Fabrication and simulation of CMOS-compatible photodiodes

Abstract

CMOS-compatible photodiodes are becoming increasinging important devices to study because of their application in combined electronic-photonic systems. They are already used as inexpensive optical transceivers in fiber optic telecommunications systems and they have the potential to be integrated in a number of applications. This thesis focuses on germanium photodiodes to be used in an integrated electronic-photonic analog-to-digital converter. It specifically studies the dark current, responsivity, and frequency response of Ge-on-Si LPCVD-grown diodes that will be used in such a system. It outlines a process that can be used to add metal contacts to pre-existing diodes and discusses characterization procedure. It was found that previously fabricated 50 pm square diodes had leakage current of 0.25 uA at -1 V, but responsivity of -5 mA/W. Diodes with higher leakage current, 1.1 piA at -1 V, had a higher responsivity of -0.5 A/W. Spreading resistance profiles (SRP) indicate that better control of the n-type contact is needed to systematically reproduce these results. Furthermore, spreading resistance analysis demonstrated that elimination of the p-type seed during growth will result in a more abrupt junction, for which simulations predict an improved frequency response. Simulations indicate that removal of the p-type seed and associated autodoping should increase the frequency response from -1.6 GHz to 1-4 GHz. Better control of the n-type profile can further increase the frequency response from '14 GHz to -27 GHz.