Effect of Doping Density on the Charge Rearrangement and Interface

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Effect of Doping Density on the Charge Rearrangement and Interface Dipole at the Molecule-Silicon Interface Omer Yaffe, Sidharam Pundlik Pujari, Ofer Sinai, Ayelet Vilan, Han Zuilhof, Antoine Kahn, Leeor Kronik, Hagai Cohen, and David Cahen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp403177e • Publication Date (Web): 17 Jun 2013 Downloaded from http://pubs.acs.org on June 17, 2013

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The Journal of Physical Chemistry

Effect of Doping Density on the Charge Rearrangement and Interface Dipole at the Molecule-Silicon Interface

Omer Yaffe,1 Sidharam Pujari,3 Ofer Sinai,1 Ayelet Vilan,1 Han Zuilhof,3 Antoine Kahn,4 Leeor Kronik,*, 1 Hagai Cohen,*,2 and David Cahen*,1

1

Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel 2

Department of Chemical Research Support, Weizmann Institute of Science, Rehovoth 76100, Israel 3

Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, the Netherlands

4

Department of Electrical Engineering, Princeton University, Princeton, New Jersey, 08544 USA

David Cahen ; tel 972-8-934-2246 ; email: [email protected] Hagai Cohen; tel 972-8-934-3442 ; email: [email protected] Leeor Kronik ; tel 972-8-934-4993 ; email: [email protected]

1 ACS Paragon Plus Environment


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Abstract The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using x-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si-C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed, but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface.

Keywords: level-alignment, organic-electronics, work-function, charge-transfer


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Introduction In organic electronics, device performance may depend strongly on the energy level/band alignment at the interface between the organic electronic material and an inorganic material, which can be the electron or hole blocking layer or the electrode.

1–5

One of the promising

methods to control and modify the band offset is to covalently bind an organic monolayer, made up of oriented polar molecules, to the electrode surface 6–9, as such monolayers can serve to adjust the work function (WF) of the electrode (see, e,g. references

10–12

, and references

therein). The electrode WF is modified by means of an electrostatic potential step, associated with a dipole layer. In the case of a molecular monolayer this potential step may arise for several reasons.1 The leading causes are the molecular dipole of the free molecule (prior to adsorption),13–15 the molecule-substrate bond dipole or interface dipole,10,13,15–17 and the modification of the original charge distribution at/near the substrate surface (also known as the “pillow” or “push back” effect).15,18–21 The relative contributions from these effects to the overall interface dipole vary according to the strength of the molecule-substrate interactions, the free molecule’s dipole, the polarizability of the molecular backbone and the intrinsic electronic properties of the solid.2,22 While the “pillow” effect is largely independent of the molecular properties, molecule-specific modification of the substrate WF can take place by either changing the molecular dipole (e.g., changing the terminal group at the end of the monolayer-forming molecule) or by changing the molecule-substrate binding group and thereby the bond dipole. For sufficiently long molecules, the bond and terminal dipole will not be coupled so that both can serve as independent “handles” for modifying the electrode WF 10,11 .



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While changing the terminal group of the molecule has proven to be an efficient and relatively easy method to modify the WF, use of the bond dipole as a handle is much more challenging. This is due in part to a limited number of binding group types that can covalently bind to a given inorganic substrate. However, even in cases where the interface dipole can be systematically modified (e.g., binding thiols to Au and Ag) it was experimentally observed and theoretically computed that the metal-molecule interface dipole variation largely compensates for the variations between the Metal WFs.11,18,23,24 In other words, while the initial WFs of two metals were substantially different prior to the adsorption of the organic monolayer, the final, postadsorption WFs were similar. Further insight into the bond dipole between an organic monolayer and an inorganic substrate can be gained by considering a silicon (Si) substrate, rather than a metal substrate. The reason is that, to a first approximation, the WF of the Si substrate can be modified via doping, while preserving the chemical bond and the structure of the monolayer on the surface. This allows to determine the extent to which the interface dipole is an intrinsic property of the covalent bond (which, to a first approximation, will be unchanged), as well as the degree to which the electrostatic potential difference at the interface (which varies with doping density) affects the interface dipole.

