19612
J. Phys. Chem. C 2008, 112, 19612–19615
Steric Effects in Dissociative Adsorption of Low-Energy CH3Cl on Si(100): Orientation and Steering Effects Michio Okada,*,†,‡,§ Seishiro Goto,‡ and Toshio Kasai‡ RenoVation Center of Instruments for Science Education and Technology, Osaka UniVersity, 1-2 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan, Department of Chemistry, Graduate School of Science, Osaka UniVersity, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan, and PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan ReceiVed: August 7, 2008; ReVised Manuscript ReceiVed: September 15, 2008
We report the rotational-state dependence of the sticking probability and a study of the steric effect for the 65 meV CH3Cl incidence on Si(100). We found that the steric effects are quite small and independent of the rotational state of |J,K,M〉 ) |111〉 or |212〉 at such a low incident energy as the thermal backfilling condition. The steering effects smear the orientation effects upon the system being trapped into a precursor state. 2. Experiments
1. Introduction The dissociative adsorption of methyl chloride (CH3Cl) on silicon (Si)1-4 is the first elementary step in silane formation.5,6 Scanning tunneling microscopy (STM) studies1,7 show that, for the dissociative adsorption of CH3Cl on Si(100) at room temperature, the ratio of adsorbed Cl to CH3 is 2:1. The deviation from the expected stoichiometric 1:1 ratio could be due to steric effects during adsorption, which motivates us to study the stereodynamics on this system. A CH3Cl molecule adsorbs dissociatively on Si(100) via precursor states,3,4,7 and the important role of the orientation of the incoming molecule was demonstrated by the discovery of steric effects on the 120 meV CH3Cl sticking probability (S) on Si(100).8,9 In our previous report, we reported no clear steric effects in the dissociative adsorption of 65 meV CH3Cl in the rotational state of |J,K,M〉 ) |111〉 on Si(100).8 For such a low incident energy near the thermal-backfilling conditions (∼25 meV),1,3,4,7 it is expected that the molecular orientation effects are very small or nonexistent due to the steering effects which reorient a molecule in the anisotropic attractive potential well.10 It is also reported that the steering effect in the strong interaction between the molecule and the surface reduces the steric effect in the oriented-beam11 and the rotationally aligned-beam12 experiment. In the present paper, we report the rotational-state dependence of the initial sticking probability (S0) and its steric effect at the low energy of 65 meV. We found that S0 and its orientation effect depend little on the rotational state |111〉 or |212〉 of an incident CH3Cl molecule. However, S0 becomes smaller for the direct beam including rotational states at higher energy and/or KM ) 0. On the basis of these data, we discuss the steering effects reorienting a molecule in the preferential direction upon being trapped into a precursor state in the low-energy region of ∼65 meV. * Corresponding author. E-mail:
[email protected] and
[email protected]. † Renovation Center of Instruments for Science Education and Technology, Osaka University. ‡ Graduate School of Science, Osaka University. § PRESTO.
