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Effect of Internal Heteroatoms on Level Alignment at Metal/molecular Monolayer/Si Interfaces Hadas Alon, Rachel Garrick, Sidharam Pundlik Pujari, Tal Toledano, Ofer Sinai, Nir Klein-Kedem, Tatyana Bendikov, Joe E. Baio, Tobias Weidner, Han Zuilhof, David Cahen, Leeor Kronik, Chaim N Sukenik, and Ayelet Vilan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09118 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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The Journal of Physical Chemistry

Effect of Internal Heteroatoms on Level Alignment at Metal/Molecular Monolayer/Si Interfaces Hadas Alon1, Rachel Garrick2, Sidharam P. Pujari3, Tal Toledano2, Ofer Sinai2, Nir Kedem2, Tatyana Bendikov4, Joe Baio5, Tobias Weidner6, Han Zuilhof3, David Cahen2, Leeor Kronik2,*, Chaim N. Sukenik1, Ayelet Vilan2,* 1

Department of Chemistry and Institute of Nanotechnology & Advanced Materials, Bar Ilan University, Ramat-Gan 52900, Israel. 2

Department of Materials & Interfaces, Weizmann Institute of Science, Rehovoth, 76100, Israel.

3

Laboratory of Organic Chemistry, Wageningen University & Research, Stippeneng 4, 6708 WE Wageningen, The Netherlands.

4

Chemical Research Support, Weizmann Institute of Science, Rehovoth, 76100, Israel.

5

School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, OR, USA 6

Department of Chemistry, Aarhus University, 8000 Århus C, Denmark.

Corresponding Authors: * Ayelet Vilan, [email protected] * Leeor Kronik, [email protected]

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ABSTRACT: Molecular monolayers at metal/semiconductor hetero-interfaces affect electronic energy level alignment at the interface by modifying the interface’s electrical dipole. On a free surface, the molecular dipole is usually manipulated by means of substitution at its external end. However, at an interface such outer substituents are in close proximity to the top contact, making the distinction between molecular and interfacial effects difficult. To examine how the interface dipole would be influenced by a single atom, internal to the molecule, we used a series of three molecules of identical binding and tail groups, differing only in the inner atom: aryl vinyl ether (PhO), aryl vinyl sulfide (PhS), and the corresponding molecule with a CH2 group - allyl benzene (PhC). Molecular monolayers based on all three molecules have been adsorbed on a flat, oxide-free Si surface. Extensive surface characterization, supported by density functional theory calculations, revealed high quality, well-aligned monolayers exhibiting excellent chemical and electrical passivation of the silicon substrate, in all three cases. Current-voltage and capacitance-voltage analysis of Hg / PhX (X=C, O, S)/Si interfaces established that the type of internal atom has a significant effect on the Schottky barrier height at the interface, i.e., on the energy level alignment. Surprisingly, despite the formal chemical separation of the internal atom and the metallic electrode, Schottky barrier heights were not correlated to changes in the semiconductor’s effective work function, deduced from Kelvin probe and ultraviolet photoemission spectroscopy on the monolayer-adsorbed Si surface. Rather, these changes correlated well with the ionization potential of the surface-adsorbed molecules. This is interpreted in terms of additional polarization at the molecule/metal interface, driven by potential equilibration considerations even in the absence of a formal chemical bond to the top Hg contact.


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1.

