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Ge(100)2×1 substoichiometric oxidation states promoted by a Cs overlayer
Surface Science 409 (1998) 46–56
Ge(100)2×1 substoichiometric oxidation states promoted by a Cs overlayer G. Faraci *, A.R. Pennisi Dipartimento di Fisica, Universita´ di Catania, Istituto Nazionale di Fisica della Materia, Corso Italia 57, 95129 Catania, Italy Received 19 December 1997; accepted for publication 24 February 1998
Abstract High-resolution photoemission spectra from the Cs/Ge(100)2×1 interface were obtained for several metal coverages from 0.1 up to 1 ML. The bonding between the Cs atoms and the Ge surface was determined from the analysis of the Cs 4d and Ge 3d core level spectra. Both covalent-like and ionic-like bonds were identified in the energy shifted components of the spectra. Exposure of the interface to O allowed the catalytic mechanism of the semiconductor oxidation (dramatically enhanced by the polarized metal 2 overlayer) to be ascertained. In fact, the onset and growth of substoichiometric oxidation states was already observed at and beyond 0.25 ML Cs coverage, and 0.1 L oxygen exposure. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Catalysis; Oxidation; Photoemission; Semiconductor; Surface
1. Introduction The dramatic enhancement of the oxidation rate of a semiconducting surface caused, in the presence of low O exposure, by a submonolayer depos2 ition of an alkali metal has long been debated [1– 8]. The understanding of the effective mechanism responsible of such a process first requires the study of the clean interface between the overlayer and the semiconductor [9–21]. In fact, the knowledge of the bonds established between the semiconductor dangling bonds and the alkali metal is of primary importance to follow the oxide formation and its evolution consequent to oxygen exposure. The catalytic enhancement of the oxidation rate is usually related to the reduction of the work func* Corresponding author. Fax: (+39) 95 383 023; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 01 9 6 -4
tion and to oxygen dissociation in the surface dipole layer due to the alkali metal deposition [1,7]. However, it is still uncertain as to how and where this dissociation takes place, the extent to which the metal is directly involved, i.e. whether it remains a spectator or participates to the process as an active player, and the manner in which the underlying semiconductor is able to capture oxygen through the metal overlayer. In fact, several papers have been published for an alkali metal overlayer on semiconducting substrates but with conflicting conclusions. Lin et al. [9] found that the chemisorption of Cs on Ge(100)2×1 was partially ionic; Soukiassian et al. [10,11] concluded that alkali metals on Si surfaces have polarized covalent bonds with a metallic Na, K, Rb or Cs overlayer. In contrast, Souda et al. [19] showed that Cs and K adatoms are ionically adsorbed, whereas the Na adatom is neutral on Ge(100) for
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coverages lower than 0.25 ML. Enhancement of the sticking probability of oxygen on Ge(100) due to the adatoms was also observed; in fact, Higasa et al. [8] studied the alkali promotion of O 2 sticking on Ge(100) and Si(100); they suggested for the alkali promotion a threshold coverage around 0.2 ML, with an abrupt transition from ionic to covalent or metallic bonding depending on the substrate. Recently, photoelectron spectroscopy studies [20] of clean and hydrogen chemisorbed Ge(001) surfaces allowed the determination of the surface configuration of Ge(001)2×1 showing a component due to the up atoms shifted by 0.5 eV towards a lower binding energy with respect to the bulk; another component at a 0.19 lower binding energy was attributed to the second-layer atoms of the substrate. Similar experiments on Cs/Si(100)2×1 by Chao et al. [18] indicated the difference between the two Si surface components attributed to up and down atoms of asymmetric Si dimers; here, the Cs saturated surface exhibited a metallic character. Of course, the surface configuration and the topology of its dangling bonds are particularly important as theoretically discussed by Pehlke and Heffler [22], who showed that the final state relaxation plays an important role with an enhanced screening at the surface. The questions raised by the previous controversial results are the aim of the present paper, which includes the investigation of the clean Cs/Ge(100)2×1 interface in order to determine their binding mechanism before any oxygen exposure. In recent papers [5,23], we studied the Cs/GaAs(110) interface and its exposure to O at 2 a photon energy of 1486.7 eV; in this case, evidence of a Cs participation to the oxidation process through hybrid or mixed links was clearly obtained. Our study used high-resolution corelevel spectroscopy at photon energies hn= 108–115 eV, which guarantees a high surface sensitivity for the substrate core level spectra, as visible in the energy-shifted features underlying each curve. Having already investigated a binary semiconductor [5,23] as substrate, we have chosen Ge(100) in order to obtain, on the surface, dangling bonds of a single element; this represents the simplest configuration in which the surface compo-
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nent of the substrate core level spectrum, at a lower binding energy with respect to the bulk, can be easily identified during the Cs deposition and the subsequent oxygen exposure. We investigated the Ge 3d and Cs 4d doublets together with the valence band, first as a function of the metal coverage at a very low co-adsorption of oxygen, in order to detect the threshold of the catalytic oxidation; after the completion of the Cs monolayer, the investigation was performed as a function of the oxygen exposure; the spectra show evidence that Cs establishes with the semiconductor both ionic and covalent bonds that evolve, in the presence of oxygen, towards mixed oxidation states (Cs–O–Ge) and Ge intermediate oxidation states (GeO ). x 2. Experimental Photoemission experiments were carried out at the VUV beamline of the Elettra synchrotron in Trieste, Italy, at a pressure lower than 10−10 Torr. The light source was used in the undulator mode in the energy range 20–850 eV. The monochromator was a spherical grating monochromator with interchangeable gratings and focusing mirrors. The electron analyzer was formed by an angle integrated 150-mm hemispherical analyzer fitted with a 16-channel multidetector. The alkali metal was evaporated from a carefully outgassed Cs chromate source supplied by the Saes Getters Company, Italy. The metal was deposited on a Ge(100)2×1 single crystal cleaned in situ by Ar ion sputtering at T=1000 K. The surface of the semiconductor was checked by LEED ( low energy electron diffraction); no contaminants were found before the Cs evaporation. After each Cs deposition, we measured the Ge 3d and Cs 4d core levels and the valence band at photon energies of 108–115 eV. During the Cs deposition and the collection of the spectra, at 10−10 Torr, the partial oxygen pressure in the vacuum chamber corresponding to a very low exposure (of the order of 0.1 L) was specified in each set of spectra. After the completion of the metal monolayer, deduced by the saturation intensity of the corresponding area of the Cs doublet, several exposures to oxygen
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were performed; the onset and the growth of additional oxidation features and/or the evolution of the components already present on the clean interface were observed in the Ge 3d and Cs 4d core level spectra. For comparison, we also investigated the Cs 4d and valence band of a thick Cs layer evaporated at 150 K. The curves were taken in the energy distribution mode, and the electrons were detected by the analyzer as a function of the electron kinetic energy. The energy resolution measured at the Fermi edge of a clean Ag surface was better than 80 meV, at a photon energy hn= 108 eV. We verified also in an independent set of measurements that Ge(100)2×1 surface did not show any oxidation feature up to 105 L of oxygen exposure, when no Cs overlayer was deposited on the surface.
3. Results and discussion Some photoemission spectra of the Cs 4d and Ge 3d core levels are displayed in Fig. 1 as a function of the binding energy for several Cs coverages and O exposures. As is well known, Cs 2 deposited at room temperature on a semiconducting substrate does not exceed 1 ML [12]; this was directly verified by the evolution of the metal photoemission intensity, normalized to the photon flux, which reached saturation as a function of the evaporation time. In this submonolayer range, of course, the metal overlayer fully contributes to the spectrum; in addition, the kinetic energy (about 80 eV ) of the photoelectrons escaping from the Ge 3d core level corresponds to the lowest electron mean free path (about 1 nm) and therefore to a high surface sensitivity. The modifications of the spectra, during the metal deposition or when exposed to O , can be easily related to the behavior 2 of the surface bonding. In fact, the dangling bonds of the clean semiconductor substrate can establish with the metal a mutual (ionic-like or covalentlike or hybrid) link, whose nature can be determined from the binding energy of the corresponding feature; similar information is expected after the oxygen exposure. In fact, some preliminary observations are evident in the raw Cs 4d spectra:
Fig. 1. Photoemission intensity of the Cs 4d and Ge 3d core level spectra, for thin Cs coverage and low oxygen exposure on Ge(100)2×1, as a function of the binding energy. The curves are normalized to the photon flux.
