Modeling of Carbon Nanotube Schottky Barrier Modulation Due to Oxidation

A model is proposed for the experimentally observed lower Schottky barrier for holes in air than in vacuum at a metallic electrode - semiconducting carbon nanotube (CNT) junction. In oxidation occurring in air, the negatively charged oxygen molecules on a material usually enhance the surface dipole and provide stronger electron confinement within the bulk. Thus the CNT electron affinity will increase in air. Then the Schottky barrier for holes will have to increase according to the standard band-alignment theory, but this is against the experiment. In order to overcome this difficulty, we propose a new Schottky barrier model, assuming there is a transition region between the electrode and the CNT and an appreciable potential can drop there. The role of the oxidation is to increase this potential drop with negatively charged oxygen molecules, leading to a lower Schottky barrier for holes. This mechanism prevails for both p- and n-CNTs. The model consistently explains all the reported CNT device experiments.