Effective Metal Top Contact on the Organic Layer via Buffer-Layer

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Effective Metal Top-Contact on Organic Layer via Buffer Layer Assisted Growth: A Multi-Scale Characterization of Au/ Hexadecanethiol/n-GaAs(100) Junctions Alexandra Junay, Sophie Guézo, Pascal Turban, Sylvain Tricot, Arnaud Le Pottier, José Avila, Soraya Ababou-Girard, Philippe Schieffer, and Francine Solal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05729 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 13, 2016

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The Journal of Physical Chemistry

Effective Metal Top-Contact on Organic Layer via Buffer Layer Assisted Growth: A Multi-Scale Characterization of Au/ hexadecanethiol/n-GaAs(100) Junctions A. Junay, S. Guézo, P. Turban, S. Tricot, A. Le Pottier, J. Avila, S. Ababou-Girard, P. Schieffer, F. Solal*

Abstract: In the field of organic and molecular electronics at monolayer coverage, the need for abrupt and well controlled top metal contacts is a key point. A general method which provides reliable molecular junctions with most metals remains to be found. In this paper we show that reliable molecular junctions Au/ hexadecanethiol/n-GaAs(100) are obtained using Buffer Layer Assisted Growth (BLAG). They show in hot electron transport measurements at the nanoscale a tunnel regime through organic monolayer with a full spatial uniformity. Using BEEM in the spectroscopic mode as well as photoemission and C(V)-transport measurements we draw a coherent band alignment scheme of the whole hetero-structure at the nanoscale and at the macroscopic scale. Through this study, BLAG method appears as a general method that should work for contacting organic monolayers with most metals.

1. Introduction Manipulating and controlling the self-organization of Organic Molecular monoLayers (OML) attract growing interest in the field of molecular electronics. OML have been considered to selectively modify the work function of metal surfaces1, or to modify the depletion layer of semiconductors2, to act as nanometer-thick tunnel barrier in both metal/OML/Semiconductor and metal/OML/metal junctions, and finally to manipulate spin polarization and magnetic anisotropy at interfaces3. In this field of organic and molecular electronics at monolayer coverage4, the need for abrupt and well controlled top metal contacts is a key point. This has been the subject of numerous studies. Focusing on vapor deposition methods, different metals have been compared, several functional terminations have been proposed, cooled substrate deposition has been used, so-called indirect evaporation has been performed ( for a review see references 5-7). Recently the successful use of lead on silicon-based hetero-structure to provide solid-state junctions controlled by organic molecules has been reported

8-9

.

However a 1

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general method which provides reliable molecular junctions with most metals remains to be found. This is a general concern. For instance, recently, in the domain of molecular spintronics, nano-indentation has been used to optimize contacts by reducing their size, even though the true transport through the molecules is not always present as IETS (inelastic electron tunneling spectroscopy) reveals

10

. Maitani et al

11

have proposed and used the so-

called Buffer Layer Assisted Growth method (BLAG), previously introduced by Wadill et al 12

in order to obtain good contacts, and hence to create reliable molecular tunnel junction. As

they suggest it could appear as a general method in which the substrate grafted with an organic monolayer is protected with a condensed noble gas which serves as a soft-landing layer to quench the kinetic energies of subsequent vapor deposited metal atoms. The rare gas condensed layer could prevent penetration of the metal atoms and chemical degradation of the underlying organic monolayer. Our aim here is to use this method in order to create a molecular tunnel junction at the GaAs(100)/Au interface. Gold (Au) is one of the most difficult metal for contacting molecules (references

5-7

), this study will then appear as a good

test for the BLAG contacting method. STM-BEEM (Scanning Tunneling Microscopy - Ballistic Electron Emission Microscopy) measurements on these BLAG junctions are used to reveal homogeneity of the contacts (no pinhole) at the nanometric level making reference to our previous work on contacts grown without BLAG

13

. Local BEEM measurements allow to demonstrate the homogeneity of the

top-contact/OML interface which is needed to make macroscopic measurements relevant (transport measurements and band alignment determination through synchrotron radiation photoemission measurements). Such band alignment determination of a whole model semiconductor-molecular monolayer-metal junction has not been performed yet to our knowledge. In addition BEEM measurements probe the hetero-structure at equilibrium in contrast to what is performed with conventional transport measurements. BEEM further allows the identification of elastic transport channels for electrons at specific energy, giving a straightforward access to the local band alignment of the heterostructure with an ultimate nanometric lateral resolution. BEEM is thus highly complementary of spatially-averaged transport and photoemission measurements. Remarkably, we finally demonstrate in the present multi-scale study on BLAG junctions an overall agreement between local and macroscopic characterizations combined with an excellent reproducibility from one junction to the other. From these observations, we validate BLAG as a general method to avoid metal

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diffusion through the molecular monolayer, allowing the preparation of macroscopically homogeneous metal/OML/semiconductor junctions.

