Isolated Silicon Dangling Bonds on a Water-Saturated n+-Doped Si(001)

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Isolated Silicon Dangling Bonds on a Water-Saturated nþ-Doped Si(001)-2 1 Surface: An XPS and STM Study J.-J. Gallet,† F. Bournel,† F. Rochet,*,† U K€ohler,‡ S. Kubsky,§ M.G. Silly,§ F. Sirotti,§ and D. Pierucci†,§ †

Laboratoire de Chimie Physique, Matiere et Rayonnement, Unite Mixte de Recherche CNRS 7614, Universite Pierre et Marie Curie, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05 France ‡ Fakult€at f€ur Physik und Astronomie, Institut f€ur Experimentalphysik IVAG Oberfl€achenphysik, Ruhr-Universit€at Bochum, NB 4/166, D-44780 Bochum, Germany § Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif sur Yvette Cedex, France ABSTRACT: Using Si 2p core-level X-ray photoelectron spectroscopy we have measured the upward band bending at the surface of nþ-doped water-saturated Si(001)-2 1 and inferred the macroscopic negative surface charge density of the surface. These macroscopic results are in excellent accord with the microscopic view provided by dual-bias scanning tunneling microscopy showing that the isolated silicon dangling bonds (∼1.2 102 defects per Si atom) bear indeed a negative charge. Noting the structural analogy between isolated dangling bonds on water-saturated Si(001) and H-terminated Si(001), in the final, prospective section of the paper, we raise the question of the possible role that these defects could play in radical chain reactions with πbonded molecules, in relationship with the hydride/hydroxyl patterns that are resolved in the scanning tunneling images.

1. INTRODUCTION Silicon surfaces modified by exposure to water vapor have been the object of numerous studies (see ref 1 for a review), due to the role of water in wet silicon oxidation,2 in the atomic layer deposition process of high-κ dielectrics,3 or even due to the use of watercovered Si(001)-2 1 as a reactive surface for the attachment of carboxylic acids4 and ethoxysilanes.5 On clean Si(001)-2 1, water dissociates into H and OH fragments that decorate the silicon dangling bonds left by surface dimerization. In fact two reaction channels, the intrarow and the on-dimer, are opened with equal probability as shown by recent calculations.6,7 In the intrarow process, H and OH are adsorbed on the same side of two adjacent Si dimers pertaining to the same row (Figure 1). At extremely low coverage, this channel leads to the formation of the so-called C-defect (two adjacent 3 SiSiOH and 3 SiSiH units, characterized by a pair of formally singly occupied dangling bonds), well documented in the scanning tunneling microscopy (STM) literature.812 For its part, the on-dimer reaction channel corresponds to the molecular cleavage over a single dimer, leading to the formation of one HSiSiOH unit (see Figure 1, middle). In contrast to the intrarow process, the on-dimer channel leaves no dangling bonds. The competition between these two channels explains why, at surface saturation, isolated dangling bonds (IDB) in 3 SiSiOH or 3 SiSiH units (Figure 1, bottom), are observed by STM as bright features in occupied state images,13,14 contrasting with the lower “density-of-state” imprint of the silicon atoms capped with OH or H. Density functional theory (DFT) calculations of the equilibrium structure of 3 SiSiOH and 3 SiSiH point to similar r 2011 American Chemical Society

SiSi bond lengths (0.233 and 0.234 nm, respectively), but the buckling is slightly more pronounced for 3 SiSiOH (12) than for 15 3 SiSiH (9). The present work is in continuity with our preceding X-ray photoelectron spectroscopy (XPS) and O K-edge X-ray absorption spectroscopy study of the hydroxyls on the water-saturated surface.16 It aims specifically at revealing the electronic structure of isolated dangling bonds (IDBs) on water-saturated, nþ-doped, Si(001)-2 1, a question unaddressed in ref 16 but that will appear central for a discussion on the chemical reactivity of this modified silicon surface. A second objective is to gain further insight into the H/OH patterning, which is also a key issue for the understanding of reactivity. Focusing on the electronic structure aspects, we note that the IDB of the water-saturated Si(001)-2 1 surface is born by a trivalent silicon, similar to the Pb defects at Si/SiO2 interfaces17 and to the IDBs in SiSiH units of the H-terminated Si(001)-2 1 surface.1820 Pb defects are known to be amphoteric.17 In the same vein, a self-consistent PoissonSchr€odinger tight binding model, using a cluster to mimic a 3 SiSiH unit on the H-terminated surface,21 and a periodic slab calculation of the H-terminated Si(001)-2 1 surface22 both show that the charge of the IDB is negative with n-doping and positive with p-doping. The structural analogies with IDBs on H-terminated surfaces, that are known to Received: February 8, 2011 Revised: March 9, 2011 Published: March 25, 2011 7686

dx.doi.org/10.1021/jp201262x | J. Phys. Chem. C 2011, 115, 7686–7693

The Journal of Physical Chemistry C

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2. EXPERIMENTAL DETAILS

Figure 1. Top and middle: H2O dissociation channels at the Si(001)(2 1) surface. Bottom: isolated dangling bonds remain on the watersaturated surface at room temperature, as OH and H fragments of the dissociated molecule randomly terminate the adjacent dangling bonds on intrarow or on-dimer sites.

