J. Phys. Chem. C 2009, 113, 6163–6168
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Tracing the Mechanism of Molecular Gated Transistors O. Shaya,† M. Shaked,† Y. Usherenko,† E. Halpern,† G. Shalev,‡ A. Doron,‡ I. Levy,‡ and Y. Rosenwaks*,† School of Electrical Engineering, Faculty of Engineering, Tel-AViV UniVersity, Ramat-AViV, 69978, Israel, and Intel Research Israel, Intel Electronics, Jerusalem 91031, Israel ReceiVed: January 14, 2009; ReVised Manuscript ReceiVed: February 3, 2009
In order to understand the mechanism of biosensing of field-effect-based biosensors and optimize their performance, the effect of each of its molecular building blocks must be understood. In this work the effect of the self-assembled linker molecules on the top of a biofield-effect transistor was studied in detail. We have combined Kelvin probe force microscopy, current-voltage measurements, and device simulations in order to trace the mechanism of silicon-on-insulator biological field-effect transistors. The measurements were conducted on the widely used linker molecules (3-aminopropyl)trimethoxysilane (APTMS) and (11-aminoundecyl)triethoxysilane (AUTES), which were self-assembled on an ozone-activated silicon oxide surface covering the transistor channel. The work function of the modified silicon oxide decreased by more then 1.5 eV, while the transistor threshold voltage increased by about 30 V following the self-assembly. A detailed analysis indicates that these changes are due to negative-induced charges on the top dielectric layer, and an effective dipole due to the polar monolayer. The results were compared with metal gated transistors fabricated on the same die, and a factor converting the molecular charge to the metal gate voltage was extracted. 1. Introduction A biosensor is an analytical device that translates a biological reaction into a signal that can be quantified and processed,1 and thus can be used to sense molecules of interest when interaction between the sensor and an analyte takes place.2 Field-effect biosensors (bioFETs) are particularly attractive for converting biological information directly to electrical signals because they are essentially sensitive to the presence of electrical charge and are inherently superior to other electronic methods because of their intrinsic gain. The vast majority of biomolecules are charged at physiological conditions, indicating that field-effect devices can provide a very general and efficient method for detecting a wide variety of biomolecules.3,4 The basic principal of the bioFET can be understood with the help of Figure 1a in which a cross-section of such a device is shown. The field-effect transistor (FET) shown is separated from the silicon bulk by a thick oxide layer, and a molecular layer replaces the top metal gate. Each molecular interaction or molecule adsorbed on the surface will result in a change in channel surface potential thus changing the threshold voltage Vth and consequently the i-V characteristics. Despite the wide interest and importance of these field-effect biosensors, the mechanism by which the sensing molecules modulate the device channel is still unclear, and correlation with molecular properties has hardly been reported. Initial attempts were made, and it was already demonstrated that charged molecules such as DNA can change the gate potential thus modulating the channel.5 In another example, the correlation between the threshold voltage of a FET and the polarity of a molecular functional group was demonstrated and explained by charge transfer from the grafted molecules to the silicon channel.6 Yang et al. have measured a positive Vth shift following * To whom correspondence should be addressed. E-mail: yossir@ eng.tau.ac.il. † Tel-Aviv University. ‡ Intel Electronics.