2. Experimental section 2.1 Samples preparation Samples are prepared under ultra-high vacuum conditions (UHV). A 1.5µm thick Si n-doped (4 × 1016cm-3) GaAs buffer layer is first grown in an independent Molecular Beam Epitaxy

chamber on a n+-GaAs(001) substrate. The GaAs(001) surface is then capped with a thick amorphous As layer and transferred in the air to the UHV preparation setup where the As protective layer is thermally removed, leading to the formation of a clean As(2x4)reconstructed GaAs(001) surface. This surface is then exposed to 10000 Langmuirs of 1hexadecanethiols (C16H33-SH, purchased from the Sigma-Aldrich ® company, purity>95%, further purified by freezing/pumping cycles) introduced under UHV through a leak valve. The sample is then annealed at 100°C in order to remove physisorbed molecules. Careful control of growth quality is performed through in situ Ultra-violet Photoemission Spectroscopy and X-ray Photoemission Spectroscopy measurements attached to the preparation chamber. The obtained monolayer corresponds to one grafted surface site out of two. According to steric hindrance

14

, this is the highest possible coverage rate on GaAs(001). The thermal stability

of this grafted surface has been checked, and no change in the C1s core level has been detected upon annealing up to 200° in agreement with reference

15

. The BLAG metal

deposition is performed though a shadow mask with 250 µm and 500 µm diameter holes which provide contacting dots. Both sample and mask are cooled down to 25K using a closecycled helium compressor. Xenon is deposited through a leak valve at a pressure of 10-6 mbar. The deposited amount (25 L) corresponds to a 3nm-thick Xe ice layer. We checked that Xe deposition does not impact the molecular monolayer in agreement with Maitani et al 11. Then gold is deposited at a rate of 0.3 nm/min, at a 10-9 mbar pressure. Depending on the measurements to be performed the metal thickness is between 2.5 and 7nm. Finally, the cooler is shut down, so that temperature raises slowly to room temperature, at a rate of 1K/min. The pressure reaches 10-6 mbar during the Xe desorption stage. These samples are defined as Au/C16MT/GaAs(001)-BLAG samples or also BLAG-samples in this text. They are air transferred to the different setups. For comparison the same samples are prepared with a usual RT-metal deposition at the same rate and through similar masks, they are called RT-samples in the text. 3 ACS Paragon Plus Environment


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2.2 STM/BEEM measurements All STM/BEEM experiments were performed in the constant-current mode of operation, with tunneling currents intensities of typically 5nA, in order to avoid tip-surface interactions and surface degradations. Details on the experimental set-up can be found elsewhere

16

. For a

detailed review on the BEEM technique, the reader might refer to the excellent recent work published by L. Douglas Bell17. Briefly, the BEEM technique is derived from the STM one. In the case of a Schottky contact (Au/GaAs), hot electrons are injected from the STM tip to the metallic surface and a small fraction of them (only a few per cent) reaches the Au/GaAs interface with no energy loss. If their energy (defined by the applied bias voltage between the tip and the sample surface) is high enough to overcome the interface potential barrier, electrons enter into the conduction band of the semiconductor and are collected at the back of the sample. Local BEEM measurements have been performed with success in the case of these GaAs based tunnel junctions and allowed to characterize the homogeneity of the obtained top-contact/molecular monolayer interface. BEEM measurements probe the heterostructure at equilibrium in contrast to what is performed with conventional transport measurements, it allows to reveal and position in energy, channels of elastic transport of electrons. Our previous studies have shown capabilities of BEEM to probe such heterostructures by determining local barrier heights and revealing an elastic transport channel through LUMO 13 or detecting pinholes at large scale.

2.3 Transport measurements Current-voltage, I(V), measurements have been performed at a 10-4 mbar pressure using a Janis cryogenic set-up. A tungsten tip has been used to contact the gold dots. Measurements were performed with a Keithley 6482 pico-ammeter. Differential capacitance measurements have been performed using a HP 4284A precision LCR-meter that allows measurements between -40V and 40V with an AC- voltage of 20mV. Here measurements have been performed with a bias voltage from -3V to 0V and an AC frequency of 1MHz.

2.4 Photoemission : band alignment determination In order to determine the band alignment of the whole hetero-structure, photoemission measurements have been performed at SOLEIL on ANTARES beamline. Taking advantage of the photoemission set-up facility in scanning samples under the beam, measurements were performed on several gold dots obtained with BLAG method. Fermi level position was determined on the gold dots and taken as reference for the band alignment displayed in Fig2. . 4 ACS Paragon Plus Environment

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As3d-core level features were fitted using symmetric Voigt function and Shirley type background. The position of the As3d5/2 component corresponding to As species of the substrate is used to deduce the position of the valence band maximum (VBM). Previous laboratory studies give As3d5/2 at 40.50eV from the VBM, this is corroborated by two studies both performed on decapped MBE grown GaAs(100) surfaces, which gives respectively 21.85±0.05 eV for the separation between bulk Ga3d5/2 and As3d5/2

18

and 18.64±0.05 eV

for the distance between Ga3d5/2 and VBM19 .

