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Real-Time XPS Study of Si(001)-2×1 Exposed to Water Vapor: Adsorption Kinetics, Fermi Level Positioning and Electron Affinity Variations Debora Pierucci, Jean-Jacques Gallet, Fabrice Bournel, Fausto Sirotti, Mathieu G. Silly, Héloïse Tissot, Ahmed Naitabdi, and Francois Rochet J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07360 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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The Journal of Physical Chemistry

Real-time XPS Study of Si(001)-2×1 Exposed to Water Vapor: Adsorption Kinetics, Fermi Level Positioning and Electron Affinity Variations

D. Pierucci§†, J.-J. Gallet§‡, F. Bournel§‡, F. Sirotti†, M. G. Silly†, H. Tissot§†, A. Naitabdi§‡ and F. Rochet §‡* §

Laboratoire de Chimie Physique Matière et Rayonnement, Sorbonne Universités, UMR 7614,

Université Pierre et Marie Curie, 11 rue P. et M. Curie, 75005 Paris, France †

‡

Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, 91192 Gif sur Yvette, France Affiliated to Synchrotron SOLEIL

* Corresponding author. Email: [email protected]

ABSTRACT

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The great advantage of X-ray photoemission spectroscopy, when performed in real-time, e.g. during the reaction of a gas with a surface, is the possibility of monitoring in a single experiment both the chemical aspects (adsorption kinetics, bond formation) and the physical ones (Fermi level positioning, variations in the electron affinity). In the present study we examine the reaction of water with Si(001)-2×1at room temperature in real time not only because water, ubiquitous in (ultra) high vacuum systems, is the main source of surface defects controlling the surface Fermi level, but also because water-saturated silicon may become an interesting starting surface in the atomic layer deposition of dielectrics on silicon. The question of water adsorption on silicon Si(001)-2×1 is renewed under the following four perspectives: (1) we propose an original kinetic analysis of the water uptake using an integrated form of the precursor model differential equations, underlying a dependence on pressure; (2) we perform a thorough analysis of the Fermi positioning within the band gap due to water-related surface defects, as a function of water coverage and for four different doping types/levels; (3) we follow the changes in the surface dipole as a function of coverage, with considerations on the dissociation channels; (4) using seven different n and p doping levels, we extract the electron affinity at saturation, a useful parameter to know, if hetero-structures are built upon the water-covered surface. Besides an applicative view, the present data can be a “bench mark” for theoretical calculations such as molecular dynamics, surface defect energy and work function calculations.

1. INTRODUCTION


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The adsorption of water on the clean Si(001) surface has attracted much attention1–6 because of the importance of wet oxidation7,8 in silicon-based device technology and in the atomic layer deposition process of the high-k dielectrics.9 Water–reaction products may also help the controlled functionalization of Si surface. Indeed the presence of surface hydroxyls enables the selective grafting of multifunctional organic molecules10,11–13 without incurring the risk of multiple adsorption geometries, observed in the case of the clean surface.14 Last but not least, water is the main pollutant in the residual gas of any ultra-vacuum chamber, and the knowledge of its reactivity, as well as the changes in the surface electronic structure it induces after reaction, is of prime importance. On clean Si (001)−2×1, water dissociates into H and OH fragments that decorate the silicon dangling bond left by surface dimerization15–20. First principles density-functional theory calculations4,5,21 (DFT), shows that two reaction channels, the intra-row and the on-dimer, are opened. In the intra-row 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-defect22,15,18,15,23,24 (two adjacent •SiSiOH and •SiSiH units, bearing a pair of formally singly occupied dangling bonds, denoted as •Si, coupled electronically). For its part, the on-dimer reaction channel corresponds to the molecular cleavage over a single dimer, leading to the formation of one HSiSiOH unit (see Figure 1, middle). In contrast to the intra-row process, the on-dimer channel leaves no dangling bonds. The competition between these two channels explains why, at surface saturation, isolated dangling bonds25,16 (IDBs) in •SiSiOH or •SiSiH units (Figure 1, bottom) are found on the surface, with a surface density of about 10-2 monolayer (ML, 1 ML=6.8 1014 atoms/cm2). In both cases, the precursor is a water molecule datively bonded to a positively charged down Si dimer atom. A


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density functional theory (DFT) calculation4 shows that the molecular adsorption energy is 0.64 eV, but the dissociated forms on-dimer and intra-row are significantly stabilized over the precursor state, with adsorption energies of 2.35 eV and 2.11 eV, respectively. The dissociation energy barriers from the precursor state to the intra-row (on-dimer) configurations are calculated to be 0.23 (0.25 eV) eV. Therefore this calculation suggests that both configurations are almost equally formed on the surface. However, using scanning tunneling microscopy (STM), Yu et al. 5 have recently challenged this view. In fact, they find that the population ratio

is equal

to ~5 at room temperature (for a molecular coverage of 0.02 L reached under a pressure of 5 × 10-10 Torr). Only by increasing the temperature (above 450 K) does the

ratio reaches

a value of ~ 0.6.


