S. Berger

Particle-size, size distribution and dislocations in nanocrystalline tungsten-carbide

Abstract Ball-milled WC powder has been compacted and sintered. The particle size has been determined by transmission and scanning electron microscopy and high resolution X-ray diffraction. The X-ray diffraction profiles were evaluated for particle size and dislocation densities by the recently developed procedures of modified Williamson-Hall plot and modified Warren-Averbach method. Log-normal particle size distributions have been obtained by a newly developed numerical method from the size parameters provided by X-ray line profile analysis. The size distributions obtained by TEM and X-rays are compared and discussed. The dislocation density was found to be 1.9 × 1017 m−2 in the ball-milled powder which decreases to 2.0 × 1015 m−2 after annealing.

Ultrathin zirconium oxide films as alternative gate dielectrics

ZrO2 films were deposited on Si(100) wafers by the rapid thermal chemical vapor deposition process using a zirconium (IV) t-butoxide Zr(OC4H9)4 precursor and oxygen. Interfacial zirconium silicate formation was observed by high resolution transmission electron microscopy and medium energy ion scattering. The intermixing of the interface can be suppressed by forming a thin silicon nitride layer on the silicon substrate prior to ZrO2 deposition. The dielectric constant of ZrO2 achieved in this work is 15–18 with very small capacitance–voltage hysteresis, ideal for metal–oxide–semiconductor field effect transistor (MOSFET) application. The NMOSFET device has good turn-on characteristics, however, the transconductance is lower than expected due to the incomplete removal of zirconium silicate at the source and drain contacts and poses integration challenges to use ZrO2 as the gate dielectric material.

Sintering study of nanocrystalline tungsten carbide powders

Abstract WC powder with an average grain size of 6 nm was obtained after high energy ball milling under protective gas atmosphere. The kinetics of densification was studied during sintering the powder in a dilatometer up to 1450 °C. The microstructure was investigated by TEM and high resolution SEM after various stages of sintering. The green density of the specimens was 45%. Three stages of sintering were defined: (a) rearrangement of particles at low temperature (850 °C) without grain or particle growth, (b) neckformation between powder particles at 1000–1250 °C and initial grain growth at 1200 °C, (c) pore elimination accompanied by massive grain growth at 1300–1450 °C.

Solid or liquid phase sintering of nanocrystalline WC/Co hardmetals

Abstract Nanocrystalline WC/Co hardmetal powder has been sintered at various temperatures in vacuum and HIP sintered under different pressures. In addition, hot stage optical microscopy (HSM) and differential thermal analysis (DTA) was performed on a similar powder. All the results indicate that the matrix of the nanopowder starts to melt at about 1150 °C; however, the melting is confined to locations that are still nanocrystalline. With increasing temperature, more material melts and at about 1330 °C the melting of the matrix is complete. High density can be achieved below this temperature, but not below 1200 °C. Compared with previous sintering results on micron sized powder, the conclusion is that the nanopowder densifies mostly in the solid state, while the latter mostly in the liquid state.

Nanoferroelectric domains in ultrathin BaTiO3 films

The microstructure and electrical properties of ultrathin BaTiO3 films (2–7 nm thick) have been studied. Multidomain structures having 180° domains and 90° domain boundaries were observed by high resolution transmission electron microscopy. The domains, which have a width as small as one unit cell, are mostly oriented in parallel to the film plane. The film exhibits ferroelectric behavior characterized by polarization hysteresis loops and Curie temperature. The leakage current, switching time, and fatigue characteristics of the films are also reported.

Dilatometric study of the sintering mechanism of nanocrystalline cemented carbides

Two types of WC-Co powders, nanometer particle size and micron particle size, were studied by means of dilatometry. The experiments showed that the sintering behavior of the nanocrystalline specimens is different from that of the micron grain-size specimen. While the sintering of the micron particle sized powder to full density is mainly done in the liquid phase, the nanoparticle sized powder can be sintered in the solid phase, at a lower temperature.

Combined effect of thickness and stress on ferroelectric behavior of thin BaTiO3 films

Thin BaTiO3 (BTO) layers were deposited by magnetron sputtering on a Si substrate between two thin LaNiO3 (LNO) electrodes (LNO/BTO/LNO/Si structure). The thickness of the BTO layer was varied between 35 nm and 1 μm. The stress, dielectric permittivity, and ferroelectric hysteresis loop were measured after deposition and during heating. It was found that the BTO layers are under tensile stress at the Curie temperature. The tensile stress increases with decreasing layer thickness leading to a shift of the Curie point to a lower temperature, a decrease of remanent polarization, and an increase of the coercive field. The effect of tensile stress on the Curie temperature is dominant within a restricted range of stresses between 300 and 450 mega Pascals (MPa), where the Curie temperature decreases linearly by about 0.16 °C/MPa. A good correlation was found between stress and dielectric measurements with respect to the temperature of the Curie point. The ferroelectric hysteresis loops showed that the ferroelect...

The Ag/Cu interface stress

Abstract The interface stress in Ag/Cu multilayered thin films of various repeat lengths on Si(100) substrates was determined from the difference between the average bulk stress in the film and the substrate curvature stress. The interface stress measured for repeat lengths between 5.3 and 11.4nm is equal to −3.19±0.43J/m2; it is tensile: elastic expansion of the interface lowers its energy. Its effect on the substrate curvature stress increases linearly with decreasing bilayer repeat length. The average film stress determined from the lattice strains is tensile, and increases by about one order of magnitude with decreasing bilayer repeat length over the range studied. Both the Ag and Cu layers have a [111] texture, which becomes gradually less pronounced as the bilayer repeat length increases from 5.3nm to 12.6nm. The interface stress found in this system has the same sign and a magnitude similar to that found by the same method in earlier work on the Ag/Ni interface.

Correlation between microstructure, particle size, dielectric constant, and electrical resistivity of nano-size amorphous SiO2 powder

Abstract Pure amorphous SiO 2 powder with nanometer size particles was exposed to various heat treatments up to 1200°C. The microstructure, particle size, dielectric constant and electrical resistivity of the powder were characterized after each heat treatment. It was found that the dielectric constant of the powder is higher compared to that of amorphous SiO 2 thin films. This enhancement is correlated with higher density of Si dangling bonds, which contribute to the polarization of the material. A major decrease in the dielectric constant takes place during heating up to 600°C where neither growth nor crystallization of the particles occur but only pronounced reduction in the density of the Si dangling bonds is observed. Pronounced growth and initial crystallization to a cristobalite phase of the powder particles occur at about 1100°C and have a minor effect on the dielectric constant. The Si dangling bonds also serve as electrical conducting centers in the powder and their annihilation due to the heat treatments is well observed as an increase in the electrical resistivity of the powder.

Parity effect in a small superconducting particle

Matveev and Larkin calculated the parity effect on the ground-state energy of a small superconducting particle in the regimes where the mean level spacing