induce radical chain reactions with π-bonded molecules,23,18,24 are a further motivation to examine the electronic structure of IDBs formed on water-saturated Si(001)-2 1. Note that the only known π-bonded molecule that attaches to this surface is a carboxylic acid (the mechanisms are not discussed in ref 4), but the analysis is complicated by the possibility of a water elimination reaction. In our combined approach, information on the electronic structure was obtained through synchrotron radiation photoemission spectroscopy (XPS) and dual-bias STM imaging. With surface-sensitive Si 2p core-level spectroscopy, we obtain a chemical characterization of the water-saturated (nþ-doped) Si(001) surface. We measure the value of the upward band bending at the surface, which in turn allows us to determine the macroscopic (negative) surface charge density.25 With respect to previous works,13,14 obtaining unoccupied state STM images of the IDB defects enables an unambiguous determination of the areal density and charge state of the defects that can then be compared to the macroscopic surface charge density deduced from Si 2p XPS binding energies. The key feature is the observation of a local upward band bending around the negatively charged defect, analogous to what is found on nþ-doped H-terminated Si(001).19 Finally, STM unveiled the local distribution of monohydrides and hydroxyls, useful in a discussion of a possible chain-reaction growth mechanism of π-bonded molecules.

2.1. Preparation of the Water-Saturated Si(001)-2 1. Water-saturated Si(001)-2 1 surfaces examined by XPS were produced in the preparation chamber of the TEMPO26 beamline end-station, SOLEIL synchrotron facility. We used highly doped nþ-type Si(001) wafers (ND ∼ 2 1019 P atoms/cm3) which are cleaned from their native oxide by flash annealing (Joule effect) at 1100 C after prolonged degassing at 600 C in ultrahigh vacuum. The silicon surface is then saturated by H2O reaction products by dosing at room temperature under a pressure of 108 mbar (ion gauge uncorrected reading27). Doses are given in langmuirs (1 L = 106 Torr s). The same wafer cleaning procedure and water exposure procedure was used in the preparation chamber of the STM setup installed in the Surface Laboratory of the SOLEIL facility. We used heavily nþ-doped Si(001) substrates (ND ∼ 4 1018 As atoms/cm3). 2.2. XPS. We have described the main features of the TEMPO beamline in a previous paper.28 The radiation source is an Apple II type Insertion Device (HU80). The photon energy is selected using a high-resolution plane grating monochromator, with a resolving power E/ΔE that can reach 15 000 on the whole energy range (501500 eV). The end-station chamber (base pressure = 1010 mbar) is equipped with a modified SCIENTA-200 electron analyzer, fitted with a delay-line 2D detector. Photoelectrons are detected at 0 from the sample surface normal and at 46 from the polarization vector E. The Si 2p levels are measured at hν = 130 eV, with an overall resolution of 70 meV. The 30 eV kinetic energy of the photoelectrons leads to a short escape depth λ of about 0.4 nm.29 After a Shirley background subtraction, the core-level spectra are fitted by sums of Voigt curves, i.e., the convolution of a Gaussian (of fullwidth at half-maximum GW) by a Lorentzian (of full-width at half-maximum LW). The Si 2p doublet is reconstituted with a 2p1/2:2p3/2 ratio of 0.5 and a spinorbit splitting of 0.6 eV.30 We take a LW of 45 meV31 for the Si 2p1/2 and Si 2p3/2 lines. The minimum “elemental silicon” GW is obtained for the water-saturated surface and is equal to 0.256 meV. The total full-width at half-maximum (fwhm) of the Voigt profile is calculated using the formula: fwhm ≈ 0.5436 LW þ (0.2169 LW2 þ GW2)1/2.32 The zero binding energy (BE) (i.e., the Fermi level) is taken at the leading edge of a clean metal surface in electrical contact with the silicon crystal (the Si 2p3/2 BE is found at 99.35 eV for the clean surface). 2.3. Scanning Tunneling Experiments. We have used a variable-temperature VTSTM OMICRON apparatus of SOLEIL surface laboratory. All STM observations reported in the present paper were obtained at room temperature by using a W tip. The dual bias images were taken on a line-by-line basis, inversing bias at each scan direction change. Images were obtained at constant current I, with the sample biased by Vbias with respect to the tip. Sample preparation was identical to that of photoemission experiments.

