Potassium tantalate niobate thin films for spaceborne pyroelectric detectors

Abstract

Potassium tantalate niobate (KTN) thin film pyroelectric detectors are being developed for broad band spaceborne sensors where passively cooled operating temperatures ∼ 90K are easily attained. The ability to tailor the phase transition temperature of KTN through composition control makes it an ideal candidate for such applications. Decreasing the operating temperature from 300K to 90K allows for over an order of magnitude increase in the background-limited sensitivity of a pyroelectric detector. Current research focuses on optimizing the composition and structure of laser deposited KTa0.91Nb0.09O3 (Tc = 93K) thin films. Preliminary studies of materials for bottom electrodes are being performed using Pt and conducting oxides with the perovskite structure: La0.5Sr0.5CoO3 (LSC), YBa2Cu3O7-∂ (YBCO) and SrRuO3. KTN films have been grown by pulsed laser deposition on LaAlO3, Pt/Si3N4/Si, LSC/LaAlO3, YBCO/SrTiO3, YBCO/LaAlO3, and YBCO/YSZ (yttria-stabilized ZrO2) substrates and heterostructures. The highest quality KTN films on (100)-pseudocubic LaAlO3 are grown at Po2 = 300–400 mTorr and 650–750°C exhibit in and out of plane orientation. The FWHM rocking curves of these films vary from 0.43–0.73°. The presence of undesirable pyrochlore phase, as evidenced by X-ray Diffraction, can be eliminated at partial oxygen pressures of 300 and 400 mTorr. Initially a potassium deficiency of 40–50% was measured in films using Rutherford Backscattering Spectrometry and X-ray Photoelectron Spectroscopy. This deficiency is eliminated by using a potassium enriched (117%) fully reacted ceramic target in addition to growing in higher oxygen partial pressures (400 mTorr). Devices have been fabricated from Pt, LSC and YBCO bottom electrode heterostructures. Results from these structures are discussed.