Evaluation of probe size in STEM imaging at 30 and 60 kV

In order to estimate the probe size on the specimen surface in a newly developed low-acceleration-voltage (30–60 kV) atomic-resolution scanning transmission electron microscopy (STEM), we compared the intensity profiles of experimentally obtained annular dark field (ADF)-STEM images of Si–Si dumbbells and those of images simulated using a multislice method which takes chromatic aberration into account. However, the simulated ADF images at 30 and 60 kV were found not to match the corresponding experimental images. Subsequently, the simulated images were convolved with probe functions (normal distributions) of different widths until a good match was obtained between the images. This allowed the probe shapes corresponding to the experimental conditions to be determined. ADF-STEM images with chromatic aberration could then be calculated by an incoherent superposition of these probe functions over a range of energies. The full widths at half maximum for the probe functions were estimated to be 99.2 pm for 30 kV and 92.8 pm for 60 kV. The D59 diameters were calculated to be 154.0 pm for 30 kV and 127.8 pm for 60 kV. This means that the 30-kV probe has a larger tail than the 60-kV probe.     

Negative index photonic crystal lenses based on carbon nanotube arrays

We report a novel utilization of periodic arrays of carbon nanotubes in the realization of diffractive photonic crystal lenses. Carbon nanotube arrays with nanoscale dimensions (lattice constant 400 nm and tube radius 50 nm) displayed a negative refractive index in the optical regime where the wavelength is of the order of array spacing. A detailed computational analysis of band gaps and optical transmission through the nanotubes based planar, convex and concave shaped lenses was performed. Due to the negative-index these lenses behaved in an opposite fashion compared to their conventional counter parts. A plano-concave lens was established and numerically tested, displaying ultra-small focal length of 1.5 μm (～2.3 λ) and a near diffraction-limited spot size of 400 nm (～0.61 λ).     

Resolution enhancement at a large convergence angle by a delta corrector with a CFEG in a low-accelerating-voltage STEM

Resolution reduction by a diffraction limit becomes severe with an increase in the wavelength of an electron at a relatively low accelerating voltage. For maintaining atomic resolution at a low accelerating voltage, a larger convergence angle with aberration correction is required. The developed aberration corrector, which compensates for higher-order aberration, can expand the uniform phase angle. Sub-angstrom imaging of a Ge [1 1 2] specimen with a narrow energy spread obtained by a cold field emission gun at 60 kV was performed using the aberration corrector. We achieved a resolution of 82 pm for a Ge–Ge dumbbell structure image by high angle annular dark-field imaging.     

Influence of Zn–Zr ions on physical and magnetic properties of co-precipitated cobalt ferrite nanoparticles

Cobalt ferrite nanoparticles having the chemical formula CoFe_2−2xZr_xZn_xO₄ with x ranging from 0.0 to 0.4 were prepared by chemical co-precipitation method. The powder X-ray diffraction pattern confirms the spinel structure for the prepared compound. The particle size was calculated from the most intense peak (3 1 1) using Scherrer formula. The particle size of the samples was found within the range of 12–23 nm for all the compositions. The magnetic and electrical properties of these materials have been studied as a function of temperature. Activation energy and drift mobility have been calculated from the DC electrical resistivity measurements. Dielectric properties such as dielectric constant and dielectric loss tangent were measured at room temperature in the frequency range 100 Hz–1 MHz.     

A study of the influence of the input electrical power on the spectral width of the 510.6 nm line of an atomic copper vapor laser

The effect of the input electrical power on the spectral width of the 510.6 nm line of an atomic copper vapor laser (CVL) is investigated. An analysis of the gas temperature inside the discharge tube and the line broadening mechanism of the CVL is reported. The input electrical power was varied from 2.0 to 4.2 kW in a cylindrical discharge tube of inner radius 2.35 cm and length 150.0 cm. A Fabry–Perot etalon and imaging camera-based setup interfaced with personal computer was used to measure the spectral width of the 510.6 nm (green) laser line. The Doppler broadened spectral profile of the laser emission varies with input electrical power and an additional broadening of almost 1 GHz at the highest operating input power was observed.     

Analysis of the response time in high-temperature LWIR HgCdTe photodiodes operating in non-equilibrium mode

The paper presents the theoretical investigation of the active region parameters, especially the influence of thickness and doping, on the response time and current responsivity of high-temperature long wavelength infrared HgCdTe photodiodes operating at 230 K in non-equilibrium mode. Results of theoretical predictions of time constant were compared to the experimental data. The response time of the devices have been characterized using Nd:YAG laser, optical parametric generator with pulse width <25 ps and fast oscilloscope with suitable transimpedance amplifier as a function of detector design, temperature and bias. The reverse bias applied to the photodiode causes Auger-suppression and improve the performances of the devices. This way the response time decreases to the value below 1 ns at the good current responsivity increased to the value of about 6 A/W and what is a promising parameter in view of potential telecommunication applications. Due to the series resistance of electrical connections, the response time of the devices is mainly limited by RC constant while the calculations show that the time constant of the Auger suppressed structures should be limited by the drift time of carriers.     

