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Angular distributions of N2 in the photodissociation of N2O adsorbed on a partially oxidized Si(100) surface at 95 K
7 July 1995
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 240 (1995) 417-422
Angular distributions of N, in the photodissociation of N,O adsorbed on a partially oxidized Si(100) surface at 95 K Jihwa Lee ‘, Hiroyuki Kato, Kyoichi Sawabe, Yoshiyasu Matsumoto
*
The Graduate Vniversi@ for Advanced Studies and Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
Received 23 February 1995; in final form 11 April 1995
Abstract The time-of-flight distributions of the N, photofragment produced in the UV photodissociation of N,O adsorbed on a partially oxidized Si(100) surface at 95 K have been measured as a function of the desorption angle. Photoinduced electron transfer initiates the dissociation of N,O to produce an adsorbed oxygen atom and energetic N, desorbing from the surface. Interestingly, the angular distribution of N, originating from chemisorbed N,O molecules is peaked at = 32” from the surface normal. The results are discussed on the basis of bonding geometry and photodissociation dynamics of N,O.
1. Introduction The photoexcitation of adsorbate-covered surfaces in the valence band region (hv< 10 eV) can lead to desorption, dissociation and surface reactions. These surface photochemical processes are not only of fundamental interest but of technological importance for applications such as in microelectronic fabrication, and therefore have attracted much attention in recent years [l-6]. If photoexcitation is accompanied by the desorption of photofragments or reaction products, molecular dynamic information can be extracted in detail by measuring the translational and internal energy distributions of the desorbing species. Furthermore, angular distribution measurements can also provide useful information on the dynamics, since the photochemical products originating from
’ Permanent address: Department of Chemical Technology, Seoul National University, Seoul 151-742, South-Korea. Corresponding author. ??
adsorbed molecules, which are usually aligned on a single crystal surface owing to interaction with the surface, are expected to desorb in an angle which is intimately related to the initial orientation of the adsorbed molecules. This intimate relation to the orientation and the bonding geometry of surface species has been successfully used in electron stimulated desorption ion angular distribution (ESDIAD) [7]. The basic principle behind this method is that the repulsive potential surface responsible for ion ejection during electron bombardment at a kinetic energy of = 100 eV is usually determined by the directionality of the chemical bonds being broken. However, it is unclear whether or not the same principle holds in the photodissociation of adsorbed molecules upon irradiation by ultraviolet photons. Therefore, it is highly desirable to clarify how angular distributions of fragments are correlated with the orientation and the bonding geometry of adsorbates for better understanding of the photodissociation dynamics of adsorbed molecules.
0009-2614/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDlOOO9-2614(95)00566-S
418
J. Lee et al. /Chemical
Physics Letters 240 (1995) 417-422
Among the numerous reports on surface photochemistry, few studies have been attempted to clarify this point so far. Bourdon et al. [8] reported that the H desorption yield in the photodissociation of HBr on LiF(100) was peaked at 55” from the surface normal, where HBr adsorbs in an end-on configuration through the H atom and the molecular axis is tilted at 21” relative to the surface plane. Although interesting, the correlation between the peak desorption angle and the orientation of HBr is not straightforward as a result of the collision between the H atom and the highly corrugated LiF(100) surface. Yang et al. [9] have shown that the photodissociation of CH,Br on GaAs(ll0) reveals highly anisotropic angular distributions of CH,. The observed two distinct angular lobes of the fragment are attributed to two different molecular orientations of CH,Br. The other relevant measurements of the angular distributions of desorbing species are the photodesorption of 0, on Pd(ll1) [lo] and the photoinitiated recombination between CH, and H on Pt(ll1) [ll]. N,O on Si(lOO)-2 X 1 is the system of choice in this study, since the chemisorption bonding to a Si atom is generally highly localized and directional. Previously we reported [12] that N,O adsorbed on Si(100) at 95 K dissociates to produce an adsorbed oxygen atom and energetic N, desorbing from the surface upon irradiation by 193 nm photons. The time-of-flight (TOF) distributions of N, have two distinct peaks characterized by nonthermal velocity distributions. In this Letter, we show a peculiar angular distribution of the N, photofragment; chemisorbed N,O gives a sharp angular distribution peaked at = 32” from the surface normal. We propose a plausible adsorption structure of chemisorbed N,O and discuss how the angular distribution of the N, photofragment is related to the initial bonding geometry of adsorbed N,O.
