Spanish Physicists Build a Silicon Chip That Can 'See' Infrared Light

Short-wave infrared (SWIR) light sits just beyond what the human eye can see. Detecting it is useful for night imaging, medical imaging, environmental monitoring and industrial inspection. But most SWIR detectors rely on expensive materials that are hard to combine with everyday electronics. Silicon, the material used in most chips, normally cannot absorb SWIR light at all, because its band gap is too wide.

Researchers at Complutense University of Madrid have built a silicon photodiode that can efficiently absorb SWIR light and is compatible with standard chip manufacturing. The device, described in a paper in Physical Review Letters, is based on silicon doped with a high concentration of tellurium atoms, a process called hyperdoping. “Our primary objective was to essentially democratize SWIR technology,” said Eric García-Hemme, the paper's first author. “By enabling silicon to see in this spectrum, we can drastically lower manufacturing costs.”

The tellurium atoms create new energy levels inside the silicon, acting as steppingstones that let low-energy SWIR photons excite electrons and generate a signal. Earlier hyperdoped silicon detectors could only be made in very thin layers, so they absorbed little light. García-Hemme's team solved this by micro-texturing the silicon surface with tiny pyramids and adding a gold mirror behind it, which traps light so it bounces back and forth inside the thin layer instead of passing through.

In tests, the photodiode absorbed about 85% of incoming light and reached a peak external quantum efficiency of 2.3% at a wavelength of 1.27 micrometers, roughly 1,000 times as high as commercially available silicon light detectors. At room temperature and a wavelength of 1.5 micrometers, it achieved a specific detectivity of 4×10¹⁰ Jones, which the researchers describe as a record for hyperdoped silicon.

Because it is based on silicon, the device can be made at the same facilities that already produce computer chips and smartphone cameras. It also works efficiently at room temperature, unlike many infrared detectors that need bulky cooling systems. “The practical implications are very exciting, as this work could bridge the gap between high-end scientific instrumentation and everyday consumer electronics,” García-Hemme said, adding that it could eventually be built into smartphones, tablets and laptops to improve night vision and facial recognition.