Monolithically Fabricated Germanium-on-SOI Photodetector and Si CMOS Circuit for Integrated Photonic Applications

In this paper, we report our design and fabrication approach towards realizing a monolithic integration of Ge photodetector and Si CMOS circuits on common SOI platform for integrated photonic applications. The approach, based on the Ge-on-SOI technology, enables the realization of high sensitivity and low noise photodetector that is capable of performing efficient optical-to-electrical encoding in the near-infrared wavelengths regime. When operated at a bias of -1.0V, a vertical PIN detector achieved a lower I-dark of similar to 0.57 mu A as compared to a lateral PIN detector, a value that is below the typical similar to 1 mu A upper limit acceptable for high speed receiver design. Very high responsivity of similar to 0.92A/W was obtained in both detector designs for a wavelength of 1550nm, which corresponds to a quantum efficiency of similar to 73%. Impulse response measurements showed that a vertical PIN photodetector gives rise to a smaller FWHM of similar to 24.4ps, which corresponds to a -3dB bandwidth of similar to 11.3GHz where RC time delay is known to be the dominant factor limiting the speed performance. Eye patterns (PRBS 2(7)-1) measurement further confirms the achievement of high speed and low noise photodetection at a bit-rate of 8.5Gb/s. In addition, we evaluate the DC characteristics of the monolithically fabricated Si CMOS inverter circuit. Excellent transfer and output characteristics were achieved by the integrated CMOS inverter circuits in addition to the well behaved logic functions. We also assess the impact of the additional thermal budget introduced by the Ge epitaxy growth on the threshold voltage variation of the short channel CMOS transistors and discuss the issues and potential for the seamless integration of electronic and photonic integrated circuits.