(1) the first is concerned with the anomalous spin–orbit branching ratio of the doublet as deduced either from the experimental curves or from their subsequent analysis; as reported elsewhere [24], we obtained at all depositions a value of 1.33±0.02 instead of the expected statistical 3/2; (2) the line shape, as in previous investigations [12,23] shows a valley whose height increases with the Cs deposition; this demonstrates an increasing line width due to the confluence of several components under the spectrum. A similar conclusion can be deduced from the
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deformation of the left tail of the Ge 3d spectra as a function of the Cs coverage. Although the present measurements were performed at a high resolution, it was not possible to clearly distinguish the components under each spectrum. However, as shown in several papers [23–29], if we assume for the core levels well known line shapes [25], it is easy by means of a statistical analysis to determine the shifted contributions that overlap to form the experimental spectrum. In other terms, in order to distinguish the different contributions under each curve, all the spectra were analyzed by a deconvolution of the raw curves into two (or more) components. Each of these was a pair of spin–orbit split curves with defined spin–orbit energy shift and branching ratio. A fitting procedure was used to determine the best values of the parameters, including the energy shift between the components, the line width and height; we used asymmetrical Doniach–Sunjic curves [25,26 ] convolved with a Gaussian curve, taking into account the experimental resolution and the thermal broadening. The asymmetry was allowed to vary so as to obtain pure Lorentzian curves when this parameter was equal to zero, as, for example, in the case of the substoichiometric oxidation components of the spectra. In general, the final fit gave a normalized x2#1 and the residuals distribution presented only random statistical fluctuations. More details on the statistical analysis can be found in Ref. [24]. The spectrum of the clean Ge(100)2×1 surface is reported in Fig. 2. Here, also, the deconvolution is displayed with the components of the best fit: the first at E =−29.4±0.05 eV binding energy, b asymmetry a=0.05±0.01, Lorentzian width W =0.050±0.002 and relative intensity 80% can l be immediately attributed to the bulk; a second component, U, exhibits a large shift DE= 0.51±0.005 eV towards a lower binding energy, whereas the third component S is located at an L intermediate energy (0.25±0.005 eV from the bulk). These small features in the clean substrate have a roughly equal intensity and confirm the expected high surface sensitivity. The Ge(100)2×1 surface configuration was long debated for establishing whether dimer rows are symmetrically or asymmetrically distributed on the surface. At pre-
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Fig. 2. Typical deconvolution of Ge 3d core level spectra showing the components obtained for each spectrum. The fit was performed by convolution of the Doniach–Sunjic curves’ spin orbit split, with a Gaussian curve taking into account the experimental resolution and the thermal broadening. The background (not shown) was fitted by a cubic spline. The solid line is the sum of the contributing curves, and closely fits the experimental points.
sent, a model has been confirmed of asymmetric dimers as demonstrated by Fontes et al. (pers. commun.) by X-ray standing waves. In this scheme, there is general agreement to attribute [20,21] the component that exhibits the largest shift from the bulk to emission from the up atoms of the Ge asymmetric dimers. However, the other component at intermediate energy was assigned by some authors [21] to the corresponding down atoms of the dimers, whereas Landemark et al. [20] attributed this component to second-layer
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atoms, and found the down atoms of the dimers to be indistinguishable from the bulk peak. In our clean sample, the equal intensity of the two small components at low binding energy clearly indicates that they belong to two different atomic planes with the same surface density of Ge atoms; we attribute the feature U at −28.9 eV to the Ge top layer (up atoms of the dimers) with a surface density of dangling bonds 3.12×1014 atoms/cm2 on the Ge(100)2×1 surface; we will demonstrate later that the other feature S is due L to second layer atoms rather than to the down atoms of the dimer rows. The previous deconvolution is consistent with that proposed by Landemark et al. [20]. The shift of the less bound component was calculated by Pehlke and Scheffler [22], taking into account the final state effects due to enhanced core hole screening. We note that a low asymmetry is present in the bulk spectrum (see Fig. 2) because of the large s–p character of the semiconducting screening [27]. This was not taken into account in Ref. [20]. In Fig. 2, some deconvolutions of the Ge 3d spectra are displayed for different Cs overlayers and low oxygen exposures. The evolution of the components obtained for Ge 3d on the clean substrate, for an increasing amount of Cs deposition, should reflect the nature of the mutual binding between the metal and the semiconductor. As can be seen in Table 1, the pristine surface component due to Ge dangling bonds of the up atoms lies at a lower binding energy with respect to the bulk (0.51 eV ), confirming some negative charging of the non-saturated Ge surface atoms; this component ( U ), however, does not modify its energy position at the maximum Cs coverage remaining of unchanged intensity up to 1 ML (see Table 2); of course, it is very unlikely that the Ge dangling bonds remain unsaturated in the presence of Cs atoms, and therefore, the invariance of the intensity of this component means that the binding energy of the Ge surface dangling bonds is very similar to that of a Cs-saturated Ge surface; this implies the preferential binding Ge–Cs with a negative charge transfer from Cs to Ge. The present interpretation is indeed the most appropriate since the alkali metal has the lowest electronegativity (0.65) with respect to Ge (1.8) and then tends
to lose its outer electron. This component of the metal–semiconductor binding is therefore of ioniclike nature and leaves the alkali metal residing on the surface positively charged; we emphasize that the ionic binding, although intrinsically local per se, is partially diffused over the whole surface and then produces an almost uniform surface polarization; this is important in relation to the catalytic promotion of the oxidation rate discussed later. The second component S attributed above to L second-layer atoms in the clean substrate remains at an unchanged intensity up to a Cs coverage of 0.4 ML; afterwards, it rapidly decreases, becoming smaller and smaller and disappearing after 1 L of oxygen exposure. The bulk feature undergoes a similar monotonic trend: its intensity decreases, whereas an additional component grows for coverages larger than 0.25 ML at a higher binding energy (E =−29.87±0.05 eV ): this feature is the 1 typical sign of the starting oxidation promoted by the Cs submonolayer in the presence of a very low oxygen exposure (0.1 L). In fact, in the subsequent Cs evaporations, this component rapidly increases in intensity with the concomitant decrease of both the bulk feature and the S component. The evoluL tion of the components found in the deconvolution analysis of the Ge spectra are presented in Table 1 where the binding energy and relative intensity of each component are reported for the Cs coverages and oxygen exposure indicated. The analysis of these spectra can be sufficient to outline the mechanism of the promoted Ge oxidation by a Cs overlayer. We indicate the following scenario: on the clean Ge surface, the two non-equivalent dimer atoms favor an ionic-like binding Cs–Ge with the dangling bonds of the up atoms, whereas the down atoms establish a covalent-like binding with cesium. These covalent bonds remain distributed in energy under the bulk peak of the semiconductor and therefore do not generate any new peak in the spectrum. Even at a low Cs coverage (0.25 ML), the oxygen atoms are attracted by the polarization layer due to ionic Cs+ and are captured by these covalent-like couples Cs–Ge which include oxygen in a hybrid configuration Cs–O–Ge giving rise to the higher binding energy component corresponding to the Ge+ oxidation state. This feature then
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Table 1 Energy position of the components obtained from the best fit of the Ge 3d spectra for the indicated Cs coverage and O exposure 2 (1 L=10− 6 Torr s) Cs (ML)
O (L) 2
Energy position (eV ) E (Ge4+)
Clean 0.1 0.25 0.4 0.75 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0 0 0.1 0.2 0.5 1 2 5 10 20 50 200
−32.28 −32.36 −32.50 −32.48
E (Ge3+)
−31.67 −31.69 −31.73 −31.71 −31.80 −31.77
E (Ge2+)
−30.74 −30.73 −30.80 −30.72 −30.77 −30.77
E (Ge1+)
−29.87 −29.89 −29.97 −30.00 −30.26 −30.26
E (bulk)
E (S ) L
E (U)
−29.40 −29.40 −29.42 −29.43 −29.34 −29.33 −29.50 −29.50 −29.54 −29.55 −29.58 −29.58
−29.15 −29.15 −29.17 −29.17 −29.09 −29.09
−28.89 −28.90 −28.91 −28.91 −28.82 −28.83 −28.91 −28.91 −29.03 −29.03 −29.04 −29.05
The uncertainty for the energies is less than ±0.1 eV. The parameters used in the fits are: spin–orbit split DE=0.58±0.01 eV; branching ratio=1.5±0.01; relative intensity of the components obtained from the best fit of the Ge 3d spectra for the indicated Cs coverage and O exposure (1 L=10−6 Torr s). The uncertainty for the intensities is less than ±2%. The parameters used in the fits 2 are: spin–orbit split DE=0.58±0.01 eV; branching ratio=1.5±0.01. Table 2 Relative intensity of the components obtained from the best fit of the Ge 3d spectra for the indicated Cs coverage and O exposure (1 L=10−6 Torr s) 2 Cs (ML)
O (L) 2
Relative intensity (%) I (Ge4+)
Clean 0.1 0.25 0.4 0.75 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0 0 0.1 0.2 0.5 1 2 5 10 20 50 200
7 10 14 20
I (Ge3+)
10 10 16 21 26 26
I (Ge2+)
17 20 27 23.5 21 20
I (Ge1+)
5.5 7 12.5 18.5 6 4
I (bulk)
I(S ) L
I( U )
80 78 75 71 69 67.5 62 60 42 38 31 28
10.5 11 9.5 12 7.5 2
9.5 11 10 10 11 12 5 6 8 7.5 8 6
The uncertainty for the energies is less than ±2%. The parameters used in the fits are: spin–orbit split DE=0.58±0.01 eV; branching ratio=1.5±0.01.
