Strain-engineered CMOS-compatible Ge photodetectors

Abstract

The development of CMOS-compatible photodetectors capable of operating throughout the entire telecommunications wavelength spectrum will aid in the integration of photodetectors with Si microelectronics, thus offering a low cost platform for high performance photoreceivers. This thesis demonstrates the first CMOS process compatible high-responsivity Ge p-i-n diodes for 1.55 [mu]m wavelengths. The thermal expansion mismatch between Ge epilayers and Si substrates was used to engineer tensile strain upon cooling from the growth temperature. This 0.2% tensile strain results in a lowering of the direct transition energy in Ge by 30 meV and extends the responsivity curve to near 1.6[mu]m. Design rules are given for high speed and high responsivity, and the advantages of waveguide integration for simultaneous achievement of high speed and high responsivity are illustrated. It is shown that waveguide integration has advantages to vertical illumination when optical saturation is considered. Optical saturation will become important as photodetector sizes shrink to the order of a few tens of microns in diameter. High Ge content SiGe could have applications for a SiGe electro-optic modulator utilizing the Franz-Keldysh effect. High Ge content SiGe films have been grown on Si substrates. The Franz-Keldysh effect has been observed in our pure Ge films as an increase in responsivity with increasing reverse bias for wavelengths longer than the bandgap energy. .