Figure 1- Schematic representation of the alkyl (C10H21, left) and alkenyl (C10H20, right) monolayer on Si(111) 4 ACS Paragon Plus Environment

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To investigate this issue and, thus, the larger question of how the energetics of the interface between a molecular monolayer and an inorganic solid are determined, we experimentally probe the interface dipole between oxide-free Si(111) substrates with different doping densities and alkyl/alkenyl monolayers (see figure 1). We use both alkyl (C10H21) and alkenyl (C10H20) monolayers: the alkyl monolayer on oxide-free Si is widely studied12 and allows us to compare and contrast our work to published results; Alkenyl on Si has a stronger photoemission signal from the Si-C interface, which is of primary interest in this study. For both alkyl and alkenyl monolayers, the 1×1 surface structure of the freshly etched Si-H is preserved.25,26. The interface dipole is probed directly by X-ray photoelectron spectroscopy (XPS)27 , where we monitor the change in electrostatic potential between the Si (Si 2p core electron binding) and organic monolayers (C 1s core electron binding). We find that for high doping densities, the level alignment depends strongly on the doping density of the Si substrate, suggesting that the interface dipole associated with the binding group depends on more than the type of atoms that comprising the covalent bond.

Experimental Methods Sample preparation and characterization followed literature descriptions.28,29 In brief, pieces of Si wafer were cleaned by sonication in acetone and oxidized by an oxygen plasma (Harrick PDC002 setup) for 3 min. Subsequently, the Si(111) substrates were etched in an Ar-saturated 40% NH4F solution for 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) or 200 °C (alkyl) and ∼10 mbar. After 16 h (alkenyl) or 4h (alkyl), the reaction was stopped and the monolayers were rinsed extensively with EtOH and CH2Cl2 and sonicated for 5 5 ACS Paragon Plus Environment


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min in CH2Cl2 to remove physisorbed molecules. Alkyl- and alkenyl-based monolayers on Si were studied extensively before. Monolayers obtained using this and similar procedures were shown to be densely packed, chemically stable, and reproducible.26,29–32 Si(111) wafers of four different doping density-type combinations were used, namely heavilydoped n-type (HDn), moderately doped n-type (MDn), moderately doped p-type (MDp), and heavily-doped p-type (HDp) silicon. The source, doping density, and type of dopant are given in the supplementary information. Measurements were conducted immediately after sample preparation and were therefore conducted on two different spectrometers. To assure that results from the two spectrometers could be compared, measurements were taken at a normal take-off angle, with some samples cross-checked. In addition to facilitating the comparison between different spectrometers, the normal take-off 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-HS spectrometer, using a monochromatic Al Kα X-ray source (60 W) with analyzer pass energies ranging between 20 and 80 eV. Initial work-function measurements under conditions that are practically free of irradiation damage (based on the method described in ref. 33) were performed on the fresh sample, followed by several repeated sets of XPS measurements over various spectral ranges, then followed by a second WF measurement to detect any radiation-induced surface potential change. Alkenyl monolayers were measured on a JPS-9200 photoelectron spectrometer (JEOL, Japan) using monochromatic Al Kα X-ray (300W), with an analyzer passenergy of 10 eV.


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Results and Discussion We begin our analysis by considering the shift of the Si 2p core level with doping. The reference Fermi energy in bulk Si is extracted from the nominal doping density, provided by the wafer manufacturer according to:

N E F = Ei + kT ln d / a  ni

  

(1)

where Ei is the intrinsic Fermi energy for Si, k is the Boltzmann constant, T is the temperature (taken as 300K), ni is the Si intrinsic carrier density and Nd/a is the nominal donor/acceptor doping density. Eq. (1) assumes that all the donor/acceptors are ionized, which is reasonable for the range of doping levels used in our experiments.34 The nominal Fermi energy in the Si bulk is generally different from the experimentally measured work-function or HOMO position, due to surface dipole and/or band-bending. In our analysis below, the doping dependence of the Fermi energy is compared to that of core levels 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 type and density (See Figure S1 in the supplementary information for a similar figure for Si(111)-C10H20 (alkenyl)). There is a clear monotonic shift in the Si 2p peak position as the doping density and type change from HDn to HDp. This is expected, because the binding energy is measured with respect to EF, which itself changes monotonically as a function of doping. Figure 2b shows the Si 2p peak position as a function of the EF position, relative to the conduction band (CB) edge. At flat-band condition, the Si 2p peak shift should simply equal the change in EF, a situation indicated by the dashed line in the figure. The deviation of the experimental measurement from the dashed line is due to residual band bending in the Si, which is typically observed even for well-passivated surfaces.25 From this deviation, we conclude 7 ACS Paragon Plus Environment


