Mixed-Alkali Effect in Li2O–Na2O–K2O–B2O3 Glasses: Infrared and Optical Absorption Studies

The mixed-alkali effect (MAE) has been investigated in the glass system (40 ? x)Li₂O–xNa₂O–10K₂O–50B₂O₃ (0 mol% ≤ x ≤ 40 mol%) through density, modulated differential scanning calorimetry (DSC), and optical absorption studies. From the absorption studies, the values of the optical band gap (E _opt) for direct transition and Urbach energy (ΔE) have been evaluated. The values of E _opt and ΔE show nonlinear behavior with the compositional parameter. The density and glass-transition temperature of the present glasses also show nonlinear variation, supporting the existence of MAE. The infrared (IR) spectra of the glasses reveal the presence of three- and four-coordinated boron atoms. The specific vibrations of Li–O, Na–O, and K–O bonds were observed in the present IR study.     

Electron Microscopy Study of Ferroelectric Memory Based on Si–SiO2–Ti–Pt–PZT Multilayer Structure

Model Si–SiO₂–Ti–Pt–PZT multilayer structures obtained by chemical solution deposition at excessive (relative to the stoichiometric composition) amounts of lead in the starting film-forming solution (x= 0–30 mol %) were studied by TEM and X-ray microanalysis techniques. In the absence of excessive lead, the films crystallize largely into the metastable phase of pyrochlore, which does not have ferroelectric properties. With an excessive amount of lead added, PZT ceramic crystallizes into the ferroelectric phase of perovskite and has columnar grains 0.2 m in size. The thermal stability of the metallization system during the formation of the ferroelectric film was investigated. A 180-nm-thick transition layer between the Pt electrode and the adhesive titanium film was discovered. This layer, resulting from high-temperature synthesis, consists of fine-grain platinum, as well as metal oxides and silicides.     

The forward bias current density–voltage–temperature (J–V–T) characteristics of Al–SiO2–pSi (MIS) Schottky diodes

Metal–insulator–semiconductor Schottky diodes were fabricated to investigate the tunnel effect and the dominant carrier transport mechanism by using current density–voltage (J–V) and capacitance–voltage (C–V) measurements in the temperature range of 295–370?K. The slope of the ln?J–V curves was almost constant value over the nearly four decades of current and the forward bias current density J is found to be proportional to J_o (T) exp(AV). The values of N_ss estimated from J–V and C–V measurements decreased with increasing temperature. The temperature dependence of the barrier heights obtained from forward bias J–V was found to be entirely different than that from the reverse bias C–V characteristics. All these behaviours confirmed that the prepared samples have a tunnel effect and the current transport mechanism in the temperature range of 295–370?K was predominated by a trap-assisted multi-step tunnelling, although the Si wafer has low doping concentration and the measurements were made at moderate temperature.     

Halogen adsorption at an As-stabilized β2–GaAs (001)–(2 × 4) surface

Halogen (F, Cl, Br, and I) adsorption at an As-stabilized GaAs (001) surface with the β2–(2 × 4) reconstruction is studied using the plane-wave projected-augmented wave method. The effect of halogens on the structural and electronic characteristics of the semiconductor surface is analyzed. The T₂′ site at the missing row edge is shown to be the energetically most favorable for the adsorption of F, Cl, and Br, whereas I prefers the H₃ site between adjacent arsenic dimers in the third layer from the surface. Ga-halogen bond formation suggests that charge is transferred via the depletion of occupied orbitals of the As-dimer surface atoms, which leads to the weakening of Ga–As bonds in the substrate. The weakening of bonds between substrate-surface atoms due to the interaction of halogens with the surface is estimated.     

Broadband Achromatic Terahertz Metalens Constituted by Si–SiO2–Si Hybrid Meta-Atoms

Metasurfaces have proved to possess extraordinary properties and propelled the development of versatile functional devices in the terahertz domain, but the intrinsic dispersion severely impedes their broadband applications. Sporadic studies on achromatic terahertz metasurfaces are either subject to specific polarization operation or challenged in the fabrication process for extended bandwidth and numerical aperture. Herein a novel meta-atom scheme made up of a silicon–silica–silicon sandwich-shaped structure is presented that enlarges the propagation phase as well as improves the dispersion engineering capability at low losses. As a classic case, an achromatic metalens is constituted by these meta-atoms and experimentally demonstrated which shows a remarkable achromatic focusing performance from 0.5 to 1.1 THz with a numerical aperture of 0.47 and an average efficiency as high as 43.1%. Continuous wavelet transformation analysis further proves the metalens has negligible group delay dispersion. This study not only demonstrates an outstanding terahertz achromatic metalens but also provides a novel meta-atom paradigm for constructing achromatic metasurfaces such as holographic plate, beam deflector, and Bessel axicon, further not limited to THz bands.     

