Extension p-Doping of Carbon Nanotube Transistors through Nitric Oxides Annealing

This work investigates the p-doping effect of Nitric Oxides (NO_X) annealing on transistors with a channel from a network of carbon nanotubes (CNTs), with a focus on extension doping of top-gated transistors and RF characterization. NO_X annealing at 100 °C for 1 h induces oxidation in CNTs, which introduces a doping band inside the nanotube band gap near the valence band. The benefits and drawbacks of NO_X annealing on the performance of back-gated (BG) CNT transistors are characterized. The doping band is beneficial in that it reduces the Schottky barrier for hole injection and increases the hole carrier concentration in the channel. However, we also note that NO_X annealing deteriorates the inverse subthreshold swing (SS) and drain-induced barrier lowering (DIBL) by introducing interface traps and worsens any existing short channel effects. Our capacitance analysis confirms that the increase in source-drain and interface-trap capacitances degrades both SS and short channel effects. To circumvent the drawbacks of NO_X annealing, we employ it to create extension doping regions in p-type, top-gated transistors with a scaled high-k gate oxide, leading to an increase in drain current from 5.09 to 13.95 μA at V_OV = -2 V and V_DS = -1 V. Because the effect of NO_X annealing is limited to the extension regions and has little impact on the channel region, negative effects on the SS and DIBL are mitigated. Finally, an RF transistor is fabricated, and NO_X annealing is used to enhance the cutoff frequency by approximately 10-fold.