- Email: [email protected]
Oxygen interaction with Si(100) and KSi(100)
ELSEVIER
Surface Science 377-379 (1997) 650-654
Oxygen interaction with Si( 100) and K/Si( 100) A. Mascaraque a, C. Ottaviani b, M. Capozi b, M. Pedio b, E.G. Michel a** aInstitute de Ciencia de Materiales ‘Niicokis Cabrera” and Departamento de Fisica de la Materia Condensada, Universidad Authoma de Madrid, 28049-Madrid, Spain b Istituto di Struttura della Materia, V. E. Fermi 38, 00044 Frascati, Italy
Received 1 August 1996; accepted for publication 15 October 1996
Abstract
The earliest stages of the oxygen interaction with clean and K-covered Si(100) surfaces (0,<0.5 ML) have been investigated using high-resolution core-level photoemission. The sequence of oxidation has been analyzed monitoring the different components of the Si 2p core level. In the case of the clean surface, the contribution coming from the upper dimer atoms is unaffected at the earliest stages of the oxidation. On the contrary, in the presence of K atoms, oxygen reacts rapidly with this type of silicon atoms, and the component is fully depleted after oxygen adsorption. The presence of K atoms induces also the formation of a new oxide component in the Si 2p core level. Its small binding energy shift supports that a fraction of oxygen atoms are bonded at sites moditied by the presence of K, forming a K-Si-0 complex. Keywords: Al!& metals; Angle resolved photoemission; Growth; Low index single crystal surfaces; Metal-semiconductor interfaces; Oxidation; Oxygen; Schottky barrier; Semiconducting surfaces; Silicon; Synchrotron radiation photoelectron spectroscopy
1. Introduction
The interaction of oxygen with alkali-metal (AM) covered silicon surfaces has been a subject of continuous interest over the last few years [ 11. One interesting feature of these interfaces is the dramatic enhancement of the silicon oxidation rate in the presence of AM [2-51. Two different ranges (corresponding to two different acting mechanisms) have been reported: below and above - 1 ML of alkali [ 561. Above 1 monolayer (ML) it was found that the AM ftlm reacts with oxygen to form alkali oxides. The substrate is oxidized after thermal decomposition of these oxides at temperatures in the range of 600°C [2]. In the case * Corresponding author. Fax: +34 13973961: e-mail: [email protected]
of submonolayer AM coverages, both local [ 5,7,8] and non-local [ 3,4,9] mechanisms have been proposed. Another controversial question was the role played by AM atoms in the oxidation process, i.e. the nature of the bonds of oxygen atoms when adsorbed on a surface partly covered by an AM [3,101. A second interesting feature of these interfaces is the strong work function decrease induced by alkali adsorption [ 111. Exposure to oxygen of a silicon surface precovered with AM, produces either an additional decrease of the work function, or an increase, depending on the initial alkali coverage deposited [ 5,12,13]. In the particular case of K/Si( 100)2 x 1, it has been found that for coverages below 0.5 ML an exposure to oxygen increases the work function, while above this coverage a decrease is found for low oxygen doses
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SOO39-6028(96)01477-X
A. Mascaraque
et al. / Surface Science 377-379 (1997) 650-654
[5]. It is this additional decrease produced by oxygen adsorption that makes it possible to achieve negative electron afhnity (NEA) conditions in some cases [ 1,141. NEA phenomena are behind infrared detectors, light boosters and image intensifiers. In spite of these applications, the atomistic processes behind NEA activation are not yet understood [ 15,161. Interestingly, both the oxygen-induced work-function change and the AM-enhanced oxidation display a turning point at a K coverage around 0.5-l .OML [ 131. Since the surface becomes metallic in this coverage range, it is probable that changes due to the surface metallization are behind both effects.
