4481
2007, 111, 4481-4483 Published on Web 03/06/2007
Strong Mechanical Stabilization and Electrical Passivation of Metal-Semiconductor Contacts by Self-Assembled Monolayer Simon Verleger, Natalie Rosenberg, Itai Lieberman, and Shachar Richter* School of Chemistry and Institute of Nanoscience & Nanotechnology, Tel AViV UniVersity, Tel AViV, 69978 Israel ReceiVed: January 7, 2007; In Final Form: February 20, 2007
Metal-semiconductor contacts play a crucial role in various applications. In the specific case of metaldichalcogenide systems, defect-free contacts are crucial for the operation of solar cells. We investigate the mechanical and electrical properties of some Hg-dichalcogenide contacts and show that introducing a single layer of organic compounds between the metal and the semiconductor dramatically improves the mechanical, electrical, and optoelectronic properties of the system. A model which explains this phenomenon is introduced.
Introduction Metal-semiconductor (MS) contacts are widely used in various micro and nanoelectronic applications.1 It is well-known that the mechanical and electronic properties of such contacts can influence or even determine the actual performance of the device.1 Although the theory and experimental study of such contacts were initiated more than a decade ago, actual tuning and control of the electronic and mechanical properties of MS contacts remain a challenge. In the past few years, it has been suggested that organic molecules spaced between the metal and the semiconductor might be used to tune the properties of the contact. Alkanethiol molecules (CH3(CH2)nSH) have been commonly used to form such metal-molecule-semiconductor (MMS) contacts as they form self-assembled monolayers (SAMs) on several types of metals and semiconductors.2-8 However, large variation has been found in the electrical properties of similar MMS contacts containing similar SAMs.4 In the case of solar cells, in which the quality of the MS contact dramatically affects the performance of the whole device,8 a defect-free surface of large area is needed. It is known that dichalcogenides, a family of layered transition metals whose structure results from stacking weakly bonded sheets of MX2 molecules (M ) metal, X ) chalcogenide molecule), are good candidates for such cells.9 Tungsten diselenide (WSe2), which belongs to this type of semiconductor compound, has attracted a great deal of interest as it is photoconductive, atomically flat, relatively inert and free of surface states.10 However, over large areas, defects, usually in the form of step edges, have been found to dramatically reduce the performance of such solar cells (Figure 1).10 Several attempts have been made to passivate these defects, mainly through deposition of a dielectric layer or by introducing a liquid layer on the semiconductor surface.10 This study investigates the mechanical and electrical properties of MMS contacts. We show that even single monolayers of simple organic compounds can passivate some of the defects in the semiconductor and eliminate chemical bonds between * To whom correspondence should be addressed. E-mail: srichter@ post.tau.ac.il.
10.1021/jp070119v CCC: $37.00
Figure 1. Suggested chemical and electrical interactions between Hg drop and WSe2 surface. The strong chemical interactions between the Hg and the Se atom polarize the topmost layer, resulting good Hg wetting. Step edges, which form electrical defects, dramatically affect the electrical properties of the macroscopic MS contact.
the metal and the semiconductor which could change the electrical properties of the contact. Using such a MMS system, we measure strong photoconductive properties, as expected from defect-free MMS contacts. Mercury was selected as the metal contact for this study since it has been proven to have atomically uniform surfaces, and to exhibit a strong affinity for thiol-based SAMs. Several types of alkanethiol and thiolated ferrocene-based molecules were examined. The almost-ideal metal uniformity enabled the alkane chains to assemble perpendicular to the Hg surface11 and parallel to one another. Figure 2 (top) shows an optical image of a Hg-WSe2 contact. Attaching a drop of Hg to the top of the WSe2 substrate resulted in an uncontrolled and deformed contact (Figure 2 top), with a contact angle of around 106°. In most of the experiments, the contact did not last for more than several minutes and the drop became completely separated from the capillary due to the large Hg/WSe2 attractive forces (Figure 1). Similar effects were found on other n-type layered compounds such as MoS2 and MoSe2. A dramatic change was observed when the contact was established after the Hg drop had been covered with a SAM (CH3(CH2)nSH, n ) 8-10, or CH3(CH2)9-Fc-(CH2)9SH, Fc ) © 2007 American Chemical Society
4482 J. Phys. Chem. C, Vol. 111, No. 12, 2007
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Figure 3. J/V measurements for different contact areas of a single drop of the MMS system. For small contact area, an almost ideal diode behavior was measured. Increment of the contact area resulted in a gradual destruction of the interface up to final collapse of the monolayers (here, ferrocene-based SAM). Inset: J/V graph of HgWSe2.
