Elsevier

Electrochimica Acta

Volume 52, Issue 8, 10 February 2007, Pages 2913-2919
Electrochimica Acta

Electrochemical impedance and solid-state electrical characterization of silicon (1 1 1) modified with ω-functionalized alkyl monolayers

https://doi.org/10.1016/j.electacta.2006.08.060Get rights and content

Abstract

In order to reveal the relationship between the dipole moment and electric properties of organically modified semiconductor, a series of ω-functionalized alkyl monolayers on oxide-free silicon (triple bondSi–(CH2)10COOH, triple bondSi–(CH2)11CH3, triple bondSi–(CH2)10COOC2H5, and triple bondSi–(CH2)11OH) was prepared and characterized. Electrochemical impedance measurements showed excellent insulating effects of these monolayers. It has been demonstrated that the dielectric constants of these organic monolayers are affected by the dipole moments introduced by the different terminal-groups. Solid-state electrical measurements showed that mercury | ω-functionalized alkyl monolayer | silicon junctions exhibit molecular tunability and a clear correlation between ideality factor and the film thickness/dielectric constant ratio. The barrier height is approximately proportional to the dipole moment of the monolayer. These results augment the possibility of fine-tuning the electrical properties of silicon-based microelectronic devices using functionalized organic monolayers.

Introduction

Since the introduction of the first metal-oxide semiconductor field-effect transistor in the 1960s [1], advances in the design of integrated circuits have been extremely rapid. They are often referred to obey Moore's law, which predicts that the number of components on a chip quadruples every 3 years [2]. At the present rate of downscaling microelectronic devices, the thickness of conventional gate oxide film will reach its fundamental physical limit within the next decade [3], [4]. Organic materials as media to control charge transport are promising alternatives to silicon oxide, because they are manipulable at scales ranging from angstroms to nanometers. Furthermore, the intrinsic optical and electrical properties of molecules with various functionalities allow them to act as either passive (tunneling junctions, rectifiers) or active (switches, transistors, and logic gates) electronic components [5]. However, the use of discrete molecules as working electronic units is challenging for several reasons. From a technical standpoint, the electrical contacts formed to single molecules must be free of current leakage (by bypassing the molecules) and robust during extensive use; therefore, a better understanding of the physical aspects of molecule | electrode interfaces is needed [6], [7].

A more realistic and easier approach is the use of groups/clusters of molecules instead of single molecule(s) or bulk materials. One of the most thoroughly explored methods is the incorporation of ultrathin organic films, such as self-assembled monolayers (SAMs) and Langmuir–Blodgett (LB) films [8], [9]. The molecular structure of these organic monolayers not only determines the rate of electron transfer across [10], but also allows fine-tuning of the bulk material properties. For example, Cahen et al. have demonstrated the tunability of Au/GaAs diodes, i.e., a correlation between the dipole moment of dicarboxylic acid derivatives adsorbed on GaAs and the barrier height/electron affinity of the diode junction [11], [12], [13].

Silicon is one of the most important semiconductors used in modern technology; it is of great interest to molecularly control its electrical properties. In the early 1990s Linford et al. prepared n-alkyl monolayers covalently bonded to silicon surfaces by photochemical methods [14], [15]. These monolayers are superior to other thin films (e.g., SAMs on oxidized silicon surface and LB films) because of their great chemical, mechanical, and thermal stability [14], [15], [16]. Several other synthetic approaches are now available to prepare organic monolayers on oxide-free silicon, such as the reaction of hydrogen-terminated silicon with diacyl peroxides, terminal olefins or Grignard reagents [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], the reduction of aryldiazonium ions at silicon electrodes, and the addition of alkylmagnesium or alkyllthium reagents to halogenated silicon surface [24], [25], [26].

The electrochemical and solid-state electric properties of these monolayers have not yet been thoroughly investigated. We have explored the improved insulating property of n-alkyl monolayers on silicon (1 1 1) compared to SiO2 films, and the chain-length dependence of its interfacial properties (reciprocal capacitance and effective barrier height) [27], [28], [29]. More recently, extended studies have been carried out by Zuilhof and co-workers on silicon (1 0 0) surfaces [30]. Meanwhile, Cheng et al. have extracted tunneling constants of electron transfer from silicon electrodes through alkyl monolayers to decamethylferricenium acceptors [31]. Wei and Zhao have observed a non-linear relationship between capacitance and composition for monolayer prepared from dodecene and methyl undecylenate [32]. By monitoring leakage currents, Miramond and Vuillaume have assessed the effect of substrate doping on the quality of octadecyl monolayers [33]. Faucheu et al. have shown that the higher the molar ratio of carboxy-terminated molecules in mixed carboxy/methyl-terminated monolayers, the larger the effective dielectric constant [34].

All previous investigations of the molecular tunability of electric properties have focused on either alkyl chain length dependence or variation of the ratio between alkyl- and carboxy-terminated molecules in the mixed monolayers. There are indications that the physical properties of the adsorbed molecules could also influence the electric properties of the system, as has been demonstrated previously in the case of GaAs or organic semiconducting materials [11], [12], [13], [34], [35], [36], [37], [38], [39]. In this paper, we describe the preparation and characterization of a series of ω-functionalized alkyl monolayers on oxide-free silicon (triple bondSi–(CH2)10COOH, triple bondSi–(CH2)11CH3, triple bondSi–(CH2)10COOC2H5, and triple bondSi–(CH2)11OH). Both electrochemical capacitance (in contact with aqueous electrolyte) and solid-state electrical measurements (by fabricating mercury | ω-functionalized alkyl monolayer | silicon junctions) have been performed and interpreted.

Section snippets

Materials

All chemicals were of reagent or the highest available commercial grade and used as received unless otherwise stated. Deionized water (>18.3 Ω cm) was obtained from a Barnstead EasyPure UV/UF compact water system (Dubuque, IA). 1-Dodecene (98%), ethyl undecylenate (97%), and sodium borohydride were purchased from Aldrich (Milwaukee, WI); tetrahydrofuran (THF) and 1,1,1-trichloroethane (99.5%) from Caledon Laboratories Ltd. (Georgetown, ON); ammonium fluoride (40%), sulfuric acid (96%),

Results and discussion

The water contact angles and ellipsometric thicknesses of the ω-functionalized alkyl monolayers on silicon are summarized in Table 1. The larger-than-90° water contact angles obtained for triple bondSi–(CH2)11CH3 are indicative of hydrophobic methyl termination although they are lower than those reported for similar monolayers prepared via other surface reactions [27], [43], [44]. The difference may be due to a lower surface density of the alkyl chains or the partial oxidation of the silicon surface upon

Conclusions

ω-Functionalized alkyl monolayers on oxide-free silicon (1 1 1) surface (triple bondSi–(CH2)10COOH, triple bondSi–(CH2)11CH3, triple bondSi–(CH2)10COOC2H5, and triple bondSi–(CH2)11OH) were prepared via photochemical reactions and characterized by electrochemical/solid-state electrical measurements. Differential capacitance–voltage studies revealed that all of the monolayers are of high insulating property and the obtained dielectric constants are influenced significantly by the dipole moments of the terminal groups. The current

Acknowledgments

The authors are grateful to the Natural Science and Engineering Research Council of Canada (NSERC) for financial support. H.Z.Y. wishes to thank Dr. Eberhard Kiehlmann for editing the manuscript.

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