Schottky barrier-based silicon nanowire pH sensor

Nano Research DOI 10.1007/s12274-013-0393-8

Schottky barrier-based silicon nanowire pH sensor with live sensitivity control Felix M. Zörgiebel1,5, Sebastian Pregl1,5, Lotta Römhildt1, Jörg Opitz3, W. Weber2,5, T. Mikolajick4,5, Larysa Baraban1 (), and Gianaurelio Cuniberti1,5 1

Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, 01062 Dresden, Germany NaMLab GmbH, 01187 Dresden, Germany 3 Fraunhofer Institute IZFP Dresden, 01109 Dresden, Germany 4 Institute for Semiconductors and Microsystems Technology, TU Dresden, 01187 Dresden, Germany 5 Dresden, Germany, Center for Advancing Electronics Dresden, TU Dresden, 01062 Dresden, Germany 2

Received: 14 September 2013

ABSTRACT

Revised: 20 November 2013

We demonstrate a pH sensor based on ultrasensitive nanosize Schottky junctions formed within bottom-up grown dopant-free arrays of assembled silicon nanowires. A new measurement concept relying on a continuous gate sweep is presented, which allows the straightforward determination of the point of maximum sensitivity of the device and allows sensing experiments to be performed in the optimum regime. Integration of devices into a portable fluidic system and an electrode isolation strategy affords a stable environment and enables long time robust FET sensing measurements in a liquid environment to be carried out. Investigations of the physical and chemical sensitivity of our devices at different pH values and a comparison with theoretical limits are also discussed. We believe that such a combination of nanofabrication and engineering advances make this Schottky barrier-powered silicon nanowire lab-on-a-chip platform suitable for efficient biodetection and even for more complex biochemical analysis.

Accepted: 21 November 2013 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2013

KEYWORDS silicon nanowires, field effect transistor, sub-threshold regime, nanosensors, pH sensor, bottom-up fabrication, maximum sensitivity of sensor

1

Introduction

Biosensors relying on electrical signal readout have attracted great attention in recent decades since they can provide rich quantitative information for medical and biotechnological assays without pre-treatment Address correspondence to [email protected]

and specific labeling of analyte solutions. Sensing of chemical and biological species using field effect transistors (FET) goes back to the 1970s [1], showing that such an electronic configuration can represent a key technology in the chemical and biodetection areas because of its high sensitivity and complementary

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metal–oxide–semiconductor (CMOS) compatibility. One prominent example, a so-called ion-sensitive field-effect transistor has been used for measuring ion concentrations, namely protons in solution. In this configuration, changes in the transistor current are detected upon changes of pH of a liquid placed on the device [2–4]. At the time, this concept was a technological novelty and represented a more sensitive alternative to the existing method, pH indicators employing halochromic compounds [5]. Biological species ranging from DNA [6–10] up to proteins (isolated, and as viral surface proteins) [11, 12], cells [13], and cultured neurons [14, 15] have since been measured using FET devices, ranging from metal oxide semiconductor field effect transistors (MOSFETs) [16] to nanoribbons [17, 18], doped nanowires [19] and carbon nanotubes [20]. During the past decade one-dimensional nanostructures, in particular semiconductor nanowires, have attracted attention as highly efficient sensor elements due to their high surface-to-volume ratio and electronic properties [21–23], which enable the detection of biochemical species down to single molecules [2, 11, 12]. Some of the main issues, which impede the straightforward commercialization of nanowire-based sensor devices are related to (i) device-to-device variations in current and sensitivity of bottom-up wires, which leads to hence calibration problems, (ii) low current

output, and (iii) electronic signal drifts and quick device degradation. Here we introduce the first bottom-up fabricated Schottky barrier FET consisting of parallel arrays of silicon nanowires, suitable for robust sensing applications in a liquid environment. Furthermore, we introduce a new measurement approach making the maximum amount of information available during the experiment. The method relies on a continuous gate sweep and allows us to follow the region of highest sensitivity during the measurement. As a first application we demonstrate the performance of Schottky barrier (SB)-based silicon nanowire FET devices for sensing the pH values of a solution.