2. Experimental The experiments were performed in a III-IV system which has been described in detail elsewhere [13,14]. Briefly, it is pumped by cascaded turbomolecular pumps and equipped with AES, LEED, XPS and a differentially pumped quadrupole mass spectrometer (QMS). The base pressure was better
than 1 X lo-” Torr. A Si(100) sample @b-doped, 0.005-0.01 Sz, 9 X 20 mm’) was mounted on a rotatable XYZ manipulator and could be cooled to 45 K with a closed cycle He refrigerator and resistively heated to 1300 K. The sample temperature was monitored by a thermocouple glued to the back of the sample using a high temperature ceramic adhesive. The surface was cleaned by repeated cycles of Ar+-sputtering and annealing followed by flashing to 1150 K, which gave a sharp c(4 X 2) LEED pattern revealing two domains. “N14N0 was used for adsorption to avoid a large signal at m/e = 28 from the background gas. In the photodissociation experiments the surface saturated with N,O at 95 K was irradiated with excimer laser pulses of 193 nm (< 2.5 mJ/cm2, 2 Hz) and the TOF distributions of the desorbed species were measured by a QMS detector. The ionizer of the QMS was located 23 cm from the surface. The maximum surface temperature rise was estimated to be = 5 K [15]. N,-TOF distributions were obtained by accumulating the desorption signal for a large enough number of laser pulses (typically = 1000) to ensure complete depletion of the adsorbed N,O. In order to obtain a better signal-to-noise ratio, this procedure was repeated 5-10 times. Since the laser beam was fixed at an angle of 45” to the QMS view axis, the variation in the surface areas irradiated and subtended by the QMS as the sample is rotated had to be properly corrected in plotting the angular distributions.
3. Results N,O adsorbs on Si(100) in two different adsorption states at low temperature: physisorbed and chemisorbed states, which have been characterized by temperature-programmed desorption and X-ray photoelectron spectroscopy (XPS) [16]. When the surface is exposed to N,O at 63 K, N,O is mostly adsorbed in the physisorbed state. Physisorbed N,O molecules partially desorb and partially dissociate to 0 and N, even at a fairly low temperature of = 85 K. While the oxygen atom is inserted in a silicon dimer bond to form 0-Si bonds [17] during the thermal oxidation process, some physisorbed N,O are converted to the chemisorbed state on the partially oxidized Si(100) surface. Thus, the
J. Lee et al. / Chemical Physics Letters 240 (I 995) 417-422 1
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.
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Fig. 1. TOF distributions of N, measured for various desorption angles, t?,,, measured with respect to the surface normal in the 193 nm photodissociation of N,O adsorbed on a partially oxidized Si(lOO). The surface was saturated with N,O at 95 K before photon irradiation. The two peaks in the distributions are denoted by P, (fast) and P, (slow), respectively.
chemisorbed N,O seems to be at a site near an oxidized Si atom, whereas the physisorbed state is more likely to be on a clean Si surface site. When the surface is exposed to N,O at 95 K, the surface is partially oxidized by the thermal decomposition of N,O and the molecularly adsorbed N,O is mostly in the chemisorbed state. All the data presented below were obtained with a Si(100) surface saturated with N,O at 95 K. When the N,O-covered surface was irradiated with 193 nm photons, N,O dissociated to produce an oxygen adatom and N, desorbing from the surface. Fig. 1 shows the TOF distributions of N, desorbed at various angles from the surface normal. While adsorbed N,O was completely depleted during N,-TOF measurements at each angle of detection, the oxygen coverage changed from = 0.7 to one monolayer, which was confirmed by post-irradiation XPS measurements. Thus, these results represent N,-TOF distributions accumulated in the range of adsorbate coverages cited above. Each distribution consists of two peaks, the fast and the slow velocity components
419
denoted by P, and Pz, respectively. One can notice that the angular dependence of the two peaks are qualitatively different. As the desorption angle, e,,,,, deviates from the surface normal, the P, peak first increases and then falls off, exhibiting a maximum at an intermediate angle, whereas the P2 component decreases monotonically with /3,,,. The TOF distribution obtained at tj,,, = - 15”, though not shown here, was the same as that at 0,,, = 15”, indicating that the velocity distributions are symmetric about the surface normal at least in the normal plane containing the [Oil] azimuth. Each TOF distribution was fitted by the sum of two shifted Maxwell-Boltzmann velocity distributions of the functional form f(u) =Au3 exp[-(u - ~,,)~/a~], where u0 and CY are parameters representing the shift and spread in velocity, respectively. The velocity-weighted average translational energy of the P, component is (E, > = 0.77-0.91 eV, increasing gradually with (I,,,, up to 30”. On the other hand, the P, component showed an almost constant (ET) of 0.28 eV. The maximum translational energy determined from the onset of the TOF distributions is 2.45 eV. The temperature of N, defined by (E,)/2k corresponds to 4400-5300 K and 1600 K for the P, and P, peaks, respectively. Angular distributions of the N,-desorption yield are plotted in Fig. 2, where the data points were obtained by velocity-weighted integration of the TOF
Fig. 2. Polar plot for the angular distributions of N, obtained by velocity-weighted integration of the TOF distributions in Fig. 1. The yields of the total (01, P, component CO), and P2 component ( A ) are shown together. The solid line shows the distribution of the total desorption yield represented by O.~~[COS*~~(~~~~ - 32”) + COS~~~(~,,,+ 32”)] + 0.25 COS~%~~~, determined from leastsquares fits. The fitted results for the P, and the P, components are also shown with broken and dotted lines, respectively.
