Dielectrophoresis (DEP) is a simple, effective and low-cost method of aligning single-crystalline nanowires between source and drain electrodes for the assembly of nanowire-based electronic devices over a large area. The technique involves the use of an electric field to place nanowires in a controlled orientation and position. It meets many of the requirements for the low-cost atmospheric manufacture of high-performance nanowire field-effect transistors (NW-FETs), but there is a technical hurdle to overcome. When an electric field is used to place nanowires on the electrodes, the electrical contacts between the nanowires and the metal electrodes can be poor.

To address this issue, researchers at National Cheng Kung University (NCKU) have included a hot-pressing step in the manufacturing process, which lowers the contact resistance and improves the performance of the devices. The team believes that the roll-to-roll compatible process should be suitable for all kinds of nanowire materials.

Using DEP, nanowires were first aligned along the substrate surface and perpendicular to the edges of the electrodes with both ends right on the electrodes. A piece of silicon coated with a diamond-like carbon film (to prevent nanowires from adhering to the surface) is then used to press the nanowires against the substrate to improve the contacts between the nanowires and electrodes at a high pressure and a high temperature (~180 °C).

Under appropriate conditions, the nanowires were shown to sink into the electrode surface. The contact resistance between the nanowires and the electrodes was effectively lowered (as shown in the graph above) and transistor behaviour was demonstrated.

Next steps

Although hot-pressing is a simple and effective method of lowering the contact resistance, the value obtained is not as low as the contact resistance of NW-FETs prepared by the direct deposition of metal onto nanowires in a vacuum system. In the lab, the NCKU group is now refining its hot-pressing method to lower the contact resistance further.