3. Results and discussion 3.1 STM images of the BLAG-deposited gold surface The BLAG metal growth is known to form clusters whose size is dependent on Xe layer thickness varying from 1nm for 18 Langmuir Xe deposition to 10 nm for 300 Langmuir Xe deposition

20

. 25 Langmuir deposition corresponds to a 3nm Xe ice layer and should fully

protect the hexadecanethiol molecular layer. Figures 1(a) and (c) display 50×50nm² STM images of the gold surface topography after 5nm metal deposition at room temperature (RT) and 7nm metal deposition under BLAG conditions. Au islands with typical lateral size of 10 nm are observed on both samples. The resulting peak-to peak roughness is different from one sample to the other. Higher corrugation of 2 nm is observed on BLAG samples compared to RT samples (1nm), for both samples the gold layer is continuous. Considering these observations, we concluded that it was possible to reduce to 2.5 nm the metal thickness and still have a continuous covering. We used this ultimate thickness in the case of samples for photoemission measurements as will be discussed in the photoemission section. 3.2 BEEM measurements Simultaneously, recorded BEEM current images (noted IBEEM in the following) show completely different features (Figures 1(b) and (d)). Contrary to RT-samples where BEEM current images present large contrasts with well-defined low IBEEM values and high IBEEM values areas, for BLAG- samples, homogeneous low BEEM current values are obtained. This is a clear indication that no pinholes are created through BLAG metal deposition, which is an important and new result. In order to analyze BEEM features quantitatively, we report in Figure 1 (e) and (f) local dependence of the normalized BEEM current versus hot-electron injection energy eUgap. Here Ugap is the applied bias voltage between the grounded sample and the tip. Several local spectroscopy curves were averaged in order to improve the signal to noise ratio. A reference BEEM spectroscopy curve obtained on a 5nm Au/GaAs(001) Schottky contact has



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been displayed on Figure 1(e). As reported previously 13 the high IBEEM areas seen in RT-samples correspond clearly to short-circuited areas due to gold diffusion through the OML as this reference perfectly matches the corresponding BEEM spectroscopy curve. No such high IBEEM areas are seen in the BLAG-samples. Contrary to RT-samples, BLAG- samples show a unique kind of spectroscopy curves, similar to the one obtained in the low IBEEM areas for RT-samples. This is interpreted as a tunneling regime through the molecular monolayer

21

? This regime is

homogeneously present when the contact is grown with BLAG method. As already performed in our previous studies13, a fit of the spectroscopy curve with the commonly used Ludeke-Prietsch 5/2 power law 22:

‫ܫ‬஻ாாெ ହ ହ = ܽ଴ + ܽଵ (‫ ܧ‬− ݁ߔଵ ) ൗଶ + ܽଶ (‫ ܧ‬− ݁ߔଶ ) ൗଶ ‫்ܫ‬

(1)

is displayed in Fig 1(f) for the unique kind of spectroscopy curves obtained in the case of BLAGsamples. It shows two conduction channels, one at Φ1= 0.97eV, the other at Φ2 = 1.34 eV from the Fermi level (see Figure 2). These two channels are attributed respectively to the injection of electrons at the bottom of GaAs conduction band and in the lowest unoccupied molecular orbitals (LUMO). Fluctuations of these two values are obtained when taken on several contacts (dots) and at different but centered parts of a given dot. Both values are mean values and the fluctuations are rather large with a dispersion of 0.2 eV. Besides, the ratio of the spectral weight ܽଵ of the first channel to the one obtained on Au/GaAs Schottky contact grown in the same way is equal to 3.0× 10-2. This value is homogeneous all along the junction and characterizes the transparency of the molecular tunnel junction. It is a confirmation of the presence of a unique tunneling regime through the whole contact in the BLAG samples. 3.3 Transport measurements Concerning BLAG-samples, Figure 3 shows the average experimental J(V) curve obtained at RT on 13 different dots of 500µm-diameter (we have measured similar ones on 40 different dots). J(V) curves obtained on Schottky junction (Au-GaAs) and on RT-molecular junction are displayed for comparison. The current density at 0.25 V applied voltage is 7. 10-4A/m2 for BLAG-samples, the standard deviation of the experimental values over the dots at this applied voltage is 1 10-4A /m2. All the dots show a current nearly two orders of magnitude lower than in the case of Schottky junction and RT-samples ones (current density of several 10-2 A/m2 at 0.25V applied voltage). On average all along the I(V) curve, the relative standard deviation is 13%, the lowest values being obtained at high direct applied voltage and the highest at reverse applied voltage. In this sense all the measured dots behave in a


