NANO LETTERS
Molecular Electronics at Metal/ Semiconductor Junctions. Si Inversion by Sub-Nanometer Molecular Films
2009 Vol. 9, No. 6 2390-2394
Omer Yaffe,† Luc Scheres,‡ Sreenivasa Reddy Puniredd,§ Nir Stein,† Ariel Biller,† Rotem Har Lavan,† Hagay Shpaisman,† Han Zuilhof,‡ Hossam Haick,§ David Cahen,*,† and Ayelet Vilan*,† Department of Materials & Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel, Laboratory of Organic Chemistry, Wageningen UniVersity, Dreijenplein 8, 6703 HB Wageningen, The Netherlands, and Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, TechnionsIsrael Institute of Technology, Haifa 32000, Israel Received March 26, 2009; Revised Manuscript Received April 30, 2009
ABSTRACT Electronic transport across n-Si-alkyl monolayer/Hg junctions is, at reverse and low forward bias, independent of alkyl chain length from 18 down to 1 or 2 carbons! This and further recent results indicate that electron transport is minority, rather than majority carrier dominated, occurs via generation and recombination, rather than (the earlier assumed) thermionic emission, and, as such, is rather insensitive to interface properties. The (m)ethyl results show that binding organic molecules directly to semiconductors provides semiconductor/metal interface control options, not accessible otherwise.
Using a semiconductor, especially Si, instead of a metal, as one of the electrodes to contact the molecules not only adds another “knob” to turn, to study transport through molecules but also can make molecular electronic junctions more relevant for potential future electronics. Implicit in most of the increasing efforts in this direction is the assumption that the molecules influence, possibly even control, the junction charge transport characteristics. In this and forthcoming reports we examine and try to define when, and in how far, this assumption holds. Specifically we use the n-Si-alkyl chain/Hg junction, often used in laboratory tests, where Hg contacts a monolayer of CnH2n+1 alkyl chain molecules, bound by direct Si-C bonds to Si 〈111〉.1 Here we focus on how molecule-semiconductor interaction can dictate the mechanisms of charge transport through these junctions. Understanding the transport mechanism(s) across molecular junctions is critical for rational design of possible future molecule-based devices. Previously charge transport across junctions with n-Si was described as thermionic emission (TE) at low forward and reverse bias and tunneling at high forward bias.1 This model was extended further, to include also p-Si.2 We present and discuss here new experimental results that point to diffusion and recombination of minority †
Department of Materials & Interfaces, Weizmann Institute of Science. Laboratory of Organic Chemistry, Wageningen University. § Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, TechnionsIsrael Institute of Technology. ‡
10.1021/nl900953z CCC: $40.75 Published on Web 05/13/2009
2009 American Chemical Society
carriers, rather than majority carrier TE, as the dominant transport mechanisms at low forward and reverse bias in n-Sialkyl chain/Hg junctions. Minority carrier transport also explains earlier experimental results that are not consistent with TE. In Figure 1 we show room temperature current densityvoltage (J-V) characteristics of n-Si-alkyl chain/Hg junctions, made with 1-10 Ω·cm Si〈111〉. The length of the alkyl chain CnH2n+1 varies from n ) 18 down to 1, which means that the monolayer thickness decreases from 2.2 down to 0.2 nm. “Short” monolayers (n e 4) were prepared and characterized in the Technion3,4 and “long” monolayers (n g 12) at Wageningen University and the Weizmann Institute,5 Monolayer preparation and characterization were as reported earlier,3-5 including contact angle, ellipsometry, XPS, and FTIR measurements. However, as reported in detail earlier, the strongest evidence for monolayer quality is the excellent low forward bias overlap of the measured J-V curves,6 J-V measurements were performed using n-SiCnH2n+1/Hg junctions, formed by placing a high-purity Hg drop on the monolayer, using a controlled growth hanging mercury drop electrode apparatus (Polish Academy of Sciences, Poland). The backside of the samples was contacted by applying In-Ga eutectic, after scratching the surface with a diamond knife. Room temperature measurements were carried out at 10% relative humidity in air and temperature-
Figure 1. ln(|J|)-V curves of n-Si-CnH2n+1/Hg (n ) 1, 2, 4, 12, 16, and 18). Bias is applied to the Hg; Si is grounded. Results are logarithmic averages of at least seven different junctions with a scan rate of 20 mV/s. The error bars represent standard deviations, which are typically less than 5% of the measured current. The insets give “artist” views of the junctions for “short” and “long” monolayers (not to scale!).
