The structure of glasses in the systems (100 – x)B2O3–xPbF2 (x = 30, 40, and 50) and 50B2O3–(50 – x)PbO–xPbF2 (x = 5, 10, 15, 20, 25, 30, 35, 40, and 45) has been studied by solid state NMR and EPR spectroscopies. On the basis of 11B and 19F high resolution solid state NMR as well as on 11B/19F double resonance results, we develop a quantitative structural description on the atomic scale. 19F NMR results indicate a systematic dependence of the fluoride speciation on PbF2 content: At low x-values, F– ions are predominantly found on BO3/2F– units, whereas, at higher x-values, fluoride tends to be sequestrated into amorphous domains rich in PbF2. In addition, both pulsed EPR studies of Yb3+ doped glasses and photophysical studies of Eu3+ doped samples indicate a mixed fluoride/borate coordination of the rare-earth ions and the absence of nanophase segregation effects.
Numerous efforts have been reported to date relative to the rapid advancement of integration in very-large-scale integrated circuits (VLSIs). A major issue involves sustaining the capacitance of a memory cell with a reduced area. As a result of this, in dynamic random access memories (DRAMs), oxide thickness has been reduced to the point where the tunneling process becomes significant and three-dimensional cell structures such as stacked or trench capacitors are used in order to obtain a large effective electrode area. However, this is expected to reach the limit in the near future with the result that ferroelectric and high dielectric constant films will probably replace SiO 2 in various memory device applications, as well as in antireflective coatings in optoelectronic devices and pyroelectric detectors. [1][2][3][4][5] However, electrical properties, such as the dielectric constant, undergoes degradation with decreasing film thickness. [6][7][8][9][10] To deal with this problem, strained superlattice structures, which are composed of two different dielectric materials, have been proposed, 11,12 in which stress is artificially introduced into the heteroepitaxial films due to lattice mismatch at the heterointerfaces between the different layers. Thus, polarization, which occurs as the result of atomic displacements, and ferroelectricity will be strongly affected. Such stress and polarization can be controlled by designing the period of a superlattice. Given this consideration, a dielectric superlattice thin film with a high dielectric constant would be expected to give rise to the dielectric properties of films. [11][12][13][14] This paper is primarily concerned with presenting experimental evidence relative to dielectric superlattice films by means of measurement of electrical characteristics such as capacitance vs. voltage, polarization vs. electric field, and current vs. voltage. Experimental SrTiO 3 /BaTiO 3 superlattice films were prepared by an atomic layer metallorganic chemical vapor deposition (ALMOCVD) method on conductive Nb-doped SrTiO 3 substrates with a [001] orientation at a temperature of 620ЊC. Details are presented in Ref. 12 and 15. The total thickness of the superlattice films was set at 90 nm. The upper electrodes consisted of evaporated gold dots which were 0.3 mm in diam. Capacitance measurements were carried out using an LCR meter. The microstructures of the superlattice films were investigated by transmission electron microscopy (TEM). Results and DiscussionThe dielectric constant, which is derived from the capacitance measurement, is shown in Fig. 1 as a function of the superlattice period. The dielectric constant increases with a decrease in the period of the superlattice. The reason for this is because the polarization
This paper presents the investigation of single-phase SrTiO3 thin films prepared by two methods: the atomic layer-by-layer metal-organic chemical vapor deposition (ALMOCVD) method and the co-deposition MOCVD method, using third generation dipivaloylmethane chelate of strontium (Sr(DPM)2·2tetraene) and titanium tetraisopropoxide as precursors. Compared with co-deposition MOCVD, the SrTiO3 films grown by ALMOCVD on MgO substrates show higher [100] orientation with a value of the full width at half maximum as small as 0.18°, investigated by X-ray diffraction θ–2θ scan, ω-rocking curve analysis and φ scan. Observations with atomic force microscopy also show that the SrTiO3 films have a very smooth surface in which the maximum distance between peaks and valleys is 2.70 nm and the average roughness is 0.37 nm. These results demonstrate that better epitaxial SrTiO3 films, in terms of crystalline quality and surface morphology, can be obtained by ALMOCVD rather than by co-deposition MOCVD. Furthermore, we have investigated that N2O supplied during deposition influences the crystallinity and surface morphology.
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