2. Experimental We set out in this work to analyze by highresolution core-level photoemission the interaction of oxygen with the clean and K-covered Si( 100)2 x 1 surfaces. We concentrate in this article on the results found in the case of alkali coverages below 0.5 ML, i.e. before the K/Si( 100)2 x 1 metallization [ 171. The experiments were performed in an ultra high vacuum (UHV) system permanently mounted at the exit of the VUV beamline of the Elettra Laboratory in Trieste (Italy). Si( 100) p-doped samples were chemically etched and cleaned by annealing to 900°C in UHV. The position of the Fermi level was determined from a Ta foil in electric contact with the sample end. An overall resolution of 90 meV at 140 eV photon energy was selected, as determined from the Ta Fermi level width. K coverages were calibrated by analyzing the shape of K 2p core level and the intensity ratio of K 2p and Si 2p core levels excited with 415 eV photons. We define in this article 1 ML as a K coverage equal to the atomic density of Si( 100)2 x 1 surface (i.e. 6.78 x 1014atoms (X-P).
6.51
[ 181. Sharp 2 x 1 low-energy electron diffraction patterns were observed at room temperature. Cooling of the surface to 85 K produced a c(4 x 2) reconstruction [ 191. Fig. 1 (bottom spectrum) shows a Si 2p core level taken with 140 eV photons at an exit angle of 60”. Fig. 2 (lower panel) presents the deconvolution of this peak in various surface and bulk components. The lowest BE peak (denoted Su in Fig. 2, lower panel) has been attributed to the upper Si dimer atoms [20]. Its intensity and BE are characteristic of a well ordered surface. The transition 2 x 1*c(4 x 2) manifests itself in an intensity increase of the highest BE component. Thus, as the transition proceeds, the shoulder at - 100.3 eV BE appears, in agreement with Ref. [20]. A Voigt line shape was used in the deconvolution, with a Lorentzian width of 190 meV and a Gaussian broadening of 130 meV. The energy splitting of all components was 0.602 eV and the branching ratio of the Si 2~~,~/2p~,~ components was 0.5. We refer the reader to Refs. 118,201 for more details on the assignment of the different components and the origin of their relative intensities.
I
L__ cleanSi I
3. Results and discussion
-103.0
-101.0
-99.0
-97.0
BINDING ENERGY (eV)
We describe now briefly the results obtained for Si(100) and K/Si(lOO). A more detailed analysis will be presented in a forthcoming publication
Fig. 1. Evolution of Si 2p core level along the K deposition and oxygen exposure process. Note the complete depletion of the surface component Su in the top spectrum.
A. Mascaraque
et al. / Surface Science 377-379
Si 2p hv=l40
eV
T=85 K
101.0
-100.0
-99.0
-98.0
BINDING ENERGY (eV)
Fig. 2. Lower panel: deconvolution of the Si 2p core level from Fig. 1 (bottom curve) in the different components characteristic of the c(4 x 2) reconstruction. Upper panel: deconvolution of the Si 2p peak after exposure to 0.7 L oxygen. Note the change in horizontal scale. The unoxidized components (analogous to the lower panel) have been grouped together iu one trace for the sake of clarity.
The adsorption of K atoms takes place along the troughs of the c(4 x 2) surface [ 17,211. It gives rise to an intensity redistribution among the different components of the core level [22]. Fig. 1 (center) shows the changes observed in the Si 2p core level when an amount of K equal to 0.3 ML is deposited. We discuss in the following the changes observed when the surface (clean or precovered with K) is exposed to oxygen. We show in Fig. 2 (upper panel) a Si 2p core level obtained after exposing the clean surface to 0.7 L oxygen. Due to the high surface sensitivity and resolution, strong changes can be observed in the core level, even at these low exposures. First, we find an intensity redistribution between the unoxidized Si components. Interestingly, the intensity of the surface component coming from upper dimer atoms does not decrease very much. We observe already at this low exposure the appearance of components due to oxide formation. The deconvolution shown in
(1997) 650-654
Fig. 2 (upper panel) has been performed using the same contributions as in the lower panel for the unoxidized components, plus four oxide components. All unoxidized components have been grouped together in the figure in only one trace for the sake of clarity. The BE shifts for the four oxide components (1.0, 1.9, 2.5 and 3.3 eV) are in agreement with the values commonly attributed to the formation of Si+, Si’+, Si3+ and Si4+ [23,24]. Only traces of Si4+ were found, but the other three components present similar intensity and denote the existence of silicon atoms bonded to several oxygen atoms already at these low exposures. We note here that intense oxide components were found, although upper dimer atoms were not much affected. This supports a preferential interaction of oxygen atoms with other types of bonds [ 181. Fig. 1 (upper curve) shows the results of exposing to oxygen a Si surface precovered with 0.3 ML K. A visual inspection of the core level allows to extract the following conclusions: the surface component Su is now fully depleted, at variance with the clean Si(100) case. Moreover, the ratio between the major peak at -99.41 eV BE and the second highest at -99.98 eV BE is now different. The core level was deconvolved (see Fig. 3) to understand these changes. The same components as in the clean surface were used for the unoxidized part of the core level, changing
Si 2p hv=140
c .z cl
eV
f+60” T&i K 0,=0.3 ML 0, exp.=0.8 L
4 i E z
-102.0
-100.0
BINDING ENERGY (eV)
Fig. 3. Deconvolution of the Si 2p core level corresponding to a 0.3 ML deposit of K exposed to 0.8 L oxygen (top curve from Fig. 1). The oxide component A is not found in the absence of K.
