Annealing and Activating Silicon Nanowires with a Laser

Hank Hogan

In electronics, smaller is almost always better. That has been further illustrated by a recent study on annealing silicon nanowires by University of California, Berkeley, researchers in collaboration with Nanosys Inc. of Palo Alto, Calif. Silicon nanowires could be deposited onto plastic substrates to make large-area and inexpensive flexible displays and sensor arrays.

Silicon nanowires melt and anneal at much lower light intensities than bulk silicon, as shown in these images. On top (a) are dark-field optical images of P-type silicon nanowires after excimer laser pulses of a given fluence and a set number of pulses. Melting takes place around 60 to 70 mJ/cm², much lower than the equivalent figure for bulk silicon. The bottom transmission electron microscope images are of silicon nanowires annealed at 35 mJ/cm² for 100 pulses (b) and at 40 mJ/cm² for 100 pulses (c). Reprinted with permission from the American Institute of Physics.

Several microns long but less than 100 nm in diameter, silicon nanowires conduct electricity and act as devices only if dopants are implanted in them and activated. That activation requires heat. Lasers have been used for years for this task in bulk silicon. Unfortunately, standard thermal annealing, which is required to electrically activate silicon nanowires, is too hot for plastic.

For their investigation, the researchers grew silicon nanowires, deposited them onto a glass substrate to form an array and selectively doped them N- and P-type. Using a KrF excimer laser operating at 248 nm and optics, they created a flat beam profile over a 10-mm² area, varied the fluence and number of laser pulses and determined the effect.

They found that the silicon nanowires annealed at an order of magnitude less fluence than that required for silicon films — a size-related effect. “Much lower fluence is required for the annealing of the nanowires because they tend to have very high absorption of the UV light,” said Costas P. Grigoropoulos, a mechanical engineering professor and research team leader at Berkeley.

Via optical inspection, the scientists found that the nanowires melted at fluences below 60 to 70 mJ/cm², less than one-fourth the fluence required for a silicon thin film and an even smaller fraction of the amount needed for bulk silicon. In measuring the electrical characteristics of an implanted nanowire annealed at 35 mJ/cm² for 100 pulses, they found performance equivalent to a device that had undergone rapid thermal annealing at 1050° C for 10 s, indicating efficient activation despite the low-temperature, plastic-friendly processing.

The researchers are investigating what happens with even smaller structures, which could lead to additional uses. “The ultimate aim of this research would be to build high-performance flexible devices based on this technology, but we are exploring several other applications as well,” Grigoropoulos said.

Applied Physics Letters, March 12, 2007, Vol. 90, 111111.