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Electrostatic Properties of Silane Monolayers in an Electrolytic Environment E. Halpern,† B. Khamaisi,† O. Shaya,† G. Shalev,‡ 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: May 18, 2009; ReVised Manuscript ReceiVed: August 9, 2009
Self-assembled silane molecules are commonly used in a wide variety of electrolyte-insulator-silicon fieldeffect transistors. We combine capacitance-voltage measurements with equivalent-circuit modeling to study such a structure, where the main steps of the self-assembly process, namely surface solvent-cleaning, surface activation, and self-assembly, were sequentially characterized. It is found that UV radiation (254 nm) used in the cleaning and hydroxylation induces interface states between the Si and SiO2 insulator with a concentration around 1013 cm-2. When the UV radiation is blocked, the interface states concentration is reduced by 1 order of magnitude. Subsequent self-assembly of polar monolayers of 3-aminopropyltrimethoxysilane and 11-aminoundecyltriethoxysilane do not induce any change in the flat-band of the Si when in contact with an electrolyte, in contrast with the same monolayers measured under dry conditions. This is attributed to electrostatic screening by the electrolyte. Introduction Chemical sensors based on field-effect transistors (FET) have been studied quite intensively in recent years.1-3 These systems are also referred to as electrolyte-insulator-semiconductor field-effect transistors (EISFET). The EISFET has a molecular functional layer instead of a top metal gate as in the conventional metal-oxide-semiconductor field-effect transistor (MOSFET); a chemical interaction or molecule adsorption on the surface will result in a surface potential change and a subsequent change in the threshold voltage and modulation of the drain-source current of the transistor. The sensitivity of the FET’s surface to analytes (usually biomolecules) is improved by covering it with an organic monolayer (ML), by the well-known self-assembly process.4,5 For the widely used top dielectric SiO2, these MLs are typically silane molecules, having different kinds of organic backbones and terminations. 3-Aminopropyltrimethoxysilane (APTMS) and 11-aminoundecyltriethoxysilane (AUTES) are two polar molecules, with a very good affinity to attach to derivatized Si, via the self-assembly reaction. The MLs they form have proved very successful for attaching a subsequent molecular layer with a high affinity for various analytes. However, their role in the sensing mechanism of field-effect devices in electrolytes is still unclear. Previous reports on ML modification of oxidized semiconductors under dry conditions have shown a large effect on the electron affinity of the underlying semiconductor as a function of the dipole moment of the individual molecule.6-10 Recently, we have shown that such linker molecules both induce a field effect, i.e., band-bending in the underlying semiconductor, and also change its electron affinity.9 However, their electrostatic behavior when in contact with an electrolyte, as in the case of ion-sensitive devices, has not been reported yet and this is the main objective of this work. We present capacitance-voltage (C-V) measurements in a phosphate-buffer electrolyte, supported by an equivalent-circuit analysis of silicon wafers, at * To whom correspondence should be addressed. E-mail: yossir@ eng.tau.ac.il. † Tel-Aviv University. ‡ Intel Electronics.
