Development of a novel thin film pyroelectric infrared detector

Passively-cooled, thin film pyroelectric infrared detectors have been developed for unique space-borne applications. Initial research involved selecting the best material for the active, sensing layer in a detector structure to be used aboard an Earth-orbiting satellite. The first critical attribute of this material is an operating, or Curie, temperature near 90 K, as this is the satellite's ambient temperature and thus the operating temperature of an onboard, passively-cooled detector. Second, the material's capacitance, or dielectric constant, must display a large nonlinear temperature dependence in the temperature region surrounding its Curie point.; The studied materials were selected from a small group of ferroelectric alloy systems that could obtain a Curie temperature of 90 K. Initial research focused on potassium tantalate niobate (KTa_1−xNb_xO ₃ or KTN). Thin KTN films were grown by pulsed laser deposition onto strontium titanate single crystal substrates, utilizing a pulsed laser deposited strontium ruthenate metallic oxide as a buffer layer. However substandard dielectric properties due to non-stoichiometric growth and the presence of a paraelectric crystalline phase led us to the best alternative alloy system which did not include any highly volatile elements: barium strontium titanate (Ba_1−xSr_xTiO₃ or BST). Pulsed laser deposited thin BST films grew stoichiometrically and phase pure. Further, initial dielectric property measurements showed significant promise toward the eventual creation of a highly sensitive pyroelectric detector utilizing BST epilayers. Thin film thermal sensitivity was optimized through PLD parameter studies. We observed a dramatic increase in thin film sensitivity when growth was slowed to produce a film with high in-plane tensile stress and large grain size.; In addition to materials optimization, detector prototypes were pursued. Detector development demanded integration with silicon-based read-out electronics. Thus depositions of BST on Si/SiN substrates were performed and device prototypes were fabricated. The electrode configuration was examined and a detector structure utilizing a single metallic oxide electrode was found to generate the highest quality detector. These first prototypes measured a detectivity figure of merit comparable to those of pyroelectric detectors commonly used in room temperature applications. More significantly, these unique detectors were designed to operate optimally in a temperature regime near 90 Kelvin.