Femtosecond Laser Promotes Rapid, Localized Nanowire Growth

Lauren I. Rugani

Optoelectronic devices, including blue laser diodes, LEDs and photodetectors, rely on the wide bandgap (2.8 eV) of semiconductor ZnSe nanowires. Because the fundamental optical and electronic properties of these nanowires depend on size, several growth techniques exist to generate them with controlled dimensions.

A team of researchers from the University of Tokyo, from Zhongshan University in Guangzhou, China, and from the Chinese Academy of Sciences in Shanghai have applied pulsed laser deposition to grow ZnSe nanowires on crystal surfaces. Using a Spectra-Physics Ti:sapphire regenerative amplifier, they irradiated crystals with 130-fs, 800-nm pulses to ablate the surface of the crystal and produce craters.

The scientists discovered significant nanowire growth within the craters on ripples formed by the laser focus, leaving the surrounding areas unaffected. “We can grow nanowires on the designated position without polluting the neighboring area, which is important in the application of optoelectronic devices,” explained Tianqing Jia, a research fellow at the University of Tokyo.

The resulting nanowires are about 1.3 μm in length, with diameters of 80 and 30 nm at the root and tip, respectively, and they grew at a rate of ~5 μm/s, which is about 10⁴ times faster than that achieved by previous methods. The nanowire dimensions can be altered by adjusting the laser parameters. For example, the lengths of the nanowires increase to about 1.5 μm, and the diameter at the middle reaches 70 nm as the pulse energy decreases to between 1 and 2 μJ. However, when the energy drops below 1 μJ, the nanowire length decreases to 0.7 μm. Similarly, changing the wavelength of the pulses from 800 to 400 nm results in shorter nanowires.

An SEM image shows an array of nanowires grown after 500 pulses from an 800-nm laser with pulse energy of 2 μJ.

Through dispersive x-ray spectroscopy analysis using an Oxford dispersive spectrometer, the researchers found the nanowire growth to be a self-catalyzed process. The laser pulses stimulate the ZnSe crystal to decompose, resulting in a zinc:selenium molar ratio greater than 1:1 and in formation of Zn:ZnSe colloid particles. Nanowire growth is initiated after these particles reach supersaturation.

Images taken with a JEOL scanning electron microscope show the nanowires’ preferential growth toward the focus center and quasi-aligned arrays grown on the sidewalls of the craters. To obtain a regular array of nanowires, they will attempt to use ZnSe crystals with a surface orientation other than 111, Jia noted. The researchers also performed micro-Raman spectroscopy and found peaks at 251 and 205 cm^–1 that correspond to the longitudinal and transverse optical modes.

With pulsed laser deposition, the team has found a rapid method for growing uniform ZnSe nanowires in localized positions. Combined with the ability to control the dimensions, this technique can be used to generate nanowires with desired optical and electronic properties in microscopic optoelectronic applications.

Applied Physics Letters, Sept. 4, 2006, 101116.