Growth and the microstructural and ferroelectric characterization of oriented BaMgF(4) thin films

The growth of ferroelectric BaMgF(4) thin films on Si(100), sapphire, and other substrates under ultrahigh vacuum (UHV) conditions is reported. Microstructural characterization of the films using transmission electron microscopy (TEM) revealed that they were oriented crystalline films, although not epitaxial. Ferroelectric hysteresis measurements yielded spontaneous polarization and coercivity values of almost 1.0 muC/cm(2) and 160 kV/cm, respectively. The discrepancy with the bulk ferroelectric values were attributed to the electrical contacts, impurities in the film, and lack of polar axis orientation. Preliminary capacitance-voltage (C-V) hysteresis measurements on a 480-nm-thick BaMgF(4) film yielded a 10.8-V threshold shift (memory window) in response to a +/-10-V programming voltage for a MIS gate structure similar to that of the ferroelectric memory field-effect transistor (FEMFET).     

Multi-objective optimization of feature selection using hybrid cat swarm optimization

With the pervasive generation of information from a wide range of sensors and devices, there always exist a large number of input features in databases, thus complicating machine learning problem formulation. However, certain features are relatively impertinent to specific problems, which may degrade the performances of classifiers in terms of prediction accuracy, sensitivity,specificity, and recall rate. The main goal of a multi-objective optimization problem is to identify the subsets of the given features. To this end, a hybrid cat swarm optimization(HCSO) algorithm is proposed in our paper for performance improvement of the basic cat swarm optimization(CSO) that incorporates guided and competitive inherent characteristics into the original CSO. The performance of HCSO has been tested by finding the optimal feature subset for 15 benchmark datasets. The number of class labels for these datasets varies between 2 and 40. The time complexity analysis of both CSO and HCSO has also been evaluated. Moreover, the performance of the proposed algorithm has been compared with that of simple CSO and other state-ofthe-art techniques. The performances obtained by HCSO have an average 2.68% improvement with a standard deviation of 2.91.The maximum performance improvement is up to 10.09% in prediction accuracy. Tested on the same datasets, CSO has yielded improvements within the range of-7.27% to 8.51% with an average improvement 0.9% and standard deviation 3.96. The statistical tests carried out in the experiments prove that HCSO manifests a moderately better feature selection capacity than that of its counterparts.     

Pulsed laser deposition (PLD) of oriented bismuth titanate films for integrated electronic applications

Abstract

In this paper we describe recent successes of growth of epitaxial bismuth titanate (BTO) films by pulsed laser deposition (PLD) suitable for electro-optic and electrical switching device structures, and fabrication of an improved gate structure for a ferroelectric memory FET (FEMFET). TEM and x-ray results indicate that excellent crystalline quality BTO films were achieved on LaAlO₃. Polarization switching was demonstrated for BTO capacitors with epitaxial superconducting YBa₂Cu₃O₇ as the lower electrode. Using an SiO₂ buffer layer, a BTO/Si structure was fabricated and direct charge modulation in the Si by polarization reversal in the BTO was demonstrated.     

UHV processing of ferroelectric barium magnesium fluoride films and devices

Barium magnesium fluoride (BaMgF4) has recently emerged as a strong candidate for application as the gate dielectric in ferroelectric random access memory (FERRAM) devices with nondestructive readout (NDRO). In earlier papers we reported the successful growth of oriented BaMgF4 films on Si(100) and other substrates in a ultrahigh vacuum (UHV) system, as well as the results of the structural and electrical characterization of these ferroelectric films. In the present paper, we review some of the earlier results, and also examine the effect of variations in the growth temperature and various post-growth anneals on the stoichiometry, crystallinity, orientation, and electrical characteristics of the BaMgF4 films. Initial attempts at integrating the ferroelectric field-effect transistor (FEMFET) with the standard CMOS VLSIC processing, as well as the effect of adding a thin capping layer of SiO2 on the BaMgF4 will also be described.     

BaMgF4 thin film development and processing for ferroelectric FETS

Abstract

A ferroelectric memory field-effect transistor (FEMFET) where a ferroelectric thin film is incorporated directly into the gate structure of the transistor is attractive, because it provides not only nonvolatility, but also nondestructive readout (NDRO). At Westinghouse, we are currently developing a FEMFET using thin film barium magnesium fluoride (BaMgF₄), a ferroelectric material that was discovered in 1969, but was not fabricated in thin film form until 1989. The BaMgF₄ films are grown by evaporation in an ultrahigh vacuum (UHV) chamber on clean Si(100). The natural tendency of these films to grow with the ferroelectric a-axis in the Si(100) plane has been overcome to obtain more random orientation with larger reversible polarization perpendicular to the film. A capping layer (SiO₂) has been found to be essential for process integrability of these BaMgF₄ films. Ti-W metallization produced only a slight reduction in the capacitance-voltage (C-V) memory window. Switching speed of these films has been measured to be 40 to 45 nanoseconds. The first FEMFET fabricated with BaMgF₄ has exhibited 18 Volt memory hysteresis window with better than 10⁵ on/off current ratio for 20 Volt programming.     