Figure 2. (Top) Optical image of an uncontrollable Hg-WSe2 contact. (Middle) MMS controlled contact. (Bottom) Contact angle vs contact diameter of the MMS system. The collapse of the contact leads to large variations in the wetting angle (inset: the Fc molecule).
Ferrocene). The Hg was clearly seen (Figure 2, middle) to not wet the WSe2 surface, and the drop kept its original spherical shape. This MMS contact remained mechanically stable for several days, and the contact area and angle did not change significantly during that time. Once the MMS contact was established, the substrate stage was slowly raised in order to increase the contact area. Figure 2 (bottom) shows the change in contact angle as a function of MMS contact area. A rapid drop in contact angle was observed when the contact area increased. At a certain value of contact area, the MMS system collapsed, and the resulting contact angle was not stable. It can be concluded that, due to the increment in mechanical pressure, the monolayer collapses or changes conformation and the contact is destroyed. This observation indicates that intimate contact was established between the Hg and the surface. The contact was then
mechanically destroyed, probably via polarization of the W-Se bond (Figure 1), resulting in good wetting of the Hg and formation of an uncontrolled contact. Possible contact formation via breakdown of the W-Se bond and creation of a Hg-Se bond was excluded by X-ray photoelectron spectroscopy (XPS) analysis of the WSe2 surface after physical removal of the Hg drop. No evidence of covalent binding of Hg was found. Current density vs voltage (J/V) measurements of the MS and MMS contacts were performed simultaneously during the variable contact area experiment. The bare Hg-WSe2 MS contact was found to be far from an ideal Schottky diode (Figure 3, inset): in the reverse bias, very rapid junction breakdown was observed, which effectively introduced a very high reverse current. This type of leaky diode has previously been attributed to the existence of step edges on the surface which act as recombination-center defects in the MS contact. Adsorption of a monolayer of alkanethiols or Fc-containing molecules along the Hg surface changes the junction’s J/V characteristics dramatically. A strong rectifying diode characteristic is measured (Figure 3). Significant changes in magnitude and shape of the current density curves were measured with the variations in contact area. Consecutive increments in contact area caused a successive approach toward the J/V characteristics of the bare Hg-WSe2 contact. Furthermore, a closer look at the current densities revealed strikingly opposite behavior for forward and reverse bias. An increment in contact area causes a decrease in the current density in forward bias. In reverse bias, the current density increases with increasing contact area, and the overall behavior resembles that of an electrical dual-polarity rectifier. Except for better stabilization for the Fc-based molecule, the shape of the graphs was identical for all measured SAMs. The simplest way to examine the J/V characteristics is to evaluate the barrier height between the metal and the semiconductor at each bias using a simple thermionic emission model1-2
φJ/V )
[
kT ln(A*T2) - ln q
((
)]
J qV exp -1 kT
)
where k is the Boltzmann constant, T is the temperature, q is the fundamental charge, A* is the effective Richardson constant
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J. Phys. Chem. C, Vol. 111, No. 12, 2007 4483
TABLE 1: Barrier Height OJ/V and Slope of OJ/V vs Voltage Obtained for Two Different Contact Areas contact diameter (µm) 234 696
reverse bias dφJ/V/dV φJ/V 0.02 0.03
0.87 0.82
forward bias dφJ/V/dV 0.94 0.95
