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Adsorption structure of K on Si(001) at various coverages
Journal of Crystal Growth 115 (1991)411—417 North-Holland
o,
CRYSTAL GROWTH
Adsorption structure of K on Si(OO1) at various coverages T. Urano, K. Sakaue, K. Nagano, S. Hongo and T. Kanaji Faculty of Engineering, Kohe Uniiersity, Rokko-dai, Nada, Kohe 657. Japan
AES signals of K atoms deposited on Si(OOl) surface saturate at every temperature. For the surface having a saturation coverage at room temperature, LEED I—V curves are measured and compared with the theoretical curves. A reliability factor (R-factor) analysis indicates that both quasi-hexagonal hollow site and cave site (H—C site) adsorbed double layer models give the smallest value of R. For the surface having nearly one-half amount of K atoms relative to a saturation coverage at room temperature it seems that the H—C site model is also most probable.
1. Introduction The adsorption system of alkali metals on semiconductor has been studied frequently because it is a simple typical adsorption system and because of its technological interest, that is, for negative-electron-affinity (NEA) devices. Spontaneous electron emission (exoemission) is observed through oxidation of this system. In spite of much effort, detailed understanding has not been obtained because of uncertainties for the amount and geometric arrangement of alkali atoms. As for alkali atoms adsorbed on Si(001) surfaces, the saturation coverage at room temperature 0 has been considered to be half a monolayer (0 1/2) [1], whereas Enta et al. conclude that 0 1.0 through the interpretation of angleresolved ultraviolet photoelectron spectroscopy (ARUPS) [21,where one monolayer (0 1.0) corresponds to the number of topmost Si atoms. However, Glander and Webb argue that in the case of Na the saturation coverage is 0 0.68 [3]. On the other hand, Oellig and Miranda observe the layer-by-layer growth of K by AES [4]. According to the thermal desorption spectra (TDS) obtained by Tanaka et al. there are two desorption peaks of K from Si(001) surface at about 600 K and 700—900 K [5]. For the Si(001) 2 x 1 surface having a saturation coverage of alkali metal, several structural =
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0022-0248/91/$03.5() © 1991
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models have been proposed. One of them is Levine’s model in which alkali atoms compose linear chains on the top-layer dimer rows of Si substrate (pedestal or quasi-hexagonal hollow (H) site, 0 1/2) [6]. The other one is the model in which alkali atoms sit on the valley site in between dimer chains (cave (C) site, 0 1/2) [71. On the other hand, Abukawa and Kono proposed the model in which alkali atoms sit on both H and C sites by X-ray photoelectron diffraction (XPD) observation (double layer model, 0 1.0) [81. We have carried out a dynamical low-energy electron diffraction (LEED) intensity analysis of the structures of K adsorbed on Si(001) 2 x 1 surface [91.For the surface having a saturation coverage at room temperature, the double layer model indicated the smallest value of the reliability factor (R-factor) among the three models mentioned above. However, the similarity between theoretical and experimental J~J/curves was not enough. In this paper we investigate the changing rate of K Auger peak height at various temperatures as a function of deposition time and decreasing rate of that through elevating temperature after sufficient adsorption at room temperature. Further, at a few stages of adsorption we measure LEED I—V curves and compare them with the theoretical curves for several structure models including the three models mentioned above. =
Elsevier Science Publishers B.V. All rights reserved
=
=
4 I2
7. L/rano ci a!.
/ Adsorption
Or/u lure 0! K on ,S’i(OOl) at anoio
SO!
erages
2. Experimental procedure ~iooF°
The experiments were carried out in a standard ultra-high vacuum system with a conventional four-grid LEED—AES system. A Si wafer (p-type. 17—23 12 cm) which was carefully chemically treated to form a thin oxide layer and cut into a strip of 5 x 15 mm is supported by a pair of tantalum foils. The sample was cleaned by flashing up to a relatively low temperature of 9000 C by a direct electric current pushed through the sample. After the cleaning, a very sharp 2 x I LEED pattern with two domains was observed and no trace of contamination was detected by AES. In our previous studies, the sample which was not given a special surface treatment was flashed up to about 120() c to eliminate carbon contamination. However, by heat treatment repetition the [113] facet pattern was observed and the hackground of the LEED pattern became a little bit brighter [9]. Therefore, this time we did not heat up the sample so many times. K atoms were deposited onto these substrates from commercial chromate dispensers (SAES Getters Co.). Fig. I shows the peak-to-peak amplitude of the AES signal of K-LMM (248 eV) with the substrate temperature as a function of deposition time. The substrate temperature was controlled by direct electric current through the sample and estimated by extrapolation of temperatures measured by both an optical pyrometer and a thermocouple at higher temperatures. 0
~iooL
/ /
~R T
300°C -
425°C
~
0C
500 0
0
° ° °
° ° ° °
o
° °
200
300
500
o o 600
Temperature (°C)
Fig. 2. The decreasing rate of K-LMM Auger signal when the substrate temperature is elevated step-h~-stepafter sufficient adsorption at room temperature.
