Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Downscaling of complementary metal-oxide-semiconductor (CMOS) technology is fraught with difficulties. As a result, novel devices and circuits, sophisticated nanomaterials, and enhanced fabrication processes have become increasingly important in recent decades. Particularly, silicon nanowires have been employed effectively in innovative electronic devices, including sensors, solar cells and in logic circuitry. Due to their high surface to volume ratio, silicon nanowires have been demonstrated as energy efficient devices, which is the key for the next generation of information processing. Field-effect-transistors based on silicon nanowires have been extensively used for sensing applications since the compact nanoscale structures allow excellent regulation of electrostatic potential across the nanowire channel. One such nanowire concept is junctionless nanowire transistor (JNT). A JNT is a highly doped nanowire channel without p-n junctions, where the gate electrode regulates the flow of charge carriers. Silicon JNTs have shown excellent sensi tivity to record-low concentrations of the protein streptavidin in liquid phase. However, they have not yet been operated as gas sensors.
In this work, we report the fabrication and characterization of silicon-based JNT devices and their initial tests as gas sensors. Intrinsic 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 implantation defect healing. Top-down approach is carried out for nanowire fabrication using electron beam lithography patterning of the negative resist HSQ followed by reactive ion etching. Successive processes of rapid thermal oxidation, nitrogen purge step and forming gas annealing are performed to create SiO2 shell around the silicon nanowires. SiO2 thickness is controlled by optimizating the time and temperature in these steps. UV lithography and metal evaporation are employed to create 50 nm thick Nickel contacts to the nanowires. Electrical characterization of these JNTs is performed by back-gating the nanowires. Unipolar device behavior is observed . However, these characteristics are changed after contact annealing leading to the ambipolarity in the devices. Two such transfer characteristics of JNTs based on unpassivated nanowires and nanowires with 3 nm SiO2 shell. These devices exhibit an on/off ratio of ~10^6. To further investigate the ambipolar nature of the silicon JNTs, output characteristics are measured, which shows Schottky barrier-based behavior of the devices. Furthermore, van der Pauw and Hall Effect measurements are performed to determine their carrier concentration and hall mobility. Successive measurements of electrical characteristics of these devices are also performed in vacuum to compare them with the usual ambient measurements. Unfunctionalized JNTs are tested as sensors in purified air and NO2 atmosphere. These sensor tests exhibited characteristic shifts in the transfer curve and a systematic increase and decrease of p- and ntype current, respectively, under the influence of NO2. These tests confirmed the potential suitability of the ambipolar JNT as sensors in gaseous environment. Additionally, these devices will be functionalized and tested for electrical detection of atmospheric free radicals.