(∼8.6 A/C m2 K2 for typical n-type semiconductors2), and ΦJ/V is the Schottky barrier height. Analysis of the data (Table 1) clearly indicates that two different processes govern the transport properties of the system. The reverse bias analysis shows very weak dependence of the barrier height on the bias voltage, as expected from thermionic emission theory.2 A dramatic reduction (50 mV) of Φ is measured when the contact area increases. Since J scales with exp(-qΦ/kT)2 the lower work function is responsible for higher J. When a forward bias is applied, a clear linear voltage dependence of the work function on the applied voltage is measured. This dependency implies that a field emission mechanism is responsible for J at this polarity. However, the field emission mechanism cannot explain the light reduction of J with contact area, which is probably related to the nature of the defects on the semiconductor. One should note that the extraordinary stabilization of the mechanical and electrical MMS contact indicates that the SAM plays several roles in this system. It serves as a chemomechanical spacer, the SAM-Hg chemical bond (Hg-S) passivates the Hg surface, and the physical spacing that exists between the Hg and the semiconductor (via the SAM) prevents polarization of the Se-W bond and wetting of the surface by the Hg drop. When the contact area increases, the monolayer gradually collapses, probably due the pressure increment on the SAM layer. This process leads to parallel mechanical and electrical collapse of the contact. Based on our observations, we conclude that the main role of the SAM is passivation of the physical and electrical defects on the surface. However, the increase in current density in forward bias in the MMS with respect to the MS indicates that the molecules actually also contribute to the electrical properties of the interface and probably change the Schottky barrier height. However, this phenomenon cannot completely explain the tendency observed in both polarities, and it has yet to be understood. To demonstrate the electrical stabilization of this MMS system, current vs voltage (I/V) and photoconductivity measurements of the MMS contact were obtained in the dark and under illumination. As expected, the electrically stabilized contact showed clear photoconductive properties (Figure 4). In conclusion, we show here that a simple SAM interface between metal and a MX2-type semiconductor can change the electrical and optoelectronic properties of the contact dramatically and mechanically stabilize it. We demonstrate that this type of contact does not obey scaling rules and suggest that the SAM acts as a mechanical spacer which helps eliminate the
Figure 4. Light response of Hg-CH3(CH2)8SH-MoS2 system at V ) 100 mV.
strong MS polarization forces and reduce electrical defects at the MMS interface. We believe that this simple MMS system could be exploited to construct simple, cheap, and efficient solar cells in the future. Acknowledgment. We thank Dr. Michael Gozin (Tel-Aviv University) for the synthesis of the Fc molecule. We thank Dr. Leeor Kronik, Prof. David Cahen (Weizmann Institute of Science, Israel) and Prof. Moshe Deutsch (Bar-Ilan University, Israel) for fruitful discussions. This work was supported by the Israel Science Foundation (Project Number 604/06) and by Clal Biotechnology Foundation. Supporting Information Available: Experimental details on the electrical and mechanical experimental procedure. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Henisch, H. K. Semiconductor Contacts; Oxford University Press: New York, 1984. (2) Hsu, J. W. P.; Loo, Y. L.; Lang, D. V.; Rogers, J. A. J. Vac. Sci. Technol. B, 2003, 21, 1928. (3) Salomon, A.; Bocking, T.; Gooding, J.; Cahen, D. Nano Lett. 2006, 6, 2873. (4) Salomon, A.; Cahen, D.; Lindsay, S.; Tomfohr, J.; Engelkes, V. B.; Frisbie, C. D. AdV. Mater. 2003, 15, 1881. (5) Haick, H.; Ghabboun, J.; Niitsoo, O.; Cohen, H.; Cahen, D.; Vilan, A.; Hwang, J. Y.; Wan, A.; Amy, F.; Kahn, A. J. Phys. Chem. B 2005, 109, 9622. (6) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075. (7) Lodha, S.; Janes, D. B. J. Appl. Phys. 2006, 100, 024503. (8) Meshulam, G.; Rosenberg, N.; Caster, A.; Burstein, L.; Gozin, M.; Richter, S. Small 2005, 1, 848. (9) Bernede, J. C.; Pouzet, J.; Gourmelon, E.; Hadouda, H. Synth. Met. 1999, 99, 45. (10) Hodes, G. Appl. Phys. Lett. 1989, 54, 2085. (11) Ocko, B. M.; Kraack, H.; Pershan, P. S.; Sloutskin, E.; Tamam, L.; Deutsch, M. Phys. ReV. Lett. 2005, 94, 017802.
