Effect of Doping Density on the Charge Rearrangement and InterfaceDipole at the Molecule−Silicon InterfaceOmer Yaffe,† Sidharam Pujari,§ Ofer Sinai,† Ayelet Vilan,† Han Zuilhof,§ Antoine Kahn,∥ Leeor Kronik,*,†

Hagai Cohen,*,‡ and David Cahen*,†

†Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel‡Department of Chemical Research Support, Weizmann Institute of Science, Rehovoth 76100, Israel§Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands∥Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, United States

*S Supporting Information

ABSTRACT: The interface level alignment of alkyl and alkenylmonolayers, covalently bound to oxide-free Si substrates of variousdoping levels, is studied using X-ray photoelectron spectroscopy.Using shifts in the C 1s and Si 2p photoelectron peaks as asensitive probe, we find that charge distribution around thecovalent Si−C bond dipole changes according to the initialposition of the Fermi level within the Si substrate. This shows thatthe interface dipole is not fixed but rather changes with the dopinglevel. These results set limits to the applicability of simple modelsto describe level alignment at interfaces and show that the interfacebond and dipole may change according to the electrostaticpotential at the interface.

■ INTRODUCTION

In organic electronics, device performance may depend stronglyon the energy level/band alignment at the interface betweenthe organic electronic material and an inorganic material, whichcan be the electron or hole blocking layer or the electrode.1−5

One of the promising methods to control and modify the bandoffset is to covalently bind an organic monolayer, made up oforiented polar molecules, to the electrode surface,6−9 as suchmonolayers can serve to adjust the work function (WF) of theelectrode (see, e.g., refs 10−14 and references therein).The electrode WF is modified by means of an electrostatic

potential step, associated with a dipole layer. In the case of amolecular monolayer this potential step may arise for severalreasons.1 The leading causes are the molecular dipole of thefree molecule (prior to adsorption),13−15 the molecule−substrate bond dipole or interface dipole,10,13,15−17 and themodification of the original charge distribution at/near thesubstrate surface (also known as the “pillow” or “push back”effect).1,15,18−21 The relative contributions from these effects tothe overall interface dipole vary according to the strength of themolecule−substrate interactions, the free molecule’s dipole, thepolarizability of the molecular backbone, and the intrinsicelectronic properties of the solid.2,22 While the “pillow” effect islargely independent of the molecular properties, molecule-specific modification of the substrate WF can take place byeither changing the molecular dipole (e.g., changing theterminal group at the end of the monolayer-forming molecule)or changing the molecule−substrate binding group and thereby

the bond dipole. For sufficiently long molecules, the bond andterminal dipole will not be coupled so that both can serve asindependent “handles” for modifying the electrode WF.10,11

While changing the terminal group of the molecule hasproven to be an efficient and relatively easy method to modifythe WF, use of the bond dipole as a handle is much morechallenging. This is due in part to a limited number of bindinggroup types that can covalently bind to a given inorganicsubstrate. However, even in cases where the interface dipolecan be systematically modified (e.g., binding thiols to Au andAg), it was experimentally observed and theoretically computedthat the metal−molecule interface dipole variation largelycompensates for the variations between the metal WFs.11,18,23,24

In other words, while the initial WFs of two metals weresubstantially different prior to the adsorption of the organicmonolayer, the final, postadsorption WFs were similar.Further insight into the bond dipole between an organic

monolayer and an inorganic substrate can be gained byconsidering a silicon (Si) substrate, rather than a metalsubstrate. The reason is that, to a first approximation, theWF of the Si substrate can be modified via doping, whilepreserving the chemical bond and the structure of themonolayer on the surface. This allows us to determine the

Special Issue: Ron Naaman Festschrift

Received: March 31, 2013Revised: June 13, 2013

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extent to which the interface dipole is an intrinsic property ofthe covalent bond (which, to a first approximation, will beunchanged), as well as the degree to which the electrostaticpotential difference at the interface (which varies with dopingdensity) affects the interface dipole.To investigate this issue and, thus, the larger question of how

the energetics of the interface between a molecular monolayerand an inorganic solid are determined, we experimentally probethe interface dipole between oxide-free Si(111) substrates withdifferent doping densities and alkyl/alkenyl monolayers (seeFigure 1). We use both alkyl (C10H21) and alkenyl (C10H20)

monolayers: the alkyl monolayer on oxide-free Si is widelystudied12 and allows us to compare and contrast our work topublished results; alkenyl on Si has a stronger photoemissionsignal from the Si−C interface, which is of primary interest inthis study. For both alkyl and alkenyl monolayers, the 1 × 1surface structure of the freshly etched Si−H is preserved.25,26

The interface dipole is probed directly by X-ray photoelectronspectroscopy (XPS),27 where we monitor the change inelectrostatic potential between the Si (Si 2p core electronbinding) and organic monolayers (C 1s core electron binding).We find that for high doping densities the level alignmentdepends strongly on the doping density of the Si substrate,suggesting that the interface dipole associated with the bindinggroup depends on more than the type of atoms that comprisethe covalent bond.