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Figure 1: Top and middle: H2O dissociation channels at the Si(001)−(2×1) surface. Bottom: isolated dangling bonds remain on the water-saturated 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. The kinetics of water adsorption on silicon surfaces were studied by various techniques, including UV photoemission (UPS). A survey can be found in the review paper by Henderson, written more than a decade ago.1 In particular, Flowers et al.26 measured the kinetics of D2O adsorption on Si(001) (via D2/SiO thermally programmed desorption), in a temperature range between 300 K and 650 K and under “an average pressure of 10-8 mbar”. Only one type/level of doping was examined (p-type, resistivity of 0.01-0.02 Ω×cm). A reactive sticking coefficient of one up to saturation was observed. The kinetics were termed “non-Langmuirian”, by reference to the precursor-mediated adsorption model.27 However, no excursion in pressure was made to verify the generality of the observation. While the work by Flowers et al.26 follows the so-called “cook-and-taste” method, Ranke and coworkers used real-time valence band UPS28,29 to monitor the oxygen coverage of silicon surfaces exposed to water. They measured the intensity of the O 2p level photoemission intensity while dosing the Si(001) surface (the sample doping type/level is unspecified) under a pressure of 2×10-9 mbar. The authors found an initial value of the reactive sticking coefficient close to one for Si(001), in accord with Ref.26. They also observed that the adsorption rate is practically constant until saturation is reached, and concluded that the adsorption curve is typical of a precursor-mediated adsorption process. Again, the investigated pressure domain is limited to one pressure.


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The present study was motivated by the fact that the collection of experimental data on water adsorption deserved to be pursued in a wider range of pressure and substrate doping. The novelty of the approach consists in the use of real-time x-ray photoemission spectroscopy (XPS) to get contemporaneously an information both on the oxygen uptake while exposing Si(001) to water vapor (via O 2s photoemission) and on the band structure of the surface exposed to the gas (precise Fermi level positioning in the band gap via XPS Si 2p binding energy measurement following the method of Himpsel and coworkers,30 variation of work-function/electron affinity via secondary electron edge measurement). We first examined whether the oxygen uptake on Si(001)-2×1 does only depend on dose Q (the product of the flux, proportional to pressure, by the exposure time). An example to the contrary is provided by the adsorption of water on Si(111)-7×7,31 where kinetics depend both on dose and pressure (or equivalently on dose and time). To examine this point we performed several kinetic studies measuring the XPS O 2s core-level intensity in real-time, at room temperature and in the pressure range 1.5×10-9–3×10-8 mbar. It also must be recognized that kinetics and electronic structure are interrelated issues. A recent theoretical work shows indeed that doping affects the adsorption energy of molecular water,32 which in turn, may have an impact on the desorption and the dissociation rate constants of a pre-dissociative molecular precursor. Therefore the influence of doping level on the kinetics was examined in a wide range of doping type and level (from highly doped n to highly doped p). However the examination of doping effects goes beyond the mere kinetic studies. Indeed realtime XPS gives the possibility of following the position of EF within the gap, as the nature of the electrically active surface defects changes with increasing coverage. While these defects have been studied at the two extreme coverages (see above), little is known at intermediate coverage.


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Assuming that the surface defects can act as electron donors or acceptors, the position of EF in the band gap at the surface likely depends both on coverage and sample doping. Thus information can be gained on the charge transition energy levels of the defects (see e.g. refs 33,34). Finally we measured the variations of the work-function/electron affinity as a function of coverage, a method typically used to follow adsorption35 and deposition processes36 with high surface sensitivity. Work function changes are sensitive to the growth mode (uniform versus island growth),37 and to conformational changes of the adsorbate.38 Finally, we provide the electron affinity of the modified Si(001) surface at water saturation.

2. EXPERIMENTAL DETAILS 2.1 Sample preparation The Si(001) samples were cleaned from their native oxide by flash annealing at 1150 ° C under ultrahigh vacuum after degassing the sample at 600 °C by Joule effect for a period of 8 to 12 hours. After cleaning the samples were cooled down to room temperature, and exposed to water vapor using a leak valve connected to glass vial. The ultra-pure water 18 MΩ×cm was purified after several freeze-pump-thaw cycles. Water dosing was carried out under three different nominal pressure (5×10-10, 1×10-9, and 1×10-8 mbar). The pressure was not measured by a gasindependent gauge (an absolute rotating-sphere gauge39 for instance). Instead we use an ionization gauge, calibrated against N2 by the manufacturer. As the relative sensitivity factor to water with respect to N2 is close to one (0.9740) the nominal pressure should be close to the real


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one. However the gauge is not directly facing the sample surface, and due to pumping, a pressure gradient between sample and gauge could exist. The oxygen coverage was measured via realtime photoemission (O 2s level), see below. In fact we must multiply the nominal pressures (given by the gauge) by a factor f=2.9 to avoid meaningless values for the initial sticking probability S0 (i.e. values greater than 1). f is deliberately chosen so that S0 is equal to one (in the fitting procedure, see below) for the exposures made at the lowest nominal pressure, that is 5×1010

mbar. The fact that f is of the order of 1 substantiates the procedure. Doses Q and pressures Pc

reported below are all corrected values. Note that this normalization was adopted by others.28 The samples used for the real-time photoemission studies had the following resistivity and doping

type/levels:

0.003

Ω×cm

(phosphorus,

denoted

5.5 Ω × ℎℎ, , 18 Ω × ", #

0.003

n+), Ω×cm

(boron, p+). Complementary doping type/level, 0.01 $×cm (phosphorus, denoted n1), 0.3 $×cm (phosphorus, denoted n2), 1 $×cm (boron, denoted p1) were used to determine the electron affinity of the clean and water-saturated surface. We have observed that the oxygen uptake at saturation is independent of doping (see Supporting Information, section S1, figure S1).

2.2 Real-time photoemission XPS Real time photoemission experiments were carried out at TEMPO beamline41 (SOLEIL French synchrotron facility). The photon source is a HU80 Apple II undulator set to deliver linearly polarized light. The end-station is ﬁtted with a modiﬁed 200 nm hemispheric electron analyzer (Scienta 200) equipped with a delay line detector. During the XPS measurements, the

photoelectrons were detected at 0° from the sample surface normal %& and at 46° from the polarization vector '%& . The acceptance angle of the analyzer is ±6°.


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Using a photon energy of 150 eV, we recorded the photoemission intensity of the O 2s level (measured in fixed mode within an energy window of 12 eV centered at a kinetic energy (KE) of 125 eV) and that of the Si 2p spectra (measured in swept mode) while dosing the surface with water. (figure 2). The variation of the secondary electron cutoff (used to measure the work function φ, see Supporting Information, Section S2, figure S2) was also followed with exposure time in the swept mode. The O 2s intensity is normalized to 0.5 ML at saturation for all doping types/levels, as saturation coverages are equal. The Si 2p core levels are used to check for the quenching of the surface-core levels and for the growth of the Si1+ (SiOH) state (see Figure 2),42 which can be correlated with the oxygen uptake. However, in the present study we rather focus on the evolution of the BE of the Si 2p3/2 level also measured at ℎ( = 150 + (see below). Indeed the

from the Si 2p3/2 binding energies ,'-./0/2 , one can obtain the positioning of the Fermi level '3

with respect to valence band maximum EVBM, '3 − '567 , as function of oxygen coverage NO,

using the known energy difference '567 − ,'-./0/2 = 98.74 +.30 The uncertainty in BE determination is ± 0.005 eV. These measurements concern the extreme surface. At a KE of ~50 eV, the inelastic mean free path λ of the photoelectrons in silicon is ~0.33 nm,43 which is at least one order of magnitude smaller than the depletion width on which the band bending changes notably. A thorough discussion and an estimation of the averaging over the probed depth is given in the Supporting Information (Section S3, equation (1)).


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Figure 2: Real-time measurements at h( = 150 eV. The Si 2p spectra are acquired in swept mode (the pass energy is 50 eV) and the O 2s spectra in fixed mode (the energy window is 12 eV wide, centered at a KE of 125 eV) during the same water dosing. As the water dose increases the signature of the “up atom” decreases and the Si 1+ component (SiOH) increases. Water dosing is stopped when the surface is saturated (the signature of the up atom has disappeared and the area of the O 2s remains constant).

3. RESULTS AND DISCUSSION

3.1 Adsorption kinetics

In the simplest case the adsorption kinetics of a molecule on a surface is described by the

langmuirian model. The molecule from the gas phase ; at infinitely low coverage (=> is the initial sticking probability). As the water molecule that dissociates upon adsorption, the desorption rate of the adsorbate ;# is null. Therefore we have the reaction scheme: ; , to the molecular flux F, and to the probability of finding an available site

1 − @. The latter is true when the adsorption is non-dissociative. In the case of water vapor,

dissociation occurs on a pair of adjacent dangling bonds. Therefore the probability of finding a dissociation site should be also 1 − @. Then one obtains: @ => × J = 1 − @ AR

Integration of (1) gives:

@ = 1 − exp V−

or equivalently as X = J × :

=> × J × W AR


(2)

(3)

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AB X = AR V1 − exp V−=> ×

X

AR

WW

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(3’)

Note that AB X depends on dose Q only, assuming that => is independent of pressure. A more

sophisticated kinetic model, the precursor-mediated adsorption model,27 goes beyond the langmuirian scheme. It assumes that the impinging water molecule is first trapped as a weakly bound mobile molecular precursor A*, and then that diffusion is fast on the surface (i.e. it is not a limiting step). The “weakly” bonded molecule then finds an available site for chemisorption to

give an adsorbed molecule ;# (here via dissociation on a pair of Si dangling bonds) with a rate

constant ka*. Chemisorption competes with desorption from the precursor state, with a rate constant kd*. The process can be written as:

where # is the rate of adsorption of the molecular precursor: #=

=> J AR

(4)