In fig. 2 the decreasing rate of AES signal of K when the substrate temperature is elevated gradually step-by-step due to heating for every I mm is shown. The amplitude is almost flat up to 3200 C, except for an initial decrease near room temperature, and decreases almost smoothly. The saturated amplitudes of every substrate temperatore shown in fig. 1 are nearly equivalent to the amplitudes corresponding to individual temperature shown in fig. 2. This result is coincident qualitatively with the wide range spectra of TDS [5].
LEED I—V curves were measured with a systern using a conventional TV camera and a microcomputer which was equipped in our lahoratory. As the surfaces having a saturation coverage of K atoms also indicated the 2 ~ 1 structure with two domains, symmetrical beams, for example. (10), (10) and (01) beams (the (01) beam could not be observed behind the sample manipulator), showed the same I—V curves. Then, after the confirmation of normal incidence of an electron beam by comparing the symmetrical beams with each other, I—V curves of three integral order beams, (10), (11) and (20), and four half order beams. (~0),(U), (~0)and (U), were measured. As examples, I—V curves at a few adsorption stages are shown in fig. 3. 3. Structure analysis
Deposition Time (mln I
Fig. I. The peak-to-peak amplitudes of AES signals of K-LMM (248 eV) with the substrate temperature as a function of deposition time.
Dynamical calculations of I—V curves for several structure models of K adsorbed on Si(001) 2 x I surfaces in which the substrate Si atoms
T Urano et a!. / Adsorption structure of K on Si(OO/) at iarlous cocci-ages
(10) beam
413
(1/2 l)beam
~
~~~cle~nSi(00I) 0
00
200
0
00 Energy (eV)
Fig. 3. Examples of experimental I—V curves measured for the surfaces on which AES signal of K atoms saturate at each temperature.
compose a symmetric dimer are carried out by using a set of subroutines presented by Van Hove and Tong [10].The four adsorption sites shown in fig. 4 (H site, C site, dimer bridge (D) site, long bridge (B) site) and 8 models, i.e., four cases D-site H-site B—site C—site
to P
Vi
ew
• •
D H
B C
~iew
where one K atom sits on every site (single-layer model) and four cases where two K atoms sit on combined sites (H—C, H—B, D—C, D—B; doublelayer model), are considered within models in which only topmost Si atoms reconstruct to compose a symmetric dimer. The similarity between experimental and theoretical I—V curves has been evaluated by using the Zanazzi—Jona reliability factor (R-factor) [11]. A set of parameters of K atom height and the displacement of the topmost Si atoms from the position of bulk crystal considered in the calculation are shown in fig. 5. First, the experimental I—V curves obtained for the surface on which K atoms saturate at room temperature have been compared with theoretical curves. In table 1 the minimum values of R for all 8 models and the parameters in those cases are indicated. It ismodel shown (H—C that the site adsorbed double layer siteH—C model) indicates the smallest value of R. This is the same as is obtained in preliminary work [9].
~
diL 2~
Fig. 4. Four adsorption sites considered in the calculations: (H) pedestal or quasi-hexagonal hollow site. (C) cave site, (D) dimer bridge site and (B) long bridge site. Four models where one K atom sits on every site (single layer model) and four models where two K atoms sit on combined sites (double layer model) are considered.
~ ‘~‘
d~
~
~Sit2>~ ‘~
‘~‘
(b)
~
~Si(2)~
‘o ‘u’ (c)
Fig. 5. A set of parameters of the K atom height and the displacement of the topmost Si atoms considered in the calculation.
414
T. Urano ci a!.
/ Adsorption
structure of K on Si(OOI) at iarious coierages
Table I The smallest numerical value of the R-faetor and the best fit parameters of 8 models within models in which only K atoms and topmost Si atoms reconstruct to compose a symmetric dimer for experimental curves obtained for the surface onwhich the K atoms were deposited at room temperature substrate R
71
H C D B Fl—C H—B D—C D—B
0.50 0.52 (1.61) 0.57 0.39 0.43 (1.62 0.51
d
d~
X1
X~
(A)
(A)
(A)
(A)
0.45 ((.45 0.45 11.55 ((.75 0.85 ((.55 0.45
0.25 0.30 0.20 0.30 0.11) 0.11) 11.20 11.20
2.1ff) 1.26 1.81 1.96 1.76 1.66 1.76 1.56
— — — —
0.4 0.6 0.4 (1.3
The Zanazzi—Jona R-factor is very sensitive to the geometry of a few surface layers; other models than the H—C site model are excluded from further consideration because of rather large values of their R-factors, as shown in table 1. Judging from its value of R-factor, the H—B site model seems to be a possible one; however, because this model does not agree with the models proposed by other researchers, we also excluded it from our consideration. Actually, it seems that the coincidence of the position of peaks and dips in the H—C site model is better than that in the H—B site model, by visual observation. Comparing the theoretical curves obtained for the structure having best fit parameters of the H—C site model with the experimental curves, although it seems to reproduce the position of the peaks and shoulders considerably, the relative intensities are not similar to each other. Then calculations for the H—C site model in which Si atoms beneath the topmost layer down to 4th layer reconstruct have been carried out within a symmetric dimer model. The best fit parameters arc shown in fig. 6, and the experimental and theoretical I—V curves shown in fig. 7 indicate that the coincidence between corresponding curves is fairly good. Next, the experimental I—V curves obtained for the surface on which K atoms were deposited at a substrate temperature of 425 C are corn0
____________
76
t 04$.