Experiments were performed with a molecular beam apparatus8,9,13 adapted for state selection and orientation of CH3Cl and the initial sticking probability of S0 determined with the King-Wells (KW) method.14 The base pressure of the surfacereaction analysis (SRA) chamber was below 1 × 10-8 Pa. A Si(100) sample (n-type, 0.01 Ωcm) was heated by passing a direct current and degassed overnight at 850 K at a pressure of 1 × 10-8 Pa. Finally, the sample was flashed to 1450 K several times and then cooled slowly from 1000 K to the experimental temperature. A sharp (2 × 1) reconstructed pattern was observed at room temperature by low-energy electron diffraction. At a repetition rate of 40 Hz, the seeded molecular beam produces CH3Cl pulses with a gate duration of 0.4 ms fwhm in the KW measurements. The CH3Cl translational energy is 65 meV for CH3Cl (25%) seeded in Kr. The angle of incidence is surface normal. The CH3Cl beam is state selected with a |J,K,M〉 ) |111〉 or |212〉 rotational state by means of an electrostatic hexapole field15-17 (electronic and vibrational ground state). The focusing curve in Figure 1a was measured with the 65 meV CH3Cl pulses with a gate duration of 0.25 ms fwhm.13 The focusing voltages for the |111〉 and |212〉 states (see arrows in Figure 1a) were 1.9 and 2.8 kV in our apparatus, respectively. The nonstate-selected CH3Cl beam along the center of the beam axis is perfectly shadowed by a beam stop and cannot enter the surface analysis chamber. Figure 1b shows the time-of-flight beam profiles for the |111〉 and |212〉 states of the 65 meV CH3Cl (seeded in Kr) and for the |111〉 state of the 120 meV CH3Cl (seeded in Ar). In addition, Figure 1b also shows the beam profile of the direct 65 meV CH3Cl beam (nonstate-selected beam) measured with the beam stop removed and the hexapole voltages off. The gate duration of the pulsed valve was 0.25 ms fwhm in Figure 1b. Since we use the pulsed beam generated by a pulsed valve as it is (no choppershaped short pulse), we cannot determine the exact velocity and its distribution in our time-of-flight measurements. However, the mean energy of 65 meV estimated from the direct-beam profile in Figure 1b is accurate enough for the present experiments and is consistent with that determined from the focusing curve shown in Figure 1a. The velocity distribution of the 65 meV |111〉 beam is narrower than that of the direct beam because of the velocity-filtering effect in the hexapole field, as seen in
10.1021/jp807052s CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
Dissociative Adsorption of CH3Cl on Si(100)
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19613
Figure 2. Ts dependence of S0 for the CH3Cl |111〉 (open circles), |212〉 (full circles), and direct beam (open squares) at an incident energy of 65 meV. Two curves are reproduced with eq 1 in the text.
direction of the orientation electric field enables us to select a Cl- or a CH3-end collision on the surface. j n ) 〈Pn(cos γ)〉 of W(cos γ) for the The Legendre moment P |111〉 state at 65 meV is as follows, taking the state purity into j 0 ) 1, P j 1 ) 0.47, and P j 2 ) 0.06, and the other consideration: P higher terms are negligibly small. Furthermore, for the |212〉 j 0 ) 1, P j 1 ) 0.33, and P j2 ) state, the Legendre moments are P j 1 and P j 2 correspond to the orientation and -0.10 at 65 meV. P the alignment of a molecule, respectively. The orientation informs us about the importance of the head and tail of a molecule in the surface chemical reactions. 3. Results and Discussions
Figure 1. (a) Focusing curves for CH3Cl(25%)/Kr. The dots and full lines indicate the measured and simulated focusing curves, respectively. Arrows correspond to the states of |JKM〉 ) |111〉 and |212〉. (b) Timeof-flight beam profiles correspond to |JKM〉 ) |111〉 (black dotted curve: CH3Cl(25%)/Ar; and black dot-dashed curve: CH3Cl(25%)/Kr) and |212〉 (red dashed curve: CH3Cl(25%)/Kr). The blue curve corresponds to the direct CH3Cl(25%)/Kr beam measured with the beam stop removed. Spectra are normalized at the maximum except for the reduced profile for the |111〉 state of CH3Cl(25%)/Kr. (c) Polar plots of estimated orientation distribution WV(cos γ) for the [111〉 (blue solid line) and the |212〉 (red dashed line) states. If the orientation electric field is applied as indicated in the figure, the Cl end of CH3Cl will be distributed as shown. The circles in the centers correspond to the random orientation distribution.
