Quantum dots (QDs) have been explored for many photonic applications, both as emitters and absorbers. Thanks to the bandgap tunability and ease of processing, they are prominent candidates to disrupt the field of imaging. This review article illustrates the state of technology for infrared image sensors based on colloidal QD absorbers. Up to now, this wavelength range has been dominated by III–V and II–VI imagers realized using flip-chip bonding. Monolithic integration of QDs with the readout chip promises to make short-wave infrared (SWIR) imaging accessible to applications that could previously not even consider this modality. Furthermore, QD sensors show already state-of-the-art figures of merit, such as sub-2- $\mu \text{m}$ pixel pitch and multimegapixel resolution. External quantum efficiencies already exceed 60% at 1400 nm. With the potential to increase the spectrum into extended SWIR and even mid-wave infrared, QD imagers are a very interesting and dynamic technology segment.

In this paper, we present a small pixel pitch image sensor optimized for high external quantum efficiency in short-wavelength infrared (SWIR). Thin-film photodiodes based on PbS colloidal quantum dot (CQD) absorber allow us to exceed the spectral limitations of silicon’s absorption while maintaining the benefits of CMOS technology. By monolithically integrating PbS CDQ thin films with CMOS readout arrays, high-pixel density SWIR image sensors can be achieved. To overcome the remaining disadvantages of the CQD-based image sensors over their bulk III-V semiconductor counterparts (lower sensitivity and reduced linearity), the thin-film photodiode stack is adapted towards the used readout circuit. A prototype image sensor with a 768x512 resolution of 5-μ m pitch pixels is fabricated by using a modified 130 nm CMOS process for readout IC, together with the new CQD thin-film photodiode on top. Thanks to the optimized photodiode stack and co-integration process, the prototype image sensor shows less than 5% linearity error while having 40% external quantum efficiency in SWIR, which enables acquisition of high-quality images.

A near-infrared (NIR) enhanced silicon single-photon avalanche diode (SPAD) fabricated in a customized 0.13 μm CMOS technology is presented. The SPAD has a depleted absorption volume of approximately 15 μm × 15 μm × 18 μm. Electrons generated in the absorption region are efficiently transported by drift to a central active avalanche region with a diameter of 2 μm. At the operating voltage, the active region contains a spherically uniform field peak, enabling the multiplication of electrons originating from all corners of the device. The advantages of the SPAD architecture include high NIR photon detection efficiency (PDE), drift-based transport, low afterpulsing, and compatibility with an integrated CMOS readout. A front-side illuminated device is fabricated and characterized. The SPAD has a PDE of 13% at wavelength 905 nm, an afterpulsing probability < 0.1% for a dead time of 13 ns, and a median dark count rate (DCR) of 840 Hz at room temperature. The device shows promising performance for time-of-flight applications that benefit from uniform NIR-sensitive SPAD arrays.

A backside-illuminated (BSI) near-infrared (NIR)-enhanced silicon single-photon avalanche diode (SPAD) for time-of-flight (ToF) light detection and ranging applications is presented. The detector contains a 2- $\boldsymbol {\mu }\text{m}$ -wide multiplication region with a spherically uniform electric field peak enforced by field-line crowding. A charge-focusing electric field extends into a 10- $\boldsymbol {\mu }\text{m}$ -deep absorption volume, whereby electrons generated in all the corners of the device can move efficiently toward the multiplication region. The SPAD is integrated with a customized 130-nm CMOS technology and a dedicated BSI process. The device has a pitch of $15 \boldsymbol {\mu }\text{m}$ , which has the potential to be scaled down without significant performance loss. Furthermore, the detector achieves a photon detection efficiency (PDE) of 27% at 905 nm, with an excess bias of 3.5 V that is controlled by integrated CMOS electronics, and a timing resolution of 240 ps. By virtue of these features, the device architecture is well-suited for large-format ToF imaging arrays with integrated electronics.

Thin-film-based image sensors feature a thin-film photodiode (PD) monolithically integrated on CMOS readout circuitry. They are getting significant attention as an imaging platform for wavelengths beyond the reach of Si PDs, i.e., for photon energies lower than 1.12 eV. Among the promising candidates for converting low-energy photons to electric charge carriers, lead sulfide (PbS) colloidal quantum dot (CQD) photodetectors are particularly well suited. However, despite the dynamic research activities in the development of these thin-film-based image sensors, no in-depth study has been published on their imaging characteristics. In this work, we present an elaborate analysis of the performance of our short-wave infrared (SWIR) sensitive PbS CQD imagers, which achieve external quantum efficiency (EQE) up to 40% at the wavelength of 1450 nm. Image lag is characterized and compared with the temporal photoresponsivity of the PD. We show that blooming is suppressed because of the restricted pixel-to-pixel movement of the photo-generated charge carriers within the bottom transport layer (BTL) of the PD stack. Finally, we perform statistical analysis of the activation energy for CQD by dark current spectroscopy (DCS), which is an implementation of a well-known methodology in Si-based imagers for defect engineering to a new class of imagers.