3. RESULTS AND DISCUSSION First we show in Figure 2 the Si 2p spectra (experimental data and fits) of the clean surface and of the water-saturated Si(001)-2 1 (water dose equal to 6.8 L), measured in surface-sensitive conditions at hν = 130 eV. Curve fitting results are collected in Table 1. The attribution of the various Si 2p3/2 components is based on the preceding Si 2p XPS works:30,33 a main structure at 99.35 eV (67% of 7687

dx.doi.org/10.1021/jp201262x |J. Phys. Chem. C 2011, 115, 7686–7693


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the Si 2p3/2 spectral weight), we denote B(þSS), as it encompasses both the bulk component B and the positively charged down silicon dimer atom (labeled SS), that presents a surface core-level shift (SCLS) of only þ0.06 eV from the bulk Si 2p3/2;30 a peak labeled S with an SCLS of 0.55 eV from the Si 2p3/2 component (15% of the Si 2p3/2 spectral weight), related to the negatively charged up silicon dimer atom; a structure labeled C with an SCLS of 0.34 eV (6% of the Si 2p3/2 spectral weight), related to subsurface silicons (3rd plane) according to ref 30; and finally the S0 peak with an SCLS of þ0.27 eV from B (12% of the Si 2p3/2 spectral weight), attributed to the silicon second plane. The exposure to water leads to substantial changes in the Si 2p spectrum, due to the capping of the silicon dangling bonds of the dimerized surface by H and OH fragments (except naturally the IDBs whose surface density is below the detection limit). As a consequence, the “up dimer” surface state line S disappears. A small peak S1 (3.2% of the Si 2p3/2 spectral weight) with a SCLS of 0.28 eV from bulk Si 2p3/2 is still present. S1 could be a

remainder of peak C of the clean surface. The formation of the hydroxyl species appears clearly as a component denoted Si1þ, at þ0.91 eV from the bulk line B, with a Si 2p3/2 spectral weight of 15%. We note the absence of the higher oxidation state Si2þ (SCLS of þ1.8 eV) that was observed after exposure to high water doses or after adsorption at cryogenic temperature.34 The S2 component (at þ0.27 eV) encompasses both the SiH (monohydride) species (shifted by þ0.26 eV from B)35 and the second silicon plane (S0 ) component of the clean surface. We note that all elemental Si fitting components (S1, B, S2) have a “narrow” fwhm of 0.280 eV and that the fwhm of the Si1þ state is only 0.375 eV. Let us now focus on the binding energy position of the bulk peak B. In the case of nþ substrates (the dopant concentration ND is ∼1019 cm3), for which the Fermi level (in the bulk) is degenerate with the bottom of the conduction band, electrons from the conduction band spill into surface states that charge the surface negatively, leaving a positive space charge region in the subsurface (the electron depleted region, consisting in ionized donors). This charge transfer produces an upward band bending | qVbb|. For the nþ-type Si(001)-2 1 surface, the Si 2p3/2 binding energy is 99.35 eV. The surface acceptor states of the clean surface can be associated either to the bottom of the π* band36 or to the C-defects, depending on their surface density.11 For the watersaturated surface, the bulk Si 2p3/2 component binding energy does not change much, as it is found at 99.40 eV (see Figure 2 and Table 1). This leads to an energy distance between the surface valence band edge (EsVB) and the surface Fermi level (EsF) EsF EsVB of 0.66 eV (the energy difference between Si 2p3/2 and EsVB is 98.74 eV37). This value places the acceptor levels of the watersaturated surface close to midgap, in excellent agreement with the calculated energy of doubly occupied tricoordinated Si on defective H-terminated silicon surfaces.21 As calculations show that SiOH and SiH bond making does not induce new gap states,38 we can take for EsCB EsVB (EsCB is the bottom of the conduction band at the surface) the bulk gap value of 1.12 eV to calculate the upward band bending |qVbb|. We find that |qVbb| ∼ 0.46 eV, as the bottom of the conduction band and the Fermi level in the bulk coincide for an nþ sample. A net (negative) surface charge density ns is associated to this band bending. Its absolute value is |ns| = (2εsND|qVbb|)1/2, where ND is the doping concentration (assumed to be constant from bulk to surface) and εs the permittivity in silicon.25 In consequence one obtains a “macroscopic” surface charge density of |ns| ∼ 1.03 102 electrons per surface Si atom (i.e., 7 102 electron/nm2) after surface saturation. The band bending occurs on the electron depletion width25 W ≈ [(2εs|qVbb|)/(q2ND)]1/2 = 5 nm. This value is an order of magnitude larger than the

Figure 2. (a) XPS Si 2p spectra (hν = 130 eV) of the clean Si(001)-2 1 surface (nþ, phosphorus-doped). (b) XPS Si 2p spectra (hν = 130 eV) of the water-saturated Si(001)-2 1 surface (the clean surface exposed to 6.8 L of water, 900 s 108 mbar). The takeoff angle of the photoelectrons, relative to the surface normal is 0. The experimental curves (dots) are fitted with sums of Voigt components (black solid lines). The Si 2p3/2 (Si 2p1/2) fitting components are the red solid (red dotted) lines. The Si 2p3/2 binding energy, the widths, and the spectral weights of the various components are given in Table 1.