dependent measurements were performed as described in ref 7. The Hg/monolayer contact area was determined using an optical microscope and was typically 0.60(5) mm in diameter. J-V measurements were done with a Keithley 6430 subfemtoamp current/voltage source measure unit. Several scans were performed for each junction with a scan rate of 20 mV/ s. At least seven junctions were made on each sample, and the results represent the average of most of the measurements ( 2ΨB, then the surface is in deep inversion.20 Figure 2 shows the three basic charge transport processes under forward bias: (1) majority carrier (electron) TE, this 2391
Figure 3. Capacitance-voltage measurements on n-Si-CnH2n+1/ Hg junctions (n ) 1, 2, 4, 12, 16, and 18), plotted as 1/C2 vs bias voltage. Dashed lines are extrapolation to (1/C2) ) 0, where Ψbi is extracted, according to the Mott-Schottky relation (see text). The slope is not identical because of small variations in doping density between the commercial wafers.
is followed, and attenuated, by tunneling through the barrier (not shown), introduced by the molecular monolayer (only the tail of the Fermi-Dirac distribution above Ψbi is available for transport), (2) recombination of electrons and holes in the depletion layer (space charge region, SCR), and (3) diffusion of minority carriers (holes) in the bulk. Processes 2 and 3 depend mainly on intrinsic semiconductor properties,21 and under specific conditions are not sensitive to the tunneling barrier, introduced by the insulator.19 Numerical calculations for charge transport through MIS tunnel diodes (Si-SiO2; SiO2 thickness 1.5-3 nm)19 showed that for a sufficiently large built-in potential, i.e., if the surface is in deep inversion, charge transport will be governed solely by hole injection and generation-recombination at reverse and low forward bias voltages. In case of very thin insulators, the junction is similar to an abrupt, one-sided p+-n junction. The flat band potential (Ψbi) can be extracted from reverse bias C-V measurements according to the Mott-Schottky relation22 C-2 )
(
)
k BT Ψbi 2 + -V 2 q q q0NDA
where C is the differential capacitance, q is the unit Coulombic charge, and 0 are the Si dielectric constant and vacuum permittivities, respectively, ND is the dopant density (cm-3), A is the junction area (cm-2), and Ψbi is the built-in potential in the Si. As shown in Figure 3, the experimentally extracted flat band potential (Ψbi) of all n-Si-alkyl chain/Hg junctions is 0.65 ( 0.02 eV and is in excellent agreement with previously published data.9,11 Since the calculated deep inversion potential (2ΨB) of the moderately doped n-Si used here (ND ∼ 1015 cm-3) is ∼0.6 eV,20 we conclude that the Si surface is indeed in deep inversion at equilibrium. The slope of the Mott-Schottky fits was used to extract the local doping density (ND) and was found to be (9 ( 3) × 1014 cm-3, within the nominal range (ND ) (5 to 50) × 1014 cm-3, for 1-10 Ω cm), confirming that the electrical contact area (A, used to extract ND) fits with the optically observed Hg contact area. 2392
Figure 4. Comparison of junctions with Hg and Au contacts to n-Si-C12H25. Au contacts were “ready-made”, using a “soft” deposition by a modified polymer-assisted lift-off (PALO) method.25,26 See Figure 1 for details on electrical measurements.
Additional support for minority instead of majority carrier transport at reverse and low forward bias comes from comparing the J-V characteristics for n-Si-C12 with Hg or Au as top metal contact (Figure 4). If an n-semiconductor is in deep inversion, further increasing the work-function of the top metal contact will not affect the current at reverse and low forward bias.23 To test if the n-Si is indeed in deep inversion, we used Au contacts, as the work function of Au is 0.2-0.5 eV higher than that of Hg.24 “Ready made” Au pads were used as contacts, adapting the method described in refs 25 and 26 to measure the J-V characteristics. The Au contact J-V curve overlaps that obtained with Hg despite the work function difference. Similar behavior was observed for other chain lengths (to be published). The reason is that with the semiconductor in deep inversion, the minority-carrier current is limited by processes in the Si (recombination or diffusion), independent of the band bending or the presence of an interfacial insulator.17-19,23 So far we focused on the semiconductor-limited J-V region, where neither the metal work function nor the insulator thickness affects the current. However, metal work function and insulator thickness are expected to dictate the bias, above which transport becomes limited by tunneling across the (molecular) insulator. Indeed, from Figure 1 we see that as the monolayer thickness decreases from C18 to C12, the currents deviate from the common curve at increasingly larger forward bias. For the short molecules this trend is overtaken by the effect of the system’s parasitic resistances, because at the transition voltage (0.6 V) the currents are already very high. For the same reason the curves for Hg and Au contacts (Figure 4) overlap over all of the measured range. Indeed, these overlaps disappear for longer chain molecules or smaller metal/molecular monolayer contact areas (to be published). The insulator in the MIS tunnel diode can have multiple effects:27 (i) suppress majority carrier TE by introducing an additional tunneling barrier; (ii) introduce an asymmetric tunneling barrier and, by that, favor minority carrierNano Lett., Vol. 9, No. 6, 2009