2 2.1

Results and discussion Fabrication of the Schottky barrier SiNW sensor

The fabrication procedure for the FET devices is summarized in Fig. 1. Sensor devices consist of parallel arrays of pre-assembled bottom-up fabricated Schottky barrier silicon nanowires (SiNWs), covered by a 6 nm thin layer of thermal oxide. Devices are produced at a p-doped silicon wafer with 100 nm and 400 nm back-gate dielectric thicknesses (see below). In contrast to top-down fabricated SB FETs [24], we fabricate Schottky junctions using a bottom-up approach, by

Figure 1 (a) Electron microscopy image of a parallel array of Schottky barrier silicon nanowire FETs. A single nanowire and the Schottky barriers between the silicon and nickel disilicide phases of the wires are highlighted. (b) Confocal microscope image of interdigitated electrodes (silver) with photoresist passivation (purple) and nanowires (vertical black lines). A single wire is highlighted with a red frame. (c) Chip integrated in a fluidic system and electrically contacted with the tips of a probe station and the reference electrode (marked with blue circle). Red arrows mark the fluid flow. The back gate voltage Vbg is applied to the metal base of the tip probe station (not shown). (d) Schematic of the electric connections to the Schottky barrier SiNW FET. The nanowire surface potential is symbolized by a battery whose voltage is given by the pH of the solution and by the pI of the surface. | www.editorialmanager.com/nare/default.asp

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thermal annealing of silicon nanowires assembled between nickel electrodes [25, 26]. A nanoscopic metal–semiconductor interface appears within the nanowire due to axial diffusion of nickel and local formation of nickel silicide. This interface is not buried below a metal electrode, but is exposed to the liquid phase during pH measurements. Figure 1(a) shows an electron microscopy image of a small part of a parallel array of SiNW FETs with two Schottky junctions marked by yellow circles. The manufacturing of such nanosized SBs is highly reproducible, since it depends only on the nanowire diameter, which is well controlled by the synthesis procedure, as well as by annealing time and temperature [26]. Therefore, the silicidation length and, thus the length of the channel of the FET is similar for all nanowires in a parallel array of SiNWs. According to the statistical analysis presented in our previous work [25], a device can consist of up to 103 contacted nanowires in parallel. More details on device fabrication are provided in the Electronic Supplementary Material (ESM) (see Fig. S3). Because of the absence of dopants during nanowire synthesis, the Debye screening length of the channel is substantially larger than the nanowire diameter [29, 30]. Therefore gate fields can efficiently penetrate into the silicon channel and Schottky contacts formed at the Si/NiSi2 interfaces, leading to FET behavior with high on/off current ratios [29]. Electrical sensitivity of the nanowire FETs to changes in the electric field in the liquid is localized at the Schottky junctions, as has been already been shown by probing SBs in dry states with top-gates, scanning gate atomic force microscopy (AFM) measurements and several theoretical investigations [26–29]. The high reproducibility of the production process enables us to contact large numbers of wires in parallel without substantially sacrificing electrical performance of the complete device. This revolutionizes bottom-up fabrication of SB-based silicon nanowire biosensors for measurements in liquid surroundings. Note that previously reported SB nanowire FETs were mainly suited for dry state measurements because the sensitive Schottky junctions were situated at the metal contact pads, which were either not electrically isolated against electrochemical reactions and thus non-usable for measurements in liquids [30, 32], or isolated and

thus inaccessible for molecules at the sensitive sites, yielding low surface charge sensitivity [24]. The electrical isolation of metal leads of SB-based nanowire sensors is provided by a 100 nm thick layer of photoresist (AR-N 4340 S5, ALL Resist) with microfabricated “windows” to expose the nanowires and SBs to the liquid environment. The photoresist passivation alignment is shown in the confocal microscope (Keyence VK-X200) image in Fig. 1(b). The alignment accuracy together with the well known length of the NiSi2 phases of the wires permits the complete exposure of Schottky barriers to the liquid to be measured. A fluidic channel manufactured using polydimethylsiloxane (PDMS, Dow Corning “Sylgard 184”) was finally attached to the chip by mechanical pressure using a custom made mechanical device, as shown in Fig. 1(c). The potential of the liquid is controlled by a commercial Ag/AgCl reference electrode (Microelectrodes Inc., USA) that is built into the fluidic capillary tubing in close vicinity to the sensor chip. The source and drain electrodes are contacted in a tip probe station. 2.2

