Description
Objective: Develop and demonstrate an array of highly sensitive, wavelength tuned, discrete photo-detectors with avalanche photodiode active regions and incorporating resonant cavity structures. Description: The Navy is developing and deploying a suite of imaging sensors (cameras) operating across both visible and infrared (IR) wavelengths. This includes wide field of view (WFOV) cameras that provide panoramic surveillance, situational awareness and, due to their high resolution, target detection. Camera sensors, even those operating in the IR, provide information (video imagery) to the ship’s crew that is fundamentally familiar, intuitive, and contextual. However, imagery in the visible band has an advantage in that it includes color. Color is useful in identifying well-resolved objects and is also useful in detecting targets that might otherwise go unnoticed, assuming that the scene is relatively clear and illuminated by visible light. The visible spectrum extends in wavelength from roughly 380 nm to 750 nm, a bandwidth of less than 0.4 µm. By contrast, the near to short wave IR bands extend from 750 nm to 3 µm (2.25 µm) and the mid-wave IR (MWIR) extends from 3 µm to 5 µm (2.0 um). The MWIR band therefore contains five times as much spectral content as the visible band, all invisible to the human eye. Though invisible, spectral content in the IR is just as useful as in the visible bands, if it is properly resolved. While imaging in the visible, near, and short-wave IR makes use of reflected light, imaging in the MWIR uses light that is emitted by bodies in the scene due to their temperature. And while MWIR images are sometimes colorized (known as false color images), the most commonly available MWIR cameras assign colors according to the infrared intensity, which corresponds to the temperature of the object being imaged. These cameras are typically referred to as thermal imagers and the images they produce as heat maps. While these instruments have obviously widespread utility, the spectroscopic information is lost. As with other bands, spectroscopic measurement in the MWIR has distinct uses, notably in detecting differences in vegetation and identifying certain minerals and manmade materials and chemicals by their characteristic spectral signature. For example, MWIR detectors with spectral discrimination have been proposed for machine vision systems intended to distinguish between and identify (for sorting) various classes of common plastics. Precise identification of specific materials (understood to include chemical compounds, minerals, gases, etc.) can be accomplished by characterization of the light reflected or emitted by the material using a spectrometer. Large, expensive, laboratory spectrometers accurately measure very narrow spectral lines across the entire width of the material’s signature. Small, portable spectrometers, with less functionality or for specific applications, are also available at much less cost (mainly in the visible spectrum). In both cases, a sample of the material to be characterized is required. Light from distant objects can be analyzed spectroscopically, as in the case of astronomical spectroscopy, but this requires a more sophisticated (and expensive) machine. In all these cases however, the spectrometer is based on separation of the light by prisms or gratings prior to detection of the individual component wavelengths. This precludes incorporation of a spectroscopy capability into an imaging sensor (camera) system without greatly increasing system complexity, size, and cost. Many applications do not require precise identification of the material composition of objects in an imaged scene however – just interpretation of sufficient spectral content to differentiate between broad classes of materials (different types of plastics, polymer versus metal, asphalt versus concrete, vegetation versus manmade materials, etc.). However, it is desirable that this be done remotely (that is, at some stand-off distance) in conjunction with an imaging sensor, especially in the MWIR band. The MWIR band has several characteristics that make it attractive for imaging under certain conditions. It also presents certain challenges. Mainly, photo-sensitive semiconductor materials in the MWIR band require cooling to cryogenic temperatures in order to achieve good performance. The materials are fundamentally broad band in nature, responding to light across the MWIR spectrum and capturing the imaged scene in terms of total intensity, regardless of finer spectral content. Finally, a great deal of the MWIR band is heavily absorbed by constituents of the atmosphere and effectively absent at any practical distance from the source. The resulting, useful portion of the MWIR band is marked by gaps and sharp discontinuities and any detector technology tuned to detect spectral content in these regions is effectively wasted. One technology for detection of MWIR radiation in a very narrow band is the resonant cavity infrared detector (RCID). The RCID uses a