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reproducible way. To estimate the band-bending ΦBB (see Figure 2) encountered by the

electron through the depletion layer of the semiconductor we have used capacitance-voltage measurements. The estimation of this value is robust as it is not sensitive to interface inhomogeneity and to any hypothesis on the transport mechanism as needed with J(V) measurements. At 1 MHz, the value of the band bending can be directly detected. Namely, the charge capacity of a narrow insulating layer (d=1.5nm) has a high value compared to the depletion layer charge capacity (120nm). If the model of series capacities is considered, as a result the measured capacity is mostly the charge capacity of the semiconductor depletion layer. These measurements are reported on figure 4, where 1/C2 is plotted with respect to the applied bias voltage. Considering the observed linear behavior, the Mott-Schottky equation is used and one can derive the flat band potential as the intercept of this straight line with the applied bias voltage axis. The flat band potential is a direct estimation of the band bending in the semiconductor region, ΦBB. Not taking into account the molecular insulating layer capacitance leads to an over estimation of the band bending. In our case, taking d=1.5 nm for the insulating layer width (molecular monolayer) and a relative permittivity ε=3 23 , ΦBB is over estimated by 5%. In the case of BLAG-samples, ΦBB is then equal to 0.95 eV taking into account this over estimation. The barrier value is then estimated ஼௏ ߔௌ஻ =1.03 eV, taking into account the difference between the bottom of the conduction

band and the Fermi level (see Figure 2) which is equal to 0.06eV for Si n-doped (4 × 1016cm-3)

GaAs(001) samples and the temperature term which is 0.02 eV. The obtained values for the BLAG- samples are displayed in Table I, with a comparison to the value obtained on BLAGSchottky barrier samples. 3.4 Photoemission measurements Photoemission experiments have been performed on BLAG-samples where gold dots are 2.5 nm thick in order to allow core-level measurements of the substrate through the molecular and metal layer. The position of the Fermi level in the gap of GaAs, and hence the barrier ௉ு ߔௌ஻ has been determined using the As 3d5/2 core level component attributed to the

substrate. This is the main purpose of this measurement and no additional information to the already performed studies 14-15-21-24-25 on this system concerning its chemistry has been obtained. Figure 5 shows As3d core level on the whole heterostructure, namely after gold deposition (on the dots) measured with a photon energy of 100eV. At this energy the escape depth of the photoelectrons is around 0.8nm, the substrate component still appears due to the corrugation of the 2.5 nm metal contact. Interface components appear shifted toward 7 ACS Paragon Plus Environment


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higher binding energy with respect to bulk component, one by 0.7 eV the other by 1.4 eV, consistent with previous studies of this interface

24-25.

These components have been

respectively related to As° and As-S in these papers although following grafting model proposed by Dubowski et al

14

two interface As species could be considered one for As

species connected to S atoms and the other remaining unconnected. Substrate and interface components are damped by the metal thickness as shown in the inset of figure 5, where As3d5/2 core level is compared when measured on the dots and off the dots. A component due to oxidized As, of comparable intensity through the whole sample, namely on and off the gold dots, is present. It corresponds to the oxidation of elemental arsenic localized at the outermost surface. This should not affect the interface. With a careful fit of As3d photoemission spectra obtained at 100eV compared to the one measured with 365 eV photon energies (not shown), the position of the component attributed to the substrate is clearly identified. The position of the substrate component is very weakly dependent on the fit, a possible additional component at lower binding energy has been also considered with no consequence on this position. The distance between As3d5/2 and the Fermi level position (see Figure 2) is measured here at ΦAs3d = 40.97eV. As3d5/2 being at 40.50eV from the valence band maximum (VBM), with a gap of 1.42eV for GaAs at room temperature, the ௉ு =0.95eV±0.06eV. electron barrier value is ߔௌ஻

3.5 Discussion We want to emphasize that we have realized here a top-contact which is reliable and gives access to band alignment values which are in agreement at the nano and macro scale. The reliability is first given by the fact that J(V) transport measurements are reproducible from one dot to the other. Clearly J(V) for BLAG-junctions are reduced in current intensity compare to Schottky contacts and RT-junctions. J(V) are similar to the one obtained by Nesher et al

21

with Hg electrodes with the same molecules on GaAs both at reverse and

forward bias regimes. Here we organize the discussion with respect to the two channels whose measurements are made available through BEEM spectroscopic measurements. BEEM measurement give access to a first channel of injection which is interpreted as injection at the bottom of GaAs conduction band (CBM) through a tunnel barrier. The ஻ாாெ distance between CBM and the Fermi level, namely the barrier, is found to be Φ1 = ߔௌ஻ =