INTRODUCTION

Molecular monolayers have been shown to be highly useful for “organic-assisted” electronics, where thin organic layers are inserted at an otherwise inorganic metal/semiconductor or semiconductor/semiconductor junction, so as to improve performance or even impart new functionality.1-8 This allows one to harness the power and flexibility of organic chemistry to engineer desired interface properties. One major role of such molecular monolayers is the control of band alignment at the interface and, therefore, of interface transport barriers. This can be understood by considering the molecular monolayer as an array of roughly aligned dipoles, such that the ensuing dipolar sheet introduces an electrostatic potential step at the interface.9-11 Chemically, this dipole may be tailored by specific functional groups in the molecule12-14 and/or by molecule-substrate interaction.15-19 At a free surface, the molecular dipole is usually manipulated by means of a substitution at the external end of the molecule,2, 19-28 so as to diminish chemical interference with the binding group and to reduce substrate-interaction effects.19 However, at an interface such outer substituents are in fact in closest proximity to the top contact, negating the above advantages. In fact, in extreme cases, the overall interface dipole may even change sign with respect to the free surface one.29-30 One way to minimize the effect of both top and bottom contacts is to insert dipolar groups inside the skeleton of the molecule.13, 31-32 Heteroatoms can serve as a minimal internal polar group.33-34 Such a design avoids steric effects as there are no pendant or bulky substituents and, therefore, can have a minimal effect on molecular orientation. For a practical realization of this concept, we choose to use a Si substrate, due to its atomic smoothness and rich possibilities for forming stable covalent bonds.14, 35 Molecules can then be bound to H-terminated Si via hydrosilylation of C=C bonds, a common route for monolayer formation on reduced Si.35-36 Steric considerations commonly limit the binding density to 50% (as in alkyl monolayers)18, 37 though higher (approaching 2/3)33,

38,

and lower (1/3)39 binding densities are also possible. Im-

portantly, even at low binding density the coverage is uniform and both the molecularly-bonded Si and the Si with bound H are extremely stable against oxidation.39 As long as the coverage is uniform, the Si-H atoms are “buried under” the molecules and their contribution to surface properties is negligible. To allow for heteroatom inser3 ACS Paragon Plus Environment


tion, we introduce a series of phenyl-vinyl monomers, CH2=CH-X-phenyl, with X = CH2, O, or S (see Figure 1), abbreviated as PhX, which upon adsorption on an oxidefree Si(111) substrate becomes Si(substrate)-CH2-CH2-X-phenyl. In this way, monolayers (MLs) with identical binding chemistry and identical exposed surface functionality can be formed. Here, we examine whether the formal chemical separation of the heteroatom from both the top surface (by the phenyl ring and absence of chemical contact) and the Si substrate (by the ethyl linker) also isolates the heteroatom from the contacts electrically. To that end, we obtain high-quality monolayers based on the monomers shown in Figure 1, characterize their electrical properties both with and without a top electrical contact made with a Hg drop, allowing us to distinguish the contribution of the top contact to the net dipole. This comparison finds that the surface dipole changes significantly upon deposition of the top contact and reveals a surprising correlation between the interface dipole and the ionization potential of the adsorbed molecules. This is attributed to subtle charge rearrangement between the Hg electrode and the phenylgroup of the molecules, which does not involve the heteroatom directly, but is affected by it via a shift in the molecular energy levels.

Figure 1. Structures of molecules used for monolayer formation. The terminal double bond of the monomers opens upon binding to Si, so as to provide the -Si-CH2CH2–X-phenyl structure.


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3.

MATERIALS AND METHODS

3.1 General Information All chemicals, unless noted otherwise, were commercially available and used as received. oriented Si wafers were P-doped (resistivity 2-12 Ω⋅cm) and polished on one side. Solvents for rinsing and cleaning the Si substrate were reagent grade materials. Mesitylene (98% purity) was dried with molecular sieves. Ultra-pure water was provided by a Milli-Q system (resistivity 18.3 MΩ cm). 3.2 Purifications and Syntheses. Allylbenzene (PhC, 98%, Sigma Aldrich) was purified by Kugelrohr distillation (b.p. 156 ºC) and handled under a nitrogen atmosphere. Phenyl vinyl sulfide (PhS, 97%, Sigma Aldrich) was purified by column chromatography (silica gel/chloroform). Phenyl vinyl ether (PhO) was synthesized by a procedure similar to that reported by Okimoto et al.,40 but on a scale of 6 grams of phenol instead of the originally reported 0.094 grams. The crude product was purified by short column chromatography (silica gel/hexane) and fractionally distilled (yield: 2 g, 30%). The product was a clear liquid (single spot on TLC); 1H-NMR (300 MHz, CDCl3) δ 7.336.97 (m, 5H), 6.12 (dd, J=13.8, 6.3 Hz, 1H), 4.75 (dd, J=13.8, 1.5 Hz, 1H), 4.40 (dd, J=6.3, 1.5 Hz).

