Ge quantum-dot polysilicon emitter heterojunction phototransistors for 1.31–1.55 μm light detection

A Si/SiGe HBT-type phototransistor with 10 Ge-dot layers (8ML in each layer, separated by 30 nm Si spacer) incorporated in the base-collector junction was grown by UHV-CVD. To achieve low dark current density and high photo-responsivity, a high-low emitter profile consisting of a heavily doped polysilicon contact on top of a thin epitaxial emitter cap has been fabricated. At room temperature, I–V measurement showed a low dark current density of 3 × 10−5 A/cm2 at 3 V. The measured breakdown voltage BVceo was about 12 V. A photo-responsivity of 1.94 and 0.028 mA/W was achieved at 1.31 and 1.55 μm for normal incidence, respectively. Compared to a p–i–n reference detector with the same quantum-dot layer, the responsivity is improved by a factor of 45 and 20 at 1.31 and 1.55 μm, respectively.