| Summary: | 碩士 === 國立東華大學 === 材料科學與工程學系 === 93 === Ferroelectric thin films of bismuth-layered perovskite family, e.g. Bi4Ti3O12 (BIT), due to its excellent fatigue endurance have attracted the great attentions of many scientists. (Bi,La)4Ti3O12 (BLnT) has been recently considered as one of the most promising materials for nonvolatile random access memory (NvRAMs) devices.
In this thesis, we investigated the ferroelectricity of modified Bi4Ti3O12 thin films prepared by chemical solution deposition. This research was divided into four parts. In the first part, the effect of the H2O2 addition to the spin-coating solution on the ferroelectric properties of Nd-substituted BIT (BNT) was investigated. We found the addition of H2O2 improved ferroelectricity of BNT thin films. In the second part: In-doped Bi4Ti3O12 thin films (Bi4-xInxTi3O12, x= 0, 0.2, 0.4, 0.5; or abbreviated as BInT thin films) were discussed. As the substitution for Bi3+ sites of the modified BIT films has been focused on the larger lanthanoids, we proposed the non-lanthanoidal In3+ with a far smaller ionic size in the hope of achieving excellent ferroelectricity. As the Nd-doped BIT films has been recognized as one of the best ferroelectrics for applications in the future. In the third part : we attempted to modified BNT films with the non-lanthanoid substitution. We investigated theBi3.15Nd0.85-yInyTi3O12 films with, y = 0, 0.2 , 0.4；In-BNT) and demonstrated their performance in this thesis. In the last part, the effect of film thickness on ferroelectric properties was studied with the focus on the [Bi3.15(Nd0.65In0.2)Ti3O12] films spin coated for 3 and 6 times.
Experimental result discovered that the addition of H2O2 in the spin-coating solution resulted in a higher remanent polarization, a better fatigue endurance, and a higher non-volatile charge density for memory applications. For the chemically derived BInT film, they started to crystallize with a layered perovskite structure at 550oC without a second phase. The best ferroelectric properties were observed for Bi3.6In0.4Ti3O12 films crystallized at 600oC in oxygen atmosphere for 15 min. It displayed a remanent polarization of 64 �媴/cm2, a coercive field of 128 kV/cm, a reduction less than 10% in remanent polarization after 1010 switching, and leakage current of 10-5 A at 3 V. For the In-doped (Bi,Nd)4Ti3O12 (In-BNT) thin films, [Bi3.15(Nd0.45In0.4)Ti3O12] films were annealed at 650oC in oxygen atmosphere for 15 min and had a remanent polarization of 76 μC/cm2, a coercive field of 160 kV/cm, and excellent fatigue resistance. For the effect of film thickness, the 6-layer-coated [Bi3.15(Nd0.65In0.2)Ti3O12] films had a remanent polarization of 81 μC/cm2, a coercive field of 198 kV/cm, lower leakage current, and improved fatigue resistance.
Our new (In, Nd)-substituted Bi4Ti3O12 films are successfully invented and have displayed an advance in developing non-volatile FRAMs.
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