Figure 1b. On the other hand, the velocity distribution of the 65 meV |212〉 beam is nearly the same as that of the direct beam. According to the simulation, the state purity of the |111〉 beam is more than 70% in Figure 1a, while that of the |212〉 beam is about 54%. The orientation of CH3Cl molecules is constructed by a homogeneous orientation electric field in front of the Si surface. On the basis of the focusing curve and its simulation,13 we determined the orientation distribution function of W(cos γ), where γ is the angle between the orientation field and the direction pointing from CH3 to Cl of CH3Cl. Figure 1c shows the estimated W(cos γ) for the state-selected beams with |J,K,M〉 ) |111〉 and |212〉 rotational states. The rotational-state distribution in the beam is taken into consideration. The distribution is drawn for the Cl end of CH3Cl. Switching the
Figure 2 shows the surface temperature (Ts) dependence of the initial sticking probability S0 for the state-selected (|111〉 or |212〉) and the direct (nonstate-selected) beam incidences with random orientations. In the direct-beam incidence, we removed the beam stop, turned off the hexapole voltages, and introduced the beam into the UHV chamber. We measured a few points with the direct beam for the comparison with the state-selected beam. In the state-selected beam, for Ts < ca. 320 K, S0 ∼ 1, while for ca. 320 K < Ts < ca. 400 K, S0 decreases steeply to ≈0. On the other hand, S0 for the direct beam is smaller than the state-selected beam, and its Ts dependence is shifted to lower temperatures. Strong Ts dependence of S0 suggests that the dissociative adsorption of CH3Cl occurs via precursors. Such a Ts dependence of S0 is not caused by the recombinative desorption of the dissociatively chemisorbed species because the temperature range is far below the thermal desorption temperature.3 In this Ts region, no desorption of dissociated species occurs. The precursor-mediated dissociative adsorption is also supported by the theoretica calculations.18,19 The initial sticking probability, S0, as a function of surface temperature, Ts, can be derived in the following equation20
S0 ) SD +
Ptrap νa Ea - Ed 1 + exp νd kTs
(
)
(1)
where SD is the direct contribution to sticking and the second term is the precursor component of sticking; Ptrap is the probability of the incident molecule trapping into the physisorption well; Ed is the activation energy for desorption; Ea is the activation for chemisorption; and νa and νd are the preexponentials for chemisorption and desorption, respectively. Figure 2 shows the resulting curves reproduced with Ed - Ea ) 0.4 eV and Ptrap ) 1 and 0.65 for the state-selected and direct
19614 J. Phys. Chem. C, Vol. 112, No. 49, 2008 beams, respectively. Here, SD is approximated to 0. The value of Ed - Ea ) 0.4 eV is larger than 0.28 eV obtained in the backfilling experiments.4 According to the theoretical calculations,18,19 the precursor state is a physisorbed state with strong charge redistribution (weakly bound state) and an attractive well of 0.27-0.32 eV in several possible geometries. Thus, the present experimental results suggest that the attractive well may be deeper than that expected in the theoretical calculations. The ratio νd/νa is 5.1 × 105 and 2.7 × 106 for the state-selected and direct beams, respectively. The value from the thermal backfilling experiments was 540.4 For a nonspherically symmetric reactant, it is expected that νd/νa lies between ca. 100 and 1000.21 Thus, the presently observed high ratios of νd/νa in the molecular beam experiments might be due to the contribution of the transiently trapped hot precursors (not stabilized in the bottom of the well), which may be able to reach the transition state with more freedoms for desorption. From the comparison of the state-selected beam with the direct beam in Figure 2, S0 for the direct beam is smaller than the state-selected beam. This result cannot be explained by the difference in the translational energy distribution because the beam profile for the direct beam is nearly the same as the |212〉 beam as shown in Figure 1b. Moreover, the 120 meV |111〉 beam that also demonstrated similar high S08 reveals the beam profile separated from the 65 meV beams in Figure 1b. Although it is expected that increasing the translational energy may reduce S0 in the precursor-mediated process, this is not the case for the energy distribution of the 65 meV direct beam. According to the previous report8 for the |111〉 state, the decease in S0 with incident energy was observed at the translational energy of 180 meV which is much higher than 65 meV in the present experiments. Therefore, the present experimental result suggests that higher rotational states and/or other rotational motions, i.e., KM ) 0, may reduce S0. Higher rotational states correspond to large hexapole voltages (compared to |111〉 and |212〉) in the focusing curve in Figure 1a, while the states with KM ) 0 cannot be focused with the hexapole fileld. The direct beam includes both contributions that may reduce S0. There are no obvious differences in S0 in comparison of |111〉 and |212〉 states. The rotational energy (Erot) for each state can be calculated from Erot ) BJ(J + 1) + (A - B)K2. Here, A and B are the rotational constants of 1.5 × 1011 and 1.33 × 1010 Hz, respectively.22 The resulting rotational energy is 0.68 and 0.90 meV for the |111〉 and |212〉 states, respectively. One cycle of rotation takes about 10-12 s in these rotational states. For the slowly moving molecule (500 ms-1) in the strong anisotropic attractive potential, subtle differences in rotational motion and energy may not strongly affect the probability of trapping into the precursor well because the steering effects reorient a molecule into the preferable direction. In all possible precursor geometries,18,19 it is expected that the Cl-end approach is more favorable than the CH3-end approach. The charge redistribution among molecules and substrates becomes important in the interaction of a precursor depending on its orientation and the adsorption site. The important role of the steering effects in this energy region can be directly demonstrated in the measurements of the orientation effects of S0, as shown in Figure 3. According to the theoretical calculation, there is a clear orientationdependent anisotropy in the weakly bound interaction due to the charge redistribution between a CH3Cl molecule and the Si(100) surface.19 The steering forces due to the anisot-
Okada et al.
