Schottky barrier-based silicon nanowire pH sensor with...
Transcript of 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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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
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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 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)
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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
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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.
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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.
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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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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 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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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
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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.