A. Mascaraque et al. / Surface Science 377-379 (1997) 650-654
only their relative intensities. These components have been grouped together in the figure in only one trace. The main difference in this part of the spectrum is obviously the full depletion of the surface component Su. In addition to these, the same oxide components as in the absence of K were used (coming from Si+’ Si2+ and Si3‘). However, to obtain a fit of comparable quality a new oxide component (denoted A in Fig. 3) with a BE shift of only 0.4 eV is now necessary. This component gives rise to the apparent change in the relative intensities of the peaks at -99.41 and -99.98 eV BE mentioned above. The adsorption of oxygen on the clean Si surface does not produce a relevant depletion of the surface component Su. In the case of the K-precovered surface, Su is rapidly destroyed. This effect indicates that the presence of K modifies drastically the interaction mechanism between oxygen and the silicon dimerized surface. While in the case of the clean surface, oxygen atoms do certainly bind to the substrate, they do not affect upper dimer atoms, indicating that other adsorption sites are preferred at the earliest stages of adsorption. The situation changes after K adsorption. K atoms, adsorbed at the surface troughs, either can block cave sites for active reaction, or alter the bond strength in such a way that now the bonding of oxygen with upper dimer atoms is much facilitated. On the other hand, we detect at the Si 2p core level the oxide component A, with a lower binding energy than Si+ . This oxide component reveals the presence on the surface of silicon atoms bonded to oxygen but in a lower oxidation state than in the absence of K. The easiest explanation for this oxide component is that oxygen binds to Si atoms and also bonds to K atoms. Since potassium is an electropositive element, one would expect from simple chemical arguments [25] that the oxidation state of Si in a complex K-0-Si was lower than in a simple Si-0-Si bridge bond. The formation of such type of bonds evidences the role of K atoms as nucleation seeds for the oxygen dissociation and reaction. It is known since a long time ago that the presence of K atoms increases the surface sticking coefficient for oxygen [ 5,261. The data presented here demonstrate that the atomistic mechanisms
653
behind silicon oxidation are different with or without potassium, as evidenced by the distinct oxidation sequence found. K atoms not only increase the sticking probability, but also allow different reaction paths. This observation is reinforced from a comparison between Figs. 3 and 2 (upper panel). The intensity distribution between the three oxide components is different after K adsorption. While in the case of the clean Si( 100)2 x 1 the intensity of the three components is approximately equal, after K adsorption the lowest oxidation state is higher, suggesting that oxygen atoms tend to affect a larger number of Si atoms. In conclusion, the presence of K atoms modifies the reaction of oxygen with silicon. The appearance of a K-0-Si complex supports the role of K atoms as local seeds where oxygen bonding to silicon is facilitated. A different oxidation sequence of silicon has been found in the presence of K.
Acknowledgements
This work was financed by NATO (grant CRG.94063 1) , DGICYT (grant PB94- 1527) and the Human Capital and Mobility Programme of the European Union (EU Contract ERBCHGECT920013).
