Strong quantum-confined Stark effect in germanium quantum-well structures on silicon

Silicon is the dominant semiconductor for electronics, but there is now a growing need to integrate such components with optoelectronics for telecommunications and computer interconnections1. Silicon-based optical modulators have recently been successfully demonstrated2, 3; but because the light modulation mechanisms in silicon4 are relatively weak, long (for example, several millimetres) devices2 or sophisticated high-quality-factor resonators3 have been necessary. Thin quantum-well structures made from III-V semiconductors such as GaAs, InP and their alloys exhibit the much stronger quantum-confined Stark effect (QCSE) mechanism5, which allows modulator structures with only micrometres of optical path length6, 7. Such III-V materials are unfortunately difficult to integrate with silicon electronic devices. Germanium is routinely integrated with silicon in electronics8, but previous silicon–germanium structures have also not shown strong modulation effects9, 10, 11, 12, 13. Here we report the discovery of the QCSE, at room temperature, in thin germanium quantum-well structures grown on silicon. The QCSE here has strengths comparable to that in III-V materials. Its clarity and strength are particularly surprising because germanium is an indirect gap semiconductor; such semiconductors often display much weaker optical effects than direct gap materials (such as the III-V materials typically used for optoelectronics). This discovery is very promising for small, high-speed14, low-power15, 16, 17 optical output devices fully compatible with silicon electronics manufacture.