0.97±0.10eV when taking into account the dispersion of the values (on several dots and on

different places on the dots). In photoemission, the corrugation of the metal layer made 8 ACS Paragon Plus Environment

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possible the measurement of the substrate As3d core level at photon energy as low as 100eV. Our measurements are then sensitive to the position of the Fermi level right at the interface between molecules and GaAs. The barrier value ࢶࡼࡴ ࡿ࡮ =0.95 ±0.06eV is in full agreement with BEEM measurements. C(V) measurements provides us with a direct value of ஼௏ =1.03 ± 0.01eV. the band bending and then to ߔௌ஻

Table 1: Comparative barrier heights for Au/GaAs(001) and Au/C16MT/GaAs(001) prepared with BLAG method, obtained by BEEM, C-V measurements and photoemission Samples

ࢶ࡮ࡱࡱࡹ (eV) ࡿ࡮

ࢶ࡯ࢂ ࡿ࡮ (eV)

ࢶࡼࡴ ࡿ࡮ (eV)

Au/GaAs(001)-BLAG Au/C16MT/GaAs(001)-BLAG

0.87 ± 0.02 0.97 ± 0.10

0.88 ± 0.01 1.03 ± 0.01

0.95±0.06

UV photoemission measurements on grafted surfaces (that will be detailed elsewhere) gave a value for the barrier prior to the gold contact of 0.74±0.1eV. Compared to the GaAs (100)

bare surface, no change of the band bending is seen for most grafted samples except sometimes a slight decrease. It looks like if no changes in the interface states responsible for the positioning of the Fermi level in the gap of GaAs occur from bare GaAs to grafted GaAs. At the same time, a modification of the work function of GaAs(100) after grafting was measured, showing a clear dipole effect at the interface, whose sign is consistent with the orientation of the molecular dipole with the S atom on the GaAs side. The effect of contacting molecules with gold leads to the most important change, an increase of the barrier height from 0.74 eV to 0.95eV. This is compatible with a metal being more electronegative than the grafted GaAs. BEEM measurements give access to a second channel of injection of hot electrons which is interpreted as the lowest energy of unoccupied molecules orbitals. Following reference and

27,

26

the unoccupied molecules orbitals have to be seen as unoccupied states extending

from the interface with the semiconductor along the molecular monolayer to the interface with the metal. In the case of a single interface between metal and molecules (polyethylene deposited on Cu), Kiguchi et al 28 have shown such states using NEXAFS. The pre-edge signal at C-K absorption edge shows components attributed to states induced by the proximity of organics and metal which are in this case in weak chemical interaction. BEEM measurements concern the whole metal-molecules-semiconductor hetero-structure, namely the induced states at both GaAs and Au interfaces with organic molecules. In particular, they reveal the lowest energy part of the density of such states which for these 9 ACS Paragon Plus Environment


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BLAG samples sets at 1.34 eV with respect to the Fermi level. The position of this channel appears to depend on the nature of the interface between Au and the molecular monolayer. Indeed, for RT-samples, in the areas where tunnel regime is observed, this channel is found at around 1.2 eV. This difference could be related to the difference of proximity between molecules and metal in these differently prepared contacts. The height difference between these two channels detected by BEEM is a measure of the tunnel barrier (ΦT=Φ2-Φ1=0.40eV) encountered by the electrons tunneling from the metal to the semiconductor conduction band. This tunnel barrier is extremely difficult to determine using macroscopic transport measurements. In the domain of molecular electronics the effective role played by the molecular orbitals in the transport processes has been and is still the subject of many studies. BEEM measurements give a signature of a second channel that is interpreted as injection into the LUMO states. Finally we want to emphasize that BEEM measurements performed for the whole system at equilibrium reveal existing channels for hot-electron elastic transport, the predominance of these channels in the case of diffusive electronic transport with low applied voltage cannot be deduced.

Conclusion

In this paper, working with gold, a metal known to induce the most wetting contacts on molecules, we obtained reliable molecular junctions using BLAG method. BLAG Au/ hexadecanethiol/n-GaAs(100) hetero-structure shows a tunnel regime through organic monolayer with a full spatial uniformity in hot electron transport measurements at the nanoscale. Macroscopic scale measurements point to a tunnel regime through the heterostructure, with reproducibility from one dot to the other. Uniformity and reproducibility make photoemission measurements on the whole junction relevant. Energy band alignment of the complete hetero-structure has been obtained by photoemission. Using BEEM, the position of the lowest unoccupied molecular orbital has been determined for the first time for the whole hetero-structure at equilibrium. This level is related to the tunnel barrier height whose determination using transport measurements is tedious and inaccurate. These unique results show a coherent band alignment at the nanoscale and at the macroscopic scale. It was possible to show a slight evolution of the semiconductor band bending after metal deposition on the molecular layer and a change in the band bending of GaAs from the Schottky junctions to the metal-molecules-semiconductor junction. Observed fluctuations at 10 ACS Paragon Plus Environment

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the nanoscale may be attributed to the nature of the obtained contact, as BLAG induces softlanding deposition of metal clusters. Through this study, BLAG method appears as a general method that should work for contacting organic monolayers with most metals.