30 A
07$. 07$.
I
I
___________
A
035$. 027~ 35$.
~~~~Oi3A ~0
027$.
0-
3$.
0
____________
)
p
Fig. 6. Best fit parameters for the Fl—C site adsorbed model.
pared with theoretical curves for the models in which only topmost Si atoms reconstruct to compose a symmetric dimer. Although these experiRT°025 (10)~
r=O32 calc OXP
____________________________ (H)
rO25
r~033
t~ —
-
_________________
-~
(1/2 0)
r=031
-~
______
~Ir027
(3/20) —-
/.\~~~028
0 16
0
100
200 Energy 1eV) Fig. 7. Comparison of best fit theoretical I—V curves with the experimental curves.
T Uraoo et a!.
/ Adsorption structure of K on
Table 2 The smallest numerical value of the R-faetor and the best fit parameters of 8 models for experimental curves for the surface on which K atoms were deposited at a substrate temperature of 4250 C
Rzj H C D B H—C H—B DC D—B
d~
d
(A)
(A)
0.42 0.43 0.47 ((.55 0.34 0.38 (1.45 0.48
2
2.06 1.16 1.96 1.86 1.76 1.66 1.86 1.86
Xi
X2
0.45 ((.45 0.45 0.55 0.75 (1.85 0.55 0.45
0.30 0.30 0.20 0.30 0.20 0.20 0.20 0.20
(A)
— — — —
0.4 0.6 0.4 0.4
(A)
mental I—V curves are not the same as those obtained for the surface adsorbed at room temperature, as shown in fig. 3, the positions of peaks and dips are rather similar to each other, As a result, the value of R of the H—C site model is also smaller than that of the single layer models shown in table 2.
Si(OOI) at tarious coi erages
415
curves saturate at temperatures higher than 200 C, at room temperature sometimes the curve showed a gradual increase after the breakpoint. 0
Therefore, we consider that whether the coverage of K atoms is saturated or not depends on the flatness of the surface. That is, in the case of high temperature heating, it is considered that there could be many defects and steps, and three-dimensional islands of K atoms might grow at such places. This is indicated by a brightness of the background of the LEED pattern. On the other hand, in the case of low temperature heat treatment, because of the sharpness of beams and low background of the LEED pattern, it is expected that the surface is rather flat. As indicated in section 3, the H—C site adsorbed model gives the smallest value of the reliability factor R within a symmetric dimer model. This structure is consistent with the socalled “double layer model” originally proposed by Enta Ct al. [2,81 and agrees with recent theoretical studies by Morikawa et al., in which they obtain an optimized structure for the coverage 0 1.0 by the first-principles molecular dynamics based on the norm-conserving pseudopotential [12]. The height of C site K atoms, the lateral displacement of top layer Si atoms and the layer distance between top layer and 2nd layer Si substrate of our model are very similar to those of the optimized structure. However, there are a few different points. One of them is the discrepancy of the interlayer spacing d5 between H site and C site K atoms in fig. 5, Morikawa Ct al. indicate this separation as 1.1 A, and this value is consis=
4. Discussion In fig. I it is indicated that AES signals of K atoms saturate at every substrate temperature. In the former case of heat treatment at high temperatures, mentioned in section 2, although the 04
~ ~
-03-
\\V II’
0
\‘iA I
I
tent proposed by Abukawa by gatedwith means carefully of that XPD the study relation [81. Therefore, betweenand d~and weKono investid2. That is, when d, is considered as a parameter and values of R are plotted as a function of d1,
~ -.~
every curve indicates a although parabolic aspect and 8.has distinct shown inminimum the figure, value, as shown the in R fig. value deAsa
0 2A
creases at the point with d1 2.25 A (d7 1.0—1.1 A), the value is larger than that at the point with =
02
0
I
20 di
I
I
30
(A)
Fig. 8. Typical example of the structure R-factor versus interlayer spacing (di) between the upper K layer and topmost Si layer with a parameter of spacing of two K layers (di).
d1
=
1.76 A (d5 0.4 A). On the other hand, although the magnitudes of displacement of Si substrate atoms down to 4th layers are not necessarily equivalent, the direc=
=
416
T (]rano et a!.
