Designer germanium quantum dot phototransistor for near infrared optical detection and amplification

We demonstrated a unique CMOS approach for the production of a high-performance germanium (Ge) quantum dot (QD) metal-oxide-semiconductor phototransistor. In the darkness, low off-state leakage (I_off ∼ 0.27 pA μm^-2), a high on-off current ratio (I_on/I_off ∼ 10⁶), and good switching behaviors (subthreshold swing of 175 mV/dec) were measured on our Ge-QD phototransistor at 300 K, indicating good hetero-interfacial quality of the Ge-on-Si. Illumination makes a significant enhancement in the drain current of Ge QD phototransistors when biased at both the on- and off-states, which is a great benefit from Ge QD-mediated photoconductive and photovoltaic effects. The measured photocurrent-to-dark-current ratio (I_photo/I_dark) and the photoresponsivities from the Ge QD phototransistor are as high as 4.1 × 10⁶ and 1.7 AW^-1, respectively, under an incident power of 0.9 mW at 850 nm illumination. A superior external quantum efficiency of 240% and a very fast temporal response time of 1.4 ns suggest that our Ge QD MOS phototransistor offers great promise as optical switches and transducers for Si-based optical interconnects.