Modeling of carbon nanotube Schottky barrier modulation under oxidizing conditions

A model is proposed for the previously reported lower Schottky barrier F Bh for hole transport in air than in vacuum at a junction between the metallic electrode and semiconducting carbon nanotube ~CNT!. We consider the electrostatics in a transition region between the electrode and CNT in the presence or absence of oxygen molecules ~air or vacuum!, where an appreciable potential drop occurs. The role of oxygen molecules is to increase this potential drop with a negative oxygen charge, leading to lower F Bh in air. The Schottky barrier modulation is large when a CNT depletion mode is involved, while the modulation is negligible when a CNT accumulation mode is involved. The mechanism prevails in both p- and n-CNT's, and the model consistently explains the key experimental findings. properties. 2 Thus oxidation changes the bulk CNT properties. Recently, oxidation effects have been studied 3 using CNT field-effect transistors 4 ~FET's !, and it has been shown that the contact properties at the electrode and CNT are modified in oxidation. The CNT was placed on a silicon dioxide layer, and a gate voltage VG was applied to the doped silicon sub- strate ~backgate! as schematically shown in Fig. 1~a!. The CNT was about 1 mm long, and the source and drain gold electrodes covered about one-third of the CNT, respectively. The channel length was a few tenths of a micrometer. They measured the drain current I D as a function of VG and the drain voltage VD , and estimated the channel conductance gd5)I D(VG ,VD)/)VD . The role of VG was to change the CNT doping effectively or modulate the Fermi level. With the application of VG , positive and negative charges were introduced as shown in Fig. 1~a!. The electric field under the source and drain electrodes was mostly vertical due to the thin CNT and wide electrode geometry. In this situation, the planar junction theory is applicable. 5,6 The CNT conduction- and valence-band edges have the shape shown in Fig. 1~b! along the arrow path in Fig. 1~a!, where A -D indicate spa- tial points in the structure. The path and electric field are parallel only at the CNT ends, and band bending occurs only there. VG determines this field strength and, therefore, the amount of band bending. Thus VG effectively changes the CNT doping and modifies the Fermi level byaVG , where a is related to the oxide capacitance and CNT capacitance. 6,7 a is usually on the order of 10 21 in experiments, 3,4 meaning that only a portion of VG is reflected in the Fermi level change.