Radial X‐Ray Diffraction Study of Superhard Early Transition Metal Dodecaborides under High Pressure

The deformation behavior of the three metal dodecaborides (YB₁₂, ZrB₁₂, and Zr_0.5Y_0.5B₁₂) is investigated using radial X‐ray diffraction under nonhydrostatic compression up to ≈60 GPa with a goal of understanding how bonding and metal composition control hardness. Zr_0.5Y_0.5B₁₂, which has the highest Vickers hardness (Hv = 45.8 ± 1.3 GPa at 0.49 N load), also shows the highest bulk modulus (K₀ = 320 ± 5 GPa). The 0.49 N hardness for ZrB₁₂ and YB₁₂ are both lower and very similar, and both show lower bulk moduli (K₀ = 276 ± 7 GPa, and K₀ = 238 ± 6 GPa, respectively). Differential stress is then measured to study the strength and strength anisotropy. Zr_0.5Y_0.5B₁₂ supports the highest differential stress, in agreement with its high hardness, a fact that likely arises from atomic size mismatch between Zr and Y combined with the rigid network of boron cages. The (200) plane for all samples supports the largest differential strain, while the (111) plane supports the smallest, consistent with the theoretically predicted slip system of {111} [ $\overline{1}\overline{1}2$ ]. Strain softening is also observed for ZrB₁₂. Finally, the full elastic stiffness tensors for ZrB₁₂ and YB₁₂ are solved. ZrB₁₂ is the most isotropic, but the extent of elastic anisotropy for all dodecaborides studied is relatively low due to the highly symmetric boron cage network.     

Photoelectrical Characteristics of a C/CNx Multiwalled Nanotube

A nanotube diode fabricated from a single C/CN_x multiwalled nanotube exhibits a large photocurrent and a large photovoltage under illumination. The current–voltage (I–V) characteristics of the diode indicate a clear rectification effect. By comparing the I–V characteristics of C, CN_x, and C/CN_x nanotube diodes, we show that the rectifying characteristics of the C/CN_x diode arises from the molecular junction formed at the C/CN_x interface where the C and CN_x segments are chemically bonded. External radiation photochemically generates electrons and holes in the C/CN_x nanotube, producing a large photocurrent because of the influence of the strong electric field in the vicinity of the C/CN_x junction. These unique photoresponsive characteristics of C/CN_x nanotube junction diodes points to potential applications such as photovoltaic devices and photodiodes.     

Strong,Tough, Electrospun Polymer–Nanotube Composite Membranes with Extremely Low Density

Electrospinning has been used to produce porous, low density, polymer–nanotube composite membranes. The membrane mechanical properties can be enhanced by tuning the nanotube content, aligning the fibers during spinning, and by post production drawing. The mechanical properties are maximized for membranes with a nanotube content of 0.43 vol %. Aligned composites at this volume fraction have been prepared by spinning onto a rotating drum collector electrode. This method results in significant increases in modulus, strength, and toughness. The best composites, produced at the maximum drum rotation rate, were post treated by a drawing step to result in further increases in modulus and strength. These methods allows the production of membranes with densities as low as ～340 kg m^?3 but with values of stiffness, strengths and toughness's more typically found in bulk thermoplastics; 1.2 GPa, 40 MPa, and 13 J g^?1.     

A novel analytical model of the vertical breakdown voltage on impurity concentration in top silicon layer for SOI high voltage devices

A novel analytical model of the vertical breakdown voltage (V_{B , V} ) on impurity concentration (N_d ) in top silicon layer for silicon on insulator high voltage devices is first presented in this article. Based on an effective ionisation rate considering the multiplication of threshold energy ε_T in the electron, a new formula of silicon critical electric field E_{S , C} on N_d is derived by solving a 2D Poisson equation, which increases with the increase in N_d especially at higher impurity concentration, and reaches up to 68.8?V/µm with N_d  = 1 × 10¹⁷?cm^?3 and 157.2?V/µm with N_d  = 1 × 10¹⁸?cm^?3 from the conventional about 30?V/µm, respectively. A new physical concept of critical energy ε_B is introduced to explain the mechanism of variable high E_{S , C} with heavy impurity concentration. From the E_{S , C} , the expression of V_{B , V} is obtained, which is improved with the increasing N_d due to the enhanced E_{S , C}. V_{B , V} with a dielectric buried layer thickness (t_I ) of 2?µm increases from 428?V of 1 × 10¹⁷?cm^?3 to 951?V of 1 × 10¹⁸?cm^?3. The dependence of N_d and top silicon layer thickness (t_S ) for an optimised device is discussed. 2D simulations and some experimental results are in good agreement with the analytical results.     

