MIR photodetection using intrinsic monolithic integrated Germanium photodiodes

Abstract

Silicon (Si) photonics as a field is about photonic integrated circuits (PIC) using group IV materials as a substrate, mainly Si. Creating PICs from Si allows circuitry to be small and inexpensive; enabled by the use of mature fabrication techniques, existing recipes and foundries. The small size of PICs is useful for applications such as free-space telecommunication and chemical sensing where many devices are needed or are made as consumables. An infrared photodetector that is responsive in the 3-4 µm is needed to create PICs for these applications (1). To keep the cost low, a detector should be monolithically grown or deposited on Si. Si defect mediated detectors have been shown to be responsive, up to wavelengths of 2.5 µm (2), and able to operate at high speeds (up to 35 GHz) (3), but have not yet demonstrated a spectral response in the 3-4 µm region. Defect mediated germanium (Ge) detectors are potential candidates for detecting 3-4 µm light. Ge can be epitaxially grown on Si and using it for defect mediated absorption currently is relatively unexplored. In this thesis, Ge PIN photodiodes were created and implanted with boron ions to create defects within a rib waveguide. The responsivity versus the length of the photodiode implanted with defeccts was investigated, at fluences of 1x1010, 1x1012 and 1x1014 ions/cm2 , using no reverse bias. These measurements showed that the boron implantation had a negative effect on the responsivity, although, the unimplanted detectors had an unexpectedly high responsivity of approximately 0.1 A/W. This prompted investigation into the responsivity with an increasing reverse bias using 2 µm and 3.8 µm light, which resulted in a maximum responsivity of 1.04 A/W at -10 V using 2 µm light and 0.1 A/W at -7 V using 3.8 µm light at room temperature. A 12.5 Gb/s pseudorandom pattern was used to modulate 2 µm light, the detection of this light resulted in an open eye diagram. Measurements were performed to find the photodetection method, the linearity of the devices suggested that there was no two photon absorption, there was minimal difference in transmission between waveguides and photodiodes suggesting that photoactive defects were not created in the PIN junction formation. Finally, Raman spectroscopy was performed and found a 0.22 % strain which may be responsible for most of the absorption at 2 µm. The absorption at 3.8 µm may be caused from threading dislocations at the Ge Si boundary as similar wafers showed a high density of defects at the boundary. These results show Ge-on-SOI as a practical detection material in an extended wavelength range. More research is needed to understand the source of infrared absorption

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ORCID for Radan Slavik: orcid.org/0000-0002-9336-4262

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