Among photodetectors, avalanche photodiodes (APDs) have an important place due to their excellent sensitivity to light. APDs transform photons into electrons and then multiply the electrons, leading to an amplified photocurrent. APDs are promising for faint light detection owing to this outstanding advantage, which will boost LiDAR applications. Although Si APDs have already been commercialized, their spectral region is very limited in many applications. Therefore, it is urgently demanded that the spectral region APDs be extended to the short-wavelength infrared (SWIR) region, which means better atmospheric transmission, a lower solar radiation background, a higher laser eye safety threshold, etc. Up until now, both Ge (GeSn) and InGaAs were employed as the SWIR absorbers. The aim of this review article is to provide a full understanding of Ge(GeSn) and InGaAs for PDs, with a focus on APD operation in the SWIR spectral region, which can be integrated onto the Si platform and is potentially compatible with CMOS technology.

This work demonstrates a high-performance photodetector with a 4-cycle Ge_0.86Si_0.14/Ge multi-quantum well (MQW) structure grown by reduced pressure chemical vapor deposition techniques on a Ge-buffered Si (100) substrate. At −1 V bias, the dark current density of the fabricated PIN mesa devices is as low as 3 mA/cm², and the optical responsivities are 0.51 and 0.17 A/W at 1310 and 1550 nm, respectively, corresponding to the cutoff wavelength of 1620 nm. At the same time, the device has good high-power performance and continuous repeatable light response. On the other hand, the temperature coefficient of resistance (TCR) of the device is as high as −5.18%/K, surpassing all commercial thermal detectors. These results indicate that the CMOS-compatible and low-cost Ge_0.86Si_0.14/Ge multilayer structure is promising for short-wave infrared and uncooled infrared imaging.

Short-wave infrared (SWIR) imaging has a wide range of applications in civil and military fields. Over the past two decades, significant efforts have been devoted to developing high-resolution, high-sensitivity, and cost-effective SWIR sensors covering the spectral range from 0.9 μm to 3 μm. These advancements stimulate new prospects across a wide array of fields including life sciences, medical diagnostics, defense, surveillance, security, free-space optics (FSO), thermography, agriculture, food inspection, and LiDAR applications. In this review, we begin by introducing monolithic SWIR image sensors and hybrid SWIR image sensors and indicate that flip-chip bump bonding technology remains the predominant integration method for hybrid SWIR image sensors owing to its outstanding performance, adaptable integration with innovative epitaxial SWIR materials, long-term stability, and long-term reliability. Subsequently, we comprehensively summarize recent advancements in epitaxial thin-film SWIR sensors, encompassing FPAs and flip-chip bump bonding technology for epitaxial InGaAs and Ge (Sn) thin-film SWIR sensors. Finally, a summary and outlook regarding the development of InGaAs and Ge (Sn) SWIR sensors are provided and discussed. The ongoing evolution of epitaxial thin-film SWIR sensors with flip-chip bump bonding technology is poised to foster new applications in both academic and industry fields.

High-resolution short-wave infrared (SWIR) imaging requires photodetectors (PDs) with simultaneously low dark current and high responsivity. To achieve this goal, we demonstrate low-defect bulk germanium-on-insulator (bulk-GeOI) PDs designed for enhanced 1550 nm absorption and suppressed dark current via a resonant cavity and low-defect material platform. Devices were fabricated by direct bonding low-defect bulk Ge and thinning it to ~1300 nm, with an intrinsic layer thickness of only 800 nm. This design avoids epitaxial defects to lower intrinsic dark current while forming a resonant cavity for enhanced responsivity at 1550 nm. Precise doping and Al₂O₃/Si₃N₄ bilayer sidewall passivation were employed. From a design perspective, using low-defect bulk Ge minimizes the defects from epitaxial growth and reduces intrinsic dark current, while thinning the Ge layer enhances the resonant cavity effect to improve 1550 nm responsivity. Experimentally, despite the thin absorbing layer, our devices achieved nA-level dark currents (e.g., 18 nA at −1 V for 10 μm devices) alongside high responsivities. Detailed analysis indicates that this dark current is predominantly attributed to surface and sidewall defects from mesa etching, with minimal contribution from low-defect bulk material defects, validating the effectiveness of the bulk-Ge approach in suppressing intrinsic bulk leakage. Optically, the devices achieved high responsivities of 0.85 A/W (1310 nm) and 0.72 A/W (1550 nm), corresponding to external quantum efficiencies of 80.6% and 57.7%, respectively. This work establishes the bulk-GeOI platform as a promising path toward high-performance SWIR PDs, successfully decoupling high responsivity from bulk leakage and paving the way for future gains through refined surface and interface engineering.