Poster on Polarity Tuning in Ambipolar Silicon Junctionless Nanowire Transistor for Gas Phase Sensing for iMNE Confernece 2024

ABSTRACT Silicon nanowires have been effectively utilized in innovative electronic devices such as sensors, solar cells, and logic circuits [1]. Field-effect transistors (FETs) based on silicon nanowires are widely used for sensing applications due to their compact nanostructures, which provides excellent control of the electrostatic potential across the nanowire channel and increased sensitivity from high surface-to-volume ratio [2]. One such example is the junctionless nanowire transistor (JNT) [3]. A JNT features a highly doped nanowire channel without pn junctions, where the gate electrode controls the flow of charge carriers. Typically, JNTs function as either p-type or n-type devices, depending on the doping type. In this study, we optimize the doping concentration of silicon JNT devices to function as ambipolar FETs by using the back gate for control. The use of both top and back gates allows for enhanced polarity tuning in JNT devices, resulting in unipolar behaviour similar to electrostatic doping in reconfigurable field-effect transistors (RFETs). The dual polarity of JNTs is advantageous for sensing applications. Previously, silicon JNTs have demonstrated excellent sensitivity to very low concentrations of the protein streptavidin in the liquid phase [4]. However, they have not yet been used as gas sensors. In this work, we also investigate the sensing properties of ambipolar JNTs in gaseous atmosphere. Lightly p-doped silicon-on-insulator (SOI) substrates are implanted with phosphorus (n-type) dopant, followed by millisecond range flash lamp annealing (FLA) for dopant activation and defect healing. Nanowires are fabricated using a top-down approach using electron beam lithography with HSQ resist and reactive ion etching. Dielectric deposition is employed using rapid thermal oxidation (RTO) for SiO2 and atomic layer deposition (ALD) for Al2O3. UV lithography and metal evaporation form 50 nm Nickel (Ni) source-drain contacts, while TiN top gate is fabricated using EBL, metal evaporation and lift-off [5,6]. Electrical characterization of JNTs is performed by back-gating after rapid thermal annealing (RTA) of the contacts leading to ambipolarity in the devices. The devices exhibit an excellent on/off ratio of ~ 10^8. To investigate the ambipolar nature of JNT, output characteristics are measured showing Schottky barrier based behaviour. Furthermore, Hall Effect measurements with van-der-Pauw configuration on n-type doped SOI confirm the carrier concentration (~ 6 x 10^17 /cm3) and hall mobility (~ 51 cm2/Vs). Tuning the unipolar behaviour of the JNT is achieved with both the back and the top gate. In this setup, the back gate acts as a program gate, adjusting the device polarity, while the top gate functions as the control gate, regulating the carrier flow. JNT sensor tests exhibited characteristic shifts in the transfer curve and systematic increase and decrease of p- and n-type current, respectively, under the influence of different gases like NO2 and NH3 confirming the potential suitability of ambipolar JNTs as gas sensors. Additionally these devices can be functionalized to display versatility of gas sensing applications, e.g. for the detection of atmospheric free radicals. [1] Amato, M., et al., Chemical Reviews, 2014. 114(2): p. 1371-1412.[2] Wan, J., et al., Microelectronic Engineering, 2009. 86(4–6): p. 1238-1242.[3] Colinge J P, et al. Nature Nanotech. 2010 5 225-229.[4] Georgiev, Y. M., et al., Nanotechnology, 2019 30 324001 (8pp).[5] Khan, M. B., et al., Applied Sciences, 2019. 9(17).[6] Ghosh, S., et al. ACS Applied Materials & Interfaces (2023) 15.34.