A GeSn-on-silicon photodetector showed a spectral cutoff at 2.6 micrometres, extending its measured infrared response beyond 2 micrometres. At 78 K, it generated photocurrent under illumination at 1.3, 1.55 and 2.3 micrometres. At a bias of 72 volts and an optical power of 165 nanowatts, its external quantum efficiency reached 163%. The authors interpret an efficiency above 100% as evidence of avalanche multiplication in the high-field silicon region. In ordinary terms, that means additional charge carriers amplify the measured signal.
Built directly on silicon
The device was built on a 4-inch silicon (100) wafer, with germanium-tin, or GeSn, grown directly on the silicon. The report does not state how many wafers or devices were tested. The reported growth conditions were 340 degrees Celsius for 20 minutes, a Ge/Sn ratio of around 1500 and a reactor pressure of 50 Torr. Fabrication used ICP RIE, a plasma-based etching method, to form circular GeSn mesas, or device islands, 25 to 200 micrometres in diameter, followed by ion implantation to dope silicon regions.
The absorber was thin and uneven
The absorber was thin and uneven. X-ray reciprocal-space mapping suggested that the layer was largely strain-relaxed. Transmission electron microscopy showed a continuous but irregular film 50 to 80 nanometres thick, with the tin content graded from 3 to 6 atomic percent. It contained both faceted crystalline regions and highly dislocated regions, or areas with many crystal defects.
Current rose near breakdown
Electrical measurements at 78 K found 1.12 nanoamps of dark current in a 100-micrometre GeSn-on-silicon device at 40 volts. The current continued to increase as the device approached breakdown at 72 volts. The paper reports that the breakdown voltage was independent of device diameter. At 0.5 volts, the reported rectification ratios were around 104 for the silicon reference device and 105 for the GeSn-on-silicon device, both with 100-micrometre diameters.
How the gain was assessed
To measure the device response, the researchers used current-voltage tests and Fourier-transform infrared spectral-responsivity measurements. They calculated external quantum efficiency, or EQE, from the photocurrent after subtracting dark current and accounting for the energy of the incoming photons. EQE exceeded 1% above 60 volts and reached 163% at the 72-volt breakdown bias, with optical power of 165 nanowatts. The authors state that an EQE above 100% confirms avalanche multiplication.
The constraints are still clear
The authors say the observed performance is constrained more by optical absorption in the thin GeSn layer than by the avalanche multiplication mechanism. A time-dependent test showed that the normalized current decayed slowly after the light was turned off. The authors associate that behaviour with relatively low bandwidth and limitations for single-photon counting.
A result that needs replication
Questions about reproducibility remain. The report does not give the number of independent replicates or uncertainty estimates. The detailed EQE and avalanche measurements were made at 78 K, so room-temperature avalanche performance was not established in this study. The document is an arXiv version 1 preprint dated 27 August 2026. Further work will need to show whether the behaviour repeats across independently fabricated devices and wafers.
Disclosure
The authors declare no conflicts of interest. They acknowledge support from NSERC Canada, Canada Research Chairs, the Canada Foundation for Innovation, Mitacs, PRIMA Québec and Defence Canada through IDEaS.
Paper data and sources
Original title: Extending Silicon Avalanche Photodetection Beyond 2 ~μ$m by Direct GeSn Integration
Authors: M. R. M. Atalla, J. Bélec, E. Rahier et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-27
DOI: Not available
Original paper · Full text