3828
J. Phys. Chem. B 2001, 105, 3828-3837
Symmetry and Electronic Structure of Benzene Adsorbed on Single-Domain Ge(100)-(2×1) and Ge/Si(100)-(2×1)† A. Fink, D. Menzel, and W. Widdra* Physik-Department E20, Technische UniVersita¨ t Mu¨ nchen, 85747 Garching, Germany ReceiVed: October 9, 2000; In Final Form: NoVember 28, 2000
The electronic structure of benzene adsorbed on single-domain Ge(100)-(2×1) and on a single-domain Ge monolayer grown on Si(100)-(2×1) has been investigated by angle-resolved ultraviolet photoemission spectroscopy (ARUPS) using linearly polarized synchrotron radiation and by temperature-programmed desorption (TPD) spectroscopy. For benzene chemisorption on both surfaces C2V adsorption symmetry is found on the basis of the ARUPS data. The detailed analysis indicates a flat-lying molecule which is di-σbonded to a Ge-Ge dimer and which exhibits a 1,4-cyclohexadiene-like electronic structure. The similarity of the ARUP spectra for benzene on Ge(100), on Ge/Si(100), and on Si(100) indicates nearly identical electronic structures. Benzene TPD spectra on a Ge monolayer on Si(100) show two desorption peaks at 311 and 369 K which are interpreted as desorption from terrace and step sites, respectively. Similarly, benzene desorption on Ge(100) leads to desorption peaks at about 230 and 250 K, again interpreted as desorption of chemisorbed benzene from terrace and step sites. Additionally, a weakly bound benzene species is found on Ge(100) which desorbs between 155 and 220 K and which is absent on the other surfaces. ARUPS data indicate C1 adsorption symmetry and an electronic structure similar to that of gas-phase benzene for this species.
I. Introduction The adsorption of simple unsaturated hydrocarbon molecules on semiconductor surfaces has been a focus of interest in recent years.1 Especially on the technologically important Si(100)(2×1) surface many experimental (e.g., refs 2-7) and theoretical (e.g., refs 8-11) studies have been reported. Also benzene as a simple model molecule with an aromatic π system has been investigated recently.12-20 By temperature-programmed desorption (TPD) experiments it was found that benzene adsorbs nondissociatively on the Si(100) surface with a saturation coverage of one adsorbate molecule per two surface unit cells.12,15 Previous high-resolution energy loss spectroscopy revealed both sp2- and sp3-hybridized carbon atoms in the adsorbed molecule as well as the existence of CdC double bonds.12 Therefore, it was proposed that benzene adsorption on Si(100) leads to the formation of σ bonds to the substrate, indicating either a 1,3-cyclohexadiene- or a 1,4-cyclohexadienelike adsorption structure.12 These two adsorption models are shown in parts a and b, respectively, of Figure 1. In contrast, theoretical studies had proposed either a tilted cyclohexenelike adsorption structure 4-fold-bonded to a single cleaved SiSi dimer13 (not shown in Figure 1) or a slightly distorted pedestal biradical C2V symmetric structure, 4-fold-bonded to two neighboring Si-Si dimers with no remaining CdC double bonds (Figure 1c).14 In a combined experimental and theoretical study15 it was unambiguously shown that benzene adsorbs in C2V symmetry and that it is di-σ-bonded on a single Si-Si dimer with a 1,4-cyclohexadiene-like electronic structure as shown in Figure 1b. By comparison of theoretical and experimental vibrational spectra which were measured by high-resolution †
Part of the special issue “John T. Yates, Jr. Festschrift”. * To whom correspondence should be addressed. E-mail: widdra@ e20.physik.tu-muenchen.de. Fax: +49-89-289-12338.
electron energy loss spectroscopy, this adsorption model has been confirmed recently.20 Much attention has been paid to the variation of hydrocarbon adsorbates on the Si(100)-(2×1) surface to understand the basic bonding mechanisms and the electronic structure of the adsorbate-substrate complex. Here we approach this goal by a variation of the semiconductor substrate: We investigated the benzene adsorption on Ge(100)-(2×1) and on a pseudomorphic Ge monolayer on Si(100)-(2×1) and will compare the results with benzene adsorption on Si(100)-(2×1). The Ge(100) surface reconstructs in a (2×1) structure as the Si(100) surface, but possesses a 4% wider lattice constant and an about 6% longer dimer Ge-Ge distance.21,22 Its geometry and its electronic structure are similar to those of the Si(100) surface, maintaining the reactive (2×1) tilted dimer structure with the characteristic dangling bond states. Again similar structures have been reported for the pseudomorphic Ge layer on Si(100).23,24 II. Experimental Details The experiments presented here were performed in an ultrahigh-vacuum two-chamber system with a base pressure of 5 × 10-11 mbar. The preparation chamber was equipped with a quadrupole mass spectrometer with a Feulner cap,25 a lowenergy electron diffraction (LEED) system, a multicapillary doser, and an argon ion sputtering gun. For the preparation of the pseudomorphic Ge layer on Si(100)-(2×1) a water-cooled electron bombardment Ge evaporator (Omicron Focus EFM3) was used, which allows a pressure below 5 × 10-10 mbar to be maintained during evaporation. In the second chamber a homebuilt multiangle electron energy analyzer allowssat a selected azimuthal anglesthe simultaneous detection of photoelectrons at emission angles between -10° and +90° with respect to the surface normal.26,27 It has a polar angular resolution of (2°, an azimuthal acceptance of (3°, and a resolving power of E/∆E
10.1021/jp003698b CCC: $20.00 © 2001 American Chemical Society Published on Web 02/08/2001
Structure of Benzene Adsorbed on Ge and Ge/Si
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3829
Figure 1. Adsorption models for benzene on Si(100)-(2×1), Ge/Si(100)-(2×1), and Ge(100)-(2×1) in side and top views: (a) tilted 1,3-cyclohexadiene-like adsorption complex with Cs adsorption symmetry; (b) 1,4-cyclohexadiene-like adsorption complex with C2V adsorption symmetry (butterfly model); (c) pedestal biradical adsorption complex with C2V adsorption symmetry (tetra-σ-bonded).