Sensor characterization

The electrical wiring scheme of the sensor is shown in Fig. 1(d). The origin and physical meaning of the elements in the scheme are introduced below. The physical mechanism for nanowire-based sensor signal is caused by surface charge induced modulation of the gating field in the nanowire, as for typical ionsensitive field effect transistors [29, 33]. Once exposed to solutions with various pH values, the gating field in the FET is generated by a back-gate potential Vbg, the liquid potential Vliquid, and the surface potential Vsurface, which is affected by the pH changes as Vsurface = Vliquid – α·59.5 mV·(pH – pI),

(1)

where α is the relative surface sensitivity according to the site-binding model with α ≤ 1 defining the Nernst limit of the surface potential kBT/e·ln(10) = 59.5 mV/pH; and pI is the isoelectric point of the surface. The electric potential in the active region of the FET can be described by coupling capacitance weighted addition of the back-gate potential Cbg and the surface potential

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Csurface with capacitances Cbg and Csurface (Fig. 1(d)) [34, 35]: Φ = (Vbg·Cbg + Vsurface·Csurface) / (Cbg + Csurface).

(2)

In the sub-threshold regime, the logarithm of the current at fixed source–drain voltage, abbreviated below as decI = log10Isd, is linearly dependent on the electrical potential Φ due to the thermal motion of electrons, with the curve steepness limited by the same numerical constant as the Nernst limit [30] ∂Φ/∂decI = –β·59.5 mV. In this equation, the gate coupling factor β, which is ≥ 1, determines the effectiveness of applied electric potentials, with β = 1 for the case of an ideal device. The minus sign results from the positive charge of the holes, which contribute to the FET conduction close to 0 V gate voltage, although the Schottky barrier-based FET devices used in our experiment are ambipolar [29]. In order to study the gate coupling efficiency, the electrical characteristics of parallel arrays of Schottky barrier SiNW FETs were measured under dry conditions and in phosphate buffer. The source–drain current Isd versus gate voltage curves under both conditions are summarized in Fig. 2. In this graph the horizontal (voltage) axis was scaled to display the two measurements according to the fitted slopes in the sub-threshold regime. The blue curve displays the I–V characteristics of the SB silicon nanowire device measured in the dry state, revealing a slope of

Figure 2 Electrical characteristics of the same FET device in phosphate buffer (100 mM sodium phosphate, pH = 7.4) and in dry surrounding. The bottom red axis indicates the liquid potential that was applied by the reference electrode in the measurement of the red curve; the top blue axis indicates the back gate voltage that was applied in dry conditions (see arrows).

about 950 mV/decI. The red curve demonstrates the Isd dependence in the liquid state with a slope of 127 mV/decI. The back-gate and liquid electrode were set to the same potential Vg = Vbg = Vliquid. The gate coupling increased by a factor of 7.5 in liquid conditions and the corresponding device quality parameter becomes β = 2.13. The gate-capacitance ratio for SB silicon nanowire devices, fabricated at wafers with back-gate dielectrics of 100 nm and 400 nm thickness (taking into account the thickness of an oxide shell of nanowires of 6 nm), is expected to be Cbg/Csurface = 0.05 and 0.0125, respectively. 2.3 Continuous gate sweeping Conventionally, in sensing measurements FET configurations are realized with a fixed gate voltage Vg. In order to carry out quantitative measurements in the optimal regime, we propose a new approach to detect signal changes in an FET sensor by continuously sweeping the gate voltage with a triangular signal and recording the source–drain current during each sweep (100 data points per sweep). This method allows the extraction of the threshold voltage at a fixed source– drain current from the recorded data. The voltage range is chosen such that the complete switching characteristic of the FET device is recorded in each sweep. The extraction of the threshold voltage at a fixed source–drain current from the recorded data is possible. The benefits of this method are: (i) all the information available in Isd (Vg) can be obtained; (ii) since a large range of currents is recorded, the threshold current with maximum sensitivity can be chosen for threshold voltage analysis; (iii) random drifts within the device hysteresis are reduced, since maxima and minima of the hysteresis are passed in each sweep (drifts from other sources are not eliminated by this procedure). We have provided comparative pH sensing measurements and sensitivity analysis using new gate sweeping approach and conventional constant-gate potential method. 2.4 2.4.1

