Stimulated desorption of Cl+ and the chemisorption of Cl2 on Si(111)-7 × 7 and Si(100)-2 × 1

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Surface Science 316 (1994) 257-266

Stimulated desorption of Cl+ and the chemisorption on Si( ill)-7 x 7 and Si( lOO)-2 x 1

of Cl,

T.D. Durbin I, W.C. Simpson, V. Chakarian 2, D.K. Shuh 3, P.R. Varekamp, C.W. Lo 4, J.A. Yarmoff * Department of Physics, University of California, Riverside, CA 92521, USA and Material Sciences Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA

Received 8 December 1993; accepted for publication 11 May 1994

Abstract The chemisorption of Cl, on Si(lll)-7 X 7 and Si(lOO)-2 X 1 and the mechanism for stimulated desorption of Cl+ from Si are studied with soft X-ray photoelectron spectroscopy (SXPS) and photon-stimulated desorption (PSD). It is shown that Cl, interacts with Si(lll)-7 X 7 at room temperature to form mono-, di- and tri-chlorides, while primarily monochlorides are formed on Si(lOO)-2 X 1. These differences are explained in terms of the reconstructions of each clean surface. The stimulated desorption of Cl+ ions has a threshold at u 20 eV that results from a direct excitation of a Cl 3s electron to an unoccupied Cl antibonding level. No direct desorption of Cl+ ions is observed at the Si 2p edge. Differences between the mechanisms for F+ and Cl+ desorption from Si are discussed.

1. Introduction

Chlorine reactions play an essential role in both the plasma etching of Si and in the growth of films on Si substrates via chemical vapor depo-

* Corresponding author. Department of Physics, University of California, Riverside, CA 92521-0413, USA. Fax: +l 909 787 4529; E-mail: yarmoff@ucrphfl .vcr.edu. 1Present address: College of Engineering, Center for Environmental Research and Technology, University of California, Riverside, CA 92521, USA. ’ Present address: Naval Research Laboratory, Code 6345, Washington, DC 20375, USA. 3 Present address: Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA. 4 Present address: Applied & Engineering Physics, Cornell University, Ithaca, NY 14853, USA. 0039-6028/94/$07.00

sition. Investigations of the chemisorption of Cl, on clean Si surfaces are thus necessary in order to understand the fundamental gas-surface interactions that occur during processing. To address these needs, there have been numerous studies of the Cl-Si system [l-16]. High-resolution soft X-ray photoelectron spectroscopy (SXPS) employing synchrotron radiation is particularly well-suited for the identification of the surface products formed upon Cl, chemisorption, as the core-level binding energy shifts associated with halogen bonding are large enough to be easily resolved. The adsorption of Cl, on Si(lll)-7 X 7 at room temperature was previously investigated with SXPS by both Schnell et al. [2] and Whitman et al. [l]. In these studies, it was shown that Cl, chemisorbs dissociatively, forming a mixture of mono-, di- and tri-chlorides. As