) 180. The angle-resolved ultraviolet photoemission (ARUP) spectra were recorded at the TGM-1 beamline (tunable energy range 20-120 eV) at the BESSY-I synchrotron radiation facility using linearly polarized light with a polarization degree of approximately 85%.28,29 The overall energy resolution for the ARUP spectra shown here was set to be better than 200 meV. The Ge and the Si samples were mounted in a multilayer bonding technique via thin platinum and silver interlayers on a tantalum plate.30 The interlayers provide good thermal contact between the tantalum plate and the semiconductor sample and allow homogeneous heating to the melting point of Ge by passing an ac current directly through the tantalum plate. Also fast cooling to 85 or 30 K is possible using liquid nitrogen or liquid helium as coolant, respectively. The temperature was measured by a chromel-alumel thermocouple spot-welded on the back of the tantalum holder and calibrated by desorption of rare gas multilayers31 and the well-investigated hydrogen desorption from the various substrates.32,33 Note that we used Ge and Si samples with an intentional miscut of 5° ( 0.5° toward the [011] direction, which allows the preparation of wellordered single-domain surfaces. Hence, after several cleaning cycles of argon ion sputtering at both low (90 K) and elevated (750-800 K) temperatures44 and with subsequent annealing (1100-1150 K) and slow (-2 K/s) cooling, a sharp (2×1) LEED pattern was observed. The suppression of the 90° rotated (1×2) minority domain, which would be equally present on a flat (100) crystal, was better than 90%. A single-domain surface is necessary to take full advantage of the possibilities of ARUPS experiments using linearly polarized synchrotron radiation. The pseudomorphic Ge layer was prepared by evaporating Ge onto a clean single-domain Si(100)-(2×1) sample, while the sample temperature was kept at 500 K. Calibration of the Ge flux was done by Ge 3d XPS (X-ray photoemission spectroscopy) measurements and titration experiments using TPD as described elsewhere.34 In all experiments presented here benzene was adsorbed at 90 K, and the sample was then heated moderately to desorb benzene multilayers. The ARUPS measurements were performed at a sample temperature of 90 K. The benzene coverages were determined by a quantitative analysis of the TPD
Figure 2. Thermal desorption spectra of benzene chemisorbed on single-domain Ge(100) (circles), 0.7 ML of Ge/Si(100) (triangles), and Si(100) (squares), respectively. The heating rate was 2.5 K/s for Ge(100) and 5 K/s for the others. The initial benzene coverages are 1.0 ML for C6D6/Ge/Si(100) and C6D6/Si(100) and approximately 0.5 ML for C6D6/ Ge(100).
spectra. After the desorption peaks of the benzene monolayer have been identified, its area can be used to calibrate other desorption peaks or spectra directly in units of monolayers (MLs). In the case of C6D6/Ge(100) the identification of the monolayer in the TPD spectra is slightly more complicated, and additional information is needed to determine the coverages. The procedure will be discussed at the end of section III.B. III. Thermal Desorption Spectroscopy A. Benzene Desorption from Various Substrates. TPD spectra for benzene (C6D6) adsorbed on Ge(100), on a Ge monolayer on Si(100), and on Si(100) at 90 K are shown in Figure 2. Benzene adsorbed on Si(100) for which the data are marked in Figure 2 by squares reveals two desorption peaks at 432 and 501 K in the monolayer regime. The two peaks have been attributed to terrace and step site adsorption species,
3830 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Figure 3. Thermal desorption spectra of benzene adsorbed on singledomain Ge(100) for different initial coverages (0.09, 0.12, 0.21, 0.47, 0.90, and 2.63 MLs) with a heating rate of 2.5 K/s.
