Flexible transducers address the limitations of conventional rigid ultrasound probes for wearable applications, such as the continuous monitoring of vital organs. In this work, we demonstrate the first flexible micromachined ultrasound transducer array based on AlScN piezoelectric film. Using flat-panel display compatible technology which enables the manufacturing of large-aperture arrays we demonstrate a piezoelectric transducer with an active area of 1.5x1.5 cm2. The rigid glass substrate is removed and replaced by a flexible acoustic backing layer. Optimizations on thickness, mechanical rigidity, and acoustic properties of the backing material were performed to maximize the flexible transducer performance leading to transmit sensitivity of 3.8 kPa/V at 1.8 MHz and capable of acoustic imaging functionalities.

Heavy-metal-free III-V semiconductor-based colloidal quantum dots (CQDs), such as InAs, are promising candidates for near- and short-wave infrared detection. However, up-to-date research efforts remain mainly limited to wavelengths below 1100 nm due to challenges in synthesis, junction formation, and passivation for large diameter InAs quantum dots. Systematic investigations into device design, reverse dark current mechanisms, and trap distributions in larger InAs quantum dots remain limited. Here, we report a thin-film PIN heterojunction colloidal InAs (1200 nm) photodiode stack with amorphous indium gallium zinc oxide and copper(I) iodide transport layers. To the best of our knowledge, the device exhibits one of the lowest reported dark current densities of 4.7 μA/cm2 at -1 V and 298 K, which decreases to 3.6 nA/cm2 at 220 K. Temperature-dependent current-voltage characteristics and activation energy analysis confirm thermally driven dark current increasing with applied field. Impedance spectroscopy reveals the dominant deep trap states within the InAs CQD layer, being tail states of the conduction band that reach down to ∼0.4 eV below the band edge, with a density of ∼2 × 1016 cm-3. The temperature-induced increase in carrier density and reduction in built-in potential within the depleted InAs layer reflect trap filling and Fermi level pinning in the N and P layers. The trapping-detrapping induced noise reduces the specific detectivity (D*) at -1 V by 1.97 orders at 1 Hz and by 1.52 orders of magnitude at 10 Hz relative to the shot-noise-limited baseline. At frequencies ∼ ≥500 Hz the D* approaches the calculated limit of 2.5 × 1011 Jones. Finally, we demonstrate infrared imaging by monolithically integrating the photodiode with a Si read-out IC, enabling imaging beyond the spectral range of CMOS sensors.

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.

Conventional Capacitive Micromachined Ultrasound Transducers (CMUTs) are generally produced on silicon wafers, limiting the size, form factor, hence applications, of the transducer arrays. In this paper, we unveil a CMUT fabrication technique that can enable the realization of high-density ultrasound arrays on large-area flexible substrates for a variety of new applications in ultrasonic sensing. We also compare optical inspection and electrical characterization methods to measure the pull-in voltage and resonance frequency of the fabricated CMUTs and show that these methods can be used to assess the yield and uniformity of the fabrication process.

This work presents a novel piezoelectric micromachined ultrasound transducer (pMUT) developed using flat panel display (FPD) technology. The pMUT phased array is a key step toward creating a large-area, flexible patch for cardiac health monitoring. By developing the pMUT on large glass and polymer substrates that can be integrated with thin film transistors (TFTs), it enables the production of large-area pMUTs controlled by TFT circuits through an inexpensive process, while maintaining high uniformity and performance levels comparable to silicon-based pMUT technology. The prototype presented in this work achieves a surface transduction efficiency of 4.9 kPa/V and a focused pressure transduction efficiency of 17 kPa/V at a distance of 10 cm from the transducer. The transducer's bandwidth is 117% at a resonance frequency of 2.4 MHz. Furthermore, cardiac imaging performed using the transducer clearly displays internal features of the heart, including the left ventricle, interventricular septum, and the aortic and mitral valves, demonstrating its potential as a cardiac output monitoring device.