Table 1. Binding Energies (Si 2p3/2), Surface Core Level Shifts (SCLS) Referenced to the Bulk Component Binding Energy, Widths, and Spectral Weights of the Voigt Components Used to Fit the Si 2p Experimental Spectra Shown in Figure 2a surface clean

water-saturated (6.8 L)

a

component 0

Si 2p3/2 binding energy (eV)

SCLS (eV)

GW (eV)

fwhm (eV)

Si 2p3/2 spectral weight (%)

attribution

S

99.62

0.27

0.256

0.289

11.7

2nd plane

B (þSS)

99.35

0

0.3

0.324

67

bulk þ down atom

C

99.04

0.31

0.256

0.28

6.2

S

98.8

0.55

0.37

0.395

15.1

up atom

100.31

0.91

0.35

0.375

15.2

SiOH

S2 B

99.67 99.4

0.27 0

0.256 0.256

0.28 0.28

21.5 60.1

SiH and 2nd plane bulk

S1

99.12

0.28

0.256

0.28

3.2

Si1þ

subsurface Si

subsurface (C)

The Lorentzian width was 0.045 eV for all components. Note that the band bending variation observed after water exposure is only 0.05 eV. 7688



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Figure 3. Wide (31 nm 44 nm) dual-bias STM images of the watersaturated (6.4 L) Si(001)-2 1 surface (nþ, As-doped, 0.01 Ω cm). In image (a) (Vbias = 1.75 eV, I = 0.54 nA), the negatively charged IDBs appear bright, while in image (b) (Vbias = þ1.97 V, I = 0.19 nA), they sit at the center of a ∼2 nm diameter circular depression (see also Figure 4). Their areal density is (1.3 ( 0.1) 102 defects/Si atom.

Si 2p electron escape depth at hν = 130 eV (0.4 nm29), which enables the use of surface sensitive Si 2p spectra binding energies. The degree of surface band bending provides a measure of the quality of the electronic passivation of a semiconductor surface. The complete passivation of the clean surface dangling bonds removes the electrically active surface gap states, and consequently the surface band bending |qVbb| goes to zero (the flatband conditions are indeed observed when a very low charge density is attained, typically below 103 electron/surface Si). Therefore, the water-saturated surface is far from ideality. Accordingly, ammonia is a better passivating agent than water: we have recently observed that ammonia adsorption on the nþ Si(001)-2 1 surface (ND ∼1019 cm3)—similar to water, the molecule breaks on the surface into H and NH2 fragments that decorate the silicon dangling bonds—leads to flatter bands, as the binding energy of the (elemental silicon) Si 2p core level increases by 0.33 eV,24 with respect to the clean surface value. This leads to EsF EVB = 0.95 eV and to an upward |qVbb| of only 0.17 eV, corresponding to |ns| ≈ 6 103 electrons per surface Si (i.e., 4 102 electron/nm2). On ammonia-saturated Si(001)-2 1, STM shows that the areal density of IDBs is about 4.4 103 defects/surface Si (∼3 102 defects/nm2).39 The comparison of the water- and ammonia-saturated surface cases is helpful, as it points to a correlation between the surface charge deduced from band bending (∼102 electron/Si atom for H2O, ∼6 103 electron/Si atom for NH3) and the IDB surface density deduced from STM images (∼1.2 102 defects/Si atom for H2O, see below and ref 13, and ∼4.4 103 defects/Si atom for NH3). One could therefore identify the gap states responsible for the observed band bending with the IDB, provided that each IDB is doubly occupied (negatively charged). This will be demonstrated by the following STM study of the water-saturated surface. STM images of a heavily nþ-doped clean Si(001) surface (ND ∼ 4 1018 cm3) saturated by water at 300 K are reported in Figures 35. A typical dual-bias wide scan (40.2 nm 40.2 nm) image is shown in Figure 3 for the occupied states (Vbias = 1.75 eV, Figure 3a) and for the unoccupied states (Vbias = þ1.97 eV,

Figure 4. Dual-bias (12 nm 16.5 nm) scan of the water-saturated (nþ) Si(001)-2 1 surface (16.5 L): (a) occupied states (I = 0.50 nA); (b) unoccupied states (I = 0.08 nA); (c) STM corrugation profiles near the IDB at positive and negative sample bias, along the lines indicated in (a) and (b).

