OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Biotechnology OBJECTIVE: Develop a room temperature photoconductive detector for the mid-infrared (2 to 4.5 m) wavelength region based on lead salt detection elements. DESCRIPTION: The chemical and biological defense community has the need for a small lightweight sensor for detection of chemical/biological agents and simulants. Infrared absorption spectroscopy has proven to be a very useful tool in the detection and identification of airborne chemicals and aerosols. Pattern recognition is used to compare the infrared spectrum of library molecules against the infrared spectra of airborne contaminants. In particular chemical warfare agents and toxic industrial chemicals have distinctive absorption lines in the infrared region. Infrared spectroscopy has been used to detect chemicals at very low concentrations. Infrared spectroscopy also holds the promise of low false alarm rates due to the spectral pattern matching over a large number of spectral bins. The size, weight, and power requirements of current infrared spectrometers have limited their utility in field environments. Current infrared spectrometers are too expensive to be deployed in large numbers. Lead sulfide (PbS) is one of the oldest and most commonly used materials in use for the fabrication of infrared detectors. PbS and lead selenide (PbSe) detector cells make use of the photoconductive effect in which electrical resistance decreases with the application of infrared radiation. PbS and PbSe photoconductive detectors have high detection performance and fast response. They also have the advantage of room temperature operation. PbS and PbSe are very good candidates to fill the existing gap in the photonic and uncooled infrared detectors sensitive to the MWIR photons. Lead Salt detectors are a serious alternative to others uncooled technologies in the low cost infrared detection market. The number of potential applications is very large and includes infrared imagers, enhanced vision systems, and chemical/biological sensors. Uncooled detectors will likely dominate the majority of the future IR detection applications. Considerable advances have taken place with thermal detectors in recent years. However it is not clear that thermal detectors are the best technology at the shorter wavelengths in the mid-infrared region. New methods of processing PbS and PbSe based on silicon technology are compatible with standard CMOS circuitry. The compatibility with CMOS allows for easy integration with electronic circuitry and fabrication based on existing lithographic methods. The goal of this effort is to examine innovative methods for fabrication of detectors based on lead salts and to examine methods for manufacturing of polycrystalline PbS and/or PbSe with good electro optical characteristics. In particular three areas need to be addressed. 1) In Chem/Bio applications PbS/PbSe detectors will be required to detect infrared radiation over a fairly large spectral range (2 to 4.5 m). Uncooled operation of the detector over this entire wavelength region will require some care. The use of external coolers is discouraged. 2) The fabrication of the new photoconductive detectors will need to be compatible with existing CMOS technology to allow for easy integration with electrical components and to minimize noise and signal loss associated with non-ideal interfaces to readout electronics. 3). Commercially available PbS/PbSe detectors are known to possess a non-linear response to the irradiance of MWIR radiation illuminating the detector. This non-linear response severely limits the utility of PbS/PbSe detectors in spectroscopic applications. The new fabrication method will need to minimize this non-linearity. PHASE I: Examine new methods for the fabrication of PbS/PbSe photoconductive detectors. The new detector elements should have good detection capability over the entire 2 to 4.5 m region of the spectrum at room temperature with no external cooling required. A detectivity (D*) of 10^10 Jones or better is desired over the entire spectral range. The new detector should be compatible with CMOS fabrication. The non-linearity of the new detector with response to the irradiance of MWIR radiation illuminating the detector should be minimized. Design a fabrication process for improved photoconductive detectors based on lead salts. Perform necessary experiments as needed. PHASE II: Based on the results of the Phase I effort, fabricate a PbS/PbSe photoconductive detector array that is suitable for spectroscopic application. The array should have at least 256 elements (16x16) with spacing and size suitable for spectroscopic applications. The detector elements should have a detectivity (D*) of 10^10 Jones or better. The detectors should be able to detect mid-IR radiation over the 2 to 4.5 m region of the spectrum at room temperature with no external cooling. Design and build all necessary readout electronics. Test and characterize the system. Use the results of the testing to update the sensor design. PHASE III DUAL USE APPLICATIONS: There are environmental applications for a small robust, chemical sensor. A rugged, inexpensive chemical sensor will benefit the manufacturing community by providing inexpensive monitoring of chemical processes. Also first responders such as Civilian Support Teams and Fire Departments have a critical need for a rugged, inexpensive sensor that can be transported to the field to test for possible contamination by CW agents. REFERENCES: 1. G. Vergara, M. T. Montojo, M. C. Torquemada, M. T. Rodrigo, F. J. S nchez, L. J. G mez, R. M. Almaz n, M. Verd , P. Rodr guez, V. Villamayor, M. lvarez, J. Diezhandino, J. Plaza, and I. Catal n, Polycrystalline lead selenide: the resurgence of an old infrared detector , Opto-Electronics Review, volume 15, number 2, pages 110-117 (2007). 2. J. Diezhandino, G. Vergara, G. P rez, I. G nova, M. T. Rodrigo, F. J. S nchez, M. C. Torquemada, V. Villamayor, J. Plaza, I. Catal n, R. Almaz n, M. Verd , P. Rodr guez, L. J. G mez, and M. T. Montojo, Monolithic integration of spectrally selective uncooled lead selenide detectors for low cost applications , Appl. Phys. Lett., volume 83, issue 14, pages 2751-2753 (2003). 3. Jun Ji, Jay P. Gore, Yudaya R. Sivathanu, and Jongmook Lim, Fast infrared array spectrometer with a thermoelectrically cooled 160-element PbSe detector , Rev. Sci. Instrum., volume 75, issue 2, pages 333-339 (2004). 4. D. E. Aspnes and M. Cardona, Electro-Optic Measurements of PbS, PbSe, and PbTe , Phys. Rev., volume 173, issue 3, pages 714-728 (1968). 5. J.E. Freeman and G.M. Hieftje, Interferometric Detection of Near-Infrared Nonmetal Atomic Emission from a Microwave-Induced Plasma , Applied Spectroscopy, volume 39, issue 2, pages 211-361 (1985). 6. L. W. Kornaszewski, N. Gayraud, J. M. Stone, W. N. MacPherson, A. K. George, J. C. Knight, D. P. Hand, and D. T. Reid, "Mid-infrared methane detection in a photonic bandgap fiber using a broadband optical parametric oscillator," Opt. Express, volume 15, issue 18, pages 11219-11224 (2007). 7. T. H. Johnson, Lead salt detectors and arrays: PbS and PbSe , SPIE Proceedings, Vol. 443, Infrared Detectors, Society for Photo-Optical Instrumentation Engineers, pages 60-94. (1984). 8. Evangelos Theocharous, Absolute linearity measurements on a PbS detector in the infrared , Applied Optics, volume 45, issue 11, pages 2381-2386, (2006). 9. S. Kouissa, M. S. Aida, and A. Djemel, Surface states simulation model for photoconductors infrared detectors , Journal of Materials Science: Materials in Electronics, volume 20, supplement 1, pages 400-406 (2009). 10. V. R. Mehta S. Shet, N. M. Ravindra, A. T. Fiory, and M. P. Lepselter, Silicon-integrated uncooled infrared detectors: Perspectives on thin films and microstructures , Journal of Electronic Materials, volume 34, number 5, pages 484-490 (2005). KEYWORDS: Chemical Detection, uncooled infrared detector, lead selenide, lead sulfide, focal-plane-array