13

C NMR (300 MHz, CDCl3) δ 156.9, 148.3, 129.7, 123.2, 117.2,

1

95.1. The H -NMR was in agreement with the spectrum reported in the literature.41 3.3 Sample preparation and adsorption procedure. Si wafers were cut to 1×2 cm and cleaned for 3 minutes in an Ar:O2 plasma, followed by 15 minutes etching in a 40% aqueous, N2 degassed NH4F solution, as previously described.33 Photochemical ML formation26 using a 10 mM solution of each molecule (Figure 1) in mesitylene was performed as reported earlier.42 Electrical characterization of the deposited MLs was performed immediately after ML preparation and all chemical and spectroscopic characterizations were done within 3 days (samples were sealed in Ar atmosphere). To minimize surface oxide formation, sample exposure to ambient was as short as possible.43 3.4 Monolayer Surface Characterization.



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The MLs were all characterized using contact angle and infrared reflectionadsorption spectroscopy (IRRAS) as described previously.33 Ellipsometry

was

performed

using

a

multiple-wavelength

ellipsometer

(550−1000 nm) at a constant incidence angle of 70° (M 2000 V from J. A. Woollam Co., Inc.) and analyzed by fitting to a Cauchy model for organic monolayers (index of refraction between 1.460 and 1.483 for the spectral range) using commercial software (WVASE32). X-ray and UV photoelectron spectroscopy (XPS, UPS) measurements were done with a Kratos AXIS ULTRA system, as detailed previously.44-45 XPS was also measured by a JEOL JPS-9200 instrument.33 In both cases XPS was performed with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) and a 10 eV pass energy. UPS employed He I (21.22 eV) and He II (40.8 eV) radiation lines. To observe the photoemission onset at low kinetic energies a –9 V bias was applied to the sample.44 Experimental monolayer thickness was extracted from the XPS measurements using an attenuation length of 39.5 Å. Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra were collected at the National Synchrotron Light Source (NSLS) U7A beamline at Brookhaven National Laboratory, using an elliptically polarized beam with ∼85% p-polarization and a monochromator (600 lines/mm grating), which provides a full width at halfmaximum (fwhm) resolution of ∼0.15 eV at the carbon K-edge. The monochromator energy scale was calibrated using the intense C 1s−π* transition (at 285.35 eV) of a graphite transmission grid placed in the path of the X-rays. Partial electron yield was monitored by a detector with the bias voltage maintained at −150 V. The NEXAFS angle is defined as the angle between the incident X-ray beam and the sample surface. Kelvin-Probe measurements of the contact potential difference (CPD) were performed in a home-built setup, based on a commercial Besocke Delta Phi Kelvin probe + controller, using a vibrating Au grid with a work function of 4.2 eV (smaller than standard due to ambient adsorbents), as calibrated relative to a freshly cleaved HOPG surface with a nominal, stable work function of 4.6 eV.46


