LETTERS Real-Time Electronic Monitoring of Adsorption Kinetics: Evidence for Two-Site Adsorption Mechanism of Dicarboxylic Acids on GaAs(100) Ayelet Vilan, ² Rachel Ussyshkin, ‡ Konstantin Gartsman, § David Cahen,* ,² Ron Naaman,* ,‡ and Abraham Shanzer* ,| Departments of Materials and Interfaces, Chemical Physics, Chemical SerVices, and Organic Chemistry, Weizmann Institute of Science, RehoVot, 76100 Israel ReceiVed: December 9, 1997; In Final Form: February 26, 1998 We show that the chemisorption of dicarboxylic acids on GaAs (100) is described well by a two-site mechanism, in contrast to benzoic acid adsorption which fits to a one-site mechanism. We do so by using a novel electrical method for direct measurement of adsorption kinetics. In the method we measure the current through a GaAs/(Al, Ga)As-based device, where the bare surface between two contacts is used as the adsorption domain. The results, which are in agreement with FTIR absorption equilibrium data, are obtained in ambient notwithstanding the notorious instability of GaAs surfaces under such conditions. We conclude that these acids chemisorb on the GaAs surface and that binding is significantly stronger for the di- than for the monocarboxylic acids. The reaction of organic molecules with semiconductor surfaces to form chemisorbed layers of molecules is a basic step in the process of semiconductor surface modification and in building of supramolecular architectures on semiconductor surfaces. As such there is great interest in understanding the process and in monitoring it in real time. The latter can be accomplished with several techniques, such as IR absorption, using FT-IR, for monitoring submonolayer concentrations. 1 A more sensitive method is the microbalance in which the frequency change of an oscillating crystal indicates the amount of material adsorbed. 2-4 In the present work, we use the effect that chemical surface modification has on semiconductor electronic properties to follow the kinetics of adsorption of dicarboxylic acids onto GaAs (100) surface. We do so by monitoring the current through a special multilayered GaAs-based device, composed of insulating outer layers and a doped inner one (Figure 1, inset) with adsorption occurring on one of the insulating layers between two metal contacts, which serve to measure conductivity. Contrary to Si, GaAs surfaces do not have a passivating native oxide layer. Hence, it is possible to chemically modify the surface states, their charge, and thus the internal field in the space charge layer. This is in contrast to common Si-based chemical sensors, which are sensitive to an external field between the gate and a reference electrode. 5 Measurements of the so-called surface conductance of semiconductors use the same geometry 6 and have been used for identifying the adsorption of simple organic molecules on Ge. 7 In the present configuration much higher sensitivity to surface processes is achieved by having a semi-insulating (Al, Ga)As layer between the semi-insulating GaAs substrate and the doped GaAs layer. Because of the space charge in the doped GaAs layer, such a structure leads to a concentration of maximum electron density at a depth of 30-50 nm from the exposed surface. 8 Therefore, ² Department of Materials and Interfaces. ‡ Department of Chemical Physics. § Department of Chemical Services. | Department of Organic Chemistry. © Copyright 1998 by the American Chemical Society VOLUME 102, NUMBER 18, APRIL 30, 1998 S1089-5647(98)00474-X CCC: $15.00 © 1998 American Chemical Society Published on Web 04/14/1998