The absolute WF value of both fl-FeSi 2 and the Si(111)(7 x 7) was measured from the UPS spectra. It was found that the respective values were (4.50 _ 0.03) eV for Si(111) (7 x 7) and (4.65 +_ 0.03) eV for fl-FeSi 2 and the former value is in agreement with that given by S6benne [12]. In fl-FeSi 2 the Auger peak-to-peak height ratio of Fe MVV (47 eV) to Si LVV (92 eV) was 0.35 and it was always the same for every new epitaxial film. The UPS spectrum showed the characteristic features of the semiconducting iron disilicide [13]. Fig. 1 shows the variation of the Auger peak-
0039-6028/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
S. Kennou, TA. Nguyen Tan / Potassium adsorption on fl-FeSi 2 thin films
L256
i i
i
•
!
K
( 252
eV }
I
•
A Si ( 9 2 eV)
Fe (47 eV) c. 13 ]_ ix. EL
j.¢
#T
~2,/J
0
,
40
,
h
80
i
,
,
120
K
i
160
I
i
I
i
200
L
240
DEPOSITION
i
b
280
Y
i ~f
k
320
450
TIME //s
Fig. 1. Auger peak-to-peak height (APPH) of K (252 eV), Si (92 eV) and Fe (47 eV) versus K deposition time.
to-peak height for K (252 eV), Si (92 eV) and Fe (47 eV) as a function of K deposition time at room temperature. Initially, the K peak increases almost linearly up to - 1 6 0 s indicating a constant sticking coefficient. Beyond 160 s it continues to increase at a slower pace up to saturation (t >_ 420 s). During deposition the substrate peaks decrease following the growth of the K peak up to 160 s of K deposition where the total attenuation of both Fe and Si is - 30%. Upon further K deposition, the peak of Fe (47 eV) remains unchanged up to saturation, whereas the Si (92 eV) continues to decrease. The total attenuation of Si (92 eV) at saturation is - 56%. Fig. 2 shows the WF change versus K deposition time. The WF initially decreases linearly up to - 80 s and then begins to level-off up to about 120 s of K deposition. At - 160 s deposition time there is a further slight decrease of the WF, lying almost within the limits of experimental error, until a plateau is reached beyond - 240 s, without a minimum. The total decrease of the W F value at saturation is 3.2 eV. In the same figure the variation of the Auger peak-to-peak height ratio of Fe(47 eV)/Si(92 eV) is also traced versus K deposition time. This ratio remains constant at its initial value of 0.35 up to - 1 6 0 s and then
increases gradually up to - 0.55 after - 320 s of K deposition. Fig. 3 shows the photoemission spectra from the valence band region of clean #-FeSi 2 and after K deposition up to saturation. The clean surface shows in particular two peaks at - 0 . 8 and - 2 eV below E v corresponding to "nonbonding 3dFe states" and "bonding F e d - S i s p
\
k X,
y_!I "~
2
b
\
\
~
-
-
-
* Ci.4
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i
I
4[~
i
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,
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i
i
t
~dlO 20: 240 2!30 DEPC!5!TI':N TIM£ / g
3i):)
Fig. 2. Variation of the work function (a) and the Auger peak-to-peak height ratio of Fe(47 eV)/Si(92 eV), (b) with K deposition time.
S. Kennou, T.A. Nguyen Tan / Potassium adsorption on fl-FeSi 2 thin films
L257
pattern of the fl-FeSi 2 at any stage of K deposition.
ia )
LK = 0
Ib)
tl~
I¢~
tK : 350s
=30s
4. Discussion (c)
113)
W
~
~
,
Z
I
I
I
I
i
I
I
10
I
I
I
5 BINDING
I
EF
ENERGY
teV )
Fig. 3. UV photoernission spectra of clean and K-covered fl-FeSi 2-
states" respectively [10]. The broad peak centered around 4 eV arises from F e 3 d - S i 3 p hybridized states [17]. Upon K deposition the shape of the spectrum suffers little change with a somewhat greater attenuation of the iron related features. No extra spots were observed in the L E E D
•" - -
7.83
A*
-~,
[]
[]
®,:, F
q . . . . ,c
x\
\
/
\ x
°,( x
/
u~
//
\
x
/~!
X
/
'd
\
O
, A
,
,, zx"/,F,,
'e'
o
Fig. 4. Atomic arrangement of the fl-FeSi 2 (110) surface and suggested sites for K adsorption. ( o ) outermost Si atoms (level 0), ([]) outermost Fe atoms (level - 0 . 6 ,~), ( ~ ) initial K,d s sites, (A) subsequent Ka0 s sites.
The correlation of the experimental results presents some evidence for the mode of K adsorption on fl-FeSi 2. In the early stages of adsorption and up to a deposition time of - 1 6 0 s adsorbed K most probably occupies sites involving both silicon and iron atoms, whereas upon further adsorption the adatoms occupy sites involving predominantly silicon. This is evidenced by the AES resuits (fig. 1 and curve b in fig. 2). Up to - 160 s deposition time both iron and silicon substrate peaks decrease in parallel and the K peak increases at a constant rate, whereas upon further deposition only the silicon substrate peak decreases while the Fe peak remains nearly constant. This picture is also supported by the rapid decrease of the Fe derived peaks in the UPS spectra in the initial stage of K deposition. An adsorption site assignment for potassium on fl-FeSi 2 which is consistent with the above observation, can be proposed in connection with fig. 4, showing the atomic arrangement of the fl-FeSi2(ll0 ) surface. This picture was obtained from data in ref. [14] assuming that the outermost layer of surface atoms consists of silicon atoms. We propose that initially potassium occupies the triangular silicon sites with an iron atom underneath. Such sites are marked by an ( X ) or ( + ) in fig. 4 and are of two types, iron I ( x ) or II ( + ) , each arranged in alternate parallel rows. Since the distance between two adjacent Fe(I) and Fe(II) atoms is - 2.5 .~, roughly equal to the diameter of a potassium ion, the first potassium adatoms, even if they are completely ionized, must occupy only one type of the above sites. We can now postulate that the saturation of all equivalent sites (either + or x ) corresponds to a K deposition time of - 1 6 0 s, thus obtaining a calibration of the absolute coverage of potassium. Taking into account the dimensions of the fl-FeSi 2 unit cell (7.8 x 12.5 ,~2) and the fact that there are two equivalent triangular Si sites with Fe underneath per unit cell we obtain a K coverage of 2.0 x 1014 atoms cm -2 at - 160 s deposition time.