The quasi-equilibrium assumption ( = 0) is adopted for the molecular precursor, (that has a Q ∗

fractional coverage θ* much smaller than θ). Then the rate equation given by Kang and Weinberg is:27 => => J ∗ θ AR JO 1 − @ AR 1 − @ = ∗ = ∗ O O Z O∗ 1 − @ Z 1 − @ O∗


(6)

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When

∗ [\ ∗ [G

≪ 1 − @, a situation observed when the desorption probability of the precursor is

negligible compared to its chemisorption probability, then = D ^ . The adsorption rate is a Q

- 3

FGH

constant until it goes abruptly to zero at saturation (@ = 1. This is a marked difference with the langmuirian kinetics, for which the rate decreases with increasing θ according to @. On the other hand, when

∗ [\ ∗ [G

Q

= D ^ 1 − - 3

FGH

≫ 1, i.e. when the desorption probability of the precursor is

greater than its chemisorption probability, then

Q

= D^

∗ - 3[G ∗ 1 FGH [\

langmuirian, with an apparent initial sticking coefficient =>` =

− @. The kinetics is pseudo-

∗ -^ [G ∗ [\

≪ =>

Generally (see Kang and Weinberg27) the adsorption rate (a derivative) is used to check for the

type of kinetics. In the present case, the experimental points in the curves “O 2s intensity against time/dose” may be too scattered to obtain a useful derivative. One could have smoothed out the curve, and then taken the derivative, but we preferred to follow an alternative approach, by calculating the integral form of equation (6), an approach that is, to our best knowledge, original. It makes use of the W-Lambert function, also called the omega function, that is the inverse function of ab = b c .44

Defining the following parameters # = D

-^

FGH

J and " = [ ∗ \ , the solution of the differential equation (6) [∗

G

is:

e C @ = 1 − "b d

1 − # I " f "

(7)

We can also write AB as a function of the dose Q and parameter b:


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=> X A R l n qq le m " k k pp k pp AB X = Aghi × k1 − "b k " k pp k p k pp k 1−

j

j

(7’)

oo

Similar to langmuirian kinetics, the precursor-mediated adsorption kinetics depends on the dose Q only, assuming that S0 and " =

∗ [\

[G∗

do not depend on pressure.

The water adsorption curves (NO against Q) are presented in figure 3 for the n+ and n-doped

samples with resistivity of 0.003 $×cm and 5.5 $×cm, respectively, and in figure 4 for the p+

and p-doped samples with resistivity 0.003 $×cm and 18 $×cm, respectively. (a) P c = 1.5×10-9 mbar P c = 3×10-9 mbar P c = 3×10-8 mbar

Sample n+ (0.003 Ω×cm)

(b) P c = 1.5×10-9 mbar P c = 3×10-9 mbar P c = 3×10-8 mbar

Sample n (5.5 Ω×cm)

Figure 3: Adsorption kinetics of phosphorus-doped Si(100): (a) n+-sample (0.003 $×cm) (b ) n-

doped (5.5 $×cm). Three different (corrected) dosing pressures Pc are explored (1.5×10-9, 3×10-


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9

, and 3×10-8 mbar). The experimental curves are fitted with equation (7’) with S0=1 (D

-^

FGH

2 ML-1) and different parameters " = [\∗ . In the inset the full dose range for Pc=3×10-8 mbar. [∗

=

G

(a)

(b) Pc = 1.5×10-9 mbar Pc = 3×10-9 mbar Pc = 3×10-8 mbar

Sample p (18 Ω×cm)

Pc = 1.5×10-9 mbar Pc = 3×10-9 mbar

Sample p+ (0.003 Ω×cm)

Figure.4: Adsorption kinetics of boron-doped Si(100): (a) p+-sample (0.003 $×cm) (b ) p-doped

(18 $×cm). The (corrected) dosing pressures Pc are 1.5×10-9, 3×10-9, and 3×10-8 mbar (only pdoped sample). The experimental curves are fitted with equation (7’) with S0=1 (D

-^

FGH

= 2 ML-1)

and different parameters " = [\∗ collected in table 1. In the inset the full dose range for Pc=3×10-8 [∗

G

mbar. As discussed before the langmuirian and the precursor-mediated adsorption kinetics depend only on Q if the kinetic parameters are themselves independent of pressure. This is verified for nominal pressures Pc=1.5×10-9 mbar and Pc=3×10-9 mbar, and for all doping types and levels:

one can see in figures 3 and 4 that the AB r X curves are superimposable. At these low

pressure, the AB X curves shape are also typical of the precursor mediated adsorption model ACS Paragon Plus Environment

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Page 16 of 38

with ka*>>kd*, as the rate is practically constant up to saturation at X = 0.7 ML (langmuirian kinetics do not fit the curves measured at Pc 5×10-9 mbar and 3×10-9 mbar). All curves in the

nominal pressure range 1.4×10-9 mbar −3×10-9 mbar are fitted with S0=1 and " = [\∗ in the range [∗

