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-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> 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.
Multispectral imaging in short‐wave infrared (SWIR) is a powerful analytical technique because of the distinctive spectral properties of many materials in this range. However, conventional SWIR image ...sensors are beyond the reach of many applications due to their high price. Image sensors based on colloidal quantum dots (CQDs) are expected to deliver affordable infrared image sensors to wider application scope. So far, the demonstrated CQD image sensors do not have a multispectral capability. Here, a dual‐band photodetector based on PbS CQDs is presented. By engineering the surface of CQDs, two oppositely facing pn junctions are fabricated in series, which enable sensing in two spectral channels. Furthermore, an optical cavity is designed that reduces the spectral crosstalk between the two channels and simultaneously enables wavelength‐tunability in one channel. Finally, an organic photodiode (OPD) is integrated with a PbS CQD photodiode in a single device, leveraging a high sensitivity in visible and near‐infrared (NIR) characteristics for OPDs. The presented photodetectors exhibit low dark current below 500 nA cm−2 at 1 V bias, a fast response measured in microseconds, as well as high external quantum efficiency, reaching 70% in NIR and 30% in SWIR.
Thin‐film semiconductors offer attractive opportunities for achieving optoelectronic functionalities that are not easily accessible with conventional bulk semiconductors. This paper presents a tunable dual‐band photodetector for multispectral sensing in the visible to short‐wave infrared range. Organic and inorganic colloidal quantum dot absorbers are integrated into a single vertically stacked structure to leverage the best features of both material systems.
Imaging in the infrared wavelength range has been fundamental in scientific, military and surveillance applications. Currently, it is a crucial enabler of new industries such as autonomous mobility ...(for obstacle detection), augmented reality (for eye tracking) and biometrics. Ubiquitous deployment of infrared cameras (on a scale similar to visible cameras) is however prevented by high manufacturing cost and low resolution related to the need of using image sensors based on flip-chip hybridization. One way to enable monolithic integration is by replacing expensive, small-scale III-V-based detector chips with narrow bandgap thin-films compatible with 8- and 12-inch full-wafer processing. This work describes a CMOS-compatible pixel stack based on lead sulfide quantum dots (PbS QD) with tunable absorption peak. Photodiode with a 150-nm thick absorber in an inverted architecture shows dark current of 10
A/cm² at -2 V reverse bias and EQE above 20% at 1440 nm wavelength. Optical modeling for top illumination architecture can improve the contact transparency to 70%. Additional cooling (193 K) can improve the sensitivity to 60 dB. This stack can be integrated on a CMOS ROIC, enabling order-of-magnitude cost reduction for infrared sensors.
Colloidal quantum dots (QDs) have attracted scientific interest for infrared (IR) optoelectronic devices due to their bandgap tunability and the ease of fabrication on arbitrary substrates. In this ...work, short‐wave IR photodetectors based on lead sulfide (PbS) QDs with high detectivity and low dark current is demonstrated. Using a combination of time‐resolved photoluminescence, carrier transport, and capacitance–voltage measurements, it is proved that the charge carrier diffusion length in the QD layer is negligible such that only photogenerated charges in the space charge region can be collected. To maximize the carrier extraction, an optical model for PbS QD‐based photodiodes is developed, and through optical engineering, the cavity at the wavelength of choice is optimized. This universal optimization recipe is applied to detectors sensitive to wavelengths above 1.4 µm, leading to external quantum efficiency of 30% and specific detectivity (D*) in the range of 1012 Jones.
An optical model for lead sulfide quantum dot thin films is developed and applied on short‐wave infrared sensitive photodetectors. Through optical engineering, the cavity is optimized at the wavelength of choice, leading to significant boost in the device external quantum efficiency, achieving values higher than 30% at wavelengths above 1.4 µm.