References [l] Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradshaw and G. Ertl (Elsevier, Amsterdam, 1989). [2] M.C. Asensio et al., Appl. Phys. Lett. 51 (1987) 1714. [3] A. Franciosi et al., Phys. Rev. B 35 (1987) 910. [4] P. Soukiassian et al., Phys. Rev. B 35 (1987) 4176. [5] E.G. Michel et al., Phys. Rev. B 38 (1988) 13 399. [6] N. Takagi et al., Phys. Rev. B 44 (1991) 12 945. [7] H.J. Ernst and M.L. Yu, Phys. Rev. B 41 (1990) 12 953. [8] T.L. Bush, TX Jones and D.O. Hayward, Surf. Sci. 307/ 309 (1994) 247. [9] HI. Smrnberg, P. Soukiassian and Z. Hurych, Phys. Rev. B 39 (1989) 12 775. [lo] G. Faraci et al., Phys. Rev. B 49 (1994) 2943. [ll] J.D. Levine, Surf. Sci. 34 (1973) 90. [ 121 J.J. Scheer and J. van Laar, Solid State Comm. 3 (1965) 189. [ 131 G.R. Castro et al., Vacuum 41 (1990) 564. [ 141 A.A. Turnbull and G.B. Evands, J. Phys. D 1 (1968) 155.
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A. Mascaraque et al. / Surface Science 377-379 (1997) 650654
[15] T. Abukawa, S. Kono and T. Sakamoto, Jpn. J. Appl. Phys. 28 (1989) L303. [16] G. Vergara et al., Surf. Sci. 278 (1992) 131. [17] E.G. Michel et al., Phys. Rev. B 45 (1992) 11 811. [ 181 A. Mascaraque et al., in preparation. [19] T. Tabata, T. Aruga and Y. Murata, Surf. Sci. 179 (1987) L63. [20] E. Landemark et al., Phys. Rev. Lett. 69 (1992) 1588. [21] H. Meyerheim, R. Sawitzki and W. Moritz, Phys. Rev. B 52 (1995) 16 830.
[22] Y.-C. Chao et al., Phys. Rev. B 52 (1995) 2579. [23] G. Hollinger and F.J. Himpsel, J. Vat. Sci. Technol. 1 (1983) 640. [24] W. Braun and H. Kuhlenbeck, Surf. Sci. 180 (1987) 279. [25] L. Pauling, The Nature of the Chemical Bond (Cornell University Press, New York, 1973). 1261 C.Y. Su et al., Surf. Sci. 86 (1979) 894.
Surface Science 377-379 (1997) 650-654
Oxygen interaction with Si( 100) and K/Si( 100) A. Mascaraque a, C. Ottaviani b, M. Capozi b, M. Pedio b, E.G. Michel a** aInstitute de Ciencia de Materiales ‘Niicokis Cabrera” and Departamento de Fisica de la Materia Condensada, Universidad Authoma de Madrid, 28049-Madrid, Spain b Istituto di Struttura della Materia, V. E. Fermi 38, 00044 Frascati, Italy
Received 1 August 1996; accepted for publication 15 October 1996
Abstract
The earliest stages of the oxygen interaction with clean and K-covered Si(100) surfaces (0,<0.5 ML) have been investigated using high-resolution core-level photoemission. The sequence of oxidation has been analyzed monitoring the different components of the Si 2p core level. In the case of the clean surface, the contribution coming from the upper dimer atoms is unaffected at the earliest stages of the oxidation. On the contrary, in the presence of K atoms, oxygen reacts rapidly with this type of silicon atoms, and the component is fully depleted after oxygen adsorption. The presence of K atoms induces also the formation of a new oxide component in the Si 2p core level. Its small binding energy shift supports that a fraction of oxygen atoms are bonded at sites moditied by the presence of K, forming a K-Si-0 complex. Keywords: Al!& metals; Angle resolved photoemission; Growth; Low index single crystal surfaces; Metal-semiconductor interfaces; Oxidation; Oxygen; Schottky barrier; Semiconducting surfaces; Silicon; Synchrotron radiation photoelectron spectroscopy
1. Introduction
The interaction of oxygen with alkali-metal (AM) covered silicon surfaces has been a subject of continuous interest over the last few years [ 11. One interesting feature of these interfaces is the dramatic enhancement of the silicon oxidation rate in the presence of AM [2-51. Two different ranges (corresponding to two different acting mechanisms) have been reported: below and above - 1 ML of alkali [ 561. Above 1 monolayer (ML) it was found that the AM ftlm reacts with oxygen to form alkali oxides. The substrate is oxidized after thermal decomposition of these oxides at temperatures in the range of 600°C [2]. In the case * Corresponding author. Fax: +34 13973961: e-mail: [email protected]