every step of the preparation of the MLs. It is found that the cleaning process involving UV radiation induces a large concentration of interface states between the dielectric and the semiconductor. Albeit, the MLs are not showing any significant influence on the semiconductor’s band-bending, in apparent contrast to the results obtained under dry conditions. Sample Preparation and Experimental Setup The samples studied are low doped (p0 ≈ 1015 cm-3) Si wafers covered with a 20 Å layer of thermal SiO2 and 90 Å of Si3N4. To remove organic contaminations from the surface, the samples were first cleaned by immersion in three dry organic solvents: hexane, acetone, and ethanol. The samples were then inserted for 45 min into an UV ozone cleaning system (UVOCS) in order to increase the surface density of hydroxyl groups, thereby increasing the surface modification efficiency.11 Two kinds of silanes were covalently attached to the surface: 3-aminopropyltrimethoxysilane (APTMS) and 11-aminoundecyltriethoxysilane (AUTES). Aminosilanes were purchased from Gelest Inc., and used as received. The UVOCS-activated samples were placed in a reactive solution comprised of 1% APTMS/AUTES and 99% ethanol (volumetric). Following 10 min of reaction the samples were placed in a N2 oven for 20 min at 100 °C, after which the self-assembled monolayers were formed. The layers were characterized by AFM, contact angle, ellipsometry, and XPS. The measurements were conducted in a three-electrode electrochemical cell, placed inside a light-sealed chamber. The reference electrode was Ag/AgCl and a Pt wire served as the counter electrode. The Pt wire had a surface area larger by a few orders of magnitude than the sample to avoid nonequilibrium phenomena. All the measurements were performed with use of an impedance analyzer (Princeton Applied Research) possessing a low noise level (∼5 pF). The main measurement frequency was 20 kHz (AC amplitude 10 mV rms), which was found to be high enough to avoid inversion in the silicon (a prerequisite for Mott-Schottky analysis12). The electrolyte was phosphate buffer, pH 7, from J. T. Backer, which was purchased in liquid
10.1021/jp904614c CCC: $40.75 2009 American Chemical Society Published on Web 09/01/2009
Electrostatic Properties of Silane Monolayers
J. Phys. Chem. C, Vol. 113, No. 38, 2009 16803 can be attributed to surface states or faradic currents,13 the other anomaly is most probably due to the presence of trapped charge induced by the UV activation process. It is well-known that UV radiation generates electron hole pairs that are trapped both at the Si/SiO2 interface and in the Si3N4 layer.12 This additional charge was modeled as a capacitor (Cit-Activated) in parallel to the space charge region capacitor (Csc).12 Our analysis is based on a standard equivalent-circuit model of a voltage sweep in the range of Vg, the applied voltage between a bulk semiconductor and the counter electrode, at a constant high frequency. The frequency is sufficiently high to prevent the formation of an inversion layer in the Si. The applied potential is distributed across the Si space-charge region, the insulator layer (SiO2 and Si3N4), and the solution diffusion-Helmholtz layer, i.e., Vg - VFB ) Ψs + Vins + VHelm + VDiff, where Ψs is the Si surface potential, VFB is the Si flatband voltage, Vins is the potential drop on the insulator, VHelm is the potential drop on the Helmholtz layer, and VDiff is the potential drop over the Diffusion layer. The voltage drop over the Helmholtz layer can be neglected due to the low ionic strength of the electrolyte.14-16 In the equivalent-circuit analysis the various static capacitances can be wisely guessed, but the space charge region capacitance,Csc, is dynamic and hence must be calculated. We have used a model for high-frequency in metal-oxidesemiconductor (MOS) devices17 that solves the 1D-Poisson equation by using a small-signal analysis to obtain the semiconductor charge, Qsc(Ψs), as a function of its surface potential. The small-signal approximation is justified for our impedance measurements due to the small rms amplitude of the applied AC voltage.17 Csc(Ψs) is obtained by differentiating Qsc(Ψs) (with respect to Ψs), but in order to perform the equivalentcircuit fitting it has to be transformed to Csc(Vg). This is done by using an equation relating Ψs to Vg, taking into account the diffusion layer capacitance, thus describing the voltage drop over the different parts of the system as:
Vg )
Figure 1. (a) The total capacitance as a function of Vg, plotted for clean (0, blue line) and activated (2, red line) samples. The lines are the values calculated by using the equivalent circuits in panels b and c, respectively. (d) Interface states density Dit as a function of band gap energies for a clean sample (dashed curve) and a UV activated (solid curve) samples.