Colloidal quantum dots based on lead sulfide (PbS) are very attractive materials for the realization of novel image sensors. They offer low cost synthesis, compatibility with a variety of substrates ...and processing on large area. The tunable band gap enables selective light detection from the visible wavelengths up to the short-wave-infrared (SWIR). This work describes the roadmap towards the integration of quantum dot photodiodes (QDPD) on top of a Si based CMOS read-out circuit. Photodiodes using an n-p junction architecture are fabricated on Si substrates, showing a dark current of 30 nA/cm 2 at −1 V reverse bias, EQE above 20% and specific detectivity above 10 12 cm Hz 1/2 W −1 at the wavelength of 940 nm. Efficiency is improved by reducing absorption in the top contact through optical design. Furthermore, photolithographic patterning of the thin-film stack is introduced for the first time, showing the feasibility of pixel pitches down to <inline-formula> <tex-math notation="LaTeX">40~\mu \text{m} </tex-math></inline-formula>, opening the way towards high resolution monolithic infrared imagers and the incorporation of infrared and visible sensitive pixels side by side.
In this letter, 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 <inline-formula> <tex-math notation="LaTeX">768\times 512 </tex-math></inline-formula> resolution of 5-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> 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.
In this article, the X-ray radiation effects on colloidal quantum dot photodiode (QDPD)-based short-wave infrared (SWIR) complementary metal-oxide semiconductor image sensors (QD-CISs) are studied. ...Individual QDPD, silicon readout IC (Si-ROIC), and QD-CIS are evaluated together for a comprehensive analysis. The dark current, activation energy, and external quantum efficiency (EQE) of samples are investigated before and after irradiating with 58.2 keV of X-ray radiation, which has a different total ionizing dose (TID) range from 22 to 220 krad. X-ray irradiation on Si-ROIC induces mid-band gap trap states and increases the dark current according to the increasing TID. However, for the QDPD, despite an increase in the TID, the dark current reduces and the EQE slightly enhances at the SWIR wavelength. The QD-CIS shows a decrease in the dark current like the QDPD results, as the TID increases. The activation energy of QD-CIS rarely changes regardless of TID amounts. The X-ray radiation effect on QDPD results in enhanced performance, and this effect continues in the integrated QD-CIS, whereas the effect of Si-ROIC degradation is minor in the current experimental range. Thus, these findings provide significant insights into the utilization of QD-CIS in various X-ray applications.
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.
We report a high-speed low dark current near-infrared (NIR) organic photodetector (OPD) on a silicon substrate with amorphous indium gallium zinc oxide (a-IGZO) as the electron transport layer (ETL). ...In-depth understanding of the origin of dark current is obtained using an elaborate set of characterization techniques, including temperature-dependent current-voltage measurements, current-based deep-level transient spectroscopy (Q-DLTS), and transient photovoltage decay measurements. These characterization results are complemented by energy band structures deduced from ultraviolet photoelectron spectroscopy. The presence of trap states and a strong dependency of activation energy on the applied reverse bias voltage point to a dark current mechanism based on trap-assisted field-enhanced thermal emission (Poole–Frenkel emission). We significantly reduce this emission by introducing a thin interfacial layer between the donor: acceptor blend and the a-IGZO ETL and obtain a dark current as low as 125 pA/cm2 at an applied reverse bias of −1 V. Thanks to the use of high-mobility metal-oxide transport layers, a fast photo response time of 639 ns (rise) and 1497 ns (fall) is achieved, which, to the best of our knowledge, is among the fastest reported for NIR OPDs. Finally, we present an imager integrating the NIR OPD on a complementary metal oxide semiconductor read-out circuit, demonstrating the significance of the improved dark current characteristics in capturing high-quality sample images with this technology.
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•A very thin and light-weight X-ray imager is made on plastic film.•All layers with the exception of the electrodes are solution processed.•The photo-sensitive layer is continuous, ...simplifying the manufacturing process.•Images are recorded at X-ray doses sufficiently low for medical applications.
We describe the fabrication and characterization of large-area active-matrix X-ray/photodetector array of high quality using organic photodiodes and organic transistors. All layers with the exception of the electrodes are solution processed. Because it is processed on a very thin plastic substrate of 25μm thickness, the photodetector is only 100μm thick. When combined with an 300-μm-thick X-ray scintillator, this gives a thin, low-weight and shatterproof X-ray detector of ca. 400μm thickness. We demonstrate X-ray imaging under conditions that are used in medical applications.
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