J. Phys. Chem. C 2008, 112, 7145-7150
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Electronic Characterization of Si(100)-Bound Alkyl Monolayers Using Kelvin Probe Force Microscopy I. Magid,† L. Burstein,‡ O. Seitz,§ L. Segev,§ L. Kronik,§ and Y. Rosenwaks*,† School of Electrical Engineering-Physical Electronics, Faculty of Engineering, Tel-AViV UniVersity, Ramat-AViV 69978, Israel, Wolfson Applied Materials Research Center, Tel AViV UniVersity, Ramat AViV 69978, Israel, and Department of Materials and Interfaces, Weizmann Institute of Science, RehoVoth 76100, Israel ReceiVed: October 13, 2007; In Final Form: February 17, 2008
Hydrogen-terminated and alkyl-chain (C18H37)-terminated Si(100) surfaces with different doping levels have been characterized using Kelvin probe force microscopy. n- and p-doped Si(100) and lateral p++n and n++p silicon junctions were hydrogenated in dilute HF solution, followed with a self-assembly deposition of organic molecules by thermally activated free-radical reaction between CdC and Si-H. The surface band bending following the two different chemical treatments was almost identical for both p-type silicon (∼0.7 eV, with a surface charge of 9.4 ( 0.5 × 1011/cm2) and n-type silicon (0.6 eV, with a surface charge of 8.7 ( 0.5 × 1011/cm2). These results indicate that the self-assembly of the C18H37 monolayer on a Si (100) surface results in electrical properties similar to those of a hydrogenated Si surface, with the advantage of longer stability in an ambient environment. The hydrogen-terminated and alkyl-chain-terminated surface do differ, however, in the surface dipole, which is lower by ∼0.6 eV for the latter, a value deduced from both the measurements and independent first principles electronic structure calculations. This dipole change is essentially due to the change in bond dipole associated with the replacement of Si-H bonds by Si-C bonds and the dipole associated with the methyl group.
1. Introduction An organic monolayer on a solid substrate offers a direct combination of molecular and solid materials, which will likely be a key component in future chemical and biological nanosensors and in molecular electronics.1 Organic monolayers attached on metals, e.g., self-assembled monolayers (SAM) on gold, have been studied and characterized in detail. Organic monolayers self-assembled on silicon may be more important in practical applications because of the use of silicon technology in microelectronics. Alkyl monolayers may be able to replace the silicon oxide layer on the silicon surface as an insulating layer; furthermore, the alkyl chain can be easily functionalized through molecular organic synthesis.2-10 The surface state concentration and energy distribution at the organic-semiconductor interface determine the behavior and performance of any electronic device based on such an interface. Indeed, many studies of the relations between the chemical and electronic properties of alkyl/Si interfaces have been performed in recent years. Royea et al.3 have measured the surface recombination velocity of Si (111) surfaces following alkylation under high and low carrier injection levels. They have calculated a residual surface trap density smaller than 3 × 109 cm-2 that has been stable for more than 4 weeks in air. Salomon et al.11 have used current-voltage (I-V) measurements in order to study the nature of electron transport through long alkyl chain molecules with different lengths in n-Si (111)/CnH2n+1//Hg junctions. Using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) techniques, they * Author to whom correspondence should be addressed. † School of Electrical Engineering-Physical Electronics, Tel-Aviv University. ‡ Wolfson Applied Materials Research Center, Tel-Aviv University. § Weizmann Institute of Science.
