A study on the Contact Property of Metal/Carbon nanotubes
Carbon nanotube (CNT) is one of the most promising materials for nano-electronics devices in the future. Since the contact resistance between metal and CNT has significant influence on the electrical performance of CNT devices, we focus on the metal/CNT contact properties in the thesis. At first, we develop a lift-off process to define metal lines as narrow as 0.2um. Random process and aligned process are developed to fabricate multiple contacts test structure. The CNTs were dispersed on SiO2/Si substrate and form ropes with diameter of around 20nm. In the measurement aspect, two terminals, three terminals, and four terminal methods are employed to measure the total resistance, CNT resistance, and contact resistance. Nine kinds of metals include Pt, Au, Ti, Cu, W, Ta, Cr, Al, Ni, are selected to form contact structure to find out which metal on the tubes could achieve the lowest contact resistance, and to investigate the main factors affecting the value of contact resistance. From the results, we find that the sputtered platinum on CNT exhibits the lowest contact resistances. The resistance value is about several hundred ohms. The extreme low chemical activity of platinum is the most dominant factor. Chemical reaction causes the oxidized interface, and it may be the insulating material resulting in high contact resistance. Because Titanium and carbon formed the Ti-C bond, and this carbide breaks the interfacial oxide between titanium and CNT, if we choose titanium as the contact metal, the contact resistance can be greatly reduced after high temperature annealing. At low temperature, we find the effect of Schottky barrier at the interface between metal and CNT ropes. The structural defects in CNT may also cause another barrier. However, the I-V characteristics show almost ohmic property at room temperature. This observation implies that the barrier height is too low to influence the carrier transport at room temperature. The conductivity of CNT ropes modulated by electric field, the current carrying capability, as well as the breakdown phenomenon are also discussed briefly in this thesis.