grows balancing first the decrease of the down atoms of the dimer (included in the bulk peak) and afterwards the second layer atoms. Another possible interpretation attributed the S feature at 0.25 eV from the bulk not to secondL layer Ge atoms as we propose, but to the down
atoms of the dimers; these could link to Cs in covalent bonds. This configuration seems to be disproved because, if it were the case, this covalent feature should be the first to be reduced when the oxidation begins, i.e. it should balance the increase of the first oxidation feature. This is not reflected
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in our experiments. In contrast, the oxidation grows at the expense of the bulk feature, demonstrating that the Ge layer involved in this process is distributed as the bulk. Since the ionic component does not change at all, this effect should be attributed to the only surface layer covalently bound to Cs, i.e. the down atom rows of the dimers. In fact, the second layer atoms begin to be involved in the oxidation process by an intensity decrease at higher Cs coverages ( larger than 0.4 ML), i.e. after the modification exhibited by the down atoms of the dimers. This is corroborated by the Cs 4d deconvolution reported in Table 3, where two components are distinguishable: the one at a higher binding energy corresponds to ionic Cs+ bound to Ge ( lower binding energy on the semiconductor), whereas the other is in a covalentlike bond with the germanium. In fact, the intensity of the ionic component progressively decreases as a function of coverage from about 85% to 38%, whereas the other grows correspondingly; this behavior can be explained by the following argument: at low coverage, the most stable sites are first saturated with an ionic binding Cs–Ge; this determines an increasing polarization at the interface with a positive charging of the surface metal atoms; subsequent addition of Cs atoms implies the growth of weaker (covalent) bonds both
because of the polarization layer and because of the progressive saturation of the closest sites Cs–Ge, depending on the mutual distance Cs–Cs at the surface layer. For comparison, in Table 3 we also reported the two components obtained on a thick Cs layer at 150 K: (1) at E =−76.1 eV, we find the bulk contribub tion of clean Cs, with 60% intensity; (2) at E =−76.6 eV, we indicate the surface peak s of clean Cs. The previous identification of the mutual binding established at the Cs/Ge(100) interface was checked by higher exposure of the surface 1 ML Cs/Ge(100) to oxygen. It is worth pointing out that the clean Ge surface is hardly oxidized at all, as we have directly verified up to 105 L oxygen exposure. The spectra as a function of the O 2 exposure are presented in Fig. 3. Here, the new features on the Ge higher binding energy tail are very evident and can be straightforwardly attributed to Ge intermediate oxidation states. Their binding energy was found by a decomposition performed as outlined above for the clean part of the spectrum; however, the high binding energy features, being due to oxidation states, were fitted by Lorentzian curves [28,29]. Typical deconvolutions are presented in Fig. 4. The onset and growth of the components can be followed in Tables 1 and
Table 3 Energy position and relative intensity of the components obtained for best-fitting the Cs 4d spectra for the indicated Cs overlayer and O exposure (1 L=10−6 Torr s) 2 Cs (ML)
Thick (150 K ) 0.10 0.25 0.40 0.75 1.0 1.0 1.0 1.0 1.0 1.0 1.0
O (L) 2
0 0 0.1 0.2 0.5 1 2 5 10 20 50 200
Energy position (eV )
Relative intensity (%)
E (cov.) 1
E (ionic) 2
I
1
I 2
−76.1 (bulk) −76.1 −76.1 −76.1 −76.2 −76.3 −76.2 −76.1 −76.0 −75.9 −75.9 −75.8
−76.6 (surface) −76.6 −76.7 −76.7 −76.6 −76.6 −76.6 −76.6 −76.6 −76.6 −76.6 −76.6
60 15 12 30 46 62 50 35 44 33 30 27
40 85 88 70 54 38 50 65 56 67 70 73
The uncertainty for the energies is less than ±0.1 eV, and that for the intensities is less than ±2%. The parameters used in the fits are: spin–orbit split DE=2.27±0.01 eV; branching ratio=1.33±0.02, for Cs bulk 1.50±0.02.
G. Faraci, A.R. Pennisi / Surface Science 409 (1998) 46–56
Fig. 3. Photoemission intensity of the Ge 3d and Cs 4d core level spectra, for a monolayer of Cs on Ge(100), as a function of the binding energy, at different oxygen exposures. The curves are normalized to the total area.
2 where the binding energy and relative intensity of each feature are reported. We observe that for an increasing O exposure, the part of the spectrum 2 due to the bulk component is reduced, whereas the surface part remains at an unchanged intensity; furthermore, the high binding energy components arising from the Ge intermediate oxidation states (Ge+, Ge2+, Ge3+) show the progressive growth of the second and third feature at the expense of the first. Note, however, that because of the low oxygen exposure, the stoichiometric Ge4+ oxide is not formed until 10 L; afterwards, this stoichiometric oxide rapidly increases in intensity, shifting in energy.
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Fig. 4. Deconvolution of Ge 3d core level spectra taken for 1ML Cs/Ge(100) at several O exposures (1 L=10−6 Torr s), 2 showing the components obtained for each spectrum. The fit was performed: (1) for the clean part of the Ge spectrum by convolution of two Doniach–Sunjic curves’ spin orbit split, with a Gaussian curve taking into account the experimental resolution and the thermal broadening; and (2) for the oxide features at higher binding energy, by Lorentzian curves. The background (not shown) was fitted by a cubic spline. The solid line is the sum of the contributing curves, and closely fits the experimental points.
