The coadsorption of cesium and potassium on Si(100)

Surface Science 227 (1990) 43-49 North-Holland

THE COADSORPTION M. KAMARATOS, Physics Department,

Received

43

OF CESIUM

S. KENNOU

University of loannina,

31 July 1989; accepted

AND POTASSIUM

ON Si(100)

and C.A. PAPAGEORGOPOULOS P.O. Box 1186, GR-451 10 loannina,

for publication

6 November

Greece

1989

An experimental investigation of K and Cs coadsorption on Si(100)2 X 1 surface at RT took place in an UHV system by LEED, AES, TDS and work function measurements. The results suggest that the sites of K and Cs on Si(lOB)2 X 1 are different and the individual saturation coverage of the two alkali is the same. During successive coadsorption the two alkali are coadsorbed collaterally in an additive manner to cover the substrate and form a mixed adlayer with a saturation coverage of - 50% higher than that of each alkali alone. When the first alkali adsorbate on Si(100) is saturated, the following deposition of the second alkali causes a decrease of the absolute coverage of the first. This is due to a partial removal of the atoms of the first alkali by those of the second. The above process is independent of the sequence of deposition of K and Cs adsorbates.

1. Introduction The interaction of alkali with semiconductor surfaces shows a growing interest in the last years because of its great technological importance [l-4]. Very recently much excitement has arisen around the behaviour of the K/Si(lOO) and Cs/Si(lOO) systems [5-131. There exists a controversy with regard to the coverage of the K and Cs overlayer on Si(100). The first model proposed by Levine [l] for the Cs/Si(lOO) system was that the Cs atoms sit on the raised sites of symmetric dimers of Si(100) 2 X 1 and form one-dimensional metallic chains corresponding to a saturation coverage at room temperature of 0.5 ML or 3.39 X 1Ol4 atoms/cm’. Later, Tochihara showed experimentally that K atoms are expected to adsorb on the same sites as Cs and thus the saturation coverage for K is the same as for Cs [6]. Abukawa and Kono proposed a double layer model for Cs- and K-saturated Si(100) [lo]. In this model Cs and K sit not only on the raised sites but also on the cave sites. As a result, the coverage of the Cs and K adlayer on Si(100) is taken as unity at saturation. Very recently Ling et al. [7] in a cluster calculation found the cave sites the most favourable adsorp0039-6028/90/$03.50 (North-Holland)

0 Elsevier Science Publishers B.V.

tion sites for K on Si(100) with alkali forming chains along the cave sites with a coverage 0.5 ML at saturation. The question whether Cs and K occupy the same sites on Si(100) can be investigated by successive coadsorption of Cs and K on the Si(100) surface. This coadsorption may also give information concerning the electronic and structural properties of the coadsorbed Cs and K system on ’ Si(100). As far as we know, there are no studies in the literature concerning binary systems in which two alkali metals are coadsorbed in submonolayer quantities on semiconductors, either collaterally or in successive superimposed layers. There exist only one study by Argile and Read on the coadsorption of Cs and K on Ag(ll1) [14]. Their experiments have shown that the two alkali metals coadsorb collaterally in a simple additive manner to cover the substrate and that approximately the same reduction in work function can be obtained by coadsorption of the two alkali metals as by adsorption of one metal alone. The results of the present work are quite different than those by Argile and Read. The view that Cs and K are absorbed on different sites on Si(100) is supported. l

44

M. Kamaratos

et al. / Coahorption

of Ce and K on Si(I#)

2. Experimental

0

The experiments were performed in an UHV system equipped with low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and facilities for work function measurements, as described previously [15]. Cesium and potassium were evaporated from commercial SAES-Getters sources, their flux was measured with a Pt ionizer and was typically - 2.0 X 1012 atoms/cm2 * s ( * lo%), the same for both alkali. The background pressure during alkali deposition was kept below lo-” Torr. The Si(lO0) crystal was n-type with resistivity of lo-* Sz cm and could be heated by passing current through a Ta strip uniformly pressed between the crystal and a Ta-foil case. The sample temperature was measured with a Ni-Cr/Ni-Al thermocouple spot-welded on the case and calibrated with an IR pyrometer. The main contamination of the surface was carbon which could be eliminated by heating up to - 1400 K. After cleaning, all contaminants were below the AES detection limit and the surface exhibited the usual r~onst~cted (2 X 1) LEED pattern. The adsorption of the alkali always took place successively.