of submonolayer AM coverages, both local [ 5,7,8] and non-local [ 3,4,9] mechanisms have been proposed. Another controversial question was the role played by AM atoms in the oxidation process, i.e. the nature of the bonds of oxygen atoms when adsorbed on a surface partly covered by an AM [3,101. A second interesting feature of these interfaces is the strong work function decrease induced by alkali adsorption [ 111. Exposure to oxygen of a silicon surface precovered with AM, produces either an additional decrease of the work function, or an increase, depending on the initial alkali coverage deposited [ 5,12,13]. In the particular case of K/Si( 100)2 x 1, it has been found that for coverages below 0.5 ML an exposure to oxygen increases the work function, while above this coverage a decrease is found for low oxygen doses
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SOO39-6028(96)01477-X
A. Mascaraque
et al. / Surface Science 377-379 (1997) 650-654
[5]. It is this additional decrease produced by oxygen adsorption that makes it possible to achieve negative electron afhnity (NEA) conditions in some cases [ 1,141. NEA phenomena are behind infrared detectors, light boosters and image intensifiers. In spite of these applications, the atomistic processes behind NEA activation are not yet understood [ 15,161. Interestingly, both the oxygen-induced work-function change and the AM-enhanced oxidation display a turning point at a K coverage around 0.5-l .OML [ 131. Since the surface becomes metallic in this coverage range, it is probable that changes due to the surface metallization are behind both effects.
2. Experimental We set out in this work to analyze by highresolution core-level photoemission the interaction of oxygen with the clean and K-covered Si( 100)2 x 1 surfaces. We concentrate in this article on the results found in the case of alkali coverages below 0.5 ML, i.e. before the K/Si( 100)2 x 1 metallization [ 171. The experiments were performed in an ultra high vacuum (UHV) system permanently mounted at the exit of the VUV beamline of the Elettra Laboratory in Trieste (Italy). Si( 100) p-doped samples were chemically etched and cleaned by annealing to 900°C in UHV. The position of the Fermi level was determined from a Ta foil in electric contact with the sample end. An overall resolution of 90 meV at 140 eV photon energy was selected, as determined from the Ta Fermi level width. K coverages were calibrated by analyzing the shape of K 2p core level and the intensity ratio of K 2p and Si 2p core levels excited with 415 eV photons. We define in this article 1 ML as a K coverage equal to the atomic density of Si( 100)2 x 1 surface (i.e. 6.78 x 1014atoms (X-P).
6.51
[ 181. Sharp 2 x 1 low-energy electron diffraction patterns were observed at room temperature. Cooling of the surface to 85 K produced a c(4 x 2) reconstruction [ 191. Fig. 1 (bottom spectrum) shows a Si 2p core level taken with 140 eV photons at an exit angle of 60”. Fig. 2 (lower panel) presents the deconvolution of this peak in various surface and bulk components. The lowest BE peak (denoted Su in Fig. 2, lower panel) has been attributed to the upper Si dimer atoms [20]. Its intensity and BE are characteristic of a well ordered surface. The transition 2 x 1*c(4 x 2) manifests itself in an intensity increase of the highest BE component. Thus, as the transition proceeds, the shoulder at - 100.3 eV BE appears, in agreement with Ref. [20]. A Voigt line shape was used in the deconvolution, with a Lorentzian width of 190 meV and a Gaussian broadening of 130 meV. The energy splitting of all components was 0.602 eV and the branching ratio of the Si 2~~,~/2p~,~ components was 0.5. We refer the reader to Refs. 118,201 for more details on the assignment of the different components and the origin of their relative intensities.
I
L__ cleanSi I
3. Results and discussion
-103.0
-101.0
-99.0
-97.0
BINDING ENERGY (eV)
We describe now briefly the results obtained for Si(100) and K/Si(lOO). A more detailed analysis will be presented in a forthcoming publication
Fig. 1. Evolution of Si 2p core level along the K deposition and oxygen exposure process. Note the complete depletion of the surface component Su in the top spectrum.
