Metal contact effect on the performance and scaling behavior of carbon nanotube thin film transistors

Abstract

Metal-tube contact is known to play an important role in carbon nanotube field-effect transistors (CNT-FETs) which are fabricated on individual CNTs. Less attention has been paid to the contact effect in network type carbon nanotube thin film transistors (CNT-TFTs). In this study, we demonstrate that contact plays an even more important role in CNT-TFTs than in CNT-FETs. Although the Schottky barrier height at the metal-tube contact can be tuned by the work function of the metal, similar to the case in CNT-FETs, the contact resistance (Rc) forms a much higher proportion of the total resistance in CNT-TFTs. Interestingly, the contact resistivity was found to increase with channel length, which is a consequence of the percolating nature of the transport in CNT films, and this behavior does not exist in CNT-FETs and normal 2D Ohmic conductors. Electrical transport in CNT-TFTs has been predicted to scale with channel length by stick percolation theory. However, the scaling behavior is also impacted, or even covered up by the effect of Rc. Once the contact effect is excluded, the covered scaling behavior can be revealed correctly. A possible way of reducing Rc in CNT-TFTs was proposed. We believe the findings in this paper will strengthen our understanding of CNT-TFTs, and even accelerate the commercialization of CNT-TFT technology.

Ultraviolet/ozone and oxygen plasma treatments for improving the contact of carbon nanotube thin film transistors

“…When this geometrical part is the dominant source of contact resistance in CNT-TFTs, the UVO treatment will not as effective as those in graphene devices for improving the contact. The geometric contact resistance can be reduced by increasing the tube density at the contact area which has been demonstrated in our previous work [2]. We believe that the contact resistance and hence the performance of CNT-TFTs will be best optimized by combining these two strategies, UVO treatment and higher density CNTs at contact area.…”

“…3, which is less than those in graphene devices [14]. We ascribe the difference to the following reasons [2]. Graphene is a two-dimensional (2D) continuous Ohmic conductor, while CNT thin film is one-dimensional (1D) percolating network which is not conventional Ohmic material.…”

“…Although each path in the channel consists of many tubes and inter-tube junctions, the number of conducting paths inside the channel is much larger than that at the contact. This results in a geometric R c in CNT-TFTs [2,23]. When this geometrical part is the dominant source of contact resistance in CNT-TFTs, the UVO treatment will not as effective as those in graphene devices for improving the contact.…”

“…As an emerging thin film transistor (TFT) technology, carbon nanotube thin film transistors (CNT-TFTs) have shown high mobility [1][2][3], high on-current (I on ) with high on/off current ratio [3][4][5]. Therefore, CNT-TFTs are regarded an promising TFT technology for future macroelectronics, such as chemical and biological sensors, optical detectors, and the backplane driving circuits for flat panel displays (FPDs) [1,6].…”

“…The metal-tube contact has been confirmed critical for the performance of CNT-TFTs [2,7,8], which should be well controlled during the device fabrication. However, the reported contact resistances of CNT-TFTs are usually very high [2]. Usually metal was deposited on top of the CNT to form side-bond contacts.…”