Phase Formation during the Surface-Diffusion Growth of Ti–Co–Si–N and Ti–Co–N Thin Films on Si and SiO2

The kinetics of phase formation in Ti–Co–Si–N and Ti–Co–N thin films on Si and SiO₂is investigated experimentally. With the deposition on Si, rapid thermal annealing (T 900°C) is shown to cause phase separation that ends in a TiN/CoSi₂/Si structure. If SiO₂is used, the alloy reacts with the substrate to produce compounds that are difficult to remove with selective etchants. This limits the potential uses of this process in the fabrication of contact systems for CMOS devices. It is shown that structure- and phase-dissimilar films can be formed on Si and SiO₂by means of the surface-diffusion reactions between a Ti–Co–Si–N or Ti–Co–N alloy and the substrate at 650–700°C. The effect of a TiN, Ti, or CoSi₂thin layer at the alloy–substrate interface on the phase separation is investigated.     

A Multifunctional Catalytic Interlayer for Propelling Solid–Solid Conversion Kinetics of Li2S2 to Li2S in Lithium–Sulfur Batteries

The theoretically high-energy-density lithium–sulfur batteries (LSBs) are seriously limited by the disadvantages including the shuttle effect of soluble lithium polysulfides (LiPSs) and the sluggish sulfur redox kinetics, especially for the most difficult solid–solid conversion of Li₂S₂ to Li₂S. Herein, a multifunctional catalytic interlayer to improve the performance of LSBs is tried to introduce, in which Fe_1–xS/Fe₃C nanoparticles are embedded in the N/S dual-doped carbon network (NSC) composed by nanosheets and nanotubes (the final product is named as FeSC@NSC). The well-designed 3D NSC network endows the interlayer with a satisfactory LiPSs capture-catalytic ability, thus ensuring fast redox reaction kinetics and suppressing LiPSs shuttling. The density functional theory calculations disclose the catalytic mechanisms that FeSC@NSC greatly improves the liquid–solid (LiPSs to Li₂S₂) conversion and unexpectedly the solid–solid (Li₂S₂ to Li₂S) one. As a result, the LSBs based on the FeSC@NSC interlayer can achieve a high specific capacity of 1118 mAh g⁻¹ at a current density of 0.2 C, and a relatively stable capacity of 415 mAh g⁻¹ at a large current density of 2.0 C after 700 cycles as well as superior rate performance.     

Effects of H2O Pretreatment on the Capacitance–Voltage Characteristics of Atomic-Layer-Deposited Al2O3 on Ga-Face GaN Metal–Oxide–Semiconductor Capacitors

Atomic layer deposition (ALD) of Al₂O₃ on Ga-face GaN is studied with respect to the effects of growth saturation, precursor injection sequence, and H₂O pretreatment. A metal–oxide–semiconductor capacitor (MOSCAP) structure is fabricated to measure the capacitance–voltage (C–V) characteristics. The origin of C–V hysteresis is explained by a model considering the different trapping behaviors of interface states and oxide border traps. The interface state density (D _it) is extracted as a function of band bending using an ultraviolet (UV)-assisted method. It is found that H₂O pretreatment followed by saturated ALD growth produces the best interface quality, with a reduced D _it compared with growth without H₂O pretreatment.     

Physicochemical Interactions in the GeSb2Te4–PbSb2Te4 System

Semiconductors - For the first time, the GeSb2Te4–PbSb2Te4 section of the GeTe–Sb2Te3–PbTe quasi-ternary system is investigated by complex methods of physicochemical analysis...     