A. Mascaraque
et al. / Surface Science 377-379
Si 2p hv=l40
eV
T=85 K
101.0
-100.0
-99.0
-98.0
BINDING ENERGY (eV)
Fig. 2. Lower panel: deconvolution of the Si 2p core level from Fig. 1 (bottom curve) in the different components characteristic of the c(4 x 2) reconstruction. Upper panel: deconvolution of the Si 2p peak after exposure to 0.7 L oxygen. Note the change in horizontal scale. The unoxidized components (analogous to the lower panel) have been grouped together iu one trace for the sake of clarity.
The adsorption of K atoms takes place along the troughs of the c(4 x 2) surface [ 17,211. It gives rise to an intensity redistribution among the different components of the core level [22]. Fig. 1 (center) shows the changes observed in the Si 2p core level when an amount of K equal to 0.3 ML is deposited. We discuss in the following the changes observed when the surface (clean or precovered with K) is exposed to oxygen. We show in Fig. 2 (upper panel) a Si 2p core level obtained after exposing the clean surface to 0.7 L oxygen. Due to the high surface sensitivity and resolution, strong changes can be observed in the core level, even at these low exposures. First, we find an intensity redistribution between the unoxidized Si components. Interestingly, the intensity of the surface component coming from upper dimer atoms does not decrease very much. We observe already at this low exposure the appearance of components due to oxide formation. The deconvolution shown in
(1997) 650-654
Fig. 2 (upper panel) has been performed using the same contributions as in the lower panel for the unoxidized components, plus four oxide components. All unoxidized components have been grouped together in the figure in only one trace for the sake of clarity. The BE shifts for the four oxide components (1.0, 1.9, 2.5 and 3.3 eV) are in agreement with the values commonly attributed to the formation of Si+, Si’+, Si3+ and Si4+ [23,24]. Only traces of Si4+ were found, but the other three components present similar intensity and denote the existence of silicon atoms bonded to several oxygen atoms already at these low exposures. We note here that intense oxide components were found, although upper dimer atoms were not much affected. This supports a preferential interaction of oxygen atoms with other types of bonds [ 181. Fig. 1 (upper curve) shows the results of exposing to oxygen a Si surface precovered with 0.3 ML K. A visual inspection of the core level allows to extract the following conclusions: the surface component Su is now fully depleted, at variance with the clean Si(100) case. Moreover, the ratio between the major peak at -99.41 eV BE and the second highest at -99.98 eV BE is now different. The core level was deconvolved (see Fig. 3) to understand these changes. The same components as in the clean surface were used for the unoxidized part of the core level, changing
Si 2p hv=140
c .z cl
eV
f+60” T&i K 0,=0.3 ML 0, exp.=0.8 L
4 i E z
-102.0
-100.0
BINDING ENERGY (eV)
Fig. 3. Deconvolution of the Si 2p core level corresponding to a 0.3 ML deposit of K exposed to 0.8 L oxygen (top curve from Fig. 1). The oxide component A is not found in the absence of K.
A. Mascaraque et al. / Surface Science 377-379 (1997) 650-654
only their relative intensities. These components have been grouped together in the figure in only one trace. The main difference in this part of the spectrum is obviously the full depletion of the surface component Su. In addition to these, the same oxide components as in the absence of K were used (coming from Si+’ Si2+ and Si3‘). However, to obtain a fit of comparable quality a new oxide component (denoted A in Fig. 3) with a BE shift of only 0.4 eV is now necessary. This component gives rise to the apparent change in the relative intensities of the peaks at -99.41 and -99.98 eV BE mentioned above. The adsorption of oxygen on the clean Si surface does not produce a relevant depletion of the surface component Su. In the case of the K-precovered surface, Su is rapidly destroyed. This effect indicates that the presence of K modifies drastically the interaction mechanism between oxygen and the silicon dimerized surface. While in the case of the clean