respectively.15 No desorption of hydrogen or other hydrocarbon fragments was found, indicating a completely reversible molecular desorption. In Figure 2 the TPD spectrum of C6D6/Ge(100) is shown for an initial benzene coverage of approximately 0.5 ML. It reveals mainly benzene desorption from a chemisorbed species, since the saturated benzene monolayer contains a chemisorbed as well as a weakly bonded benzene species, as will be shown in section III.B and as is supported by the photoemission data presented in section IV.B. Two peaks at 202 and 234 K and a shoulder on the high-temperature side are discernible. The peak at 234 K and the shoulder are attributed again to chemisorbed benzene species adsorbed at terrace and step sites, respectively. The peak at 202 K belongs to a weakly bound species within the monolayer range (see below). Again, no desorption of hydrogen or other hydrocarbon fragments is found. The curve marked with triangles in Figure 2 represents TPD data for benzene adsorbed on a Ge monolayer (approximately 0.7 ML of Ge), which has been evaporated on a single-domain Si(100)-(2×1) surface. Four desorption features are clearly discernible. The desorption peaks at 311 and 369 K represent benzene adsorbed on the Ge monolayer. The two peaks are attributed to benzene adsorbed on the terrace or at the steps of the vicinal Ge/Si(100) substrate as proposed for benzene on Si(100).15 The peak at 422 K is attributed to benzene molecules adsorbed directly on the Si(100) surface, which is still present due to an incomplete Ge layer. Furthermore, a small shoulder at approximately 225 K is visible in the desorption spectra which we attribute to a minority benzene species desorbing from thicker Ge islands, which result from imperfect Ge growth on Si(100). The significantly different desorption temperatures of about 430, 310, and 230 K as shown in Figure 2 for benzene on Si(100), Ge/Si(100), and Ge(100), respectively, demonstrate rather different desorption energies if one assumes similar45 desorption preexponentials. For an assumed constant preexponential of 1013 s-1, which is the result of a more detailed TPD study for benzene on Si(100),35 the corresponding desorption energies are about 1.2, 0.8, and 0.5 eV, respectively. The assumption of an equal preexponential for all three systems is of course a rough simplification. B. Benzene on Ge(100). Benzene TPD spectra for different initial benzene coverages on Ge(100) with a heating rate of 2.5 K/s are presented in Figure 3. We divide the desorption spectra into three desorption regions as indicated by the dashed lines in Figure 3. In the region below 155 K two peaks at 143 and 151 K are discernible which are attributed to multilayer and
Fink et al.
Figure 4. Benzene coverage versus exposure for benzene adsorption on Ge(100)-(2×1) at 90 K. The data (solid circles) correspond to the spectra in Figure 3. The solid line represents a linear fit.
bilayer desorption of physisorbed benzene, respectively. Similar multilayer desorption has been observed for benzene on Si(100).15 For temperatures above 155 K we find four main desorption peaks at 183, 202, 234, and 252 K. On the basis of the photoemission results (see below), we assign the desorption above 220 K to chemisorbed benzene and the desorption between 155 and 220 K to a weakly bound benzene species. The two desorption peaks at 234 and 252 K are tentatively attributedsin analogy to benzene on Si(100)sto chemisorbed benzene on terrace and step sites, respectively. The sum of the coverages of the chemisorbed and the weakly bound species on Ge(100) equals approximately the coverage of the benzene monolayer on Si(100) (one benzene molecule per two (2×1) unit cells36). The chemisorbed benzene contributes approximately 0.4 ML and the weakly bound species about 0.6 ML. This absolute coverage calibration is discussed in the following together with the sticking coefficients. The benzene coverage on Ge(100) as a function of the benzene exposure at a surface temperature of 90 K is shown in Figure 4. The benzene coverage increases linearly with the benzene exposure within an experimental error of 0.05 ML as marked by the solid line. Therefore, we conclude that the sticking coefficient remains constant for all coverages including multilayer adsorption similarly as has been found for benzene on Si(100).35 Assuming equal sticking coefficients (most likely close to unity) for multilayer adsorption on both surfaces, we can determine the absolute coverage with an absolute error below 20% from the respective exposures since the experimental dosing conditions are equal in both cases. IV. Angle-Resolved Photoemission A. Chemisorbed Benzene on Ge(100)-(2×1) and Ge/ Si(100)-(2×1). 1. Energy LeVels and Peak Assignment. In Figure 5 UPS data for chemisorbed C6D6 on Ge(100), Ge/Si(100), and Si(100) are compared. In the lower part of Figure 5 a spectrum of the clean Si(100)-(2×1) surface is shown for comparison. All spectra were recorded at a photon energy of 50 eV with normal light incidence and polar-angle-integrated photoelectron detection. The UP spectra for benzene adsorbed on the three different semiconductor substrates are rather similar with respect to the peak positions and even the relative intensities. Note that the coverage for the C6D6/Ge(100) is only 0.4 of that of the two other systems. For benzene chemisorbed on Si(100) it has been unambiguously shown that the adsorbate electronic structure is significantly altered from that of gas-phase benzene but close to that of gas-phase 1,4-cyclohexadiene.15 To avoid
Structure of Benzene Adsorbed on Ge and Ge/Si
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3831 TABLE 1: Binding Energy and Symmetry of the Molecular Orbitals of Weakly Bound and Chemisorbed Benzene on Ge(100) and Ge/Si(100)a physisorbed C6H6 D6h 1e2u (π*) 1e1g (π)
chemisorbed C6H6
BE (eV)
D2h
3e2g
∼5.8
1a2u
∼6.9
BE (eV)
a1
∼2.3
1b2g(σ: Si-C) b1 1b3g b2
∼8.4 4.0
a1 a2
6.5 5.7
1b1u (σ: Si-C) a1
∼8.9
4b2u 5b3u
a1 b1
∼8.4 ∼7.9
2b1u (π) 3.9
C2V
{6a3b
g 1g
{
3e1u
∼8.1
1b2u
∼8.8
3b2u
b2
∼8.9
2b1u
∼9.9
4b3u
b1
10.2
3a1g
11.3
5ag
a1
11.2
4ag 2b1g
a1 a2
14.1 12.9
3u
b1 b2
∼17.1 ∼17.0
a1
19.9
2e2g
13.4
2e1u
17.1
2a1g
∼20.3
{ {3b2b
2u
3ag
a
Figure 5. Angle-integrated UP spectra of chemisorbed benzene on Ge(100), Ge/Si(100), and Si(100) recorded at a photon energy of 50 eV with normal light incidence (ExDx). The benzene coverages correspond to 1.0 ML for C6D6/Ge/Si(100) and C6D6/Si(100) and approximately 0.4 ML for C6D6/Ge(100). A spectrum of the clean Si(100)-(2×1) surface is shown for comparison in the lower part. The molecular orbitals are labeled according to the nomenclature of gasphase 1,4-cyclohexadiene.
