Germanium photodetector integrated with silicon-based optical receivers
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With the development of fiber optics communication systems and optical interconnects, there is an increased demand for low-cost, high-speed, highsensitivity optical receivers. Previously, our group has demonstrated Si photodiodes integrated with CMOS preamplifier circuits. In order to extend the operating wavelength to 1300nm, Ge photodetectors integrated with Si has been studied for Si based optical receivers in this work. Ge has the advantage of compatability with much of Si process technology, as well as the high mobility and large absorption coefficient at 1300 nm. The key challenge for Ge photodetector integrated with Si is the growth of high quality Ge layer on Si. In this work, a successful Ge growth technique has been developed by using a UHV-CVD system. The preliminary integration of Ge photodetector with Si CMOS circuits has also been demonstrated. To further improve the device performance, a SiGe buffer layer technique has been investigated to reduce the dark current of the photodetector. Directly growing Ge on Si generates many dislocations which increase dark current. By using the SiGe buffer layers, many threading dislocations can be “trapped” at the heterojunction interface, thereby reducing the dislocation density in the Ge layer and the photodetector dark current. A backsideilluminated photodetector has been fabricated with the dark current as low as 12 mA/cm2 at 1 V reverse bias, as well as the responsivity of 0.57 A/W and the bandwidth of 8.7 GHz. To improve the speed of these devices, another device with thinner SiGe buffer layers were demonstrated and achieved 21.5 GHz bandwidth at 1.31μm, resulting in a record high efficiency-bandwidth product of 12.9 GHz.