Methane free biohydrogen production from carbon monoxide using a continuously operated moving bed biofilm reactor

Biological water-gas shift (WGS) reaction is a green and sustainable alternative to thermochemical-catalytic WGS process for hydrogen production from carbon monoxide (CO). However, CO tolerant carboxydotrophic microbes for hydrogen production and scaling up the technology using a bioreactor system present challenges in successful application of this technology. This study demonstrated the capability of anaerobic microbial consortium for biohydrogen production from CO using a moving bed biofilm reactor (MBBR). The CO conversion pathway followed by the anaerobic biomass was first elucidated by inhibiting the methanogens present using 2-bromoethanesulfonate (BES) at an optimum concentration of 10 mmol/L. An increase in inlet CO concentration to the MBBR enhanced the H₂ production, but the CO conversion efficiency was low. More than 80% CO conversion efficiency was obtained only for a low inlet CO concentration. A maximum H₂ concentration of 19.5 mmol/L along with 2 mmol/L of acetate were obtained for 36 mmol/L of inlet CO concentration in the bioreactor. The carbon flux analysis showed that the CO was mainly utilized for methane free H₂ production, and only <10% of carbon flux was diverted towards acetate formation. Overall, this study demonstrated that MBBR system can be used for steady state biohydrogen production over a prolonged operation period.     

The adhesion of thin plasma-polymerized organic films to metal substrates

The direct pull method was used to study the adhesion of thin (<1 μm) plasma-polymerized films on metal substrates. Application of this method provides information on the cohesive, as well as the adhesive, properties of these films. After breakage of the films by the pull method, the interface was analyzed by scanning electron microscopy to determine the nature of the breakage at the interface. Electron spectroscopy for chemical analysis was used to determine whether any chemically adsorbed layers of organic films remained after the films had been pulled from the metal substrate. The adhesive or cohesive strengths of plasma-polymerized tetrafluoroethylene and chlorotrifluoroethylene films on 304 stainless steel, aluminum and silver were measured by the pull method. Adhesive strength was found to be higher than cohesive strength for plasma-polymerized tetrafluoroethylene films on the various substrates. The result for plasma-polymerized chlorotrifluoroethylene films was not as expected, indicating that considerable care is required in the interpretation of tensile tests of laminates such as those conducted in this study.     

The observation of pseudodiffusion of nickel in single-crystal silicon by in-depth Auger electron spectroscopy

Nickel films r.f. sputtered onto the (100) surface of single-crystal silicon diffuse into the substrate when annealed at temperatures between 250 and 350°C. The change in the concentration gradient is measured by recording the Ni and Si Auger electron signals during ion sputtering. The diffusion coefficient is given by the equation

$\text{D = 10}{}^{-13}\text{exp(-0.27 eV/kT>) cm}{}^2\text{s}{}^{-1}$

     

Deformation and strength behavior of two nickel-base turbine disk alloys at 650 °C

Two powder metallurgy nickel-base turbine disk alloys, RENE’95* and KM4, were studied for strength and deformation behavior at 650 °C. Two classes of microstructures were investigated: unimodal size distributions of γ′ precipitates with particle sizes ranging from 0.1 to 0.7 μm and commercially heat-treated structures with bimodal or trimodal size distributions of γ′ precipitates. The strength and deformation mechanisms were heavily influenced by the microstructure. In both alloys, deformation during compression tests consisted of a combination of a/2〈110〉 antiphase boundary (APB)-connected dislocation pairs and a/3〈112〉 partials creating superlattice intrinsic stacking faults (SISFs). In unimodal alloys, the fault density increased with decreasing particle size and decreasing strain rate. These trends, observed in compression testing, are consistent with earlier studies of similar alloys, which were tested in creep. As the γ′ size was reduced, the nature of the faults changed from being isolated within single precipitates to being extended across entire grains. Commercially heat-treated alloys, containing a bimodal distribution of γ′ particles, exhibited significantly more faulting than unimodal alloys at the same cooling γ′ size. This augmentation of the faulting in commercial alloys was apparently due to the presence of the fine, aging γ′ particles. The two typical commercial heat treatments (supersolvus and subsolvus) resulted in different deformation structures: the subsolvus behavior was similar to that of unimodal alloys with γ′ sizes between 0.2 and 0.35 μm, while the supersolvus deformation was similar to that of unimodal alloys with the 0.1 μm γ′ size. These differences were attributed to differences in the size of the fine, aging γ′ particles. Creep deformation in a commercially heat-treated material at 650 °C occurred solely by SISF-related mechanisms, resulting in a macroscopic slip vector of 〈112〉. The effects of alloy chemistry, APB energy, and microstructure on the deformation and mechanical behavior are discussed in detail, and possible effects of the faulting mechanisms on the mechanical behavior are explored. Finally, models for yield strength as a function of microstructure for bimodal alloys with large volume fractions of precipitates are found to be in need of development. RENE′95 is a trademark of General Electric Company, Fairfield, CT.