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3.5 Junction Formation and Electrical Characterization Electrical contacts: A top contact to the molecularly modified Si was made using a controlled growth hanging mercury (99.9999% purity) drop electrode (Polish Academy of Sciences). A back, Ohmic contact to the Si was made on the opposite side of the Si wafer by rubbing a eutectic mixture of 1:3 In:Ga and scribing it into the Si with a diamond pen. Measurements were carried out in a controlled environment glove box with 10% relative humidity. The Hg contact area was optically measured (diameter ∼ 0.5 ± 0.05 mm) and used to extract the barrier height from the C-V and I-V measurements (section 3.5). Current-Voltage (I-V) characteristics were measured with a Keithley 4200 parameter analyzer. Bias scans from −1 V to +1 V (applied to Hg) were measured for each junction with a scan rate of 20 mV/s. Between 3 to 5 samples were prepared for each monolayer type, and on each sample, I-V traces were recorded at 5-10 different spots. Capacitance-Voltage (C-V) measurements were done with a Keithley 4200 parameter analyzer using a 30 mV ac amplitude and 1 MHz frequency. C−V scans were recorded over 5-10 spots on two different samples for each monolayer. 3.6 Computations Density functional theory (DFT) calculations for the electronic structure, total energy, and geometry of all systems described above were obtained by solving the Kohn–Sham equations within the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional,47 as described previously.44 Total energies were corrected for dispersive interactions, important for covalently bound monolayers on Si32 but typically severely underestimated in PBE, using the pairwise-correction scheme of Tkatchenko and Scheffler (TS-VdW).48-49 Surfaces were modeled by a periodic slab containing 4 Si surface atoms, of which two are hydrogen bonded and two bonded to PhX molecules, reflecting a 50% coverage.18, 37-38

All calculations were carried out using the plane wave-based Vienna Ab Initio

Simulation Package (VASP).50



4.

RESULTS

4.1

Molecular adsorption and monolayer quality

The PhX monomers (Figure 1) bind to H-Si by opening of the double bond to form a saturated C-Si bond and progress via radical reaction.36, 51 Monolayer quality

600

CPS

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a) PhC

b) PhO

c) PhS

C+, 23%, +1.6 eV

C+, 9%, +1.5 eV

400

200

C+, 5%, +1.0 eV

C-Si, 9%, -0.9 eV

0 288

285

282

Binding Energy [eV]

288

285

282

Binding Energy [eV]

288

285

282

Binding Energy [eV]

Figure 2. XPS C1s peak for PhC (a), PhO (b), and PhS (c), showing raw data (symbols), composing peaks (shaded areas) and their sum (blue line) with respect to the baseline (red line). Legends give the fractional areas (%) and the energy shifts with respect to the main (C-C) peak; see text for peak assignments.

was verified by a variety of surface analyses. First, monolayer thickness was measured by ellipsometry and independently calculated from XPS measurements52 and compared to DFT-extracted ML thickness (Table 1). For all monolayers the ellipsometric thickness was found to be ~ 11Å, while the XPS thickness was lower and closer to that which was theoretically predicted. This is consistent with formation of a single-molecule thick layer, though the ambient ellipsometry may include more physisorbed contaminates. The correlation between DFT and experiment is reasonable and qualitatively correct. The 1-3 Å variations in the experiment are attributed to empirical parameters (refractive index for ellipsometry and attenuation length for XPS), which are generic and cannot be calibrated for each specific ML. On the theoretical side, the DFT calculations assume an ideal monolayer and do not take into account any local disorder, which is inevitably present experimentally. Finally, thickness determination for similar MLs by a parameter-free X-ray reflectance (XRR) method revealed that the XRR-determined thickness is smaller by up to 2 Å as compared to ellipsometry.27 This may further explain why the DFT results yield smaller values. The 8 ACS Paragon Plus Environment

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ratio between the XPS thickness and the DFT-predicted one can be further used to extract the binding density,52 which yields ~ 62% of surface Si atoms bonded to PhX, while the rest are H-terminated. This is a fairly high coverage, in agreement with formally reported binding densities of analogous alkyl-X-C=C33 and bromo-styrene,38 and may reflect a hexagonal binding pattern.39

Table 1. Thicknesses (Å) and coverage (%) for PhC, PhO, and PhS monolayers.