surface, oxygen atoms do certainly bind to the substrate, they do not affect upper dimer atoms, indicating that other adsorption sites are preferred at the earliest stages of adsorption. The situation changes after K adsorption. K atoms, adsorbed at the surface troughs, either can block cave sites for active reaction, or alter the bond strength in such a way that now the bonding of oxygen with upper dimer atoms is much facilitated. On the other hand, we detect at the Si 2p core level the oxide component A, with a lower binding energy than Si+ . This oxide component reveals the presence on the surface of silicon atoms bonded to oxygen but in a lower oxidation state than in the absence of K. The easiest explanation for this oxide component is that oxygen binds to Si atoms and also bonds to K atoms. Since potassium is an electropositive element, one would expect from simple chemical arguments [25] that the oxidation state of Si in a complex K-0-Si was lower than in a simple Si-0-Si bridge bond. The formation of such type of bonds evidences the role of K atoms as nucleation seeds for the oxygen dissociation and reaction. It is known since a long time ago that the presence of K atoms increases the surface sticking coefficient for oxygen [ 5,261. The data presented here demonstrate that the atomistic mechanisms
653
behind silicon oxidation are different with or without potassium, as evidenced by the distinct oxidation sequence found. K atoms not only increase the sticking probability, but also allow different reaction paths. This observation is reinforced from a comparison between Figs. 3 and 2 (upper panel). The intensity distribution between the three oxide components is different after K adsorption. While in the case of the clean Si( 100)2 x 1 the intensity of the three components is approximately equal, after K adsorption the lowest oxidation state is higher, suggesting that oxygen atoms tend to affect a larger number of Si atoms. In conclusion, the presence of K atoms modifies the reaction of oxygen with silicon. The appearance of a K-0-Si complex supports the role of K atoms as local seeds where oxygen bonding to silicon is facilitated. A different oxidation sequence of silicon has been found in the presence of K.
Acknowledgements
This work was financed by NATO (grant CRG.94063 1) , DGICYT (grant PB94- 1527) and the Human Capital and Mobility Programme of the European Union (EU Contract ERBCHGECT920013).
References [l] Physics and Chemistry of Alkali Metal Adsorption, Eds. H.P. Bonzel, A.M. Bradshaw and G. Ertl (Elsevier, Amsterdam, 1989). [2] M.C. Asensio et al., Appl. Phys. Lett. 51 (1987) 1714. [3] A. Franciosi et al., Phys. Rev. B 35 (1987) 910. [4] P. Soukiassian et al., Phys. Rev. B 35 (1987) 4176. [5] E.G. Michel et al., Phys. Rev. B 38 (1988) 13 399. [6] N. Takagi et al., Phys. Rev. B 44 (1991) 12 945. [7] H.J. Ernst and M.L. Yu, Phys. Rev. B 41 (1990) 12 953. [8] T.L. Bush, TX Jones and D.O. Hayward, Surf. Sci. 307/ 309 (1994) 247. [9] HI. Smrnberg, P. Soukiassian and Z. Hurych, Phys. Rev. B 39 (1989) 12 775. [lo] G. Faraci et al., Phys. Rev. B 49 (1994) 2943. [ll] J.D. Levine, Surf. Sci. 34 (1973) 90. [ 121 J.J. Scheer and J. van Laar, Solid State Comm. 3 (1965) 189. [ 131 G.R. Castro et al., Vacuum 41 (1990) 564. [ 141 A.A. Turnbull and G.B. Evands, J. Phys. D 1 (1968) 155.
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A. Mascaraque et al. / Surface Science 377-379 (1997) 650654
[15] T. Abukawa, S. Kono and T. Sakamoto, Jpn. J. Appl. Phys. 28 (1989) L303. [16] G. Vergara et al., Surf. Sci. 278 (1992) 131. [17] E.G. Michel et al., Phys. Rev. B 45 (1992) 11 811. [ 181 A. Mascaraque et al., in preparation. [19] T. Tabata, T. Aruga and Y. Murata, Surf. Sci. 179 (1987) L63. [20] E. Landemark et al., Phys. Rev. Lett. 69 (1992) 1588. [21] H. Meyerheim, R. Sawitzki and W. Moritz, Phys. Rev. B 52 (1995) 16 830.
[22] Y.-C. Chao et al., Phys. Rev. B 52 (1995) 2579. [23] G. Hollinger and F.J. Himpsel, J. Vat. Sci. Technol. 1 (1983) 640. [24] W. Braun and H. Kuhlenbeck, Surf. Sci. 180 (1987) 279. [25] L. Pauling, The Nature of the Chemical Bond (Cornell University Press, New York, 1973). 1261 C.Y. Su et al., Surf. Sci. 86 (1979) 894.