Figure 3b), corresponding to an exposure of 6.4 L. In agreement with previous studies,13 the dimer rows are still visible on the saturated surface showing that the adsorption of the H2O fragments does not break the dimer bond. A smaller scanned area (13.4 nm 13.4 nm) of a surface exposed to 16.5 L is presented in Figure 4a (negative bias) and 4b (positive bias), along with corrugation profiles near an IDB (Figure 4c). Bright protrusions sitting on the water-reacted dimer rows are observed at negative sample bias (Figures 3a and 4a). The corrugation profile (Figure 4c) indicates an apparent height of about 0.3 nm and a diameter of ∼1 nm. At positive bias (an energy domain that was not explored in the previous STM works13,14), the STM topography of the single dangling bond shows a small protrusion surrounded by a dark halo (Figures 3b and 4b). The line profile (Figure 4c) indicates that the diameter of the depression is ∼1.8 nm. The observed contour is characteristic of a negatively charged defect:4043 while the central peak is dominated by changes in the local density of states directly associated with the bonding at the defect site, at larger distances, the contour is dominated by electrostatic effects. Indeed the negative charge on the Si IDB repels the electrons around the defect and increases their potential energy (Stroscio et al.40 call it a local upward band bending). Therefore, the number of unoccupied (occupied) states accessible to the tunneling process at positive (negative) sample bias decreases (increases). Profiles like that of Figure 4c are identically observed in the case of Si IDBs present on nþ-type H-terminated Si(001)-2 1.19,20,44 Neutral IDBs on p-doped19 or on low-doped n-type44 H-terminated Si(001)-2 1 do not induce an electric perturbation around them and therefore do not 7689



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Figure 5. (a) Image (8.5 nm 4.0 nm) of the water-saturated nþ-doped surface (16 L) scanned at Vbias = 2.3 V and I = 0.57 nA, showing occupied states. (b) Same as (a) but with enhanced contrast to show the aligned (red rectangles) and zigzag (green rectangles) patterns made of HSiSiOH units. (c) Line profile through the location marked by the red arrows in (a), encompassing from left to right, HSi-SiOH, HOSi-SiOH, and HSi-SiOH units; (d) H and OH distribution along a dimer row, showing the coexistence of HSiSi-OH, HSiSi-H, and HOSiSiOH units.

Table 2. Dangling Bond Surface Concentration as a Function of Exposure to Watera size of scanned area (nm nm)

a

√

water dose (L)

number of IDB N in scanned area

N

60 60

0.9

429

21

(1.7 ( 0.05) 102

73 73 40.2 40.2

1.8 6.4

424 140

21 12

(1.2 ( 0.06) 102 (1.3 ( 0.1) 102

number of dangling bonds per surface Si atoms

We recall that the areal density of silicon atoms is 6.8 1014 atoms/cm2.

exhibit a characteristic circular depression at Vbias > 0. The radius of the depressed area (∼1 nm45 at Vbias = þ2 V) in Figure 4c is an estimate of the characteristic screening length of the electrostatic potential created by the charged center.46,41,42 In the present case, screening is due to holes accumulating around the negatively charged IDB, and the 1 nm radius is indicative of a hole density in the 10181019 cm3 range.46 This density is several orders of magnitude greater than the hole free carrier density at the very surface (1091010 cm3 range), deduced from the upward band bending47 at the surface measured by XPS. In fact local inversion is due both to the local band bending around the defect and to the applied positive Vbias.48 We have examined how the IDB areal density varies with the exposure to water vapor after surface saturation (between 0.9 and

6.4 L, see Table 2). Above 1.8 L the density remains stable at ∼1.2 102 defects per surface Si atoms. In Figures 3a,b and 4a,b, some features, bright at Vbias < 0, do not change aspect at Vbias > 0, and thus they cannot be attributed to IDBs. As their areal concentration is extremely low (∼103 defects per surface Si atoms), they cannot enter into the discussion on the electrically active surface defects. We present in Figure 5ad atomically resolved STM images giving complementary information on the H/OH patterns on the water-saturated surface. The profile of Figure 5c, drawn along a line passing through the dimer axes, shows that it is possible to make a distinction between OH and H groups. On the same dimer, H and OH present an apparent height difference of ∼8 pm. This is a confirmation of the recent STM work by Skliar et al.,49 and as the 7690



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Figure 6. Radical chain reaction (alkene case) involving H-abstraction.

preceding authors, we attribute the highest (lowest) feature to SiOH (SiH). Figure 5d shows the assignment along one exemplary dimer row. The bright protrusions (Figure 5a,d) spanning the length of the dimer (“bright bars”) are attributed to HOSiSiOH units. The line profile in Figure 5c, and the high contrast image of Figure 5b, show that the apparent height of the OH pair is enhanced with respect to that of single OH, but nevertheless a shallow dip separates the two hydroxyls. With respect to the distance of 0.336 nm measured between the two protrusions in the HSiSi-OH unit, the distance separating the two protrusions is reduced to 0.215 nm in the HOSiSiOH unit. We attribute this reduction to the formation of a H bond between the two hydroxyls that brings them closer to each other.50 With respect to all the reacted dimers, the percentage of HOSiSiOH dimers appearing under the form of “bright bars” is (19 ( 3)%. The enhanced contrast image (Figure 5b) highlights the H/OH distribution patterns. We can see “aligned” patterns (red rectangles), where OH moieties all sit along the same side of the dimer row, spanning up to four dimers. We see also “zigzag” patterns (green rectangles) where the OH sits alternatively on opposite sides of the dimer row and spanning two dimers. The occurrence of aligned patterns is definitely above a pure statistical one, meaning that during the adsorption process there is a preference within the dimer row to adopt the OH/H ordering of the neighboring dimer. On the other hand, as seen in Figure 5a,b, the ordering is interrupted by the formation of the on-dimer OH pair. Therefore, no long-range ordering of the OH moieties on the same side of the dimer row can develop, in contrast to previous claims.51,52

