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 interconnections(1). Silicon-based optical modulators have recently been successfully demonstrated(2,3); but because the light modulation mechanisms in silicon(4) are relatively weak, long ( for example, several millimetres) devices(2) or sophisticated high-quality-factor resonators(3) 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) mechanism(5), which allows modulator structures with only micrometres of optical path length(6,7). Such III-V materials are unfortunately difficult to integrate with silicon electronic devices. Germanium is routinely integrated with silicon in electronics(8), but previous silicon - germanium structures have also not shown strong modulation effects(9-13). 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-speed(14), low-power(15-17) optical output devices fully compatible with silicon electronics manufacture.