Fabrication and characterisation of the PiN Ge photodiode with poly-crystalline Si:P as n-type region

Germanium (Ge) PiN photodetectors are fabricated and electro-optically characterised. Unintentionally and p-type doped Ge layers are grown in a reduced-pressure chemical vapour deposition tool on a 200 mm diameter, <001>-oriented, p-type silicon (Si) substrates. Thanks to two Ge growth temperatures and the use of short thermal cycling afterwards, threading dislocation densities down to 10⁷ cmˉ² are obtained. Instead of phosphorous (P) ion implantation in germanium, the authors use in situ phosphorous-doped poly-crystalline Si (poly-Si) in the n-type regions. Secondary ion mass spectrometry revealed that P was confined in poly-Si and did not diffuse in Ge layers beneath. Over a wide range of tested device geometries, production yield was dramatically increased, with almost no short circuits. At 30 °C and at −0.1 V bias, corresponding to the highest dynamic resistance, the median dark current of 10 μm diameter photodiodes is in the 5–20 nA range depending on the size of the n-type region. The dark current is limited by the Shockley-Read-Hall generation and the noise power spectral density of the current by the flicker noise contribution. A responsivity of 0.55 and 0.33 A/W at 1.31 and 1.55 μm, respectively, is demonstrated with a 1.8 μm thick absorption Ge layer and an optimized anti-reflection coating at 1.55 μm. These results pave the way for a cost-effective technology based on group-IV semiconductors.