0 1994 Elsevier Science B.V. All rights reserved

SSDI 0039-6028(94)00287-J

258

TD. Durhin et al. /Surface Science 316 (1994) 257-266

observed with scanning tunneling microscopy (STM) [8,9], these species form as a rest.& of Cl bonding at different sites of the dimer-adatomstacking fault reconstruction of the Si(lll)-7 X 7 surface. The chemisorption of atomic Cl and molecular Cl, on Si(lOO), on the other hand, has been investigated by photoelectron spectroscopy only with a laboratory X-ray source [17], which has lower resolution and less surface sensitivity than synchrotron-based SXPS. During plasma processing, chlorinated surfaces are exposed to electrons and photons that can induce the desorption of Cl. Thus, studies of the stimulated desorption of surface species are also useful for delineating mechanisms operative during processing. The electron stimulated desorption ion angular distributions (ESDIAD) of Cl+ ions desorbed from Cl,/Si(lOO)-2 X 1 have been investigated by two groups [6,7,11,13]. Both groups reported that Cl” is emitted from the surface in both normal and off-normal directions. There was disagreement, however, as to the origin of these ions and their relative distributions as a function of temperature. These results differ somewhat from ESDIAD of F+ desorption from Si(lOO), in which ions desorb principally in offnormal directions, consistent with F bonding at surface dimer sites in two orthogonal domains of the 2 x 1 reconstruction [13,18]. Furthermore, the ESDIAD studies did not address the details of the mechanism for Cl+ desorption. The mechanism for the stimulated desorption of F+ from Si has been previously investigated with photon-stimulated desorption (PSD) [19,20] and electron-stimulated desorption (ESDI [la]. It was shown that the threshold for desorption occurs at an incident electron or photon energy of - 27.5 eV, which corresponds to an excitation of the F2s level to the Si conduction band minimum (CBM). In addition, it was shown that PSD at the Si2p edge occurs primarily via the direct excitation of the bonding Si atom. This means that the specific photon energy at which ions desorb via excitation of the Si2p core level depends on the oxidation state of the bonding Si, i.e., the ion desorption is chemically selective. In this paper, the chemisorption of Cl, on Si(lll)-7 x 7 and Si(lOO)-2 X 1 and the mecha-

nism for stimulated desorption of Cl” from Si are studied with SXPS and PSD. The SXPS results show that, in contrast to the Si(lll)-7 x 7 surface, Cl, chemisorption on Si(lOO)-2 x 1 at room temperature forms primarily monochlorides. PSD measurements show that the threshold for stimulated desorption of Cl+ ions is at N 20 eV and occurs via a direct process involving a transition of a C13s core-level electron to unoccupied Cl antibonding levels that are located 6-8 eV above the valence band maximum [21]. In contrast to F+ desorption from Si, however, no direct desorption of Cl+ occurs at the Si2p edge.

2. Experimental procedure The experiments were performed on beamline UV-8a at the National Synchrotron Light Source (NSLS) located at Brookhaven National Laboratory. The SXPS and PSD spectra were collected with an ellipsoidal mirror analyzer (EMA) [22] operated in an angle-integrated mode, and monochromatic photons were generated with a 3 m toroidal grating mono~hromator. The Si 2p core-level spectra were collected using a photon energy of 130 eV in order to maximize the surface sensitivity of the measurement, as this produces photoelectrons with a kinetic energy of - 25 eV, which corresponds to the minimum escape depth [23]. At this energy, the combined resolution of the monochromator and spectrometer is better than 0.2 eV. To measure the desorption of positively charged ions, the polarity of the electron optics was reversed, as described elsewhere 2201.To acceIerate the ions into the detector, - 1000 V was applied to the front of the microchannel plate array, while a bias voltage in the range of - 25 to - 120 V was applied to the first grid of the EMA in order to collect ions emitted form the sample over all angles. Single-crystal Si(lll) and Si(100) wafers (ndoped, N 1 R. cm), oriented to a precision of +0.25”, were employed for these measurements. The samples were cleaned by in situ resistive heating to 1050°C. The temperature of the sample was measured with an optical pyrometer. The cleanness and crystallinity of the samples was

T. D. Durbin et al. /Surface

verified by the presence of features in both the Si2p core-level and valence band spectra indicative of clean, well-ordered surfaces [24]. The Cl, exposures were performed by backfilling a turbomolecular-pumped dosing chamber through a sapphire leak valve, and are reported in langmuirs (1 L = 10d6 Torr * s), uncorrected for the sensitivity of the cold cathode ion gauge employed. In all cases, a sufficient exposure was used to reach saturation coverage at room temperature. In order to determine the binding energies and relative areas of the various core-level components, the Si2p SXPS spectra were deconvolved in the following manner. First, the secondary electron background was removed via a two-step process, as discussed in Ref. [25]. The resulting data were then fit to a sum of Gaussian-broadened Lorentzian line shapes using a least-squares fitting procedure. The line shape employed has a spin-orbit splitting of 0.61 eV and a branching ratio of 0.52 [26], with the Lorentzian contribution to the width held constant at 0.12 eV [27]. In order to determine the identity of the positively charged ions that desorb from chlorinated Si, ESD experiments were performed in a separate III-IV chamber. This chamber is equipped with low-energy electron diffraction (LEED) optics that are also used as a retarding field analyzer for Auger electron spectroscopy @ES), an electron gun, a quadrupole mass spectrometer (QMS) and a hemispherical electrostatic analyzer (ESA). An electron beam energy of 300 eV was used to collect the ESD spectra. This energy is greater than the highest energy used for the collection of the PSD data, so that all of the desorption channels that contributed to the PSD measurements were excited by the electron beam. In order to mass-analyze the desorbing ions, the QMS was operated with the ionizer turned off. Using this method, the stimulated desorption products from Si(lll)-7 x 7 saturated with Cl,, XeF, and H,O were identified as Cl+, F+ and H+, respectively. ESD from XeF,- and H,Osaturated surfaces were collected because F+ and H+ are likely contaminants in a PSD measurement. Once the identity of the positive ion was determined with the QMS, the corresponding