pH sensing with SB SiNW device Physical aspects: Maximizing sensitivity

As introduced in the previous section and Eqs. (1)

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and (2), the influence of the pH of the liquid on the surface potential Vsurface determines the physical basis of the sensitivity of the nanowire-based devices. The sensitivity of current change to pH change is represented as S = ∂decI /∂pH = (α/β)·(1 + Cbg/Csurface)–1

(3)

Thus, the maximum current sensitivity Smax = 1 can be achieved only for a fully activated surface (α = 1), an ideal FET device (β = 1), and a dominant surface capacitance, Csurface Cbg. The interesting consequence of Eq. (3) is that use of the ideal FET device with a large nanowire surface capacitance leads to a linear scaling of the current with the ion concentration in solution. The estimated current sensitivity of FET devices fabricated for our experiments is limited to S = 0.95÷0.98 of the linear limit due to the high backgate capacitance. We applied the gate sweeping method to the detection of pH changes with our silicon nanowire sensor devices (see Fig. 3). Plots of the source–drain current versus gate voltage Vg and time during the course of a pH sensing experiment on a sensor chip with a 100 nm back-gate dielectric are demonstrated in Fig. 3(a). In order to better visualize the modulation of the current upon pH and gate voltage changes, we employed color mapping of the recorded signal. Source–drain current Isd was extracted from these data at Vg = 0 V and plotted as a function of pH (Fig. 3(b), blue crosses). Linear fitting of the obtained curve (the dashed line) for low pH values and low currents, i.e., in the sub-threshold regime, shows that the maximum sensitivity of the SB-based device is S ≈ 1/3. This is on the order of the magnitude of the theoretical limit S = 1 (or decI pH), displayed in Fig. 3(b) by the dot-dashed line and greatly exceeds sensitivities previously reported for top-down fabricated Schottky barrier silicon nanowire pH sensors [24]. The non-linearity of the Isd obtained at higher pH and current values is caused by the typical nonlinearity of the FET switching behavior. In order to investigate in detail the sensitivity of the device in solutions with pH = 5.7–8.0, we fabricated a device with a 400 nm back gate dielectric, which gives to a more linear current response. The current

sensitivity versus pH change was determined for all applied gate voltages by linear fitting of S = ∂decI /∂pH to the measured data. The evolution of the sensitivity versus gate voltage Vg is plotted in Fig. 3(c), and exhibits a maximum at Vg = 0.25 V (red circles in Figure 3(c)), in the sub-threshold regime, similar to values reported by Gao et al. [30]. Plots of current versus pH for three gate voltages (0.2 V, 0.5 V, and 0.7 V) are shown in the insets with the respective gate voltage indicated. Naturally, the sensitivity of the device can be only judged in relation to the standard deviation σS and signal to noise ratio S/σS, which were analyzed from the fitting procedure based on the standard deviation of the currents measured for each pH value. The signal to noise ratio has a plateau-like shape for low values of gate voltages Vg (from –0.2 V to –0.2 V), and sharply declines for higher voltages (see the gray plot in Fig. 3(c)). The maximum sensitivity S of the reported device and signal-to-noise ratio S/σS thus only overlap for a small gate voltage range. The reason for this behavior is related to the absolute values of the Isd current. The highest sensitivity is measured at the highest slope of the FET switching characteristics; however this point coincides with low Isd levels. On the other hand, lower sensitivities S in conjunction with higher current levels lead to the same quality of sensing. This statement allows us to conclude that the previously assumed importance of the sub-threshold regime for optimized sensing [30], is rather relative. In order to demonstrate the efficiency of our gate sweeping approach for FET sensing, we further compared our technique with the conventional constantgate potential sensing method. This is realized by consecutive gate sweep and constant liquid gate potential (Vg = 0 V) experiments, applied to the same device for the solutions of the same pH. The responses of the device are summarized in Fig. 3(d), where source–drain currents Isd are plotted versus pH. A sensitivity of S = 0.08 was determined for the fixed gate voltage measurement, while the sensitivity extracted from the gate sweep at Vg =0 V was higher (S = 0.122). It must be noted that the current levels for both measurements were different. The difference in the sensitivity values can therefore be explained by a signal drift between the two measurements, and not by a general change of experimental conditions, which