Also the Cs 4d spectra were analyzed as specified above. Again, we obtained two components whose energy and intensity are reported in Table 3. These values indicate that the energy position E is quite 2 stable, whereas the covalent feature increases to 0.2 eV with the Cs coverage and decreases afterwards to 0.5 eV for increasing oxygen exposure. The intensity of the ionic component is quite
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high (85%) at a low Cs coverage, reflecting the preferential ionic binding; it decreases at the completion of the monolayer when saturation is reached; further O exposure increases this compo2 nent again: this behavior demonstrates the conversion of the covalent Cs link into ionic-like bonds caused by oxygen inclusion: Ge–CsGe–O–Cs. The present results confirm the mechanism proposed in recent papers [5,23] for explaining the catalytic enhancement of the oxidation rate of a semiconductor promoted by an alkali metal overlayer. The actual Ge surface after the Cs deposition clearly shows an ionic-like binding with the metal: in fact, the surface component U of the clean Ge spectrum reflects the presence of dangling bonds; these belonging to the up atoms of the dimers are responsible of the ionic bonds with the alkali metal as confirmed by the corresponding intensity of the U component vs. Cs coverage; this component shows minor changes both in height and in binding energy up to the completion of the Cs overlayer. The ionic strength of the binding Cs–Ge attributed to the U component is also demonstrated by its behavior in the presence of O ; this feature is 2 reduced only partially, even at the highest oxygen exposure, whereas the S component is already L washed out at 1 L, and the bulk feature undergoes a minor decrease during the Cs coverage but is attenuated of a factor 3 at 200 L. The bulk Ge component does not change either in intensity or in binding energy up to 0.25 ML; at this coverage, an intensity decrease is observed for a new higher (at −29.9±0.1 eV ) component. This feature signs the growth of the first oxidation state catalyzed by the Cs overlayer; an increase of this feature is caused by low oxygen exposure, followed by a strong reduction at higher exposures. This implies the O -induced disruption of the 2 covalent bonds and the growth of the other semiconductor substoichiometric oxidation states. A model was proposed by Abukawa and Kono [30] for K deposition on Si(100)2×1. According to this model, which can be extended to Cs on Ge(100)2×1, the cesium atoms could reside both on top and between the Ge dimer rows. However, the lattice parameter of Ge bidimensional rows is 0.4 nm; this separation in not compatible with Cs–Cs mutual distance (0.523 nm) in the Cs metal,
but it is if the Cs loses its outer electron, establishing an ionic bond. In fact, Cs+ the ionic radius is 0.183 nm in the alkali halides configuration. Taking into account the cesium atomic radius, a full monolayer of Cs could be disposed as claimed by the model of Chao [17] and Abukawa [30] only if the Cs–Ge bonds are of ionic character. The same holds for K on Si. In contrast, the Cs 4d spectrum exhibits two components at different binding energies; the same result was obtained in Ref. [17] for the deconvolution of K on Si(100)2×1. In this context, TEM measurements [31] of Cs on GaAs(110) or InSb(110) with lattice constant 0.4 and 0.458 nm, respectvely, showed zigzag chains at low coverage, whereas the first complete Cs overlayer seems to consist of compressed (0.49 nm) five atoms polygons arranged in a c(4×4) superlattice. The analysis of the Cs 4d spectra reveals two components in agreement with the previous scenario: the first E at low binding energy can be 1 attributed to the weak covalent bond Cs–Ge, the other to the ionic Cs binding with the Ge substrate. The relative intensity confirms the previous interpretation that the dangling bonds of the substrate partly establish ionic links with the metal as long as the coverage is low; for increasing deposition, saturation of this component is reached with a progressive increase in the covalent component. This effect is also corroborated by the corresponding surface densities of Ge(100) and Cs: on the semiconductor surface, we find 6.25×1014 atoms/cm2, whereas for 1 ML Cs, the Cs–Cs bulk distance (5.23) corresponds to 3.6×1014 atoms/cm2, even though scanning electron microscopy [31] gives an interatomic distance up to 7% closer in five-atom Cs polygons induced by the underlying lattice on which Cs is adsorbed. In any case, the lattice mismatch between Cs and Ge suggests that only the closest Ge–Cs couples can produce an ionic binding; the others on different site arrangements are very likely distributed as covalent bonds. The fast modifications observed with O exposure both on the Ge 3d and Cs 4d 2 spectra are the evident symptoms of the catalytic enhancement of the semiconductor oxidation rate promoted by the metal overlayer. In fact, the growth of the Ge oxidation states with the concom-
G. Faraci, A.R. Pennisi / Surface Science 409 (1998) 46–56
itant weakening of the bulk component (see Table 3) can be mainly attributed to a disruption of covalent Ge–Cs bonds capturing an oxygen atom in a double (GeO ) or mixed (Cs–O–Ge) x configuration. These states are in fact more stable not only for the Ge partner but also for the Cs element whose I component (see Table 3) results 2 were amplified just because of the transfer of covalent disrupted bonds (reduction of I as a 1 function of the exposure) towards the ionic Cs+ component (Cs–O–Ge). Furthermore, it is worth observing that the previous effect, as expected, modifies the Cs component at E , shifting up to 1 0.5 eV towards a lower binding energy. As for other substrates [23,24], the metal plays its prevalent role through the polarized surface layer, positively charged as a consequence of the electron transfer of the ionic bonds. This non-local and diffused polarization strongly attracts oxygen, enhancing its sticking coefficient and determining the dramatic oxidation of the substrate. We add some comments about the lineshape of the spectra: as far as the Ge 3d core level is concerned, we get a Lorentzian full width at half maximum (FWHM ) for the bulk contribution c =0.072 eV, b and for the surface c =0.17 eV; for both, the s Gaussian broadening (FWHM ) s =0.45 eV. For G the Cs spectra, the intrinsic line shape is lower than 0.17 eV, whereas the Gaussian extra-broadening goes from 0.80 up to 1.35 eV as a function of the oxygen exposure. This last result is again in the direction indicated above, i.e. the Cs spectra tend to broaden as much as Cs adsorbs oxygen in mixed configuration. In conclusion, we demonstrated by an accurate statystical analysis, the presence of both ionic-like and covalent-like bonds at the Cs/Ge(100)2×1 interface. The first ones are responsible for the polarization layer established at the surface, which, by attracting oxygen, promotes the catalytic oxidation of the substrate. The oxidation proceeds by driving oxygen towards the covalent bonds that are, therefore, modified by the inclusion of an oxygen atom in a mixed link; the mechanism for explaining the enhanced oxidation rate of the Ge(100)2×1 surface catalyzed by a Cs overlayer and the growth of substoichiometric oxidation states was followed at its very beginning.
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Acknowledgements We wish to thank the staff of The Sincrotrone Trieste for the excellent assistance during the experiments. In particular, we appreciated the collaboration of C. Comicioli, C. Crotti, S. La Rosa, M. Peloi, and M. Zacchigna. This work is supported by the Istituto Nazionale di Fisica della Materia and the Istituto Nazionale di Fisica Nucleare.
References [1] L. Surnev, M. Tikhov, Surf. Sci. 123 (1982) 519. [2] H.I. Starnberg, P. Soukiassian, M.H. Bakshi, Z. Hurych, Phys. Rev. B 37 (1988) 1315. [3] H.I. Starnberg, P. Soukiassian, M.H. Bakshi, Z. Hurych, Surf. Sci. 224 (1989) 13. [4] H.I. Starnberg, P. Soukiassian, S.K. Tim, A. Papageorgopoulos, S. Kapoor, Surf. Sci. 269/270 (1992) 934. [5] G. Faraci, A.R. Pennisi, G. Margaritondo, Phys. Rev. B 53 (1996) 13851. [6 ] P. Soukiassian, M.H. Bakshi, Z. Hurych, T.M. Gentle, Phys. Rev. B 35 (1987) 4176. [7] E.G. Michel, J.E. Ortega, E.M. Oellig, M.C. Asensio, J. Ferron, R. Miranda, Phys. Rev. B 38 (1988) 13399. [8] J. Higasa, F.H. Geuzebroek, Y. Yamashita, A. Namiki, Surf. Sci. 297 (1993) L55. [9] D.S. Lin, T. Miller, T.-C. Chiang, Phys. Rev. B 44 (1991) 10719. [10] P. Soukiassian, J.A. Kubby, P. Mangat, Z. Hurych, K.M. Schirm, Phys. Rev. B 46 (1992) 13471. [11] P. Soukiassian, M.H. Bakshi, Z. Hurych, T.M. Gentle, Surf. Sci. 221 (1989) L759. [12] T. Kendelewicz, P. Soukiassian, M.H. Bakshi, Z. Hurych, I. Lindau, W.E. Spicer, Phys. Rev. B 38 (1988) 7568. [13] B. Reihl, R. Dudde, L.S.O. Johansson, K.O. Magnusson, S.L. Sorensen, S. Wiklund, Appl. Surf. Sci. 56 (1992) 123. [14] J. Ortega, R. Perez, F.J. Garcia-Vidal, F. Flores, Appl. Surf. Sci. 56 (1992) 264. [15] J.E. Ortega, M. Miranda, Appl. Surf. Sci. 56 (1992) 211. [16 ] T.M. Grehk, M. Go¨thelid, U.O. Karlsson, L.S.O. Johansson, S.M. Gray, K.O. Magnusson, Phys. Rev. B 52 (1995) 11165. [17] Y.-C. Chao, L.S.O. Johansson, C.J. Karlsson, E. Landemark, R.I.G. Uhrberg, Phys. Rev. B 52 (1995) 2579. [18] Y.-C. Chao, L.S.O. Johansson, R.I.G. Uhrberg, Phys. Rev. B 54 (1996) 5901. [19] R. Souda, W. Hayami, T. Aizawa, Y. Ishizawa, Phys. Rev. B 48 (1993) 17255. [20] E. Landemark, C.J. Carlsson, L.S.O. Johansson, R.I.G. Uhrberg, Phys. Rev. B 49 (1994) 16523.