Science 316 (1994) 257-266

259

kinetic energy distribution was measured with the ESA. For Cl,-saturated Si(lll)-7 x 7, from which only Cl+ desorbed, a single feature was observed in the kinetic energy distributions. The kinetic energy distributions obtained from Cl,-saturated Si(ll1) at the NSLS were identical to those obtained via ESD, so that it was concluded that Cl+ is the major desorption product in the PSD spectra collected from Si(ll1). For the PSD spectra collected at the NSLS from chlorinated Si(lOO), however, there were, at times, two features observed in the kinetic energy distributions, as shown in Fig. la. This indicates a considerable contribution from a second ionic species. By comparison to ESD spectra collected from fluorinated Si, it was determined that the high kinetic energy feature results from desorbing F+ ions. Some small amount of F was likely to be present on these samples due to contamination from background gases in the dosing chamber. It is well-known that desorption of F+ ions can be detected for very small surface concentrations, since the stimulated desorption cross section of F+ is much higher than that of other ions, such

b) + 63O’C

a) 10 kL Cl,

-2

0 Ion

2

4

Kinetic Energy (eV)

Fig. 1. Kinetic energy distributions of ions desorbed by irradiation with 101 eV photons from a Cl,-saturated Si(lOO)-2x 1 surface at room temperature and following a 630°C anneal.

T.D. Durbin et al. /Surface

260

as oxygen or other halogens [28]. In fact, F+-ion desorption is often observed from surfaces that were not int~ntionalIy exposed to F or F-containing compounds [13,18]. These surfaces showed no measurable fluorine signal with either SXPS or AES. By annealing the Cl,-saturated Si(100) surface to temperatures above N 525*C, which is the temperature at which F desorbs from Si 1291,the F+ contribution to the PSD signal is removed, as shown in Fig. lb. Thus, only Cl+ PSD spectra collected from Si000) surfaces that have been post-annealed are shown in this paper. It should be noted that since the level of F contamination was well below the SXPS detection Emit, the presence of F did not affect the SXPS results.

Science 316 (1994) 257-266

t b) 300 L ~l~Si(~~)

a) 5 kL Cl&3(111)

u-_-.._A -4

-2

0

Binding Energy (eV, Relative to bulk Si 2~32)

3. Resuits and discussion 3.1.

SoftX-ray p~ot~~~ctro~ spectroscopy {SXPSJ

SXPS spectra of the valence band and shallow core levels collected with 86 eV photons from clean and Cl,-saturated Si(lll)_7 x 7 and Si(lOO)-

20

Ill

0

Binding Energy (eV, Relative to VBM) Fig. 2. SXPS survey spectra collected with a photon energy of 86 eV for (a) clean Si(lll)-7x7, (b) Cl,-saturated Si(lll)7X7, (c) clean Si(iOO)-2X 1, and (d) Cl,-saturated Si(lOO)2 x 1. The binding energies are given relative to the valence band maximum.

Fig. 3. Si 2p core-level spectra collected from Si(lll)-7 X 7 and Si(lOO)-2x 1 saturated by Cl, at room temperature. The dots show the raw data after background subtraction, the dashed line shows the individual com~nents of the numerical fit and the solid line is the sum of the components.