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Figure 3 (a) Source-drain currents for different pH values in a gate sweep measurement as a function of gate voltage and time. Dashed lines mark lines of constant gate voltages, representing constant gate voltage measurements with different sensitivities. (b) Sourse-drain current Isd is extracted from data shown in (a), at Vg = 0 V. The sensitivity for pH values below 5.7 was fitted to S=0.3 (dashed line), while the charge sensing limit of S=1 is indicated with a dash dotted line. (c) The fitted current sensitivity versus pH change S=∂decI /∂pH is shown as blue line on the left axis with the standard deviation σS as error bars. Current versus pH graphs for three exemplary gate voltages are shown in the insets with the respective gate voltage indicated. The respective points in the sensitivity curve are marked accordingly with a red square, a red circle and a red triangle. The signal-to-noise ratio S/σS is shown as grey shading on the right axis. (d) A constant liquid gate potential of Vg = 0V was applied to the same device for the same pH solutions. Source-drain current Isd is shown for the “clamped gate” (green squares) and the gate sweep (blue circles) measurements. Corresponding sensitivities (dashed lines) are indicated.

were held constant. This result underlines that fixing a constant gate voltage might result in measurements out of the range of the optimal gate voltage regime. 2.4.2

Chemical aspects: Surface potential measurement

More suitable for pH sensing experiments is the measurement of the surface potential on the ionsensitive FET. In such a configuration the threshold gate voltage Vt, that fixes the source–drain current Isd at a constant threshold value It, is measured continuously. In our setup, where the back-gate and the liquid electrode are set to the same potential, the

change in threshold voltage with pH Vt, can be represented as ∂Vt /∂pH = α·59.5mV·(1 + Cbg/Csurface)–1, according to Eqs. (1) and (2) [36]. Changes in the surface potential in the ion-sensitive FET are therefore given by ΔVsurface = – ΔVt·(1 + Cbg/Csurface)

(4)

The absolute value of the surface potential is obtained by determining the isoelectric point of the nanowire surface pI, which defines Vsurface (pI) = 0 V and thus Vsurface = ΔVsurface – ΔVsurface (pI). In order to determine the pI value, we measured the zeta potential of the silicon nanowires in solution at different pH