adsorption of spiropyran and have attributed this to a protonation and consequently positive charging of the SiO2 surface.7 However, for the nonpolar monolayer, the molecular effect is still unclear. In nanowire-based FETs, the mechanisms may be different and other mechanisms may dominate; in addition, the current depends on the nanowire diameter and exact geometry.8-14 Several electrostatic scenarios are plausible when a molecule is adsorbed on the top dielectric layer of a field-effect transistor. First, a change in the channel surface band bending can be due to molecule-induced surface states.15 In addition, adsorption of a molecular monolayer on a semiconductor can create a surface dipole, which will change the electron affinity depending on the dipole orientation.16 The surface dipole is governed by two main factors: (a) the molecular permanent dipole; (b) a formation of a polar chemical bond between the molecule and the surface. We note that an ideal dipole layer will not change the channel band bending and the transistor threshold voltage; however, organic monolayers often form domains,17 and it has been shown that finite-domain monolayers can readily explain experimentally observed field effects, unexpected with ideal polar monolayers.18 In addition, when devices are operated in an ionic liquid, the redistribution of ions in the intermolecular spaces within the adsorbed molecular layer19 has also to be considered. One more possibility is the depolarization of the self-assembled molecules due to the very large electric fields induced in the layers.16 The prime goal of this manuscript is to understand the operating mechanism of such molecular gated transistors. To achieve this goal we combine nanoscale potential measurements using Kelvin probe force microscopy (KPFM) of the transistors channel with macroscopic electrical measurements, appropriate device simulations, and comparison with identical metal gated devices. We focus on (3-aminopropyl)trimethoxysilane (APTMS) and (11-aminoundecyl)triethoxysilane (AUTES) which are the most widely used linker molecules. It is found that the work function (WF) of the device after self-assembly of these common
10.1021/jp900382v CCC: $40.75 2009 American Chemical Society Published on Web 03/20/2009
6164 J. Phys. Chem. C, Vol. 113, No. 15, 2009
Shaya et al.
Figure 1. A schematic cross-section of (a) a SOI field-effect biosensor and (b) a typical SOI MOSFET.
silane linkers is decreased considerably while the threshold voltage is increased. 2. Sample Preparation and Experimental Setup A. Sample Preparation and APTMS Modification. The samples studied in this work included low doped (p ∼ 1 × 1015 cm-3) bare Si 〈100〉 wafers covered with a 50 Å layer of thermal SiO2 and 90 Å of Si3N4, and fully depleted silicon-on-insulator (SOI) bioFET devices (fabricated by Intel Research Israel Laboratory, Jerusalem), as shown schematically in Figure 1a. Two types of devices were used. In type I devices the SOI thickness was 40 nm, the buried oxide (BOX) layer was 400 nm, and the top gate dielectric was 40 nm SiO2. In type II devices, SOI thickness was 30 nm, BOX was 1000 nm, and the top gate dielectric was a 50 Å layer of thermal SiO2 covered with 15 nm of Si3N4 and 15 nm of SiO2. In order to remove organic contamination from the surface, the samples were first cleaned by immersion in three different organic solvents: hexane, acetone, and ethanol; for the type I devices, this was followed by 20 s etching in dilute (2%) HF (to remove the oxide) followed by a 5 min immersion in deionized (DI) water to remove HF residues. The samples were then inserted for 30 min into an ultraviolet ozone cleaning system (UVOCS) in order to increase the surface density of hydroxyl (OH) groups, thereby increasing the APTMS surface modification efficiency. For the type II devices, the solvent cleaning was followed by 45 min in the UVOCS. The UVOCS activated samples were then placed in a 1% v/v APTMS or AUTES in a 95%/5% v/v EtOH/DI solution and subsequently placed into a N2/vacuum oven for 20 min at 100 °C. This resulted in a self-assembled molecular gated device. B. Samples Characterization. The wafers’ and devices’ surface potential was measured following each modification step by KPFM. The KPFM measures the contact potential difference, VCPD, defined as: VCPD ≡ - (φt - φS)/q where q is the elementary charge, φt is the WF of the tip, and φS is the WF of the sample.20 The measured work functions were calibrated against that of highly oriented freshly peeled pyrolytic graphite
(HOPG), with a known work function of 4.6 eV.21,22 The KPFM setup was based on two commercial atomic force microscopes (NTMDT Solver 47H pro, VEECO Dimension 3100) where the electrostatic force is measured in the ’lift mode’.20 The devices i - V characteristics were measured simultaneously by using a semiconductor parameter analyzer (Agilent 4155C). The i - V curves were used to extract the transistor back gate threshold voltage, Vth-b, by extrapolating the linear region of the drainto-source current. All measurements were carried out in a glovebox maintaining a dry nitrogen environment (