Analog and radio-frequency performance analysis of silicon-nanotube MOSFETs

This work presents a comparative study of the influence of various parameters on the analog and RF properties of silicon-nanotube MOSFETs and nanowire-based gate-all-around (GAA) MOSFETs. The important analog and RF performance parameters of SiNT FETs and GAA MOSFETs, namely drain current (I_d), transconductance to drain current ratio (g_m/I_d), I_on/I_off, the cut-off frequency (f_T) and the maximum frequency of oscillation (f_MAX) are evaluated with the help of Y- and H-parameters which are obtained from a 3-D device simulator, ATLAS^TM. It is found that the silicon-nanotube MOSFETs have far more superior analog and RF characteristics (g_m/I_d, f_T and f_MAX) compared to the nanowire-based gate-all-around GAA MOSFETs. The silicon-nanotube MOSFET shows an improvement of~2.5 and 3 times in the case of f_T and f_MAX values respectively compared with the nanowire-based gate-all-around (GAA) MOSFET.     

Electronic properties and bulk moduli of new boron nitride polymorphs, i.e., hyperdiamond B12N12 and simple cubic B24N24, B12N12 fulborenites

The energy-band structure, density of states, electron density distribution, equation of state, and bulk moduli of three boron-nitride fulborenite crystals, i.e., B₁₂N₁₂ with diamond lattice and B₂₄N₂₄, B₁₂N₁₂ with simple cubic lattice, whose sites contain fulborene B₁₂N₁₂ and B₂₄N₂₄ molecules, are calculated for the first time using the full-potential linearized augmented plane wave method. The following hyperdiamond B₁₂N₁₂ parameters were obtained: the equilibrium lattice parameter a = 1.1191 nm, the B-N bond length a _BN = 0.1405 nm, the number of atoms per unit cell Z = 192, the density ρ = 2.823 g/cm³, the bulk modulus B ₀ = 658 GPa, and the band gap ΔE _g = 3.05 eV. This is a previously unknown unique light superhard semiconductor faujasite with a recorded bulk modulus higher than that of diamond. There are reasons to assume that it is a E phase. The characteristics of B₂₄N₂₄ with simple cubic lattice are as follows: the equilibrium lattice parameter a = 0.7346 nm, the B-N bond length a _BN = 0.1521 nm, the number of atoms per unit cell Z = 48, the density ρ = 2.495 g/cm³, the bulk modulus B ₀ = 367 GPa, and the band gap ΔE _g = 3.76 eV. This material is a heteropolar semiconductor or insulator with a bulk modulus comparable with that of cubic boron nitride, as well as a new boron-nitride zeolite with channel diameter of 0.46 nm. B₁₂N₁₂ with simple cubic lattice is a molecular semimetal.     

非傍轴部分相干厄米-余弦-高斯光束传输特性

为了研究非傍轴部分相干厄米-余弦-高斯光束传输特性,运用Wigner分布函数法,从空间域和频率域对非傍轴部分相干厄米-余弦-高斯光束传输特性进行了理论分析,得出了该光束1阶情况下在空间、频率域系统的解析传输公式及光强分布表达式,分析了这些表达式中3个主要参量f,fσ和g对厄米-余弦-高斯光束在自由空间传输的影响。结果表明,在调制参量g不变时,束腰参量f和相干参量fσ对非傍轴部分相干厄米-余弦-高斯光束的非傍轴性起了至关重要的作用;非傍轴部分相干厄米-余弦-高斯光束在传输过程中随着g的改变,不能保持其光强分布形状,有前移趋势。     