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values and found that the isoelectric point of the silicon nanowires is reached at pH = 4.8 (see Fig. S2 in the ESM). With the new gate sweep approach we can, in parallel to current measurements, extract the shift of the threshold voltage from each measured curve, and therefore determine the surface potential in the timedomain. We developed and employed an analysis method that enables us to extract automatically all the necessary parameters of the measurements (namely threshold current, sensitivity and signal to noise ratio), utilizing the full gate sweep data in the regions with highest sensitivity to gate voltage (see Figs. S1, S3 and S4 in the ESM). To derive the surface potential changes, silicon nanowire sensor devices were exposed to buffer solutions between pH = 1 and pH = 12 using the gate sweep regime of measurements. The source–drain current Isd was recorded at a frequency of 0.81 s–1 as a function of gate voltage. Figure 4 displays the surface potentials, which are plotted for two devices with 100 nm thick (main plot) and 400 nm thick (inset) back-gate dielectrics. Dashed lines are linear fits to the data, and the dash-dotted lines mark the Nernstlimit of 59.5 mV/pH. Two principal regimes can be discerned: below pH = 6, the slope was fitted to –37.17 mV/pH, while above pH = 6 the corresponding value is only –20.76 mV/pH for the 100 nm back-gate dielectric and –20.86 mV/pH for the 400 nm back-gate dielectric. Accordingly, the surface activation parameters α for the two regimes can be estimated as α = 0.625 and α = 0.350, respectively, showing that α does not vary markedly as a function of back-gate dielectric thickness. Note that the previously reported value [30, 37, 38] of the relative surface sensitivity for silicon α ≈ 0.5 is comparable to our estimates. The sensitivity values are also consistent with the measurements of current sensitivity and device quality shown in Fig. 3. Measurements of zeta potential of SiNWs in solution for pH values below 6 are also in good agreement with our measurements of surface potential changes Vsurface (Fig. 4) (see Fig. S2 in the ESM). Furthermore, a low slope of the surface potential has been reported for low pH values and a higher slope for larger pH values [2]. However one has to respect that silicon

Figure 4 Surface potential Vsurface versus pH value calculated from threshold voltage change, gate capacitance ratio and the pI of silicon nanowires. Blue squares and red circles correspond to devices with 100 nm and 400 nm back-gate dielectric, respectively. All data was adapted to the pI of silicon nanowires determined in a zeta potential measurements (see the ESM). Dashed lines are fits to the data, the respective slopes are indicated in the figure. Dash dotted lines represent the Nernst-limit of –59.5 mV/pH.

oxide shows a hysteretic behaviour for pH sweeping, i.e., a remanence of the surface potential, which leads to a higher slope in a range of low pH values.

3

Conclusions

We have demonstrated the bottom-up manufacture of parallel arrays of Schottky barrier silicon nanowire field effect transistors, which can be used for pH sensing with high sensitivity [24] and accuracy. The excellent device performance results from the sensitive nanosize atomically sharp Si/NiSi2 metal–semiconductor junctions (Schottky barriers), formed within silicon nanowires by thermal annealing, and their being exposed to the liquid environment during sensing. We introduced and employed the new measurement concept of continuous gate sweeps, which incorporates optimum current sensitivity to pH and, in parallel, accurate potentiometric measurements allowing quantitative information to be obtained. Remarkably, a combined analysis of the sensitivity S and signal to noise ratio S/σS enabled us to conclude that the sub-threshold regime—commonly considered as the optimal one [30]—is not obligatory for the best sensing measurements. We showed that lower sensitivities in conjunction with higher Isd current levels yield comparable or higher signal-to-noise ratios.


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Our bottom-up manufactured architecture relies on assembled parallel arrays of silicon nanowires, helping to increase the current output and to decrease the device-to-device variation, and is thus a good candidate to be integrated into existing bio-nanoelectronic detection chips. In particular, the fabrication of FETs using the nanowire printing technique enables the easy transfer of such sensor technology onto flexible and stretchable substrates [39, 40]. Finally we believe that the proposed highly sensitive platform, representing a smart conjunction of bottom-up nanofabrication techniques and measurement concepts represents a promising future alternative for state-of-the-art technology in the area of biodetection and diagnostics.

Acknowledgements This work was supported by the European Union (European Social Fund) and the Free State of Saxony (Sächsische Aufbaubank) in the young researcher group ‘InnovaSens’ (SAB-Nr. 080942409). Further we acknowledge support from the German Excellence Initiative via the Cluster of Excellence EXC1056 “Center for Advancing Electronics Dresden” (cfAED). We thank Kai Meine (Keyence Deutschland GmbH) for providing the laser scanning microscope, Anja Caspari and Dr. Cornelia Bellmann (Leibniz Institute, IPF) for their support in zeta potential measurements. Finally, we thank Dr. Robin Ohmann for his comments and fruitful discussions. Electronic Supplementary Material: Supplementary material about device fabrication (printing and lithography, electrical measurements, and zeta-potential measurements) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-013-0393-8.

