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Nano Res
1
Ionic effect on the transport characteristics of nanowire-based FETs in liquid environment
Daijiro Nozaki1 (), Jens Kunstmann1,2, Felix Zörgiebel1, Sebastian Pregl1, Larysa Baraban1,
Walter M. Weber3, Thomas Mikolajick3, and Gianaurelio Cuniberti1,4,5 Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-013-0404-9
http://www.thenanoresearch.com on December 18 2013
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Nano Research
DOI 10.1007/s12274-013-0404-9
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Ionic effect on the transport characteristics of
nanowire-based FETs in liquid environment
Daijiro Nozaki*, Jens Kunstmann, Felix
Zörgiebel, Sebastian Pregl, Larysa Baraban,
Walter M. Weber, Thomas Mikolajick, and
Gianaurelio Cuniberti
Technical University of Dresden, Germany
Department of Chemistry, Columbia University
NaMlab gGmbH
Page Numbers. The font is
ArialMT 16 (automatically
inserted by the publisher)
A simulation platform for quantum charge transport through 1D
nanostructures in liquid environments is established and applied to silicon
nanowire field effect transistors. The platform is supposed to be used for the
design and the optimization of nanowire-based chemical or biosensors. The
reduction of the sensitivity of the sensor due to the formation of an electric
double layer could be successfully reproduced.
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Ionic effect on the transport characteristics of nanowire-based FETs in liquid environment
Daijiro Nozaki1 (), Jens Kunstmann1,2, Felix Zörgiebel1, Sebastian Pregl1, Larysa Baraban1, Walter M. Weber3, Thomas Mikolajick3, and Gianaurelio Cuniberti1,4,5 1 Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, 01062 Dresden, Germany 2 Department of Chemistry, Columbia University, 3000 Broadeway, New York, NY 10027, USA 3 NaMlab gGmbH, Nöthinger Str. 64, 01187 Dresden 4 Center for Advancing Electronics Dresden (cfAED), TU Dresden, 01062 Dresden, Germany 5 Dresden Center for Computational Materials Science (DCCMS), TU Dresden, 01062 Dresden, Germany
SiNW-FETs [38] to a parallel array of SiNW-FETs [39]
that allows to decrease device-to-device variations
and to increase a total current output. The surface of
the device is covered with the layer of Al2O3 to
protect the FET from electrochemical reactions. The
procedure to create the parallel array of the
SiNW-SBFETs and the experimental setup and
measurement are shown in Ref. 39 and in the ESM.
Figure 7(a) presents the ionic concentration
dependence of the measured current through the
device with different ionic concentrations at a fixed
source-drain bias (VSD = 0.25 V) and a negative
gate-field (VG = -1.0 V). We can see that the
SiNW-FETs in liquid show a weak dependence of the
drain current to ionic concentrations and that the
drain current (hole current) is slightly reduced with
the increase of the ionic strength. In order to
demonstrate that this reduction of the hole current is
due to the formation of electric double layers at the
surface of the device as discussed with the band
diagram in Fig. 6(b), we have modeled the
SiNW-SBFETs and calculated the hole current
through the device with different ionic
concentrations. The device geometry and the setting
of parameters are shown in the ESM. Figure 7(b)
shows the numerical result of the ionic dependence
of the current through the SiNW-SBFETs. We can
clearly see that the hole current is reduced with the
increase of the ionic concentrations due to the shift of
the valence band involved with the formation of the
electric double layer at the surface of the device. The
electric potential shown in the inset of Fig. 7(b) also
supports the band diagram suggested in Fig. 6(b).
This is how we could elucidate the weak dependence
of the hole current through the SiNW-SBFETs to the
ionic concentration and the origin of current
reduction using our multi-scale simulation platform.
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Figure 2 Electric double layer at a 2D liquid-solid interface: (a) The geometry of the interface. Electric potential landscapes for ionic concentrations of (b) 0.001 M, (c) 0.01 M, and (d) 0.1 M. (e) The same potentials plotted along the axis perpendicular to the surface. (f) Accumulated charges as a function of distance from the surface. More charge is accumulated in the case of higher ionic concentrations resulting in a stronger screening effect.