form (Sigma Aldrich Inc.) and diluted from ionic strength (aion) 200 mM to aion ) 40 mM. Results and Discussion A. UV Activated Si Wafer. Figure 1a shows the measured C-V curves for clean (squares) and activated (triangles) wafers. The solid lines represent the calculated capacitance from the equivalent-circuit for clean (Figure 1b) and activated wafers (Figure 1c). The C-V curve of the clean wafer exhibits a typical high-frequency behavior that saturates in the accumulation region (-2 V < Vg < -1 V). On the other hand, the activated wafer shows no saturation at negative gate voltage, and a large stretching of the depletion region. While the lack of saturation
[
]
εsemidins 1 Qsc(Us) kT ¯ Us + U F(Us) + UFB + s · q εins LD CDiff (1)
where k is Boltzmann’s constant, T is the absolute temperature, and q is the elementary charge. F(Us) is the function describing the dimensionless electric field at the Si surface,18 Us is the dimensionless surface potential of the Si (normalized to kT/q), j s is a parameter determining whether Vg is above or below U j s ) +1, otherwise U js the flat-band voltage (if Vg > VFB then U ) -1), LD is the extrinsic Debye length in the Si, CDiff is the diffusion capacitance, dins is the width of insulator, and ε is the dielectric constant. The combination of eq 1 with the smallsignal MOS model can be solved numerically to obtain Ψs(Vg), which is then used to transform Csc(Ψs) into Csc(Vg). The presence of interface states affects the variation of Ψs as a function of Vg. Charge neutrality in the system requires Qele + Qit + Qsc ) 0, assuming no charges are present in the insulator (Qele, Qit, Qsc are the charges in the electrolyte, Si/ SiO2 interface, and the Si space charge region, respectively). This means that driving the Si from accumulation to depletion requires a larger gate voltage due to the interface states, which results in stretching of the C-V curve.12 To account for interface states, the term Qsc(Ψs)/CDiff in eq 1 is replaced by (Qsc + Qit)/ CDiff (both terms in the nominator depend on Ψs). Our numerical simulations showed that any reasonable value assigned to CDiff has no significant effect on the total capacitance:
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J. Phys. Chem. C, Vol. 113, No. 38, 2009
CT ) (Csc + Cit)
Cins Csc + Cit + Cins
Halpern et al.
(2)
where Cins is the known insulator capacitance. Hence, the only fitting parameter is the interface states capacitance (Cit). Cit consists of two parallel capacitors: Cit-Clean and Cit-Activated, where Cit-Clean is the interface states capacitance prior to the UV activation. Dividing Cit by qA (A is the sample area) we obtain the density of interface states Dit [in eV-1 cm-2]. The function Dit(Vg) is transformed to Dit(Ψs), using the function Vg(Ψs), which is the interface states distribution inside the Si band gap. This distribution was calculated only up to the flat-band potential (200 mV above VFB); this is because at larger applied potentials the surface Fermi level of the Si is pinned, as evident from the C-V plots.19 Finally an estimation of the total interface states, Nit [in cm-2], is obtained by integration over the aforementioned energy range as ∫Dit(E) dE. We note that this analysis accounts only for fast interface states, i.e., those that respond to frequencies higher than 20 kHz. C-V measurements conducted at lower frequencies showed clear evidence for additional states, but this is outside the scope of this work. It is possible that the UV radiation induces trapped charges inside the Si3N4 layer. However, in such a case a large hysteresis is expected in the measured C-V curves;12 since no hysteresis was observed at high frequencies, we conclude that the UV radiation induced states mainly at the Si/SiO2 interface. As shown in Figure 1a, the fit obtained with the equivalent-circuit Figure 1c is excellent. Figure 1d shows the interface states distribution within the Si band gap. The largest states concentration for both cases at negative gate voltages corresponds to the accumulation regime. ≈ 1012 cm-2 and NActivated Integration of this curve gives NClean it it ≈ 1013 cm-2 for the clean and activated samples, respectively; both results are in good agreement with previous reports.20-22 The excellent fit validates that these traps are interfacial, rather than at the sample surface. This implies that their charge is affecting mainly the semiconductor and not the insulator-solution interface. Electronic states located at the top surface would encourage electrochemical reactions and also will have a weaker effect on the Si band bending.12,15 Another consequence of the fit is that any attempt to extract the doping and the flat-band potential (VFB) from the linear C-V curve by the Mott-Schottky method would result in a large error, as explained below. The Mott-Schottky (M-S) analysis is commonly used to extract doping and VFB from electrolyte-insulator-semiconductor (EIS) systems.12,15 It relies on the following relation, based on the depletion approximation:
CT-2 )
-2 kT V - VFB + 2 g q qεSiNAA
(
)
(3)
where NA is the doping concentration. By plotting CT-2 versus Vg, the doping concentration is extracted from the slope of the curve, and VFB from its intersection with the Vg axis. The M-S relation is based on the assumption that in depletion, the semiconductor space-charge capacitance is the smallest one, and thus the most dominant. Various authors23-25 have described the difficulties in EIS systems that distort or modify the linear M-S regime. We ensured that the M-S regime indicated the correct doping and flat-band voltage by using a thin dielectric and a low ionic strength electrolyte. The thin dielectric ensures a high constant capacitance, which does not affect the depletion capacitance, and the low ion concentration ensures that the
Figure 2. (a) The total capacitance as a function of Vg, plotted for clean (0, blue line), APTMS without UV-induced interface states (2, red line), and APTMS with UV-induced interface states (b, black line) samples. The lines indicate the values calculated by using the equivalent circuits in panels b and c in Figure 1 and panel b, respectively.