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Research

Nano Res.

Table of contents

We demonstrate a pH sensor based on ultrasensitive nanosized Schottky junctions formed within bottom-up grown dopant-free arrays of assembled silicon nanowires and present a new measurement concept allowing experiments to be performed in the optimum sensitivity regime.

Nano Res.

Electronic Supplementary Material

Schottky barrier-based silicon nanowire pH sensor with live sensitivity control Felix M. Zörgiebel1,5, Sebastian Pregl1,5, Lotta Römhildt1, Jörg Opitz3, W. Weber2,5, T. Mikolajick4,5, Larysa Baraban1 (), and Gianaurelio Cuniberti1,5 1

Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, 01062 Dresden, Germany NaMLab GmbH, 01187 Dresden, Germany 3 Fraunhofer Institute IZFP Dresden, 01109 Dresden, Germany 4 Institute for Semiconductors and Microsystems Technology, TU Dresden, 01187 Dresden, Germany 5 Dresden, Germany, Center for Advancing Electronics Dresden, TU Dresden, 01062 Dresden, Germany 2

Supporting information to DOI 10.1007/s12274-013-0393-8

1

Determination of the threshold voltage shift

Beyond the trivial method to find the intersect of the curve Isd(Vg) and the threshold current It we can find the shift between two curves Isd(Vg, ti) and Isd(Vg, tj) by fitting the mean squared displacement MSDij(Δ) =〈[pIsd(Vg, tj) – pIsd(Vg, ti)]2〉using a parabolic function. The fitted function has its minimum at the shift of the threshold voltage between the curves, ΔVt. Our method is exemplified by two measurement curves for different pH values in Fig. S1(a). Figure S1(b) shows the resulting threshold voltage shifts for changing pH measurement between

Figure S1 (a) Source–drain currents for gate sweeps at two different pH values. The shape of the curves is close to identical, if they are shifted with respect to each other on the gate voltage axis. We determine this shift from the mean square deviation of the shifted curves and—in order to gain accuracy beyond voltage step resolution of the measurement—we fit the resulting curve with a parabolic function (inset). The position of the minimum mean square deviation determines the shift in threshold voltage. (b) Cumulative sum (integral) of threshold voltage shifts versus time for a pH measurement with pH values between 5.7 and 8.0, as indicated. Address correspondence to [email protected]

Nano Res.

pH 5.7 and pH 8 versus time. The pH solutions were exchanged by pumping through the microfluidic channel, so that the sensor response is step-wise.

2

Investigations of zeta potential of silicon nanowires

In order to obtain absolute values of the surface potential, we determined the zeta potentials (ZP) of silicon nanowires in solution by dynamic light scattering. For low salt concentrations and in a small range around the isoelectric point pI, the ZP is equivalent to the surface potential. Thus, ZP measurements can be efficiently used to determine the isoelectric point pI of the surface, by measuring the pH at which the surface potential becomes zero. Hence the surface potential change with pH can be determined from the ZP measurements as well. Figure S2 summarizes the investigations of the ZP of silicon nanowires in solutions with different pH values. In order to estimate the isoelectric point of SiNWs, we determined that the linear fit (adapted from Fig. 4) of the measured data intersects the abscissa at pH = 4.87. This corresponds to a zero value of zeta potential ZP = 0 V. Zeta potential at pH ≈ 5 is in a good agreement with the surface potential change of –37.2 mV/pH, measured by our SB-based nanowire device below pH6. The decrease of the slope for lower pH values can be explained by increase of the ionic strength of a solution with decreasing pH value (pH values were tuned by addition of HCl to distilled water), caused by evolution of electric double layer at the surface of the nanowires.

Figure S2 Zeta potential measurements of silicon nanowires dispersed in an aqueous solution with tuned pH values. The dashed line with slope –37.2 mV/pH is adapted from Fig. 4.