4. COULD IDB INDUCE RADICAL REACTION ON THE WATER-SATURATED SI(001)-2 1? Having examined both the issue of IDB electronic structure and that of the H/OH arrangements, it is worthy of interest to have a prospective vision on the chemical reactivity of the watersaturated Si(001)-2 1 surface, based on the electronic and topological similarities/dissimilarities of this surface with the H-terminated Si(001)-2 1 surface. The main finding of this paper is the similar electronic structure of IDBs on watersaturated and H-terminated surfaces. This may have a strong impact, hitherto neglected, on the chemical reactivity of the water-saturated surface with unsaturated organic functionalities. As a matter of fact, on the H-terminated Si(001)-2 1, Si radicals, left by incomplete Si surface passivation, or created by current pulses under an STM tip, react with π-bonded molecules via a chain reaction. The latter involves the bonding of an intermediate radical species to the IDB and the subsequent jump of an adjacent H atom onto the β-C of the adsorbate to give the hydrogenated product,18,53 as depicted in Figure 6, drawn for the special case of an alkene. The possible occurrence of chain reactions involving the IDB and a π-bonded molecule on water-saturated Si(001)-2 1 raises a series of questions.

1. What are the respective reactivities of H and OH with the radical adduct? On the water-saturated surface, could an OH be abstracted by the radical adduct to form a COH bond with the β carbon? 2. Assuming that only H-abstraction by the radical adduct is possible, could the disorder of the H patterns impede a chain reaction? As shown in Figure 5, this is a major difference with the H-terminated Si(001)-2 1 surface, for which aligned H patterns allow the progress of the radical reaction, via H abstraction from a neighbor hydride sitting on the same side of the dimer row, and the directed growth of molecular lines (as is the case of styrene18). Still, then could “nonreactive” OH, the only nearest-neighbors (such as the “on-dimer” paired OH) to the radical adduct, put an end to the chain reaction? It is clear that the radical adduct (less stable than the hydrogenated product) will desorb from the surface,54 if no available H is present in due time at a jump distance. The IDB must diffuse away from the hydroxyl area55 in a region where hydrides are available to immobilize a new radical adduct that will be subsequently hydrogenated. If the OH pairs tend to cluster (for instance, after mild surface heating49), then some parts of the surface may be not reacted. 3. Does the charge state of the IDB have an impact on reactivity? Negatively charged IDB could repel electronrich functionalities, increasing the energy barrier to form the radical adduct (Figure 6) with respect to the case of neutral and positively charged IDB.56 These questions open up avenues for future experiments. We feel that the observed grafting of a carboxylic acid on the water-reacted surface4 cannot prove (nor disprove) that a radical chain reaction operates on this surface, as a water elimination reaction between the surface SiOH and the carboxyl functionality can give an ester. Therefore, we suggest to use first alkenes (R-CHdCH2) as “test molecules”. In particular, if the molecule reacts, one could easily check, by C 1s XPS or vibrational spectroscopies, whether OH can bind to the β carbon, or not.

5. CONCLUDING REMARKS Using synchrotron radiation Si 2p core-level photoemission, we have shown that a sizable upward band bending remains after saturation of the Si(001)-2 1 surface by the dissociation products of water and estimated, for a nþ-doped sample, the corresponding macroscopic (negative) surface charge density (about 102 electron/Si atom). Comparison with similar systems (ammonia-saturated Si(001)-2 1) is indicative that isolated dangling bonds act as acceptor centers. Indeed, using dual-bias STM, we have demonstrated that the isolated dangling bonds that amount to about 1.2 102 defects/Si atom are doubly occupied (negatively charged) when the substrate is nþdoped. In the unoccupied state images, the negatively charged 7691