unnecessary relabeling, we will denote in the following all adsorbate-derived photoemission peaks for chemisorbed benzene using the nomenclature of the 1,4-cyclohexadiene molecule, which has D2h symmetry in the gas phase. The correspondence of the molecular orbital nomenclature for D6h and D2h symmetries is indicated in Table 1. For the adsorbed benzene molecule one expects to find 16 valence photoemission peaks according to the 30 valence electrons of benzene and the 2 dangling bond electrons of the substrate dimer which are involved in the chemisorption. However, only 14 emission features are resolvable due to broadening by integrating over the dispersion, lifetime broadening, and the finite line width of the experiment. In detail 12 peaks are clearly resolvable, while the peak position of the 5b3u orbitals and the splitting of the 2b2u and 3b3u orbitals can only be estimated in some spectra under certain emission angles (not shown here). The energetic positions of the orbitals on all three investigated substrates are determined identically within experimental error (∼0.15 eV) to 19.9 eV for 3ag, 17.1 eV for 3b3u, 17.0 eV for 2b2u, 14.1 eV for 4ag, 12.9 eV for 2b1g, 11.2 eV for 5ag, 10.2 eV for 4b3u, 8.9 eV for 1b1u and 3b2u, 8.4 eV for 1b2g and 4b2u, 7.9 eV for 5b3u, 6.5 eV for 6ag, 5.7 eV for 3b1g, 4.0 eV for 1b3g, and 2.3 eV for 2b1u. The peak positions are summarized in Table 1 together with the expected orbital symmetries in the C2V adsorption symmetry. As can be seen in Figure 5 the degeneracy of the benzene gas-phase e orbitals is lifted upon adsorption on all three substrates. The former gas-phase 2e2g and 3e2g orbitals now split into the 4ag, 2b1g and 6ag, 3b1g orbitals, respectively. Also the other gas-phase e orbitals are split upon chemisorption as has been discussed in detail for C6D6/Si(100).15,19 Here the peak assignment is based on a symmetry-resolved comparison
The nomenclature of gas-phase benzene in D6h symmetry and 1,4cyclohexadiene in D2h symmetry is used for the weakly bound and chemisorbed benzene, respectively. The experimental error for the binding energies is approximately (0.15 eV.
between theoretical and experimental data for Si(100).15 The two emission features visible in Figure 5 between 4.0 and about 2.3 eV are denoted as 1b3g and 2b1u orbitals; they correspond to the two highest occupied molecular orbitals (HOMOs). They dominantly consist of the symmetric and antisymmetric linear combinations of the remaining two π orbitals attributed to the CdC double bonds on the opposite sides of the carbon ring. They are characteristic of the 1,4-cyclohexadiene-like electronic structure. Only one of them, 1b3g, stems from the degenerate gas-phase 1e1g orbital. The other corresponds to the formerly unoccupied 1e2u π* orbital. Note that the HOMOs of the benzene adsorbate do not represent the adsorbate-substrate bonds as is the case for ethylene on Si(100).7,37 For benzene, the 1b1u and 1b2g orbitals have significant SisC bonding character, which are derived from two of the gas-phase benzene π orbitals.15 The emission feature at approximately 0.9 eV denoted with DB for C6D6/Si(100) is attributed to the remaining unreacted dangling bonds since only every second surface dimer is occupied. In the case of benzene on Ge(100) and Ge/Si(100) this feature is not visible because the Ge-DB states are slightly lower in energy and therefore experimentally more difficult to detect. 2. Adsorption Symmetry of the Chemisorbed Species. Before we start to analyze the angle-resolved photoemission data according to dipole selection rules, we introduce the experimental measurement geometries used here. For all experimental data shown in the following, light incidence was parallel to the surface normal. The linear polarization (E B vector) was aligned either parallel (Ex) or perpendicular (Ey) to the surface dimers. The photoelectrons were detected in a plane perpendicular to the surface, and either parallel (Dx) or perpendicular (Dy) to the surface dimers. Out of these four highly symmetric experimental geometries two, ExDx and EyDy, are often referred to as “even” and two, ExDy and EyDx, are often referred to as “odd” photoemission geometries depending on the light polarization vector being within the detection plane or perpendicular to it. This nomenclature stems from the fact that only orbitals which are eVen (odd) with respect to the plane formed by the
3832 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Fink et al.
Figure 6. ARUP spectra of chemisorbed benzene on Ge(100)-(2×1) (coverage approximately 0.4 ML) measured at a photon energy of 50 eV in even (ExDx and EyDy, solid lines) and odd (ExDx and EyDx, symbols) geometries. Pairs of spectra are shown for emission angles ranging from 0° (normal emission, topmost pair of spectra) to 80° (grazing emission).