■ EXPERIMENTAL METHODSSample preparation and characterization followed literaturedescriptions.28,29 In brief, pieces of Si wafer were cleaned bysonication in acetone and oxidized by an oxygen plasma(Harrick PDC-002 setup) for 3 min. Subsequently, the Si(111)substrates were etched in an Ar-saturated 40% NH4F solutionfor 15 min. After etching the samples were rinsed with water,blown dry with nitrogen, and immersed in Ar-saturated neat 1-alkyne or 1-alkene (GC purity >99.9%) at 120 °C (alkenyl) or200 °C (alkyl) and ∼10 mbar. After 16 h (alkenyl) or 4h(alkyl), the reaction was stopped, and the monolayers wererinsed extensively with EtOH and CH2Cl2 and sonicated for 5min in CH2Cl2 to remove physisorbed molecules. Alkyl- andalkenyl-based monolayers on Si were studied extensively before.Monolayers obtained using this and similar procedures wereshown to be densely packed, chemically stable, andreproducible.26,29−32

Si(111) wafers of four different doping density-typecombinations were used, namely, heavily doped n-type(HDn), moderately doped n-type (MDn), moderately dopedp-type (MDp), and heavily doped p-type (HDp) silicon. The

source, doping density, and type of dopant are given in theSupporting Information.Measurements were conducted immediately after sample

preparation and were conducted on two different spectrom-eters. To ensure that results from the two spectrometers couldbe compared, measurements were taken at a normal takeoffangle, with some samples cross-checked. In addition tofacilitating the comparison between different spectrometers,the normal takeoff angle enhances the signal from the “buried”carbon atom at the Si−C interface, which is of interest here.The alkyl monolayers were measured on a Kratos AXIS-HSspectrometer, using a monochromatic Al Kα X-ray source (60W) with analyzer pass energies ranging between 20 and 80 eV.Initial work-function measurements under conditions that arepractically free of irradiation damage (based on the methoddescribed in ref 33) were performed on the fresh sample,followed by several repeated sets of XPS measurements overvarious spectral ranges, then followed by a second WFmeasurement to detect any radiation-induced surface potentialchange. Alkenyl monolayers were measured on a JPS-9200photoelectron spectrometer (JEOL, Japan) using a mono-chromatic Al Kα X-ray (300 W), with an analyzer pass energyof 10 eV.

■ RESULTS AND DISCUSSIONWe begin our analysis by considering the shift of the Si 2p corelevel with doping. The reference Fermi energy in bulk Si isextracted from the nominal doping density, provided by thewafer manufacturer according to

= +⎛⎝⎜

⎞⎠⎟E E kT

Nn

lnF id/a

i (1)

where Ei is the intrinsic Fermi energy for Si; k is the Boltzmannconstant; T is the temperature (taken as 300 K); ni is the Siintrinsic carrier density; and Nd/a is the nominal donor/acceptor doping density. Equation 1 assumes that all thedonor/acceptors are ionized, which is reasonable for the rangeof doping levels used in our experiments.34 The nominal Fermienergy in the Si bulk is generally different from theexperimentally measured work function or HOMO position,due to surface dipole and/or band bending. In our analysisbelow, the doping dependence of the Fermi energy is comparedto that of core level peaks to elucidate interface effects.Figure 2a shows the XPS Si 2p peak of the Si(111)−C10H21

(alkyl) surfaces with all four combinations of Si doping typeand density (see Figure S1 in the Supporting Information for asimilar figure for Si(111)−C10H20 (alkenyl)). There is a clearmonotonic shift in the Si 2p peak position as the doping densityand type change from HDn to HDp. This is expected becausethe binding energy is measured with respect to EF, which itselfchanges monotonically as a function of doping. Figure 2bshows the Si 2p peak position as a function of the EF position,relative to the conduction band (CB) edge. At flat-bandconditions, the Si 2p peak shift should simply equal the changein EF, a situation indicated by the dashed line in the figure. Thedeviation of the experimental measurement from the dashedline is due to residual band bending in the Si, which is typicallyobserved even for well-passivated surfaces.25 From thisdeviation, we conclude that the Si band bending varies between100 and 200 meV. These values agree well with previousreports, where band bending (relative to Si−H) was measuredby XPS and by photosaturated contact potential differ-

Figure 1. Schematic representation of the alkyl (C10H21, left) andalkenyl (C10H20, right) monolayer on Si(111).