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[21] G. Le Lay, J. Kanski, P.O. Nilsson, U.O. Karlsson, K. Hricovini, Phys. Rev. B 45 (1992) 6692. [22] E. Pehlke, M. Scheffler, Phys. Rev. Lett. 71 (1993) 2338. [23] G. Faraci, A.R. Pennisi, F. Gozzo, S. La Rosa, G. Margaritondo, Phys. Rev. B 53 (1996) 3987. [24] G. Faraci, A.R. Pennisi, Y. Hwu, Phys. Rev. B 54 (1996) 4515. [25] S. Doniach, M. Sunjic, J. Phys. C 3 (1970) 285. [26 ] G.K. Wertheim, D.M. Riffe, P.H. Citrin, Phys. Rev. B 45 (1992) 8703.
[27] D.M. Riffe, G.K. Wertheim, D.N.E. Buchanan, P.H. Citrin, Phys. Rev. B 45 (1992) 6216. [28] G. Faraci, S. La Rosa, A.R. Pennisi, G. Margaritondo, Phys. Rev. B 50 (1994) 1965. [29] G. Faraci, S. La Rosa, A.R. Pennisi, Y. Hwu, G. Margaritondo, J. Appl. Phys. 78 (1995) 4091. [30] T. Abukawa, S. Kono, Phys. Rev. B 37 (1988) 9097. [31] L.J. Whitman, J.A. Stroscio, R.A. Dragoset, R.J. Celotta, Phys. Rev. Lett. 66 (1991) 1338.
Ge(100)2×1 substoichiometric oxidation states promoted by a Cs overlayer G. Faraci *, A.R. Pennisi Dipartimento di Fisica, Universita´ di Catania, Istituto Nazionale di Fisica della Materia, Corso Italia 57, 95129 Catania, Italy Received 19 December 1997; accepted for publication 24 February 1998
Abstract High-resolution photoemission spectra from the Cs/Ge(100)2×1 interface were obtained for several metal coverages from 0.1 up to 1 ML. The bonding between the Cs atoms and the Ge surface was determined from the analysis of the Cs 4d and Ge 3d core level spectra. Both covalent-like and ionic-like bonds were identified in the energy shifted components of the spectra. Exposure of the interface to O allowed the catalytic mechanism of the semiconductor oxidation (dramatically enhanced by the polarized metal 2 overlayer) to be ascertained. In fact, the onset and growth of substoichiometric oxidation states was already observed at and beyond 0.25 ML Cs coverage, and 0.1 L oxygen exposure. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Catalysis; Oxidation; Photoemission; Semiconductor; Surface
1. Introduction The dramatic enhancement of the oxidation rate of a semiconducting surface caused, in the presence of low O exposure, by a submonolayer depos2 ition of an alkali metal has long been debated [1– 8]. The understanding of the effective mechanism responsible of such a process first requires the study of the clean interface between the overlayer and the semiconductor [9–21]. In fact, the knowledge of the bonds established between the semiconductor dangling bonds and the alkali metal is of primary importance to follow the oxide formation and its evolution consequent to oxygen exposure. The catalytic enhancement of the oxidation rate is usually related to the reduction of the work func* Corresponding author. Fax: (+39) 95 383 023; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 01 9 6 -4
tion and to oxygen dissociation in the surface dipole layer due to the alkali metal deposition [1,7]. However, it is still uncertain as to how and where this dissociation takes place, the extent to which the metal is directly involved, i.e. whether it remains a spectator or participates to the process as an active player, and the manner in which the underlying semiconductor is able to capture oxygen through the metal overlayer. In fact, several papers have been published for an alkali metal overlayer on semiconducting substrates but with conflicting conclusions. Lin et al. [9] found that the chemisorption of Cs on Ge(100)2×1 was partially ionic; Soukiassian et al. [10,11] concluded that alkali metals on Si surfaces have polarized covalent bonds with a metallic Na, K, Rb or Cs overlayer. In contrast, Souda et al. [19] showed that Cs and K adatoms are ionically adsorbed, whereas the Na adatom is neutral on Ge(100) for
G. Faraci, A.R. Pennisi / Surface Science 409 (1998) 46–56
coverages lower than 0.25 ML. Enhancement of the sticking probability of oxygen on Ge(100) due to the adatoms was also observed; in fact, Higasa et al. [8] studied the alkali promotion of O 2 sticking on Ge(100) and Si(100); they suggested for the alkali promotion a threshold coverage around 0.2 ML, with an abrupt transition from ionic to covalent or metallic bonding depending on the substrate. Recently, photoelectron spectroscopy studies [20] of clean and hydrogen chemisorbed Ge(001) surfaces allowed the determination of the surface configuration of Ge(001)2×1 showing a component due to the up atoms shifted by 0.5 eV towards a lower binding energy with respect to the bulk; another component at a 0.19 lower binding energy was attributed to the second-layer atoms of the substrate. Similar experiments on Cs/Si(100)2×1 by Chao et al. [18] indicated the difference between the two Si surface components attributed to up and down atoms of asymmetric Si dimers; here, the Cs saturated surface exhibited a metallic character. Of course, the surface configuration and the topology of its dangling bonds are particularly important as theoretically discussed by Pehlke and Heffler [22], who showed that the final state relaxation plays an important role with an enhanced screening at the surface. The questions raised by the previous controversial results are the aim of the present paper, which includes the investigation of the clean Cs/Ge(100)2×1 interface in order to determine their binding mechanism before any oxygen exposure. In recent papers [5,23], we studied the Cs/GaAs(110) interface and its exposure to O at 2 a photon energy of 1486.7 eV; in this case, evidence of a Cs participation to the oxidation process through hybrid or mixed links was clearly obtained. Our study used high-resolution corelevel spectroscopy at photon energies hn= 108–115 eV, which guarantees a high surface sensitivity for the substrate core level spectra, as visible in the energy-shifted features underlying each curve. Having already investigated a binary semiconductor [5,23] as substrate, we have chosen Ge(100) in order to obtain, on the surface, dangling bonds of a single element; this represents the simplest configuration in which the surface compo-
47
nent of the substrate core level spectrum, at a lower binding energy with respect to the bulk, can be easily identified during the Cs deposition and the subsequent oxygen exposure. We investigated the Ge 3d and Cs 4d doublets together with the valence band, first as a function of the metal coverage at a very low co-adsorption of oxygen, in order to detect the threshold of the catalytic oxidation; after the completion of the Cs monolayer, the investigation was performed as a function of the oxygen exposure; the spectra show evidence that Cs establishes with the semiconductor both ionic and covalent bonds that evolve, in the presence of oxygen, towards mixed oxidation states (Cs–O–Ge) and Ge intermediate oxidation states (GeO ). x 2. Experimental Photoemission experiments were carried out at the VUV beamline of the Elettra synchrotron in Trieste, Italy, at a pressure lower than 10−10 Torr. The light source was used in the undulator mode in the energy range 20–850 eV. The monochromator was a spherical grating monochromator with interchangeable gratings and focusing mirrors. The electron analyzer was formed by an angle integrated 150-mm hemispherical analyzer fitted with a 16-channel multidetector. The alkali metal was evaporated from a carefully outgassed Cs chromate source supplied by the Saes Getters Company, Italy. The metal was deposited on a Ge(100)2×1 single crystal cleaned in situ by Ar ion sputtering at T=1000 K. The surface of the semiconductor was checked by LEED ( low energy electron diffraction); no contaminants were found before the Cs evaporation. After each Cs deposition, we measured the Ge 3d and Cs 4d core levels and the valence band at photon energies of 108–115 eV. During the Cs deposition and the collection of the spectra, at 10−10 Torr, the partial oxygen pressure in the vacuum chamber corresponding to a very low exposure (of the order of 0.1 L) was specified in each set of spectra. After the completion of the metal monolayer, deduced by the saturation intensity of the corresponding area of the Cs doublet, several exposures to oxygen
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G. Faraci, A.R. Pennisi / Surface Science 409 (1998) 46–56
were performed; the onset and the growth of additional oxidation features and/or the evolution of the components already present on the clean interface were observed in the Ge 3d and Cs 4d core level spectra. For comparison, we also investigated the Cs 4d and valence band of a thick Cs layer evaporated at 150 K. The curves were taken in the energy distribution mode, and the electrons were detected by the analyzer as a function of the electron kinetic energy. The energy resolution measured at the Fermi edge of a clean Ag surface was better than 80 meV, at a photon energy hn= 108 eV. We verified also in an independent set of measurements that Ge(100)2×1 surface did not show any oxidation feature up to 105 L of oxygen exposure, when no Cs overlayer was deposited on the surface.