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J. Lee et al. / Chemical Physics Letters 240 (1995) 417-422
distributions per unit irradiated surface area. Interestingly, the P, component has a sharp angular distribution peaked at e,,, = 32”, whereas the P, component is peaked at the surface normal. The distribution of the total desorption yield was approximately represented by 0.75[cos8.0(0,,es - 32”) + cos8.0(eddes+ 32”)] + 0.25 cos2.20des, where the C, symmetry ’ of the P, component has been taken into account.
4. Discussion Although not shown here, irradiation with 351 nm laser pulses resulted in a qualitatively similar angular distribution. N,-TOF distributions showed an interesting wavelength dependence, but the maximum N, kinetic energy was the same at two widely different photon energies (6.4 versus 3.5 eV) [18]. Thus, the photodissociation of N,O on Si(100) is believed to be initiated by a substrate-mediated mechanism [19]. According to a relevant study on N,O production by the surface reaction of NO on GaAs(100) at 77 K [20], N,O also weakly adsorbs on GaAs(lOO) with a desorption temperature of 110 K. Metastable quenching spectroscopy showed that the highest occupied molecular orbital of N,O, the 27~ orbital, lies 7.8 eV below the Fermi level. If we assume a similar situation for cx,-N,O on Si(lOO), the 2~ level would lie at least 7 eV below the valence band maximum of silicon. Thus, it is not energetically possible to create such hot holes at a photon energy of 3.5 eV (A = 351 nm), and hence dissociation would not occur. Therefore, it is quite likely that photo- and/or ‘hot’ electrons produced by substrate excitation are responsible for dissociation rather than hot holes. By analogy to the gas-phase dissociative electron attachment of N,O [21], a negative ion state of the adsorbed N,O relevant to the excitation process could be correlated with either the bend N,O-( 2’) ground state or the linear first excited state of the gas phase N,O-. During a short lifetime of the negative ion state the N-O bond is elongated owing to the repul-
2
Strictly speaking, of the two equivalent contributions by the current measurements
it should show fourfold symmetry because domains of the Si(lOO)-2 X 1 surface, but the two out-of-plane lobes are small in the and therefore neglected.
sive potential surface along the N-O bond and the molecule begins to fall apart. The photofragment 0 atom is ultimately bonded to the surface Si atoms owing to a strong attractive interaction with the surface atoms, whereas N, is ejected from the surface, carrying some excess energy. As shown in Fig. 1, the P, component in the N,-TOF distributions dominates over the P, component in all the desorption angles. However, we found that the relative contribution of the P, component increases when N,O is adsorbed on Si(100) at 63 K [16], where N,O is physisorbed and the coverage of N,O is = 4 times larger than the surface saturated with N,O at 95 K. Thus, a possible origin for the P, component could be the energy dissipation of the N, photofragment through an inelastic collision with substrate or neighboring N,O when it leaves the surface. Alternatively, the P2 component could originate from the dissociation of physisorbed N,O that likely oriented randomly. In either case, the angular distribution of N, would be broad and isotropic. On the other hand, if N,O were more rigidly bonded to the surface in a specific orientation and isolated from neighboring adsorbates, the angular distribution of N, could be more anisotropic and narrower. Thus, we conclude that the P, component is attributed to N,O chemisorbed on a partially oxidized Si(100) surface. The bimodality in the translational energy distribution of the N, photofragment has also been observed in photodissociation of N,O on Pt(ll1) at 193 nm [22]. We now ask how the angular distribution of the N, photofragment is related to the initial bonding geometry of chemisorbed N,O. The molecular structure and configuration of N,O chemisorbed on Si(100) are not known. However, it is interesting to note that N,O adsorbed on clean GaAs(100) lies flat, but in contrast N,O produced by the reaction of NO is oriented vertically with the O-end down on oxidized GaAs(lOO) based on their He* quenching spectra of N,O, in which the electron emission yield is sensitive to the overlap between the He*(2s) and N,O valence orbitals [20]. Adapting this view to the present system, we suggest the following model for the adsorption structure of N,O chemisorbed on Si(100) as shown in Fig. 3. When randomly oriented physisorbed N,O is converted to the chemisorbed state by partial oxidation of the surface, the
J. Lee et al. / Chemical Physics Letters 240 (1995) 417-422
,Si,
,Si,
,.Si,
si I
Si I
Fig. 3. Proposed model of adsorption geometry N,O on a partially oxidized Si(100) surface.