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ence.25,35,36 Relatively speaking, these are small values, whichindicate that the Si under the molecular monolayer is well-passivated. We note, however, that the broader Si 2p and C 1speaks observed for the HDp Si adsorbed monolayers (Figure 2aand 2c, respectively) are a result of the unusual structural andchemical properties of the freshly etched HDp Si surface. It hasbeen shown that such surfaces are much rougher, compared tothe n-Si surface, and that they are not always H-terminated.39−41 As a result, this Si−C interface is not as wellordered or defined as that for other Si doping levels. For furthercharacterization of the HDp samples, see the SupportingInformation.If we assume that the monolayer energy levels are fixed with

respect to those of the substrate (Si), the C 1s core level shift(Figure 2c) should parallel the Si 2p shift. However, as Figure2c shows, the shift of the C 1s core levels of alkyl monolayersadsorbed on the surfaces of differently doped Si substrates isvery small (∼285.2 eV). To illustrate this point, Figure 2dpresents the difference between the C 1s and Si 2p peakpositions as a function of the nominal bulk-Si EF. Indeed, thisdifference is identical (within experimental error) for the twoMD Si samples (n-type vs p-type), but it is smaller by 0.4 eV forthe HDn Si and larger by about the same amount for the HDpSi. A similar result was obtained for alkenyl monolayers on Si(see Figure S1 of Supporting Information). The fact that theposition of the C 1s peak is not affected by the doping densityof the Si substrate is remarkable, considering that the thicknessof the organic layer (the only possible source for the C 1s peak)is only 1.5 nm. Therefore, these results imply that the chargedistribution differences between MD and HD Si are highly

localized at the interface between the Si and the organic chain.In other words, the Si−C bond dipole strongly depends on theinitial doping density of the Si substrate. Indeed, a similardependence was observed in previously published ultravioletphotoemission spectroscopy (UPS) results on similar alkyl,alkenyl, and phenanthrenequinone monolayer−Si sam-ples.28,36−38 However, in the UPS experiments of these earlierstudies it was not possible to determine whether the initial Sidoping density affects the whole molecular monolayer or islocalized on the bond only. Here, by comparing the C 1s and Si2p emission peaks, we are able to extract the potential drop onthe Si−C bond dipole itself and find that it varies significantlywith the initial doping density of the Si (Figure 2d).To learn more about the charge distribution in the

immediate vicinity of the interface between the Si and theorganic chain, we focus on the C 1s peak of the alkenylmonolayer of MDn Si, shown in Figure 3a. The alkenylmonolayer possesses a double bond, adjacent to the Si surface(Figure 1), and was shown to have a prominent photoelectronsignal from the C atom, bound to the Si (CSi in Figure3a),28,42,43 which is typically shifted by ∼1.4 eV toward lowerbinding energies with respect to the main C 1s peak (CCH2 inFigure 3a).25,44 This reduced (lower binding energy) C 1sshoulder is also observed for the alkyl monolayers (see arrowsin Figure 2c). However, there it is less pronounced than foralkenyl monolayers (Figure 3). Note that a small thirdcomponent (Cvib) is attributed to losses to vibrationalexcitations and is therefore associated with higher energypeaks.25,45 The area of the CSi peak, presented in Figure 3a, is∼7% of the total C 1s peak, consistent with the stoichiometric

Figure 2. (a) Si 2p core level from Si−C10H21 samples with different Si doping density and type. (b) Si 2p peak position as a function of the Fermilevel, EF, relative to the Si conduction band (CB) edge, the position of which was extracted from the nominal doping type and density of each Sisample. (c) C 1s core level from Si−C10H21 samples with different Si doping density and type. Arrows mark the C 1s component from C attached tothe Si (see text). (d) Difference between the C 1s and Si 2p peak positions as a function of EF, relative to the Si CB edge. The dashed line is a guideto the eye that represents an ideal case, in which the charge rearrangement is unaffected by the doping density of the Si substrate. The error bars forthe binding energies represent the measurement accuracy, and those of the EF, computed from eq 1, reflect the uncertainty in doping densityprovided by the Si wafer manufacturer.