electrolyte behavior is static (with respect to the chemical modification of the surface). The measured accumulation capacitance of 0.47 µF · cm-2 is exactly the static capacitance of the dielectric; this supports our assumption that the capacitance of Helmholtz and diffusion layers is negligible. Moreover, the reliability of the wet C-V data was checked by changing the pH of the electrolytes and measuring the change in the flatband voltage. We have found a pH sensitivity of 55 mV/pH in the flat-band voltage, which is in agreement with previously reported results.26 The M-S curve of the clean wafer corresponds to a doping of 4.5 × 1014 cm-3, which is in good agreement with the manufacturer’s estimation. The M-S curve of the activated wafer shows that the slope of the depletion regime is changed significantly, though the linear relation is preserved. This is a clear consequence of the induced interface states. B. Self-Assembled Monolayers. Following the UVOCS treatment, the wafers were self-assembled with APTMS and AUTES layers. Figure 2a shows the measured (symbols) and calculated (solid lines) C-V plots for the wafer modified with APTMS, clean and activated wafers. The organic ML is modeled by a static capacitance, CML, with a dielectric constant εML ) 2.327 connected in series to CDiff and Cins, as represented by the equivalent-circuit of Figure 2b, and given by: CT )
(
1 1 1 1 + + + CDiff Cins CML Csc + Cit-Clean + Cit-Activated
)
-1
(4)
The results clearly indicate that the interface states were not suppressed following the self-assembly, as evident from the
Electrostatic Properties of Silane Monolayers
J. Phys. Chem. C, Vol. 113, No. 38, 2009 16805 the ML, or both.28,29 By fitting these curves, it was found that the value of CDiff required to obtain a good fit is unreasonably low, while a ML capacitance requires a much more conceivable value. We note that for measurements conducted at higher frequencies (up to 400 kHz) the accumulation capacitance decreased by up to an order of magnitude. While normally this could be attributed to electronic states anywhere in the structure, the undistorted general shape of the C-V curves requires a different explanation. It was previously suggested30 that electrode roughness can be a major cause for frequency dispersion. AFM images of our samples showed low (0.5 nm rms) roughness, which cannot explain these results, and this issue is currently under investigation. As can be seen in the M-S plot (Figure 3c), the slopes of all curves are the same, indicating no change in the doping concentration. All the M-S curves have the same interception with the Vg axis, i.e., the flat-band voltage does not change with the ML deposition, within the measuring error. The flat-band voltage is given by:29
VFB ) Eref + χsol + χML - Ψ0 -
ΦSi Qins + Qss q Cins
(5)
Figure 3. (a) The total capacitance as a function of Vg, plotted for clean (0, blue line), AUTES (2, light green line), and APTMS (b, dark green line) samples, without UV-induced interface states. The lines indicate the values calculated by using the equivalent circuit in Figures 1b and 3b, respectively. (c) Mott-Schottky plots for clean (blue line), AUTES (light green line), and APTMS (dark green line) without UVinduced interface states.