L258
S. Kennou, T.A. Nguyen Tan /Potassium adsorption on fl-FeSi 2 thin films
Upon further K deposition new sites are occupied as evidenced by the break in the AES and WF curves for K deposition (figs. 1 and 2) and saturation is obtained at a K coverage of approximately 4.0 x 1014 atoms cm -2. Since the new sites must involve only silicon atoms in the outermost surface layer (only the SiLVV signals decreases with the deposition time) a natural choice for the position of adsorbing K atoms beyond the coverage of 2 x 1014 atoms c m - 2 is on top of the triangular silicon sites marked with a (zx) in fig. 4. This type of sites lie near the center of the squares formed by the Fe(II) atoms marked by ( + ) in fig. 4 and have no iron atoms underneath. In contrast, the centers of the squares formed by the Fe(I) atoms marked by ( × ) in fig. 4 are on top of the first layer Si atoms. Assuming K adatoms always prefer hollow triangular adsorption sites it appears as the most likely arrangement that the K adatoms initially occupy the ( + ) sites up to a coverage of 2 x 1014 atoms cm -z and then continue to adsorb on (zx) sites. The average distance between (zx) marked sites and ( + ) marked sites is about 5 #,, that is slightlYolarger than the K bulk metal diameter of - 4.6 A. As there are two (zx) sites per unit cell, the coverage after complete saturation of these sites becomes 4 x 1014 atoms cm -2 in agreement with the intensity of the K Auger signal. The above arrangement of adsorbed K is consistent with the absence of extra spots in the LEED pattern of fl-FeSi 2. Using the above coverage calibration and the constancy of the sticking coefficient in the early stages of adsorption one can obtain from the initial slope of the W F curve (fig. 2) an initial dipole moment of - 5 . 4 D for adsorbed potassium. This value can be compared with the initial dipole moments for K on Fe(100) 4.4 D [15], on Fe(ll0) 6.0 D [16], on F e ( l l l ) 3.9 D [15] and on S i ( l l l ) 3.9 D [4]. It is in better agreement with the
initial dipole m o m e n t of K on Fe than that of K on Si. On the other hand, the final W F change at saturation, - 3 . 2 eV, is the same as that observed for K on S i ( l l l ) [2]. These two observations are also consistent with adsorbed K at low coverage interacting with iron lying slightly below the outermost layer of silicon atoms. As the K coverage increases the interaction with the substrate is mainly limited to the outermost layer of silicon atoms.
References [1] A. Franciosi, P. Soukiassian, P. Philhp, S. Chang, A. Wall, A. Raisanen and N. Troulier, Phys. Rev. B 35 (1987) 910. [2] Y. Enta, T. Kinoshita, S. Suzuki and S. Kono, Phys. Rev. B 36 (1987) 9801. [3] U.A. Ditzinger, Ch. Lunau, B. Schieweck, St. Tosch, H. Neddermeyer and M. Hanbiicken, Surf. Sci. 211/212 (1989) 707. [4] E.M. Oeilig and R. Miranda, Surf. Sci. 1977 (1986) L974. [5] S. Kennou, S. Ladas, M. Kamaratos and C.A. Papageorgopoulos, Surf. Sci. 216 (1989) 462. [6] M.G. Beni, U. Del Pennino, C. Mariani, S. Valeri and J.A. Schaefer, Surf. Sci. 211/212 (1989) 659. [7] H. Starnberg, P. Soukiassian, M.H. Bakshi and Z. Hurych, Surf. Sci. 224 (1989) 13. [8] S. Kennou, T.A. Nguyen Tan and R.C. Cinti, ECOSS-11, Surf. Sci., in press. [9] R.C. Cinti, T.A. Nguyen Tan, Y. Capiomont and S. Kennou, Surf. Sci. 134 (1983) 755. [10] N. Cherief, R.C. Cinti, M. De Crescenzi, J. Derrien, T.A. Nguyen Tan and J.Y. Veuillen, Appl. Surf. Sci. 41/42 (1989) 241. [11] N. Cherief, C. D'Anterroches, R.C. Cinti, T.A. Nguyen Tan and J. Derrien, Appl. Phys. Lett. 55 (1989) 1671. [12] C.A. S~benne, Nuovo Cimento 39 (1977) 768. [13] B. Egert and G. Panzer, Phys. Rev. B 29 (1984) 3293. [14] Y. Dusansoy, J. Protas, R. Nandji and B. Roques, Acta Cryst. B 27 (1971) 1209. [15] S.B. Lee, M. Weiss and G. Ertl, Surf. Sci. 108 (1981) 357. [16] G. Brod~n and H.P. Bonzel, Surf. Sci. 84 (1979) 106. [17] N. CherieL Doctoral Thesis, Grenoble, 1989.