conventional photodetector “sandwiched” between two reflective interfaces. The reflective interfaces form an optical resonant cavity with the cavity length (and hence, the resonant wavelength) determined by the intrinsic thickness of the detector layers and any spacer layer added to adjust the cavity length. The active region of the detector is sensitive to the broad MWIR band but the resonant cavity structure supports only those wavelengths in a very narrow band determined by the quality factor of the resonant structure and centered at the resonant wavelength. If the length of the cavity can be carefully varied and controlled by design and process, then an entire range of photodetectors can be produced, each at a desired wavelength, and carefully chosen for regions of particular interest in the parts of the MWIR band exhibiting maximum atmospheric transmission. Conceivably, the outputs from individual RCID elements can then be processed to interpret the spectral content present in a particular imaged scene. Individual RCID elements have been successfully demonstrated with detectors of n-type absorption layer, Barrier layer, and n-type contact layer (nBn) construction. While the spectral information obtained from a series of wavelength-specific detectors is significant, the energy received is now spread over all the detector elements with any wavelengths falling between the resonances of the individual detector elements not detected. So, the IR light received by any one detector is now greatly reduced. This presents the risk that the MWIR signature emitted by dim or distant objects might no longer be detectable. This fundamental reduction in incident signal power can only be compensated for by increasing the sensitivity of the detector. The avalanche photodetector (APD) has been proposed as a means of increasing detector sensitivity as it is inherently more sensitive than nBn type photodetectors. The Navy desires an innovative detector technology that combines the relatively new RCID topology with the proven APD to demonstrate fixed wavelength-selectable detectors with high sensitivity in the MWIR band. The goal is to achieve a minimum increase in sensitivity of two orders of magnitude over that possible with state-of-the-art nBn type detectors. This may be shown by direct comparison to published work. Wavelength selection shall be shown possible by design and process control over the full MWIR band, excluding those sub-bands of atmospheric absorption. Demonstration of a full two-dimensional (NxM) focal plane array of detector elements is not expected from this effort. Demonstration of linear arrays or partial two-dimensional arrays of elements is sufficient. However, the ability to construct multiple RCID-APD detectors, each with different tuned wavelengths, together and closely spaced on the same substrate is required. The ability to arbitrarily tune individual RCID elements, independent of position in the array and independent of adjacent elements is highly desirable. Incorporation of a read-out integrated circuit in the solution is not required. However, electrical contacts must be included in the design and fabricated into the prototype such that the performance of each individual RCID element can be measured. The prototype is expected to be a single substrate (chip) containing multiple RCID elements with all required electrical contacts integrated, and with the substrate mounted on a suitable test structure. Since operation in MWIR requires the detector to be cooled, the solution shall also include a means to test the prototype at the designed operating temperature. A minimum of four prototypes must be produced and tested. At completion of the effort, the prototypes will be delivered to the Naval Research Laboratory. Work produced in Phase II may become classified. Note: The prospective contractor(s) must be U.S. owned and operated with no foreign influence as defined by 32 U.S.C. § 2004.20 et seq., National Industrial Security Program Executive Agent and Operating Manual, unless acceptable mitigating procedures can and have been implemented and approved by the Defense Counterintelligence and Security Agency (DCSA) formerly Defense Security Service (DSS). The selected contractor must be able to acquire and maintain a secret level facility and Personnel Security Clearances. This will allow contractor personnel to perform on advanced phases of this project as set forth by DCSA and NAVSEA in order to gain access to classified information pertaining to the national defense of the United States and its allies; this will be an inherent requirement. The selected company will be required to safeguard classified material during the advanced phases of this contract IAW the National Industrial Security Program Operating Manual (NISPOM), which can be found at Title 32, Part 2004.20 of the Code of Federal Regulations. Keywords: Resonant Cavity Infrared Detector; Avalanche Photodetector; nBn Photodetector; Spectroscopy; Mid-Wave Infrared; Wavelength Selection CMMC Level: Level 2 (Self)