A two-step, chlorination/alkylation procedure has been used to convert the surface Si-H bonds on NH 4 F ͑aq͒ -etched ͑111͒-oriented Si wafers into Si-alkyl bonds of the form Si-C n H 2nϩ1 (nу1). The electrical properties of such functionalized surfaces were investigated under high-level and low-level injection conditions using a contactless rf apparatus. The charge carrier recombination velocities of the alkylated surfaces were Ͻ25 cm s Ϫ1 under high-level and low-level injection conditions, implying residual surface trap densities of Ͻ3ϫ10 9 cm Ϫ2 . Although the carrier recombination velocity of hydrogen-terminated Si͑111͒ surfaces in contact with aqueous acids is Ͻ20 cm s Ϫ1 , this surface deteriorates within 30 min in an air ambient, yielding a high surface recombination velocity. In contrast, methylated Si͑111͒ surfaces exhibit low surface recombination velocities in air for more than 4 weeks. Low surface recombination velocities were also observed for Si surfaces that had been modified with longer alkyl chains.

Fermi's golden rule is used to formulate rate expressions for charge transfer of delocalized carriers in a nondegenerately doped semiconducting electrode to localized, outer-sphere redox acceptors in an electrolyte phase. If the charge-transfer rate constant is known experimentally, these rate expressions allow computation of the value of the electronic coupling matrix element between the semiconducting electrode and the redox species. This treatment also facilitates comparison between charge-transfer kinetic data at metallic and semiconducting electrodes in terms of parameters such as the electronic coupling to the electrode, the attenuation of coupling with distance into the electrolyte, and the reorganization energy of the charge-transfer event. Within this framework, rate constant values expected at representative semiconducting electrodes have been evaluated from experimental data for charge transfer from Au electrodes to ferrocene-terminated thiols, to Ru(NH3)5 3+/2+-terminated thiols, and through blocking layers to dissolved [Fe(2,2‘-bipyridine)2(CN)2]+/0. Based on the experimental parameters determined for these systems, the maximum rate constant (i.e. at optimal exoergicity) for outer-sphere processes at semiconducting electrodes is computed to be in the range 10-17−10-16 cm4 s-1. These values are in excellent agreement with prior theoretical models and experimental results for charge-transfer kinetics at semiconductor/liquid interfaces and thus serve to unify the theoretical and experimental descriptions of electrochemical processes at semiconducting and metallic electrodes.

The Fermi golden rule formalism has been used to derive the rate constant for interfacial electron transfer from a semimetallic electrode, such as highly ordered pyrolytic graphite (HOPG), to a redox couple in solution. A simple expression is presented that semiquantitatively relates the electron-transfer rate constant at a semimetallic electrode to that at a metallic electrode. The approach allows for the estimation of the value of the rate constant for interfacial charge transfer to nonadsorbing outer-sphere redox species at semimetallic electrodes. Rate constants for interfacial electron transfer for a variety of one-electron redox couples at semimetallic electrodes have been calculated relative to the rate constant of the ferrocenium/ferrocene redox couple at a gold electrode. Good agreement is found, in general, between the calculated and observed rate constants.

Photoconductivity decay lifetimes have been obtained for NH4F(aq)-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of CH3OH or tetrahydrofuran (THF) containing either ferrocene+/0 (Fc+/0), bis(pentamethylcyclopentadienyl) Fe+/0, or I2. Si surfaces in contact with electrolytes having Nernstian redox potentials >0 V versus the standard calomel electrode exhibited low effective surface recombination velocities regardless of the different surface chemistries, whereas those exposed only to N2(g) ambients or to electrolytes containing mild oxidants showed differing rf photoconductivity decay behavior depending on their different surface chemistry. The data reveal that formation of an inversion layer, and not a reduced density of electrical trap sites on the surface, is primarily responsible for the long charge-carrier lifetimes observed for Si surfaces in contact with CH3OH or THF electrolytes containing I2 or Fc+/0.

Photoconductivity decay data have been obtained for NH4F(aq)-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of methanol, tetrahydrofuran (THF), or acetonitrile containing either ferrocene+/0 (Fc+/0), [bis(pentamethylcyclopentadienyl)iron]+/0 (Me10Fc+/0), iodine (I2), or cobaltocene+/0 (CoCp2 +/0). Carrier decay measurements were made under both low-level and high-level injection conditions using a contactless rf photoconductivity decay apparatus. When in contact with electrolyte solutions having either very positive (Fc+/0, I2/I-) or relatively negative (CoCp2 +/0) Nernstian redox potentials with respect to the conduction-band edge of Si, Si surfaces exhibited low effective surface recombination velocities. In contrast, surfaces that were exposed only to N2(g) ambients or to electrolyte solutions that contained a mild oxidant (such as Me10Fc+/0) showed differing rf photoconductivity decay behavior depending on their different surface chemistry. Specifically, surfaces that possessed Si−OCH3 bonds, produced by reaction of H-terminated Si with CH3OH−Fc+/0, showed lower surface recombination velocities in contact with N2(g) or in contact with CH3OH−Me10Fc+/0 solutions than did NH4F(aq)-etched, air-exposed H-terminated Si(111) surfaces in contact with the same ambients. Furthermore, the CH3OH−Fc+/0-treated surfaces showed lower surface recombination velocities than surfaces containing Si−I bonds, which were formed by the reaction of H-terminated Si surfaces with CH3OH−I2 or THF−I2 solutions. These results can all be consistently explained through reference to the electrochemistry of Si/liquid contacts. In conjunction with prior measurements of the near-surface channel conductance for p+−n−p+ Si structures in contact with CH3OH−Fc+/0 solutions, the data reveal that formation of an inversion layer (i.e., an accumulation of holes at the surface) on n-type Si, and not a reduced density of surface electrical trap sites, is primarily responsible for the long charge carrier lifetimes observed for Si surfaces in contact with CH3OH or THF electrolytes containing I2 or Fc+/0. Similarly, formation of an accumulation layer (i.e., an accumulation of electrons at the surface) consistently explains the low effective surface recombination velocity observed for the Si/CH3OH−CoCp2 and Si/CH3CN−CoCp2 contacts. Detailed digital simulations of the photoconductivity decay dynamics for semiconductors that are in conditions of inversion or depletion while in contact with redox-active electrolytes support these conclusions.