2.8.3 Environmental Influences on the Performance of CNT-FETs

The effect of ambient air on the performance and functionality of CNT-FETs can be also understood within the framework of the SCHOTTKY barrier model of conduction. In particular, this model helps to clarify and separate the effects due to the bulk of the CNT channel from those arising from the effects at the contact between the metal electrode and the CNT.

It has been proposed that e.g. oxygen adsorption leads to doping of CNTs [82]. However, as shown in Fig. 2.19-a, the effect of oxygen on the transport properties of a CNT-FET is a reversible transition from p-type (devices prepared in air) to n-type after annealing the transistor in vacuum [83]. In contrast, the deposition of an n-type dopant such as potassium (Fig. 2.19-b) shifts the transfer characteristics with respect to the gate voltage. It is known that the work function of a metal surface is altered significantly upon the adsorption of gases due to the formation of interface dipoles. Thus, the local work function of the metal electrode can be modified considerably by the adsorption of oxygen at the contacts. If the work function of the metal electrode changes the line-up of the metal FERMI energy with the CNT, bands will shift2.5 [73].

Figures 2.19-c and 2.19-d compare the effect of doping with that of a shift in the line-up, i.e. a reduction of the SCHOTTKY barrier height to the conduction band and an increase of that to the valence band or vice versa. While n-type doping shifts the transport curves to more negative gate voltages, a change in the work function promotes either the p-type or the n-type branch of conduction and reduces the other.

Figure 2.19: Effect of gas adsorption and doping on the operation of CNT-FETs. a) and b) are experimental data and c) and d) numerical calculations. In a) a vacuum annealed n-type FET has been exposed to increasing amounts of oxygen until the ambient is reached. In b) the curves from right to left correspond to increasing deposited amounts of potassium. In c) the work function difference between metal and CNT is changed from $ -0.2$ eV to $ +0.2$ eV in steps of $ +0.1$ eV [78,84].
M. Pourfath: Numerical Study of Quantum Transport in Carbon Nanotube-Based Transistors