Effects of trench oxide and field plates on the breakdown voltage of SOI LDMOSFET

To improve the breakdown voltage, we propose a SOI-based LDMOSFET with a trench structure in the drift region. Due to the trench oxide and underneath boron implanted layer, the surface electric field in the drift region effectively reduced. These effects resulted in the increment of breakdown voltage for the trenched LDMOS more than 100 V compared with the conventional device. However, the specific on-resistance, which has a trade-off relationship, is slightly increased. In addition to the trench oxide on the device performance, we also investigated the influence of n− drift to n+ drain junction spacing on the off-state breakdown voltage. The measured breakdown voltages were varied more than 50 V with different n− to n+ design spaces and achieved a maximum value at L_DA = 2.0 μm. Moreover, the influence of field plate on the breakdown voltage of trench LDMOSFET was investigated. It is found that the optimum drain field plate over the field oxide is 8 μm.     

18.2 nm Schwarzschild microscope for diagnostics of hot electron transport

Schwarzschild microscope at 18.2 nm for diagnostics of hot electron transport in femtosecond laser-plasma interaction has been developed. Based on the third-order aberration theory the microscope is designed for numerical aperture of 0.1 and magnification of 10. Mo/Si multilayer films with peak throughput at 18.2 nm is designed and deposited by magnetron sputtering method. The 24 lp/mm copper mesh is imaged via Schwarzschild microscope, and resolutions of less than 3 μm are measured in 1.2 mm field. The diagnostics experiment of hot electron transport is performed on 286 TW SILEX-I laser facilities, and the spatial distribution of radiation caused by hot electron is imaged by Schwarzschild microscope.     

Size dependence of ground-state energy of the excitons in spherical Y2O3

The exciton energies of rare earth oxides (Ln₂O₃) have rarely been calculated by the theory. Experimentally, the blue-shift of exciton energy in nanocrystals deviates from the traditional size confinement effect. Herein, the dependence of the ground-state energy of an exciton in Y₂O₃ spheres on particle radius was calculated by using a variational method. In the model, an exciton confined in a sphere surrounded by a dielectric continuum shell was considered. The ground-state energy of exciton comprises kinetic energy, coulomb energy, polarization energy and exciton–phonon interaction energy. The kinetic and coulomb energy were considered by the effective mass and the dielectric continuum and the exciton–phonon interaction energy was given by the intermediate coupling method. The numerical results demonstrate that the present model is roughly consistent with the experimental results. The confinement effect of the kinetic energy is dominant of the blue-shift of the exciton energy in the region of R < 5 nm, while confinement effect of the coulomb energy is dominant of the blue-shift of the exciton energy in the region of R > 5 nm. The polarization energy contributes largely to the exciton energy as the particle size is smaller than ~ 10 nm, while the exciton–phonon interaction energy takes only a little contribution in all the range.     

Fiber-optic distributed sensor based on phase-sensitive OTDR and wavelet packet transform for multiple disturbances location

A novel location scheme based on the wavelet packet transform (WPT) for the phase-sensitive optical time-domain reflectometry (OTDR) is proposed. The wavelet packet energy of the backscattered signal at a time interval over the sensing fiber is provided to discriminate the disturbances regions from other regions. It is insensitive to the frequency drift of the laser. The simultaneous measurement of two disturbances at 5 km and 7 km positions is achieved by the location technique based on the WPT over a 9 km monitored length with a 50 m spatial resolution even when a laser with a frequency drift rate of 230 MHz/min is utilized. During 100 times experiments the false alarm rate is only 2% and the maximum location error is 150 m.     

Kinetics and thermodynamics of sucrose crystallization from pure solution at different initial supersaturations

Growth rates of sucrose crystallization from pure solutions of initial relative supersaturation levels between 0.094 and 0.181 were studied in agitated crystallizer at 313.13 K. Birth and spread model was applicable for the obtained growth rate data in this range of supersaturation and used to estimate the principal growth parameters. The estimated interfacial free energy varied inversely with supersaturation from 0.00842 to 0.00461 J/m², respectively. The obtained kinetic coefficient changed with the initial supersaturation from 9.45 × 10^? 5 to 2.79 × 10^? 7 m/s. The corresponding radius of the 2D (two dimensional) critical nucleus varied from 7.47 × 10^? 9to 1.46 × 10^? 9 m. Predominance of surface integration or volume diffusion mechanism during the growth process was assessed using the calculated activation free energies of the 2D nucleation process. An acceptable confirmation of the calculated radius of the critical 2D nucleus was found using atomic force microscopy (AFM) technique. The calculated interfacial free energy between the saturated sucrose solution and the crystal surface was found to be 0.02325 J/m².     

