Schottky barrier-based silicon nanowire pH sensor with...

Schottky barrier-based silicon nanowire pH sensor withlive 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 2 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

Received: 14 September 2013

Revised: 20 November 2013

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

ABSTRACT

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.

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

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

Nano Research

DOI 10.1007/s12274-013-0393-8

Address correspondence to [email protected]

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2 Nano Res.

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 nano-

structures, 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 bio-

chemical species down to single molecules [2, 11, 12].

Some of the main issues, which impede the straight-

forward 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 app-

lications 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 app-

lication we demonstrate the performance of Schottky

barrier (SB)-based silicon nanowire FET devices for

sensing the pH values of a solution.

2 Results and discussion

2.1 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 theSchottky barriers between the silicon and nickel disilicide phases of the wires are highlighted. (b) Confocal microscope image ofinterdigitated electrodes (silver) with photoresist passivation (purple) and nanowires (vertical black lines). A single wire is highlightedwith a red frame. (c) Chip integrated in a fluidic system and electrically contacted with the tips of a probe station and the referenceelectrode (marked with blue circle). Red arrows mark the fluid flow. The back gate voltage Vbg is applied to the metal base of the tipprobe station (not shown). (d) Schematic of the electric connections to the Schottky barrier SiNW FET. The nanowire surface potential issymbolized by a battery whose voltage is given by the pH of the solution and by the pI of the surface.

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3 Nano Res.

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 theo-

retical 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 polydimethyl-

siloxane (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 (Micro-

electrodes 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 ion-

sensitive 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


4 Nano Res.

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 confi-

gurations 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 mea-

surements and sensitivity analysis using new gate

sweeping approach and conventional constant-gate

potential method.

2.4 pH sensing with SB SiNW device

2.4.1 Physical aspects: Maximizing sensitivity

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


5 Nano Res.

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 back-

gate 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 com-

pared our technique with the conventional constant-

gate 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


6 Nano Res.

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 ion-

sensitive 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 con-

tinuously. 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

Figure 3 (a) Source-drain currents for different pH values in a gate sweep measurement as a function of gate voltage and time. Dashedlines 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 currentIsd is shown for the “clamped gate” (green squares) and the gate sweep (blue circles) measurements. Corresponding sensitivities (dashed lines) are indicated.


7 Nano Res.

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 time-

domain. 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 Nernst-

limit 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 para-

meters 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 com-

parable or higher signal-to-noise ratios.


8 Nano Res.

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, represen-

ting 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 litho-

graphy, 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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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.

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Electronic Supplementary Material

Schottky barrier-based silicon nanowire pH sensor withlive 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 2 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

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]


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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


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.


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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.

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