have also measured the band bending of a Si-C14H29 structure and found that the band bending of the n-Si was 0.55 eV. Seitz et al.12 have used I-V measurements, contact angle measurements, and XPS to demonstrate the importance of the alkyl monolayer quality on an oxide-free Si (111) surface for interpreting current transport through alkyl chains in n-Si/ CnH2n+1//Hg junctions (n ) 12, 14, 18). They have shown that solid-state current transport characterization is a very important quality control tool, which is very sensitive to surface oxidation and contamination at the semiconductor molecule interface, as well as to the packing of alkyl chains in the monolayer. Kar et al.13 and Faucheux et al.14 have used capacitancevoltage (C-V) measurements to measure the electronic traps at the interface between Si(111) and an octadecene-based organic monolayer (Si-C18H37). They have found that for n-Si, with a doping level of about 1017/cm3, the interface charge was ∼1012/cm2, whereas for p-Si with the same doping level, the interface charge was 2 × 1011/cm2. Miramond et al.15 have shown that octadecene-based monolayers on n-, p-, and p+Si(111) were densely packed and that the monolayers formed on n+-type silicon were more disordered and therefore exhibited larger leakage current densities when embedded in a silicon/ monolayer/metal junction. Despite the extensive studies of electronic properties and current transport through alkyl monolayers grown directly on silicon substrates (a small sample of which has been surveyed above), to the best of our knowledge there are hardly any studies which determine the band bending and surface state densities at Si/alkyl interfaces in a contactless mode, without using a metal top contact layer on top of the alkyl monolayer. In this paper, we present band bending and surface states concentration of Si(100)-alkyl interfaces with different Si doping levels measured using a contactless Kelvin probe force microscope
10.1021/jp709973d CCC: $40.75 © 2008 American Chemical Society Published on Web 04/15/2008
7146 J. Phys. Chem. C, Vol. 112, No. 18, 2008
Magid et al.
Figure 1. Schematic sample structure, following deposition of an octadecyl monolayer.
(KPFM). We found the band bending and surface state concentration at the hydrogen-passivated Si(100) surface to be insensitive to the silicon doping level and to be identical to that found at the Si(100)-C18H37 interface. We interpret this as an indication of an almost perfect substitution of the Si-H bonds that are replaced by Si-C bonds. 2. Experimental Details Four different silicon wafers were used: n-type Si with a P dopant concentration of 1-1.3 × 1017/cm3, p- type silicon with a B dopant concentration of 4 × 1017/cm3, n++p Si lateral junctions with a degenerate n++ (P) concentration of 4 to 6 × 1019/cm3, and p++n Si lateral junctions with a degenerate p++ (B) concentration of 1-1.3 × 1019/cm3. The Si lateral junctions were supplied by the Institute of Electron Technology (ITE Warszawa, Poland) and consisted of highly doped silicon strips (5 µm wide), separated by 20 µm wide low-doped stripes on the sample surface. Prior to chemical functionalization, each sample was cleaned by a sequential rinse with hexane, acetone, and ethanol, followed by 1 h dipping in piranha solution (H2O2:H2SO4 3:7 v:v) heated to 90 °C, rinsing in distilled water, and drying with a stream of N2. The samples were then immediately placed in 10% HF solution for about 15 s to etch the top oxide layer and produce an H-terminated Si (100) surface. The samples were then rinsed thoroughly with distilled H2O, dried under a stream of N2, and transferred immediately into a N2 glove box for the KPFM measurements. For alkylation, the H-terminated Si(100) samples were immersed in C18H36 inside a glove box under nitrogen environment, in order to prevent the formation of SiO2 at the silicon/monolayer interface; the reaction solution was heated to 200 °C for 4 h. The samples were then removed from the reaction solution, ultrasonically cleaned with ethanol, immersed in boiled trichloromethane for 3 min, and dried with a stream of N2; afterward the samples were transferred to the KPFM glove box for electrical measurements. Figure 1 shows a typical asymmetric junction sample structure following adsorption of octadecene molecules. XPS measurements were performed in an ultrahigh vacuum (UHV) system (2 × 10-10 Torr base pressure) using a 5600 Multi-Technique System (PHI, Eden Prairie, MN). The samples were irradiated with an Al KR monochromatic source (1486.6 eV). A hemispherical capacitor analyzer was used to analyze the electrons coming through a slit aperture of 800 µm in diameter. High resolution measurements were performed with a pass energy of 11.75 eV. KPFM measurements were conducted using a commercial atomic force microscope (Autoprobe CP II, Veeco, Inc.) operating in noncontact mode based on a setup described previously16 inside a home-built nitrogen-containing glove box (