Third order susceptibility enhancement using GaN based composite nanoparticle

In this work, we are going to introduce a new composite nanostructure (metal-dielectric) based on GaN to manage optical and electrical properties. For doing and evaluation of these properties, first, we evaluated and calculated the wave functions and energy levels of the introduced structure by solving the Schrodinger equation analytically. Then using wave function nonlinear optical properties such as third order susceptibility are studied. We observed that with control of nanoparticle parameters different behavior is obtained. For example with increase of the well width, third order susceptibility is decreased too but with more increasing the well width we observed increasing the nonlinear susceptibility. This effect depends on different displacement of ground and first excited stated energy levels and consequently wave functions due to changing of defect radius. Also, we show that the third order susceptibility in this case is very larger than bulk cases. Finally it should mention that the reported results are for wavelengths larger than 30 μm.     

Multichannel wavelength division multiplexing system based on silicon rods of periodic lattice constant of hetero photonic crystal units

Characteristics of two different multichannel wavelength division multiplexing (WDM) systems composed of two-dimensional (2D) hetero photonic crystals (HPCs) are introduced. One utilizes five photonic crystal (PC) units, each fabricated with triangular and rectangular lattice. The other consists of five PC units in rectangular lattice. Both systems have a lattice constant difference of 4 nm between adjacent PC units, and both systems apply silicon rods with a radius of 120 nm. Finite-difference time-domain (FDTD) method and plan wave expansion (PWE) method reveal the ability of wavelength spacing ～8 nm with high quality factor (Q) in a system based on triangular and rectangular lattice; and ～8 nm with almost constant transmission efficiency based on rectangular lattice.     

Millimeter-wave scattering from neutral and charged water droplets

We investigated 94 GHz millimeter-wave (MMW) scattering from neutral and charged water mist produced in the laboratory with an ultrasonic atomizer. Diffusion charging of the mist was accomplished with a negative ion generator (NIG). We observed increased forward- and backscattering of MMW from charged mist, as compared to MMW scattering from an uncharged mist. In order to interpret the experimental results, we developed a model based on classical electrodynamics theory of scattering from a dielectric sphere with diffusion-deposited mobile surface charge. In this approach, scattering and extinction cross-sections are calculated for a charged Rayleigh particle with effective dielectric constant consisting of the volume dielectric function of the neutral sphere and surface dielectric function due to the oscillation of the surface charge in the presence of applied electric field. For small droplets with radius smaller than 100 nm, this model predicts increased MMW scattering from charged mist, which is qualitatively consistent with the experimental observations. The objective of this work is to develop indirect remote sensing of radioactive gases via their charging action on atmospheric humid air.     

Theoretical ultraslow bright and dark optical solitons in cascade-type GaAs/AlGaAs multiple quantum wells

We show the formation of ultraslow bright and dark optical solitons in a cascade-type three-level system of GaAs/AlGaAs multiple quantum wells (MQWs) structure based on the biexciton coherence in the transient optical response, and study analytically and numerically with Maxwell–Schrödinger equations. The calculated velocity of bright and dark optical solitons are V_g = 2.7 × 10⁴ ms^? 1 and V_g = 8.91 × 10⁴ ms^? 1, respectively. Such investigation of ultraslow optical solitons in MQWs may provide practical applications such as high-fidelity optical delay lines and optical buffers in semiconductor quantum wells structure, because of its flexible design.     

Temperature dependence of microwave resistances of n++np++ Si X band IMPATT diode

The change of both microwave negative resistance (R) and its parasitic series resistance (R_s) on the rise of junction temperature in the range of 100–220 °C of HP n⁺⁺np⁺⁺ Si IMPATT [M. Mitra, M. Das, S. Kar, S.K. Roy, IEEE Trans. Electron. Dev. 40 (1993) 1890] diode at X band (8–12 GHz) have been simulated. The studies followed by Gummel–Blue Technique [H.K. Gummel, J.L. Blue, IEEE Trans. Electron. Dev. 14 (1967) 569] show that for a constant experimental bias current of 25 mA [Mitra et al., 1993], for which the space charge effect is not prominent, the values of negative conductance and negative resistance degrade taking into account the changes in the ionization rates and drift velocities due to rise of temperature. Also observed that the critical series resistance increases with the increase of temperature up to 2.23 Ω, slightly higher than the realistic limit of 2 Ω [Mitra et al., 1993].     