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that the Si band bending varies between 100 and 200 meV. These values agree well with previous reports, where band bending (relative to Si-H) was measured by XPS and by photosaturated contact potential difference.25,35,36 Relatively speaking, these are small values, which indicate that the Si under the molecular monolayer is well-passivated. We note, however, that the broader Si 2p and C 1s peaks observed for the HD p Si adsorbed monolayers (figure 2a and 2c, respectively), are a result of the unusual structural and chemical properties of the freshly etched HD p Si surface. It has been shown that such surfaces are much rougher, compared to the n Si surface, and that they are not always H-terminated. 39–41 As a result, the Si–C interface is not as well ordered or defined as it is for other Si doping levels. For further characterization of the HD p samples, see the supplementary information.

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 Fermi level, EF, relative to the Si conduction band (CB) 8 ACS Paragon Plus Environment

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edge, the position of which was extracted from the nominal doping type and density of each Si sample. (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 to the 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 guide to 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 for the binding energies represent the measurement accuracy and those of the EF, computed from Eq. (1), reflect the uncertainty in doping density provided by the Si wafer manufacturer.

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 Figure 2c shows, the shift of the C 1s core levels of alkyl monolayers adsorbed on the surfaces of differently doped Si substrates is very small (~ 285.2eV). To illustrate this point, Figure 2d presents the difference between the C1s and Si2p peak positions as a function of the nominal bulk-Si EF. Indeed, this difference is identical (within experimental error) for the two MD Si samples (n-type vs. p-type), but it is smaller by 0.4 eV for the HD n-Si and larger by about the same amount for the HD p-Si. A similar result was obtained for alkenyl monolayers on Si (see Figure S1 of supplementary information).The fact that the position of the C 1s peak is not at all affected by the doping density of the Si substrate is remarkable, considering that the thickness of the organic layer (the only possible source for the C1s peak) is only 1.5 nm. Therefore, these results imply that the charge distribution 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 the initial doping density of the Si substrate. Indeed, a similar dependence 9 ACS Paragon Plus Environment


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was observed in previously published ultraviolet photoemission spectroscopy (UPS) results on similar alkyl, alkenyl and phenanthrenequinone monolayer-Si samples.28,36–38 However, in the UPS experiments of these earlier studies it was not possible to determine whether the initial Si doping density affects the whole molecular monolayer or is localized on the bond only. Here, by comparing the C 1s and Si 2p emission peaks, we are able to extract the potential drop on the SiC bond dipole itself and find that it varies significantly with 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 the organic chain, we focus on the C 1s peak of the alkenyl monolayer of MD n-Si, shown in Figure 3a. The alkenyl monolayer possesses a double bond, adjacent to the Si surface (Figure 1), and was shown to have a prominent photoelectron signal from the C atom, bound to the Si (CSi in Figure 3a)28,42,43, which is typically shifted by ~1.4 eV toward lower binding energies with respect to the main C1s peak (CCH2 in Figure 3a).25,44 This reduced (lower binding energy) C 1s shoulder is also observed for the alkyl monolayers (see arrows in Figure 2c). However, there it is less pronounced than for alkenyl monolayers (Fig. 3). Note that a small third component (Cvib), is attributed to losses to vibrational excitations and is therefore associated with higher energy peaks.25,45 The area of the CSi peak, presented in Figure 3a, is ~7 % of the total C 1s peak, consistent with the stoichiometric ratio (1:10) multiplied by the exponential attenuation of the signal intensity with depth, with a photoelectron inelastic mean free path of 3.3 nm. 46


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Figure 3 – (a) XPS C 1s core level from an alkenyl monolayer (Si-CH=CH-R ; R=(CH2)7CH3) on MD nSi, with decomposition into peaks that are further described in the text. (b) Overlay of the C 1s peak of alkenyl on MD and HD n-Si. The gray area in (a) marks the peak due to the C atom that 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 to 1.15 eV. The C-Si peak is clearly observed for the monolayer on the MD n-Si, but is greatly reduced for the alkenyl monolayer on the HD nSi (dashed line in (b)). Curve-fitting to HD samples resulted in smeared peaks of an ambiguous designation.