3. Results and discussion Some photoemission spectra of the Cs 4d and Ge 3d core levels are displayed in Fig. 1 as a function of the binding energy for several Cs coverages and O exposures. As is well known, Cs 2 deposited at room temperature on a semiconducting substrate does not exceed 1 ML [12]; this was directly verified by the evolution of the metal photoemission intensity, normalized to the photon flux, which reached saturation as a function of the evaporation time. In this submonolayer range, of course, the metal overlayer fully contributes to the spectrum; in addition, the kinetic energy (about 80 eV ) of the photoelectrons escaping from the Ge 3d core level corresponds to the lowest electron mean free path (about 1 nm) and therefore to a high surface sensitivity. The modifications of the spectra, during the metal deposition or when exposed to O , can be easily related to the behavior 2 of the surface bonding. In fact, the dangling bonds of the clean semiconductor substrate can establish with the metal a mutual (ionic-like or covalentlike or hybrid) link, whose nature can be determined from the binding energy of the corresponding feature; similar information is expected after the oxygen exposure. In fact, some preliminary observations are evident in the raw Cs 4d spectra:
Fig. 1. Photoemission intensity of the Cs 4d and Ge 3d core level spectra, for thin Cs coverage and low oxygen exposure on Ge(100)2×1, as a function of the binding energy. The curves are normalized to the photon flux.
(1) the first is concerned with the anomalous spin–orbit branching ratio of the doublet as deduced either from the experimental curves or from their subsequent analysis; as reported elsewhere [24], we obtained at all depositions a value of 1.33±0.02 instead of the expected statistical 3/2; (2) the line shape, as in previous investigations [12,23] shows a valley whose height increases with the Cs deposition; this demonstrates an increasing line width due to the confluence of several components under the spectrum. A similar conclusion can be deduced from the
G. Faraci, A.R. Pennisi / Surface Science 409 (1998) 46–56
deformation of the left tail of the Ge 3d spectra as a function of the Cs coverage. Although the present measurements were performed at a high resolution, it was not possible to clearly distinguish the components under each spectrum. However, as shown in several papers [23–29], if we assume for the core levels well known line shapes [25], it is easy by means of a statistical analysis to determine the shifted contributions that overlap to form the experimental spectrum. In other terms, in order to distinguish the different contributions under each curve, all the spectra were analyzed by a deconvolution of the raw curves into two (or more) components. Each of these was a pair of spin–orbit split curves with defined spin–orbit energy shift and branching ratio. A fitting procedure was used to determine the best values of the parameters, including the energy shift between the components, the line width and height; we used asymmetrical Doniach–Sunjic curves [25,26 ] convolved with a Gaussian curve, taking into account the experimental resolution and the thermal broadening. The asymmetry was allowed to vary so as to obtain pure Lorentzian curves when this parameter was equal to zero, as, for example, in the case of the substoichiometric oxidation components of the spectra. In general, the final fit gave a normalized x2#1 and the residuals distribution presented only random statistical fluctuations. More details on the statistical analysis can be found in Ref. [24]. The spectrum of the clean Ge(100)2×1 surface is reported in Fig. 2. Here, also, the deconvolution is displayed with the components of the best fit: the first at E =−29.4±0.05 eV binding energy, b asymmetry a=0.05±0.01, Lorentzian width W =0.050±0.002 and relative intensity 80% can l be immediately attributed to the bulk; a second component, U, exhibits a large shift DE= 0.51±0.005 eV towards a lower binding energy, whereas the third component S is located at an L intermediate energy (0.25±0.005 eV from the bulk). These small features in the clean substrate have a roughly equal intensity and confirm the expected high surface sensitivity. The Ge(100)2×1 surface configuration was long debated for establishing whether dimer rows are symmetrically or asymmetrically distributed on the surface. At pre-
49
Fig. 2. Typical deconvolution of Ge 3d core level spectra showing the components obtained for each spectrum. The fit was performed by convolution of the Doniach–Sunjic curves’ spin orbit split, with a Gaussian curve taking into account the experimental resolution and the thermal broadening. The background (not shown) was fitted by a cubic spline. The solid line is the sum of the contributing curves, and closely fits the experimental points.
sent, a model has been confirmed of asymmetric dimers as demonstrated by Fontes et al. (pers. commun.) by X-ray standing waves. In this scheme, there is general agreement to attribute [20,21] the component that exhibits the largest shift from the bulk to emission from the up atoms of the Ge asymmetric dimers. However, the other component at intermediate energy was assigned by some authors [21] to the corresponding down atoms of the dimers, whereas Landemark et al. [20] attributed this component to second-layer
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G. Faraci, A.R. Pennisi / Surface Science 409 (1998) 46–56
atoms, and found the down atoms of the dimers to be indistinguishable from the bulk peak. In our clean sample, the equal intensity of the two small components at low binding energy clearly indicates that they belong to two different atomic planes with the same surface density of Ge atoms; we attribute the feature U at −28.9 eV to the Ge top layer (up atoms of the dimers) with a surface density of dangling bonds 3.12×1014 atoms/cm2 on the Ge(100)2×1 surface; we will demonstrate later that the other feature S is due L to second layer atoms rather than to the down atoms of the dimer rows. The previous deconvolution is consistent with that proposed by Landemark et al. [20]. The shift of the less bound component was calculated by Pehlke and Scheffler [22], taking into account the final state effects due to enhanced core hole screening. We note that a low asymmetry is present in the bulk spectrum (see Fig. 2) because of the large s–p character of the semiconducting screening [27]. This was not taken into account in Ref. [20]. In Fig. 2, some deconvolutions of the Ge 3d spectra are displayed for different Cs overlayers and low oxygen exposures. The evolution of the components obtained for Ge 3d on the clean substrate, for an increasing amount of Cs deposition, should reflect the nature of the mutual binding between the metal and the semiconductor. As can be seen in Table 1, the pristine surface component due to Ge dangling bonds of the up atoms lies at a lower binding energy with respect to the bulk (0.51 eV ), confirming some negative charging of the non-saturated Ge surface atoms; this component ( U ), however, does not modify its energy position at the maximum Cs coverage remaining of unchanged intensity up to 1 ML (see Table 2); of course, it is very unlikely that the Ge dangling bonds remain unsaturated in the presence of Cs atoms, and therefore, the invariance of the intensity of this component means that the binding energy of the Ge surface dangling bonds is very similar to that of a Cs-saturated Ge surface; this implies the preferential binding Ge–Cs with a negative charge transfer from Cs to Ge. The present interpretation is indeed the most appropriate since the alkali metal has the lowest electronegativity (0.65) with respect to Ge (1.8) and then tends
