Silicon photonics
What is the main cause of the tendency toward high dark current in germanium photodetectors grown on silicon?
- AGe is a direct-gap material, so part of the absorbed light is re-emitted and reabsorbed, increasing the current
- BGe cannot absorb telecom-wavelength light, so all the current that flows is dark current
- CSi and Ge differ in lattice constant by about 4%, so crystal defects such as dislocations readily form in the grown film
Show answer and explanation
Answer: C. Si and Ge differ in lattice constant by about 4%, so crystal defects such as dislocations readily form in the grown film
Dark current is current that flows even when no light is incident; it adds noise to the photodetector and degrades sensitivity. Because Si and Ge differ in lattice constant by about 4%, growing Ge on Si creates defects such as (threading) dislocations as strain is relieved, and carriers are readily generated through defect levels. In addition, Ge's band gap (about 0.66 eV) is smaller than Si's, so more carriers are thermally generated. Dark current is suppressed by reducing dislocations with post-growth annealing or by shrinking the detector area. Ge is an indirect-gap material, and light emission has almost no effect.