Silicon photonics
Which is a correct reason germanium (Ge) is widely used for telecom-wavelength photodetectors in silicon photonic circuits?
- AIt is a direct-gap material, so it can serve both as a photodetector and as an efficient laser
- BIt can absorb light in the 1.31 µm and 1.55 µm bands, and it can be grown on silicon and readily integrated in CMOS fabs
- CIts band gap is larger than silicon's, so almost no dark current flows
Show answer and explanation
Answer: B. It can absorb light in the 1.31 µm and 1.55 µm bands, and it can be grown on silicon and readily integrated in CMOS fabs
Silicon cannot absorb light at wavelengths longer than about 1.1 µm, so telecom-wavelength detection needs a material that absorbs lower-energy light. Ge is an indirect-gap material, but its direct gap is close by at about 0.8 eV, so it absorbs strongly up to around 1.55 µm. As a group-IV element like Si, it can be grown selectively by epitaxy on Si and handled with CMOS fab equipment, which is a major advantage. InGaAs photodetectors perform well but are difficult to build directly on silicon. Ge's band gap (about 0.66 eV) is smaller than Si's, so its dark current is actually on the high side.