Germanium on Silicon Near-Infrared Photodetectors

Abstract

In recent years, the of Germanium on Silicon approach has been recognized as the best alternative to the well-established III-V technology for the fabrication of high performance near-infrared photodetectors. Recent results demonstrate that Ge heteroepitaxy on Si is by now mature to compete with standard III-V devices. Unfortunately, the integration of Ge-on-Si technology in standard CMOS process flows is still an open challenge due to the sophisticated growth techniques as well as the high thermal budget involved. This work proposes an alternative approach to the growth of Ge on Si for NIR optoelectronics applications. The first chapter introduces NIR detection for optical communication systems, with particular emphasis on Ge as a suitable material for the monolithic integration into NIR photodetectors on a Si platform. In the second chapter, the deposition process is described. Ge is deposited on Si by thermal evaporation, a very simple and low temperature (300◦ C ) technique suitable for both streamline process and back-end monolithic integration of Ge on Si CMOS electronics. Material characterization, both morphological and electrical, is also discussed. Raman and X-Ray analysis, as well as Transmission Electron Microscopy evidenced that Ge is epitaxially grown in a monocrystalline form with a high dislocation density. Hall measurements demonstrated high unintentional p -type doping (1017 ÷ 1018 cm−3 ) associated to the acceptor-like levels due to the large defect density. The transport and detection properties of evaporated Ge on Si heterojunctions are presented in the third chapter. Results demonstrated a trapassisted conduction mechanism explained by energy band pinning at the Ge/Si interface. The NIR detection properties were also investigated by illumination at normal incidence. The high doping together with the short diffusion length were found to drastically limit the responsivity of normal incidence devices. The last part of this work is dedicated to the design and fabrication of optimized NIR photodetector and their integration on SOI optical chips. Waveguide photodetectors (WPD) were fabricated to take advantage from the distributed absorption of light in guiding structures. WPD exhibit very promising performance with typical responsivities exceeding 0.2 A/W at 1 V reverse bias and 1.55 µm wavelength. These devices were monolithically integrated on SOI optical chips for the realization of channel monitors. The integrated devices exhibit very promising performance, with sensitivity of 10 nW and good linearity over about four orders of magnitude.