Ge shape engineering unlocks ultra-low back reflection in high-speed Ge-on-Si photodetectors
Shape‑engineered Ge‑on‑Si photodetectors deliver ‑36 dB optical return loss, ~60 GHz bandwidth, 0.95 A/W responsivity, and <30 nA dark current ‑ achieved through innovative Ge shaping in a 300 mm CMOS silicon
photonics flow, enabling robust, IEEE‑compliant solutions for AI data centers and co‑packaged optics.
BY YUSHENG BIAN, SUJITH CHANDRAN, ABDELSALAM ABOKETAF, WON SUK LEE, QIDI LIU, MASSIMO SORBARA, BOB MULFINGER AND RYAN SPORER, GLOBALFOUNDRIES; EDGAR HUANTE-CERON, RANOVUS
As link speeds surge for artificial intelligence (AI) clusters and cloud data center interconnects (DCI), back reflection from optical components has emerged as a subtle but escalating threat to link stability and signal integrity. Reflections drive standing waves, jitter, and even laser instabilities; in dense links, they also inflate bit error rate (BER) and degrade signal to noise ratio (SNR). IEEE 802.3 standards enforce tight specifications on both transmitter (TX) and receiver (RX) reflectance to ensure link stability and compliance [1] ‑ shifting some margin from TX to RX can relieve costly TX‑side constraints at 100G+/lane. From the field’s perspective, ORL is the system‑level sum of Fresnel reflections and Rayleigh backscatter along the light path; controlling it at the component level (including photodetectors (PDs) is essential to tame end‑to‑end reflection budgets in modern links.
Our contribution: shape engineered Ge on Si PIN PDs
In a CMOS monolithic silicon photonics flow [2-9], we redesigned the Ge absorption geometry ‑ moving beyond the conventional rectangular mesa (Fig.1(a)) to angled, convex/concave, quadrilateral, and pentagon shapes that steer reflected light away from the Si waveguide taper, as shown in Figs.1(b)‑(e)). The result is a family of PDs that achieve ORL down to ‑36 dB while preserving high responsivity (~0.95 A/W), low dark current (<30 nA median at ‑1 V), and ~60 GHz 3 dB opto‑ electrical (OE) bandwidth (BW) in the O band [10]. Why it matters: Reducing PD originated reflections relaxes upstream laser/ isolator demands and makes it easier to meet stringent ORL budgets in pluggables and future co packaged optics (CPO) ‑ without exotic process steps.
Figure 1. Ge-on-Si PD shape variations 3D perspective views of Ge-on-Si photodetector designs illustrating geometry-driven reflection control: (a) Reference PD with rectangular Ge; (b) convex-shaped Ge; (c) concave-shaped Ge; (d) quadrilateral-shaped Ge; (e) pentagon-shaped Ge. These alternative geometries redirect reflected light away from the Si waveguide taper to minimize ORL
- Reference (rectangular Ge): higher reflection at PD-WG interface (time domain peaks.
- Shape engineered PDs: multi dB ORL reductions across all variants; best -36 dB => -41 dB at 1310 nm; tight lot to lot dispersion.
- Dark current: median <30 nA; most devices <100 nA, consistent with high quality Ge epitaxy and junction control (Fig.3 (c)).
- Responsivity: >0.85 A/W median; up to ~0.95 A/W, matching or exceeding the reference PD (Fig.3 (d)).
- Photocurrent remains linear up to several milliwatts; as expected, response begins to roll off at higher powers due to carrier recombination/screening, but increasing reverse bias expands the linear region (e.g., 10% roll off point shifts from ~3.35 mW to ~4 mW at room temperature) (Figs.3 (e) – (g)). The power handling vs. temperature trade off follows known high carrier density dynamics in Ge PDs.
- Normalised OE frequency response shows ~60 GHz 3 dB bandwidth at ~0.2 mA photocurrent; still >50 GHz at ~1 mA (Fig.4). Bandwidth rolls off moderately at higher photocurrents, consistent with carrier screening; we’re exploring implant/profile refinements to further stabilise high power bandwidth.
- Across ORL, DC, and RF metrics, no speed/responsivity penalty accompanies reflection suppression the trade off is effectively neutralised through geometric shaping.
- Co-optimisation with couplers, junctions, and electrical parasitics to further improve responsivity and bandwidth across a wider operating range.
- Integration with TIA and eyediagram analysis to assess link-level performance and confirm compliance with high-speed standards.
- Module-level validation (e.g., with isolator-free or reduced-isolation TX) to quantify system-level penalties recovered by low-reflection PDs particularly in O-band pluggables and CPO engines.
Figure 4. High-speed performance (a) Normalised opto-electric frequency response of the angled PD at varying input optical powers (photocurrent range: 0.2 mA to 2 mA). (b) Extracted 3 dB EO bandwidth as a function of photocurrent, demonstrating ~60 GHz at low photocurrent and >50 GHz at 1 mA, with moderate roll-off at higher powers due to carrier screening.
- Start with geometry: If you see stubborn reflection peaks at the PD-WG interface in OFDR, try angled or asymmetric Ge tips to dump reflected power off axis. This is a layout-level adjustment using the existing mask set and remains fully compatible with foundry design rules.
- Validate statistically: Measure dozens of dies; reflection is phase sensitive and can hide behind path length variations. OFDR’s spatial mapping reveals true contribution.
- Co-design bias & power: Expect bandwidth roll off at higher photocurrent due to screening; allocate headroom in reverse bias to extend linearity and BW in deployment.
- Think system budgets: A few dB better ORL at the PD can shift margin in IEEE style allocations and de-risk both RX and TX. Coordinate with optics + packaging teams early.