The Journal of Physical Chemistry C dangling bond induces a local upward band bending, inducing an apparent circular depression around the defect. The silicon dangling bonds, which are uniformly distributed on the surface, and whose areal density is reproducible after water saturation, are created with great economy of means, e.g., without any external intervention (i.e., STM current injection) as is the case for H-terminated Si(001)-2 1. In the final, prospective section of the paper, we examine the role that isolated dangling bonds on water-saturated Si(001)-2 1 could play in radical chain reactions with π-bonded molecules, analogous to what is observed for H-terminated surfaces, and we suggest experiments to examine this issue.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT D.P. gratefully acknowledges grant support from the Institute of Chemistry, Centre National de la Recherche Scientifique (CNRS). ’ REFERENCES (1) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1–308. (2) Mott, N. F.; Rigo, S.; Rochet, F.; Stoneham, A. M. Philos. Mag. Part B 1989, 60, 189–212. (3) Widjaja, Y.; Musgrave, C. B. Appl. Phys. Lett. 2002, 81, 304–306. (4) Ihm, K.; Kang, T.-H.; Moon, S.; Hwang, C. C.; Kim, K.-J.; Hwang, H.-N.; Jeon, C.-H.; Kim, H.-D.; Kim, B.; Park, C.-Y. J. Electron Spectrosc. Relat. Phenom. 2005, 144147, 397–400. (5) Fan, C.; Lopinski, G. P. Surf. Sci. 2010, 604, 996–1001. (6) Lee, J.-Y.; Cho, J.-H. J. Phys. Chem. B 2006, 110, 18455–18458. (7) Warschkow, O.; Schofield, S. R.; Marks, N. A.; Radny, M. W.; Smith, P. V.; McKenzie, D. R. Phys. Rev. B 2008, 77, 201305. (8) Hamers, R. J.; Kohler, U. K. J. Vac. Sci. Technol. A 1989, 7, 2854. (9) Hossain, M. Z.; Yamashita, Y.; Mukai, K.; Yoshinobu, J. Phys. Rev. B 2003, 67, 153307. (10) Choi, J.-H.; Cho, J.-H. Phys. Rev. B 2009, 80, 125314. (11) Tanaka, S.; Tanimura, K. Phys. Rev. B 2008, 77, 195323. (12) Scanning tunneling spectroscopy (STS) shows that this defect is metallic at room temperature (ref 8 of the present paper). Twophoton photoemission, combined with STM imagery, brings also strong evidence that the C-defect is responsible for the pinning of the surface Fermi level, at least for densities above 0.05 defect/Si atom (ref 11 of the present paper) (13) Andersohn, L.; K€ohler, U. Surf. Sci. 1993, 284, 77–90. (14) Kato, H. S.; Akagi, K.; Tsuneyuki, S.; Kawai, M. J. Phys. Chem. C 2008, 112, 12879–12886. (15) Vittadini, A.; Selloni, A.; Casarin, M. Phys. Rev. B 1995, 52, 5885–5889. (16) Carniato, S.; Gallet, J.-J.; Rochet, F.; Dufour, G.; Bournel, F.; Rangan, S.; Verdini, A.; Floreano, L. Phys. Rev. B 2007, 76, 085321. (17) Lenahan, P. M.; Dressendorfer, P. V. J. Appl. Phys. 1983, 54, 1457–1460. (18) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48. (19) Liu, L.; Yu, J.; Lyding, J. W. NanoptterningFrom UltralargeScale Integration to Biotechnology, MRS Symposia Proceedings 705; Materials Research Society: Pittsburgh, 2002; Y6.6.1. (20) Bellec, A.; Riedel, D.; Dujardin, G.; Boudrioua, O.; Chaput, L.; Stauffer, L.; Sonnet, P. Phys. Rev. B 2009, 80, 245434. (21) Blomquist, T.; Kirczenow, G. Nano Lett. 2006, 6, 61–65.