3

Preparation of pH buffer solutions

Buffer solutions were exchanged by a syringe pump (Harvard Apparatus, PHD2000) with a pumping rate of 500 μL·s–1. Phosphate buffers were used to set the range of pH values between 5.7 and 8.0. This was achieved by mixing two solutions containing 100 mM·L–1 Na2HPO4 and 100 mM·L–1 NaH2PO4 in the ratio given in a sodium phosphate buffer table. In order to increase or decrease the pH beyond these values, NaOH or HCl were added to the buffer until the desired pH was reached. pH values were controlled with a pH meter (InoLab).

4

Characterization of the SB FET device for sensing applications

The growth of the wires was performed on SiO2 coated silicon wafers using gold nanoparticles as seeds (BB International), with an average diameter of 19 nm. Devices were produced at the p-doped silicon wafer with 100 nm and 400 nm back-gate dielectric thicknesses. We employed the parallel array concept, where the nanowires are contacted between source and drain interdigitated electrodes (see Fig. S3, right panel). Within the | www.editorialmanager.com/nare/default.asp

Nano Res.

parallel array approach we can overcome typical shortcomings of single nanowire devices, related mostly to the low transconductance and high device-to-device variability. The inter-electrode distance was fixed at 4 m (the yellow arrow in Fig. S3, left panel) for demonstration purposes, and 10 m in real experiments, respectively. Longer inter-electrode distances resulted in better electrical characteristics (i.e., on/off ratio) and simplified electrical isolation procedure. In contrast to top-down fabricated SB FETs, we fabricated Schottky junctions using a bottom-up approach, by thermal annealing of silicon nanowires assembled between nickel electrodes. Therefore the length of the charge carrier channel (the red arrow in Fig. S3, left panel) is typically substantially shorter than the inter-electrode distance (the yellow arrow in Fig. S3, left panel). Because multiple wires up to 103, with polydispersity of their diameters of about 20% ([25] in the main text), were electrically contacted in parallel, the silicidation lengths (and thus channel lengths) in the array also deviate from wire to wire. We investigated the silicidation lengths of the nanowires within a single FET device (calculated to be around 30%) and reported it in our previous work (see Ref. [25] in the main text). Finally, the electrical isolation step was performed in order to expose the device to a liquid environment. The electrical isolation of metal leads of SB-based nanowires sensors was provided by a 100 nm thick layer of photoresist (AR-N 4340 S5, ALL Resists) with microfabricated “windows” to expose the nanowires and SBs to the liquid environment.

Figure S3 Left panel: Sketch of the silicon nanowires FET device, demonstrating the inter-electrode distance (between source and drain), Ni–Si phases, formed within nanowires and undoped silicon (charge carrier channel). Right panel: interdigitated electrodes, used for the formation of a FET for sensing

5

Biosensor pre-testing

In a preparatory step for DNA recognition experiments we used silane-functionalized silicon nanowire FET devices after ALD deposition of 10 nm Al2O3. This surface treatment leads to surface potential changes with pH value comparable to the Nernst-limit, as shown in Fig. S4. The measurements reveal the high reproducibility of the threshold voltage shift for scanning pH from lower to higher values ones and back (the inset in Fig. S4) and corresponding sensitivities (the red and black curves in Fig. S4). The measurement conditions (source–drain voltage, liquid electrodes, measurement time) were the same as for the measurements in the main text. The electrical device characteristics were similar in a similar way to that employed for the devices shown in Figs. 2 and S2. This demonstrates that our devices are indeed capable of measuring at the Nernst-limit, if the surface is sufficiently chemically activated. This experiment underlines the reproducibility of our devices even for different post-treatment procedures.


Research

Nano Res.

Figure S4 pH sensitivity of SiNW FETs functionalized with ssDNA after ALD deposition of 10 nm Al2O3. The data in red are for increasing pH, while the data in black are for decreasing pH. The linearly fitted slopes of both datasets are indicated in the figure and differ only slightly from each other and from the Nernst-limit of 59.5 mV/pH.