-1

3 -2 .% ::

-3

4

1

TIME

I

0

(mtn)

02

-3.2 eV. Similarly, when the amount of predeposited Cs corresponds to a value of WF before the ~nimum subsequent adsorption of K causes

3. Results 3. I. Work function

6

10

2 CL) Fig. 1. Work function change versus successive deposition of Cs and K on Si(100) and subsequent oxygen exposure. (The first alkali is saturated.) 5 DEPOSITION

depositton

measurements

Fig. 1 shows the variation of the work function upon Cs or K deposition on a saturated adlayer of K or Cs, respectively, on Si(100). In both curves the deposition of one kind of alkali on the saturated predeposited other kind causes a slight decrease of the WF of about 0.1 eV. Fig. 2 shows the variation of the work function upon Cs or K deposition on a submonolayer of K or Cs, respectively, on Si(100). When the amount of the predeposited K on Si(lO0) is smaller than that corresponding to the WF minimum subsequent deposition of Cs causes a further decrease of the WF which passes through a minimum at a A+ value of about - 3.25 eV and upon increasing Cs deposition reaches a saturation vafue at A$ =

. x

CS K

0

02

Fig. 2. Work function change versus successive deposition of Cs and K on Si(100) and subsequent oxygen exposure. (The coverage of the first alkali corresponds to a submonolayer.)

45

M. Kamaratos et al. / Coadwrption of Ce and K on Si(lo0)

further decrease of the WF which passes through a ~nimum value at A+ = - 3.45 eV (smaller than that of Cs deposition alone, at A+ = - 3.50 eV), and upon further K adsorption it reaches the same saturation value as in the case of Cs deposition alone, A+ - -3.35 eV (fig. 1). In both cases the saturation WF values after adsorption of the second alkali are the same (Cs first) or - 0.1 eV lower (K first) as those achieved upon saturation with the first alkali alone (fig. 1). Both figs. 1 and 2 indicate that when the coadsorbed K and Cs system on Si(100) is exposed to 0, the WF initially decreases to a minimum value and with further oxygen exposure it increases. The WF variation upon 0, exposure is independent of the sequence of K and Cs deposition and the initial amount of predeposited alkali adsorbate. It is quite similar to the WF variation during oxygen adsorption on saturated Cs or K/Si(lOO) [16,11]. The initial decrease during oxygen adsorption on K + Cs/Si(lOO) is substantially lower than that observed on K + Cs/Ag(lll) system [14].

lot

(bi

I

0

5 K DEPOSITION

TIME

10 (mln)

L

15

3.2. Auger electron spectroscopy

Fig. 3. Auger peak-to-peak height of Cs(47 eV), K(252 eV) and Si(92 eV) versus K-deposition time (a) OR a submonolayer of Cs coverage on Si(lOO), (b) on a Cs saturated Si surface.

Figs. 3a and 3b show the variation of the Auger peak-to-peak height of Cs(47 ev), K(252 eV) and Si(92 eV) versus K-deposition time when different amounts of Cs were predeposited on Si(100). As seen in fig. 3a, when a fractional monolayer of predeposited Cs on Si was followed by K deposition the peak of Cs remains almost unchanged while the peak of K increases linearly up to 3 min of K deposition when a break is observed. After that it continues to increase up to saturation at a decreasing rate. The peak of Si decreases following the behaviour of the growth of the I( peak. Its attenuation is - 30% with respect to its initial value (l/4 ML of Cs). When Cs predeposition on Si(100) up to saturation is followed by K adsorption (fig. 3b) there is a continuous decrease of Cs(47 eV) following the increase of the K(252 eV) peak. The total decrease at saturation is I~OW - 50%. The decrease of Si(92 eV) is - 10% with respect to its initial value (Cs saturation). Figs. 4a and 4b show the variation of the Auger peak-to-peak height of Cs(47 eV), K(252 eV) and Si(92 eV) versus Cs deposition time when different

amounts of K were predeposited on Si(lOO), The general features are about the same as in figs. 3a and 3b. When K was preadsorbed up to saturation coverage (fig. 4b) the Si(92 eV) exhibited a very small decrease (- 10%) while the decrease of the Auger peak-to-peak height for K was - 50%. The Auger curve for Cs does not show any layer-bylayer growth. When a submonolayer of K was first adsorbed followed by Cs (fig. 4a) the Auger curve for Cs increases linearly until a break which indicates completion of a binary coadsorbed monolayer. It is important to mention that in both figs. 3 and 4 the ratio of the Auger peak height of the second alkali to that of the first is the same, - 2, the completion of adsorption of second alkali. Figs. 5a and 5b show the variation of the Auger peak heights of Cs(47 eV), K(252 eV) and Si(92 eV) upon heating at a linear temperature rate of the K + Cs/Si(lOO) system. In fig. 5a, K was adsorbed first at saturation coverage followed by Cs deposition up to 10 min. In fig. 5b the process was similar but reversible in deposition. As seen in

46

M. Kamaratos

et al. / Coaakorption of Ce and K on Si(100)