/ Adsorption structure oI K
tions are similar to those of the optimized structure mentioned above and those in the case of adsorption geometry of Na on Si(00l) obtained by Wei et al. [13]. The near neighbour bond lengths between K and Si atoms are 2.88 A for H site K atom and 3.50 A for C site K atom, which are different from the value of 3.14 A of the K—Si bond length obtained by surface extended X-ray absorption fine structure spectroscopy (SEXAFS) for the half layer adsorbed K/Si(00l)2 x I interface presented by Kendelewicz et al. [14]. This might he possible if electrons go away froni the upper K atoms and come in to Si atoms of the surface layer. On the other hand, the Si—Si bond length between top layer dimer atoms is 2.45 A, which is slightly larger than the value of hulk Si atoms. and the bond length between top layer and 2nd layer Si atoms is equivalent to the value of hulk crystal. For the surface on which K atoms are adsorbed by nearly one-half of the saturation coverage at room temperature, the H—C site adsorbed double layer model also indicates the smallest R-factor. It is supposed that there are two regions: one in which K atoms exist abundantly and form a nearly H—C site adsorbed structure, and another in which K atoms scarcely exist and do not form any structure. Actually, this surface showed slightly weak beam intensities and a little hit bright background. The most remarkable diiference between two I—V curves obtained for surfaces saturated at room temperature and at 425 C is the existence of a peak at the energy of about 65 eV in the latter curve of the (10) beam in fig.3. This peak exists in the curve of clean Si(OOl) surface. On the other hand, the peak shown in the middle curve of the (~1) beam at an energy of around 60 eV might he due to the clean surface which shows the broad tail. For the adsorption geometry of Na on Si(00 I) of saturation coverage. Wei et al. indicate that the H site adsorbed model is best. On the other hand, however, it is also mentioned that the H—C site adsorbed double layer model is possible, by means of LEED analysis [13], and they did not accept the H—C site model because of saturation coverage 0 0.68 due to the Auger analysis [4]. Then. 0
=
osi ,Si(OOl) at arioio cot 5,1st/es
it seems that such an interpretation as mentioned above could he possible. Other order structures such as 2 x 2 at the coverage less than half of K on Si(0() I). and 2 >< 3 and c(4>< 2) for Cs on Si(00l) have been found by Ahukawa and Kono [15]. Wei et al. observe 4 x I and 2 x 3 structures for Na on Si(OOl) surface. We tried carefully to observe such a structure at several stages of the deposition: however, a distinct structure could not he observed by LEEI). According to recent scanning tunneling microscopy (STM) observations, K atoms sit on one site of Si atoms composing an asymmetric dinier [16]. However, it seems that this structure is possihle only for a surface with very low coverage of K less than 0.1 monolayer.
5. Summary AES signals of K atoms deposited ~rt Si(00I surface saturated at every temperature. LEED I—V curves were measured for the surface having a different coverage and were compared with the theoretical curves. For the surface having a satLiration coverage at room temperature, a reliability factor analysis indicated that both quasi-hexagonal hollow site and cave site (I-I—C site) adsorbed double layer models gave the smallest value of I? among 8 models. The agreement between experimental and theoretical curves was improved remarkably by taking into account the shift of the interior Si atoms up to 4th layers within a symmetrical dimer model. For the surface having nearly one-half the amount of K atoms relative to a saturation coverage at room temperature, it seems that the H—C site model is also most probable.
Acknowledgements The authors would like to thank Professor A. Kawazu of the University of Tokyo for his supply of beneficial information about the I—V curve measurement system. The authors wish to express their thanks to Professor J.B. Pendry of Imperial College, Professor K. Muller and Professor K.
T. Urano et a!.
/ Adsorption
structure 01K on Si(OOl) at iarious cot era/es
Heinz of Universität Erlangen—Nurnberg, and Dr. S.P. Tear of York University for helpful discussions and encouragement.
References Ill 121 [3] [4]
15] 161
Fl. Toehihara. Surface Sci. 126 (1983) 523. Y. Enta. T. Kinoshita. S. Suzuki and S. Kono, Phys. Rev. B 36(1987)98(11. G.S. Glander and MB. Webb. Surface Sei. 222 (1989) 64; 224 (1989) 60. EM. Oellig and R. Miranda, Surface Sci. 117 (1986) L947. 5. Tanaka. N. Takagi. N. Minami and M. Nishijima. Phys. Rev. B 42 (1990) 1868. iD. Levine. Surface Sci .34 (1973) 90.
417
[7] M.C. Asencio. E.G. Miehel. J. Alvarez. C. Ocal. R. Miranda and S. Ferrer, Surface Sci. 211/212 (1989) 31. T. Ahukawa and S. Kono, Phys. Rev B 37 (1988) 9097. T. Urano. Y. Uchida, S. Hongo and T. Kanaji. Surface Sci. 242 (1991) 39. [10] MA. Van Hove and S.Y. Tong, Surface Crystallography by LEED (Springer, Berlin. 1979). [II] E. Zanazzi and F. Jona. Surface Sci. 62 (1977) 61. [12] Y. Morikawa. K. Kohayashi. K. Terakura and S. Blogel, Phys. Rev. B. to he published. 1131 C.M. Wei. H. Haung. S.Y. Tong. (IS. Glander and MB. Webb, Phys. Rev. B. to he published. [141 T. Kendelewicz, P. Soukiassian, R.S. List. J.C. Woieik, P. Pianetta, 1. Lindau and J.C. Spicer, Phys. Rev. B 37 (1988)7115. 115] T. Abukawa and S. Kono. Surface Sci 214 (1989) 141. 116] T. Hashizume. Y. Ilasegawa, 1. Kamiya. T. Ide. I. Sumita, S. Hyodo. Sakurai. Tochihara, Kuhota Murata, J. T. Vacuum Sci.II.Technol. A $ M. (1990) 223. and Y.