Figure 3. Ts dependence of SCl/S0 (full circles) and SCH3/S0 (open circles) for the [111〉 (upper panel) and the |212〉 (lower panel) states at an incident energy of 65 meV. The value of 1 corresponds to no steric effects.
ropy of the interaction are constant. The molecular orientation can be relevant or washed out by steering effects depending on the collision energy. We expect that orientation effect would appear at higher translational energy. That is the case of the 120 meV CH3Cl incidence on Si(100) where a clear steric effect was found.8 That is attributed to the anisotropy in the trapping probability due to the different energy dissipation caused by the anisotropy in the interaction potential. On the other hand, the orientation effects would be smeared out at low translational energy due to the reorientation of the molecule steered by the anisotropy in the interaction potential. In Figure 3, SCl and SCH3 representing S0 for the Cl-end and CH3-end incidences, respectively, are normalized by S0 for random orientation. No orientation effects correspond to the value of 1. Thus, no clear orientation effects are observed both for the |111〉 and |212〉 beam incidence within the experimental errors in the whole range of Ts. These results are different from those in the 120 meV CH3Cl incidence.8 Therefore, the strong steering effects reorient the slowly moving molecule into favorable orientation. This consideration can be rationalized by the high sticking probability of ∼1 at low temperatures below 320 K, as shown in Figure 2. Moreover, according to the theoretical calculations,18 the depth of the precursor state is expected to be 270-321 meV. The present experiments suggest more than ∼400 meV. The kinetic energy of 65 meV is small enough that the steering effects may be induced on the |111〉 and |212〉 states. The geometry of the possible stable precursor state into the dissociative adsorption is inclined to the surface with Cl close to the Si of a dimer. Steering effects will reorient the incident molecule into such a stable geometry. The anisotropy of the interaction potential also induces the orientation dependence of the rotational excitation which causes the steric effect. In the scattering of CH3Cl from the highly oriented pyrolytic graphite (HOPG) surface (a weak physisorption system),23 the trapping probability for the Cl end is larger than the CH3 end. Thus, even if the rotational
Dissociative Adsorption of CH3Cl on Si(100) excitation might contribute to the dynamical interaction, it is expected that the same sign of steric effect as in the 120 meV CH3Cl on Si(100)8 will be observed. Thus, the observed negligible steric effects suggest that the steering effect smears out such dynamical anisotropic effects and plays an important role at low incident energy. In the previous STM reports on the thermal CH3Cl exposure of Si(100) by backfilling,1,7 the ratio of adsorbed Cl to CH3 was 2:1. As we discussed above, in the low energy of 65 meV, the steering effect, smearing out molecular orientation effects in the initial sticking probability S0, becomes dominat. Thus, because no steric effects were observed in S0, the initial molecular orientation does not seem to contribute to the Cl /CH3 ratio. The ratio of Cl:CH3 2:1 may come from the transient dynamical-trapping process where the selection of the reaction route, depending on the molecular orientation, can take place.9 The dynamical trapping state in the potential well may depend on the initial molecular orientation, even though the trapping probability is the same due to the steering effect. Further experiments of monitoring the products induced by the oriented molecular beam will be required for more detailed arguments. 4. Conclusions We measured, using an oriented-molecular beam, the rotational-state dependence of the sticking probability and its steric effect on the CH3Cl/Si system. The steric effects are quite small, independent from the rotational state |J,K,M〉 ) |111〉 or |212〉 for the 65 meV CH3Cl incidence on Si(100). The results clearly demonstrate that the strong steering effects play an important role upon the system being trapped into a precursor state in the low-energy region of 65 meV. The contribution of other higher rotational states and/ or KM ) 0 reduces the sticking probability as determined by measurements with a direct molecular beam. Acknowledgment. We gratefully acknowledge MEXT for a Grant-in-Aid for Scientific Research (No. 20350005). M.O.