of chemisorbed
chemisorbed N,O may be bonded to a Si dangling bond through the oxygen atom by making a weak covalent bond. Thus, the orientation of the chemisorbed N,O could be tilted with respect to the surface normal. This bond between N,O and a Si atom could be realized with some electron transfer from the Si atom to the lowest unoccupied molecular orbital of the adsorbate; this electron transfer makes the N-O bond weaker. This model can be supported by the following observations: First, as shown in Fig. 4, while N,O adsorbed at 63 K predominantly showed an O(ls)
SiO
I
530
*
,
N;O
,
,
I,
1
,
,
,
535
1
,
540
Binding Energy (eV) Fig. 4. O(ls) X-ray photoelectron spectra of N,O adsorbed on Si(100). The surface was exposed to (a) 2.5 L of N,O at 63 K and (b) 9.6 L of N,O at 95 K. The peaks at 535.9 and 535.3 eV are attributed to oxygen in N,O and the peak at 532.3 eV to oxygen adatoms produced by the dissociation of N,O.
421
peak at 535.9 eV, the surface exposed to N,O at 95 K revealed two O&s) peaks at 532.3 and 535.3 eV, originating from oxygen atoms bonded to silicon atoms [23] and chemisorbed N,O, respectively. Thus, when the physisorbed N,O is converted to the chemisorbed state, the O(ls) peak is shifted toward lower binding energy and the amount of shift, 0.6 eV, is larger than that of the terminal N(ls), 0.2 eV; this indicates that some electron transfer takes place from the substrate to the chemisorbed N,O and its bonding configuration is likely to be O-end down. In addition, ab initio MO calculations [24] based on a silicon cluster model also indicate that the O-end down configuration is more stable than the N-end down configuration. Second, preliminary results of near-edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis of adsorbed N,O at 95 K is tilted from the surface normal [25]. Third, a weakened N-O bond can be inferred from the facile thermal dissociation of the chemisorbed N,O below 300 K. It is interesting to note that the dangling bond of the symmetric Si(lOO)-2 X 1 dimer atoms is inclined by 36” from the surface normal, which has been directly seen from the F+ ion emission angle in the electron-stimulated desorption of the F/Si(lOO) system [27]. Although oxygen insertion into a dimer bond may alter the tilting angle of the dangling bond to some extent, these observations bring the following conjecture; N, originating from the chemisorbed N,O is ejected in the direction of the N-O bond which is collinear to the Si dangling bond to result in an angular distribution peaked at = 32” from the surface normal. Thus, the present case seems to be quite similar to that of ESDIAD. However, one needs some caution to discuss the UV-photon-induced dissociation dynamics of N,O adsorbate on the same basis as used in ESDIAD. In particular, if the dissociation dynamics were mostly governed by the excited state with a bent configuration as N,O-(Z+) ground state in the gas phase, the relation between the angular distributions of N, and the bonding geometry of N,O would not be so straightforward. Thus, in order to clarify this point further in detail, it is necessary to perform more thorough experimental and theoretical studies, such as NEXAFS [25] and ab initio MO calculations [24], now in progress in our laboratory.
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Acknowledgement
We thank Dr. H. Oshima for providing the highly doped Si(100) samples. This work was supported in part by Grants-in-Aid for Scientific Research on New Programs (06NP0301) and on Priority Areas (06228232, 06239263) from the Ministry of Education, Science and Culture of Japan. JL gratefully acknowledges the hospitality of the Institute for Molecular Science during his sabbatical leave from Seoul National University.
References [l] T.J. Chuang, Surface Sci. Rept. 3 (1983) 1. [2] W. Ho, Comments Cond. Mat. Phys. 13 (1988) 293. [3] P. Avouris and R.E. Walkup, Ann. Rev. Phvs. Chem. 40 (1989) 173. [41 X.-L. Zhou, X.-Y. Zhu and J.M. White, Surface Sci. Rept. 13 (1991) 73. El J.C. Polanyi and H. Rieley, in: Dynamics of gas-surface interactions, eds. CT. Rettner and M.N.R. Ashfold (The Royal Society of Chemistry, Cambridge, 1991) p. 329. I61J.G. Eden, in: Chemical analysis series, Vol. 22. Photochemical vapor deposition (Wiley, New York, 1992), and references therein. [71 R.D. Ramsier and J.T. Yates Jr., in: Dynamics of gas-surface interactions, eds. CT. Rettner and M.N.R. Ashfold (The Royal Society of Chemistry, Cambridge, 1991) p. 257.
