High bandwidth Ge p-i-n photodetector integrated on Si

The authors present a germanium on silicon $p\text{-}i\text{-}n$ photodiode for vertical light incidence. For a Ge $p\text{-}i\text{-}n$ photodetector with a radius of $5\mu \mathrm{m}$ a $3\mathrm{dB}$ bandwidth of $25\mathrm{GHz}$ is measured at an incident wavelength of $1.55\mu \mathrm{m}$ and zero external bias. For a modest reverse bias of $2\mathrm{V}$ , the $3\mathrm{dB}$ bandwidth increases to $39\mathrm{GHz}$ . The monolithically integrated devices are grown on Si with solid source molecular beam epitaxy. The complete detector structure consisting of a highly $p$ -doped Ge buried layer, an intrinsic absorption region, and a highly $n$ -doped top contact layer of $\mathrm{Ge}∕\mathrm{Si}$ is grown in one continuous epitaxial run. A low growth temperature sequence was needed to obtain abrupt doping transitions between the highly doped regions surrounding the intrinsic layer. A theoretical consideration of the $3\mathrm{dB}$ bandwidth of the Ge detector was used to optimize the layer structure. For a photodiode with $5\mu \mathrm{m}$ mesa radius the maximum theoretical $3\mathrm{dB}$ frequency is $62\mathrm{GHz}$ with an intrinsic region thickness of $307\mathrm{nm}$ .