G

0.2−0.4, see table 1. Doping/resistivity

Corrected

(Ω Ω×cm)

pressure

s=

S0

t∗u t∗v

(mbar) n+-type/0.003

n-type/5.5

p-type/18

p+-type/0.003

1.4 ×10-9

1

0.2

3×10-9

1

0.2

3×10-8

1

1.0

1.4 ×10-9

1

0.2

3×10-9

1

0.2

3×10-8

1

1.5

1.4 ×10-9

1

0.4

3×10-9

1

0.3

3×10-8

1

1.3

1.4 ×10-9

1

0.3

3×10-9

1

0.3

Table 1: Fitting result using equation (9): S0=1 (the initial sticking probability of the precursor after pressure correction, see text), and " = [\∗ . [∗

G

However the situation changes under a pressure of 3×10-8 mbar. The AB X curves are no

more superimposable to the ones obtained under pressures an order of magnitude smaller. We see a much lower value of

DE w

in the initial adsorption stage for the n+, n, and p-doped samples

(we lack data for the highly doped p+ sample at this pressure). Keeping an interpretation scheme


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coherent with the kinetics obtained at lower pressure (equation (7’)), we fit the AB X curves

obtained under a pressure Pc of 3×10-8 mbar with S0=1 (the initial sticking coefficient of the

precursor remains constant) and " = [\∗ in the range 1−1.5, a value greater by a factor of ~5 than [∗

G

the b values in the 10-9 mbar range.

The increase of b above a threshold pressure may be interpreted in terms of an increase in O∗ ,

assuming that ka* is constant. We can envisage an enhanced desorption due to an increase in the

collision frequency between gas phase molecules impinging on the surface and the weakly adsorbed precursor molecules according to the scheme: ∗ [\

; ν∗ permits to estimate a minimum value for '∗ '∗ > O6 P × lnν> /νz {{

(8)

(9)

With O6 P=0.025 eV and νz {{ =310 s-1 one gets '∗ > 0.61 eV. This value is comparable to the

adsorption energy of the precursor datively bound to a down dimer atom i.e. 0.64 eV (the adsorption is barrier-less).4 Other sites involving H-bonding with molecular water could be thought of. The impact of doping of the kinetics constituted one of the motivations of the present study. In

fact, the comparison of the AB X curves shows no clearcut difference. The fitting parameters in

table 1, for a given range of pressure, are indeed very close. Doping has no significant effect on


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the kinetics. This negative result is surprising. Indeed DFT calculations32 show that boron doping increases the molecular adsorption energy with respect to phosphorus doping (0.87 eV versus 0.80 eV). This in turn should impact both desorption and dissociation rate constants of the molecular precursor, which is not the case.

3.2 Band bending variation monitored via Si 2p real time XPS

During the same water-dosing runs under a pressure Pc of 1.5×10-9 mbar, we monitored the BE variation of the Si 2p as a function of time and/or of oxygen coverage (via O 2s region measurement), for the four Si doping type/levels: n+ (0.003 Ω×cm), n (5.5 Ω×cm), p (18 Ω×cm) and p+ (0.003 Ω×cm). The Si 2p spectra are reconstructed with an automatic fitting procedure that is described in the Supporting Information, section S4 (figure S4). Briefly the main “bulk” Si 2p3/2 component BE is let free, and the standard deviation on its binding energy is better than 2.5 meV. Then the positioning of the Fermi level with respect to the valence band maximum

'567 at the surface using the known energy difference '567 − ,'-./0/2 = 98.74 + given by

Himpsel and coworkers.45 In figure 5, we show '3 − '567 as a function of the oxygen

coverage NO. The zero of oxygen coverage corresponds to the starting point of water dosing under 1.5×10-9 mbar. Between the Joule cleaning of the silicon surface and the establishment of the working pressure, the surface was exposed to an estimated maximum dose Q of 5×1012

molecules/cm2, corresponding to the adsorption of ~0.015 ML of water molecules. This may influence the positioning of the Fermi level at the surface, as Tanaka et al. find that C-defects start to pin '3 from a concentration of 0.05 ML.23


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Figure 5: '3 − '567 as a function of surface oxygen density NO for four Si doping levels:

(a) n+(0.003 Ω×cm), (b) n (5.5 Ω×cm), (c) p (18 Ω×cm) and (d) p+ (0.003 Ω×cm). The

experiment was carried out a water pressure Pc= 1.5×10-9 mbar. For all water coverages, band bending is present at the surface, as the position of the fermi level

in the bandgap is not the same, in the bulk ('3 − '567 {[ , see Table 3, and at the surface ('3 − '567 ), see Figure 5. Band bending +66 writes as:


20

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+66 = '3 − '567 {[ −'3 − '567 10.

From the '3 − '567 {[ values given in Table 3, one sees that the bands of the n-type samples

are bent upward as '3 − '567 < '3 − '567 {[ . Conversely, the bands are bent

downward for the p-type ones, as '3 − '567 > '3 − '567 {[ . Negative (positive)

charge appears at the surface for n-type (p-type) samples. The surface charge density σ can be deduced from +66 and the bulk doping concentration:46

= −L2 > A +66 for the n-type (11)

= L2 > A |+66 | for the p-type (11’)

where > is the vacuum permittivity, the relative permittivity of silicon (11.946), ND/A the density of donors/acceptors.