[S] E.B.D. Bourdon,
C.-C. Cho, P. Das, J.C. Polanyi, C.D. Stanner and G.-Q. Xu, J. Chem. Phys. 95 (1991) 1361. [9] Q.Y. Yang, W.N. Schwartz, P.J. Lasky, S.C. Hood, N.L. Loo and R.M. Osgood Jr., Phys. Rev. Letters 72 (1994) 3068. [lo] A. de Meijere, H. Hirayama and E. Hasselbrink, Phys. Rev. Letters 70 (1993) 1147. [ll] V.A. Ukraintsev and I. Harrison, J. Chem. Phys. 98 (1993) 5971. [12] Y. Matsumoto, J. Lee, H. Kato, and K. Sawabe, SPIE Proc. 2125 (1994) 303. [13] K. Sawabe and Y. Matsumoto, Chem. Phys. Letters 194 (1992) 45. [14] K. Sawabe and Y. Matsumoto, Surface Sci. 283 (1993) 126. [15] L.J. Richter and R.R. Cavanagh, Progr. Surface Sci. 39 (1992) 155. [16] H. Kato, K. Sawabe, J. Lee and Y. Matsumoto. to be submitted. [171 T. Engel, Surface Sci. Rept. 18 (1993) 91. I181J. Lee, H. Kato, K. Sawabe and Y. Matsumoto, in preparation. D91 E.P. Marsh, T.L. Gilton, W. Meier, M.R. Schneider and J.P. Cowin, Phys. Rev. Letters 61 (1988) 2725. DO1 C.P. Dehnbostel, A. Ludviksson, C. Huang, H.J. JBnsch and R.M. Martin, Surface Sci. 265 (1992) 305. 1211P.J. Chantry, J. Chem. Phys. 51 (1969) 3369. D21 K. Sawabe, J. Lee and Y. Matsumoto, J. Chem. Phys. 99 (1993) 3143. Dl G. HoRinger, J.F. Morar, F.J. Himpsel, G. Hughes and J.L. Jordan, Surface Sci. 168 (1986) 609. [241 K. Sawabe and Y. Matsumoto, to be submitted. La K. Sawabe, S. Tanaka, H. Kato, M. Kamada and Y. Matsumoto, to be published. [261 M.J. Bozack, M.J. Dresser, W.J. Choyke, P.A. Taylor and J.T. Yates Jr., Surface Sci. 184 (1987) L332.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 240 (1995) 417-422
Angular distributions of N, in the photodissociation of N,O adsorbed on a partially oxidized Si(100) surface at 95 K Jihwa Lee ‘, Hiroyuki Kato, Kyoichi Sawabe, Yoshiyasu Matsumoto
*
The Graduate Vniversi@ for Advanced Studies and Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
Received 23 February 1995; in final form 11 April 1995
Abstract The time-of-flight distributions of the N, photofragment produced in the UV photodissociation of N,O adsorbed on a partially oxidized Si(100) surface at 95 K have been measured as a function of the desorption angle. Photoinduced electron transfer initiates the dissociation of N,O to produce an adsorbed oxygen atom and energetic N, desorbing from the surface. Interestingly, the angular distribution of N, originating from chemisorbed N,O molecules is peaked at = 32” from the surface normal. The results are discussed on the basis of bonding geometry and photodissociation dynamics of N,O.
1. Introduction The photoexcitation of adsorbate-covered surfaces in the valence band region (hv< 10 eV) can lead to desorption, dissociation and surface reactions. These surface photochemical processes are not only of fundamental interest but of technological importance for applications such as in microelectronic fabrication, and therefore have attracted much attention in recent years [l-6]. If photoexcitation is accompanied by the desorption of photofragments or reaction products, molecular dynamic information can be extracted in detail by measuring the translational and internal energy distributions of the desorbing species. Furthermore, angular distribution measurements can also provide useful information on the dynamics, since the photochemical products originating from
’ Permanent address: Department of Chemical Technology, Seoul National University, Seoul 151-742, South-Korea. Corresponding author. ??
adsorbed molecules, which are usually aligned on a single crystal surface owing to interaction with the surface, are expected to desorb in an angle which is intimately related to the initial orientation of the adsorbed molecules. This intimate relation to the orientation and the bonding geometry of surface species has been successfully used in electron stimulated desorption ion angular distribution (ESDIAD) [7]. The basic principle behind this method is that the repulsive potential surface responsible for ion ejection during electron bombardment at a kinetic energy of = 100 eV is usually determined by the directionality of the chemical bonds being broken. However, it is unclear whether or not the same principle holds in the photodissociation of adsorbed molecules upon irradiation by ultraviolet photons. Therefore, it is highly desirable to clarify how angular distributions of fragments are correlated with the orientation and the bonding geometry of adsorbates for better understanding of the photodissociation dynamics of adsorbed molecules.
