High Speed Ge Photodetectors on Si Platform for GHz Optical Communications in C+L Bands
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High Speed Ge Photodetectors on Si Platform for GHz Optical Communications in C+L Bands Jifeng Liu1, Jurgen Michel1, Douglas D. Cannon1, Wojciech Giziewicz2, D. Pan1, David T. Danielson1, Samerkhae Jongthammanurak1, John Yasaitis3, Kazumi Wada1, Clifton G. Fonstad2 and Lionel C. Kimerling1 1
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 2 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 3 Analog Devices, Inc., Cambridge, Massachusetts 02139 ABSTRACT We present a high speed, high responsivity, tensile strained Ge p-i-n photodetector selectively grown on Si platform that covers the whole C band and a large part of the L band for high capacity optical communications. The device shows a 3dB bandwidth of 2.5GHz and its responsivities at 1310nm and 1550nm are comparable to commercial InGaAs photodetectors currently used in telecommunications. The device has promising applications in Si microphotonics such as the fiber-to-the-home technology. INTRODUCTION Near infrared (NIR) photodetectors are indispensable devices in optical communications to convert optical signals into electronic ones [1]. III-V semiconductors such as InGaAs are currently used as photodetectors in telecommunications, but these devices are not compatible with Si complimentary metal oxide semiconductor (CMOS) technology and require growth on InP or GaAs substrates, which leads to much higher costs. Currently, the wavelength range used in the dense wavelength division multiplexing (DWDM) technology is expanding from C band (1528-1560nm) to also including L-band (15611620nm), requiring photodetectors that can cover C+L bands. In this paper, we demonstrate a CMOS compatible, tensile strained Ge p-i-n photodiode selectively grown on Si that covers the C band and a large part of the L band. By introducing 0.20% tensile strain into the epitaxial Ge material, the direct band edge of the Ge film was shifted from 1550nm (0.80eV) to 1605nm (0.77eV) [2-5], significantly enhancing the responsivity of the device in the L band. The responsivities at 1310nm and 1550nm are comparable to commercial InGaAs photodetectors currently used in optical communications [6]. With high quality selectively grown Ge epitaxial layers and adequate device design, the responsivity of the device at 0V bias is almost the same as that at -2V reverse bias, a desirable feature for low voltage operations. The device shows a 3dB frequency of 2.5GHz, applicable for a bit rate of 5Gb/sec. Unlike InGaAs photodiodes, the fabrication of the presented device is fully compatible with Si CMOS technology, therefore enabling monolithically integrated photodiodes with Si circuitry.
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EXPERIMENTAL DETAILS Smooth Ge epitaxial layers with a root mean square roughness of ~0.7nm (determined by atomic force microscopy) were selectively grown directly on Si windows surrounded by a SiO2 isolating layer, where a ~60nm Ge buffer layer was grown at 3
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