Europium-substituted bismuth titanate (Bi3.25Eu0.75Ti3O12) thin films were deposited on the Pt/Ti/SiO2/Si(111) substrates by metal-organic decomposition (MOD) method using a repeated coating/drying cycle. Effect of crystallization temperature on microstructure of Bi3.25Eu0.75Ti3O12(BET) thin films was investigated by X-ray diffractometry(XRD), scanning electron microscopy (SEM) and Raman spectroscopy, and ferroelectric property was studied by Precision Workstation Ferroelectric Tester. The crystallinity of BET thin films is improved and the average grain size increases with the crystallization temperature from 600 to 750 ℃. Under 9 V applied voltage, the remnant polarization (2Pr) of BET thin films annealed at 700 ℃ is 50.7 μm/cm2, which is higher than that of the films annealed at 600, 650 and 750℃.
The possibility of ultra-thin Y2O3 (yttrium sesquioxide) films as insulator of metal ferroelectric insulator semiconductor (MFIS) structure was investigated. The ultra-thin Y2O3 films with thickness of 10-40 nm were fabricated on p-type Si (100) substrates by molecular beam epitaxy(MBE) in vacuum and subsequently submitted to rapid thermal processing (RTP) in air ambient at 700, 800 and 900 ℃ for 30 min, respectively. The films were characterized by X-ray diffractometry and Raman spectroscopy. High frequency capacitance—voltage (C—V) characteristics and current—voltage (I—V) characteristics of the Y2O3/Si structure were analyzed. A Raman peak of the Y2O3 thin films was observed at 378 cm-1. From the C—V data, these films exhibit dielectric constants ranging from 13 to 17.28, the hysteresis width (△VFB) ranging from 0.07 to 0.22 V and the density of trapped charges ranging from 1.65×1011 to 4.01×1011 cm-2. A leakage current of 4.75×10-8 -9.0×10-6 A/cm2 at 1.5 MV/cm was observed. The results show that the Y2O3 buffer layers are suitable for non-volatile MFIS structure field-effect-transistors (FETs) memory application.
A Landau-Ginsburg-Devonshire(LD)-type thermodynamic theory was used to describe the effect of external stress on phase diagrams and dielectric properties of epitaxial ferroelectric thin films grown on orthorhombic substrates which induce nonequally biaxial misfit strains in the films plane. The “misfit strain-external stress” and “external stress-temperature” phase diagrams were constructed for single-domain BaTiO3(BT) and PbTiO3(PT) thin films. It is shown that the external stress may lead to the rotation of the spontaneous polarization and a gradual change of its magnitude, which may result in phase transition. Nonequally biaxial misfit strains dependence of the stability of polarization states may be governed by external stress. At room temperature, stress-induced ferroelectric/paraelectric phase transition which occurs in film on cubic substrate does not take place in the ferroelectric thin film grown on orthorhombic substrate. It is also shown that the nonequally misfit strains in the film plane may lead to the appearance of new phases which do not form in films grown on cubic substrates under external stress. The dependence of the dielectric response on the external stress is also studied. It is shown that the dielectric constants of single-domain PT and BT films are very sensitive to the external stress under the given anisotropic misfit strains-temperature conditions. It presents theoretical evidence that the external stress and anisotropic misfit strains can be employed for improving the thin films physical properties.