controlled charge transport; (iii) decrease the voltage drop on the semiconductor depletion layer; (iv) contribute an additional dipole and by that alter the semiconductor band bending; (v) reduce the surface recombination rates by passivating the surface states, especially those induced by direct metal-semiconductor interactions, i.e., act as buffer between the metal and the semiconductor.28 The results with methyl and ethyl (n ) 1, 2) films, which are ineffective as insulators under our experimental conditions, show that in the case of the Si-C interface, effects i, ii, and iii are negligible. The low and nearly bias-independent current under reverse bias indicates negligible surface recombination, supporting the importance of effect v. However, because Hg does not interact electronically with Si,29 no interface states are expected in Hg/Si junctions to begin with. Therefore, we cannot deduce from the Hg result only whether the organic insulating layer passivates metal-Si interactions in general. However, additional, preliminary results with “ready-made” Au contacts suggest that these molecules do not completely passivate such interactions. Therefore, we conclude that (iv), the change in electrostatic potential due to the dipole layer introduced by chemically binding a molecular monolayer (∼-0.5 eV for Si-CH330 and ∼-0.35 eV for Si-C2H531 compared to Si-H), suffices to increase the built-in potential (Ψbi) to the level where the n-Si is in deep inversion and by that to make any majority carrier charge transport negligible.32-34 The high reproducibility of Si-alkyl/Hg junctions with given alkyl monolayers as reported here, as well as elsewhere,1,2,9 is quite remarkable. For example, it was very difficult to explain why the reported large change in coverage (and, therefore, dipole density) does not affect the barrier for transport.9 Having an inverted Si surface answers this puzzle because both the J-V and the C-V “saturate” at some threshold value and are indifferent to further increases in work function difference.18,23,35 Thus, even though the interfacial dipole with a C2 layer is not necessarily the same as that with a C18 one, both are sufficiently high so that the transport and capacitance are practically identical. Apparently, most of the dipole effect is due to the Si-C bond formation, while the rest of the alkyl tail, and a possible very small number of Si-OH sites,10 as well as its packing details, make minor contributions, below the sensitivity of either the width of the space charge (C-V) or the semiconductor transport mechanism (J-V). The ability of such a subnanometer layer (as in C1-C2) to invert the Si is of great interest for photovoltaics.19,36,37 In principle, close to ideal MIS-like behavior would be possible if we could passivate the Si surface with a “Si oxide monolayer”. However, in practice this has not been possible until now, while our results clearly show that even mixed ethyl-/H-coverage or more uniform methyl-coverage can provide sufficient electrostatic change to passivate the Si and, in this case, to invert it, without the need for a (thicker) insulating layer. Minority carrier-controlled transport clarifies two previous experimental observations, which seemed inconsistent with Nano Lett., Vol. 9, No. 6, 2009
(majority carrier) TE. The first one is that the ∼0.85 eV (effective) TE barrier height, derived from either room temperature J-V or reverse bias C-V (φb ) Ψbi + (EC EF)) measurements,1,2,9,11 is almost twice the 0.45-0.47 eV activation energy, extracted from the temperature dependence (250-330 K) of current-voltage characteristics (I-V-T) for both “long” monolayers7 (using the same set of Si wafers as reported here) and “short” monolayers, as measured in the current study (see Supporting Information). I-V-T analysis allows experimental extraction of the charge transport activation energy regardless of any a priori assumed transport mechanism (via the Arrhenius relation).22 While the saturation current for TE is proportional to exp(-qφb/ kT), where q is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, and φb is the barrier height, the saturation current for recombination is proportional to exp(-Eg/2kT), where Eg is the semiconductor band gap.38 If we extract a “barrier height” from I-V-T data there is an implicit assumption that the charge transport is TE controlled. For n-Si-alkyl chain/Hg junctions, the experimentally