2 x I are shown in Fig. 2. The spectra, which show the valence band region from 0 to 10 eV and the Cl3s at 15-16 eV, are similar to those reported elsewhere [2,3]. For the clean surfaces, there are three large peaks in the valence band that originate from the Si substrate, and a small contribution from surface states visible near the Fermi level. After CL, adsorption, the surface states disappear, and, in addition to the CI 3s level, there are three new states created at binding energies of approximately 3.4, 4.7 and 6.3 eV. Si2p SXPS spectra collected after the adsorption of saturation coverages of Ci, on Si(lll)7 x 7 and Si(lOO)-2 x 1 are shown, after background subtraction, in Fig. 3, along with numerical fits to the data. The spectrum collected from Si(lll), shown in Fig. 3a, is similar to those previously published, and is adequately fit with four components [1,2]. The largest component, which is at the lowest binding energy, originates from bulk, i.e., substrate, Si. The three components shifted 0.9, 1.7 and 2.7 eV with respect to the bulk component have been previously considered to result from adsorbed SiCl, SiQ, and

T.D. Durbin et al. /Surface

SiCl, moieties, respectively. There may, however, be a contribution to the 0.9-eV-shifted component from substrate Si atoms that are bonded to SiCl, groups, as discussed below. Note that the width of each of the components increases with oxidation state, as also observed previously, due to an increase in the disorder associated with the local bonding environment of each surface species. After Cl, chemisorption on Si(lOO)-2 x 1, as seen in Fig. 3b, the Si2p spectrum shows primarily the 0.9-eV-shifted component, due to adsorbed SiCl, with only a trace amount of SiCl,. Although the component shifted 0.9 eV from the substrate the Si2p component has previously been identified as resulting from Si+, i.e. SiCl, there is new evidence to suggest that the nearest-neighbor Si to an Si3+ group would also show a binding energy shift. This evidence comes from studies of Si oxide systems, which show that an Si3+-O3 unit has a sufficient group electronegativity to induce a large core-level shift of its own [30-321. Following that formalism, the group electronegativity of SiCl, is found to be close to the electronegativity of a Cl atom, and the core-level binding energy shift for a bulk Si atom bonded to SiCl, is calculated to be approximately 75% that of the binding energy shift due to direct bonding to Cl. Thus, bulk Si bonded to SiCl, may be indistinguishable from an Sic1 group. If this were the case, in order to quantitatively determine the amount of Sic1 that is adsorbed on the surface, the intensity of the 0.9eV-shifted component needs to be reduced by the area of the SiCl, component. Although nearestneighbor effects will be explored in greater detail in future work, the analysis presented here is given with and without consideration of these effects. Note that nearest-neighbor shifts calculated for Sic1 and SiCl, are smaller than the

Table 1 Intensities of the SiCI, SiCl, and SiCI, Si2p components given as a fraction of the total Si2p core-level intensity for Cl,-saturated Si(lll)-7 x 7 and Si(lOO)-2x 1 Si(lll)-7x7 Si(lOO)-2X 1

Sic1

SiCl,

SiCl,

0.26 0.38

0.11 0.04

0.07 -

Science 316 (1994) 257-266

261

Table 2 Coverages of SiCl, SiCl, and SiCl, for Cl,-saturated Si(lll)7X7 and Si(lOO)-2X 1; the coverages are determined as described in the text; for Si(ll1) the results are calculated both with and without consideration of nearest-neighbor shifts; on the right, the total Cl and SiCl, coverages are given Coverages of SiCl, (ML)

Total coverage (ML)

Sic1

SiCl,

SiCl,

O,,

Osia,

Si(lll)-7X7 0.74 Si(lll)-7 x 7 - with 0.53 nearest-neighbor shiftsconsidered 1.11 Si(lOO)-2X 1