2D Ruddlesden–Popper Perovskite Single Crystal Field-Effect Transistors

2D Ruddlesden–Popper perovskites (2D PVKs) have attracted huge interest because of their excellent optoelectronic properties, yet the understanding of their electrical properties is inadequate due to the difficulties in obtaining 2D PVK field-effect transistors (FETs) with decent performance. Herein, the fabrication and characterization of 2D PVK ((BA)₂(MA)_n−1Pb_nI_3n+1) single crystal FETs are reported, which exhibit reliable field effect electrical characteristics at low temperatures. Kelvin probe force microscopy (KPFM) results reveal that both ion migration and contact resistance seriously degrade device performance. While ion migration can be suppressed at low temperatures, contact resistance seems to fundamentally determine device performance. On one hand, Schottky contacts are observed to form at the metal/2D PVK interface because of Fermi level pinning, resulting in significant charge injection resistance, although this can be remarkably improved by replacing Au electrodes with Ca. On the other hand, the out-of-plane mobility is found to be three orders of magnitude lower than the in-plane mobility in 2D PVKs, causing large interlayer transport resistance. Thus, a low work-function metal and a thin crystal are important for achieving high device performance. This work provides important experimental insights into fabrication and electrical properties of 2D PVK FETs.     

New insights into acidic wet chemical silicon etching by HF/H2O–NOHSO4–H2SO4 solutions

Acidic wet chemical etching of crystalline silicon has been examined by utilization of HF–NOHSO₄–H₂SO₄ mixtures. In light of our previous studies the effects of nitrosyl ion concentrations on etching rates were studied time- and temperature resolved. The reactivity of crystalline silicon surfaces in HF/H₂SO₄ solutions is determined by NO⁺-ion concentrations at the silicon/electrolyte interface, measured by ion chromatography. Quantitative solution analysis proofed accumulation of ammonium ions and indicated the conversion of NO⁺ as limiting for the overall etching process. Direct participation in the rate-limiting step was confirmed by calculation of activation energies. Increasing NO⁺-ion contents cause transition from reaction (E_A=55 kJ mol^?1) to diffusion controlled (E_A=10 kJ mol^?1) etching procedures. In combination with time and concentration dependent studies of produced structures a convenient regime for selective texturing or polishing polycrystalline silicon surfaces is reported. Qualitative analysis by ¹⁹F-NMR and Raman spectroscopy identified SiF₅^?/HF₂^? complexes as well as elementary hydrogen (H₂) as hitherto unknown products of silicon dissolution reactions in HF–NOHSO₄–H₂SO₄ mixtures. Based on our findings a strategy for fundamental investigations of relevant reaction pathways is presented and discussed with regard to reported mechanistic concepts.     

Enhanced formaldehyde-sensing properties of mixed Fe2O3–In2O3 nanotubes

Pure In₂O₃ and mixed Fe₂O₃–In₂O₃ nanotubes were prepared by simple electrospinning and subsequent calcination. The as-prepared nanotubes were characterized by scanning electron microscopy, powder X-ray diffraction, and energy-dispersive X-ray spectrometry. Gas sensors were fabricated to investigate the gas-sensing properties of In₂O₃ and Fe₂O₃–In₂O₃ nanotubes. Compared to pure In₂O₃, Fe₂O₃–In₂O₃ nanotubes exhibited better gas-sensing properties for formaldehyde at 250 °C. The response of the Fe₂O₃–In₂O₃ nanotube gas sensor to 100 ppm formaldehyde was approximately 33, which is approximately double the response of the pure In₂O₃ nanotube gas sensor. In both cases the response time was ~5 s and the recovery time was ~25 s.     

Capacitance–conductance–current–voltage characteristics of atomic layer deposited Au/Ti/Al2O3/n-GaAs MIS structures

We have studied the admittance and current–voltage characteristics of the Au/Ti/Al₂O₃/n-GaAs structure. The Al₂O₃ layer of about 5 nm was formed on the n-GaAs by atomic layer deposition. The barrier height (BH) and ideality factor values of 1.18 eV and 2.45 were obtained from the forward-bias ln I vs V plot at 300 K. The BH value of 1.18 eV is larger than the values reported for conventional Ti/n-GaAs or Au/Ti/n-GaAs diodes. The barrier modification is very important in metal semiconductor devices. The use of an increased barrier diode as the gate can provide an adequate barrier height for FET operation while the decreased barrier diodes also show promise as small signal zero-bias rectifiers and microwave. The experimental capacitance and conductance characteristics were corrected by taking into account the device series resistance R_s. It has been seen that the non-correction characteristics cause a serious error in the extraction of the interfacial properties. Furthermore, the device behaved more capacitive at the reverse bias voltage range rather than the forward bias voltage range because the phase angle in the reverse bias has remained unchanged as 90° independent of the measurement frequency.     

