Article pubs.acs.org/JPCC
Spectroscopic Characterization and Transport Properties of Aromatic Monolayers Covalently Attached to Si(111) Surfaces Yosuke Harada, Takanori Koitaya, Kozo Mukai, Shinya Yoshimoto, and Jun Yoshinobu* The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan S Supporting Information *
ABSTRACT: We fabricated self-assembled monolayers (SAMs) composed of aromatic molecules with different anchor groups on Si(111) surfaces by wet chemical reactions. We investigated the bonding structures and transport properties by spectroscopic and electrical measurements, respectively. By using simple aromatic molecules (phenol, styrene, and phenylacetylene) as initial precursors, we successfully fabricated aromatic SAMs covalently bonded to Si(111) surfaces through different anchor structures (Si−O−, Si− CH2−CH2−, and Si−CHCH−). Transmission infrared spectroscopy clarify that the phenyl rings in the SAMs are oriented almost perpendicular to the Si surfaces. High-resolution X-ray photoelectron spectroscopy reveals that the aromatic molecules attach to the Si surface with the surface coverage of ∼0.5. The experimental results of these spectroscopies lead to a conclusion that the aromatic SAMs form densely packed monolayers on Si(111). Current density−voltage measurements of Hg/aromatic SAM−Si(111) sandwiched structures revealed that the “Si(111)−O−Ph” (SAM from phenol) show higher conductivity compared with the long-chain alkyl SAM on Si(111).
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INTRODUCTION Fabrication of organic self-assembled monolayers (SAMs) on semiconductor surfaces is now a central issue in surface science and device chemistry because of its ability to impart renewed functionalities on the surfaces. Since well-ordered and dense organic SAMs can be fabricated without severe reaction conditions and expensive equipments, SAMs on a semiconductor have become attractive materials that are easily accessible model systems for both fundamental scientific research and development of practical molecular devices. Among various semiconductor−organic interface structures, silicon (Si)-organic SAM systems are undoubtedly promising candidates for future applications because of the actual proven performance of Si in today’s electronics.1 Since a simple wet etching method was established to prepare atomically flat, hydrogen-terminated Si(111)(1 × 1) surfaces,2 many papers on organic SAMs covalently attached to Si(111) surfaces have been published.3−7 Long-chain alkyl SAMs (−CH2−CH2−···−CH3) on Si(111) surfaces are the most well-known organic SAM−semiconductor systems, and their structures and properties have been thoroughly investigated.3,8−19 While alkyl chain−Si interface structures can be obtained via several synthetic routes (alkylation of halide-terminated surfaces, using Grignard reagents, photochemical hydrosilylation, etc.),4−7 the simplest way is through the thermal hydrosilylation of hydrogen-terminated Si surfaces using an unsaturated molecule (typically 1-alkene) as a precursor. Such alkyl monolayers are highly stable against © 2013 American Chemical Society
thermal and chemical decomposition due to not only the toughness of −CH2− bonds but also the direct Si−C covalent bond.9−12 From the viewpoint of device engineering, one important property of alkyl SAMs is that alkyl SAMs act as electrical insulators.13−19 The insulation properties of alkyl SAMs result from their bonding structure; an alkyl SAM consists of only σ-bonds (−CH2−). Alkyl SAMs are considered to be suitable for developing electronic devices such as metal− insulator−semiconductor (MIS) diodes,15,16 because the thickness of the insulating layer, that is, the thickness of the monolayer, can be easily controlled by using 1-alkene molecules of different lengths as the precursor. The surface properties of semiconductors modified with a SAM are governed by the intrinsic properties of organic molecules composing the SAM. Therefore, the choice of organic molecules in fabricating SAMs is very important when developing high-efficiency organic-semiconductor hybrid devices. If unsaturated bond structures such as aromatic moieties with well-defined configuration are introduced to the Si surface, noble interfacial and electrical properties may be enabled.20−25 The synthesis and structures of “aromatic SAMs” on Si(111) have been reported by several authors.26−31 However, to our knowledge there are few reports on detailed analysis for Received: October 8, 2012 Revised: March 19, 2013 Published: March 20, 2013 7497
dx.doi.org/10.1021/jp309918p | J. Phys. Chem. C 2013, 117, 7497−7505
The Journal of Physical Chemistry C
Article
Figure 1. Schematic representations of the aromatic monolayers on Si(111) studied in this article [Si(111)−O−Ph, Si(111)−C−C−Ph, and Si(111)−CC−Ph], hydrogen-terminated Si(111) [Si(111)−H], and long-chain alkyl SAM [Si(111)−C12H25].
solution (1:3 with a volume ratio of 30% H2O2/conc. H2SO4) at 90 °C for 10 min, followed by rinsing with water. The cleaned wafer was sequentially immersed in 5% HF aqueous solution (20 s) and 40% NH4F aqueous solution (17 min) to obtain the Si(111)−H wafer. The NH4F solution was deaerated by argon bubbling for 1 h before use and continuously purged with argon during the etching process. Prior to fabrication of aromatic SAMs, the solution containing the precursor aromatic compound [phenol (neat), styrene (diluted from 0.3 M in mesitylene), or phenylacetylene (diluted from 0.7 M in mesitylene)] was deoxygenated by argon bubbling for at least 30 min with heating to ∼130 °C. A freshly prepared Si(111)−H substrate was transferred into the solution from the etching solution without rinsing. The temperature of the solution was maintained at 130−140 °C for 3 h with continuous argon bubbling. After cooling, the wafer was picked up from the reaction vessel and then washed with organic solvents (for Si(111)−O−Ph, ethanol and acetone; for Si(111)−C−C−Ph and Si(111)−CC−Ph, mesitylene, 1,2dichloroethane, and ethanol) and dried under a nitrogen gas stream. We confirmed the step and terrace structures on all the prepared sample surfaces using an atomic force microscope (AFM), which indicated that homogeneous, highly ordered monolayers were fabricated on the Si(111) surface without polymerization of the aromatic molecules. An alkyl (dodecyl, −C12H25) monolayer was also fabricated in a similar manner using 1-dodecene as the precursor (reaction temperature, 180− 190 °C). Transmission FT-IR Spectroscopy. Transmission FT-IR measurements were performed using a FT-IR spectrometer (FT/IR-6100, JASCO Co.). The incident IR light was ppolarized and detected by a mercury−cadmium−telluride (MCT) detector cooled by liquid nitrogen. The sample chamber of the spectrometer was evacuated by a scroll dry pump to prevent oxidation and contamination of the sample. Double-side-polished, moderately doped n-Si(111) (1−3.5 Ωcm, 600 μm thickness) wafers cut into 10 × 25 mm2 were used as the sample substrate. The incident angle was controlled by a home-built angle-adjustable sample holder.36,37 The spectra were obtained by an average of 100 repeated alternate measurements of the sample and a reference [Si(111)/thin SiO2, prepared by treatment with Piranha solution of the Si(111)−H wafer at 90 °C], where each measurement consisted of an average of 10 scans (the total number of scans being 1000 scans) with a resolution of 4 cm−1, using a commercial shuttle device (SSH-4000, Jasco Co.). HR-XPS Measurements. HR-XPS measurements were performed at Photon Factory BL-13A (KEK-PF PAC 2009S2-007). HR-XPS spectra were measured using a Phoibos 100 (Specs) under an ultrahigh vacuum (UHV) condition (