Engineering Highly Ordered Iron Titanate Nanotube Array Photoanodes for Enhanced Solar Water Splitting Activity

Highly ordered iron titanate (Fe₂TiO₅) nanotube array photoanode is synthesized on F:SnO₂ glass with ultrathin anodized aluminum oxide as a hard template. Highly crystalline, yet the nanotube array morphology‐preserved Fe₂TiO₅ is fabricated by hybrid microwave annealing (HMA). The effects of the synthesis parameters on photoelectrochemical (PEC) water splitting activity under simulated sunlight are systematically studied including HMA time, pore size, wall thickness, and length of the nanotubes to optimize the nanotube array photoanode. In addition, triple modification strategies of TiO₂ underlayer, hydrogen treatment, and FeNiO_x cocatalyst loading effectively improve the PEC activity further. The systematically engineered nanotube array photoanode achieves a photocurrent density of 0.93 mA cm^?2 at 1.23 V_RHE under 1 sun (100 mW cm^?2) irradiation, which corresponds to 2.6 times that of the previous best Fe₂TiO₅ photoanode. In addition, the photocurrent onset potential shifts cathodically by ≈280 mV relative to the pristine nanotube array electrode.     

Determination of the Elastic Properties of Cu<Subscript>3</Subscript>Sn Through First-Principles Calculations

Nine elastic constants of single-crystal Cu₃Sn were determined from first-principles calculations to characterize its polycrystalline elastic behavior and elastic anisotropy. The ideal elastic (E = 147 GPa), shear (G = 56 GPa) and bulk modulus (K = 132 GPa), and Poisson’s ratio (v = 0.315), were determined using the Voigt–Reuss–Hill method and were very close to the range of experimental results. Cu₃Sn exhibits distinct anisotropy in Young’s modulus, with a 44 GPa difference between its maximum and minimum values, which may be partially responsible for the discrepancy in the reported experimental results.     

Flicker noise in MOSFETs with gate-voltage-dependent mobility

Flicker noise measurements in MOSFETs at low drain bias are explained in terms of the dependence of the carrier mobility on the gate voltage of the form μ₀₀[1 + β(V_G ? V_T ? V₀)]^?1. Excellent agreement, both for the (I_d, V_g) characteristic and for the flicker noise, is obtained. The noise current spectrum is expressed in the normalized functions f(y₀, y₁) and f(y₀, y₁)/y₀ in terms of the bias parameters y₀ = β(V_g ? V_T) and y₁ = β(V_g ? V_T ? V_d).     

dV/dt effect in high-voltage (1.5 kV) 4H–SiC thyristors

The switching of high-voltage (1.5 kV) 4H–SiC gate turn-off thyristors (GTOs) by the dV/dt effect has been studied in the temperature range from 300 to 504 K. At a 30 ns rise time of the forward bias V(t), the characteristic bias at which the structure under investigation can be switched on by the dV/dt effect decreases steadily from 289 V at room temperature (dV/dt 9.7 kV/μs) to 137 V at T = 504 K. The characteristic critical charge per unit area, Q_cr, equal to 1.9 × 10⁻⁷ C/cm² at room temperature, also decreases steadily as the temperature increases. The main physical mechanisms that contribute to Q_cr formation and the temperature dependence of the critical charge are qualitatively analyzed. The influence exerted by two-dimensional processes on the dV/dt switching is examined by making analytical estimates and using a computer simulation. The results obtained agree well with experimental data.     

Experimental investigation of the dielectric-semiconductor interface with scanning capacitance microscopy

An experimental investigation of how interface states effect scanning capacitance microscopy (SCM) measurements is presented. Different sample polishing procedures were used to make SCM samples that would have different interface state densities, but identical oxide thicknesses. By comparing SCM signals of these samples, the effect of interface states could be singled out. The interface states of these SCM samples were found to have an amphoteric energy distribution. The magnitude of the maximum SCM signals (maximum dC/dV in dC/dV versus dc bias, V_dc, plots) is independent of the interface-trapped charges, while the full width at half maximum (FWHM) of the dC/dV–V_dc curves is broadened with the interface states. The physics of SCM interface states effect is also discussed.     