The CSi component of alkenyl on HD Si is considerably attenuated compared to MD Si case, as shown by dashed and solid lines in Figure 3b, respectively. The lack of a clearly resolved CSi peak on HDn is because of both less net charging (as indicated by Fig. 2) and more delocalization of the charging upward into the organic chain, such that more carbon atoms are slightly shifted in energy. The more extended delocalization of the charge is probably due to the extremely strong



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potential gradient at the surface of HD Si. Yet, the high concentration of negative charges at the surface of HDn induced less net charging of the organic layer compared to MDn. We show that the doping level and type affect the position of C1s with respect to Si 2p (Fig. 2), as well as the internal shape of the C 1s peak. This serves as direct experimental evidence that the charge distribution of the interfacial Si-C bond is strongly doping-dependent. Furthermore, this result indicates that the excess charge is localized at the Si surface, rather than on the carbon chain atoms. This large surface charge is not expressed as band bending (Figure 2b) because the depletion layer width of the HD (i.e., spatial width of the region at which band bending exists) in Si is smaller than the escape depth of electrons in the XPS measurement. Thus, for all practical purposes the distinction between band bending and bond dipole is lost. This shows that, for a sufficiently high doping level, the effective bond dipole is “flexible” rather than a rigidly predetermined property. It appears to change according to the electrostatic potential difference between the bulk of the solid surface and the molecular backbone. Consequently, the result for the alkyl monolayers on differently doped Si is surprisingly similar to that obtained for thiols on Au and Ag: initial (before monolayer adsorption) differences in the substrate Fermi level are by-and-large compensated by changes in the interface dipole, so that the final work function (after monolayer adsorption) is the same for both metals. The Si substrates studied here shows that differences in bond charging are not only due to different elemental electro-negativities (e.g., Au cf. Ag) but are 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 a microscopic, localized property such as the bond dipole.


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Conclusions In conclusion, we have studied the bond dipole between organic molecules and Si substrates of different doping densities and polarities. By avoiding a metallic substrate, we could isolate the effect of the substrate’s initial work function on the bond dipole. Using shifts in the XPS core levels we could accurately locate the charge distribution around the covalent Si-C bond as the most sensitive region to changes in the initial position of the Fermi level within the Si substrate. These results show that the charging across the interface bond and subsequent dipole may change as a function of the electrostatic potential at the interface, and thus set limits outside of which simple models may not be a sufficient description of band alignment at interfaces.

Acknowledgements The authors are grateful for frequent discussions with Prof. Ron Naaman – an early and consistent advocate of the importance and role of charge transfer between molecular monolayers and conducting substrates. 47–51 AV, LK and DC thank the Israel Science Foundation via its centers of Excellence program, for partial support. DC and AK thank the US-Israel Science Foundation for partial support. Work at Princeton University was further supported by the National Science Foundation (DMR-1005892). DC and LK are grateful to the Wolfson Family Trust and the Kimmel Centre for Nanoscale Science for support. LK thanks the Lise Meitner Minerva Center for Computational Chemistry. OY thanks the Azrieli Foundation for the award of a Fellowship. HZ is grateful for the award of the 2013-



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2014 Joseph Meyerhoff Visiting Professorship. DC holds the Rowland and Sylvia Schaefer chair in Energy research. Supporting information is Available: Sample preparation, alkenyl monolayer XPS results and HD p type monolayer characterization. This Material is available free of charge via the internet at http://pubs.acs.org REFERENCES

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Graphic TOC Entry


Effect of Doping Density on the Charge Rearrangement and Interface

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