Theoretical

a

PhC

PhO

PhS

7.5

8.4

8.7

10.5 8.9 63

11.6 9.3 63

Ellipsometry 10.7 XPSb 7.7 c Coverage 61 a

distance from middle of Si-C bond to outermost H, determined from the DFT calculations. computed using an attenuation length of 39.5Å for Si 2p photoelectrons emitted through an organic monolayer.52 c the fraction of surface atoms which are molecularly-bonded; computed following Ref. 52 using the physical constants reported there.

b

The chemical composition of the monolayers was confirmed using infrared (IR) spectroscopy. The IR spectrum exhibited characteristic peaks of aromatic and aliphatic C-H stretching vibrations (see Section S1 of the SI), with no detectable effect of the heteroatom. Aromatic C-H stretch peaks were observed at 3030 and 3070 cm-1, in agreement with past findings for similar monolayers.38,

53-54

An aliphatic C-H anti-

-1

symmetric stretch peak was observed at 2929 ± 1 cm . This value is 10 cm-1 higher (blue-shifted) than that of a well-packed 16-carbon long alkyl monolayers on a reduced silicon surface,55 indicating weak interactions between neighboring chains, which is reasonable for alkyl chains consisting of no more than three methylene units.53 This underscores the importance of the interactions between the phenyl rings as those that drive the ordering of the MLs (see NEXAFS results below). The surface elemental composition was determined from XPS measurements, which exhibited peaks corresponding to silicon (Si 2p at 99.62 eV), carbon (C 1s at 9 ACS Paragon Plus Environment


285.0 eV), oxygen (O 1s at 532-533 eV), and for PhS, sulfur (S 2s at 228.0 eV). The monolayers were mostly uncontaminated, though excess O did suggest some (unavoidable, adventitious) contamination (see Fig. S3 of the SI). The source of the oxygen excess is not in SiO2, as no peak was observed at a binding energy of 103.5 eV (see Section S2 of the SI), indicating negligible SiO2 formation.56 To assess the possible presence of other sub-oxides, we performed a decomposition of the Si 2p peak, which is reported in Fig. S2 and Table S1 of the SI. The analysis reveals the presence of a single positively-charged surface species at ~0.8 eV higher binding energy than that of the main Si peak, similar to a species appearing at H-terminated Si. Using attenuation relations,52 the relative intensity of the sub-peaks corresponds to approximately half the Si surface atoms, suggesting that this peak is related to remaining Si-H surface atoms. The absence of oxidized Si is crucial for reliable electrical characterization of the system. Sulfur was observed only for PhS monolayers (see Fig. S3d of the SI) at a relative amount of [S]:[C] = 1:9, in a good agreement with the stoichiometric ratio (1:8), taking into account depth attenuation. XPS data of the C1s peak for all three monolayer types are given in Figure 2. For PhO the C1s peak is split because of the presence of oxygen near two carbon atoms (Figure 2b). The intensity of the main peak (C0, blue-colored) is 3.3 times larger than that of the sub-peak (C+, green-colored), compared to a stoichiometric ratio of 3 (two carbons next to an oxygen atom relative to the six remaining carbon atoms). However, the C+ atoms are underneath the C0 ones, and due to signal attenuation with increasing depth, the observed ratio is indeed expected to be somewhat larger than the stoichiometric one (~3.2). This agreement supports our notion that PhO comprises most of the surface layer, with a negligible amount of unknown contaminants. The other molecules also show slight broadening to the left, comprising ~6% of the net signal. This may be due to residual physisorption or due to other effects that we cannot resolve. The C 1s peak of PhC exhibits a small low binding energy shoulder due to the C-Si bond.57 No such a shoulder exists for PhO, though for PhS a C– shoulder may hide in the noisy baseline. This implies that the heteroatom alters the C-Si bond dipole. Indeed, it has been shown that the charging of the C atom, bonded to Si, is sensitive to long-range polarization, as demonstrated for alkyl monolayers adsorbed on a Si substrate with varying doping type and concentration.57