Channel current limitations in GaAs MESFETS

The maximum channel current, I_m, the maximum forward gate bias voltage, V_f, and the corresponding knee voltage, V_kf, play an important role in determining the maximum power handling capability of a GaAs MESFET. The definition of these parameters is given in a practical manner. Simple and yet accurate enough expressions for these parameters derived from a theoretical model are shown in terms of the geometrical and material parameters of the active channel of a device. A very simple expression for I_m obtained on an empirical basis is also shown. For the zero-gate-bias channel current, I_o, simple theoretical and empirical expressions are presented. Calculated values of I_m, V_f, V_kf and I_o using these expressions are in excellent agreement with their measured values for sample devices chosen from a variety of channel properties. A graphical presentation of I_m and that of the maximum channel-current enhancement ratio, I_m/I_o, are given as functions of basic channel parameters for practical purposes.     

Classification of Energy Levels in Quantum Dot Structures by Depleted Layer Spectroscopy

The coexistence of quantum confined energy levels and defect energy levels in quantum dot (QD) structures may cause difficulties in distinguishing between their different origin when using deep-level transient spectroscopy (DLTS). Using InAs/GaAs QDs as demonstration vehicles, we present methodologies to obtain such a classification by DLTS. QD-related spectra measured as a function of repetition frequency of electrical pulses, f, or temperature, T, and reverse voltage, V _R, are depicted as contour plots on (f, V _R) and (T, V _R) planes, thus reflecting the complex thermal and tunneling emission of electrons from the ground and excited states. Defect-related levels give rise to different contour patterns and undergo modification, exhibiting double-peak structured emission when defects are agglomerated in the vicinity of the QD plane. This effect is interpreted in terms of an interaction between electron states in traps and the confined QD states.     

Low-field current transport mechanisms in rf magnetron sputter deposited boron carbide (B5C)/p-type crystalline silicon junctions in the dark

Measurements of the forward and reverse currents in an undoped rf magnetron sputter deposited boron carbide (B₅C)/p-type Si(111) junction have been made in the dark in the temperature range 120–300K at low-bias voltages (0–0.3V). A diode-like behaviour of the junction current has been observed in this low-bias region at all temperatures but with a rather large reverse (leakage) current I _R, particularly at high temperatures (I _R≈2 μA at V = ?0:3V and T = 290 K). The forward ‘voltage factor’ A (T) was found to decrease with increasing temperature as A (T)≈q/ηk _B T, with relatively high values of the ‘ideality factor’ η(about 3.5–4), probably due to the existence of an interfacial layer. The temperature dependence of the measured junction current (forward and reverse) flowing at low bias voltages and of the forward ‘current factor’ I _0F can be described satisfactorily by a model of the tunnelling of thermally excited carriers, including tunnelling via impurity localized levels, of the form I (T) ∝ exp [?C/T ^1/3] over the entire temperature range studied (120–300K). A high density of ‘localized’ energy states as large as 10¹⁸ cm^?3 eV^?1 was estimated, which can be attributed in part to ‘extrinsic’ interface states that could have been formed throughout the fabrication procedures of the rf sputter deposited B₅C/p-crystalline silicon junction studied. Another possible cause of such large concentration of ‘localized’ states is the ‘intrinsic’ interface states produced by the lattice mismatch between the polycrystalline boron carbide and crystalline silicon semiconductors as well as of the high intrinsic defect concentration caused by structural imperfections that often exist in boron carbide compounds.     

First-principle calculation on mechanical and thermal properties of B2-NiSc with point defects

Using the first-principles plane-wave pseudo-potential method based on density functional theory, the effect of vacancy and anti-position defect on the mechanical and thermal properties of B2-NiSc intermetallics were discussed in detail. Several parameters, such as the shear modulus, bulk modulus, modulus of elasticity, C₁₁-C₁₂, the Debye temperature and Poisson''s ratio, have been calculated to evaluate the effect of vacancy and anti-position defect on the hardness, ductility and thermal properties of B2-NiSc intermetallics. The results show that V_Ni, Sc_Ni, V_Sc and Ni_Sc the four point defects all make the crystal hardness decrease and improve plasticity of B2-NiSc intermetallics. The entropy, enthalpy and free energy of V_Ni, Sc_Ni, V_Sc and Ni_Sc are monotonously changed as temperature changes. From the perspective of free energy, Ni_Sc is the most stable, while Sc_Ni is the most unstable. Debye temperature of NiSc intermetallics with four different point defects shows V_Ni, Sc_Ni, V_Sc and Ni_Sc the four point defects all reduce the stability of B2-NiSc intermetallics.     