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ratio (1:10) multiplied by the exponential attenuation of thesignal intensity with depth, with a photoelectron inelastic meanfree path of 3.3 nm.46

The CSi component of alkenyl on HD Si is considerablyattenuated compared to the MD Si case, as shown by dashedand solid lines in Figure 3b, respectively. The lack of a clearlyresolved CSi peak on HDn is likely related to the absence of aclear distinction between the band bending in the Si and thebond dipole at the Si−C interface, owing to the extremelystrong potential gradient at the surface of the HD Si.We show that the doping level and type affect the position of

C 1s with respect to Si 2p (Figure 2), as well as the internalshape of the C 1s peak. This serves as direct experimentalevidence that the charge distribution of the interfacial Si−Cbond is strongly doping-dependent. Furthermore, this resultindicates that the excess charge is localized at the Si surface,rather than on the carbon chain atoms. This large surfacecharge is not expressed as band bending (Figure 2b) becausethe depletion layer width of the HD (i.e., spatial width of theregion at which band bending exists) in Si is smaller than theescape depth of electrons in the XPS measurement. Thus, forall practical purposes the distinction between band bending andbond dipole is lost. This shows that, for a sufficiently highdoping level, the effective bond dipole is “flexible” rather than arigidly predetermined property. It appears to change accordingto the electrostatic potential difference between the bulk of thesolid surface and the molecular backbone. Consequently, theresult for the alkyl monolayers on differently doped Si issurprisingly similar to that obtained for thiols on Au and Ag:initial (before monolayer adsorption) differences in thesubstrate Fermi level are by-and-large compensated by changesin the interface dipole, so that the final work function (aftermonolayer adsorption) is the same for both metals. The Sisubstrates studied here show that differences in bond chargingare not only due to different elemental electronegativities (e.g.,Au cf. Ag) but also strongly affected by the solid’s Fermi energy.This is a unique example of the translation of a bulk,macroscopic property, such as the Fermi energy, into amicroscopic, localized property such as the bond dipole.

■ CONCLUSIONS

In conclusion, we have studied the bond dipole betweenorganic molecules and Si substrates of different doping

densities and polarities. By avoiding a metallic substrate, wecould isolate the effect of the substrate’s initial work functionon the bond dipole. Using shifts in the XPS core levels wecould accurately locate the charge distribution around thecovalent Si−C bond as the most sensitive region to changes inthe initial position of the Fermi level within the Si substrate.These results show that the charging across the interface bondand subsequent dipole may change as a function of theelectrostatic potential difference at the interface and, thus, setlimits outside of which simple models may not be a sufficientdescription of band alignment at interfaces.

■ ASSOCIATED CONTENT*S Supporting InformationSample preparation, alkenyl monolayer XPS results, and HDptype monolayer characterization. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Leeor Kronik. Tel.: 972-8-934-4993. E-mail: leeor.kronik@weizmann.ac.il. Hagai Cohen. Tel.: 972-8-934-3442. E-mail:hagai.cohen@weizmann.ac.il. David Cahen. Tel.: 972-8-934-2246. E-mail: david.cahen@weizmann.ac.il.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful for frequent discussions with Prof. RonNaaman − an early and consistent advocate of the importanceand role of charge transfer between molecular monolayers andconducting substrates.47−51

AV, LK, and DC thank the Israel Science Foundation via itscenters of Excellence program, for partial support. DC and AKthank the US-Israel Science Foundation for partial support.Work at Princeton University was further supported by theNational Science Foundation (DMR-1005892). DC and LK aregrateful to the Wolfson Family Trust and the Kimmel Centrefor Nanoscale Science for support. LK thanks the Lise MeitnerMinerva Center for Computational Chemistry. OY thanks theAzrieli Foundation for the award of a Fellowship. HZ thanksthe Weizmann Institute of Science for the award of the 2013−

Figure 3. (a) XPS C 1s core level from an alkenyl monolayer (Si−CHCH−R; R = (CH2)7CH3) on MDn Si, with decomposition into peaks thatare further described in the text. (b) Overlay of the C 1s peak of alkenyl on MD and HDn Si. The gray area in (a) marks the peak due to the C atomthat is bound to the Si (C−Si). The synthetic peaks are of Gaussian−Lorentzian shape with 30% Lorentzian character and a full-width at half-maximum of 0.85−1.15 eV. The C−Si peak is clearly observed for the monolayer on the MDn Si but is greatly reduced for the alkenyl monolayer onthe HDn Si (dashed line in (b)). Curve-fitting to HD samples resulted in smeared peaks of an ambiguous assignment.

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2014 Joseph Meyerhoff Visiting Professorship. DC holds theRowland and Sylvia Schaefer chair in Energy research.

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The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp403177e | J. Phys. Chem. C XXXX, XXX, XXX−XXXE

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The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp403177e | J. Phys. Chem. C XXXX, XXX, XXX−XXXF