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4.3

Orientation and order of the molecules in the monolayer Molecular orientation directly affects the surface-normal dipole.14 NEXAFS spec-

troscopy can be used to probe the molecular tilt of surface-adsorbed species by measuring characteristic X-ray absorption resonances, corresponding to electronic transitions from atomic core levels to unoccupied molecular orbitals.58 Carbon K-edge spectra collected from PhX layers on silicon, acquired at a 55° incident angle of the X-ray beam (see experimental section), are presented in Figure 3a. The π1*(C=C) emission near 285.7 eV,59-61 present in all spectra, stems from the resonance of the aromatic head groups. The related π2*(C=C) resonances near 288 eV overlap with the strong Rydberg (R*) and σ*(C−H) resonances near 287.9 eV. For the PhO and PhS MLs, a contribution from σ*(C–O) and σ*(C–S) resonances near 288 eV cannot be excluded. In addition, all MLs show the expected broad σ* resonances at 293 eV and higher photon energies, which are related to C−C bonds in general within the monolayer.60-64 Molecular orientation was subsequently assessed from the variation in resonance intensity with incident angle. Figure 3b shows the difference spectra between normal (90°) and glancing (20°) incident angles obtained for the PhX layers on silicon. Disordered monolayers with random orientation are expected to yield featureless difference spectra. The difference spectra related to the PhC, PhO, and PhS monolayers possess a significant positive dichroism for the π1*(C=C) molecular orbitals (near 285.7 eV, see arrows in Figure 3b), indicating that the rings within these monolayers are ordered.60, 62-63 For ring resonances, a positive dichroism shows an upright ring orientation. Such a high degree of ordering is unusual for aromatic monolayers with only one phenyl group65-67 and merely 2-3 methylene units.45 The dichroism observed near 287.9 eV and 289.5 eV can in principle be related to π2* resonances of the aromatic rings, or to Rydberg and σ* resonances of the aliphatic chains. As mentioned above, a low chain order was observed for the methylene units in the IR analysis. Therefore, the strong dichroism observed by NEXAFS is likely related to ring π2* contributions. The evolution of the C=C π* resonance with incidence angle (using five spectra between 90° and 20°, see Section S4 of the Supporting Information) was used to compute the averaged orientation of the phenyl rings,58 α (see inset to Figure 3b, α=0° 12 ACS Paragon Plus Environment

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if the ring plan is vertical to the surface), as listed in Table 2. Within the ±7° experimental error for the angle calculations, the MLs have similar orientations (< 30°), with a slightly more upright orientation of the rings for the oxygen substituted species and a slightly more tilted configuration for PhC. The ring orientation of PhX is similar to that of monolayers of bi-phenyl (24°) and tetra-phenyl (28°), linked via O to Si(111)61 while a monolayer of bromo-styrene on Si(111) has a slightly more up-right orientation (14°).38 This difference reflects the odd-even tether effect on ring orientation45 (the tether length is 2, 1, and 3 for styrene,38 phenol61 and PhX). The similar ring orientation for the three PhX monolayers is in qualitative agreement with results of water contact angle measurements (elaborated below) and DFT calculations (see Table 2). We note that agreement between DFT and NEXAFS was significantly degraded if a smaller unit cell containing only two Si surface atoms, one of which is bonded to a molecule, was used instead of the four Si surface-atom cell used throughout this work (see section S5 of the SI). The reason can be that the extra degrees of freedom, afforded by the larger unit cell, facilitate the improved agreement by allowing some degree of inter-molecular disorder within the otherwise perfectly periodic structure of the theoretical model system.

Table 2: Data related to molecular orientation, obtained by different methods Water contact angle (± 1°) NEXAFS dichroic ratioa (± 0.2) NEXAFS Ring plane,b α (± 7°) A (°) DFT Ring plane,c α B (°)

PhC 83 0.13

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