Ethan Klem

PbS colloidal quantum dot photodiodes for low-cost SWIR sensing

RTI has developed a photodiode technology based on solution-processed PbS colloidal quantum dots (CQD). These devices are capable of providing low-cost, high performance detection across the Vis-SWIR spectral range. At the core of this technology is a heterojunction diode structure fabricated using techniques well suited to wafer-scale fabrication, such as spin coating and thermal evaporation. This enables RTI’s CQD diodes to be processed at room temperature directly on top of read-out integrated circuits (ROIC), without the need for the hybridization step required by traditional SWIR detectors. Additionally, the CQD diodes can be fabricated on ROICs designed for other detector material systems, effectively allowing rapid prototype demonstrations of CQD focal plane arrays at low cost and on a wide range of pixel pitches and array sizes.

Buckled topography to enhance light absorption in thin film organic photovoltaics comprising CuPc/C60 bilayer laminates:

Abstract Organic photovoltaic (OPVs) devices are promising due to their low cost, light weight, and compatibility with high throughput processing on flexible substrates. This paper demonstrates a simple process utilizing thin-film instabilities to enhance light absorption in OPVs in a way that is compatible with planar processing and the customary thermal annealing steps. Placing a thin, transparent polystyrene (PS) film between the glass substrate and the transparent conductive indium tin oxide (ITO) electrode results in the formation of periodic surface buckles in the PS layer due to induced strain caused by thermal expansion mismatch between the ITO and PS films. OPVs comprising bilayer laminates of copper phthalocyanine (CuPc) and fullerene (C60) deposited onto buckled the ITO/PS substrate show enhanced light absorption due to the longer path-length and improved power conversion efficiency (20%) relative to a similar planar device. This approach is appealing because it takes advantage of naturally-occurring surface topography (i. e., buckling) without the need for any sophisticated patterning. This work is distinguished from other buckling strategies for OPVs by the use of ITO as a transparent, conductive electrode and the absence of additional processing steps.

Towards low-cost infrared imagers: how to leverage Si IC ecosystem

Although performance remains paramount, especially in defense and security applications, cost increasingly drives the implementation strategy for existing and emerging infrared imaging systems. In this paper, we review two technologies that leverage mainstream Si IC technology and infrastructure to increase manufacturability and throughput and decrease the cost of infrared focal plane arrays. In the first example, we review a wafer-level vacuum packaging approach which replaces die-at-a-time serial approaches with a parallel process in which vacuum enclosures for hundreds of microbolometer arrays are formed simultaneously. The second example is a novel nanostructured short-wave infrared sensor developed at RTI International that can be monolithically integrated with Si CMOS at wafer scale.

Colloidal quantum dot Vis-SWIR imaging: demonstration of a focal plane array and camera prototype (Presentation Recording)

RTI has developed a photodiode technology based on solution-processed PbS colloidal quantum dots (CQD). These devices are capable of providing low-cost, high performance detection across the Vis-SWIR spectral range. At the core of this technology is a heterojunction diode structure fabricated using techniques well suited to wafer-scale fabrication, such as spin coating and thermal evaporation. This enables RTI’s CQD diodes to be processed at room temperature directly on top of read-out integrated circuits (ROIC), without the need for the hybridization step required by traditional SWIR detectors. Additionally, the CQD diodes can be fabricated on ROICs designed for other detector material systems, effectively allowing rapid prototype demonstrations of CQD focal plane arrays at low cost and on a wide range of pixel pitches and array sizes. We will show the results of fabricating CQD arrays directly on top of commercially available ROICs. Specifically, the ROICs are a 640 x 512 pixel format with 15 µm pitch, originally developed for InGaAs detectors. We will show that minor modifications to the surface of these ROICs make them suitable for use with our CQD detectors. Once completed, these FPAs are then assembled into a demonstration camera and their imaging performance is evaluated. In addition, we will discuss recent advances in device architecture and processing resulting in devices with room temperature dark currents of 2-5 nA/cm^2 and sensitivity from 350 nm to 1.7 μm. This combination of high performance, dramatic cost reduction, and multi-band sensitivity is ideally suited to expand the use of SWIR imaging in current applications, as well as to address applications which require a multispectral sensitivity not met by existing technologies.

Low-cost SWIR sensors: advancing the performance of ROIC-integrated colloidal quantum dot photodiode arrays

RTI has developed a novel photodiode technology based on solution-processed PbS colloidal quantum dots (CQD) capable of providing low-cost, high performance detection across the Vis-SWIR spectral range. The most significant advantages of the CQD technology are ease of fabrication, small pixel size, and extended wavelength range. The devices are fabricated directly onto the ROIC substrate at low temperatures compatible with CMOS, and arrays can be fabricated at wafer scale. We will discuss recent advances in device architecture and processing that result in measured dark currents of 15 nA/cm2 at room temperature and enhanced SWIR responsivity from the UV to ~1.7 μm, compare these results to InGaAs detectors, and present measurements of the CQD detectors temperature dependent dark current.

Room temperature SWIR sensing from colloidal quantum dot photodiode arrays

While InGaAs-based focal plane arrays (FPAs) provide excellent detectivity and low noise for SWIR imaging applications, wider scale adoption of systems capable of working in this spectral range is limited by high costs, limited spectral response, and costly integration with Si ROIC devices. RTI has demonstrated a novel photodiode technology based on IR-absorbing solution-processed PbS colloidal quantum dots (CQD) that can overcome these limitations of InGaAs FPAs. The most significant advantage of the CQD technology is ease of fabrication. The devices can be fabricated directly onto the ROIC substrate at low temperatures compatible with CMOS, and arrays can be fabricated at wafer scale. Further, device performance is not expected to degrade significantly with reduced pixel size. We present results for upward-looking detectors fabricated on Si substrates with sensitivity from the UV to ~1.7 µm. We further show devices fabricated with larger size CQDs that exhibit spectral sensitivity that extends from UV to 2 µm.