0.31 0.31

0.21 0.21

1.98 1.78

1.26 1.05

0.11

-

1.33

1.22

width of the bulk component and are therefore not observable. The relative proportions of each of the shifted components to the total Si2p area are given in Table 1 for both surfaces. By only employing ratios within a given spectrum, any discrepancies due to changes in the beam or sample positions between Cl, doses are eliminated. A quantitative analysis of the coverages of adsorbed chlorine is presented in Table 2. The SiCl, coverages are determined from the ratios given in Table 1 by comparison to the ratio of the surface core-level shift (SCLS) to the total Si2p intensity measured on the clean surface, as discussed in Refs. 123,331. Based on this calibration, shifted core-level intensity ratios of 0.35 and 0.34 indicate that 1.0 ML of Si is bonded to an adsorbate on Si(ll1) and Si(lOO), respectively. In Table 2, the Sic1 coverage is shown both before and after reducing its intensity by the area of the SiCl, component to account for the nearestneighbor shifts. Note that cross-sectional enhancements of the shifted Si components [25] are not accounted for, as they are unknown for the Cl-Si system. Although these enhancements should be small, they would act to reduce the calculated coverages, primarily for SiCl,. The SiCl, SiCl,, SiCl, and total SiCl, (Osic,,> coverages calculated in the above manner are given in Table 2. The Cl coverage CO,,) is calculated by summing the SiCl, coverages after multiplying the SiCl, and SiCl, contributions by two and three, respectively.

262

T.D. Durbin et al. /Surface

It is reasonable to assume that the differences in the distributions of adsorbed chlorides between the (111) and (100) surfaces are a consequence of the geometry of the initial surface reconstructions. The Si(lll)-7 X 7 structure is more open than Si(lOO)-2 X 1, and can therefore more easily accommodate adsorbed chlorine. The manner in which the structure of the Si(lll)-7 X 7 surface is responsible for the formation of adsorbed chlorides was investigated with STM by Boland and Villarrubia [8]. They showed that upon the initial reaction, Sic1 formation occurs as Cl attaches at the dangling bond (DB) sites of both the rest atoms and adatoms. Di- and tri-chlorides then form from the adatoms when the highly strained bonds to the second layer are broken. If one of these bonds is broken by a Cl atom attaching to Sic1 located at an adatom site, the adatom then converts to SiCl, bonded at a bridge site. When an additional Cl atom attaches to the adatom, a second Si-Si bond is broken and SiCl, is formed at an atop site. It should be noted, however, that the combined coverage of the higher chlorides, SiCl, and SiCI, (0.52 ML), is well in excess of the number of adatoms (0.24 ML), as has been previously noted for Cl, [5] and I, [33] adsorption on Si(lll)-7 X 7. This shows that, in addition to higher chloride formation at surface adatom sites, there is further disruption of substrate Si-Si bonds by the chemisorption reaction. This bond breaking exposes additional sites at which higher chlorides can attach. The adsorption of SiCl, and SiCl, in excess of the surface adatom concentration is explained in the following manner. When a Cl-Cl bond is ruptured by attachment of Cl to a surface DB, the other Cl atom is liberated. Liberated atomic Cl can then react with the surface by either attaching at another DB site or by breaking a substrate Si-Si bond. The breaking of substrate Si-Si bonds by atomic Cl was demonstrated in a study which showed that the adsorbed layer due to the reaction of Si with atomic Cl is much thicker than the layer formed by reaction with Cl, molecules [171. This difference was explained by asserting that Cl atoms can break substrate Si-Si bonds, while Cl, molecules cannot. When a

