SrBi 2 Ta 2 O 9 thin films made by liquid source metal-organic chemical vapor deposition
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SrBi2 Ta2 O9 thin films made by liquid source metal-organic chemical vapor deposition Yongfei Zhu, Seshu B. Desu,a) Tingkai Li, and Sasangan Ramanathan Department of Materials Science and Engineering, Virginia Tech, Blacksburg, Virginia 24061
Masaya Nagata Functional Devices Laboratories, Sharp Corporation, Chiba 277, Japan (Received 10 May 1996; accepted 19 August 1996)
A liquid source metal-organic chemical vapor deposition system was installed to deposit SrBi2 Ta2 O9 (SBT) thin films on sapphire and PtyTiySiO2ySi substrates. The process parameters such as deposition temperature and pressure, and ratio of Sr : Bi : Ta in the precursor solutions were optimized to achieve stoichiometric films with good reproducible ferroelectric properties. It was found that the nucleation of SBT started at a deposition temperature close to 500 ±C and grain growth dominated at 700 ±C and higher temperatures. With increasing deposition temperatures, the grain size of SBT thin films increased from 0.01 mm to 0.2 mm; however, the surface roughness and porosity of the films also increased. To fabricate specular SBT films, the films had to be deposited at lower temperature and annealed at higher temperature for grain growth. A two-step deposition process was developed which resulted in high quality films in terms of uniformity, surface morphology, and ferroelectric properties. The key to the success of this process was the homogeneous nucleation sites at lower deposition temperature during the first step and subsequent dense film growth at higher temperature. The two-step deposition process resulted in dense, homogeneous films with less surface roughness and improved ferroelectric properties. SBT thin films with a grain size of about 0.1 mm exhibited the following properties: thickness: 0.16–0.19 mm; 2Pr : 7.8 –11.4 mCycm2 at 5 V; Ec : 50–65 kVycm; Ileakage : 8.0 –9.5 3 1029 A cm–2 at 150 kVycm; dielectric constant: 100 –200; and fatigue rate: 0.94–0.98 after 1010 cycles at 5 V.
I. INTRODUCTION
In recent years, ferroelectric thin films have been fabricated onto silicon-integrated circuits to provide high speed, high read/write endurance, high radiation hardness, and low power consumption nonvolatile memory.1 However, commercially available ferroelectric random access memory (FRAM) devices are currently low density devices.1 Although ferroelectric thin films have great potential for high-density (.Mbit) FRAM’s, commercial usage has been hindered largely by serious degradation problems such as fatigue, imprint, retention, and aging that reduce the lifetime of the devices.2 The most widely investigated for ferroelectric materials for nonvolatile memory applications have been PbZrx Ti12x O3 (PZT) because of their high Curie temperature and good ferroelectric properties.3 However, PZT films on PT electrode have been known to suffer from serious degradation problems such as fatigue (loss of switchable polarization with increasing reversal of polarization), aging, and leakage current.4–9 An alte
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