Acknowledgment: Support from Europe (FEDER) is expressly acknowledged. This study has benefitted from valuable scientific discussions with Bruno Lépine. References:

(1) de Boer, B.; Hadipour, A.; Mandoc, M. M.; van Woudenbergh, T.; Blom, P. W. M. Tuning of Metal Work Functions with Self-Assembled Monolayers. Adv. Mater. 2005, 17, 621-625. (2) Vilan, A.; Shanzer A.; Cahen, D. Molecular Control over Au/GaAs Diodes. Nature, 2000, 404, 166-168. (3) Petta, J.R.; Slater, S. K.; Ralph, D. C. Spin-dependent Transport in Molecular Tunnel Junctions. Phys. Rev. Lett. , 2004, 93, 136601. (4) Vilan A.; Visoly-Fisher, I. Molecular Functionalization of Surfaces for Device Applications. Special Issue of Journal of Physics: Condensed Matter, 2016, 28. (5) Haick, H.; Cahen, D. Review Making contact: Connecting Molecules Electrically to the Macroscopic World. Prog. Surf. Sci. 2008, 83, 217–261. (6) Popoff, R. T. W.; Kavanagh, K. L.; Yu, H-Z. Preparation of Ideal Molecular Junctions: Depositing Non-invasive Gold Contacts on Molecularly Modified Silicon. Nanoscale, 2011, 3, 1434-1445. (7) Maitani, M. M.; Allara, D. L. Issues and Challenges in Vapor-Deposited Top Metal Contacts for Molecule-Based Electronic Devices. Top. Curr. Chem. , 2011, 312, 239−273. (8) Lovrincic, R.; Kraynis, O.; Har-Lavan, R.; Haj-Yahya, A-E. ; Li, W.; Vilan, A.; Cahen, D. A New Route to Nondestructive Top-Contacts for Molecular Electronics on Si: Pb Evaporated on Organic Monolayers. J. Phys. Chem. Lett. 2013, 4, 426−430. (9)Yu , X.; Lovrincic, R.; Kraynis , O.; Man , G.; Ely , T.; Zohar , A.; Toledano, T.; Cahen , D.; Vilan, A. Fabrication of Reproducible, Integration-Compatible Hybrid Molecular/Si Electronics. Small 2014, /smll.201400484. (10) Galbiati, M.; Tatay, S.; Delprat, S.; Le Khanh, H.; Servet, B.; Deranlot C.; , Collin, S.; Seneor, P.; Mattana, R.; Petroff, F. Is Spin Transport through Molecules Really Occurring in Organic Spin Valves? A Combined Magnetoresistance and Inelastic Electron Tunnelling Spectroscopy Study. Appl. Phys. Lett. 2015, 106, 082408. (11) Maitani, M. M.; Allara, D. L.; Ohlberg, D. A. A.; Li,; Stanley Williams, Z. R.; Stewart, D. R. High Integrity Metal/Organic Device Interfaces via Low Temperature Buffer Layer Assisted Metal Atom Nucleation. Appl. Phys. Lett. 2010, 96, 173109. 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(12) Wadill, G. D.; Vitomirov, I. M.; Aldao, C. M.; Weaver, J. H. Cluster Deposition on GaAs(110): Formation of Abrupt, Defect-Free Interfaces. Phys. Rev. Lett. , 1989, 62, 15681571. (13) Junay, A.; Guézo, S.; Turban, P.; Delhaye, G.; Lépine, B.; Tricot, S.; Ababou-Girard, S.; Solal, F. Spatially-Resolved Band Alignments at Au-Hexadecanethiols Monolayer-GaAs(001) Interfaces by Ballistic Electron Emission Microscopy. J. Appl. Phys., Am. Inst. Phys., 2015, 118, 085310. (14) Voznyy, O.; Dubowski, J. J. Structure of Thiol Self-Assembled Monolayers Commensurate with the GaAs (001) Surface. Langmuir, 2008, 24, 13299-13305. (15) Huang, T.P.; Lin, T.H.; Teng, T.F.; Lai, Y.H.; Hung, W.H. Adsorption and Thermal Reaction of Short-Chain Alkanethiols on GaAs(1 0 0). Surf. Sci. , 2009, 603, 1244–1252. (16) Guézo, S.; Turban, P.; Lallaizon, C.; Le Breton, J. C.; Schieffer, P.; Lépine, B.; Jézéquel, G. Spatially Resolved Electronic Properties of MgO/GaAs (001) Tunnel Barrier Studied by Ballistic Electron Emission Microscopy, Appl. Phys. Lett., 2008, 93,172116. (17) Douglas Bell, L. Ballistic Electron Emission Microscopy and Spectroscopy: Recent Results and Related Techniques, J. Vac. Sci. Technol., B , 2016, 34, 040801. (18) Le Lay, G.: Mao, D.; Kahn, A.; Hwu, Y.; Margaritondo, G. High-Resolution Synchrotron-Radiation Core-level Spectroscopy of Decapped GaAs(100) Surfaces. Phys. Rev. B , 1991, 43, 14301-14304. (19) Lu, Y.; Le Breton, J. C.; Turban, P.; Lépine, B.; Schieffer, P.; Jézéquel, G. Measurement of the Valence-Band Offset at the Epitaxial MgO-GaAs(001) Heterojunction by X-ray Photoelectron Spectroscopy. Appl. Phys. Lett., 2006, 88, 042108. (20) Haley, C.; Weaver, J.H. Buffer-Layer-Assisted Nanostructure growth via TwoDimensional Cluster–Cluster Aggregation. Surf. Sci., 2002, 518, 243–250. (21) Nesher, G.; Vilan, A.; Cohen, H.; Cahen, D.; Amy, F.; Chan, C.; Hwang, J.; Kahn, A. Energy Level and Band Alignment for GaAs/Alkylthiol Monolayer/Hg Junctions from Electrical Transport and Photoemission Experiments. J. Phys. Chem. B, 2006, 110, 14363– 14371. (22) Prietsch, M. Ballistic-Electron Emission Microscopy (BEEM): Studies of MetalSemiconductor Interfaces with Nanometer Resolution. Phys. Rep., 1995, 253, 163 – 233. (23) Faber, E. J.; de Smet, L. C. P. M.; Olthuis, W.; Zuilhof, H.; Sudholter, E. J. R.; Bergveld, P.; Van den Berg. A. Si-C Linked Organic Monolayers on Crystalline Silicon Surfaces as Alternative Gate Insulators. Chem. Phys. Chem. 2005, 6, 2153-2166.