181 191
o,
CRYSTAL GROWTH
Adsorption structure of K on Si(OO1) at various coverages T. Urano, K. Sakaue, K. Nagano, S. Hongo and T. Kanaji Faculty of Engineering, Kohe Uniiersity, Rokko-dai, Nada, Kohe 657. Japan
AES signals of K atoms deposited on Si(OOl) surface saturate at every temperature. For the surface having a saturation coverage at room temperature, LEED I—V curves are measured and compared with the theoretical curves. A reliability factor (R-factor) analysis indicates that both quasi-hexagonal hollow site and cave site (H—C site) adsorbed double layer models give the smallest value of R. For the surface having nearly one-half amount of K atoms relative to a saturation coverage at room temperature it seems that the H—C site model is also most probable.
1. Introduction The adsorption system of alkali metals on semiconductor has been studied frequently because it is a simple typical adsorption system and because of its technological interest, that is, for negative-electron-affinity (NEA) devices. Spontaneous electron emission (exoemission) is observed through oxidation of this system. In spite of much effort, detailed understanding has not been obtained because of uncertainties for the amount and geometric arrangement of alkali atoms. As for alkali atoms adsorbed on Si(001) surfaces, the saturation coverage at room temperature 0 has been considered to be half a monolayer (0 1/2) [1], whereas Enta et al. conclude that 0 1.0 through the interpretation of angleresolved ultraviolet photoelectron spectroscopy (ARUPS) [21,where one monolayer (0 1.0) corresponds to the number of topmost Si atoms. However, Glander and Webb argue that in the case of Na the saturation coverage is 0 0.68 [3]. On the other hand, Oellig and Miranda observe the layer-by-layer growth of K by AES [4]. According to the thermal desorption spectra (TDS) obtained by Tanaka et al. there are two desorption peaks of K from Si(001) surface at about 600 K and 700—900 K [5]. For the Si(001) 2 x 1 surface having a saturation coverage of alkali metal, several structural =
=
=
=
0022-0248/91/$03.5() © 1991
—
models have been proposed. One of them is Levine’s model in which alkali atoms compose linear chains on the top-layer dimer rows of Si substrate (pedestal or quasi-hexagonal hollow (H) site, 0 1/2) [6]. The other one is the model in which alkali atoms sit on the valley site in between dimer chains (cave (C) site, 0 1/2) [71. On the other hand, Abukawa and Kono proposed the model in which alkali atoms sit on both H and C sites by X-ray photoelectron diffraction (XPD) observation (double layer model, 0 1.0) [81. We have carried out a dynamical low-energy electron diffraction (LEED) intensity analysis of the structures of K adsorbed on Si(001) 2 x 1 surface [91.For the surface having a saturation coverage at room temperature, the double layer model indicated the smallest value of the reliability factor (R-factor) among the three models mentioned above. However, the similarity between theoretical and experimental J~J/curves was not enough. In this paper we investigate the changing rate of K Auger peak height at various temperatures as a function of deposition time and decreasing rate of that through elevating temperature after sufficient adsorption at room temperature. Further, at a few stages of adsorption we measure LEED I—V curves and compare them with the theoretical curves for several structure models including the three models mentioned above. =
Elsevier Science Publishers B.V. All rights reserved
=
=
4 I2
7. L/rano ci a!.
/ Adsorption
Or/u lure 0! K on ,S’i(OOl) at anoio
SO!
erages
2. Experimental procedure ~iooF°
The experiments were carried out in a standard ultra-high vacuum system with a conventional four-grid LEED—AES system. A Si wafer (p-type. 17—23 12 cm) which was carefully chemically treated to form a thin oxide layer and cut into a strip of 5 x 15 mm is supported by a pair of tantalum foils. The sample was cleaned by flashing up to a relatively low temperature of 9000 C by a direct electric current pushed through the sample. After the cleaning, a very sharp 2 x I LEED pattern with two domains was observed and no trace of contamination was detected by AES. In our previous studies, the sample which was not given a special surface treatment was flashed up to about 120() c to eliminate carbon contamination. However, by heat treatment repetition the [113] facet pattern was observed and the hackground of the LEED pattern became a little bit brighter [9]. Therefore, this time we did not heat up the sample so many times. K atoms were deposited onto these substrates from commercial chromate dispensers (SAES Getters Co.). Fig. I shows the peak-to-peak amplitude of the AES signal of K-LMM (248 eV) with the substrate temperature as a function of deposition time. The substrate temperature was controlled by direct electric current through the sample and estimated by extrapolation of temperatures measured by both an optical pyrometer and a thermocouple at higher temperatures. 0
~iooL
/ /
~R T
300°C -
425°C
~
0C
500 0
0
° ° °
° ° ° °
o
° °
200
300
500
o o 600
Temperature (°C)
Fig. 2. The decreasing rate of K-LMM Auger signal when the substrate temperature is elevated step-h~-stepafter sufficient adsorption at room temperature.