extracted activation energy for both “short” and “long” monolayers is ∼0.5 eV, which is much closer to half the band gap of Si (0.51 eV20) than to the φb value, extracted from either room temperature J-V or C-V measurements (0.82-0.85 eV1,9,11). This indicates that recombination in the depletion region is the dominant charge transport mechanism. The second result, which can be explained by minority carrier-controlled charge transport, but not by TE, is the variation of I-V-T characteristic of an n-Si-alkyl chain/ Hg junction upon low-energy electron irradiation.7 Irradiation was found to considerably increase the number of electronic states in the monolayer (“dope” the monolayer), and this is expressed as a considerable net increase in current. Yet, I-V-T analysis showed that the transport activation energy also increases (instead of decreasing to explain the higher current).7 With the new understanding that junction transport before irradiation is minority carrier controlled, we suggest that irradiation changes the equilibrium electronic state of the junction from deep inversion to depletion. In such a case the charge transport mechanism changes from minority to majority carrier controlled, i.e., to TE, attenuated by tunneling. Although the activation energy for TE is larger than that for recombination, the net current still increases because of the much higher concentration of majority than minority carriers. Irradiation also decreased the ideality factor toward that for ideal TE ()1), from the originally higher value, which would fit recombination.39 We conclude that the well-studied n-Si-alkyl chain/Hg is in deep inversion at equilibrium (0 V) and at room temperature. Therefore, charge transport at reverse and low forward bias is controlled by minority carrier generation (at reverse bias) and by recombination and diffusion (at low forward bias) rather than by majority carrier thermionic emission. The indications for the minority carrier control are as follows: • The current is independent of molecular length, at reverse and low forward bias (see Figure 1). 2393
• The experimentally measured built-in potential (Ψbi) is larger than the minimal potential for deep inversion (2ΨB) (see Figure 3). • The current is independent of the work function of the metal electrode (which will hold for any metal with a work function, higher than that of Hg, see Figure 4). • The activation energy for charge transport of both “short” (see Supporting Information) and “long”7 monolayers is about half the Si band gap.
While interface energetics (built-in potential) can be extracted from C-V measurements, the shape of the J-V curves is independent of interface properties over most of the bias range considered here. Recognizing inversion is a crucial step toward understandingthebasicscienceofhybridmetal-molecules-semiconductor (MMS) devices and mapping which molecular aspects control device performance under different conditions. Realizing that transport is by minority carriers is directly relevant for solar-cell applications.36,37 Acknowledgment. We thank L. Kronik, L. Segev (WIS), A. Kahn (Princeton), J. Pelz (Ohio State), and R. Tung (CUNY) for fruitful discussions, the Israel Science Foundation, ISF (Jerusalem), through its Converging Technology and Centre of Excellence programs and the Minerva Foundation (Munich) for partial support, and 21 Ventures for a generous research grant. H.H. holds the Horev Chair for leader in Science and Technology and thanks the US-Israel BSF (Jerusalem) for financial support. H.S. holds an ISF converging technology predoctoral fellowship. H.Z. thanks NanoNed, funded by the Dutch Ministry of Economic Affairs (project WSC.6972) for financial support. D.C. holds the Rowland and Sylvia Schaefer chair of Energy Research. Supporting Information Available: Comparison between J-V curves of oxide-free ethyl monolayer, oxidized ethyl monolayer, and H terminated Si and I-V-T data, Arrhenius plot, and thermal activation energy as function of bias, on 〈111〉 n-Si-C2H5/Hg junction. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Salomon, A.; Bo¨cking, T.; Seitz, O.; Markus, T.; Amy, F.; Chan, C.; Zhao, W.; Cahen, D.; Kahn, A. Phys. ReV. Lett. 2005, 95, 266807. (2) Salomon, A.; Bo¨cking, T.; Seitz, O.; Markus, T.; Amy, F.; Chan, C.; Zhao, W.; Cahen, D.; Kahn, A. AdV. Mater. 2007, 19, 445–450. (3) Puniredd, S. R.; Assad, O.; Haick, H. J. Am. Chem. Soc. 2008, 130, 13727–13734. (4) Puniredd, S. R.; Assad, O.; Haick, H. J. Am. Chem. Soc. 2008, 130, 9184–9185. (5) Scheres, L.; Arafat, A.; Zuilhof, H. Langmuir 2007, 23, 8343–8346. (6) Seitz, O.; Bocking, T.; Salomon, A.; Gooding, J. J.; Cahen, D. Langmuir 2006, 22, 6915–6922. (7) Seitz, O.; Vilan, A.; Cohen, H.; Haeming, M.; Schoell, A.; Umbach, E.; Kahn, A.; Cahen, D. AdV. Funct. Mater. 2008, 18, 2102–2113. (8) Salomon, A.; Shpaisman, H.; Seitz, O.; Bo¨cking, T.; Cahen, D. J. Phys. Chem. C 2008, 112, 3969–3974.
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