Photodiode Design Using Germanium Solves Key Challenge in on-Chip Light Monitoring

janrinok writes:

https://phys.org/news/2025-09-photodiode-germanium-key-chip.html

Programmable photonics devices, which use light to perform complex computations, are emerging as a key area in integrated photonics research. Unlike conventional electronics that transmit signals with electrons, these systems use photons, offering faster processing speeds, higher bandwidths, and greater energy efficiency. These advantages make programmable photonics well-suited for demanding tasks like real-time deep learning and data-intensive computing.

A major challenge, however, lies in the use of power monitors. These sensors must constantly track the optical signal's strength and provide the necessary feedback for tuning the chip's components as required. However, existing on-chip photodetectors designed for this purpose face a fundamental tradeoff. They either have to absorb a significant amount of the optical signal to achieve a strong reading, which degrades the signal's quality, or they lack the sensitivity to operate at the low power levels required without needing additional amplifiers.

As reported in Advanced Photonics, Yue Niu and Andrew W. Poon from The Hong Kong University of Science and Technology have addressed this challenge by developing a germanium-implanted silicon waveguide photodiode. Their approach overcomes the tradeoffs that have hindered existing on-chip power monitoring technologies.

A waveguide photodiode is a small light detector that can be integrated directly into an optical waveguide, which confines and transports light. Its purpose is to convert a small portion of the light traveling through the waveguide into an electrical signal that can be measured via more conventional electronics. One way to enhance this conversion is through ion implantation, a process that introduces controlled defects into the photodiode's silicon structure by bombarding it with ions.

If executed properly, these defects can absorb photons with energies too low for pure silicon, enabling the photodiode to detect light across a broader range of wavelengths.

Previous attempts to build such detectors used boron, phosphorus, or argon ions. While these approaches improved performance in some respects, they also introduced many free carriers into the silicon lattice, which in turn degraded optical performance. In contrast, the team implanted germanium ions. Germanium, a Group IV element like silicon, can replace silicon atoms in the crystal structure without introducing significant numbers of free carriers. This substitution allows the device to extend its sensitivity without compromising signal quality.

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