Kiyomi Nakajima

Oxygen Vacancy Induced Substantial Threshold Voltage Shifts in the Hf-based High-K MISFET with p+poly-Si Gates -A Theoretical Approach

A theoretical investigation has been made of the origin of substantial threshold voltage (Vth) shifts observed in p+poly-Si gate Hf-based metal insulator semiconductor field effect transistors (MISFETs), by focusing on the effect of oxygen vacancy (VO) formation in HfO2. It has been found that VO formation and subsequent electron transfer across the interface definitely causes substantial Vth shifts, especially in p+poly-Si gate MISFETs. Moreover, the theory also systematically reproduces recent experimental reports that large flat band (Vfb) shifts are observed, even in intrinsic poly-Si gates, and that the Vfb shifts exhibit a high dependence on HfSiOx thickness.

Cobalt valence states and origins of ferromagnetism in Co doped TiO2 rutile thin films

Co doped rutile thin films were fabricated on α-Al2O3 (10-12) substrates by laser molecular beam epitaxy. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy indicated that the rutile thin films are (101) oriented and have smooth surfaces with no impurity phases. Co K-edge x-ray absorption near-edge structure, extended x-ray absorption fine structure, and x-ray photoelectron spectroscopy revealed the coexistence of different valence states of Co in the film. Magnetic circular dichroism studies suggest that the observed ferromagnetism is uniform and is related to the electron band structure of TiO2 rutile. Contribution of oxidized (Co2+) and metallic (Co0) cobalt to the ferromagnetism is discussed.

Cavity formation on an optical nanofiber using focused ion beam milling technique.

We present the experimental realization of nanofiber Bragg grating (NFBG) by drilling periodic nano-grooves on a subwavelength-diameter silica fiber using focused ion beam milling technique. Using such NFBG structures we have realized nanofiber cavity systems. The typical finesse of such nanofiber cavity is F ∼ 20 - 120 and the on-resonance transmission is ∼ 30 - 80%. Moreover the structural symmetry of such NFBGs results in polarization-selective modes in the nanofiber cavity. Due to the strong confinement of the field in the guided mode, such a nanofiber cavity can become a promising workbench for cavity QED.

Nanostructured LaB6 Field Emitter with Lowest Apical Work Function

LaB(6) nanowires are ideal for applications as an electrical field-induced ion and electron point source due to their miniature dimensions, low work function, as well as excellent electrical, thermal, and mechanical properties. We present here a reliable method to fabricate and assemble single LaB(6) nanowire-based field emitters of different crystal orientations. The atomic arrangement, emission brightness from each crystal plane, and field emission stability have been characterized using field ion microscopy (FIM) and field emission microscopy (FEM). It is found that the 001 oriented LaB(6) nanowire emitter has the highest field emission symmetry while the 012 oriented LaB(6) nanowire has the lowest apical work function. The field emission stability from the single LaB(6) nanowire emitter is significantly better than either the LaB(6) needle-type emitter or W cold field emitters.

Magneto-Optical Spectroscopy of Anatase TiO2 Doped with Co

Magneto-optical spectroscopy of a transparent ferromagnetic semiconductor, anatase TiO2 doped with Co, is carried out at room temperature. A large magneto-optical response with ferromagnetic field dependence is observed throughout from ultraviolet to visible range and increases with increasing Co content or carrier concentration. The magnitude of magnetic circular dichroism (MCD) per unit thickness has a peak around the absorption edge such a huge value of ~10400 deg/cm at 3.57 eV for a 10 mol% Co-doped specimen. Although the results are not sufficient to prove that the ferromagnetism is in the ordinary framework of diluted magnetic semiconductors, the coexistence of Co impurity and mobile carrier is shown to transform the band structure of host TiO2 to generate ferromagnetism.

Physics in Fermi level pinning at the polySi/Hf-based high-k oxide interface

We report that O vacancy (Vo) formation in ionic Hf-based dielectrics and subsequent electron transfer into poly Si gates across the interface, definitely cause substantial flat band (Vfb) shifts especially for p+ gate MISFETs. Our theory can systematically reproduce experiments related to Hf-based dielectrics, and gives a guiding principle towards gate/high-k oxide interface control.

Recombination dynamics of localized excitons in cubic InxGa1−xN/GaN multiple quantum wells grown by radio frequency molecular beam epitaxy on 3C–SiC substrate

Recombination dynamics of localized excitons in strained cubic (c-)InxGa1−xN/GaN multiple quantum wells (MQWs) grown on 3C–SiC (001) were summarized in terms of well thickness L, InN molar fraction x, and temperature T. Photoluminescence (PL) peak energy of c-In0.1Ga0.9N/GaN MQWs showed a moderate blueshift as L decreased, and the low-temperature PL lifetime did not change remarkably by changing L. These results proved that the quantum-confined Stark effect due either to spontaneous or piezoelectric polarization was inactive in cubic polytypes. Consequently, time-resolved PL (TRPL) data of c-InGaN MQWs reflect the intrinsic exciton dynamics. The TRPL signal showed stretched exponential decay and spectral redshift with time after excitation up to 300 K. The results are fingerprints that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures forming extended and localized states. Effective localization depth increased with the increase in x,...

An ultrabright and monochromatic electron point source made of a LaB6 nanowire.

Electron sources in the form of one-dimensional nanotubes and nanowires are an essential tool for investigations in a variety of fields, such as X-ray computed tomography, flexible displays, chemical sensors and electron optics applications. However, field emission instability and the need to work under high-vacuum or high-temperature conditions have imposed stringent requirements that are currently limiting the range of application of electron sources. Here we report the fabrication of a LaB6 nanowire with only a few La atoms bonded on the tip that emits collimated electrons from a single point with high monochromaticity. The nanostructured tip has a low work function of 2.07 eV (lower than that of Cs) while remaining chemically inert, two properties usually regarded as mutually exclusive. Installed in a scanning electron microscope (SEM) field emission gun, our tip shows a current density gain that is about 1,000 times greater than that achievable with W(310) tips, and no emission decay for tens of hours of operation. Using this new SEM, we acquired very low-noise, high-resolution images together with rapid chemical compositional mapping using a tip operated at room temperature and at 10-times higher residual gas pressure than that required for W tips.

Field-effect modulation of the transport properties of nondoped SrTiO3

We have fabricated SrTiO3 (100) single crystal field-effect transistors with amorphous and epitaxial CaHfO3 gate insulator layers. The devices with amorphous insulator layers showed nearly temperature independent behavior. The transistors with epitaxial interfaces exhibited a large improvement over the amorphous devices. The field-effect mobility was found to increase at low temperature, reaching 35cm2∕Vs at 50K. This result shows that the carriers accumulated by the field effect on the SrTiO3 side of the gate interface behaved as would be expected for electron-doped SrTiO3. An insulator-metal transition, induced by field-effect doping, was observed in epitaxial SrTiO3-based transistors.

Ductile fracture mechanism in fine-grained magnesium alloy

The ductile fracture mechanism in a fine-grained magnesium alloy has been investigated by transmission electron microscopy-focused ion beam techniques. In coarse-grained or conventional magnesium alloys, twins form at the very beginning of the deformation process, and crack propagation occurs through the twin boundaries. However, in the alloy used in this study, subgrain structures were found instead of twins at the crack tip. Nanoscale twins formed subsequently owing to large stress in the crack propagation route. The fine-grained alloys showed high fracture toughness resulting from resistance to the twins at the beginning of the deformation.