In fig. 2 the decreasing rate of AES signal of K when the substrate temperature is elevated gradually step-by-step due to heating for every I mm is shown. The amplitude is almost flat up to 3200 C, except for an initial decrease near room temperature, and decreases almost smoothly. The saturated amplitudes of every substrate temperatore shown in fig. 1 are nearly equivalent to the amplitudes corresponding to individual temperature shown in fig. 2. This result is coincident qualitatively with the wide range spectra of TDS [5].
LEED I—V curves were measured with a systern using a conventional TV camera and a microcomputer which was equipped in our lahoratory. As the surfaces having a saturation coverage of K atoms also indicated the 2 ~ 1 structure with two domains, symmetrical beams, for example. (10), (10) and (01) beams (the (01) beam could not be observed behind the sample manipulator), showed the same I—V curves. Then, after the confirmation of normal incidence of an electron beam by comparing the symmetrical beams with each other, I—V curves of three integral order beams, (10), (11) and (20), and four half order beams. (~0),(U), (~0)and (U), were measured. As examples, I—V curves at a few adsorption stages are shown in fig. 3. 3. Structure analysis
Deposition Time (mln I
Fig. I. The peak-to-peak amplitudes of AES signals of K-LMM (248 eV) with the substrate temperature as a function of deposition time.
Dynamical calculations of I—V curves for several structure models of K adsorbed on Si(001) 2 x I surfaces in which the substrate Si atoms
T Urano et a!. / Adsorption structure of K on Si(OO/) at iarlous cocci-ages
(10) beam
413
(1/2 l)beam
~
~~~cle~nSi(00I) 0
00
200
0
00 Energy (eV)
Fig. 3. Examples of experimental I—V curves measured for the surfaces on which AES signal of K atoms saturate at each temperature.
compose a symmetric dimer are carried out by using a set of subroutines presented by Van Hove and Tong [10].The four adsorption sites shown in fig. 4 (H site, C site, dimer bridge (D) site, long bridge (B) site) and 8 models, i.e., four cases D-site H-site B—site C—site
to P
Vi
ew
• •
D H
B C
~iew
where one K atom sits on every site (single-layer model) and four cases where two K atoms sit on combined sites (H—C, H—B, D—C, D—B; doublelayer model), are considered within models in which only topmost Si atoms reconstruct to compose a symmetric dimer. The similarity between experimental and theoretical I—V curves has been evaluated by using the Zanazzi—Jona reliability factor (R-factor) [11]. A set of parameters of K atom height and the displacement of the topmost Si atoms from the position of bulk crystal considered in the calculation are shown in fig. 5. First, the experimental I—V curves obtained for the surface on which K atoms saturate at room temperature have been compared with theoretical curves. In table 1 the minimum values of R for all 8 models and the parameters in those cases are indicated. It ismodel shown (H—C that the site adsorbed double layer siteH—C model) indicates the smallest value of R. This is the same as is obtained in preliminary work [9].
~
diL 2~
Fig. 4. Four adsorption sites considered in the calculations: (H) pedestal or quasi-hexagonal hollow site. (C) cave site, (D) dimer bridge site and (B) long bridge site. Four models where one K atom sits on every site (single layer model) and four models where two K atoms sit on combined sites (double layer model) are considered.
~ ‘~‘
d~
~
~Sit2>~ ‘~
‘~‘
(b)
~
~Si(2)~
‘o ‘u’ (c)
Fig. 5. A set of parameters of the K atom height and the displacement of the topmost Si atoms considered in the calculation.
414
T. Urano ci a!.
/ Adsorption
structure of K on Si(OOI) at iarious coierages
Table I The smallest numerical value of the R-faetor and the best fit parameters of 8 models within models in which only K atoms and topmost Si atoms reconstruct to compose a symmetric dimer for experimental curves obtained for the surface onwhich the K atoms were deposited at room temperature substrate R
71
H C D B Fl—C H—B D—C D—B
0.50 0.52 (1.61) 0.57 0.39 0.43 (1.62 0.51
d
d~
X1
X~
(A)
(A)
(A)
(A)
0.45 ((.45 0.45 11.55 ((.75 0.85 ((.55 0.45
0.25 0.30 0.20 0.30 0.11) 0.11) 11.20 11.20
2.1ff) 1.26 1.81 1.96 1.76 1.66 1.76 1.56
— — — —
0.4 0.6 0.4 (1.3
The Zanazzi—Jona R-factor is very sensitive to the geometry of a few surface layers; other models than the H—C site model are excluded from further consideration because of rather large values of their R-factors, as shown in table 1. Judging from its value of R-factor, the H—B site model seems to be a possible one; however, because this model does not agree with the models proposed by other researchers, we also excluded it from our consideration. Actually, it seems that the coincidence of the position of peaks and dips in the H—C site model is better than that in the H—B site model, by visual observation. Comparing the theoretical curves obtained for the structure having best fit parameters of the H—C site model with the experimental curves, although it seems to reproduce the position of the peaks and shoulders considerably, the relative intensities are not similar to each other. Then calculations for the H—C site model in which Si atoms beneath the topmost layer down to 4th layer reconstruct have been carried out within a symmetric dimer model. The best fit parameters arc shown in fig. 6, and the experimental and theoretical I—V curves shown in fig. 7 indicate that the coincidence between corresponding curves is fairly good. Next, the experimental I—V curves obtained for the surface on which K atoms were deposited at a substrate temperature of 425 C are corn0
____________
76
t 04$.