Figure 7. ARUP spectra of chemisorbed benzene on Ge/Si(100)-(2×1) (benzene coverage 1.0 ML) measured at a photon energy of 50 eV in even (ExDx and EyDy, solid lines) and odd (ExDy and EyDx, symbols) geometries. Spectra are shown for emission angles ranging from 0° (topmost pair of spectra) to 80° (grazing emission). The Ge coverage is about 0.7 ML.
photoelectron and the light polarization show nonzero photoemission intensity. For details see ref 15. In the following we simultaneously analyze the ARUP spectra of chemisorbed C6D6 on Ge(100) and on the Ge monolayer on Si(100) (Ge coverage 0.7 ML). The spectra which are presented in Figures 6 and 7 for even (ExDx and EyDy, marked by solid lines) and odd (ExDy and EyDx, marked by open symbols)
measurement geometries were recorded at a photon energy of 50 eV. The spectra are shown for polar emission angles ranging from 0° (topmost spectra) to 80° (grazing emission) with respect to the surface normal. Similar series of spectra have been recorded for photon energies of 41 and 35 eV and have been included in the considerations for the complete symmetry analysis but are not shown here.
Structure of Benzene Adsorbed on Ge and Ge/Si For benzene adsorption on any of the three semiconductor (100) surfaces the highest possible adsorption symmetry is C2V. In this case the molecular orbitals can be characterized by their a1, a2, b1, or b2 symmetry. The four experimental measurement geometries for which the data are shown in Figures 6 and 7 are chosen in such a way that a symmetry determination of the molecular orbitals is possible based on dipole selection rules. In the following we will focus on orbitals which are clearly resolved in the spectra and do not coincide with other orbitals of different symmetry. We start with the deepest 3ag orbital at 19.9 eV, which is a1 symmetric in the case of the highest possible adsorption complex symmetry of C2V. According to Figures 6 and 7, it possesses intensity in off-normal emission only in the even geometries (ExDx and EyDy), while in odd geometries (ExDy and EyDx) hardly any emission intensity is observable. This observation is compatible with C2V adsorption symmetry of a flat-lying benzene molecule. Although the subsequent 3b3u and 2b2u orbitals at about 17.0 eV, which are of b1 and b2 symmetry type, are not resolved, their emission characteristics can be used to exclude a C2V symmetric benzene molecule standing on edge. For a standing benzene molecule, i.e., with the carbon ring aligned perpendicular to the surface, a splitting of the degenerate gas-phase 2e1u orbital into a1 and b1 states is predicted and not into b1 and b2 states as for a flatlying molecule. Since equal intensities are observed in both odd geometries in Figures 6 and 7, a standing benzene molecule can be ruled out for C6D6 on both Ge(100)-(2×1) and Ge/Si(100)-(2×1). The formerly degenerate gas-phase 2e2g and 3e2g orbitals split into the pairs 4ag, 2b1g at 14.1 and 12.9 eV and 6ag, 3b1g at 6.5 and 5.7 eV. Both pairs show strong complementary photoemission intensities with respect to the even and odd geometries as seen in Figures 6 and 7. The a1 symmetric 4ag and 6ag states are predominantly visible in the even spectra. In contrast the a2 symmetric 2b1g and 3b1g states show stronger emission in the odd geometry. This is also compatible with a C2V adsorption symmetry of a flat-lying benzene molecule. We note that besides this dominant emission the a1 (a2) symmetric orbitals also show weak emission (with an intensity below approximately 20% of the dominant one) in the odd (even) geometries. On the basis of the dipole selection rules, this emission is not allowed for a C2V symmetric adsorption complex. However, there are several reasons for our specific situation which can lead to weak emission in these forbidden geometries as have been discussed in detail in ref 15. Briefly, due to the incomplete linear light polarization of the synchrotron radiation28,29 and the necessary use of vicinal surfaces for which the microscopic and macroscopic surface normals deviate by about 5° and for which the dimer rows are shifted from one terrace to the next by a quarter (2×1) unit cell, the weak photoemission intensities can be understood.15 Additionally, remnants of the (1×2) rotated domains can also contribute to some of the intensities. Keeping these considerations in mind, we return to the discussion of the adsorbate symmetry. The 5ag (11.2 eV) orbital exhibits strong emission in the even geometries, as expected for a1 symmetric orbitals. The 4b3u orbital at 10.2 eV can be seen nearly exclusively in the ExDx and ExDy spectra of Figures 6 and 7, where the excitation was done with light polarized parallel to the surface dimers (Ex). This corresponds to the expected b1 orbital symmetry for a flatlying di-σ-bonded benzene molecule. The five orbitals in the energy range of 8.9-7.9 eV (3b2u, 1b1u, 4b2u, 1b2g, and 5b3u) are not clearly resolvable and complicate a symmetry analysis for these orbitals. The discussion of the ARUPS data so far revealed evidence
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3833
Figure 8. Comparison of ARUP spectra in even (ExDx and EyDy) and odd (ExDy and EyDx) geometries for chemisorbed C6D6 on Ge(100)(2×1), Ge/Si(100)-(2×1), and Si(100)-(2×1). The spectra were measured with a photon energy of 50 eV, normal incidence, and an electron emission angle of 50°. The benzene coverages are 1.0 ML for C6D6/ Ge/Si(100) and C6D6/Si(100) and approximately 0.4 ML for C6D6/Ge(100). Each pair of a1 and a2 orbitals (4ag (a1), 2b1g (a2) and 6ag (a1), 3b1g (a2)) is indicated by arrows.
