Fabrication and characteristics of high performance SOI-based Ge PIN waveguide photodetector

Abstract

Silicon-based photonics has aroused an increasing interest in the recent year, mainly for optical telecommunications or optical interconnects in microelectronic circuits. The waveguide photodetector is one of the building blocks needed for the implementation of fast silicon photonics integrated circuits. The main considerations for designing such a device are the bandwidth, the power consumption and the responsivity. Germanium is now considered as an ideal candidate for fully integrated receivers based on silicon-on-insulator (SOI) substrates and complementary metal oxide semiconductor (CMOS)-like process because of its large optical absorption coefficient at the wavelength for optical communication. Therefore, the study of high speed and high responsivity Ge waveguide photodetectors is necessary. In this paper, high concentration phosphor doped SOI substrate is achieved by using solid-state source diffusion at first. Secondly, the high quality epitaxial germanium (Ge) is grown on phosphor doped SOI substrate by using low temperature Ge buffer layer technique based on the UHV/CVD system. The surface profile, crystal quality and strain of epitaxial Ge film are characterized by using atomic force microscopy, X-ray diffraction (XRD), and Raman scattering spectrum. The results show that the Ge film has a smooth surface of 1.12 nm roughness and about 0.2% tensile strain, which is verified by XRD characterization result. Thirdly, ptype Ge region is formed by BF2+ implantation, and rapid thermal annealing to repair the implantation damages and activate impurity. Finally, the highperformance Ge PIN waveguide photodetectors with different sizes are fabricated by standard COMS technology. Moreover, the device performances, in terms of dark current versus voltage characteristics, photocurrent responsivity and 3 dB bandwidth, are well studied. The results show that the detector with a size of 4 m20 m demonstrates a dark current density of 75 mA/cm2 at -1 V and a photocurrent responsivity of 0.58 A/W for 1.55 m optical wavelength. In addition, an optical band width of 5.3 GHz at -2 V for 1.55 m is also demonstrated, which is far below theortical value of about 40 GHz. This can mainly be attributed to two aspects. On the one hand, Ge PIN structure contains low temperature Ge buffer layer, which has highdensity dislocation because of large lattice mismatch between Si and Ge. Those dislocations or defects can trap and release the photo-generated carrier, which increases the transit time. On the other hand, the contact characteristics of Al with n+-Si and p+-Ge are not very good, leading to a large contact resistance and RC delay. Through improving the above two aspects, the performance of Ge PIN waveguide photodetector will be further enhanced.