to lose its outer electron. This component of the metal–semiconductor binding is therefore of ioniclike nature and leaves the alkali metal residing on the surface positively charged; we emphasize that the ionic binding, although intrinsically local per se, is partially diffused over the whole surface and then produces an almost uniform surface polarization; this is important in relation to the catalytic promotion of the oxidation rate discussed later. The second component S attributed above to L second-layer atoms in the clean substrate remains at an unchanged intensity up to a Cs coverage of 0.4 ML; afterwards, it rapidly decreases, becoming smaller and smaller and disappearing after 1 L of oxygen exposure. The bulk feature undergoes a similar monotonic trend: its intensity decreases, whereas an additional component grows for coverages larger than 0.25 ML at a higher binding energy (E =−29.87±0.05 eV ): this feature is the 1 typical sign of the starting oxidation promoted by the Cs submonolayer in the presence of a very low oxygen exposure (0.1 L). In fact, in the subsequent Cs evaporations, this component rapidly increases in intensity with the concomitant decrease of both the bulk feature and the S component. The evoluL tion of the components found in the deconvolution analysis of the Ge spectra are presented in Table 1 where the binding energy and relative intensity of each component are reported for the Cs coverages and oxygen exposure indicated. The analysis of these spectra can be sufficient to outline the mechanism of the promoted Ge oxidation by a Cs overlayer. We indicate the following scenario: on the clean Ge surface, the two non-equivalent dimer atoms favor an ionic-like binding Cs–Ge with the dangling bonds of the up atoms, whereas the down atoms establish a covalent-like binding with cesium. These covalent bonds remain distributed in energy under the bulk peak of the semiconductor and therefore do not generate any new peak in the spectrum. Even at a low Cs coverage (0.25 ML), the oxygen atoms are attracted by the polarization layer due to ionic Cs+ and are captured by these covalent-like couples Cs–Ge which include oxygen in a hybrid configuration Cs–O–Ge giving rise to the higher binding energy component corresponding to the Ge+ oxidation state. This feature then
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G. Faraci, A.R. Pennisi / Surface Science 409 (1998) 46–56
Table 1 Energy position of the components obtained from the best fit of the Ge 3d spectra for the indicated Cs coverage and O exposure 2 (1 L=10− 6 Torr s) Cs (ML)
O (L) 2
Energy position (eV ) E (Ge4+)
Clean 0.1 0.25 0.4 0.75 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0 0 0.1 0.2 0.5 1 2 5 10 20 50 200
−32.28 −32.36 −32.50 −32.48
E (Ge3+)
−31.67 −31.69 −31.73 −31.71 −31.80 −31.77
E (Ge2+)
−30.74 −30.73 −30.80 −30.72 −30.77 −30.77
E (Ge1+)
−29.87 −29.89 −29.97 −30.00 −30.26 −30.26
E (bulk)
E (S ) L
E (U)
−29.40 −29.40 −29.42 −29.43 −29.34 −29.33 −29.50 −29.50 −29.54 −29.55 −29.58 −29.58
−29.15 −29.15 −29.17 −29.17 −29.09 −29.09
−28.89 −28.90 −28.91 −28.91 −28.82 −28.83 −28.91 −28.91 −29.03 −29.03 −29.04 −29.05
The uncertainty for the energies is less than ±0.1 eV. The parameters used in the fits are: spin–orbit split DE=0.58±0.01 eV; branching ratio=1.5±0.01; relative intensity of the components obtained from the best fit of the Ge 3d spectra for the indicated Cs coverage and O exposure (1 L=10−6 Torr s). The uncertainty for the intensities is less than ±2%. The parameters used in the fits 2 are: spin–orbit split DE=0.58±0.01 eV; branching ratio=1.5±0.01. Table 2 Relative intensity of the components obtained from the best fit of the Ge 3d spectra for the indicated Cs coverage and O exposure (1 L=10−6 Torr s) 2 Cs (ML)
O (L) 2
Relative intensity (%) I (Ge4+)
Clean 0.1 0.25 0.4 0.75 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0 0 0.1 0.2 0.5 1 2 5 10 20 50 200
7 10 14 20
I (Ge3+)
10 10 16 21 26 26
I (Ge2+)
17 20 27 23.5 21 20
I (Ge1+)
5.5 7 12.5 18.5 6 4
I (bulk)
I(S ) L
I( U )
80 78 75 71 69 67.5 62 60 42 38 31 28
10.5 11 9.5 12 7.5 2
9.5 11 10 10 11 12 5 6 8 7.5 8 6
The uncertainty for the energies is less than ±2%. The parameters used in the fits are: spin–orbit split DE=0.58±0.01 eV; branching ratio=1.5±0.01.
grows balancing first the decrease of the down atoms of the dimer (included in the bulk peak) and afterwards the second layer atoms. Another possible interpretation attributed the S feature at 0.25 eV from the bulk not to secondL layer Ge atoms as we propose, but to the down
atoms of the dimers; these could link to Cs in covalent bonds. This configuration seems to be disproved because, if it were the case, this covalent feature should be the first to be reduced when the oxidation begins, i.e. it should balance the increase of the first oxidation feature. This is not reflected
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in our experiments. In contrast, the oxidation grows at the expense of the bulk feature, demonstrating that the Ge layer involved in this process is distributed as the bulk. Since the ionic component does not change at all, this effect should be attributed to the only surface layer covalently bound to Cs, i.e. the down atom rows of the dimers. In fact, the second layer atoms begin to be involved in the oxidation process by an intensity decrease at higher Cs coverages ( larger than 0.4 ML), i.e. after the modification exhibited by the down atoms of the dimers. This is corroborated by the Cs 4d deconvolution reported in Table 3, where two components are distinguishable: the one at a higher binding energy corresponds to ionic Cs+ bound to Ge ( lower binding energy on the semiconductor), whereas the other is in a covalentlike bond with the germanium. In fact, the intensity of the ionic component progressively decreases as a function of coverage from about 85% to 38%, whereas the other grows correspondingly; this behavior can be explained by the following argument: at low coverage, the most stable sites are first saturated with an ionic binding Cs–Ge; this determines an increasing polarization at the interface with a positive charging of the surface metal atoms; subsequent addition of Cs atoms implies the growth of weaker (covalent) bonds both
because of the polarization layer and because of the progressive saturation of the closest sites Cs–Ge, depending on the mutual distance Cs–Cs at the surface layer. For comparison, in Table 3 we also reported the two components obtained on a thick Cs layer at 150 K: (1) at E =−76.1 eV, we find the bulk contribub tion of clean Cs, with 60% intensity; (2) at E =−76.6 eV, we indicate the surface peak s of clean Cs. The previous identification of the mutual binding established at the Cs/Ge(100) interface was checked by higher exposure of the surface 1 ML Cs/Ge(100) to oxygen. It is worth pointing out that the clean Ge surface is hardly oxidized at all, as we have directly verified up to 105 L oxygen exposure. The spectra as a function of the O 2 exposure are presented in Fig. 3. Here, the new features on the Ge higher binding energy tail are very evident and can be straightforwardly attributed to Ge intermediate oxidation states. Their binding energy was found by a decomposition performed as outlined above for the clean part of the spectrum; however, the high binding energy features, being due to oxidation states, were fitted by Lorentzian curves [28,29]. Typical deconvolutions are presented in Fig. 4. The onset and growth of the components can be followed in Tables 1 and
Table 3 Energy position and relative intensity of the components obtained for best-fitting the Cs 4d spectra for the indicated Cs overlayer and O exposure (1 L=10−6 Torr s) 2 Cs (ML)
Thick (150 K ) 0.10 0.25 0.40 0.75 1.0 1.0 1.0 1.0 1.0 1.0 1.0
O (L) 2
0 0 0.1 0.2 0.5 1 2 5 10 20 50 200
Energy position (eV )
Relative intensity (%)
E (cov.) 1
E (ionic) 2
I
1
I 2
−76.1 (bulk) −76.1 −76.1 −76.1 −76.2 −76.3 −76.2 −76.1 −76.0 −75.9 −75.9 −75.8
−76.6 (surface) −76.6 −76.7 −76.7 −76.6 −76.6 −76.6 −76.6 −76.6 −76.6 −76.6 −76.6
60 15 12 30 46 62 50 35 44 33 30 27
40 85 88 70 54 38 50 65 56 67 70 73
The uncertainty for the energies is less than ±0.1 eV, and that for the intensities is less than ±2%. The parameters used in the fits are: spin–orbit split DE=2.27±0.01 eV; branching ratio=1.33±0.02, for Cs bulk 1.50±0.02.
G. Faraci, A.R. Pennisi / Surface Science 409 (1998) 46–56
Fig. 3. Photoemission intensity of the Ge 3d and Cs 4d core level spectra, for a monolayer of Cs on Ge(100), as a function of the binding energy, at different oxygen exposures. The curves are normalized to the total area.