for a flat-lying benzene-derived molecule with C2V symmetry. There are two adsorption models in Figure 1, the butterfly (b) and the pedestal model (c), which are compatible with these experimental results. The electronic structures of both adsorption complexes have been studied previously15,19 for adsorption on Si(100). The most striking difference is the sequence of the a1 and a2 orbitals originating from the formerly degenerate gasphase 2e2g and 3e2g orbitals. Their splitting upon chemisorption into the 4ag and 2b1g orbitals as well as into the 6ag and 3b1g orbitals turns out to be reversed for the two different models. This can be understood by the fact that in the butterfly model the a1 orbital can interact with the σ bonds to the substrate and is therefore lowered in energy, while the a2 orbital cannot.15,19 The opposite situation is encountered for the pedestal adsorption model. In Figure 8 ARUPS data are compared for chemisorbed C6D6 on Ge(100)-(2×1), Ge/Si(100)-(2×1), and Si(100)-(2×1). All spectra were measured with a photon energy of 50 eV, normal incidence, and an electron emission angle of 50°. The formerly degenerate gas-phase 2e2g and 3e2g orbitals are indicated by arrows. On the basis of dipole selection rules, we identify in Figure 8 the corresponding a1 orbitals to be lower in energy than the a2 orbitals for benzene adsorption on all three investigated substrates. This is clear evidence that benzene is chemisorbed in a 1,4-cyclohexadiene-like adsorption structure as shown in Figure 1b, di-σ-bonded to a single surface dimer, on Ge(100)-(2×1) and Ge/Si(100)-(2×1). 3. HOMOs of Chemisorbed Benzene. The two highest molecular orbitals of chemisorbed benzene, 1b3g and 2b1u, which correspond to the symmetric and antisymmetric linear combinations of the two π orbitals in the 1,4-cyclohexadiene-like adsorption complex, will be discussed in more detail now. The ARUPS data in Figures 6 and 7 show significant emission for the 1b3g orbital in both even, ExDx and EyDy, geometries. The 2b1u orbital, which is located at about 2.3 eV, seems to show emission in all measurement geometries. However, on closer
3834 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Figure 9. Comparison of ARUP spectra for C6D6 chemisorbed on Ge(100)-(2×1), Ge/Si(100)-(2×1), and Si(100)-(2×1) (solid symbols) and the corresponding clean surface (open symbols). The spectra were recorded at a photon energy of 50 eV, normal light incidence (ExDx), and an electron emission angle of 50°. The benzene coverages are 1.0 ML for C6D6/Ge/Si(100) and C6D6/Si(100) and approximately 0.4 ML for C6D6/Ge(100).
inspection of the spectra one finds different peak positions in the different spectra. This can best be seen in Figure 6. Note that dispersion of this orbitalsespecially for the dilute chemisorbed benzene on the Ge monolayerscan be excluded as an explanation for the different energies. Rather the shift has to be explained by photoemission from different states. Note that the strongest relative emission in the range from 1 to 3 eV for the odd, ExDy and EyDx, geometries is found in Figure 6, which corresponds to a lower benzene coverage (about 0.4 ML). The photoemission intensity in this range is at least partly related to substrate and back-bond emission. E.g., for the clean Si(100) surface the strong emission around 2 eV in odd geometries is attributed to the B1 state.38 In general substrate emission is rather strong in the energy region between 0.5 and 5.5 eV.38,39 For one measurement geometry, ExDx, the direct comparison of the photoemission of the three clean substrates (open symbols) with the corresponding adsorbate systems (solid symbols) is shown in Figure 9. It shows ARUP spectra recorded at a photon energy of 50 eV with normal light incidence and an electron emission angle of 50°. For the clean Si(100), Ge/Si(100), and Ge(100) surfaces the dimer dangling bond states can be nicely seen under these experimental conditions at about 0.9, 1.4, and 1.5 eV, respectively. Figure 9 shows clearly that there are no significant changes in the energy splitting of the HOMOs within the experimental error visible for the three different benzene adsorption systems. The differences which seem to exist in Figure 5 are most likely due to varying emission intensity from substrate states. Since the HOMOs are formed out of the two π bonds on both sides of the 1,4-cyclohexadiene-like carbon ring, they can be taken as an indicator of the carbon ring structure. Due to adsorption onto a surface dimer the two halves of the carbon ring are bent away from the surface as shown in Figure 1b. The dimer length is different for Si(100) and Ge(100),21,22 and the bond strength varies considerably for the investigated substrates as has been shown above (section III.A). Because tetrahedral bonding angles of the two sp3-hybridized carbon
Fink et al.
Figure 10. ARUP spectra in even (ExDx) and odd (ExDy) geometries for the chemisorbed (coverage 0.4 ML) and weakly bound species (coverage 1.0 ML) on Ge(100)-(2×1) compared with an angleintegrated spectrum of benzene multilayers. All spectra were recorded at 50 eV photon energy, normal incidence, and an electron emission angle of 50° for the angle-resolved spectra. The energy positions of the split a1 and a2 orbitals or the corresponding degenerate gas-phase e orbitals are marked with arrows. The positions of the benzene multilayer peaks are marked with bars and labeled according to the gas-phase benzene nomenclature.
