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Nanospikes sharpen germanium's appeal

11 May 2007

Femtosecond laser treatment could improve infrared photodetectors, solar cells and field emission displays.

Laser-textured germanium (Ge) could lead to a new breed of high-performance photodetectors, solar cells and electron emitters, claim scientists in the US. Inspired by microstructured silicon, researchers have used a laser to create performance-enhancing nanospikes on the germanium surface.

By increasing absorption and reducing reflection, the structured surface can make a detector more efficient. "At visible and infrared wavelengths, the tip diameter should be less than 1 µm," Mool Gupta of the University of Virginia's department of electrical and computer engineering told nanotechweb.org. "For field emission or biomedical applications such as drug delivery it would be beneficial to have tips measuring 10-100 nm."

To treat the surface, the researchers expose chipped portions of Ge wafer to 1.4 mJ pulses of 800 nm emission from a Ti:sapphire laser. Pulses are 130 fs in duration and fired at a repetition rate of 1 kHz on to a scanned sample surface. The process takes place in a chamber filled to 400 mbar with sulphur hexafluoride, a gas with excellent insulating and cooling properties.

"The vacuum requirement for our process is not very high (typically 10-3 mbar), so it should be very easy to automate the laser-surface treatment," explained Gupta. "Even if we had to go for high vacuum to control impurity levels then a load-lock system could be implemented to allow quick and easy sample loading."

SEM images reveal a surface covered with conical structures that have a base diameter of about 5 µm, a height of 10-15 µm and a tip diameter of just 100 nm. Tips can be sharpened further by a brief chemical-etching process that lasts just 10 seconds and gives a tip radius of approximately 10 nm.

The group is looking at the laser-texturing process in detail to understand better the formation of its so-called nanospikes. According to Gupta, the most significant parameters include laser fluence, the number of laser shots, gas pressure and sample scanning speed.

The researchers reported their work in Nanotechnology.

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