Figure 3 Electric double layer at a 2D liquid-solid interface of a SiNW on a SiO2 insulator: (a) device geometry, (b) profiles of the electric potential along the red line in panel (a), and (c)-(d) the space charge density as a function of distance from the gate electrode along (c) the red line and (d) the blue line in panel (a) at different ionic concentrations. The gate voltage is 5 V in (b)-(f). 2D surface plots of the space charge density for two ionic concentrations are shown in (e) and (f). The applied potential almost completely drops in the insulator and it drops even more rapidly in the case of high ionic concentrations. Although the accumulated charge looks small for 10 mM in panel (d), this is due to the fact that charge accumulates only in close proximity of the SiO2 water interface.
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Figure 4 Sensitivity of a SiNW sensor device to a model charge as a function of charge and ionic concentration in a 2D model. Electric potential profiles of SiNWs on insulators with a single model charge calculated in a 2D model: (a) the device geometry is the same as Fig. 3(a), except for a model charge that is placed at a distance of 2.5 nm above the surface of the SiNW. (b) Electric potential along the red line in (a) for different charges and ionic concentrations. The gate voltage is VG = 5 V and q is the elementary charge. The influence of the model charge on the electric potential is reduced for higher electronic concentrations. The results indicate that thin insulators, low ionic concentrations, and short separations between the attached species and the NW are desired to have a high sensitivity of the sensor device.
Figure 5 The electric double layer in a 3D SiNW-FET device. Charge densities and potential profiles: (a) the device geometry, (b) FEM mesh used for the calculations, (c)-(f) cross-sectional plots of the space charge density of NW-FET for different source-drain voltages and ionic concentrations, (g)-(h) 1D electric potential along NW axis for different ionic concentrations with/without source-drain voltages VD. In all calculations, the gate field is VG = 5 V. The surface of the SiNW is covered with a native SiO2 layer of 3nm SiO2 (not visible in (a)). Note that the color scales in (c)-(f) are not the same. In the presence of a
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non-zero source-drain voltage, the charge of the double layer accumulates asymmetrically. A cross-sectional view of the device geometry including the oxide shell covering the SiNW is shown in Electronic Supplementary Material (See Fig. S4).
Figure 6 The ionic concentration dependence of the drain current through the NW-FET device shown in Fig. 5(a) with positive gate fields. (b) The corresponding energy band diagram for the NW-FET explains why hole currents are enhanced and electron currents are reduced with increasing ionic concentration. The Schottky-barrier height for electrons is larger than that for holes (ΦSB
h = 0.44 eV, ΦSBe= 0.68 eV), In all calculations, the gate voltage and the voltages at source and drain are
fixed to VS = 0 V, and VD = 0.5 V, respectively.
Figure 7 Ionic concentration dependence of the drain current through the parallel array of the SiNW-SBFETs under negative gate field in (a) experiment and (b) theory. Panel (a) is 2D histogram of measured drain current with different ionic concentration. In the both results, the hole currents are slightly reduced with increasing ionic concentration. The Schottky-barrier for holes is set to (ΦSB
h = 0.15 eV). In all calculations, the voltages at source and drain are fixed to VS = 0 V, and VD = 0.25 V, respectively. The gate voltage in both calculations and experiment is VG = -1.0 V. In numerical calculations, we assumed that there are 1000 NW-FETs between electrodes. Corresponding electric potentials along the silicon channel are shown in inset.
3. Conclusion
In summary, in order to investigate the influence of
ions in liquid environments on the transport
characteristics of NW-FETs for sensor applications,
we have implemented a modified
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Poisson-Boltzmann model into the
previously-developed multi-scale model. The model
correctly describes the formation of the electric
double layer at the solid-liquid interface. It can
explain and quantify the experimentally well-known
reduction of the sensitivity of the device to surface
charges in the case of high ionic concentrations, and
the weak dependence of the drain current on the
ionic concentration of the buffer solutions. As a
demonstration, we have fabricated NW-FETs,
measured the current in the ionic solutions, and
compared the measured current with our model
showing a good agreement.
We have established a simulation platform for
NW-based FET devices in liquid environments. It can
be used for the interpretation and elucidation of
experimental observations, as guidelines for the
planning of future experiments, as well as for the
optimal design of nanowire-based sensors.
Acknowledgements We thank Kannan Balasubramanian for inspiring
discussions. This work is funded by the European
Union (ERDF) and the Free State of Saxony via the
ESF project 080942409 InnovaSens, and by the World
Class University program funded by the Ministry of
Education, Science and Technology through the
National Research Foundation of Korea (R31-10100).
We also gratefully acknowledge support from the
German Excellence Initiative via the Cluster of
Excellence EXC 1056 “Center for Advancing
Electronics Dresden" (cfAED).
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