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Short-distance optical communications and large-scale networks are central in telecom, datacom, and information processing technologies. Such applications require high transmission capacity, low energy consumption, implementation at the chip-scale level and cost-effective production. Silicon photonics (SiP) is well-positioned to tackle these growing needs. Indeed, photonic chips enable dense integration of complex functions through light-driven, compact, and high-performance devices. SiP is boosted by complementary metal-oxide-semiconductor (CMOS) processes and toolsets used for decades in the microelectronic industry. CMOS-friendly manufacturing, mature germanium (Ge) epitaxy and open-access foundry model are advantageous for monolithic integration, where a low-loss silicon (Si) platform is associated with advanced opto-electrical efficiency and fast response time. Optical detectors are key building blocks in the SiP library. They leverage progresses made in design strategies, material processing and seamless Si CMOS integration. In particular, chip-integrated silicon-germanium (Si-Ge) photodetectors are aiming for fast, sensitive, and energy-aware operation, with performances that are becoming competitive or even superior to existing and well-adopted III/V-based counterparts. In this work, we present recent advances in waveguide-integrated p-i-n photodetectors based on double silicon-germanium-silicon (Si-Ge-Si) heterojunctions. Hetero-structured Si-Ge-Si photodetectors with a butt-waveguide-coupling and a lateral p-i-n electrical junction are most promising to detect light on a silicon chip at near-infrared (near- IR) wavelengths. They rely on an unique integration approach that yields structures with compact footprints and properly engineered waveguide geometries. Such devices then benefit from improved modal confinement and overall control over the intrinsic region. They are also easy to fabricate and integrate with other passive and active components on a single chip. Si-Ge-Si p-i-n photodetectors were fabricated on 200 mm silicon-on-insulator (SOI) substrates using industrial-scale semiconductor manufacturing processes. Fabricated p-i-n structures were operated under low-voltages and in avalanche regime, respectively. In both situations, devices showed up a reliable high-speed, low-noise, and sensitive signal detection within a mainstream telecom waveband around 1.55 μm.
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