The Microstructure and Properties of Layered Oxide Thin Films Fabricated by MOCVD
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ABSTRACT For the first time, layered oxide thin films of SrBi 2 Ta 2 O 9 (SBT) with very good ferroelectric properties have been prepared by direct-liquid-injection MOCVD technique. The SBT films were deposited onto both Pt/Ti/Si0 2 /Si wafers and single-crystal sapphire substrates to measure their phase formation, microstructure and ferroelectric properties. Crystalline SBT phase had been observed at temperatures as low as 5000 C. With increasing deposition temperature above 500 0 C, the grain size of SBT thin films was increased from 0.01 ýtm to 0.2 ýtm. The films were found to be dense and homogenous. Typically, SBT thin films of 200 nm thick with grain size about 0. 1ltm have 2Pr around 10 PC/cm 2 at 5V, Ec 55.7 kV/cm, and dielectric constant around 100. The leakage 9 2 at 150 kV/cm. The films also showed fatigue-free currents were as low as 8 x 10-9 A/cm characteristics: no fatigue was observed up to 1.4 x 1010 switching cycles. This development of MOCVD technique for good quality SBT films make their integration into high density nonvolatile memories relatively easy. INTRODUCTION In recent years, the interest in ferroelectric materials for nonvolatile random access memory application (NvRAMs) has intensified dramatically1- 4 . In order to meet the requirements for these memory applications, the ferroelectric capacitors should have small size, low coercive field, high remanent polarization, low fatigue rate and low leakage current. So far, ferroelectric materials such as PbZrl_xTixO 3 (PZT) or doped PZT had been investigated. This is because of their high Curie temperature and promising electrical properties such as large remanent polarization and low switching field. However, these perovskite ferroelectrics, in general, are known to suffer from serious degradation problems such as fatigue (loss of switchable polarization with increasing reversal of polarization), aging and leakage current that affect the lifetime of the devices 5 -10 . An alternative approach is to find new ferroelectric materials. Recently, SBT films were successfully prepared by several research groups 1 1-16 using metalorganic decomposition (MOD) 1 1-15 or pulsed laser deposition (PLD) techniques 16 . All the reported data on these materials showed very attractive ferroelectric properties and fatigue free characteristics. However, neither MOD nor PLD is suitable for high density device applications due to their poor step coverage. In this paper, we present a comprehensive study of the preparation, characterization and ferroelectric properties of SBT thin films on various substrates. This is the first time that metalorganic chemical vapor deposition (MOCVD) technique has been used to prepare SBT films. 189 Mat. Res. Soc. Symp. Proc. Vol. 415 ©1996 Materials Research Society
EXPERIMENT The SBT films were prepared on both Pt/Ti/SiO 2/Si substrates and sapphire disks by metalorganic chemical vapor deposition. Bis(2,2,6,6-tetramethyl-3, 5-heptanedionato) strontium hydrate [Sr(TMHD) 2 ], Triphenylbismuth [Bi(C 6 H 5 )3 ], Tantalum ethoxide [Ta
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