Crystal growth in Bi–Sr–Ca–Cu–O system

We report growth of Bi₂Sr₂CaCu₂O_8+δ single crystals with dimensions of 6×2×0.03 mm³ using a melt and growth technique. The oxygen content is determined to be δ≈0.13 by iodometric titration. The crystal is shown to be homogeneous and close to stoichiometric cation ratio. The superconducting temperature with a sharp transition width (10–90% level) of 6–8 K was determined to be T_c=92 K from resistivity and dc susceptibility measurements. The predominant impurity phase is a Cu-free crystal, whose composition is identified as Bi₁₀Sr₁₁Ca₅O_x. The crystal structure of Bi₁₀Sr₁₁Ca₅O_x is monoclinic with a=11.108(1) Å, b=5.9487(1) Å; c=19.838 (3) Åand β=101.5° (P2_1/c space group).     

Real-time measurements on a four stage RSFQ-counter with Nb–Al2O3–Nb Josephson junctions

RSFQ-toggle-flipflops with a SFQ-trigger circuit a Josephson transmission line at the input and a SFQ/dc-circuit at the output of each stage are implemented in the Nb–Al₂O₃–Nb Josephson junction technology on a single chip having coplanar wave guides at input and output. The counter is tested successfully at 4.2 K via coplanar/coaxial transitions using a bit pattern generator and a digital oscilloscope at room temperature up to f_I≈2 GHz pulse repetition frequency at the input. The highest test frequency f_I is limited by the available pattern generator.     

Calibration algorithm for 16-bit voltage-mode R–2R DAC

A novel calibration algorithm is presented for the 16-bit voltage-mode R–2R Digital-to-Analog converter (DAC). The proposed calibration can be realized with only some digital circuits and an additional calibrating DAC (CaliDAC) identical to the main DAC (MDAC) added. With the weighing-coefficient compressing technology (WCT) adopted, the nonlinearity of the CaliDAC can be compressed when the calibration is implemented, therefore leaving almost no effect on the output. Adopting the segment-calibration technology (SCT), the integral nonlinearity (INL) errors of the output can be calibrated segment by segment. With the proposed calibration algorithm, the INL errors in the final output can be calibrated successfully in the range of [−0.5LSB, 0.5LSB] for 16-bit voltage-mode R–2R DAC.     

MR images reconstruction based on TVWL2–L1 model

Compressive sensing (CS) theory, which has been widely used in magnetic resonance (MR) image processing, indicates that a sparse signal can be reconstructed by the optimization programming process from non-adaptive linear projections. Since MR Images commonly possess a blocky structure and have sparse representations under certain wavelet bases, total variation (TV) and wavelet domain ?₁ norm regularization are enforced together (TV-wavelet L1 method) to improve the recovery accuracy. However, the components of wavelet coefficients are different: low-frequency components of an image, that carry the main energy of the MR image, perform a decisive impact for reconstruction quality. In this paper, we propose a TV and wavelet L2–L1 model (TVWL2–L1) to measure the low frequency wavelet coefficients with ?₂ norm and high frequency wavelet coefficients with ?₁ norm. We present two methods to approach this problem by operator splitting algorithm and proximal gradient algorithm. Experimental results demonstrate that our method can obviously improve the quality of MR image recovery comparing with the original TV-wavelet method.     

Bioinspired High Tolerant Vein–Membrane Al2O3 Coating

The vein–membrane structure inspired by insect wings is applied to the artificial structural design of materials, with the aim of enhancing the intrinsic low strain tolerance and bonding strength of Al₂O₃ coating. Al₂O₃ platelets with high aspect ratio are used as veins, forming a bioinspired vein–membrane structure along with the Al₂O₃ membrane. Further, Al₂O₃ platelets take “root” in the substrate, forming a bioinspired root–soil interlocking structure. The greatest highlight of this coating is that it can restrict the cracks to the submicron area enclosed by the vein. The multi-bioinspired structure enabled the Al₂O₃ coating to sustain strains as high as 8%, which is 2 orders of magnitude higher than the reported failure strain of Al₂O₃ film. The bonding strength is also increased by at least five times. The vein–membrane structure opens a novel technological space for the artificial structural design of materials.