The surface charge density is given at water saturation (AB =0.5 ML) in Table 3. For the degenerate samples, the surface charge corresponds to about 10-2 ML of negatively (n+) or

positively (p+) charged defects. For an n+ sample, this density corresponds to that of the isolated dangling bonds observed by STM, with a double electron occupancy.16,47 For the p+ sample the isolated dangling bonds should be emptied, assuming that their surface density is the same as that

of the n+ sample. For the n and p sample is minute, less than 10-4 unit charge per Si atom,

pointing to quasi neutral defects. Considering that the isolated dangling bonds are Pb-like

defects,33 the neutrality level of the isolated dangling bond is at '3 − '567 ≈ 0.52 +, close to mid gap.


21


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'3 − '567 {[

Type/resis tivity

'3 − '567 at saturation

+66

Page 22 of 38

σ

at saturation

at saturation

(Ω×cm))

unit charge per Si atom

n+/0.003

1.12

0.67

+0.45

-1.7×10-2

n/5.5

0.85

0.52

+0.33

-8.6×10-5

p/18

0.25

0.51

-0.26

+7.6×10-5

p+/0.003

0

0.28

-0.28

+1.8×10-2

Table 3. Fermi level position within the band gap in the bulk and at the surface, band bending (+66 ) and surface charge density for the four doping levels/types at water saturation. The strong added value of the real-time study is to highlight changes in Fermi level

positioning that depend on coverage. Away from saturation, the '3 − '567 curve of the n+ sample “mirrors” that of the p+ sample (see Figure 5). For a coverage A B of ~0.2 ML,

'3 − '567 reaches a minimum at 0.615 eV for the n+ sample and a maximum at 0.325 eV

for the p+ sample. We attribute this positioning to the C-defects (or to clusters of C-defects)

present at the surface. The C-defects formed at room temperature are semiconducting according to calculation,24 with a band gap in the range 0.42 eV -0.46 eV. These gap values compare well with the Fermi energy difference between the n+ and p+ samples (~0.3 eV) measured at AB ~0.2

ML. For the lightly doped samples, '3 − '567 ranges between 0.444 eV (p) and 0.467 eV (n) at AB ~0.2 ML, halfway between the n+ and p+ values for the same coverage. The typical

semiconducting character of these surface defects explains why the Fermi level positioning depends on doping, in contrast with the case of the metallic Si(111)-7×7 surface.45

22 ACS Paragon Plus Environment

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Figure 5 shows that the positions of the Fermi level in the intermediate water coverage regime are distinctly different from those reached at saturation. This is due to a profound change in the electronic nature of the surface defects, passing from coupled pairs of dangling bonds (Cdefects), at low and intermediate coverage, to isolated dangling bonds at water saturation. Indeed, for the isolated dangling bonds, which are very similar to the Pb defects of the Si/SiO2 interface, on-site correlation effects become prominent.33 The n+ and p+ Fermi energy difference at water saturation gives an (upper bound) estimate of the charge transfer energy difference (i.e. the correlation energy U ~0.4 eV) for the isolated dangling bonds.33

3.3 Work-function and electron affinity variations during water adsorption Using a photon energy of 150 eV, and a sample bias of -10 V, we have measured the time variation of the work-function φ of a n+ sample exposed to water at room temperature under a pressure Pc of 3×10-9 mbar. The time evolution of the secondary electron (SE) edges (measured in swept mode) was monitored, while measuring the Si 2p spectrum (in swept mode) and the O 2s region (in fixed mode). Before discussing the relationship between the oxygen coverage and the work-function, or the electron affinity (EA), a detailed examination of the SE edge is necessary. In fact its shape can give hints on surface work-function inhomogeneity.37,48 The raw SE edge intensity is plotted against the KE in figure 6 (a), for various values of NO (NO = 0, 0.25, 0.5). We observe that the SE cut-off KE decreases with increasing NO: the work function of the water-covered surface is lower by 0.38 eV than that of the clean one. Concomitantly, we notice an increase of the SE yield as χ decreases. With decreasing χ, the number of inner SE able to escape into the vacuum increase, largely due to a change in refraction at the surface (a lower χ means a greater inner 23 ACS Paragon Plus Environment


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solid angle for external emission, assuming that the distribution of SE is isotropic in the solid).49 After normalization to their maximum heights, the SE edge curves versus KE are given in figure 6 (b). One observes that with increasing coverage the energy spectra just shift down along the KE axis without changing their shape. The SE onsets remain sharp: the derivative

F

of these

step functions is fitted (around the onset) with a Gaussian of constant fwhm 140 meV (see inset of figure 6 (b)), from the clean to the water-saturated surface. This observation is of particular importance in discussing the issue of surface uniformity (see below). On semiconductor surfaces, only the EA χ (defined as the energy difference between the

vacuum level 'z and the bottom of the conduction band '67 is intrinsically related the

surface dipole layer, and not φ (the energy difference between the vacuum level Evac and EF) that is affected by band bending. One has:

= 'z − '3 = Z ' Z '567 − '3 = Z ' Z ,'-./0/2 − 98.74 + (12)

Defining Δ as the variation between the covered surface work-function and that of the clean

surface, on the one hand, and Δ as the variation between the covered surface EA and that of the clean surface, on the other hand, one gets: ∆ =∆χ+Δ,'-./0/2 (13), And then

∆ = ∆φ − ∆,'- ./0/2 (14)


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Figure 6. Raw (a) and normalized (b) secondary electron edges measured for various oxygen coverages NO. The photon energy is 150 eV, and the sample is biased by -10 V with respect to the analyzer. The sample normal is aligned with the analyzer entry lens axis, and the acceptance angle is ±6°.

−∆ is plotted as a function of time (or as a function of NO) in figure 7 to get an information

on the formation of the chemisorbed layer.



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Figure 7 : Time evolution of NO and −∆ as a function of time.( Pc = 3×10-9 mbar). The

sample is n+ doped (ρ = 0.003 Ω × cm). Negative times correspond to measurements made before water introduction (clean Si(001) −2×1 surface). The two curves are superimposable after

normalization to their saturation values, showing that Δ (accuracy of ± 0.01 eV) is proportional

to NO (accuracy of ± 0.028 ML).

The −∆ curve and NO(t) curves have a linear time dependence until they saturate at 0.44

eV and 0.5 ML, respectively, after 420 s. It is clear from figure 8(b) that ∆ is proportional to NO.

This proportionality can be explained in two ways, according to the surface patterns adopted by the reacted dimers. One can first consider a non-uniform surface. Indeed STM indicates that water-reacted dimers tend to form islands on the surface.50 For a patchy surface consisting of water-covered areas

(with a local “saturation” coverage of one molecule/dimer and a work-function ) and of 2B 26


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clean areas (with a work-function z{ ), the lower SE cut-off corresponds to the surface averaged work function (in the so-called “small patch limit”). Then the work function variation Δ is:48 Δ = − z{ = D

D

FGH

¡ − z{ ¢ (15). 2B

Taking into account that ∆qVBB < 0.06 eV , Δ ≈ Δ. Then Δ should be trivially

proportional to NO (which is observed here), but no information on the coverage dependence of

the dipole moment can be extracted. However the SE edges of figure 6 (a,b) are indicative of a uniform surface. In particular we do not see the presence of two different onsets, associated to

and z{ , respectively.48 The increase of SE intensity (a factor of 1.8 between the clean and

saturated surface, see figure 6 (b)) is insufficient to “drown” the onset from the clean areas. Therefore we shall consider a uniform surface in the following analysis. Borriello and coworkers38 published a theoretical work on the electron affinity change due to

the adsorption of alkenes cyclo-added on the dimers of the clean Si(001) surface. To our knowledge, this is the only study on EA variation induced by molecular adsorption on Si(001) using a modern approach (the EA variation is calculated from the total charge density, that is obtained through DFT). These authors showed in all generality that: Δ = − ¥ Δ (16)

^

where ¦> is the vacuum permittivity, Δ is the variation of the dipole moment density of the reacted surface, at a molecular fractional coverage θ, and of clean surface.

In the case of the adsorption of water vapor, we find that Δ is proportional to the oxygen

coverage NO (or equivalently to the molecular fractional coverage θ). In fact this observation is more the exception than the rule. For instance, among the alkene molecules studied by Boriello

et al.38 only for the small pseudo-diatomic molecule C2H4 Δ is found roughly proportional to the 27



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coverage, as it increases by a factor of 1.82, passing from a coverage of 0.5 molecule/dimer to a coverage of one molecule/dimer. The other, bigger molecules (based on a cyclopentene ring) are prone to structural relaxation with increasing coverage, which in turn induces a strong non-

linearity with coverage. Δ can be fractioned between ∆ (the variation of the surface dipole

moment density due to the surface structural relaxation, as some of the buckled dimers become

symmetric after reaction with water) and Δ (that includes the dipole moment of the adsorbate itself and the induced dipole moment density due to the formation of the chemical

bond). The periodic slab DFT calculation of Borriello et al.38 shows that for C2H4, Δ is roughly proportional to the coverage, while this is clearly not the case for ∆ . In fact ∆

accounts for −0.005 eV in ∆ (via equation (16)) at 0.5 molecule/dimer, but for −0.149 eV at

one molecule/dimer. As expected, the calculation shows that ∆ at full coverage is independent

of the adsorbed molecule. Considering the on-dimer adsorption mode, the attachment of H/OH fragments to the silicon dimer makes it also symmetric. Therefore the “de-buckling” contribution (~−0.15 eV) should be a sizable part (30%) of Δ at water-saturation (where Δ = −0.44 eV),

but almost zero at half coverage, and thus the proportionality of Δ with θ should not be verified.

