We have developed a new low temperature growth technique for Bi4Ti3O12 thin films using a MOCVD method in which an ultra-thin double buffer layer (5-nm thick Bi4Ti3O12/5-nm thick TiO2) is used to control the crystallization and fine grain structure. The 100-nm thick Bi4Ti3O12 thin films fabricated at 400°C showed an extremely smooth surface morphology and good electrical properties, namely, a large remanent polarization of P r=11 µC/cm2, a coercive field of E c=90 kV/cm and a low leakage current I L=7×10-9 A/cm2 at 3 V. Moreover, we successfully fabricated 50-nm ultra thin Bi4Ti3O12 films with P r=9 µC/cm2 and E c=120 kV/cm at 3 V. For the first time, the fatigue free property, which is very important for nonvolatile ferroelectric memory (NVFRAM) applications, was confirmed up to 1×1012 switching cycles.

RNA participates both in functional aspects of the cell and in gene regulation. The interactions of these molecules are mediated by their secondary structure which can be viewed as a planar circle graph with arcs for all the chemical bonds between pairs of bases in the RNA sequence. The problem of predicting RNA secondary structure, specifically the chemically most probable structure, has many useful and efficient algorithms. This leaves RNA folding, the problem of predicting the dynamic behavior of RNA structure over time, as the main open problem. RNA folding is important for functional understanding because some RNA molecules change secondary structure in response to interactions with the environment. The full RNA folding model on at most O(3 n ) secondary structures is the gold standard. We present a new subset approximation model for the full model, give methods to analyze its accuracy and discuss the relative merits of our model as compared with a pre-existing subset approximation. The main advantage of our model is that it generates Monte Carlo folding pathways with the same probabilities with which they are generated under the full model. The pre-existing subset approximation does not have this property.

This letter describes the heteroepitaxy of InP on Si by MOCVD. A new epitaxial structure with a thin GaAs intermediate layer (InP/GaAs/Si) is proposed to alleviate the large lattice mismatch (8.4%) between InP and Si. Using this structure, a 4-inch InP single crystal with a mirror-like surface and good thickness uniformity (Δd/d=±10%) was obtained. Residual stress in the InP film was 5.7±108 dyn/ cm2 for the InP/GaAs/Si structure, as compared to 8.3×108 dyn/cm2 for the InP directly grown on Si. This shows that the GaAs intermediate layer is also effective in reducing the residual stress in the InP epilayer.

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