Sebastian Pregl

Parallel arrays of Schottky barrier nanowire field effect transistors： Nanoscopic effects for macroscopic current output

AbstractWe present novel Schottky barrier field effect transistors consisting of a parallel array of bottom-up grown silicon nanowires that are able to deliver high current outputs. Axial silicidation of the nanowires is used to create defined Schottky junctions leading to on/off current ratios of up to 106. The device concept leverages the unique transport properties of nanoscale junctions to boost device performance for macroscopic applications. Using parallel arrays, on-currents of over 500 μA at a source-drain voltage of 0.5 V can be achieved. The transconductance is thus increased significantly while maintaining the transfer characteristics of single nanowire devices. By incorporating several hundred nanowires into the parallel array, the yield of functioning transistors is dramatically increased and deviceto-device variability is reduced compared to single devices. This new nanowirebased platform provides sufficient current output to be employed as a transducer for biosensors or a driving stage for organic light-emitting diodes (LEDs), while the bottom-up nature of the fabrication procedure means it can provide building blocks for novel printable electronic devices.

Schottky barrier-based silicon nanowire pH sensor with live sensitivity control

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.

Light Weight and Flexible High‐Performance Diagnostic Platform

A flexible diagnostic platform is realized and its performance is demonstrated for early detection of avian influenza virus (AIV) subtype H1N1 DNA sequences. The key component of the platform is high-performance biosensors based on high output currents and low power dissipation Si nanowire field effect transistors (SiNW-FETs) fabricated on flexible 100 μm thick polyimide foils. The devices on a polymeric support are about ten times lighter compared to their rigid counterparts on Si wafers and can be prepared on large areas. While the latter potentially allows reducing the fabrication costs per device, the former makes them cost efficient for high-volume delivery to medical institutions in, e.g., developing countries. The flexible devices withstand bending down to a 7.5 mm radius and do not degrade in performance even after 1000 consecutive bending cycles. In addition to these remarkable mechanical properties, on the analytic side, the diagnostic platform allows fast detection of specific DNA sequences of AIV subtype H1N1 with a limit of detection of 40 × 10(-12) m within 30 min suggesting its suitability for early stage disease diagnosis.

Optoelectronic switching of nanowire-based hybrid organic/oxide/semiconductor field-effect transistors

A novel photosensitive hybrid field-effect transistor (FET) which consists of a multiple-shell of organic porphyrin film/oxide/silicon nanowires is presented. Due to the oxide shell around the nanowires, photoswitching of the current in the hybrid nanodevices is guided by the electric field effect, induced by charge redistribution within the organic film. This principle is an alternative to a photoinduced electron injection, valid for devices relying on direct junctions between organic molecules and metals or semiconductors. The switching dynamics of the hybrid nanodevices upon violet light illumination is investigated and a strong dependence on the thickness of the porphyrin film wrapping the nanowires is found. Furthermore, the thickness of the organic films is found to be a crucial parameter also for the switching efficiency of the nanowire FET, represented by the ratio of currents under light illumination (ON) and in dark conditions (OFF). We suggest a simple model of porphyrin film charging to explain the optoelectronic behavior of nanowire FETs mediated by organic film/oxide/semiconductor junctions.

Compact Nanowire Sensors Probe Microdroplets

The conjunction of miniature nanosensors and droplet-based microfluidic systems conceptually opens a new route toward sensitive, optics-less analysis of biochemical processes with high throughput, where a single device can be employed for probing of thousands of independent reactors. Here we combine droplet microfluidics with the compact silicon nanowire based field effect transistor (SiNW FET) for in-flow electrical detection of aqueous droplets one by one. We chemically probe the content of numerous (∼10(4)) droplets as independent events and resolve the pH values and ionic strengths of the encapsulated solution, resulting in a change of the source-drain current ISD through the nanowires. Further, we discuss the specificities of emulsion sensing using ion sensitive FETs and study the effect of droplet sizes with respect to the sensor area, as well as its role on the ability to sense the interior of the aqueous reservoir. Finally, we demonstrate the capability of the novel droplets based nanowire platform for bioassay applications and carry out a glucose oxidase (GOx) enzymatic test for glucose detection, providing also the reference readout with an integrated parallel optical detector.