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19615 was also financially supported by PRESTO of JST. We are also thankful to Steven Kleyn for his critical reading of the manuscript. References and Notes (1) Bronikowski, M. J.; Hamers, R. J. J. Vac. Sci. Technol. 1995, A13, 777. (2) Colaianni, M. L.; Chen, P. J.; Gutleben, H.; Yates, J. T., Jr. Chem. Phys. Lett. 1992, 191, 561. (3) Brown, K. A.; Ho, W. Surf. Sci. 1995, 338, 111. (4) Lee, J. Y.; Kim, S. Surf. Sci. 2001, 482–485, 196. (5) Rochow, E. G. J. Am. Chem. Soc. 1945, 67, 963. (6) Frank, T. C.; Falconer, J. L. Langmuir 1985, 1, 104. (7) Woelke, A.; Imanaka, S.; Watanabe, S.; Goto, S.; Hashinokuchi, M.; Okada, M.; Kasai, T. J. Electron Microscopy 2005, 54 (Supplement 1), i21. (8) Okada, M.; Goto, S.; Kasai, T. Phys. ReV. Lett. 2005, 95, 176103. (9) Okada, M.; Goto, S.; Kasai, T. J. Am. Chem. Soc. 2007, 129, 10052. (10) Gross, A. Theoretical Surface Science: A Microscopic PerspectiVe; Springer: Berlin, 2003. (11) Komrowski, A. J.; Ternow, H.; Razaznejad, B.; Berenbak, B.; Sexton, J. Z.; Zoric, I.; Kasemo, B.; Lundqvist, B. I.; Stolte, S.; Kleyn, A. W.; Kummel, A. C. J. Chem. Phys. 2002, 117, 8185. (12) Gerbi, A.; Savio, L.; Vattuone, L.; Pirani, F.; Cappelletti, D.; Rocca, M. Angew. Chem., Int. Ed. 2006, 45, 6655. (13) Okada, M.; Moritani, K.; Goto, S.; Kasai, T. Jpn. J. Appl. Phys. 2005, 44, 8580. (14) King, D. A.; Wells, M. G. Surf. Sci. 1972, 29, 454. (15) Brooks, P. R. Science 1976, 193, 11. (16) Stolte, S.; Reuss, J.; Schwartz, H. L. Physica 1973, 66, 211. (17) Kramer, K. H.; Bernstein, R. B. J. Chem. Phys. 1965, 42, 767. (18) Romero, A. H.; Sbraccia, C.; Silvestrelli, P. L.; Ancilotto, F. J. Chem. Phys. 2003, 119, 1085. (19) Preuss, M.; Schmidt, W. G.; Bechstedt, F. J. Phys. Chem. B 2004, 108, 7809. Preuss, M.; Schmidt, W. G.; Seino, K.; Bechstedt, F. Appl. Surf. Sci. 2004, 234, 155. (20) Sullivan, D. J. D.; Flaum, H. C.; Kummel, A. C. J. Phys. Chem. 1993, 97, 12051. (21) Weinberg, W. H. In AdVances in Gas-Phase Photochemistry and Kinetics, Dynamics of Gas-Surface Interactions; Rettner, C. T., Ashfold, M. N. R., Eds.; Royal Society of Chemistry: Cambridge, 1991; p 171. (22) Townesand, C. H.; Schawlow, A. L. MicrowaVe Spectroscopy; McGraw-Hill: New York, 1955. (23) Fukuyama, T.; Okada, M.; Kasai, T., in preparation.
JP807052S
