Nat. Photonics: Low-Noise, High-Efficiency Avalanche Photodiodes Enabled by Photon Trapping

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Highlights

Recently, the team of Joe C. Campbell of the University of Virginia(UVA) published a study.

Photon trapping enables high efficiency in thin absorber to suppress dark current and improve SNR.
Designed SACM APD structure achieves low noise and dark current density 3 orders lower than HgCdTe APDs.
Measured 22-24% quantum efficiency with photon trapping, first demonstration for mid-IR APDs.
Gain reaches 700 at 240K, 4x higher than prior 2 μm APDs. Bandwidth 7 GHz with gain-bandwidth product >200 GHz.
Overall 70x higher SNR than previous AlInAsSb APDs, 20x higher than HgCdTe APDs. Breaks tradeoff between dark current and efficiency.

Background

Mid-infrared (2-5μm wavelength) photonics shows promise for sensing, spectroscopy, diagnostics, and communications applications. This spurs demand for high-performance mid-infrared photodetectors. A key figure of merit is the signal-to-noise ratio (SNR), limited by the high dark current in narrow bandgap absorber materials like HgCdTe that detect mid-infrared light. To suppress dark current, these materials require cryogenic cooling. Avalanche photodiodes (APDs) provide internal gain to amplify weak signals, but also amplify shot noise. Further, narrow bandgap absorbers in APDs suffer from excessive dark current that degrades SNR. Reducing absorber thickness suppresses dark current but also reduces quantum efficiency and SNR. Approaches like photonic crystals or plasmonic structures enable light trapping to enhance absorption in thin layers, but have not been shown for mid-infrared APDs. Developing APDs that suppress dark current yet maintain efficiency would improve mid-infrared detection SNR.

Results

The designed separate absorption, charge, and multiplication (SACM) avalanche photodiode (APD) achieved a maximum gain of ~700 at 240K, the highest reported for 2μm APDs. The excess noise was comparable to Si APDs. The ultrathin 200nm absorber layer enabled reduced dark current density approximately 3 orders of magnitude lower than state-of-the-art HgCdTe APDs and 2 orders lower than previous AlInAsSb APDs.

Incorporating metal grating photon trapping structures yielded high quantum efficiency while maintaining the ultrathin absorber. Measured external quantum efficiency reached 22-24%, over 3 times higher than the planar device without photon trapping. This is the first demonstration of photon trapping for mid-infrared APDs.

The APDs also demonstrated high speed performance, with a 3dB bandwidth of ~7GHz and a gain-bandwidth product exceeding 200GHz. These values surpass previous 2μm APDs by over 4 times.

Overall, the photon-trapping enhanced SACM APD achieved a signal-to-noise ratio approximately 70 times higher compared to previous AlInAsSb APDs and 20 times higher than state-of-the-art HgCdTe APDs. This performance improvement is attributed to the combination of low excess noise, ultralow dark current enabled by the thin absorber, and high quantum efficiency provided by the photon trapping structures. This breaks the traditional tradeoff between low dark current and high quantum efficiency for mid-infrared photodetectors.

Methods

Designed and grew by MBE a separate absorption, charge, and multiplication (SACM) APD structure with AlInAsSb digital alloy materials lattice-matched on GaSb substrate.
200 nm thick AlInAsSb absorber layer to suppress dark current. Incorporated metal grating arrays for photon trapping.
Fabricated circular mesa devices by photolithography and wet etching. Added metal gratings by e-beam lithography and lift-off.
Characterized I-V curves, gain, excess noise, dark current, quantum efficiency, and frequency response. Compared to prior HgCdTe and AlInAsSb APDs.
Simulated band structure, electric field, and quantum efficiency enhancement.

Conclusions

This work shows photon trapping can achieve low dark current and high efficiency in mid-IR APDs, breaking a key performance tradeoff. The approach could extend to longer wavelength APDs. Further optimization of the photon trapping design may improve efficiency beyond 22-24% attained here. Using a semi-insulating substrate instead of GaSb would increase bandwidth past 7GHz. The low excess noise and dark current match state-of-the-art HgCdTe APDs at far higher temperatures, reducing cooling needs. This enables high-performance mid-IR detection without cryogenic cooling, expanding potential applications in sensing, spectroscopy, diagnostics, and communications. By combining low noise, dark current, and high speed and efficiency, photon trapping provides a path to realize the promise of mid-IR photonics across fields.

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Fig. 3: Current–voltage, measured gain and excess noise measurements.

Fig. 5: EQE and bandwidth of the edge-coupled waveguide APD.

References:

https://www.nature.com/articles/s41566-023-01208-x