10

(b)

lb)

Cs(47eV)X~ o K(252eV)X

I 0.10 h 4

Fig. 4. Auger peak-to-peak height of Cs(47 eV), K(252 eV) and Si(92 eV) versus G-deposition time (a) on a submonolayer of K coverage on Si(lOO), (b) on a K-saturated Si surface.

both figures, independently of the succession, K always completely desorbs from the surface at - 900 K before Cs which is desorbed completely at - 1100 K. 3.3. Thermal desorption spectroscopy Fig. 6a shows a series of thermal desorption curves of K when different quantities of Cs were deposited on a K-saturated Si(100) surface. Fig. 6b shows a series of thermal desorption curves of Cs when different quantities of K was deposited on a Cs-saturated Si(100) surface. As seen in both cases of fig. 6 when the amount of the second deposited alkali increases there is a continuous decrease of the total area under the thermal desorption curve of the alkali which was deposited first. The area under the desorption curve is roughly proportional to the absolute coverage of the surface. Fig. 7a shows the variation of the area under the Cs thermal desorption curve and the variation of the Cs(565 eV) Auger peak height with deposi-

x 51(92eV)X (a)

Fig. 5. Auger peak-to-peak height of Cs(47 eV), K(252 eV) and Si(92 ev) versus annealing temperature (a) after Cs deposition on a K-saturated Si surface, (b) after K deposition on a Cs-saturated Si surface.

tion time of K on Cs-saturated Si(100). Fig. 7b shows the variation of area under the K thermal desorption curve and the variation of the K(252 ev) Auger peak height with deposition time of Cs on K-saturated Si(100). These two curves exhibit quite analogous behaviour. In both figures (7a and 7b), the TDS area and consequently the absolute coverage of the alkali which was deposited first up to saturation decreases continuously as the deposition time of the second alkali increases. The variation of the Auger curve follows that of the TDS area. Fig. 8a shows thermal desorption spectra of Cs which was deposited on K-saturated and on clean Si(lO0) surface. In both spectra there two states. As indicated in this figure, the predeposited K causes a shift of the high-T Cs peak to higher temperatures while the low-T peak is shifted to lower temperatures. Fig. 8b shows thermal desorption spectra of K which was deposited on a Cssaturated and on a clean Si(100) surface. In both spectra there are three desorption states. As seen

iU. Kamaratos

-2

(a)

2 OO,r

IA 2

7

d

1

0

. CsTDS AREA o Cs(565eVl Ap-pH

0

h60-

Cs (mln) 1

; 0

5 z 40-

.

-8

r;i 2

-6

,? h I

“go

0 .

4

2 c 20-

% Q

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I 5 K DEPOSITION

I 10 TIME

(mln)

K (t-m)

1

0

2 3 4 5

2 4 6

.

Cs

700

900

1100

K (252eV)Ap.pH

10

0

500

47

et al. / Coaakorption of Ce and K on Si(100)

1300

T (K)

5 DEPOSITION

10 TIME (mln)

Fig. 7. (a) Total area under the thermal desorption curves of Cs and Auger peak height of Cs(565 eV) versus deposition time of K on a Cs-saturated Si(100) surface. (b) Total area under thermal desorption curves of K and Auger peak height of K(252 eV) versus deposition time of Cs on a K-saturated Si(100) surface.

Fig. 6. Thermal desorption curves of (a) K after Cs deposition on K-saturated Si(100) surfaces (b) Cs after K deposition on a Cs-saturated Si(100) surface.

in this figure, K deposition causes a shift of the high-T peaks to lower temperatures, whereas the low-T peak remains almost unchanged. Also the desorption of K from a Cs-precovered surface is completed earlier than that from the clean surface. It appears that the amount of the second deposited alkali is not affected substantially by the saturated preadsorbed alkali on the Si(lO0) surface in agreement with the AES results (figs. 3 and 4).

K/Cs/Si

ln

5

(100)

Cslmin)

4. Discussion A comparison of figs. 1 and 2 suggests that the WF minimum and maximum correspond to about the same deposition time. If we combine this with the fact that both alkali have the same flux we may assume that the sticking coefficient and the maximum coverage is about the same. The AES data in figs. 3 and 4 in connection with the TDS area curves in fig. 7 indicate that the

500

700

900

1100

T (K)

Fig. 8. Thermal desorption curves of (a) Cs on a clean and K-saturated Si(lO0) surface, (b) K on a clean and Cs-saturated Si(lO0) surface.

