The fabrication of p-Ge/n-Si photodetectors, compatible with back-end Si CMOS processing, by low temperature (< 400 o
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The fabrication of p-Ge/n-Si photodetectors, compatible with back-end Si CMOS processing, by low temperature (< 400 oC) molecular beam epitaxy and electron-beam evaporation Prabhakar Bandaru, Subal Sahni and Eli Yablonovitch Department of Electrical Engineering, University of California at Los Angeles, Los Angeles, CA 90095 Hyung-Jun Kim and Ya-Hong Xie Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA 90095 ABSTRACT We report on the low temperature growth, by molecular beam epitaxy (375 oC) and electron-beam evaporation (300 oC), of p-Ge films on n-Si substrates for fabricating p-n junction photodetectors, aimed at the integration of opto-electronic components with back-end Si CMOS processing. Various surface hydrogen and hydrocarbon removal treatments were attempted to improve device properties. We invoke Ge diffusion and growth modes as a function of deposition temperature and rate to correlate structural analysis with the device performance. INTRODUCTION Silicon has been the mainstay of the micro-electronics industry, and a very large infrastructure has been built around it, both in terms of the materials research and the capital equipment that is used for fabrication. In recent years, the advent of the Silicon-On-Insulator (SOI) technology, the feasibility of constructing Photonic Integrated Circuits (PICs) and the continuing trend towards miniaturization and multi-functionality has provided impetus for the construction of Silicon based micro-photonics as well. In addition to the practical advantages of a monolithic platform with integrated electronic and optical circuitry for information processing and communication, Si based photonics and opto-electronics offers a way to overcome problems, such as increased power dissipation and reduced speed of operation due to cross-talk, inherent to miniaturization and the consequent scaling down of interconnect density1. We investigate photonic materials, processes and devices that are amenable to integration with existing CMOS (Complementary Metal Oxide Semiconductor) based electronic circuitry. This scheme of integration is preferred as it avoids changes in standard foundry processes, and could lead to easier realization and quicker adoption of optics vis-à-vis electronics. However, such back-end processing is restricted to less than 450 oC to avoid degrading the Al/Ti metallization. A projected application is in fiber based (λ:1.3- 1.55 µm) communication systems. One could envisage fiber coupling in the optical signals directly onto a substrate, where the light is guided and electronically detected using waveguides and photodetectors respectively. For Si based photonics, the refractive index difference between Si and SixNy/SiO2 can be used for waveguides, while Germanium on Si could work well as an electronic p-n junction photodetector. Ge is a good choice at 1.55 µm due to a direct band gap of 0.8 eV. (Although, GaAs and InP based semiconductors have higher efficiencies they are excluded from our study du
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