Liquid Delivery MOCVD of Ferroelectric PZT
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infrared radiation detectors, by itself, or with doping/modification with various cations. While there have been reports on pyroelectric effects in thin films deposited by several methods [1-4], there has been no systematic investigation of the variation of pyroelectric properties with Zr content in PZT films produced by MOCVD. Previous results in MOCVD [5-7] of PZT using a traditional bubbler approach have described regimes where PbO volatility can act to compensate for excess Pb in the precursor gas stream, resulting in a stoichiometric film. This results in an apparent process window over a certain range of A/B (Pb/(Zr+Ti)) ratios. The liquid delivery technique [8-10] was used in the present study to explore wide ranges of composition space. In the course of process development aimed at examining the effect of Zr content on pyroelectric coefficient and other properties for films produced by MOCVD, we have observed similar effects. In this paper, we describe the effect of Zr content in the film on pyroelectric properties as well as on the A/B process window and report on the variation of film properties with A/B ratio in the precursor gas stream. EXPERIMENT PZT films were deposited on Pt/Si substrates in a vertical tube reactor with heated walls to prevent condensation of the precursors. The substrate was heated by a quartz halogen lamp through a SiC susceptor; deposition temperature was calibrated with a Pt/Si wafer with embedded thermocouples. A liquid delivery system was used to vaporize Pb(thd)2, Zr(thd)4, Ti(OiPr)2(thd)2 dissolved in organic solvent. The precursor gas stream (Ar carrier gas + 123
Mat. Res. Soc. Symp. Proc. Vol. 415 ©1996 Materials Research Society
metalorganics) was mixed with oxidizing gases (02 and N20) 0.5 meters upstream of the substrate. Total reactor pressure was maintained at 2 torr with 450 sccm flow of 02 and 320 sccm flow of N20. The precursor ratios were varied by mixing three solutions each containing different concentrations of Pb, Zr, and Ti metalorganic compounds. Composition was measured by wavelength dispersive x-ray fluorescence (XRF) using standards and thin film software. Crystallinity was characterized by x-ray diffraction using Cuka radiation in the Bragg-Brentano geometry. Pt top electrodes were deposited by evaporation through a shadow mask for electrical characterization. Ferroelectric hysteresis was measured using a Radiant Technologies RT6000. Pyroelectric characterization was carried out using a electrometer to measure pyroelectric current by the Byer-Roundy method [11]. This method measures pyroelectric current while temperature is ramped at a known rate. The pyroelectric coefficient is calculated by the following relationship [12]: p = - I / A (dt/dT)
(1)
where I = pyroelectric current, A = area of the capacitor, T = temperature, and t = time. Before carrying out the measurement, the sample was heated under +3V bias to 45'C, held for 5 minutes, and cooled under bias. The internal power supply of the electrometer was then shorted momentarily to eliminate str
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