48

A4. Kamaratos et al. / Coaakorption of Ce and K on Si(lO0)

adsorption of the second alkali causes a partial (- 50%) removal of the saturated layer of the first one. On the contrary, it does not affect substantially the first alkali when its coverage is less than that which corresponds to the WF minimum. In both cases the saturation coverage of the second alkali is the same regardless of the amount of the preadsorbed alkali. This saturation coverage is, within experimental error, the same as that obtained upon its adsorption on clean Si(100). If we consider that the saturation coverage for both K and Cs alone on Si(100) is - 0.5 ML [17] the saturation coverage of the mixed adlayer should be about 0.75 ML. The TDS curve in fig. 8 also indicates that the maximum coverage of the second alkali is always the same regardless of the amount of preadsorbed alkali. The same figure shows that the shape of peaks remains the same independently of the presence of the first alkali. The small shifts of desorption peaks in the presence of the alkali could be due to lateral interactions between the adsorbates in their neighbouring sites. This observation indicates that most probably K and Cs initially occupy different sites. According to Levine’s model [l] and recent report [7], Cs and K initially occupy the dimer and cave sites, respectively. A plausible model to explain the adsorption behaviour when K and Cs are successively adsorbed on Si(100) is the following. Upon K or Cs saturation of the surface the cave or dimers sites are filled respectively with K or Cs. As the second alkali is adsorbed it begins to fill up its corresponding sites at the same time causing a partial removal of the first alkali from the neighbouring sites mainly due to a transfer of momentum. This removal stops when the binding energy of the first alkali is increased, as the coverage decreases, to a certain value. When the binary adlayer is saturated only about 50% of the first alkali coverage is left. This is also consistent with the fact that the AES ratio of two alkali is - 2 at the saturation coverage (section 3.2). The above model is quite different than that reported by Argile and Read on the coadsorption of Cs and K on Ag(ll1). Probably this is due to the fact that the nature of metallic substrate is different than that of semiconductor and thus the

interaction of the alkali overlayer with the substrate is not the same.

5. Conclusion The study of K and Cs coadsorption on a Si(100)2 X 1 surface at RT leads to the following conclusions. Adsorption of each alkali alone on Si(100)2 x 1 gives the typical A+ versus ealkali curve, i.e., an initial decrease to a $B,,,~,,and a subsequent increase to a &,,,, with a greater initial dipole moment in the case of Cs than that of K. This result, in correlation with AES and TDS measurements, suggests that the sites of K and Cs on Si(100) are different and the individual saturation coverage of the two alkali is the same. When the first alkali adsorbate on Si(100) is saturated the following deposition of the second alkali causes a decrease of the absolute coverage of the first. This is due to a partial removal (- 50%) of the atoms of the first alkali by those of the second. The atoms of the second alkali form a mixed adlayer, with the remaining atoms of the first alkali, with a saturation coverage higher than that of each alkali alone. The above process is independent of the sequence of deposition of K and Cs adsorbates.

References [l] J.D. Levine, Surf. Sci. 34 (1973) 90. [2] L. Surnev, Surf. Sci. 110 (1981) 458. [3] D. Edwards, Jr. and W.J. Peria, Appl. Surf. Sci. 52 (1975) 40. [4] J. Derrien and F. Arnaud d’Avitaya, Surf. Sci. 65 (1977) 668. [5] R. Holtom and P.M. Gundry, Surf. Sci. 63 (1977) 263. [6] H. Tochihara, Surf. Sci. 126 (1983) 523. [7] Y. Ling, A.J. Freeman and B. Delley, Phys. Rev. B 39 (1989) 10144. [S] S. Ciraci and I.P. Batra, Phys. Rev. Lett. 56 (1986) 877. [9] Y. Enta, T. Kinoshita, S. Suzuki and S. Kono, Phys. Rev. B 36 (1987) 9801. [lo] T. Abukawa and S. Kono, Phys. Rev. B 37 (1987) 9097. [ll] E.G. Michel, J.E. Ortega, E.M. Oelling, M.C. Asensio, J. Ferron and R. Miranda, Phys. Rev. B 38 (1988) 13399. [12] T. Kendelewicz, P. Soukiassian, R.-S. List, J.C. Woicik, P. Pianetta, I. Lindau and W.E. Spicer, Phys. Rev. B 37 (1988) 7115.

h4. Kamaratos

49

et al. / Coaakorption of Ce and K on Si(lO0)

1131 S. Kennou, M. Kamaratos, S. Ladas and C. Papageorgopoulos, Surf. Sci. 216 (1989) 462. [14] C. Argille and G.E. Rhead, Surf. Sci. 203 (1988) 175. [15] S. Kennou, S. Ladas and C. Papageorgopoulos, Surf. Sci. 152/153 (1985) 1213.

[16] M. Kamaratos, S. Kennou, poulos, J. Phys. Condensed

S. Ladas and C. PapageorgoMatter 1 (1989) 6071.