J. Phys. Chem. C 2008, 112, 9081–9088
9081
Nanoscale Electrical and Structural Characterization of Gold/Alkyl Monolayer/Silicon Diode Junctions Marcus A. Kuikka,† Wenjie Li,‡ Karen L. Kavanagh,‡,* and Hua-Zhong Yu†,* Department of Chemistry and Department of Physics, Simon Fraser UniVersity, Burnaby, British Columbia V5A 1S6, Canada ReceiVed: March 28, 2008
The effect of molecular modification on gold-silicon diode junctions has been investigated by electrical and structural measurements at both the macroscale and the nanoscale levels. Diode junctions prepared with Si-C bonded, organic monolayers on silicon and thermally evaporated gold contacts yield identical macroscale barrier heights and ideality factors, irrespective of the alkyl chain length and end group of the molecular layer. Electrical and structural measurements on the nanoscale level, using ballistic emission electron microscopy (BEEM) and cross-sectional transmission electron microscopy (TEM), indicate laterally uniform penetration of deposited gold atoms into n-alkyl monolayers (20 nm resolution), whereas inhibited penetration occurs for thiol-terminated monolayers on silicon. Average BEEM transmission is reduced via scattering associated with the presence of an organic monolayer, with the largest effects observed for the thiol-terminated system. 1. Introduction With the ever-increasing demand for miniaturization of electronic devices, much focus has recently been devoted to research in the molecular modification of traditional semiconductor materials. Organic monolayers directly attached to silicon have been eyed initially as an alternative insulator to traditional oxide films,1 which are approaching their theoretical thickness limit.2,3 More significantly, combining the electrical or optical functionality of the wide library of organic molecules, with wellcharacterized semiconductor substrates, is expected to grant novel applications, for example, in biochemical sensing devices (i.e., the preparation of silicon-based DNA hybridization microarrays).4–6 Covalently bonded, organic monolayers on silicon can be readily prepared from the photochemical, thermal, or electrochemical reactions of hydrogen-terminated silicon (H-Si) with diacyl peroxides, 1-alkenes, or Grignard reagents.1,7–11 In the past decade, the structural properties of such organically modified silicon surfaces have been investigated thoroughly via atomic force microscopy (AFM) to examine surface topology,9a ellipsometry to evaluate the monolayer thickness,8b wetting measurements to check the surface polarity,12 X-ray photoelectron spectroscopy to determine the composition of the monolayer,8,9a and vibrational spectroscopy (FT-IR, sum frequency generation spectroscopy) to probe their structural order.12,13 Electrical characterization has been explored as a sensitive tool to examine organically modified semiconductor surfaces and the effect of organic monolayers on the interfacial electron transport. In particular, current density-voltage (J-V) and capacitancevoltage (C-V) measurements using mercury drops as the top electrode contact have shown that alkyl monolayer-modified junctions possess higher effective barrier heights, lower ideality factors (i.e., close to unity), and better reproducibility than those * To whom correspondence should be sent. E-mail:
[email protected] (H.Y.);
[email protected] (K.K.). † Department of Chemistry. ‡ Department of Physics.
of native oxide thin films.14–16 The high surface tension of the mercury drop apparently inhibits metal film interaction and subsequent disruption of the monolayer.14d For electronic devices to attain widespread practical applications, however, a stable, reliable method of making electrical contacts to the monolayers must be found. The most popular option is to use vacuum deposition to create a metal pad on the surface. There is much spectroscopic evidence that organic monolayers remain after such metal evaporation, although structural disruption was detected;17 and in some reports thermal deposition processes have destroyed the monolayer.18 However, most of these studies were based on macroscale measurements, which can only determine spatially averaged structural/electrical properties. Monolayers may contain defects such as pinholes, which can provide pathways of lower resistance for flow of electrical charge, sites for oxidation, and locations where deposited metals may penetrate the monolayer. The effects of the molecule-substrate bonding has yet not been investigated thoroughly for organic monolayers, and the majority of previous studies have focused on oxidized silicon where the molecular bonding is via C-O-Si bridges.17–24 In contrast, studies of metal contacts to Si-C bonded monolayers on oxide-free silicon are very few.18 One technique that is capable of probing the nanoscale electrical properties of such devices is ballistic electron emission microscopy (BEEM), a modification to scanning tunneling microscopy (STM).25 For a BEEM experiment, STM measurements are carried out on a grounded, thin metallic layer (typically