Scaling and Graphical Transport-Map Analysis of Ambipolar Schottky-Barrier Thin-Film Transistors Based on a Parallel Array of Si Nanowires

Si nanowire (Si-NW) based thin-film transistors (TFTs) have been considered as a promising candidate for next-generation flexible and wearable electronics as well as sensor applications with high performance. Here, we have fabricated ambipolar Schottky-barrier (SB) TFTs consisting of a parallel array of Si-NWs and performed an in-depth study related to their electrical performance and operation mechanism through several electrical parameters extracted from the channel length scaling based method. Especially, the newly suggested current-voltage (I-V) contour map clearly elucidates the unique operation mechanism of the ambipolar SB-TFTs, governed by Schottky-junction between NiSi2 and Si-NW. Further, it reveals for the first-time in SB based FETs the important internal electrostatic coupling between the channel and externally applied voltages. This work provides helpful information for the realization of practical circuits with ambipolar SB-TFTs that can be transferred to different substrate technologies and applications.

Ionic effects on the transport characteristics of nanowire-based FETs in a liquid environment

AbstractFor the development of ultra-sensitive electrical bio/chemical sensors based on nanowire field effect transistors (FETs), the influence of the ions in the solution on the electron transport has to be understood. For this purpose we establish a simulation platform for nanowire FETs in the liquid environment by implementing the modified Poisson-Boltzmann model into Landauer transport theory. We investigate the changes of the electric potential and the transport characteristics due to the ions. The reduction of sensitivity of the sensors due to the screening effect from the electrolyte could be successfully reproduced. We also fabricated silicon nanowire Schottky-barrier FETs and our model could capture the observed reduction of the current with increasing ionic concentration. This shows that our simulation platform can be used to interpret ongoing experiments, to design nanowire FETs, and it also gives insight into controversial issues such as whether ions in the buffer solution affect the transport characteristics or not.

Printable Parallel Arrays of Si Nanowire Schottky-Barrier-FETs With Tunable Polarity for Complementary Logic

In this paper, we present a novel technology of printable bottom-up grown Si nanowire parallel arrays for low-dissipation power electronics. Parallel aligned layers of monocrystalline Si nanowires can be deposited on arbitrary substrates over large areas by the printing process. The presented transistors consist of parallel arrays of longitudinal NiSi2-Si-NiSi2 nanowire heterostructures, which naturally show ambipolar transistor behavior when a single gate is employed. By a double gate architecture, a reconfigurable transitor component is created, for which unipolar p- or n-type characteristics can be obtained depending on the polarity of the second gate. Transfer and output characteristics of these transistors on a Si/SiO2 substrate with back gate, top gate, and reconfigurable double gate architecture are presented here in detail. Very high on/off-current ratios of over 108 are achieved with very low off-currents. Due to the high number of nanowires incorporated into individual parallel arrays, output currents of 0.5 mA and a high yield of functional transistors of close to 100% at nanowire coated areas are demonstrated.

Channel length dependent sensor response of Schottky-barrier FET pH sensors

We present a multi channel Schottky-barrier (SB) field effect transistor (FET) based platform for chemical sensor applications and investigate its sensitivity on channel length. Designed transistors consist of parallel assembled bottom up grown silicon nanowires with a mean diameter of 20 nm. Focusing on investigations of devices with different channel lengths, we demonstrate that different optimum sensing regimes exist and they are determined by the device geometry. These target at different realizations and operation schemes. The sensitivities of the SB-FETs in linear and subthreshold regime are extracted from analysis of the pH response of silicon nanowire sensor devices.

P1.4.6 Investigations on the sensing mechanisms in silicon nanowire Schottky-barrier field effect sensors

Surface functionalization of NiSi2-Si-NiSi2 nanowire heterostructures, acting as Schottky-junction field effect transistors (SB-FETs) represent a promising route for biosensor applications. Axially Nisilicidized silicon nanowires exhibit a very sharp metal-semiconductor interface, thus forming a well defined and reproducible Schottky barrier. These barriers determine the current through the wire and can be changed by small molecules chemiadsorbed on the nanowire surface. We report that surface modifications can alter the polarity of the devices. This severe influence on the charge transport implies ultrahigh sensitivity for nanowire SB-FETs.