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(24) McGuiness, C. L.; Diehl, G. A.; Blasini, D.;Smilgies, D. M.; Zhu,M.; Samarth,S.; Weidner, T.; Ballav, N.; Zharnikov, M.; Allara, D. L. Molecular Self-Assembly at Bare Semiconductor Surfaces: Cooperative Substrate−Molecule Effects in Octadecanethiolate Monolayer Assemblies on GaAs(111), (110), and (100). ACS Nano, 2010, 6, 3447-3465. (25) Marshall, G. M.; Lopinski, G. P.; Bensebaa, F.; Dubowski, J. J.; Surface Dipole Layer Potential Induced IR Absorption Enhancement in n-Alkanethiol SAMs on GaAs(001). Langmuir, 2009, 25, 13561-13568. (26) Yaffe, O.; Scheres, O.; Puniredd, S. R. ; Stein, N.; Billier, A.; Lavain, R. H.; Shaisman, H.; Zuilhof, H.; Haick, H.; Cahen, D.; Vilan. A. Molecular Electronics at Metal/Semiconductor Junctions. Si Inversion by Sub-Nanometer Molecular Films. Nano Lett. 2009, 9, 2390-2394. (27) Segev, L.; Salomon, A.; Natan, A.; Cahen, D.; Kronik, L.; Amy, F.; Chan, C. K.; Kahn, A. Electronic Structure of Si (111)-bound Alkyl Monolayers: Theory and Experiment. Phys. Rev. B, 2006, 74,165323. (28) Kiguchi, M.; Arita, R.; Yoshikawa, G.; Tanida, Y.; Ikeda, S.; Entani, S.; Nakai, I.; Kondoh, H.; Ohta, T.; Saiki,K.; Aoki, H. Metal-Induced Gap States in Epitaxial OrganicInsulator/Metal Interfaces. Phys. Rev. B, 2005, 72, 075446.



