Germanium-on-silicon photodetector

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 bandgap (0.66 eV) gives an absorption cutoff near 1880 nm, covering all standard telecom wavelengths (1260 – 1625 nm).

Why Ge-on-Si is the dominant silicon-photonic detector. Silicon's bandgap is 1.12 eV, with absorption cutoff at 1107 nm — making 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 fully CMOS-compatible (already used as transistor channel material in some Intel processes) and can be deposited in standard front-end-of-line processes

Modern silicon photonic foundries (AIM Photonics, IMEC iSiPP50G, GlobalFoundries 9WG, Tower) all offer Ge-on-Si waveguide photodetectors as a standard PDK component.

Standard structure.

Layer	Composition	Thickness
Top contact	Heavily-doped p+ Si or Ge:B	100 – 200 nm
Absorber	Pure Ge (intrinsic)	200 – 500 nm
Buffer / strain-relaxation	SiGe gradient	100 – 300 nm
Bottom contact	Heavily-doped n+ Si	200 – 500 nm
Silicon waveguide	SOI strip / rib	220 nm SOI

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 $-2$ V	10 – 200 nA
Active area	5 – 30 μm length × 1 – 4 μm width
Operating bias	$-1$ to $-3$ V

Bandwidth-vs-responsivity tradeoff. Longer Ge regions give higher absorption (higher responsivity) but more transit-time delay and higher capacitance (lower bandwidth). Modern designs use:

• Lateral p-i-n structures with electrodes alongside the absorber to reduce transit distance
• Uni-traveling carrier (UTC) designs where only electrons (faster carriers) drift through the absorber
• 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 $\sim 100$ nA per Ge-on-Si PD — significantly higher than InGaAs PDs ($\sim 0.5 - 5$ nA), but acceptable for telecom receivers operating at $> -20$ 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. Gain × bandwidth products exceed 270 GHz; effective sensitivity at 25 Gb/s reaches $-28$ dBm — competitive with InGaAs APDs at significantly lower cost.

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: Vivien & Vakarin (eds.), Silicon Photonics III: Systems and Applications (Springer, 2018), Ch. 9 on Ge-on-Si photodetectors; Michel, Liu, Kimerling, High-performance Ge-on-Si photodetectors, Nature Photonics 2010.