Mechanisms of temperature dependence of threshold voltage in high-k/metal gate transistors with different TiN thicknesses

The change in temperature coefficient of the threshold voltage (=dV_th/dT) for poly-Si/TiN/high-k gate insulator metal–oxide–semiconductor field-effect transistors (MOSFETs) was systematically investigated with respect to various TiN thicknesses for both n- and p-channel MOSFETs. With increasing TiN thickness, dV_th/dT shifts towards negative values for both n- and p-MOSFETs. A mechanism that changes dV_th/dT, depending on TiN thickness is proposed. The main origins are the work function of TiN (Φ_TiN) and its temperature coefficient (dΦ_TiN/dT). These are revealed to change when decreasing the thickness of the TiN layer, because the crystallinity of the TiN layer is degraded for thinner films, which was confirmed by ultraviolet photoelectron spectroscopy (UPS), transmission electron microscopy (TEM) and X-ray diffraction (XRD).     

一种氮化镓高迁移率晶体管的栅电流噪声模型

This work presents a theoretical and experimental study on the gate current 1/f noise in Al Ga N/Ga N HEMTs. Based on the carrier number fluctuation in the two-dimensional electron gas channel of Al Ga N/Ga N HEMTs, a gate current 1/f noise model containing a trap-assisted tunneling current and a space charge limited current is built. The simulation results are in good agreement with the experiment. Experiments show that, if Vg Vx, gate current 1/f noise comes from not only the trap-assisted tunneling RTS, but also the space charge limited current RTS. This indicates that the gate current 1/f noise of the Ga N-based HEMTs device is sensitive to the interaction of defects and the piezoelectric relaxation. It provides a useful characterization tool for deeper information about the defects and their evolution in Al Ga N/Ga N HEMTs.     

ZnO-Coated TiO2 Nanotube Arrays for a Photoelectrode in Dye-Sensitized Solar Cells

In dye-sensitized solar cells, highly ordered TiO₂ nanotube arrays as a photoelectrode have higher charge collection efficiencies than a nanoparticle-based structure due to their faster charge percolation and slower recombination of electrons. Highly ordered TiO₂ nanotube arrays were grown by anodic oxidation of 0.5-mm-thick titanium foil. To increase the conversion efficiency of dye-sensitized solar cells with TiO₂ nanotube arrays, the surface of the TiO₂ nanotube arrays was modified by zinc oxide thin films. The ZnO thin film was formed by atomic layer deposition. The thin film was conformal on the inner and outer walls of TiO₂ nanotube arrays. ZnO thin film improved the short circuit current (J _sc) and open circuit voltage (V _oc) due to increasing specific surface area from particulates of ZnO thin film and increasing the surface charge induced from the isoelectric point. The power conversion efficiency of dye-sensitized solar cells with ZnO thin film on 4.5-μm-thick TiO₂ nanotube arrays was 1.43%. Microstructure and phase were observed by scanning electron microscopy, x-ray diffractometry, and transmission electron microscopy.     

Many-body effects and electron tunneling in metal-insulator-p-type semiconductor structures

Investigation of the tunneling conductivity σ _d(V) of structures made on a highly doped, narrow-gap p-type semiconductor HgCdTe reveals an abrupt increase in this quantity at voltages corresponding to the start of tunneling into the conduction band. It is shown that the observed functions σ _d(V) cannot be described in the framework of a model based on single-particle tunneling. It is proposed that the abrupt increase in σ _d(V) is attributable to tunneling into exciton states. Fiz. Tekh. Poluprovodn. 32, 1069–1072 (September 1998)