What is the Barrier for Tunneling Through Alkyl Monolayers? Results from n‐ and p‐Si–Alkyl/Hg Junctions

Abstract

Current transport by tunneling through molecular devices is thought to be dominated by the height and width of the barrier(s) resulting from the presence of molecules between the electrodes. To a first approximation, the barrier height in metal/molecule junctions is given by the energy difference between the Fermi level of the electrode and the closest molecular energy levels, the highest occupied molecular orbital (HOMO) and/or the lowest unoccupied molecular orbital (LUMO). For semiconductor/molecule junctions, the corresponding barrier height is the energy difference between the edge of the conduction or valence band and the LUMO or HOMO, respectively, depending on the semiconductor doping type, and can be tuned by changing the semiconductor doping type.[1] Experimentally the position of the molecules' HOMO and LUMO relative to the electrodes' Fermi level or band edges can be determined using ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) measurements. Therefore, the tunneling-barrier height through a molecular layer can, in principle, be deduced by using this method.[2]Here, we compare and analyze the electronic transport through alkyl chains, C n H 2n+1 with n = 12, 14, 16, and 18, bound directly to p-or n-Si, via C-Si bonds, and contacted by Hg to form Si-alkyl/Hg junctions. In these molecular junctions the alkyl chains are connected via the same Si-C bonds to either n-or p-Si, with presumably the same amount of charge transfer between the molecule and the electrode as a result of this bond formation. This feature allows us to isolate and ascertain the effect of the electrode Fermi-level position on charge transport through the junction. [3,4] As carried out earlier for the n-Si system only, [5] we now deduce the barriers for charge transport through the alkane monolayers, both from transport through the junctions and from spectroscopic measurements of the corresponding p-and n-Si-C n H 2n+1 interfaces. The main differences in analyses with our earlier n-Si work [5] are that we use a more complete model for transport analysis and that we can now interpret the photoemission data with the help of complementary theoretical computations. [6] In this way, we find that whereas the spectroscopic measurements show a tunnel barrier of approximately 3-4 eV (rather than the smaller one derived earlier [5] without the help of theory to interpret the IPES and UPS data), fitting the current-voltage (I-V) curves to transport by tunneling yields a barrier of only approximately 0.7-1 eV. We show that this difference, which is ascribed to the presence of states at the interface caused by Si molecule interactions beyond Si-C bond formation, forces us to revise our view of tunneling through such molecular junctions. As shown earlier, in a semiconductor/saturated-molecule/ metal junction, two transport barriers can exist simultaneously, a Schottky barrier inside the semiconductor, caused by band bending near the interface, and a tunnel barrier formed by the insulating, r-bonded molecu...

Molecules on Si: Electronics with Chemistry

“…Only at higher forward bias do the currents vary according to the thickness of the insulator, i.e., the molecular layer width here. Such length-independence of current at low forward bias was observed for several moderately-doped n-type MOMS junctions with n-Si [29,56,88,134,135,138,139] and n-GaAs, [35,36,140] contacted with medium-high work-function metals such as Hg.…”

“…[29,30] Chemically, binding organic molecule(s) onto Si is directional and stable, i.e., the semiconductor surface can act as template for molecular adsorption. [31] This is very different from the case of thiols on Au, where the energy barrier between the two chemisorbed modes on Au (the corrugation energy) is about 2.5 kcal mol À1 ($0.11 eV mol À1 ), allowing the thiolate to move at room temperature rather easily from one Au site to another, even without applying a voltage.…”

“…The coincidence of the experimentally measured C 1 , C 2 , and C 4 (methyl, ethyl and butyl) curves at high forward bias is due to parasitic resistance, an effect that is expected to decrease as the contact area gets smaller, i.e., with decreasing currents. b) Simulation, using ultra-thin MIS (UT-MIS) model of log(J)-V behavior of minority carrier-controlled device, for metal/organic-monolayer/Si junction, with varying insulator widths (in Å, corresponding to C 12 to C 18 alkanes), using N D ¼ 5 Â 10 15 cm À3 ; f ¼ 4.9 eV; x ¼ 3.9 eV; tunneling barriers for electrons (h e ) and holes (h h ) are taken as 2.6 and 3.6 eV, respectively [29,47]. A relative effective mass of m* ¼ 0.4 was found to best reproduce the experimental data.…”

“…The difference between Hg work function (f ¼ 4.49 eV) [19,29] and Si electron affinity (x ¼ 4.05) [83] yields a theoretical Schottky barrier height (SBH, as defined in Eq. (1)) of $0.4 eV for the n-SiÀH/Hg junction.…”

“…also Refs. [20,29,56,97,138]). This difference can be explained by minor changes in interface electrostatics, e.g., a minute amount of residual chemicals can increase the Si work function and drive the MOMS from inversion (Fig.…”