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Transistor-Based Ge/SOI Photodetector for Integrated Silicon Photonics


This dissertation describes our effort on developing a technology of photodetectors for application in chip-level optical communication. The photodetector proposed in this thesis work is the Ge/SOI Photo-Hetero-JFET. It is based on a silicon junction-FET in which the traditional electrical gate is replaced by a photo-active germanium mesa. The silicon channel conductance is then modulated by near-infrared light signal incident on the germanium gate.

The limitations of traditional electrical wires which restrict the performance of microelectronic information systems drive researchers to look at optical interconnects as a good alternative for inter-chip data communication. One of the major challenges that the optics solution faces is to achieve as low energy consumption as 100aJ/bit. This in turn sets stringent requirements on the sensitivity of photodetectors, which can only be achieved when the photodetector can be highly integrated and has extremely small device capacitance (<1fF). The Ge/SOI Photo-Hetero-JFET is seamlessly integratable with microelectronic circuitry and also scalable to achieve extremely small capacitance. It was therefore proposed as a promising photodetector design for the application to inter-chip-scale optical links.

Ge/SOI Photo-Hetero-JFETs with gate length of 100nm are fabricated. They were then characterized as near-infrared photodetectors both under continuous-wave laser and pulsed laser at 1550nm. Geminate recombination together with severe SRH recombination of photocarriers in the germanium gate is found to significantly limit the responsivity of the photodetector. Nonetheless, after correcting for the poor internal quantum efficiency, we found that one collected photons can lead to the generation of ~750 electrons in the silicon channel, which indicates a DC secondary photoconductive gain of 750 on top of primary responsivity.

Time-resolved measurement done on the Photo-Hetero-JFET further reveals that the photodetector can respond to laser pulses as short as 4ps. Although the observed risetime of transient photoresponse is 50ps which is currently limited by bandwidth of the measurement circuit, it is believed that when the photodetector is fully-integrated it can achieve its inherent risetime of ~1ps! One caveat regarding the Photo-Hetero-JFET is that its transient photoresponse has a long tail (~26ns fall-time). This was originally attributed to dielectric relaxation process of trapped holes in the gate, but is later found to result from the dispersive nature of photocarrier transport in the defective germanium mesa. In the analysis of peak transient amplitude through JFET model based on trapped charges, we found that with the design of Photo-Hetero-JFET only ~50 photo-holes on the gate/channel junction of 0.1ìm2 can induce channel current of ~5ìA! This proves that Photo-Hetero-JFETs can also achieve high sensitivity under pulsed illumination.

The attributes of the Photo-Hetero-JFET design that makes the device highly sensitive is its extraordinarily small device capacitance (~52aF) and its seamless integrability with silicon circuitry. Currently, the fabricated Photo-Hetero-JFETs suffer from poor quantum efficiency and slow gain which were brought about by the poor germanium quality. Nonetheless, the device still presents impressive secondary photoresponsivity and great potential in its bandwidth improvement. It is believed that with reasonable germanium film quality, (diffusion length of ~100nm already available in the industry), the Photo-Hetero-JFET is capable of demonstrating great sensitivity and fast speed in the application of chip-level optical communications.

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