0009-2614/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDlOOO9-2614(95)00566-S
418
J. Lee et al. /Chemical
Physics Letters 240 (1995) 417-422
Among the numerous reports on surface photochemistry, few studies have been attempted to clarify this point so far. Bourdon et al. [8] reported that the H desorption yield in the photodissociation of HBr on LiF(100) was peaked at 55” from the surface normal, where HBr adsorbs in an end-on configuration through the H atom and the molecular axis is tilted at 21” relative to the surface plane. Although interesting, the correlation between the peak desorption angle and the orientation of HBr is not straightforward as a result of the collision between the H atom and the highly corrugated LiF(100) surface. Yang et al. [9] have shown that the photodissociation of CH,Br on GaAs(ll0) reveals highly anisotropic angular distributions of CH,. The observed two distinct angular lobes of the fragment are attributed to two different molecular orientations of CH,Br. The other relevant measurements of the angular distributions of desorbing species are the photodesorption of 0, on Pd(ll1) [lo] and the photoinitiated recombination between CH, and H on Pt(ll1) [ll]. N,O on Si(lOO)-2 X 1 is the system of choice in this study, since the chemisorption bonding to a Si atom is generally highly localized and directional. Previously we reported [12] that N,O adsorbed on Si(100) at 95 K dissociates to produce an adsorbed oxygen atom and energetic N, desorbing from the surface upon irradiation by 193 nm photons. The time-of-flight (TOF) distributions of N, have two distinct peaks characterized by nonthermal velocity distributions. In this Letter, we show a peculiar angular distribution of the N, photofragment; chemisorbed N,O gives a sharp angular distribution peaked at = 32” from the surface normal. We propose a plausible adsorption structure of chemisorbed N,O and discuss how the angular distribution of the N, photofragment is related to the initial bonding geometry of adsorbed N,O.
2. Experimental The experiments were performed in a III-IV system which has been described in detail elsewhere [13,14]. Briefly, it is pumped by cascaded turbomolecular pumps and equipped with AES, LEED, XPS and a differentially pumped quadrupole mass spectrometer (QMS). The base pressure was better
than 1 X lo-” Torr. A Si(100) sample @b-doped, 0.005-0.01 Sz, 9 X 20 mm’) was mounted on a rotatable XYZ manipulator and could be cooled to 45 K with a closed cycle He refrigerator and resistively heated to 1300 K. The sample temperature was monitored by a thermocouple glued to the back of the sample using a high temperature ceramic adhesive. The surface was cleaned by repeated cycles of Ar+-sputtering and annealing followed by flashing to 1150 K, which gave a sharp c(4 X 2) LEED pattern revealing two domains. “N14N0 was used for adsorption to avoid a large signal at m/e = 28 from the background gas. In the photodissociation experiments the surface saturated with N,O at 95 K was irradiated with excimer laser pulses of 193 nm (< 2.5 mJ/cm2, 2 Hz) and the TOF distributions of the desorbed species were measured by a QMS detector. The ionizer of the QMS was located 23 cm from the surface. The maximum surface temperature rise was estimated to be = 5 K [15]. N,-TOF distributions were obtained by accumulating the desorption signal for a large enough number of laser pulses (typically = 1000) to ensure complete depletion of the adsorbed N,O. In order to obtain a better signal-to-noise ratio, this procedure was repeated 5-10 times. Since the laser beam was fixed at an angle of 45” to the QMS view axis, the variation in the surface areas irradiated and subtended by the QMS as the sample is rotated had to be properly corrected in plotting the angular distributions.
3. Results N,O adsorbs on Si(100) in two different adsorption states at low temperature: physisorbed and chemisorbed states, which have been characterized by temperature-programmed desorption and X-ray photoelectron spectroscopy (XPS) [16]. When the surface is exposed to N,O at 63 K, N,O is mostly adsorbed in the physisorbed state. Physisorbed N,O molecules partially desorb and partially dissociate to 0 and N, even at a fairly low temperature of = 85 K. While the oxygen atom is inserted in a silicon dimer bond to form 0-Si bonds [17] during the thermal oxidation process, some physisorbed N,O are converted to the chemisorbed state on the partially oxidized Si(100) surface. Thus, the
J. Lee et al. / Chemical Physics Letters 240 (I 995) 417-422 1
I
I
1
i ;.“.. ‘$1
3, 1
5
0
. . .
.
.
~
I
100
I
I
200
Time of flight
300
1
400
(ps)
Fig. 1. TOF distributions of N, measured for various desorption angles, t?,,, measured with respect to the surface normal in the 193 nm photodissociation of N,O adsorbed on a partially oxidized Si(lOO). The surface was saturated with N,O at 95 K before photon irradiation. The two peaks in the distributions are denoted by P, (fast) and P, (slow), respectively.
