Roughing Up Silicon Improves Near-Infrared Performance

Hank Hogan

Despite its electronic and visible spectrum prowess, silicon is an infrared weakling. Beyond 1100 nm, the material absorbs little radiation and, therefore, the response of silicon-based photodiodes fades to almost nothing.

Now a research team from Harvard University in Cambridge and Radiation Monitoring Devices Inc. in Watertown, both in Massachusetts, have developed a technique that uses a femtosecond laser to significantly improve the near-IR performance of avalanche photodiodes, boosting the quantum efficiency of these devices by almost half at 1064 nm.

Microstructuring the silicon surface of an avalanche photodiode with femtosecond laser pulses boosts near-IR quantum efficiency from ~25 percent at 1064 nm to >45 percent without harming gain or noise characteristics. In this image, half the surface of an avalanche photodiode has been microstructured (dark region). Courtesy of Radiation Monitoring Devices Inc.

The company makes avalanche photodiodes that achieve quantum efficiencies of up to 80 percent in the visible wavelength range. However, that number drops to 30 percent or less at 1064 nm. Techniques such as the use of a thicker detector or a semiconductor made of a material other than silicon could boost efficiency. However, these approaches suffer from increased noise, higher cost, more difficulty in manufacturing or a lessened ability to integrate with silicon-based electronics.

The company is collaborating with a group led by Eric Mazur, a professor of physics and applied physics, to see whether surface microstructuring could improve the near-IR performance of its avalanche photodiodes. His team had discovered that femtosecond laser bursts roughened the silicon surface, producing what is known as black silicon and leading to changes in optical performance.

In the partnership, the company not only supplied the devices, but also tested and packaged them. The university researchers used a custom Ti:sapphire laser operating at 800 nm with 100-fs pulses for the microstructuring. They created a 150-μm-diameter focal spot on the avalanche photodiode being treated, moving the spot so that the entire 2 × 2-mm device’s surface was subjected to laser pulses. They varied the power and number of pulses in atmospheres of sulfur hexafluoride, air, hydrogen sulfide or nitrogen.

The group found that, under the right conditions, it could boost avalanche photodiode quantum efficiency at 1064 nm to 58 percent while preserving the device’s gain and noise characteristics. The increase indicated fundamental changes in the material’s optoelectronic properties, which Mazur said ultimately resulted from a high concentration of surface impurities. The devices with the highest quantum efficiency had close to 1 percent of sulfur in a 200-nm-thick layer at the surface. “It is that heavy doping that changes the absorption characteristics,” Mazur said.

The enhancement was accomplished by irradiating the devices on a wafer in a blanket fashion, making the process commercially interesting. However, it might be that modifying the avalanche photodiode structure will result in further enhancement. Determining that will require further experimentation.

The method has potential use in free-space optical communications, for example, noted Richard A. Myers, a senior scientist at the company. “A line of enhanced avalanche photodiodes and avalanche photodiode arrays can readily be fabricated. Commercial markets are currently being explored,” he said.

Applied Optics, Dec. 10, 2006, pp. 8825-8831.