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High-speed avalanche photodiodes for low light level detection


Extremely sensitive optical receivers operating at high bandwidth are critical for optical communications technologies and other emerging applications that require the detection of an extremely low number of photons. Light imaging, detection and ranging (LIDAR), quantum communications, biophotonics, medical imaging systems, and other emerging applications will require a new generation of photodetectors that can detect extremely low power levels and be mass manufacturable. Today, Avalanche Photodetectors (APDs) and Single Photon Avalanche Photodetectors (SPADs) are the only detectors that can meet those requirements, but their bandwidth, sensitivity, and noise limitation must be overcome.

This thesis presents the development of new silicon and germanium-on-silicon-based APDs and SPADs with high speed and absorption efficiency at visible and near-infrared wavelengths. Through the modeling, fabrication, and characterization we show that the implementation of photonic nanostructures in photodetectors enhances their responsivity and gain by guiding the light parallel to its surface, greatly enhancing the interaction with the semiconductor material. These nanostructures also allow enhancing their time response by two methods: (i) use of thinner absorption layers to reduce the transit time of the photogenerated carriers, without sacrificing their absorption capabilities and (ii) reduction of junction capacitance by reducing the effective area of the device through the removal of material. This thesis also develops the concept of penetration depth engineering. We show that it is possible to guide photons to a critical depth in semiconductors and maximize the gain-bandwidth performance and absorption efficiency in avalanche-based photodetectors by integrating photon-trapping nanoholes. Our new Si APDs have shown superior amplification gain and speed compared to their conventional counterparts.

The advantages of photon-trapping are exploited in Germanium (Ge) on Si photodetectors. Our Ge-on-Si PDs show enhanced absorption capabilities at the O (original band: 1260-1360 nm) and C (conventional band: 1530 nm to 1565 nm) optical wavelengths of bands and enable their use at longer wavelengths in the L band (long wavelength: 1565 nm to 1625 nm). These PDs with photon-trapping holes have the potential to be monolithically integrated with CMOS/BiCMOS ASICs and offer a promising solution for waveguide PDs required for Photonic Integrated Circuits (PICs).

A new generation of APDS and SPADs are now designed with the implementation of appropriate doping profiles for high amplification, thin semiconductor layers for high speed, and the accurate design of micro/nanoholes for highly sensitive and ultrafast optical receivers.

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