Colloidal quantum dots (CQDs) are cutting-edge optoelectronic semiconductor nanocrystals that enable short-wave infrared (SWIR) vision by a widely tunable SWIR light absorption. Thanks to the advances in CQD surface ligand engineering, SWIR detectors and emitters will soon find their way into products. The CQD-based optoelectronic devices are being optimized by adapting the size of CQDs and selection of the ligands, and yet, the measurement schemes of energy band structure based on different ligands and processes of ligand exchange are not systematically studied. In this work, we systematically characterize the energy band structure of PbS (absorbing at different SWIR wavelengths) and InAs with various ligands for both solid-state and liquid-phase ligand exchange (LPLE) processes [solid-state ligand exchange (SSLE) and LPLE] by using ultraviolet photoelectron spectroscopy. The deduced energy band structures reveal that the apparent energy difference between the Fermi and valence band maximum, |E F - E VBM|, largely depends on the physical density and distribution of the CQDs within the probing area. Transmission electron microscopy images, X-ray photoelectron spectroscopy, atomic force microscopy, and variable angle spectroscopic ellipsometry reveal details of the CQD distribution, surface elemental profile, and topologies and how they affect the observed energy band structure. We demonstrate that the multistep coating improves the CQD distribution and packing density, resulting in more reliable and reproducible results that represent the bulk CQD film energy band structure. The comparison of solid and liquid phase ligand-exchanged PbS and InAs SWIR CDQs energetics indicates that the LPLE ensures more uniform dispersion and a high packing density of CQDs regardless of the solution concentration. The photoemission-deduced energy band structures are validated by fabricating thin-film photodiodes using SWIR SSLE PbS and LPLE In(As,P) CQDs. The Fermi-referenced band structures of the fabricated full photodiode stacks including band offsets and bending are discussed to improve our understanding of the device working principles and to further optimize the devices.

Image sensors are must-have components of most consumer electronics devices. They enable portable camera systems, which find their way into billions of devices annually. Such high volumes are possible thanks to the complementary metal-oxide semiconductor (CMOS) platform, leveraging wafer-scale manufacturing. Silicon photodiodes, at the core of CMOS image sensors, are perfectly suited to replicate human vision. Thin-film absorbers are an alternative family of photoactive materials, distinguished by the layer thickness comparable with or smaller than the wavelength of interest. They allow design of imagers with functionalities beyond Si-based sensors, such as transparency or detectivity at wavelengths above Si cutoff (e.g., short-wave infrared). Thin-film image sensors are an emerging device category. While intensive research is ongoing to achieve sufficient performance of thin-film photodetectors, to our best knowledge, there have been few complete studies on their integration into advanced systems. In this paper, we will describe several types of image sensors being developed at imec, based on organic, quantum dot, and perovskite photodiode and show their figures of merit. We also discuss the methodology for selecting the most appropriate sensor architecture (integration with thin-film transistor or CMOS). Application examples based on imec proof-of-concept sensors are demonstrated to showcase emerging use cases.

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.

Sensors based on quantum dot photodiodes promise quality and accessibility improvement of infrared imaging. We demonstrate miniaturization by sub-2-mu m pixel pitch arrays. Functionality is confirmed with external quantum efficiencies above 40% at 1450 nm. Monolithic integration enables high throughput and wide deployment of short-wave infrared (SWIR) imagers in applications that previously could not afford them.

Image sensors made using silicon complementary metal–oxide–semiconductor technology can be found in numerous electronic devices and typically rely on pinned photodiode structures. Photodiodes based on thin films can have a high absorption coefficient and a wider wavelength range than silicon devices. However, their use in image sensors has been limited by high kTC noise, dark current and image lag. Here we show that thin-film-based image sensors with a pinned photodiode structure can have comparable noise performance to a silicon pinned photodiode pixel. We integrate either a visible-to-near-infrared organic photodiode or a short-wave infrared colloidal quantum dot photodiode with a thin-film transistor and silicon readout circuitry. The thin-film pinned photodiode structures exhibit low kTC noise, suppressed dark current, high full-well capacity and high electron-to-voltage conversion gain, as well as preserving the benefits of the thin-film materials. An image sensor based on the organic absorber has a quantum efficiency of 54% at 940 nm and read noise of 6.1e – .

This letter presents an organic thin-film photodiode (OPD) based photogate (PG) pixel for near-infrared image sensors with improved linearity and reduced dark current. The proposed image sensor is based on the conventional 3T pixel readout with an additional PG electrode below the photodiode structure. By including the PG below the OPD, which is being separated by a thin dielectric layer, the potential bias is kept constant during integration, allowing the photodiode to be biased with low potential. Compared to the conventional capacitive transimpedance amplifier pixel, which uses an in-pixel amplifier to fix the bias of the photodiode, the proposed pixel architecture provides an effective solution for affordable high-resolution, high-performance thin-film image sensors by keeping the simple pixel structure. The proposed image sensor is designed and processed using a 130nm complementary metal-oxide semiconductor process and an OPD process. The proposed pixel structure demonstrated a 72.01 % reduction in dark current while maintaining a 3.56 times higher conversion gain. In addition, the linearity error was reduced by 59.3 %.