Design optimization of porous fibrous material for maximizing absorption of sounds under set frequency bands

In this paper, a methodology is proposed for designing porous fibrous material with optimal sound absorption under set frequency bands. The material is assumed to have a rigid frame and a hexagonal arrangement of fibers, and the analytical model derived by Johnson, Champoux and Allard (“JCA model”) is used to investigate the influences of the micro-structural parameters (fiber radius r and gap w) on sound absorption performance, and the macro-acoustic parameters used in JCA model is determined via finite element analysis for the hexagonal micro-structure. Moreover, a mathematical model is constructed to obtain the optimized micro-structure design, with fiber radius and gap as design parameters and average absorption performance of the porous fibrous material under set frequency band as target. Utilizing the constructed optimization model, the microstructure parameters are derived with optimal sound absorption under low frequency (20 ⩽ f < 500 Hz), medium frequency (500 ⩽ f < 2000 Hz) and high frequency (2000 ⩽ f < 15,000 Hz), respectively. On top of that, for a given thickness of porous fibrous material layer, the analytical relationship between fiber radius and optimal porosity under set frequency bands is constructed.     

Medium-high frequency ultrasound and ozone based advanced oxidation for amoxicillin removal in water

In this study, treatment of an antibiotic compound amoxicillin by medium-high frequency ultrasonic irradiation and/or ozonation has been studied. Ultrasonic irradiation process was carried out in a batch reactor for aqueous amoxicillin solutions at three different frequencies (575, 861 and 1141 kHz). The applied ultrasonic power was 75 W and the diffused power was calculated as 14.6 W/L. The highest removal was achieved at 575 kHz ultrasonic frequency (>99%) with the highest pseudo first order reaction rate constant 0.04 min⁻¹ at pH 10 but the mineralization achieved was around 10%. Presence of alkalinity and humic acid species had negative effect on the removal efficiency (50% decrease). To improve the poor outcomes, ozonation had been applied with or without ultrasound. Ozone removed the amoxicillin at a rate 50 times faster than ultrasound. Moreover, due to the synergistic effect, coupling of ozone and ultrasound gave rise to rate constant of 2.5 min⁻¹ (625 times higher than ultrasound). In the processes where ozone was used, humic acid did not show any significant effect because the rate constant was so high that ozone has easily overcome the scavenging effects of natural water constituents. Furthermore, the intermediate compounds, after the incomplete oxidation mechanisms, has been analyzed to reveal the possible degradation pathways of amoxicillin through ultrasonic irradiation and ozonation applications. The outcomes of the intermediate compounds experiments and the toxicity was investigated to give a clear explanation about the safety of the resulting solution. The relevance of all the results concluded that hybrid advanced oxidation system was the best option for amoxicillin removal.     

Control of electric field in 4H-SiC UMOSFET: Physical investigation

In this paper, we show how breakdown voltage (V_BR) and the specific on-resistance (R_on) can be improved simply by controlling of the electric field in a power 4H-SiC UMOSFET. The key idea in this work is increasing the uniformity of the electric field profile by inserting a region with a graded doping density (GD region) in the drift region. The doping density of inserted region is decreased gradually from top to bottom, called Graded Doping Region UMOSFET (GDR-UMOSFET). The GD region results in a more uniform electric field profile in comparison with a conventional UMOSFET (C-UMOSFET) and a UMOSFET with an accumulation layer (AL-UMOSFET). This in turn improves breakdown voltage. Using two-dimensional two-carrier simulation, we demonstrate that the GDR-UMOSFET shows higher breakdown voltage and lower specific on-resistance. Our results show the maximum breakdown voltage of 1340 V is obtained for the GDR-UMOSFET with 10 µm drift region length, while at the same drift region length and approximated doping density, the maximum breakdown voltages of the C-UMOSFET and the AL-UMOSFET structures are 534 V and 703 V, respectively.     

Characterization of Young’s modulus of silicon versus temperature using a “beam deflection” method with a four-point bending fixture

Young’s modulus (E) and Poisson’s ratio (ν) are dependent upon the direction on the silicon surface. In this work, E and ν of silicon have been calculated analytically for any crystallographic direction of silicon by using compliance coefficients (s₁₁, s₁₂, and s₄₄), and the values of E are confirmed experimentally by using a “beam deflection” method with a four-point bending fixture. Experimental results for E as a function of temperature from −150 °C to +150 °C are presented for (0 0 1) and (1 1 1) silicon wafers.