chemisorbed N,O seems to be at a site near an oxidized Si atom, whereas the physisorbed state is more likely to be on a clean Si surface site. When the surface is exposed to N,O at 95 K, the surface is partially oxidized by the thermal decomposition of N,O and the molecularly adsorbed N,O is mostly in the chemisorbed state. All the data presented below were obtained with a Si(100) surface saturated with N,O at 95 K. When the N,O-covered surface was irradiated with 193 nm photons, N,O dissociated to produce an oxygen adatom and N, desorbing from the surface. Fig. 1 shows the TOF distributions of N, desorbed at various angles from the surface normal. While adsorbed N,O was completely depleted during N,-TOF measurements at each angle of detection, the oxygen coverage changed from = 0.7 to one monolayer, which was confirmed by post-irradiation XPS measurements. Thus, these results represent N,-TOF distributions accumulated in the range of adsorbate coverages cited above. Each distribution consists of two peaks, the fast and the slow velocity components
419
denoted by P, and Pz, respectively. One can notice that the angular dependence of the two peaks are qualitatively different. As the desorption angle, e,,,,, deviates from the surface normal, the P, peak first increases and then falls off, exhibiting a maximum at an intermediate angle, whereas the P2 component decreases monotonically with /3,,,. The TOF distribution obtained at tj,,, = - 15”, though not shown here, was the same as that at 0,,, = 15”, indicating that the velocity distributions are symmetric about the surface normal at least in the normal plane containing the [Oil] azimuth. Each TOF distribution was fitted by the sum of two shifted Maxwell-Boltzmann velocity distributions of the functional form f(u) =Au3 exp[-(u - ~,,)~/a~], where u0 and CY are parameters representing the shift and spread in velocity, respectively. The velocity-weighted average translational energy of the P, component is (E, > = 0.77-0.91 eV, increasing gradually with (I,,,, up to 30”. On the other hand, the P, component showed an almost constant (ET) of 0.28 eV. The maximum translational energy determined from the onset of the TOF distributions is 2.45 eV. The temperature of N, defined by (E,)/2k corresponds to 4400-5300 K and 1600 K for the P, and P, peaks, respectively. Angular distributions of the N,-desorption yield are plotted in Fig. 2, where the data points were obtained by velocity-weighted integration of the TOF
Fig. 2. Polar plot for the angular distributions of N, obtained by velocity-weighted integration of the TOF distributions in Fig. 1. The yields of the total (01, P, component CO), and P2 component ( A ) are shown together. The solid line shows the distribution of the total desorption yield represented by O.~~[COS*~~(~~~~ - 32”) + COS~~~(~,,,+ 32”)] + 0.25 COS~%~~~, determined from leastsquares fits. The fitted results for the P, and the P, components are also shown with broken and dotted lines, respectively.
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distributions per unit irradiated surface area. Interestingly, the P, component has a sharp angular distribution peaked at e,,, = 32”, whereas the P, component is peaked at the surface normal. The distribution of the total desorption yield was approximately represented by 0.75[cos8.0(0,,es - 32”) + cos8.0(eddes+ 32”)] + 0.25 cos2.20des, where the C, symmetry ’ of the P, component has been taken into account.
4. Discussion Although not shown here, irradiation with 351 nm laser pulses resulted in a qualitatively similar angular distribution. N,-TOF distributions showed an interesting wavelength dependence, but the maximum N, kinetic energy was the same at two widely different photon energies (6.4 versus 3.5 eV) [18]. Thus, the photodissociation of N,O on Si(100) is believed to be initiated by a substrate-mediated mechanism [19]. According to a relevant study on N,O production by the surface reaction of NO on GaAs(100) at 77 K [20], N,O also weakly adsorbs on GaAs(lOO) with a desorption temperature of 110 K. Metastable quenching spectroscopy showed that the highest occupied molecular orbital of N,O, the 27~ orbital, lies 7.8 eV below the Fermi level. If we assume a similar situation for cx,-N,O on Si(lOO), the 2~ level would lie at least 7 eV below the valence band maximum of silicon. Thus, it is not energetically possible to create such hot holes at a photon energy of 3.5 eV (A = 351 nm), and hence dissociation would not occur. Therefore, it is quite likely that photo- and/or ‘hot’ electrons produced by substrate excitation are responsible for dissociation rather than hot holes. By analogy to the gas-phase dissociative electron attachment of N,O [21], a negative ion state of the adsorbed N,O relevant to the excitation process could be correlated with either the bend N,O-( 2’) ground state or the linear first excited state of the gas phase N,O-. During a short lifetime of the negative ion state the N-O bond is elongated owing to the repul-
2
Strictly speaking, of the two equivalent contributions by the current measurements
it should show fourfold symmetry because domains of the Si(lOO)-2 X 1 surface, but the two out-of-plane lobes are small in the and therefore neglected.