“stretching” of the C-V curve. Fitting the bottom curve in Figure 2a resulted in a negligible change in the concentration of the interface states. To eliminate UV-induced interface states, the UVOCS treatment was carried out with a UV filter in front of the sample, allowing sample exposure to the ozone atmosphere. Figure 3 shows the calculated (solid line) and measured (symbols) C-V curves for self-assembled APTMS and AUTES, in comparison with the UVOCS-cleaned wafer. It is observed that no interface states were induced during the UVOCS treatment, and also that the MLs do not induce additional interface or surface states. A decrease in accumulation capacitance is observed for the modified samples. This shift can be due to a change in the diffusion capacitance, addition of a series capacitance due to
where Eref is the contribution of the reference electrode, χsol is the potential drop over the surface dipole of the solution (in contact with the dielectric), χML is the potential drop over the organic ML, ΦSi is the silicon work function, Ψ0 is the potential drop in the liquid interface, and Qins and Qss are the insulator and surface-state charge density, respectively. All the terms apart from χML, χsol, and Ψ0 are not expected to change following the self-assembly process. In the later, the dipolar term χML is considered positive when the dipole is oriented outward from the solid surface. CDiff and CML were found very large compared to the depletion capacitance (0˜.015 µF · cm-2). If we assume that χsol does not change significantly following self-assembly, then only χML and Ψ0 can cause changes in the flat-band voltage. The fact that VFB did not change following the self-assembly means that the band-bending did not change; this is an apparent contradiction to previous reports9,31,32 that have shown that an organic ML on the same dielectric under dry conditions induces surface charges and a dipolar term of 1 V.33 However, reports on various MLs self-assembled on Si, in contact with an aqueous electrolyte, do not show any marked change in VFB; results range between zero13,34 and tens of millivolts.35,36 The flat-band voltage change can be affected by the electron affinity change of the Si (or, equivalently, of the electrolyte) and the electrolyte surface potential change, due to the difference between the two point-of-zero charges (PZC) of the measured Clean surfaces: ∆VFB ) VML FB - VFB ) χML + ∆Ψ0. If the two surfaces are measured at the same pH and ionic strength, the difference in the flat-band voltage between the measurements, due to proton association-dissociation equilibrium, follows from the sitePZC PZC - AClean · pHClean , where we binding model37 as AML · pHML assume for simplicity a constant pH-sensitivity (A) for both surfaces. Using this assumption, and reported values for the PZC of each surface (7.5 for APTMS38 and 3 for Si3N426), then this term can contribute around 55mV/pH · (pH 3 - pH 7.5) ≈ -250 mV for the change in VFB; a pH-sensitivity of 55 V/pH was measured for our samples. In addition, the contribution of the ML dipole may be screened by dipolar and ionic interactions with the electrolyte;
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Halpern et al. of-zero charge of the measured surface following the selfassembly process. References and Notes
Figure 4. Schematic representation of the filling of monolayer vacancies by the electrolytesa possible reason of depolarization of the organic layer.
if the ML’s possess large dipole moments then the water mobile dipoles will tend to depolarize them (see Figure 4). This effect will be significant at moderate to low ML coverage, so we cannot quantify this effect and further experiments are underway to study this phenomenon. According to our measurements conducted under dry conditions, the dipolar term χML contributes a positive value,33 which means that it has the opposite effect on VFB relative to the PZC difference (∆Ψ0). If the dipolar term is indeed lower than the estimated 1 V under wet conditions (due to screening), then ∆Ψ0 and χML can cancel each other to give a negligible net change in the flat-band voltage. In eq 5 we assume that the ML is a continuous perfect layer. However, we have observed using AFM measurements that the MLs are composed of domains separated by pinholes. The ML coverage can be estimated from the measured C-V curves: since the accumulation capacitance represents only the static capacitors in the system, any change in its value (taken from the most negative applied voltage) can be interpreted as a change in the effective area of the ML, as deduced from CML ) ε0εMLAML/ dML, compared with the physical contact of the electrolyte with the sample.13 As stated earlier, the dielectric constant of the MLs is assumed to be 2.3, while its thickness (dML) is 0.8 nm for APTMS and 1.5 nm for AUTES, as was measured by using ellipsometry. It is found that both MLs are covering around 98% of the Si3N4 surface, as shown schematically in Figure 4. Conclusions We have conducted wet capacitance-voltage (C-V) measurements of Si/SiO2/Si3N4 wafers, modified with self-assembled organic polar silane monolayers. The main steps of the selfassembly process, namely surface solvent-cleaning, surface activation, and self-assembly, were sequentially characterized. When comparing the solvent-clean and activated stages, we have found that the UV radiation used in the activation process has introduced electronic states between the SiO2 and the Si. By fitting the C-V data to an equivalent-circuit, these interface states amount to 1013 cm-2; they were eliminated by using a glass UV filter. The polar monolayers, APTMS and AUTES, did not introduce interface states, and also no surface charge, in contrast with measurements conducted under dry conditions.9 In addition, it was found that the monolayers are covering 98% of the wafer’s surface. As a consequence, we conclude that the band-bending of the Si did not change following the selfassembly process. We attribute this to the combined effect of dipolar screening by the electrolyte, and the change in the point-
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