Perspectives paper: First principles modeling of high-k gate dielectrics

Aggressive scaling has led to silicon dioxide (SiO₂) gate dielectrics as thin as 15 Å in state-of-the-art CMOS technologies. As a consequence, static leakage power due to direct tunneling through the gate oxide has been increasing at an exponential rate. As technology roadmaps call for sub-10 Å gate oxides within the next five years, a variety of alternative high-k materials are being investigated as possible replacements for SiO₂. The higher dielectric constants in these materials allow the use of physically thicker films, potentially reducing the tunneling current while maintaining the gate capacitance needed for scaled device operation. Recognizing that the current Si/SiO₂ system benefits from nearly 30 years of research, developing a replacement material for SiO₂ presents an immense challenge. This has prompted recent interest in novel computational approaches, such as first principles density functional theory (DFT) simulations, to computationally screen candidate dielectrics by predicting their properties based on the microscopic interactions within the system.This paper provides perspectives on the application of DFT simulations to address challenging problems of high-k gate dielectric research. We provide background and motivation for the development of high-k materials and highlight opportunities for theoretical study of such materials. We also describe specific examples of recent first principles work related to two particularly promising materials systems: silicates and aluminates.     

A method to measure shearing modulus of the foamed core for sandwich plates

This paper reports on a test method to evaluate the shearing modulus of foamed core which is used for sandwich plates. Although this kind of relatively soft core is widely used for various applications, a suitable test method has not necessarily been established. In the present study, a symmetric test sample with four pieces of core is used to eliminate undesirable deformation due to eccentric load. By varying the core length and the core thickness, we found that the shearing modulus strongly depends on the specimen configuration. This tendency was also recognized from the results of FEM and beam theory analyses where the shearing deformation is incorporated. Finally we propose a methodology for obtaining a reliable shearing modulus with the aid of FEM analysis.     

Acoustic destruction of a microcapsule having a hard plastic shell

Acoustic destruction of a microcapsule having a hard plastic shell is discussed. In an ultrasonic drug delivery system, microcapsules having thin elastic shells release drugs that are contained therein when the shell is destroyed. In this paper, two subjects related to capsule destruction are discussed: the driving pulse duration for capsule destruction and the frequency dependence of capsule destruction. Optical observation of microcapsule destruction is performed with a high-speed video camera. In the case of capsule destruction by a pulse wave, the internal gas of the microcapsule cannot be ejected completely, and a portion of the internal gas remains inside the broken shell. It is found that capsule destruction by pulse waves depends on both the amplitude of the driving pressure and the pulse duration. The frequency dependence of microcapsule destruction also is investigated. In the case of capsule destruction by a low-amplitude acoustic wave, the destruction rate under the resonance condition is higher than under nonresonance conditions. By controlling the driving frequency, selective capsule destruction can be achieved.     

Temperature‐Controlled High‐Speed AFM: Real‐Time Observation of Ripple Phase Transitions

With nanometer lateral and Angstrom vertical resolution, atomic force microscopy (AFM) has contributed unique data improving the understanding of lipid bilayers. Lipid bilayers are found in several different temperature‐dependent states, termed phases; the main phases are solid and fluid phases. The transition temperature between solid and fluid phases is lipid composition specific. Under certain conditions some lipid bilayers adopt a so‐called ripple phase, a structure where solid and fluid phase domains alternate with constant periodicity. Because of its narrow regime of existence and heterogeneity ripple phase and its transition dynamics remain poorly understood. Here, a temperature control device to high‐speed atomic force microscopy (HS‐AFM) to observe dynamics of phase transition from ripple phase to fluid phase reversibly in real time is developed and integrated. Based on HS‐AFM imaging, the phase transition processes from ripple phase to fluid phase and from ripple phase to metastable ripple phase to fluid phase could be reversibly, phenomenologically, and quantitatively studied. The results here show phase transition hysteresis in fast cooling and heating processes, while both melting and condensation occur at 24.15 °C in quasi‐steady state situation. A second metastable ripple phase with larger periodicity is formed at the ripple phase to fluid phase transition when the buffer contains Ca²⁺. The presented temperature‐controlled HS‐AFM is a new unique experimental system to observe dynamics of temperature‐sensitive processes at the nanoscopic level.     

Active optical interconnect based on liquid-crystal grating

We propose a transparent plate of a liquid-crystal grating to be used as a light guide for optical interconnection. To this end, we are empowering the connection itself with active functions, such as switching, wavelength division, power adjustment, etc. In experiments, we built a grating based on in-plane switching. It contains vertically-aligned nematic liquid-crystal molecules between a glass plate with a high refractive index (light guide) and another glass plate with a pair of interdigitated electrodes. Entering a TM wave from an edge of the light guide, we have demonstrated that the activation of diffraction and intensity adjustment for the guided light are both possible. Because a TE wave is barely diffracted, the device also exhibits polarization division capability.     

