Presentation on "Polarity Control in Doped Silicon Junctionless Nanowire Transistor for Sensing Application" for 82nd DRC
Creators: name: Ghosh, Sayantan
name: Echresh, Ahmad
name: Khan, M. B.
name: Bhattacharya, D.
name: Kentsch, Ulrich
name: Prucnal, Slawomir
name: Vardhan, Vaishali
name: Biswas, Subhajit
name: Hellebust, Stig
name: Wenger, John
name: Holmes, Justin D
name: Erbe, Artur
name: Georgiev, Yordan M.
Publisher: name: Zenodo
Year: 2024
Subjects: radical
junctionless nanowire transistor
environmental monitoring
gas sensor
air quality monitoring
atmospheric radicals
air chemistry
electronic sensors
rapid thermal annealing
Schottky barrier
Field-effect-transistors
flash lamp annealing
rapid thermal oxidation
atmospheric free radicals

description: ABSTRACT:

Introduction: 

Silicon nanowires have been employed effectively in innovative electronic devices including sensors, solar cells and logic circuitry [1]. Field-effect-transistors (FET) based on silicon nanowires are extensively used for sensing applications, benefitting from their compact nanoscale structures that allow excellent regulation of electrostatic potential across the nanowire channel and enhanced sensitivity owing to a high surface-to-volume ratio[2]. One such nanowire FET concept is the junctionless nanowire transistor (JNT) [3]. A JNT is a highly doped nanowire channel without p-n junctions, where the gate electrode regulates the flow of charge carriers. Usually, JNTs act as either a p- or an n-type device depending on the type of doping. In this work, we optimize the doping concentration of the silicon JNT devices to perform as ambipolar FETs by controlling through the back gate. The useof both top and back gates enables enhanced polarity control in the JNT devices, leading to unipolar behaviour much like electrostatic doping [4]. Such functionality is similar to Schottky barrier-based FETs known as reconfigurable field effect transistors (RFET) [5]. Furthermore, these properties of dual polarity in JNT are advantageous for a widerange of sensing applications. Previously, silicon JNTs have shown excellent sensitivity to record-low concentrations of the protein streptavidin in the liquid phase [6]. However, they have not yet been operated as gas sensors. Here we also explore the sensing properties of ambipolar JNT in NO2 atmosphere.

Fabrication:

Lightly p-doped silicon-on-insulator (SOI) substrates are ion-implanted with phosphorus (n-type) dopant. Millisecond range flash lamp annealing (FLA) is used for dopant activation and defect healing. Top-down approach is carried out for nanowire fabrication using electron beam lithography (EBL) of the negative tone resist HSQ, followed by reactive ion etching (RIE) for pattern transfer [7,8]. Rapid thermal oxidation (RTO) and atomic layer deposition (ALD) are utilized for the formation of a dielectric shell made of SiO2 and Al2O3 around thenanowires, respectively. UV lithography and metal evaporation are employed to create 50 nm Nickel (Ni) sourcedrain contacts to the nanowires. TiN top gate is fabricated using EBL, metal evaporation and lift-off technique. The schematic diagram and top view SEM micrograph of one such nanowire array-based JNT device are shown in Fig. 1.

Results:

Electrical characterization of JNTs is initially performed by back-gating giving unipolar device behaviour. However, these characteristics are altered by rapid thermal annealing (RTA) of the contacts leading to the ambipolarity in the devices (Fig. 2a). The back gate voltage (VBG) is swept from -30 to 30 V with a constant drain to source voltage (VDS) of 0.5 V. The devices exhibit an excellent on/off ratio of ~108. To investigate the ambipolar nature of JNT, output characteristics are measured, which show the Schottky barrier-based behaviour of the devices (Fig. 2b). Furthermore, van der Pauw and Hall Effect measurements on n-doped SOI confirm the carrier concentration (~6 x 1017 /cm3) and hall mobility (~51 cm2/Vs) of the devices. The unipolar behaviour of JNT is tuned by controlling the device with both the back and top gate. Here, the back gate acts as a program gate (tuning the device polarity) whilethe top gate acts as a control gate (modulating the flow of the carriers). Fig 3 shows the band diagram and the transfer characteristics of one such device. The top gate voltage (VTG) is swept from -5 to 5 V while keeping VDS constant at 1 V. VBG is either kept at 30 or – 30 V to tune the n- or p-characteristics, respectively. Unfunctionalized back-gated JNTs are tested as sensors in purified air (termed as zero air) and NO2 atmosphere using a gas sensing probe station. Successive measurements are conducted in the chamber by introducing (in steps) zero air and different mixing ratios of NO2 (25 and 50 ppm) for fixed time intervals. The sensor tests exhibited characteristics shifts in the transfer curve and a systematic increase and decrease of p- and n-type on-currents, respectively, under the influence of NO2 (Fig. 4).

Conclusion:

In conclusion, we demonstrate the polarity control and tuning of JNT transfer characteristics with one or two-gate biasing leading to a high on/off ratio of 8 orders of magnitude. Additionally, these devices can be functionalized to display versatility in gas sensing applications, e.g. for the detection of atmospheric free radicals.

References:[1] Amato, M., et al., Chemical Reviews, (2014). 114(2). [2] Wan, J., et al., Microelectronic Engineering, (2009). 86(4–6).[3] Colinge J. P, et al. Nature Nanotechnology (2010) 5 225-229. [4] Mikolajick, T., et al., Semiconductor Science and Technology,(2017) 32.4. [5] Heinzig, A., et al., Nano Letters, (2011).12(1). [6] Georgiev, Y. M., et al., Nanotechnology, (2019) 30. [7] Khan,M. B., et al. 2020 DRC. IEEE, (2020). [8] Ghosh, S., et al. ACS Applied Materials & Interfaces (2023) 15.34.
description: Additional Information: 

PhD student Sayantan Ghosh delivered this presentation for 82nd Device Research Conference.

RADICAL represents a 'Fundamental Breakthrough in Detection of Atmospheric Free Radicals'.

Find out more on the RADICAL project website: radical-air.eu

The RADICAL project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 899282.