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Near-infrared response for laser etched silicon

23 Aug 2006

A microstructured silicon photodetector that exhibits a photoresponse at 1.31 and 1.55 microns could inspire a host of specialized imaging applications.

Scientists are using the same microstructuring approach that enhances the efficiency of silicon solar cells to widen the spectral response of photodetectors. (Appl. Phys. Lett. 89 033506)

"The challenges lay in understanding the material's physical properties and how they can be controlled and used to improve device performance," Jim Carey of Harvard University, US, told optics.org.

Conventionally, silicon is transparent to wavelengths longer than 1 µm, which makes it unsuitable for many near-infrared applications. However, the researchers have found a way of modifying the material's band-gap to make it absorb at longer wavelengths.

Using a Ti:sapphire laser, Carey and his colleagues irradiate n-doped Si wafers with a 1 kHz train of 100 fs pulses in an sulfur-rich atmosphere to generate a surface covered with 2-3 micron-sized structures. According to the team, the laser causes ablation and melting of the silicon surface, which evolves and interacts with the gas before re-solidifying with an altered morphology.

The detector's microstructured surface encourages multiple reflections, which promote the absorption of light. However, this is only part of the picture. "It is a combination of increased absorption in the infrared and large gain that leads to the extension of the operating wavelength," said Carey. "The incorporation of large amounts of sulfur during laser irradiation is responsible for significant absorption beyond 1100 nm."

Photodetectors made from the textured silicon were found to have a responsitivity of 92 A/W at 850 nm and 119 A/W at 960 nm (3V reverse bias in both cases). What's more, the devices continued to exhibit a photoresponse at 1.31 and 1.55 µm.

The group, which also includes scientists from the University of Texas and the University of Virginia, both US, is now looking to commercialize its technology. Carey expects that the first big market will be in specialized imaging applications such as security and surveillance. He thinks that it is unlikely that devices will make their way into consumer items such as camera phones. "Margins are too low and the end customer doesn't care enough about the infrared to drive a premium," he commented.

Author

James Tyrrell is News Editor on Optics.org and Opto & Laser Europe magazine.

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