Effect of a Ge Barrier on the Microstructure of BaTiO 3 Deposited on Silicon By Pulsed Laser Ablation
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EFFECT OF A Ge BARRIER ON THE MICROSTRUCTURE OF BaTiO 3 DEPOSITED ON SILICON BY PULSED LASER ABLATION ELIZABETH G. JACOBS, YOUNG G. RHO, RUSSELL F. PINIZZOTTO, SCOTT R. SUMMERFELT" AND BRUCE E. GNADE" Center for Materials Characterization, University of North Texas, P.O. Box 5308, Denton, TX 76203 "Materials Science Laboratory, Texas Instruments, Dallas, TX 75243 ABSTRACT BaTiO 3 (BT) thin films deposited using pulsed laser ablation on substrates of (100) Si and (100) Si with 0.3 pm Ge were examined. For one set of samples, approximately 2000 A of BT was deposited at 7000 C at 02 pressures of 0, 1.0 and 10.0 mT. 'n a second set, 02 pressures of 0 and 1.0 mT were used during deposition of 200 A of BT at 450'C followed by a 5 minute anneal and deposition of an additional 2000 A at 750°C. This gave a total sample matrix of 10 samples. Cross-sectional TEM revealed that an interfacial layer formed in the BT on Si samples but not in the BT on Ge samples. HREM analysis of the interfaces showed that the interfacial layer was amorphous. On Ge, the BT films were found to have large areas of epitactic growth along the interface. This was confirmed by diffraction tilt angle experiments which showed a strong preferred orientation of BT on Ge. No preferred orientation was found for BT on Si. Statistical grain size analysis of the films using multiple regression showed that the film microstructures were affected most strongly by the substrate type (Ge or Si) followed by the deposition temperature of the substrate. Only a weak effect of 02 pressure was observed. INTRODUCTION Ferroelectric materials show potential for a number of practical electronic applications, including their use as high dielectric constant replacements for gate dielectrics in high density capacitors, such as the 256M and larger DRAM devices. Interest is especially keen in this area since the need for this technology is immediate.1 In an earlier study of barium strontium titanate (BST) deposited on Si, an amorphous interfacial layer was observed at the BST/Si interface that increased slightly with increasing 02 overpressure during deposition by pulsed laser ablation. 2 The films were polycrystalline, columnar and there was evidence of TiSi 2 in the films. The significance of these results was that the formation of an amorphous layer at the interface would prevent the BST layer from contacting capacitor electrodes, which could greatly effect the electrical characteristics and fatigue behavior of a ferroelectric capacitor. In this work, a Ge barrier layer was placed at the interface between the ferroelectric and the Si substrate to prevent the formation of the amorphous layer. The effect of the Ge barrier on the film microstructure was studied for several deposition conditions. In this paper, we report the results of our TEM analysis in cross-section and plane view, and statistical grain size measurements of thin films of BT deposited onto Si and Si/Ge substrates using pulsed laser ablation. EXPERIMENTAL PROCEDURES Pulsed Laser Ablation Deposition: Thin films of
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