Thin-cell sub-Doppler spectroscopy by spatially separated beam method and pump-probe method

We experimentally demonstrate two methods that improve the resolution of sub-Doppler spectroscopy using a 1 mm thick vapor cell. The linewidths of the observed spectra are approximately 1 order of magnitude narrower than the Doppler width. The first method involves using a 1 mm thick cell filled with Rb atomic vapor and two spatially separated laser beams. By employing the same principle, we also demonstrate that it is possible to achieve the same resolution by using the pump and probe pulses of a single beam. The latter method enables us to construct a simple and robust optical setup for sub-Doppler spectroscopy.     

A comparative study of different solvothermal methods for the synthesis of Sn<Superscript>2+</Superscript>-doped BaTiO<Subscript>3</Subscript> powders and their dielectric properties

Ternary oxides containing Sn²⁺ are rare and difficult to prepare by the conventional solid state reactions due to the disproportionation of Sn²⁺ to Sn⁴⁺ and Sn at high temperatures. In this article, Sn²⁺-doped barium titanate, Ba_1−x Sn _x TiO₃ (x = 0.00, 0.02, 0.05, and 0.10) nanopowders were successfully synthesized at a moderate temperature by a microwave-assisted solvothermal reaction (MSR) and a solvothermal reaction with rolling (SRR). The powders obtained using the MSR and SRR consisted of nanoparticles of 20–50 nm and 100–120 nm in diameter, respectively. The dielectric constant of the sample increased by doping with a small amount of Sn²⁺ (x ≤ 0.05), but decreased by doping in excess amounts of it.     

Synthesis and properties of ceramic fibers from polycarbosilane/polymethylphenylsiloxane polymer blends

We synthesized ceramic fibers based on silicon carbide (SiC) from polymer blends of polycarbosilane (PCS) and polymethylphenylsiloxane (PMPhS) by melt-spinning and radiation curing. PMPhS was compatible with PCS up to 30 mass%, and formed a transparent melt at temperatures higher than 513 K. The softening point was also lowered by adding PMPhS and 15 mass% of PMPhS to PCS was the most suitable condition for obtaining thin fibers with an average diameter of 14.4 μm. Due to the lowered softening point of the PCS–PMPhS fibers, γ-ray curing in air was adopted. The ceramic yield of the cured fiber was 85.5% after pyrolysis at 1273 K. In spite of the small diameter, the resulting tensile strength at 1273 K was rather limited at 0.78 GPa. Blooming of the PMPhS component during pyrolysis may have caused surface defects. After high-temperature pyrolysis at 1673–1773 K, a porous nanocrystalline SiC fiber with a unique microstructure was obtained with surface area of 70–150 m²/g. When the fiber was pyrolyzed at the same temperature under a highly reductive atmosphere, wire bundle-shaped fibers were obtained by gas evolution and reactions.     

Deposition of unhydrogenated amorphous carbon films by sublimation of C60 fullerene in electron beam excited plasma

C₆₀ fullerene clusters are used as a carbon source to deposit unhydrogenated amorphous carbon films. C₆₀ clusters are sublimated by heating up to 850 °C. The sublimated fullerene powders are injected to an electron beam excited argon plasma and dissociated to be active species. Consequently, the carbon species condense as a thin film on the negatively biased substrates that are immersed in the plasma. Deposition rates of approximately 1.0 µm/h and the average surface roughness of 0.2 nm are achieved. X-ray diffraction pattern reveals that the microstructure of the film is amorphous while fullerene film deposited without the plasma shows crystal structure. Raman spectroscopic analysis shows that the films are one of the types of diamond-like carbon films. The nano-indentation technique is used for hardness measurement of the films and results in hardness up to about 20 GPa.     

Spatial resolutions for applying photorefractive polymers to diffractive elements

Spatial resolutions of diffracted beams by photorefractive polymers are investigated in detail for applying the polymers to diffractive optical elements. The coupled wave equations for the four-wave mixing in photorefractive polymers and their solutions by applying the two-beam coupling approximation are derived taking the orientational enhancement effect into consideration. Characteristics of the diffraction efficiency of the polymers for the applied electric field, the incident angles of beams, the incident radiation intensity and the polarization are clarified. Furthermore, resolutions of diffracted images with the polymers are discussed by a numerical analysis from the viewpoint of the Bragg diffraction in a spatial frequency domain. An experiment with the TPD (tetra-phenyl-diphenyldiamine) acrylate based photorefractive polymers and the USAF test target is also performed to measure the spatial resolution of two-dimensional images diffracted by the polymer and to verify the calculated results. It is revealed that high resolution over 50 lp mm^?1 can be obtained with sufficient diffraction efficiency in the case of the polymer thickness of several 10 µm.