High-performance SWIR sensing from colloidal quantum dot photodiode arrays

RTI has demonstrated a novel photodiode technology based on IR-absorbing solution-processed PbS colloidal quantum dots (CQD) that can overcome the high cost, limited spectral response, and challenges in the reduction in pixel size associated with InGaAs focal plane arrays. The most significant advantage of the CQD technology is ease of fabrication. The devices can be fabricated directly onto the ROIC substrate at low temperatures compatible with CMOS, and arrays can be fabricated at wafer scale. Further, device performance is not expected to degrade significantly with reduced pixel size. We present results for upward-looking detectors fabricated on Si substrates with sensitivity from the UV to ~1.7 μm, compare these results to InGaAs detectors, and present measurements of the CQD detectors temperature dependent dark current.

ROIC for 3 um Pixel Pitch Colloidal Quantum Dot Detectors

The DARPA Wafer-scale Infrared Detectors (WIRED) program requires the development of low cost detector technologies with high quantum efficiency and low dark current based on colloidal quantum dots (CQDs) of compound semiconductors. The program targets the SWIR (900 – 1700 nm) spectral range. This paper focuses on the design of 3 μm pitch, low noise ROICs for interfacing with SWIR CQD detectors. So far, the team has developed and demonstrated PV detector technology based on thin film CQDs IR-absorbing materials. The CQD films are deposited by spin-coating from solution directly on ROIC and fanout wafers at room temperature. The photodiode fabrication is fully compatible with CMOS fabrication at the wafer-level, allowing for large format FPAs limited only by the ROIC size. This paper outlines the preliminary design of a 3 μm pitch 1920 x 1080 modular ROIC, easily scalable to larger formats by mask stitching. DRS has designed a full HD (1920x1080) readout integrated circuit (ROIC) specifically for cost-effective waferscale infrared (WIRED) detectors on 3 μm pitch, for best theoretical image quality optical system performance. The sensor’s pixels use a capacitive trans-impedance amplifier (CTIA) and a metal-insulator-metal (MIM) integration capacitor, to achieve 22 Ke- well capacity, 0.7 V output swing and 37 e- or 18 e- equivalent readout noise, operating at 60 Hz ripple readout mode or 30 Hz correlated double sampling (CDS) mode, respectively. The fully digital ROIC consumes approximately 0.5 W of power, allowing it to be fielded in battery-powered applications.

Performance evaluation and modeling of a conformal filter (CF) based real-time standoff hazardous material detection sensor

Hyperspectral imaging (HSI) systems can provide detection and identification of a variety of targets in the presence of complex backgrounds. However, current generation sensors are typically large, costly to field, do not usually operate in real time and have limited sensitivity and specificity. Despite these shortcomings, HSI-based intelligence has proven to be a valuable tool, thus resulting in increased demand for this type of technology. By moving the next generation of HSI technology into a more adaptive configuration, and a smaller and more cost effective form factor, HSI technologies can help maintain a competitive advantage for the U.S. armed forces as well as local, state and federal law enforcement agencies. Operating near the physical limits of HSI system capability is often necessary and very challenging, but is often enabled by rigorous modeling of detection performance. Specific performance envelopes we consistently strive to improve include: operating under low signal to background conditions; at higher and higher frame rates; and under less than ideal motion control scenarios. An adaptable, low cost, low footprint, standoff sensor architecture we have been maturing includes the use of conformal liquid crystal tunable filters (LCTFs). These Conformal Filters (CFs) are electro-optically tunable, multivariate HSI spectrometers that, when combined with Dual Polarization (DP) optics, produce optimized spectral passbands on demand, which can readily be reconfigured, to discriminate targets from complex backgrounds in real-time. With DARPA support, ChemImage Sensor Systems (CISS™) in collaboration with Research Triangle Institute (RTI) International are developing a novel, real-time, adaptable, compressive sensing short-wave infrared (SWIR) hyperspectral imaging technology called the Reconfigurable Conformal Imaging Sensor (RCIS) based on DP-CF technology. RCIS will address many shortcomings of current generation systems and offer improvements in operational agility and detection performance, while addressing sensor weight, form factor and cost needs. This paper discusses recent test and performance modeling results of a RCIS breadboard apparatus.

Solution-processed colloidal quantum dot photodiodes for low-cost SWIR imaging

While InGaAs-based focal plane arrays (FPAs) provide excellent detectivity and low noise for SWIR imaging applications, wider scale adoption of systems capable of working in this spectral range is limited by high costs, limited spectral response, and costly integration with Si ROIC devices. RTI has demonstrated a novel photodiode technology based on IR-absorbing solution-processed PbS colloidal quantum dots (CQD) that can overcome these limitations of InGaAs FPAs. We have fabricated devices with quantum efficiencies exceeding 50%, and detectivities that are competitive with that of InGaAs. Dark currents of ~2 nA/cm2 were measured at temperatures compatible with solid state coolers. Additionally, by processing these devices entirely at room temperature we find them to be compatible with monolithic integration onto readout ICs, thereby removing any limitation on device size. We will show early efforts towards demonstrating a direct integration of this sensor technology onto a Si ROIC IC and describe a path towards fabricating sensors sensitive from the visible to 2200 nm at a cost comparable to that of CMOS based devices. This combination of high performance, dramatic cost reduction, and multispectral sensitivity is ideally suited to expand the use of SWIR imaging in current applications, as well as to address applications which require a multispectral sensitivity not met by existing technologies.