October 26, 2007 – Researchers at the National Institute of Standards and Technology (NIST) say they have developed a technique suitable for mass production of nanowire-based devices, that enables selective growth of nanowires on sapphire wafers in specific positions and orientations, to be attached to contacts and layered with other circuit elements using conventional lithography techniques.
Typical nanowire production involves growing them like blades of grass on a substrate, mow them down and mix them in a fluid to transfer them to a test surface, where they are given a preferred orientation, and when the fluid dries the nanowires are left behind, scattered in piles. Scanning probe microscopy or other test tools are used to find isolated nanowires, or just place electrical contacts with no clear knowledge of the nanowire’s orientation.
The new method builds on earlier work growing nanowires horizontally on the surface of wafers. Using “conventional semiconductor manufacturing techniques,” small amounts of gold were deposited in precise locations on a sapphire wafer, exposed to a high-temperature process which beaded them up into nanodroplets, acting as nucleation points for crystals of zinc oxide. “A slight mismatch” in the zinc oxide and sapphire crystal structures results in growth of a narrow nanowire in one particular direction across the substrate; both the start and endpoints are well defined, and can receive electrical contacts and other features using additional lithography steps, NIST explained in a statement.
The researchers say they’ve created >600 nanowire-based transistors in a single process. Small bunches of up to 8 nanowires at a time have been prototyped, though finer control of the initial gold deposits should allow more precise selection of the number of wires in each position.
Nanowire electronics: (Top) Optical image shows metal electrodes attached to zinc oxide nanowires using the NIST technique. Dark spots near the center are the gold pads that start nanowire growth; red arrow shows direction of growth. Scale bar is 50 micrometers long. (Bottom) Scanning electron microscope image shows electrodes connected to group of nanowires. Scale bar is five micrometers long.