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0952-F10-08

Ferroelectric PTCR Films for Photonic Crystal Gas Sensor J. RaviPrakash1, Susan Trolier-McKinstry1, Jing-Gong Cheng1, Mark McNeal2, Anton Greenwald2, Irina Puscasu2, Edward Johnson2, Martin Pralle2, and Ashak Shah2 1 Pennsylvania State University, 151 Materials Research Laboratory, University Park, PA, 16802 2 Ion Optics, 411 Waverley Oaks Road, Suite 144, Waltham, MA, 02452

ABSTRACT We examined lanthanum doped lead barium titanate films for temperature measurements of photonic crystal - MEMS devices. Films were deposited by sol-gel techniques and crystallized in air above 650ºC. X-ray diffraction spectra consistent with the perovskite structure were detected. The room temperature dielectric constant was ~570 at 10 kHz of La-doped (0.3 mol%) Pb0.3Ba0.7TiO3 films. These films had a remanent polarization of ~20µC/cm2 and a coercive field of 145 kV/cm. The leakage current density was ~ 2×10−7 amps/cm2 at 100 kV/cm field. The resistivity of the films extracted from the linear region of the I-V data (electric fields in excess of 100 kV/cm) measured as a function of temperature shows an increase in resistivity at temperatures above Tc (120ºC for BaTiO3 and 240ºC for Pb0.3Ba0.7TiO3) of the film consistent with positive coefficient of resistance (PTCR) effect. However, the change in resistance was small when compared to bulk samples of similar compositions. INTRODUCTION We have developed a single chip MEMS device for infrared spectroscopic sensing of vapors based on the use of photonic crystal (PC) emission.[1] A thin heated filament emits IR radiation limited to a narrow band by the PC. The light passes through vapors to be sensed and is re-imaged on the filament where the PC now acts a preferential absorber. Optical absorption is detected as a temperature change that is measured as a change in resistance of a Pt film. Replacing the metal with a material that has a very high positive temperature coefficient of resistance (PTCR) would boost signal and greatly improve overall device sensitivity. The program goal was to fabricate a small, inexpensive, high precision carbon dioxide sensor for environmental measurements now requiring bulky apparatus. What we were trying to achieve is shown is Figure 1. Just above the Curie temperature some ferroelectric PTCR materials show an increase of resistance between 4 and 7 orders of magnitude. The temperature coefficient of resistance or TCR = (∆R/R)/∆T is about 20 to 100/ºC. This compares to a value of 0.0037/ºC for platinum now used in the sensor. The cause of this effect in bulk materials is thought to be a grain boundary phenomenon [3]. In BaTiO3 based compounds, a barrier is formed at the grain boundary due to presence of acceptor states at the interface. These states trap the carriers (electrons from the conduction band) leading to formation of a depletion region. At temperatures above Tc, the decrease in the permittivity of the ferroelectric leads to an increase in the barrier height. The grain boundary resistance has an exponential dependence on the barrier height.