Science 316 (1994) 257-266

Si-Si bond is broken by a Cl atom, one Si-Cl bond is formed while the other Si atom is left with a DB. This DB then acts to propagate the reaction, as long as it is located in a region accessible to incoming Cl, molecules. The overall reaction terminates when all of the reactive DBs have been saturated, so that no further Cl, dissociation can take place. The room temperature chemisorption of Cl, on Si(lOO)-2 X 1, in contrast to Si(lll)-7 X 7, forms primarily SiCl. Again, this result can be explained in the context of the geometry of the reconstructed surface. For the Si(lOO)-2 X 1 structure, Cl, will most likely attach at the DBs of a Si-Si dimer, thus forming two Sic1 groups. This interpretation is in accord with other experimental results. A combination of ESDIAD and high-resolution electron energy loss spectroscopy (HREELS) has shown that Cl bonded at dimer sties is the majority species after Cl, adsorption at 120 K [6,7]. LEED has shown that the 2 X 1 LEED pattern of the clean surface remains after Cl, saturation, indicating that Si-Si dimer bonds are largely not broken [3,7,34], although some reduction of the half-integral spots has been reported [7]. Previous experiments using polarization-dependent photoemission have also concluded that Cl forms covalent bonds to Si(100) in an off-normal direction [3]. Finally, STM has directly shown that the initial Cl, adsorption occurs at surface dimer sites without disruption of the 2 x 1 reconstruction [lo]. Because dimer sites are located near to each other on the Si(lOO)-2 X 1 surface, when most of the Cl, molecules dissociate, they will attach at two dimers. Thus, only a small amount of liberated atomic Cl is available for Si-Si bond breaking, which leads to a saturation Cl coverage close to 1 ML, and very little adsorbed SiCl,. The trace amount of SiCl, observed on Si(100) is likely to have formed via the breaking of some of the Si-Si bonds by atomic Cl liberated from Cl, dissociation at a surface DB. The total number of SiCl, groups that could form on Si(100) is limited, however, by lateral repulsion between neighboring Cl atoms. The lateral repulsion between adsorbed halogens was investigated theoretically for F on Si(100) by Wu and Carter [35].

T. D. Durbin et al. f Surface Science 316 (1994) 257-266

These studies showed that a surface with a uniform coverage of SiF, could not form due to the large activation barriers resulting from the repulsive interaction between neighboring F atoms. Wu and Carter predicted a large activation barrier for the addition of F atoms beyond a coverage of 1.5 ML. It is expected that the latera repulsion will be even greater between adjacent Cl atoms, since the ionic radius of a Cl atom (1.81 ;i> is greater than that of F (1.33 A>, and the Cl-Si bond length (_ 2.0 A> is greater than the F-Si bond length (N 1.6 ;i> [36]. Another possible explanation for the presence of SiCl, is that it is formed at step edges or defects. Although this cannot be entirely ruled out, the density of step edges on these well-oriented samples is small, so that adsorption at step edges is an unlikely explanation for the formation of SiCI,. It should be noted that in a recent STM study of Cl, on Si(lOO), a small concentration of species composed of two Cl atoms was also identified, consistent with the present results [lo]. As a further note, there is no indication of a thermal conversion of higher chlorides to monochlorides as suggested in Refs. [l&13], since after annealing to 6Oo”C, both Sic1 and SiCl, species remained on the surface. Although Cl bonding at dimer DB sites is the predominant species on the (100) surface, there is evidence for other bonding structures as well. For example, ESDIAD [6,7,11,13] studies have observed normally emitted Cl” ions, which is evidence for Cl bonding in a ~nfiguration that is bonded normally to the surface. STM has shown that some Cl bonds in a bridge site 1101, i.e. as S&Cl, which is likely to be the species responsible for the normal Cl+ desorption [6,7]. For this structure, however, it is expected that the chemical shift in the Si2p core level would be, to first order, half that of SiCl, so that its contribution to the SXPS spectra would be difficult to determine via deconvolution.

energies. Note that the kinetic energy distributions have been normalized to equal peak height in the figure. The data show that the shape of the kinetic energy distribution of Cl+ ions is independent of photon energy over the range employed. These distributions are similar to those reported for ESD of Cl’ ions from Si(lO0) [11,13] and are peaked at N 0.4 eV. The observation of only a single kinetic energy component with a shape that is independent of the photon energy suggests that there is only a single repulsive state from which desorption occurs. This conclusion is borne out by analysis of the photon energy dependence of the PSD yield, presented below. The kinetic energy distribution is asymmetric, which is a result of a cut-off for ions whose energy is lower than that of the surface potential experienced by the ion [371. The cut-off is observed in these spectra because the kinetic energy distribution of Cl” ions is small enough that it

