Fabrication and characterization of sensitive vertical P-i-N Germanium Photodiodes as infrared detectors

Germanium on silicon P-i-N photodetectors fabricated using standard CMOS tools are now successfully used as uncooled detectors in the near-infrared (NIR), which extends from 0.75 to 1.4 mu m and in the short-wave infrared (SWIR), which extends from 1.4 to 3 mu m. They feature a remarkably high responsivity up to 1550 nm and a low dark current when they are operated at reverse biases. The aim is to achieve a very low dark current density and a high responsivity with a small Germanium photodiode pitch. In this paper we discuss the fabrication and the characterization of vertical P-i-N photodiodes with the epitaxy of Germanium on Silicon. The Germanium epilayer is a 1.3 mu m thick Ge film with a bottom Ge layer P-type doped with boron at 10(19) cm(-3) and a top Ge layer N-type doped with phosphorus at 10(20) cm(-3). Secondary ion mass spectroscopy was used to assess the doping level of the Phosphorus doped N+region. The strain in the Germanium epilayer on Silicon substrate was investigated. It was tensile, with a value around +0.15% from x-ray diffraction (XRD), in good agreement with a +0.12% value from Raman spectroscopy. In this paper, we focus on P-i-N photodiodes with a circular shape and a diameter of 10 mu m. Electrical characterizations were performed in dark and under NIR-SWIR radiation (1310 nm, 1550 nm), with very low dark current of 0.45 nA and enhanced photocurrent at -1 V. The external responsivities were measured at 0.275 and 0.133 AW(-1) for 1310 nm and 1550 nm, respectively. Finally, internal quantum efficiencies of the fabricated vertical P-i-N photodiodes were extracted at 66% and 52% at 1310 nm and 1550 nm, respectively, in good agreement with TCAD simulations. Finally, a measurement of the noise in dark conditions is presented.