Tuning of Au/n-GaAs Diodes with Highly Conjugated Molecules Deng Guo Wu, ² Jamal Ghabboun, ² Jan M. L. Martin, ‡ and David Cahen* ,² Department of Materials and Interfaces, and Department of Organic Chemistry, Weizmann Institute of Science, RehoVot, 76100 Israel ReceiVed: July 13, 2001; In Final Form: September 7, 2001 Bifunctional conjugated molecules, consisting of electron donating or accepting groups that are connected, via a conjugated bridge, to a carboxylic acid group, were adsorbed as monomolecular carboxylate films on n-GaAs (100) and characterized by reflection FTIR, ellipsometry, and contact angle techniques. The way the donors and acceptors affected the electronic properties of the semiconductor was investigated. In agreement with theory, we find a linear relation between the calculated dipole moment of the molecules and the change in electron affinity of the moleculary modified surface, as well as between the barrier height of Au/molecule on n-GaAs junctions, extracted from their current-voltage characteristics and the dipole moment. The experimental results show little effect of the nature of the conjugated bridge in the molecules. Comparison with earlier work shows a clear decrease in the effect of the dipole of the free molecule on the semiconductor surface and interface behavior, notwithstanding the strongly conjugated link between the donor or acceptor groups of the molecule and the semiconductor surface. The simplest way to understand this is to consider the higher polarizability of the intervening bonds. Such effect needs to be considered in designing molecules for molecular control over devices. Introduction Although molecular organic (semi)conductors have been studied for more than fifty years, we are far from understanding them as well as we do their nonmolecular counterparts. Much work was and is done to use molecular organic semiconductors as the active component in (opto)electronic and electrooptical devices. 1-3 Photovoltaic devices, using organics have been fabricated with, for example, conjugated polymers such as polyacetylene, poly(N-vinylcarbazole), and various derivatives of polyacetylene. Much of the recent interest is in molecular and polymeric organic or organic-based light-emitting devices. 4-6 A major driving force in this area is the continuous quest to free microelectronics from the limitations imposed on it by Si and III-V technologies. 7 Indeed, large efforts are made toward fully molecular electronics, including use of individual molecules as switches or memory units. 8 We are pursuing a hybrid approach, using molecules to extend the properties of conventional semiconduc- tors, and in this way to use the advantages of both. The idea is to control the electronic properties of semiconductor devices, via molecular control over the interface(s) in such devices. To do so, semiconductor or metal surfaces are modified by adsorbing molecules on the free surface, which can subsequently be made into an interface when it is used to make a device. 9 The efficacy of such molecular control is assessed by using series of molecules, rather than a single type. In that series, a property (mostly we have used the dipole moment) is varied systematically. We then search for a corresponding systematic trend in semiconductor and device properties. This approach circumvents the problem of basing observations of a molecular effect on comparisons of a system with a molecule to one without one. Mostly the molecules that are used are composed of a binding group, a group with variable dipole, and a bridge that connects those. 10-13 If the molecules form at least a partial monolayer on the surface of the solid (in our work we find generally between 0.75 and 1 monolayer 14-17 ), the resulting dipole layer will affect the work function of the moleculary modified solid. For a semiconductor, the relevant quantities are the electron affinity and the band bending (built-in potential). Changes in electron affinity can come about without charge transfer or polarization of the bond of the molecule with the solid’s surface atoms, based just on the intrinsic dipole moment of the molecules [cf. Figures 4 and 5 in ref 18]. If charge transfer does occur, additional modifications take place (see below, discussion of Table 3 and Figure 5). Figure 1 illustrates how dipole layers can affect the energetics at n-GaAs surfaces and interfaces. Figure 1a represents a situation of the energy profiles for bare n-GaAs contacted by a metal. Figure 1b,c shows the effect of a monolayer of adsorbed molecules on the electron energetic properties of the surface and interface. If molecules with a donor group at the end opposite the binding group are adsorbed on the semiconductor (Figure 1b), the work function ( s ) and effective electron affinity ( s ) of the semiconductor will decrease. This means that after adsorption of the molecule less external energy needs to be provided (as in a UPS experiment) than before to take an electron from the solid through the surface to a point just outside the range of the crystal forces. If molecules with an acceptor group are adsorbed (Figure 1c), the effective electron affinity ( s ) and the work function ( s ) will increase; that is, it will cost more energy to remove an electron from the surface, than if no molecules were present. As the surface becomes an interface, the opposite effect is obtained; that is, if molecules with an acceptor group are adsorbed on the semiconductor * To whom correspondence should be addressed. ² Department of Materials and Interfaces, Weizmann Institute of Science. ‡ Department of Organic Chemistry, Weizmann Institute of Science. 12011 J. Phys. Chem. B 2001, 105, 12011-12018 10.1021/jp012708l CCC: $20.00 © 2001 American Chemical Society Published on Web 11/01/2001