30 A
07$. 07$.
I
I
___________
A
035$. 027~ 35$.
~~~~Oi3A ~0
027$.
0-
3$.
0
____________
)
p
Fig. 6. Best fit parameters for the Fl—C site adsorbed model.
pared with theoretical curves for the models in which only topmost Si atoms reconstruct to compose a symmetric dimer. Although these experiRT°025 (10)~
r=O32 calc OXP
____________________________ (H)
rO25
r~033
t~ —
-
_________________
-~
(1/2 0)
r=031
-~
______
~Ir027
(3/20) —-
/.\~~~028
0 16
0
100
200 Energy 1eV) Fig. 7. Comparison of best fit theoretical I—V curves with the experimental curves.
T Uraoo et a!.
/ Adsorption structure of K on
Table 2 The smallest numerical value of the R-faetor and the best fit parameters of 8 models for experimental curves for the surface on which K atoms were deposited at a substrate temperature of 4250 C
Rzj H C D B H—C H—B DC D—B
d~
d
(A)
(A)
0.42 0.43 0.47 ((.55 0.34 0.38 (1.45 0.48
2
2.06 1.16 1.96 1.86 1.76 1.66 1.86 1.86
Xi
X2
0.45 ((.45 0.45 0.55 0.75 (1.85 0.55 0.45
0.30 0.30 0.20 0.30 0.20 0.20 0.20 0.20
(A)
— — — —
0.4 0.6 0.4 0.4
(A)
mental I—V curves are not the same as those obtained for the surface adsorbed at room temperature, as shown in fig. 3, the positions of peaks and dips are rather similar to each other, As a result, the value of R of the H—C site model is also smaller than that of the single layer models shown in table 2.
Si(OOI) at tarious coi erages
415
curves saturate at temperatures higher than 200 C, at room temperature sometimes the curve showed a gradual increase after the breakpoint. 0
Therefore, we consider that whether the coverage of K atoms is saturated or not depends on the flatness of the surface. That is, in the case of high temperature heating, it is considered that there could be many defects and steps, and three-dimensional islands of K atoms might grow at such places. This is indicated by a brightness of the background of the LEED pattern. On the other hand, in the case of low temperature heat treatment, because of the sharpness of beams and low background of the LEED pattern, it is expected that the surface is rather flat. As indicated in section 3, the H—C site adsorbed model gives the smallest value of the reliability factor R within a symmetric dimer model. This structure is consistent with the socalled “double layer model” originally proposed by Enta Ct al. [2,81 and agrees with recent theoretical studies by Morikawa et al., in which they obtain an optimized structure for the coverage 0 1.0 by the first-principles molecular dynamics based on the norm-conserving pseudopotential [12]. The height of C site K atoms, the lateral displacement of top layer Si atoms and the layer distance between top layer and 2nd layer Si substrate of our model are very similar to those of the optimized structure. However, there are a few different points. One of them is the discrepancy of the interlayer spacing d5 between H site and C site K atoms in fig. 5, Morikawa Ct al. indicate this separation as 1.1 A, and this value is consis=
4. Discussion In fig. I it is indicated that AES signals of K atoms saturate at every substrate temperature. In the former case of heat treatment at high temperatures, mentioned in section 2, although the 04
~ ~
-03-
\\V II’
0
\‘iA I
I
tent proposed by Abukawa by gatedwith means carefully of that XPD the study relation [81. Therefore, betweenand d~and weKono investid2. That is, when d, is considered as a parameter and values of R are plotted as a function of d1,
~ -.~
every curve indicates a although parabolic aspect and 8.has distinct shown inminimum the figure, value, as shown the in R fig. value deAsa
0 2A
creases at the point with d1 2.25 A (d7 1.0—1.1 A), the value is larger than that at the point with =
02
0
I
20 di
I
I
30
(A)
Fig. 8. Typical example of the structure R-factor versus interlayer spacing (di) between the upper K layer and topmost Si layer with a parameter of spacing of two K layers (di).
d1
=
1.76 A (d5 0.4 A). On the other hand, although the magnitudes of displacement of Si substrate atoms down to 4th layers are not necessarily equivalent, the direc=
=
416
T (]rano et a!.