atoms which are bonded to the substrate dimers are strongly favored, one might speculate that the different dimer lengths impose different strains on the carbon ring. This should affect the butterfly-like upward bending of the two halves of the carbon ring and also influence the splitting or the amount of chemical shift of the 1b3g and 2b1u π bonds. However, since no significant energetic differences can be determined for these orbitals, we conclude that the carbon ring structure for chemisorbed benzene is electronically rather similar on all investigated substrates. B. Weakly Bound Benzene on Ge(100)-(2×1). 1. Comparison of Weakly Bound and Chemisorbed Benzene. As shown in section III.B a weakly bound benzene species exists on Ge(100), in contrast to C6D6 on Si(100) and on Ge/Si(100). It clearly differs from the second layer species found also on Si(100) and on Ge/Si(100) since it desorbs at higher temperatures (see Figure 3). To illuminate its electronic structure and to analyze the differences compared to the chemisorbed benzene species on Ge(100), ARUP spectra in the even and odd geometries (solid lines and circles, respectively) are shown in Figure 10. The spectra are compared with an angle-integrated spectrum for benzene multilayers which is shown in the upper part of Figure 10. For the benzene multilayer spectrum the peak positions are marked according to gas-phase data.40 All spectra were recorded at 50 eV photon energy with normal light incidence and an electron emission angle of 50° for the angle-resolved spectra. Whereas the spectra for the chemisorbed species correspond to about 0.4 ML, the spectra containing the weakly bound species correspond to 1 ML. The energy positions of the degenerate gas-phase e2g orbitals, which split upon chemisorption into orbitals with a1 and a2 symmetry as discussed above, are indicated with arrows in Figure 10. Note for the following discussion that the spectra for the weakly bound species actually represent the sum of the photoemission intensities of the chemisorbed and weakly bound species. By a comparison of
Structure of Benzene Adsorbed on Ge and Ge/Si
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3835 benzene species which is adsorbed in C1 symmetry and is close to gas-phase benzene. V. Discussion and Summary
Figure 11. ARUP spectra for the benzene monolayer (weakly bound + chemisorbed species) on Ge(100)-(2×1) measured at a photon energy of 50 eV and normal incidence in even (ExDx and ExDy, solid lines) and odd (ExDy and EyDx, symbols) geometries. Spectra are shown for emission angles ranging from 0° (topmost) to 80° (grazing emission).
the spectra for the three different systems in Figure 10 we find that the spectra of the weakly bound species resemble more closely the multilayer spectrum of physisorbed benzene than the spectra of chemisorbed benzene. As can be best seen for the 2e2g orbital the e2g orbitals are still degenerate for weakly bound benzene on Ge(100). From this comparison it is concluded that the geometric and electronic structures of the weakly bound benzene species, which desorbs in a temperature range of 155-220 K, closely resemble those of physisorbed benzene in the multilayer. Since we do not detect any significant differential shift of the π orbitals with respect to the σ orbitals and also no splitting of the π 1e1g orbital, only weak π bonding can be present. 2. Adsorption Symmetry of the Weakly Bound Species. In Figure 11 ARUP spectra for a benzene monolayer (weakly bound plus chemisorbed species) on Ge(100) are shown for the same experimental conditions as in Figure 6 for the dilute benzene layer. The azimuthal and polarization dependencies are much weaker than in the spectra of the chemisorbed benzene species presented in Figure 6. Since we have shown in Figure 10 that the spectra of the weakly bound benzene species closely resemble the electronic structure of gas-phase benzene, we will use heresin contrast to the situation for chemisorbed benzenes the nomenclature of the gas-phase molecule. All benzenederived orbitals show significant emission for all geometries and electron emission angles. There is hardly any azimuthal dependence seen in Figure 11, especially considering the oriented chemisorbed species which is also present here. This rules out C2V or Cs symmetry for the weakly bound species because for both symmetries at least one measurement geometry exists where emission is observed but not allowed according to the selection rules. Especially the 3a1g, 2b1u, and 1b2u orbitals, which are not degenerate and clearly resolved at 11.3, 9.9, and 8.8 eV, substantiate this statement. To decide whether C2 symmetry is compatible with the ARUPS data in Figure 11, one has to examine the normal emission spectra. Dipole selection rules allow normal emission in C2 symmetry for b orbitals but not for a orbitals. The 3a1g orbital at 11.3 eV, which possesses a symmetry (regardless of an adsorption geometry parallel or perpendicular to the surface), shows normal emission in all geometries. Thus, C2 symmetry must be excluded also. We conclude that the ARUPS data give evidence for a weakly bound
On all three substrates, Si(100), Ge/Si(100), and Ge(100), we find chemisorbed benzene with a 1,4-cyclohexadiene-like electronic structure. The binding energies of the molecular orbitals are identical within experimental error for adsorption on the three different substrates (Table 1). Therefore, we conclude that the different lattice constants and especially the changed dimer bond lengths have no significant influence on the adsorbate electronic structure here. However, the exact binding energies of the 1b1u and the 1b2g orbitals, which are expected to correspond in a simplified picture to the symmetric and antisymmetric linear combinations of the two Si-C and Ge-C bonds, respectively, are experimentally difficult to extract due to the overlap of several orbitals in this energy region. On the other hand, significantly different desorption temperatures are found for benzene on Si(100), Ge/Si(100), and Ge(100), which we attribute to different Si-C (Ge-C) bond strengths. The observed sequence of the adsorbate-substrate bond strength, stronger on Si than on Ge, is as one might expect for the variation within a column of the periodic table; e.g., the homonuclear C-C, Si-Si, and Ge-Ge bond enthalpies amount to 606, 327, and 264 kJ/mol, respectively.41 The hydrogen (monohydride) desorption temperatures on C(100), Si(100), and Ge(100) shift similarly down in this sequence from above 1000 K to about 800 and 600 K, respectively.32,33 However, the benzene desorption temperature on the Ge monolayer on Si(100),42,43 