Hg/Molecular Monolayer-Si Junctions: Electrical Interplay between Monolayer Properties and Semiconductor Doping Density Omer Yaffe, †,‡ Luc Scheres, †,§ Lior Segev, ‡ Ariel Biller, ‡ Izhar Ron, ‡ Eric Salomon, | Marcel Giesbers, § Antoine Kahn, | Leeor Kronik, ‡ Han Zuilhof, § Ayelet Vilan,* ,‡ and David Cahen* ,‡ Department of Materials and Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel, Laboratory of Organic Chemistry, Wageningen UniVersity, Dreijenplein 8, 6703 HB Wageningen, The Netherlands, and Department of Electrical Engineering, Princeton UniVersity, Princeton, New Jersey, 08544 ReceiVed: February 24, 2010; ReVised Manuscript ReceiVed: April 29, 2010 Metal-organic molecule-semiconductor junctions are controlled not only by the molecular properties, as in metal-organic molecule-metal junctions, but also by effects of the molecular dipole, the dipolar molecule-semiconductor link, and molecule-semiconductor charge transfer, and by the effects of all these on the semiconductor depletion layer (i.e., on the internal semiconductor barrier to charge transport). Here, we report on and compare the electrical properties (current-voltage, capacitance-voltage, and work function) of large area Hg/organic monolayer-Si junctions with alkyl and alkenyl monolayers on moderately and highly doped n-Si, and combine the experimental data with simulations of charge transport and electronic structure calculations. We show that, for moderately doped Si, the internal semiconductor barrier completely controls transport and the attached molecules influence the transport of such junctions only in that they drive the Si into inversion. The resulting minority carrier-controlled junction is not sensitive to molecular changes in the organic monolayer at reverse and low forward bias and is controlled by series resistance at higher forward bias. However, in the case of highly doped Si, the internal barrier is smaller, and as a result, the charge transport properties of the junction are affected by changing from an alkyl to an alkenyl monolayer. We propose that the double bond near the surface primarily increases the coupling between the organic monolayer and the Si, which increases the current density at a given bias by increasing the contact conductance. Introduction Incorporating molecular elements into electronic devices poses a fascinating scientific challenge. 1 By varying the mo- lecular chemistry, we hope to tailor the device’s electrical properties, possibly leading to flexible and scalable fabrication schemes. Much of the work in this direction focuses on single molecules or monolayer ensembles on metal electrodes. 2-4 Using a semiconductor instead of a metal provides significant physical and technological advantages, 5-9 among which are possibly tunable interactions between the semiconductor bands and the molecular energy levels that may lead to novel electrical behavior. 5 Semiconductor (SC) surfaces, as well as metal ones, can be functionalized with organic molecules to yield stable and high-quality monolayers. 10-13 However, unlike metals, the bulk electronic properties of semiconductors can be tailored through doping and the (near-)surface properties can be modified via electrical dipoles and (monopole) charges, thereby consider- ably expanding the possibilities for tuning the device performance. 14,15 Adsorbing molecules on the SC surface generally changes the surface potential and, thus, the SC work function (and electron affinity). 16 This potential change at the SC surface can extend from roughly a few nanometers to a few micrometers into the semiconductor, forming a space charge region (SCR), which constitutes an internal barrier for charge transport. Therefore, if a metal contact is made to the SC, the presence of molecules at the interface can change the internal charge transport barrier across the resulting junction. 9,17 This internal SC barrier changes the current-voltage (J-V) characteristics of the junction, in addition to the specific charge transport barrier imposed by the molecules. Hence, the “molecular effect” of hybrid metal/organic molecule/semiconductor (MOMS) junc- tions can be divided into (a) the overall dipole of the molecules on the surface, plus any molecule-substrate charge transfer that affects the effective SC electron affinity; (b) the electronic transport barrier, presented by the molecules, especially if they form a continuous monolayer. In addition, the introduction of surface/interface states can also have a large effect on the electrical properties of the junction. However, it was shown in the past that the interface state density of well-prepared Si-organic monolayer interfaces is very low. 18,19 The doping density of the SC affects the relative importance of effects (a) and (b), because an increase in doping density can (1) induce image charge lowering of the barrier, (2) decrease the SC depletion layer width 20,21 and, thereby, increase the probability of tunneling through the SCR barrier (field emission), and (3) affect the magnitude of the surface dipole, induced by the monolayer. 22 Therefore, the molecular properties (e.g., degree of conjugation, presence of redox active centers, mo- lecular length) will have different overall effects for different doping levels of otherwise identical semiconductors. To study and comprehend this interplay between the molec- ular and Si properties, we compare here the electrical charac- * Author for correspondence. E-mail david.cahen@weizmann.ac.il. † These authors contributed equally to this work. ‡ Weizmann Institute of Science. § Wageningen University. | Princeton University. J. Phys. Chem. C 2010, 114, 10270–10279 10270 10.1021/jp101656t  2010 American Chemical Society Published on Web 05/13/2010