sive potential surface along the N-O bond and the molecule begins to fall apart. The photofragment 0 atom is ultimately bonded to the surface Si atoms owing to a strong attractive interaction with the surface atoms, whereas N, is ejected from the surface, carrying some excess energy. As shown in Fig. 1, the P, component in the N,-TOF distributions dominates over the P, component in all the desorption angles. However, we found that the relative contribution of the P, component increases when N,O is adsorbed on Si(100) at 63 K [16], where N,O is physisorbed and the coverage of N,O is = 4 times larger than the surface saturated with N,O at 95 K. Thus, a possible origin for the P, component could be the energy dissipation of the N, photofragment through an inelastic collision with substrate or neighboring N,O when it leaves the surface. Alternatively, the P2 component could originate from the dissociation of physisorbed N,O that likely oriented randomly. In either case, the angular distribution of N, would be broad and isotropic. On the other hand, if N,O were more rigidly bonded to the surface in a specific orientation and isolated from neighboring adsorbates, the angular distribution of N, could be more anisotropic and narrower. Thus, we conclude that the P, component is attributed to N,O chemisorbed on a partially oxidized Si(100) surface. The bimodality in the translational energy distribution of the N, photofragment has also been observed in photodissociation of N,O on Pt(ll1) at 193 nm [22]. We now ask how the angular distribution of the N, photofragment is related to the initial bonding geometry of chemisorbed N,O. The molecular structure and configuration of N,O chemisorbed on Si(100) are not known. However, it is interesting to note that N,O adsorbed on clean GaAs(100) lies flat, but in contrast N,O produced by the reaction of NO is oriented vertically with the O-end down on oxidized GaAs(lOO) based on their He* quenching spectra of N,O, in which the electron emission yield is sensitive to the overlap between the He*(2s) and N,O valence orbitals [20]. Adapting this view to the present system, we suggest the following model for the adsorption structure of N,O chemisorbed on Si(100) as shown in Fig. 3. When randomly oriented physisorbed N,O is converted to the chemisorbed state by partial oxidation of the surface, the
J. Lee et al. / Chemical Physics Letters 240 (1995) 417-422
,Si,
,Si,
,.Si,
si I
Si I
Fig. 3. Proposed model of adsorption geometry N,O on a partially oxidized Si(100) surface.
of chemisorbed
chemisorbed N,O may be bonded to a Si dangling bond through the oxygen atom by making a weak covalent bond. Thus, the orientation of the chemisorbed N,O could be tilted with respect to the surface normal. This bond between N,O and a Si atom could be realized with some electron transfer from the Si atom to the lowest unoccupied molecular orbital of the adsorbate; this electron transfer makes the N-O bond weaker. This model can be supported by the following observations: First, as shown in Fig. 4, while N,O adsorbed at 63 K predominantly showed an O(ls)
SiO
I
530
*
,
N;O
,
,
I,
1
,
,
,
535
1
,
540
Binding Energy (eV) Fig. 4. O(ls) X-ray photoelectron spectra of N,O adsorbed on Si(100). The surface was exposed to (a) 2.5 L of N,O at 63 K and (b) 9.6 L of N,O at 95 K. The peaks at 535.9 and 535.3 eV are attributed to oxygen in N,O and the peak at 532.3 eV to oxygen adatoms produced by the dissociation of N,O.
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peak at 535.9 eV, the surface exposed to N,O at 95 K revealed two O&s) peaks at 532.3 and 535.3 eV, originating from oxygen atoms bonded to silicon atoms [23] and chemisorbed N,O, respectively. Thus, when the physisorbed N,O is converted to the chemisorbed state, the O(ls) peak is shifted toward lower binding energy and the amount of shift, 0.6 eV, is larger than that of the terminal N(ls), 0.2 eV; this indicates that some electron transfer takes place from the substrate to the chemisorbed N,O and its bonding configuration is likely to be O-end down. In addition, ab initio MO calculations [24] based on a silicon cluster model also indicate that the O-end down configuration is more stable than the N-end down configuration. Second, preliminary results of near-edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis of adsorbed N,O at 95 K is tilted from the surface normal [25]. Third, a weakened N-O bond can be inferred from the facile thermal dissociation of the chemisorbed N,O below 300 K. It is interesting to note that the dangling bond of the symmetric Si(lOO)-2 X 1 dimer atoms is inclined by 36” from the surface normal, which has been directly seen from the F+ ion emission angle in the electron-stimulated desorption of the F/Si(lOO) system [27]. Although oxygen insertion into a dimer bond may alter the tilting angle of the dangling bond to some extent, these observations bring the following conjecture; N, originating from the chemisorbed N,O is ejected in the direction of the N-O bond which is collinear to the Si dangling bond to result in an angular distribution peaked at = 32” from the surface normal. Thus, the present case seems to be quite similar to that of ESDIAD. However, one needs some caution to discuss the UV-photon-induced dissociation dynamics of N,O adsorbate on the same basis as used in ESDIAD. In particular, if the dissociation dynamics were mostly governed by the excited state with a bent configuration as N,O-(Z+) ground state in the gas phase, the relation between the angular distributions of N, and the bonding geometry of N,O would not be so straightforward. Thus, in order to clarify this point further in detail, it is necessary to perform more thorough experimental and theoretical studies, such as NEXAFS [25] and ab initio MO calculations [24], now in progress in our laboratory.
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Acknowledgement
We thank Dr. H. Oshima for providing the highly doped Si(100) samples. This work was supported in part by Grants-in-Aid for Scientific Research on New Programs (06NP0301) and on Priority Areas (06228232, 06239263) from the Ministry of Education, Science and Culture of Japan. JL gratefully acknowledges the hospitality of the Institute for Molecular Science during his sabbatical leave from Seoul National University.
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