Ge/Si quantum dot photodetectors for midinfrared applications

Ge/Si quantum dots fabricated by molecular-beam epitaxy at 500 C are overgrown with Si at different temperatures T_cap, and their mid-infrared photoresponse is investigated. The photocurrent maximum shifts from 2.3 to 3.9 μm with increasing T_cap from 300 to 750 C. The best performance is achieved for the detector with T_cap= 600 C in a photovoltaic mode. At a sample temperature of 90 K and no applied bias, a responsivity of 0.43×10^-3 A/W and detectivity of 6:2×10¹⁰ cm Hz^1/2 /W at λ= 3 μm were measured under normal incidence infrared radiation. The device exhibits very low dark current (I_dark= 2 nA/cm² at T= 90 K and U= - 0:2 V) and operates until 200 K.