Therefore to match the observed Δ ∝ @ dependence, ∆ should be also proportional to the coverage. The competition of the on-dimer (one buckled dimer eliminated per dissociative adsorption event) and of the intra-row adsorption mode (two buckled dimer eliminated per dissociative adsorption event) may explain why the buckled dimer contribution (with a moment oriented inward) to the surface dipole is eliminated earlier than in the case of the alkene adsorption that is strictly on-dimer.

If we define ΔK as the dipole moment variation per reacted silicon pair, one gets: ∆K = QD

¨/

FGH

= −¦> QD

¨©

FGH

(17). 28 ACS Paragon Plus Environment

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With Δ = −0.44 eV at @ = 1 and AR =3.4×1014 cm-2 one finds ΔK equal to ~10-30 Cm (~0.3 Debye).

Until now we have dealt with variations as a function of coverage (examining specifically an

, which is, as stated before, an intrinsic n+ doped sample). The value of the EA at saturation 2B

property of the surface, can be extracted from the work-function measurements and from the correlative positioning of EF in the band gap. The work-functions = 'z − '3 of the clean

and water-saturated (@ = 1) surfaces were measured for seven n and p doping levels (see the summary table S3 in Supporting Information, section S5). With '67 denoting the conduction

band minimum, '67 − '3 (at the surface) is obtained via the measurement of the Si 2p3/2 BE (that gives '3 − '567 ), assuming a band gap of 1.12 eV. Then = − '67 − '3 is plotted against '67 − '3 , as shown in figure 8. The linear dependence of

with '67 − '3 (the slope is equal to one) is verified, and the intercept of at '67 −

=3.91 '3 = 0, gives the EA for the clean (χcl=4.30 eV ± 0.02 eV) and water-saturated ( 2B

± 0.01 eV) surfaces. A Δ value of ~−0.4 eV is therefore verified in a wide range of doping.



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Figure 8. Plot of versus '67 − '3 . The linear fit is made with a slope equal to one. The

intercept at '67 − '3 = 0 eV gives the EA. The resistivity, doping and '67 − '3

values are collected in Table S3 of the Supporting Information, section S5.

SUMMARY With real-time XPS, we have monitored the oxygen uptake NO and changes in the band structure due to the adsorption of water on Si(001)-2×1 at room temperature. In the pressure range 1.5×10-9 – 3×10-8 mbar, the adsorption kinetics (NO versus dose Q) are characteristic of a precursor-mediated chemisorption. We have worked out an integrated solution for the rate equation given by Kang and Weinberg,27 involving only two parameters, the initial sticking coefficient S0, and a second parameter " = [\∗ , where O∗ is the desorption rate constant of the [∗

G


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precursor and O∗ its chemisorption rate constant. While under pressures below 3×10-9 mbar NO(Q) curves are practically superimposable, at 3×10-8 mbar the fitting parameter b increases

notably. We interpret this observation as due to a pressure threshold effect on O∗ that increases

(due to collisions between the weakly bound precursor and molecules from the gas phase), while O∗ remains constant. We also find that doping has no impact on the kinetics, despite the fact that

the precursor adsorption via dative bonding could be influenced by the electronic occupation of the dangling bonds of the clean surface. The position of the Fermi level was also studied, via Si 2p core-level photoemission, as a

function of time and coverage, for four doping levels ranging from highly doped n+ (phosphorus) to highly doped p+ (boron). The position of the Fermi level within the gap depends on doping at all coverages, and in no ways is a flat band situation observed at water saturation. Thanks to the high accuracy in the binding energy measurement, two regimes of Fermi level positioning are distinguished. The regime dominated by C-defects (pairs of coupled dangling bonds) ends abruptly at water saturation due to the formation of Pb-like, isolated dangling bond left on the surface. Our doping dependent analysis enables the determination of defect band gaps for the first type of defect and of charge transition levels, for the second one. The secondary electron edges were measured in real time together with the oxygen coverage and the Si 2p binding energy. By taking into account variations in the band bending we have determined the variation of the electron affinity as a function of coverage, and found it is proportional to the latter. This linearity may stem from the nature of the dissociation channel itself (intra-row versus on-dimer). By measuring the work functions for a wide range of doping



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(from degenerate p to degenerate n) the electron affinity of the water-saturated surface was determined to be 3.91 ± 0.01 eV.

The present study demonstrates that real-time XPS, besides adsorption kinetic information, gives valuable information on how the main electron structure characteristics (Fermi level positioning, electron affinity) depend on water-coverage, given the dominance of this molecule in the residual gas of ultra-high vacuum systems and its potential usefulness as a silicon surface functionalizing agent.

SUPPORTING INFORMATION The Supporting Information contains XPS data enabling the determination of the oxygen coverage, details on the measurement of the work function via the cutoff of the secondary electron distribution curve, an original calculation leading to the estimate of the surface band bending contribution to the determination of the elemental silicon Si 2p binding energy, and details on the automatic Si 2p curve fitting.

ACKNOWLEDGMENTS Debora Pierucci thanks Centre National de la Recherche Scientifique (CNRS) and Synchrotron SOLEIL for jointly providing her a PhD grant. REFERENCES (1)

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