Imaging sensors (cameras) have widespread commercial, scientific, and military applications. In the visible band, such cameras are commonplace, easy to integrate and use, and generally produce a high-quality image. They are also relatively cheap due to the fortuitous properties of silicon-based semiconductors which allows imaging sensors in the visible band to be realized in the widely available and well proven silicon complementary metal–oxide–semiconductor (CMOS) system. Imaging sensors in the infrared (IR) lack many of these advantages as the light sensitive semiconductors are not suitable for fabrication of the circuitry necessary to capture and transmit the photo-electrical response. Consequently, IR imaging sensors are comprised of focal plane arrays (FPAs) that are fabricated in semiconductor systems such as InSb and HgCdTe and then bonded to ROICs that are fabricated in the more familiar Si/SiO2 semiconductor system. This hybrid construction is inherently more costly, not just because of the increased parts count, but also due to yield issues related to the individual components and to the bonding process itself. An imaging sensor architecture that eliminates the FPA to ROIC bonding process would fundamentally reduce cost. One straight-forward way of accomplishing this is by the direct deposition of light sensitive colloidal quantum dot (CQD) films onto the Si/SiO2 based ROIC. In this way, the ROIC also serves as the substrate for the deposited QCD film. Therefore, the FPA and the ROIC essentially become a single device, albeit fabricated with two separate processes. Nonetheless, this architecture eliminates the FPA as a separate (and expensive) component and eliminates the traditional semiconductor to semiconductor bonding process. In addition, because the photo-sensitive CQD material is applied as a continuous film layer (with additional film layers added for electrical function and mechanical protection), etching process or discrete pixel deposition processes requiring highly accurate registration with the underlying ROIC are also eliminated. While QCD photodetectors have been successful in the near-to short-wave infrared, they struggle to compete with traditional detectors (InSb and HgCdTe) in the mid-wave IR (MWIR) band. The main obstacle limiting the performance of these devices in the MWIR is low photoluminescence quantum yield (PLQY) – the ratio of photons emitted to photons absorbed in an ensemble of CQDs (either in solution or in a deposited film). Low PLQY in the CQDs that make up the active layer of the overall layered film structure leads to large dark currents when these films are incorporated with a substrate to form photodiodes. And it is the dark current that essentially establishes the noise floor of the device, which limits sensitivity. In particular, the PLQY of CQD films is inversely proportional to the dark current. Thus, low PLQY in the CQD film leads to low dynamic range which reduces the ability to detect dim targets. Therefore, integration times increase, potentially decreasing camera frame rates. While some of these effects can be compensated for by novel ROIC designs, nothing can take the place of inherently low dark current, which, in the case of CQDs, is a function of the semiconductor chemistry and the associated process for CQD deposition. Currently, the PLQY of CQDs drops from about 60% in the near IR band (~1 µm) to < 1% at 5 µm, following a similar “energy gap law” as seen in organic dyes. The Navy needs a novel quantum colloidal dot nanocrystal-based photo-detector technology with improved PLQY in the MWIR band, suitable for deposition on Si/SiO2 substrates. This technology is currently not commercially available. Nominally, a five-fold increase in PLQY, over the current state of the art at 5 µm is desired. The technology must be suitable for large-scale and uniform deposition for the eventual formation of photo-detector arrays of size and performance comparable to current FPA-ROIC sensors produced by conventional means. While the ROIC design is not part of this effort, the CQD based technology shall be compatible with electrical readout. That is, the colloidally deposited nanocrystal active layer shall not inhibit electrical contact with circuit features of the ROIC structure. Nor shall the ROIC require non-electrical elements such as additional photonic or magnetic elements to affect the read-out function. Likewise, while the ROIC design will determine pixel pitch, suitability of the technology for an effective pixel size of no greater than 20 µm (diameter or longest linear dimension) shall be demonstrated with a feasible path to 5 µm pixels identified. PLQY, measured at specific operating temperatures, is the key metric for this effort. Specifically, the demonstrated PLQY is assumed to be the intrinsic PLQY of the CQD active layer, when deposited as a stable film, and representative of an actual device. Means of enhancing the PLQY, such as incorporation of optical resonance cavities, though potentially of interest in some applications, are not valid in determining the true PLQY improvement. Likewise, solutions that are photo-sensitive only across extremely narrow wavelength ranges are not of interest. The prototype solution should be demonstrated across the entire MWIR sub-band of 3-5 µm. Solutions that combine increased PLQY with higher operating temperatures are highly desirable. This effort anticipates a technical solution that combines new (or improved) material with a corresponding set of manufacturing processes that will ultimately be used to produce low cost MWIR imaging sensors. While demonstrations of the technology in affecting large FPA-like sensors are beyond the scope of this effort, demonstration that the technology is compatible with this goal is expected. Therefore, fabrication and demonstration of individual photodetectors is expected and fabrication of small-scale, multi-detector, test structures is desirable. In any case, demonstration of the technology shall be accompanied by suitable evidence that the process is scalable. Demonstration in an actual camera is not required and the choice of test structures and methods for demonstration of prototype devices is not restricted. However, it is necessary to measure and report intrinsic PLQY, photo-electrical performance, and the manufacturing quality. Therefore, innovative measurement techniques (and perhaps new instruments) may need to be developed as well. The solution that demonstrates the highest PLQY while meeting the other requirements, as defined above, is desired and this effort is anticipated to include many build-test-build cycles as both the colloidal nanocrystal chemistry and process are refined.
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