...-.- hv

a~ lO@eV

-hv=106eV hv = 110 eV

I -I

I-

I+-, + hv=ZleV * hv=lOOeV Ahv=l(MeV + hv=llOeV . hv = 125 eV a) UN)L ~l~Si(lll)

-1

Kinetic energy distributions obtained for Cl+ ions desorbed from Si(lll) and Si000) are shown in Fig. 4 for excitation at a number of photon

263

2 0 1 Ion Kinetic Energy (eV)

3

Fig. 4. Kinetic energy distributions of Cl+ ions desorbed from a t&-saturated Si(ll@7x 7 surface after annealing to _ 500°C and from a Cl,-saturated Si(lOO)-2x I surface after annealing to 630°C collected with a variety of incident photon energies. The data are normalized to equal peak heights.

T.D. Durbin et al. /Surface

264

overlaps the minimum energy that is necessary to overcome the surface potential. Since the dipole contribution to the surface potential is different for ions and electrons, electron cut-off spectra cannot be used to locate the zero of energy for ions. Thus, because the contact potential difference between the sample and the analyzer experienced by ions cannot be directly measured, the energy scale in Fig. 4 was calibrated by setting the observed cut-off energy in each spectrum to zero. There is a certain absolute minimum excitation energy required for direct ion desorption [381. This threshold energy can be considered as the difference between the energy of an atom bonded to the surface and an unbound ion having the measured kinetic energy. For the Cl-Si system, this threshold is the energy required to break a Cl-Si bond (4.6 eV) [36], ionize the Cl atom (13 eV) [36] and give the Cl+ ion 0.4 eV of kinetic energy. Thus, the absolute threshold for desorption of Cl+ from Si is N 18 eV. The observed threshold then corresponds to the first electronic transition that produces ion desorption, which must occur at an energy greater than this absolute threshold. The Cl+ PSD yield as a function of photon energy in the region of the threshold is shown in Fig. 5. This spectrum shows an edge with an onset at N 20 eV. Note that the background seen prior to the edge is due to PSD induced by second- and higher-order light. The edge in the PSD yield spectrum occurs at the energy required for the absorption of a photon via a direct transition of a C13s core-level electron to an unoccur

1

ZOOLCl2 on Si(ll1)

gi

fiti

+-500°C

1

Photon Energy (eV) Fig. 5. PSD spectrum Cl,-saturated Si(lll)-7

x

collected at the Cl3s edge from a 7 surface annealed to - 500°C.

Science 316 (1994) 257-266

pied level. Following photon absorption, the Cl 3s hole is filled by an intratomic Auger process, which creates a two-hole state on the Cl atom, thus changing the oxidation state from - 1 to + 1 [39]. Coulomb repulsion between the Cl+ ion and the Si cation is then responsible for the desorption of the ion. Since the first step involves the absorption of a photon, PSD yield spectra, such as that shown in Fig. 5, provide the absorption spectrum of the surface atoms. The position of the PSD threshold corresponds with promotion of a C13s electron to an unoccupied state. The unoccupied levels of the Cl-Si system have been previously investigated both theoretically and experimentally. Calculations of the Cl-Si surface band structure show the presence of Cl antibonding orbitals which show dispersion from 6 to 8 eV above the VBM [21]. These unoccupied levels have also been observed experimentally with constant initial state photoemission [31. In Ref. [31, levels located - 6.5 and N 6.2 eV above the VBM were observed for Cl,-saturated Si(lll)-7 X 7 and Si(lOO)-2 X 1, respectively. A transition from the C13s core level, which is located approximately 15-16 eV below the VBM (as shown in Fig. l), to these unoccupied orbitals corresponds well with the PSD feature whose maximum is located at N 22 eV in Fig. 5. PSD spectra of Cl+ collected at the Si2p edge from Cl,-saturated Si(lll)-7 x 7 and Si(lOO)-2 x 1 are shown in Figs. 6a and 6b, respectively. The spectra were normalized to equal intensity at the edge. The onset for PSD at the Si2p edge corresponds to a transition from the bulk Si2p to the CBM, which is illustrated by a pair of vertical lines that indicate the Si2p,,, and the Si 2p,,, initial states. The measured PSD yield is essentially identical to the bulk absorption, which is shown by the total electron yield (TEY) spectrum of Fig. 6c. This indicates that the dominant mechanism for Cl+ desorption at the Si2p edge occurs via the excitation of a bulk Si atom, which indirectly leads to desorption, rather than the excitation of a Si atom that is directly bonded to Cl. The subsequent relaxation of the bulk Si2p core hole creates a cascade of secondary electrons which induce ion desorption via the excitation of