/ Adsorption structure oI K
tions are similar to those of the optimized structure mentioned above and those in the case of adsorption geometry of Na on Si(00l) obtained by Wei et al. [13]. The near neighbour bond lengths between K and Si atoms are 2.88 A for H site K atom and 3.50 A for C site K atom, which are different from the value of 3.14 A of the K—Si bond length obtained by surface extended X-ray absorption fine structure spectroscopy (SEXAFS) for the half layer adsorbed K/Si(00l)2 x I interface presented by Kendelewicz et al. [14]. This might he possible if electrons go away froni the upper K atoms and come in to Si atoms of the surface layer. On the other hand, the Si—Si bond length between top layer dimer atoms is 2.45 A, which is slightly larger than the value of hulk Si atoms. and the bond length between top layer and 2nd layer Si atoms is equivalent to the value of hulk crystal. For the surface on which K atoms are adsorbed by nearly one-half of the saturation coverage at room temperature, the H—C site adsorbed double layer model also indicates the smallest R-factor. It is supposed that there are two regions: one in which K atoms exist abundantly and form a nearly H—C site adsorbed structure, and another in which K atoms scarcely exist and do not form any structure. Actually, this surface showed slightly weak beam intensities and a little hit bright background. The most remarkable diiference between two I—V curves obtained for surfaces saturated at room temperature and at 425 C is the existence of a peak at the energy of about 65 eV in the latter curve of the (10) beam in fig.3. This peak exists in the curve of clean Si(OOl) surface. On the other hand, the peak shown in the middle curve of the (~1) beam at an energy of around 60 eV might he due to the clean surface which shows the broad tail. For the adsorption geometry of Na on Si(00 I) of saturation coverage. Wei et al. indicate that the H site adsorbed model is best. On the other hand, however, it is also mentioned that the H—C site adsorbed double layer model is possible, by means of LEED analysis [13], and they did not accept the H—C site model because of saturation coverage 0 0.68 due to the Auger analysis [4]. Then. 0
=
osi ,Si(OOl) at arioio cot 5,1st/es
it seems that such an interpretation as mentioned above could he possible. Other order structures such as 2 x 2 at the coverage less than half of K on Si(0() I). and 2 >< 3 and c(4>< 2) for Cs on Si(00l) have been found by Ahukawa and Kono [15]. Wei et al. observe 4 x I and 2 x 3 structures for Na on Si(OOl) surface. We tried carefully to observe such a structure at several stages of the deposition: however, a distinct structure could not he observed by LEEI). According to recent scanning tunneling microscopy (STM) observations, K atoms sit on one site of Si atoms composing an asymmetric dinier [16]. However, it seems that this structure is possihle only for a surface with very low coverage of K less than 0.1 monolayer.
5. Summary AES signals of K atoms deposited ~rt Si(00I surface saturated at every temperature. LEED I—V curves were measured for the surface having a different coverage and were compared with the theoretical curves. For the surface having a satLiration coverage at room temperature, a reliability factor analysis indicated that both quasi-hexagonal hollow site and cave site (I-I—C site) adsorbed double layer models gave the smallest value of I? among 8 models. The agreement between experimental and theoretical curves was improved remarkably by taking into account the shift of the interior Si atoms up to 4th layers within a symmetrical dimer model. For the surface having nearly one-half the amount of K atoms relative to a saturation coverage at room temperature, it seems that the H—C site model is also most probable.
Acknowledgements The authors would like to thank Professor A. Kawazu of the University of Tokyo for his supply of beneficial information about the I—V curve measurement system. The authors wish to express their thanks to Professor J.B. Pendry of Imperial College, Professor K. Muller and Professor K.
T. Urano et a!.
/ Adsorption
structure 01K on Si(OOl) at iarious cot era/es
Heinz of Universität Erlangen—Nurnberg, and Dr. S.P. Tear of York University for helpful discussions and encouragement.
References Ill 121 [3] [4]
15] 161
Fl. Toehihara. Surface Sci. 126 (1983) 523. Y. Enta. T. Kinoshita. S. Suzuki and S. Kono, Phys. Rev. B 36(1987)98(11. G.S. Glander and MB. Webb. Surface Sei. 222 (1989) 64; 224 (1989) 60. EM. Oellig and R. Miranda, Surface Sci. 117 (1986) L947. 5. Tanaka. N. Takagi. N. Minami and M. Nishijima. Phys. Rev. B 42 (1990) 1868. iD. Levine. Surface Sci .34 (1973) 90.
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[7] M.C. Asencio. E.G. Miehel. J. Alvarez. C. Ocal. R. Miranda and S. Ferrer, Surface Sci. 211/212 (1989) 31. T. Ahukawa and S. Kono, Phys. Rev B 37 (1988) 9097. T. Urano. Y. Uchida, S. Hongo and T. Kanaji. Surface Sci. 242 (1991) 39. [10] MA. Van Hove and S.Y. Tong, Surface Crystallography by LEED (Springer, Berlin. 1979). [II] E. Zanazzi and F. Jona. Surface Sci. 62 (1977) 61. [12] Y. Morikawa. K. Kohayashi. K. Terakura and S. Blogel, Phys. Rev. B. to he published. 1131 C.M. Wei. H. Haung. S.Y. Tong. (IS. Glander and MB. Webb, Phys. Rev. B. to he published. [141 T. Kendelewicz, P. Soukiassian, R.S. List. J.C. Woieik, P. Pianetta, 1. Lindau and J.C. Spicer, Phys. Rev. B 37 (1988)7115. 115] T. Abukawa and S. Kono. Surface Sci 214 (1989) 141. 116] T. Hashizume. Y. Ilasegawa, 1. Kamiya. T. Ide. I. Sumita, S. Hyodo. Sakurai. Tochihara, Kuhota Murata, J. T. Vacuum Sci.II.Technol. A $ M. (1990) 223. and Y.
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