InP Nanowires Act as Detectors

Hank Hogan

They have been shown to work as LEDs, but indium-phosphide nanowires may also find a place in optical interconnects. A research team at Harvard University in Cambridge, Mass., has discovered that the nanowires exhibit different optical properties depending on their direction, making them suitable for use as photodetectors.

The length of a nanowire is much greater than its diameter, which ranges from 10 to 50 nm; hence, the moniker. The researchers created the structures using laser-assisted catalytic growth. A Spectra-Physics Nd:YAG laser operating at 1064 nm vaporized indium-phosphide targets to supply the raw material.

They first performed measurements of photoluminescence spectra with a nitrogen-cooled CCD camera from Roper Scientific Inc., focusing the output of a Melles Griot argon-ion laser on a quartz substrate on which the nanowires were dispersed. The same focusing objective, filtered to remove the excitation light, collected the signal from the nanowires and fed it to the camera.

Widely varying response

The researchers found that the nanowires responded very differently, depending on the polarization of the incoming 488- or 514-nm light from the argon-ion laser. The difference was so great that the intensity of the photoluminescence response at the parallel polarization was 20 to 50 times that at the perpendicular, said Charles M. Lieber, a professor of chemistry at the university.

This high polarization anisotropy enables the nanowires to act as detector elements. By monitoring changes in the photoconductivity of a single wire that was extended between contact electrodes made of layers of nickel, indium and gold, the researchers constructed single-nanowire photodetectors.

The manufacturing technique can be extended to create other optically interesting devices. Previously, Lieber's team reported the fabrication of nanoscale LEDs by crossing indium-phosphide nanowires -- one a doped N-type and the other P-type.

Lieber said the group hopes to incorporate the LEDs with nanowire-based photodetectors on the same chip. The pairs of photodetectors and LEDs may find an application as high-density optical interconnects and switches in nanoelectronics.