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Figure 1: 50×50nm² STM images of gold overlayer surfaces ((a) 5 nm-thick gold deposited at room temperature and (c) 7 nm-thick gold deposited by BLAG method). Corresponding color scale are 0 to 1.1 nm and 0 to 1.7 nm. Simultaneously recorded BEEM images (grid of 80×80 points) at (b) 1.80V (color scale: 0 to 0.16 nA) and (d) 1.70V (color scale: 0 to 0.10 nA). (e) BEEM spectra recorded on “bright” areas (orange squares), on “dark” areas (black squares) and on a Schottky contact (dashed red line) for Au(5nm)/C16MT/GaAs(001)-RT samples. (f) Similar BEEM spectra recorded for Au(7nm)/C16MT/GaAs(001)-BLAG samples, inset : the same spectra scaled to show properly the two component fit using Ludeke-Prietsch 5/2 power law (13)

Figure 2: Schematic of band alignment for the Au/C16MT/n-GaAs(001) hetero-structure. In blue, quantities measured by BEEM-spectroscopy, in purple, quantities measured by photoemission measurements and in green quantities derived from C(V) transport measurements. In black values from literature or from our previous studies.

Figure 3: Experimental J-V curves for Au/C16MT/GaAs(001)-RT samples (filled squares, average of 11 different measures), Au/C16MT/GaAs(001)-BLAG samples (empty squares, average of 12 different measures) and Au/GaAs(001) reference (red solid line, average of 5 different measurements) obtained for T=293K. (black broken lines are guide for eyes at 0.25 bias applied voltage as discussed in the text)

Figure 4: Comparison of C(V) measurements on Au/hexadecanethiol-GaAs (001)-BLAG and Au/GaAs(001)-BLAG. The diameter of the dots on which measurements have been performed is 300µm.

Figure 5: As 3d core level at hν=100 eV for Au(2.5nm)/C16MT/GaAs(001)-BLAG. Experiment is represented by blue dots, the red line is the result of the fit. The component at lower kinetic energy is an oxide component. Interfaces components appear which are consistent with the existence of molecule-As bond. The Fermi level position has been determined on the Au dots using the same photon energy, and even more precisely with no change in monochromator setting between As 3d and Fermi level measurements. The Fermi level is at 92.15 eV, the As 3d5/2 is at 51.18 eV. The inset contains the same spectrum compared to the one obtained for C16MT/GaAs(001) namely off dots (upper spectrum), it reveals the damping of all components except the oxide one.


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Figure 1: 50×50nm² STM images of gold overlayer surfaces ((a) 5 nm-thick gold deposited at room temperature and (c) 7 nm-thick gold deposited by BLAG method). Corresponding color scale are 0 to 1.1 nm and 0 to 1.7 nm. Simultaneously recorded BEEM images (grid of 80×80 points) at (b) 1.80V (color scale: 0 to 0.16 nA) and (d) 1.70V (color scale: 0 to 0.10 nA). (e) BEEM spectra recorded on “bright” areas (orange squares), on “dark” areas (black squares) and on a Schottky contact (dashed red line) for Au(5nm)/C16MT/GaAs(001)-RT samples. (f) Similar BEEM spectra recorded for Au(7nm)/C16MT/GaAs(001)BLAG samples, inset : the same spectra scaled to show properly the two component fit using LudekePrietsch 5/2 power law (13) 3.2 BEEM Measurements



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Figure 2: Schematic of band alignment for the Au/C16MT/n-GaAs(001) hetero-structure. In blue, quantities measured by BEEM-spectroscopy, in purple, quantities measured by photoemission measurements and in green quantities derived from C(V) transport measurements. In black values from literature or from our previous studies. it shows two conduction channe


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Figure 3: Experimental J-V curves for Au/C16MT/GaAs(001)-RT samples (filled squares, average of 11 different measures), Au/C16MT/GaAs(001)-BLAG samples (empty squares, average of 12 different measures) and Au/GaAs(001) reference (red solid line, average of 5 different measurements) obtained for T=293K. (black broken lines are guide for eyes at 0.25 bias applied voltage as discussed in the text) 3.3 Transport Measurements



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Figure 4: Comparison of C(V) measurements on Au/hexadecanethiol-GaAs (001)-BLAG and Au/GaAs(001)BLAG. The diameter of the dots on which measurements have been performed is 300µm. The measurements are reported 160x112mm (220 x 220 DPI)


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Figure 5: As 3d core level at hν=100 eV for Au(2.5nm)/C16MT/GaAs(001)-BLAG. Experiment is represented by blue dots, the red line is the result of the fit. The component at lower kinetic energy is an oxide component. Interfaces components appear which are consistent with the existence of molecule-As bond. The Fermi level position has been determined on the Au dots using the same photon energy, and even more precisely with no change in monochromator setting between As 3d and Fermi level measurements. The Fermi level is at 92.15 eV, the As 3d5/2 is at 51.18 eV. The inset contains the same spectrum compared to the one obtained for C16MT/GaAs(001) namely off dots (upper spectrum), it reveals the damping of all components except the oxide one. 3.4 Photoemission Measurements 276x196mm (96 x 96 DPI)



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Table of Contents Graphic


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Effective Metal Top Contact on the Organic Layer via Buffer-Layer

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