We present a thin‐film piezoelectric micromachined ultrasonic transducer (PMUT) technology compatible with flat‐panel manufacturing methods. Using the developed flow which is based on a low temperature AlScN piezoelectric layer, we fabricate large area 48x48 element PMUT arrays on glass. Beam steering and ultrasound medical imaging are demonstrated. The developed transducer technology can be combined with a TFT backplane and has the potential of direct integration on top of display size glass sheets, expanding the boundaries of ultrasound application domains.

Thin-film photodiodes (TFPD) monolithically integrated on the Si Read-Out Integrated Circuitry (ROIC) are promising imaging platforms when beyond-silicon optoelectronic properties are required. Although TFPD device performance has improved significantly, the pixel development has been limited in terms of noise characteristics compared to the Si-based image sensors. Here, a thin-film-based pinned photodiode (TF-PPD) structure is presented, showing reduced kTC noise and dark current, accompanied with a high conversion gain (CG). Indium-gallium-zinc oxide (IGZO) thin-film transistors and quantum dot photodiodes are integrated sequentially on the Si ROIC in a fully monolithic scheme with the introduction of photogate (PG) to achieve PPD operation. This PG brings not only a low noise performance, but also a high full well capacity (FWC) coming from the large capacitance of its metal-oxide-semiconductor (MOS). Hence, the FWC of the pixel is boosted up to 1.37 Me- with a 5 μm pixel pitch, which is 8.3 times larger than the FWC that the TFPD junction capacitor can store. This large FWC, along with the inherent low noise characteristics of the TF-PPD, leads to the three-digit dynamic range (DR) of 100.2 dB. Unlike a Si-based PG pixel, dark current contribution from the depleted semiconductor interfaces is limited, thanks to the wide energy band gap of the IGZO channel material used in this work. We expect that this novel 4 T pixel architecture can accelerate the deployment of monolithic TFPD imaging technology, as it has worked for CMOS Image sensors (CIS).

We present a near-infrared (NIR) imager based on high-performance organic photodiode in terms of dark current, specific detectivity and response time. A carefully designed interfacial layer is introduced in the thin-film organic photodiode stack to reduce trap assisted carrier emission leading to sub-nA/cm2 dark current and external quantum efficiency above 50% in the NIR range. The developed imager chip benefits from this improved dark current-voltage characteristic (high light signal to dark noise ratio) and enables high-resolution, monolithic NIR image sensors.

Quantum dot (QD) thin-film photodiodes (TFPDs) are studied extensively in the image sensor field as they can pave the way toward the cost-efficient implementation of short-wave infrared (SWIR) cameras. Interestingly, the QD TFPD image sensors can be operated in the global shutter (GS) mode by turning on the photodiode (PD) only during integration time and subsequently turning it off during the readout. This offers the substantial advantage of reducing the pixel size as it eliminates the need for additional transistors or capacitors that are otherwise typically used in conventional GS pixels. So far, no comprehensive study has yet been performed on this PD turn on/off operation mode. Therefore, in this work, we investigated the PD turn on/off GS operation mode in comparison with the conventional voltage domain (VD) GS operation—a first in-depth report of its kind. We confirm that the PD turn on/off GS mode has the advantage of a small pixel size but comes at the cost of an increasing nonlinearity as the integration time approaches the PD speed limitation. We report a parasitic light sensitivity (PLS) of −70 dB over the visible (VIS) and SWIR range and moreover demonstrate that the PLS has the potential to reach $< -100$ dB based on the discrete PD measurement.

Hyperspectral and multispectral imaging enable augmented reality experience by collecting spectral information of a scene and mapping it onto a 2D image. This imaging method is especially powerful if done in short-wave infrared (SWIR) because of the unique spectral fingerprints of many molecules found in this region of the spectrum. Despite its high potential, this technology has not been widely adopted due to the high price of standard SWIR cameras. Recently, image sensors based on colloidal quantum dot thin films have gained a lot of attention due to their potential to enable affordable and high-resolution SWIR imaging. In this work, we present the latest results of our efforts to leverage imec's thin-film SWIR imaging platform for spectral imaging. We present the measurement results of our multispectral photodetectors, as well as the results of optical simulations demonstrating new concepts for light filtering in the SWIR region, compatible with the thin-film technology.

Short-wave infrared (SWIR) range carries information vital for augmented vision. Colloidal quantum dots (CQD) enable monolithic integration with small pixel pitch, large resolution and tunable cut-off wavelength, accompanied by radical cost reduction. In this paper, we describe the challenges to realize manufacturable CQD image sensors enabling new use cases.

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.

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.