2 where the binding energy and relative intensity of each feature are reported. We observe that for an increasing O exposure, the part of the spectrum 2 due to the bulk component is reduced, whereas the surface part remains at an unchanged intensity; furthermore, the high binding energy components arising from the Ge intermediate oxidation states (Ge+, Ge2+, Ge3+) show the progressive growth of the second and third feature at the expense of the first. Note, however, that because of the low oxygen exposure, the stoichiometric Ge4+ oxide is not formed until 10 L; afterwards, this stoichiometric oxide rapidly increases in intensity, shifting in energy.
53
Fig. 4. Deconvolution of Ge 3d core level spectra taken for 1ML Cs/Ge(100) at several O exposures (1 L=10−6 Torr s), 2 showing the components obtained for each spectrum. The fit was performed: (1) for the clean part of the Ge spectrum by convolution of two Doniach–Sunjic curves’ spin orbit split, with a Gaussian curve taking into account the experimental resolution and the thermal broadening; and (2) for the oxide features at higher binding energy, by Lorentzian curves. The background (not shown) was fitted by a cubic spline. The solid line is the sum of the contributing curves, and closely fits the experimental points.
Also the Cs 4d spectra were analyzed as specified above. Again, we obtained two components whose energy and intensity are reported in Table 3. These values indicate that the energy position E is quite 2 stable, whereas the covalent feature increases to 0.2 eV with the Cs coverage and decreases afterwards to 0.5 eV for increasing oxygen exposure. The intensity of the ionic component is quite
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high (85%) at a low Cs coverage, reflecting the preferential ionic binding; it decreases at the completion of the monolayer when saturation is reached; further O exposure increases this compo2 nent again: this behavior demonstrates the conversion of the covalent Cs link into ionic-like bonds caused by oxygen inclusion: Ge–CsGe–O–Cs. The present results confirm the mechanism proposed in recent papers [5,23] for explaining the catalytic enhancement of the oxidation rate of a semiconductor promoted by an alkali metal overlayer. The actual Ge surface after the Cs deposition clearly shows an ionic-like binding with the metal: in fact, the surface component U of the clean Ge spectrum reflects the presence of dangling bonds; these belonging to the up atoms of the dimers are responsible of the ionic bonds with the alkali metal as confirmed by the corresponding intensity of the U component vs. Cs coverage; this component shows minor changes both in height and in binding energy up to the completion of the Cs overlayer. The ionic strength of the binding Cs–Ge attributed to the U component is also demonstrated by its behavior in the presence of O ; this feature is 2 reduced only partially, even at the highest oxygen exposure, whereas the S component is already L washed out at 1 L, and the bulk feature undergoes a minor decrease during the Cs coverage but is attenuated of a factor 3 at 200 L. The bulk Ge component does not change either in intensity or in binding energy up to 0.25 ML; at this coverage, an intensity decrease is observed for a new higher (at −29.9±0.1 eV ) component. This feature signs the growth of the first oxidation state catalyzed by the Cs overlayer; an increase of this feature is caused by low oxygen exposure, followed by a strong reduction at higher exposures. This implies the O -induced disruption of the 2 covalent bonds and the growth of the other semiconductor substoichiometric oxidation states. A model was proposed by Abukawa and Kono [30] for K deposition on Si(100)2×1. According to this model, which can be extended to Cs on Ge(100)2×1, the cesium atoms could reside both on top and between the Ge dimer rows. However, the lattice parameter of Ge bidimensional rows is 0.4 nm; this separation in not compatible with Cs–Cs mutual distance (0.523 nm) in the Cs metal,
but it is if the Cs loses its outer electron, establishing an ionic bond. In fact, Cs+ the ionic radius is 0.183 nm in the alkali halides configuration. Taking into account the cesium atomic radius, a full monolayer of Cs could be disposed as claimed by the model of Chao [17] and Abukawa [30] only if the Cs–Ge bonds are of ionic character. The same holds for K on Si. In contrast, the Cs 4d spectrum exhibits two components at different binding energies; the same result was obtained in Ref. [17] for the deconvolution of K on Si(100)2×1. In this context, TEM measurements [31] of Cs on GaAs(110) or InSb(110) with lattice constant 0.4 and 0.458 nm, respectvely, showed zigzag chains at low coverage, whereas the first complete Cs overlayer seems to consist of compressed (0.49 nm) five atoms polygons arranged in a c(4×4) superlattice. The analysis of the Cs 4d spectra reveals two components in agreement with the previous scenario: the first E at low binding energy can be 1 attributed to the weak covalent bond Cs–Ge, the other to the ionic Cs binding with the Ge substrate. The relative intensity confirms the previous interpretation that the dangling bonds of the substrate partly establish ionic links with the metal as long as the coverage is low; for increasing deposition, saturation of this component is reached with a progressive increase in the covalent component. This effect is also corroborated by the corresponding surface densities of Ge(100) and Cs: on the semiconductor surface, we find 6.25×1014 atoms/cm2, whereas for 1 ML Cs, the Cs–Cs bulk distance (5.23) corresponds to 3.6×1014 atoms/cm2, even though scanning electron microscopy [31] gives an interatomic distance up to 7% closer in five-atom Cs polygons induced by the underlying lattice on which Cs is adsorbed. In any case, the lattice mismatch between Cs and Ge suggests that only the closest Ge–Cs couples can produce an ionic binding; the others on different site arrangements are very likely distributed as covalent bonds. The fast modifications observed with O exposure both on the Ge 3d and Cs 4d 2 spectra are the evident symptoms of the catalytic enhancement of the semiconductor oxidation rate promoted by the metal overlayer. In fact, the growth of the Ge oxidation states with the concom-
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itant weakening of the bulk component (see Table 3) can be mainly attributed to a disruption of covalent Ge–Cs bonds capturing an oxygen atom in a double (GeO ) or mixed (Cs–O–Ge) x configuration. These states are in fact more stable not only for the Ge partner but also for the Cs element whose I component (see Table 3) results 2 were amplified just because of the transfer of covalent disrupted bonds (reduction of I as a 1 function of the exposure) towards the ionic Cs+ component (Cs–O–Ge). Furthermore, it is worth observing that the previous effect, as expected, modifies the Cs component at E , shifting up to 1 0.5 eV towards a lower binding energy. As for other substrates [23,24], the metal plays its prevalent role through the polarized surface layer, positively charged as a consequence of the electron transfer of the ionic bonds. This non-local and diffused polarization strongly attracts oxygen, enhancing its sticking coefficient and determining the dramatic oxidation of the substrate. We add some comments about the lineshape of the spectra: as far as the Ge 3d core level is concerned, we get a Lorentzian full width at half maximum (FWHM ) for the bulk contribution c =0.072 eV, b and for the surface c =0.17 eV; for both, the s Gaussian broadening (FWHM ) s =0.45 eV. For G the Cs spectra, the intrinsic line shape is lower than 0.17 eV, whereas the Gaussian extra-broadening goes from 0.80 up to 1.35 eV as a function of the oxygen exposure. This last result is again in the direction indicated above, i.e. the Cs spectra tend to broaden as much as Cs adsorbs oxygen in mixed configuration. In conclusion, we demonstrated by an accurate statystical analysis, the presence of both ionic-like and covalent-like bonds at the Cs/Ge(100)2×1 interface. The first ones are responsible for the polarization layer established at the surface, which, by attracting oxygen, promotes the catalytic oxidation of the substrate. The oxidation proceeds by driving oxygen towards the covalent bonds that are, therefore, modified by the inclusion of an oxygen atom in a mixed link; the mechanism for explaining the enhanced oxidation rate of the Ge(100)2×1 surface catalyzed by a Cs overlayer and the growth of substoichiometric oxidation states was followed at its very beginning.
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Acknowledgements We wish to thank the staff of The Sincrotrone Trieste for the excellent assistance during the experiments. In particular, we appreciated the collaboration of C. Comicioli, C. Crotti, S. La Rosa, M. Peloi, and M. Zacchigna. This work is supported by the Istituto Nazionale di Fisica della Materia and the Istituto Nazionale di Fisica Nucleare.
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