Suppression of hole relaxation in small-sized Ge/Si quantum dots

We study the effect of quantum dot size on the mid-infrared photocurrent, photoconductive gain, and hole capture probability in ten-period $p$-type Ge/Si quantum dot heterostructures. The dot dimensions is varied by changing the Ge coverage during molecular beam epitaxy of Ge/Si(001) system in the Stranski–Krastanov growth mode while keeping the deposition temperature to be the same. A device with smaller dots is found to exhibit a lower capture probability and a higher photoconductive gain and photoresponse. The integrated responsivity in the mid-wave atmospheric window ($\lambda =(3{-}5)\,\mu$m) is improved by a factor of about $8$ when the average in-plane dot dimension changes from $18$ to $11$ nm. The decrease of the dot size is expected to reduce the carrier relaxation rate due to phonon bottleneck by providing strong zero-dimensional quantum mechanical confinement.