T.D. Durrbin et al. /Surface

*i

mm

I si2p----*cBM L

- 98

102

104

106

100

lli

Photon Energy (eV) Fig. 6. PSD spectra collected

at the SiZp edge from (a) a t&-saturated Si(lll$7X7 surface, and fb) a &-saturated Si(lOOI-2x 1 surface annealed to 630°C. A total electron yield WY1 spectrum is shown in (cl.

a lower-lying transition [40]. Since the number of secondary electrons created is proportional to the absorption, the indirect process produces a PSD yield curve that is essentially the same as the TEY spectrum, as observed. In this particular case, the lower energy transition that is excited is the one that originates from the Cl 3s initial state, as discussed above, since it is the only possibility, The fact that photons over this entire energy range all act to excite this same transition is consistent with the observation of only a single feature in the kinetic energy distributions with a shape that is independent of photon energy. These results differ from those reported for the desorption of F+ ions at the Si 2p edge [19,20]. For F-Si it was found that the primary channel for ion desorption at the Si 2p edge is through the direct excitation to the CBM of an electron from the Si2p level of a Si atom directly bonded to F. Since F atoms are very electronegative, the adsorbed F is initially in an F- oxidation state. The decay of this excited state, which occurs via an interatomic Auger process 1411,creates two holes on the F atom, thereby producing an Ff ion, Subsequent Coulomb repulsion between the F4

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and Si, both now in + 1 oxidation states, leads to F+ desorption. The differences in the mechanisms for ion desorption of F+ and Cl+ at the Si2p edge are due primarily to differences in the natures of Si-Cl and Si-F bonds. Because of the high electronegativity of F, there is a larger amount of charge transfer from Si to adsorbed F. Thus, the electron density near the Si atom is limited, so that the only electrons available to fill the Si2p core hole are those that are localized at the F atom. Consequently, the core hole is filled via the interatomic Auger process. The shorter Si-F bond length, as compared to Si-Cl, also facilitates the interatomic process. For the Si-Cl bond, however, the degree of charge transfer between Si and Cl is less than that for Si-F. Ab initio calculations of Si clusters found that for an on-top adsorption site the net charge transfer from Si to F is -OSSe, whereas the net charge transfer to Cl is only -0.36e [42]. It is therefore more likely that the Si2p core hole can be filled by a faster intratomic Auger decay channel for Cl bonded to Si. Since this intratomic process places two holes on the Si atom, rather than on Cl, the repulsive state required for ion desorption is never created,

4. Conclusions The chemisorption of Cl, with S#lll) and Si(l~) and the mechanism for stimulated desorption of Cl+ from Si have been studied with SXPS and PSD. Cl, chemisorption on the Si@O) surface forms primarily monochlorides at room temperature, in contrast to the chemisorption of Cl, on Si(lll&7 x 7 which forms SiCl, SiCl, and SiCl,. These differences are related to the geometries of each surface reconstruction. PSD measurements show that Cl+ deso~tion is initiated by a transition from the C13s core level to unoccupied Cl antibonding levels. This process has a threshold at an energy of N 20 eV. At the Si2p edge, Cl+ desorption occurs via an indirect process in which secondary electrons induce ESD, in contrast to the direct process observed for F+ desorption from Si.

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Acknowledgements

This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the US Department of Energy under Contract No. DEAC03-76SF00098. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. This work was carried out, in part, at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the Department of Energy (Division of Materials Sciences and Division of Chemical Sciences, Basic Energy Sciences) under contract No. DE-AC02-76CH 00016.

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