Germanium-on-silicon photodetector
A photodiode using a germanium absorber layer epitaxially grown on a silicon substrate, integrated with silicon photonic waveguides. The standard receiver element for silicon photonic integrated circuits at telecom wavelengths.
A germanium-on-silicon photodetector uses pure or strain-engineered germanium as the absorber layer, epitaxially grown on a silicon substrate and integrated with silicon photonic waveguides. Germanium's indirect bandgap (0.66 eV) sets the absorption edge near 1880 nm, but strong absorption comes from the 0.80 eV direct gap (about 1550 nm), so responsivity is high across the O- to C-bands and falls off in the L-band unless tensile strain shifts the edge to longer wavelengths.
Why Ge-on-Si is the dominant silicon-photonic detector. Silicon's bandgap is 1.12 eV, with absorption cutoff at 1107 nm. That makes it transparent (and useless as an absorber) for telecom O-, S-, C-, and L-band wavelengths. The two natural alternatives are:
- Bond III-V detectors onto silicon photonics: works but is expensive (per-die bonding) and adds process complexity
- Grow Ge directly on silicon: Ge is CMOS-compatible (silicon-germanium is already used in strained transistors) and can be grown selectively on the silicon by epitaxy within a silicon photonic process flow
Silicon photonic foundries (for example AIM Photonics, imec, GlobalFoundries and Tower) offer Ge-on-Si waveguide photodetectors as a standard PDK component.
Standard structure.
| Region | Composition | Notes |
|---|---|---|
| Silicon waveguide | SOI rib or strip, typically 220 nm SOI | Carries light into the detector |
| Absorber | Intrinsic Ge, a few hundred nm thick | Grown selectively on the silicon, starting with a low-temperature seed layer that takes up the lattice mismatch |
| Contacts, lateral p-i-n | p⁺ and n⁺ implants in the silicon on either side of the Ge | A common foundry layout |
| Contacts, vertical p-i-n | Doped silicon below the Ge and a doped region on top of it | Used in some processes |
Light enters from the side via a tapered waveguide that gradually overlaps with the Ge region. The Ge absorbs the photon, generating an electron-hole pair that is swept by the reverse-bias electric field to the contacts.
Performance specifications for typical foundry-PDK waveguide-integrated Ge-on-Si PDs at 1550 nm:
| Parameter | Typical value |
|---|---|
| Responsivity | 0.6 – 1.0 A/W |
| Bandwidth | 25 – 50 GHz (high-speed designs to 67 GHz) |
| Dark current at V | 10 – 200 nA |
| Active area | 5 – 30 μm length × 1 – 4 μm width |
| Operating bias | to V |
Bandwidth-vs-responsivity tradeoff. Longer Ge regions give higher absorption (higher responsivity) but higher capacitance (lower RC bandwidth); the carrier transit time is set by the width of the intrinsic region the carriers cross, not by the length. Modern designs use:
- Lateral p-i-n structures with electrodes alongside the absorber to reduce transit distance
- Uni-traveling carrier (UTC) designs, in which only electrons, the faster carriers, cross the depleted collection region
- Tapered/distributed designs that distribute light absorption along a long Ge section while keeping individual electrical segments short
Key challenge: dark current. Pure Ge on Si has lattice mismatch of 4%, producing high threading-dislocation density at the Ge/Si interface unless careful buffer engineering is used. Defects act as generation-recombination centers, raising dark current. Mature foundry processes have nA per Ge-on-Si PD, significantly higher than InGaAs PDs ( nA) but acceptable for telecom receivers operating at dBm signal levels.
Avalanche operation. Ge-on-Si avalanche photodiodes (Ge-Si APDs) using silicon's separate-absorption-multiplication (SAM) architecture combine Ge absorption with avalanche multiplication in the silicon region. Monolithic Ge/Si APDs reported in 2009 reached a gain-bandwidth product of 340 GHz and a sensitivity of dBm at 10 Gb/s (Kang et al., 2009), competitive with InGaAs APDs, and later designs have gone well beyond that gain-bandwidth product.
Applications.
- Datacom transceivers (100G, 400G, 800G): Ge-on-Si is the default receiver
- Coherent transceivers: balanced Ge-on-Si receivers for 100G/400G/800G coherent
- LIDAR receivers at 1.55 μm
- High-speed instrumentation
- On-chip photonic computing (matrix multiplications, optical interconnects)
References: J. Michel, J. Liu and L. C. Kimerling, "High-performance Ge-on-Si photodetectors," Nature Photonics 4, 527 (2010); Y. Kang et al., "Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain–bandwidth product," Nature Photonics 3, 59 (2009).