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(22) Pei, Y.; Ma, J. J. Phys. Chem. C 2008, 112, 16078–16086. (23) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145–3155. (24) Miramond, C.; Vuillaume, D. J. Appl. Phys. 2004, 96, 1529– 1536. (25) Sze, S. M. Physics of Semiconductor Devices; Wiley Interscience: New York, 1981. (26) http://www.synchrotron-soleil.fr/portal/page/portal/Recherche/LignesLumiere/TEMPO. (27) Note that the ion gauges in the XPS and STM chambers are not calibrated against each other. (28) Mathieu, C.; Bai, X.; Bournel, F.; Gallet, J.-J.; Carniato, S.; Rochet, F.; Sirotti, F.; Silly, M. G.; Chauvet, C.; Krizmancic, D.; Hennies, F. Phys. Rev. B 2009, 79, 205317. (29) Himpsel, F. J.; McFeely, F. R.; Taleb-Ibrahimi, A.; Yarmoff, J. A.; Hollinger, G. Phys. Rev. B 1988, 38, 6084–6096. (30) Landemark, E.; Karlsson, C. J.; Chao, Y.-C.; Uhrberg, R. I. G. Phys. Rev. Lett. 1992, 69, 1588–1591. (31) Bozek, J. D.; Bancroft, G. M.; Cutler, J. N.; Tan, K. H. Phys. Rev. Lett. 1990, 65, 2757–2760. (32) Olivero, J. J.; Longbothum, R. L. J. Quant. Spectrosc. Radiat. Transfer 1977, 17, 233–236. (33) Koh, H.; Kim, J. W.; Choi, W. H.; Yeom, H. W. Phys. Rev. B 2003, 67, 073306. (34) Poncey, C.; Rochet, F.; Dufour, G.; Roulet, H.; Sirotti, F.; Panaccione, G. Surf. Sci. 1995, 338, 143–156. (35) Uhrberg, R. I. G.; Landemark, E.; Chao, Y. -C. J. Electron Spectrosc. Relat. Phenom. 1995, 75, 197–207. (36) Martensson, P.; Cricenti, A.; Hansson, G. V. Phys. Rev. B 1986, 33, 8855–8858. (37) Himpsel, F. J.; Hollinger, G.; Pollak, R. A. Phys. Rev. B 1983, 28, 7014–7018. (38) Nishida, M. Appl. Phys. Lett. 2002, 81, 1827–1829. (39) Gardener, J.; Owen, J. H. G.; Miki, K.; Heutz, S. Surf. Sci. 2008, 602, 843–851. (40) Stroscio, J. A.; Feenstra, R. M.; Fein, A. P. Phys. Rev. Lett. 1987, 58, 1668–1671. (41) Hamers, R. J. J. Vac. Sci. Technol. B 1988, 6, 1462–1467. (42) Ebert, Ph. Surf. Sci. Rep. 1999, 33, 121–303. (43) Brown, G. W.; Grube, H.; Hawley, M. E.; Schofield, S. R.; Curson, N. J.; Simmons, M. Y.; Clark, R. G. J. Vacuum Sci. Technol., A 2003, 21, 1506–1509. (44) Haider, M. B.; Pitters, J. L.; DiLabio, G. A.; Livadaru, L.; Mutus, J. Y.; Wolkow, R. A. Phys. Rev. Lett. 2009, 102, 046805. (45) The screening length is 1 order of magnitude smaller than the depletion width W, equal to 10 nm for ND = 4 1018 As/cm3. The band bending normal to the surface has hence little contribution to the discussion on the contrast variation around the defect. (46) Dingle, R. B. Philos. Mag. 1955, 46, 831–840. (47) The electron concentration n at the surface is n = n(¥)exp[ (|qVbb|)/(kT)], where n(¥) is the electron concentration in the bulk far away from the surface (n(¥) ≈ ND); k the Boltzmann constant; and T the temperature in Kelvin. The hole concentration p is deduced via the mass action law np = ni2 where ni = 1.45 1010 cm3. See Sze, ref 25. (48) In the case of negatively charged dangling bonds on H-terminated Si(001)-2 1, Liu et al. (ref 19) show that the screening radius diminishes with increasing positive Vbias. This effect is due to an increased density of holes, considering the tip-induced band bending effect. (49) Skliar, D. B.; Willis, B. G. J. Phys. Chem. C 2008, 112, 9434–9442. (50) Density functional theory calculations using the B3LYP functional indicate that two OH placed on the same side of the dimer row pair up due to H bond formation. The OO distance is reduced to 0.337 nm, smaller than the [011] lattice spacing of 0.384 nm. In the case of the water dimer (H2O)2, the calculation gives a still shorter OO distance of 0.29 nm. The distance between protrusions measured by STM suggests that that H bond between hydroxyls on the same dimer is strong. See ref 16 of the present paper. 7692



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(51) Larsson, C. U. S.; Johnson, A. L.; Flodstrom, A.; Madey, T. E. J. Vac. Sci. Technol.A 1987, 5, 842–846. (52) Johnson, A. L.; Walczak, M. M.; Madey, T. E. Langmuir 1988, 4, 277–282. (53) Takeuchi, N.; Kanai, Y.; Selloni, A. J. Am. Chem. Soc. 2004, 126, 15890–15896. (54) In the case of styrene reacting with a dangling bond of the H-terminated surface, DFT calculations indicate that the radical adduct adsorption energy is 0.8 eV, less than that of reacted product (after abstraction of a nearby H), that is, 1 eV. See ref 53 of the present paper. (55) The mobility of the IDB (considered as a H or an OH vacancy) on the surface should facilitate the progress of the reaction by enabling the defect to move away from hydroxyl clusters. STM images13 show that the IDB can change position on a dimer (on-dimer jump), but the nature of the partner is unknown (H or OH). DFT calculations (ref 15) find that both OH and H are indeed mobile via a vacancy diffusion mechanism (OH is even more mobile than H): the activation energy of OH (H) is 0.9 eV (1.1 eV) and 1.4 eV (1.6 eV) for on-dimer and intrarow jumps (on the same side of the dimer row), respectively. (56) In the case of H-terminated Si surfaces, the influence of the charge is unclear. On one hand, a negative charge does not impede the grafting of styrene on H-terminated nþ-doped Si(001)-2 1 (ref 18). On the other hand, an increased nucleophilicity of the silicon dangling bond due to nþ doping may explain the decrease of its reaction rate with 1-octadecene on H-terminated Si(111)-1 1, when compared to n- and p-doping (see ref 24).

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Isolated Silicon Dangling Bonds on a Water-Saturated n+-Doped Si(001)

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