which lies between the values found for Si(100) and Ge(100), shows further that the strength of the adsorbatesubstrate bond is controlled not only by the local Si-C or Ge-C bonds. The strain and the differences of the substrate electronic structures have an effect as well. In addition to the chemisorbed 1,4-cyclohexadiene-like benzene we have found a weakly bound benzene species on Ge(100). Similar species are absent on the Ge/Si(100) and Si(100) surfaces. The occurrence of the weakly bound species is related to the absence of a full monolayer of chemisorbed benzene (which is expected to be one molecule per two (2×1) unit cells) on Ge(100).46 The higher desorption temperature of the weakly bound benzene compared to bilayer and multilayer benzene as shown in Figure 3 could have different explanations. Different sticking probabilities, which also affect via detailed balance the desorption, can be ruled out as has been discussed in connection with Figure 4. Different desorption preexponentials, which leadsat a given desorption energysto different desorption temperatures, are expected if the adsorbate mobility is different. However, one would expect that the bilayer benzene is more mobile than a weakly bound benzene which is located between the chemisorbed molecules. This would lead to a higher preexponential and to a lower desorption temperature for the same energy for the weakly bound benzene. The opposite behavior is found experimentally. Therefore, we interpret the higher desorption temperature as an indication of a higher desorption energy of the weakly bound benzene species compared to the second-layer benzene. From its electronic structure we can rule out any strong π bonding contribution, which should show up as differential shifts or splitting of the 1a2u and 1e1g π orbitals. Therefore, we conclude that a stronger physisorption due to a higher polarizability of the nearby substrate compared to the benzene monolayer, a weak additional π interaction with the substrate, or a combination of both is responsible for the higher desorption energy. The absence of
3836 J. Phys. Chem. B, Vol. 105, No. 18, 2001 such a species on the two other surfaces is then simply due to the fact that these sites may sterically not be available there. One remaining question is left which we cannot answer fully on the basis of the experimental data presented here: Why does the chemisorbed benzene phase saturate on Ge(100) at about 0.4 ML, in contrast to Ge/Si(100) and Si(100)? In the following we will briefly discuss a possible explanation. The constant benzene sticking probability at about 90 K indicates that the chemisorption process proceeds via an adsorption precursor. The benzene molecule is initially trapped in the shallow potential well of the intrinsic or extrinsic precursor, which is most likely related to the weakly bound or physisorbed species directly on the surface. For the subsequent chemisorption process the details of the potential energy surface are important. On Si(100) and on Ge/Si(100) the benzene converts fully from the precursor state to the chemisorbed state. On Ge(100) about 60% of the benzene molecules desorb back into the gas phase and do not chemisorb. The fact that chemisorption of benzene on Ge/ Si(100) and Si(100) is much stronger corresponds to the picture of a deeper chemisorption potential. If the chemisorption potential is made deeper, the crossing of the chemisorption and precursor potentials will occur at lower energy. On Ge(100) the crossing from the physisorption to the chemisorption potentials will occur at a higher energy than on the other surfaces. This will influence the branching ratio between the precursor-to-chemisorption and precursor-to-gas-phase transitions and might even introduce an adsorption barrier toward chemisorption. However, further experiments are necessary to identify the adsorption mechanism and to further characterize both benzene species and their temperature-dependent properties. To summarize, we have shown that the desorption temperature of chemisorbed benzene on the elemental semiconductor surfaces decreases strongly on going from Si(100)-(2×1) via Ge/Si(100)-(2×1) to Ge(100)-(2×1) with peak temperatures of about 430, 310, and 230 K, respectively. On the basis of a detailed ARUPS data analysis, we find that on Ge(100) and on the Ge monolayer benzene chemisorbs with C2V symmetry and a 1,4-cyclohexadiene-like electronic structure. It is di-σ-bonded to a single Ge-Ge dimer via two carbon atoms in opposite ring (1,4) positions. The comparison of benzene adsorption on Ge(100), on the Ge monolayer, and on Si(100) did not show any significant differential shifts of the benzene-derived orbitals, indicating similar electronic structures. For benzene adsorption on Ge(100), the saturation coverage of the chemisorbed species is significantly lower, and an additional weakly bound species which desorbs in a temperature range between 155 and 220 K has been identified spectroscopically. This is in contrast to the two other systems where no such phase is observed. ARUPS data reveal only C1 adsorption symmetry for this species and an electronic structure which is gas-phase benzene-like. This indicates a physisorbed species with only weak π bonding to the substrate. A possible explanation of why the weakly bound benzene species can only be observed on Ge(100) but not on Si(100) or Ge/Si(100) has been given; it is based on kinetic hindrance of saturation of the chemisorbed species. Further theoretical and experimental studies are required to test this proposition. Acknowledgment. We thank our technical co-workers Karl Eberle and Max Glanz and the BESSY staff for support during the synchrotron beamtimes. This work has been supported by the German Federal Ministry of Research via Grant No. 05SF8WOA8.
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Structure of Benzene Adsorbed on Ge and Ge/Si (44) Ar ion sputtering at low and elevated temperatures has been used to remove contaminants at low temperatures, especially carbon from the dissociation of the hydrocarbon molecules. The high-temperature sputtering on the other hand avoids the buildup of a thicker amorphous Si layer and leads to layer-by-layer removal. (45) More quantitatively, similar means here that the preexponentials
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3837 should not deviate by more than a factor of 100. (46) This system is a good example of the importance of the experimental determination of the absolute saturation coverage, as one would otherwise wrongly assume that the saturation coverage for chemisorbed benzene should be equal on all three surfaces.
