Si Structures Research Articles

High‐Sensitivity Infrared Photoelectric Detection Based on WS2/Si Structure Tuned by Ferroelectrics

AbstractAs one of the typical transition‐metal dichalcogenides with distinct optical and electrical properties, WS2 exhibits tremendous potential for optoelectronic devices. However, its inherent band gap range limits the application in the infrared region. To overcome this draw‐back and improve the sensitivity, P(VDF‐CTFE) is used as a ferroelectric gate to control the states of WS2/Si junctions and achieve an enhanced infrared photodetection. The polarization electric field not only broadens the range of absorption wavelength (405–1550 nm) but also greatly promotes the sensitivity of lateral photovoltaic effect (LPE) (from 198.6 to 503.2 mV mm−1). This phenomenon is attributed to the reduction of WS2 band gap and the change of potential barrier at the interface of the junction. Meanwhile, the response speed is improved significantly due to the increase of carrier initial kinetic energy. This new scheme for ferroelectric tuned LPE opens up a way to realize high‐sensitivity, ultrafast, and stable infrared photodetection.

Strain—Engineered Asymmetrical Si/Si1–xGex IR-Photo-Detector: Theoretical Reliability and Experimental Feasibility Studies

This paper reports the prospect of strained Si/Si1-xGex nano-scale p++-i-n++ photo-detector in the infrared (IR) wavelength region (1000nm-2200nm). The proposed device is capable of offering high photo-responsivity and quantum efficiency at 1600 nm wavelength in comparison to the unstrained conventional Si based pin photo-detector device. The authors have developed a Quantum Modified Drift Diffusion Model to analyze the nano-scale properties of strained Si photo-detector device. The analysis reveals that the strained Si based pin photo-detector offers high photo responsivity 0.67 A/W at 1600nm wavelength and quantum efficiency 0.60 at the same wavelength compared to the conventional unstrained Si based photo-detector. The enhancements of the photo electric characteristics of the Si photo device are obtained by incorporating selective amount of Ge into the Si active region. The electrical properties in the strained Si semiconductor at room temperature (300K), have made this material more attractive for being used as a photodetector. Due to the incorporation of this Ge layer, in-plane biaxial strain is produced in the active region. This phenomenon enhances the out-of-plane mobility of charge carriers. In this current research, in-plane strain is used to boost up the outplane mobility depending on the geometrical structure of the active region. The validity of the proposed model is established by comparing the simulated data with the corresponding experimental results. The authors also report the reliability study of the proposed device in details. Further, the authors have developed a 4x4 array type photo-detector based on the strained Si structure and the results are compared with the corresponding single array type photo-detector. In case of the 4x4 array type photo detector, the photo electric characteristics i.e. photo responsivity and quantum efficiency increase to 0.74 A/W and 0.69 respectively at 1600nm wavelength. In IR wavelength region, the designed photo-sensor depicts a significant improvement of performance compared to the available photo-detector in published research work. The proposed photo-detector can play an important role in Biomedical, Defence and Agro industries. According to the authors’ knowledge, this is the first report on strain engineered nano scale asymmetrical Si/Si1-xGex based photo-detector array.

Chirality-selective all-dielectric metasurface structural color display

The polarization dependent switchable structural coloration has shown a prominence for its unnecessity of changing the structure itself to achieve tunable color displays. Nevertheless, a chirality-selective structural color display has been rarely elucidated. Here, we suggest a chirality-selective reflective structural color display under perpendicularly incident light based on all-dielectric metasurfaces. We first investigate a chiral response of a subwavelength thickness two-dimensional (2D) amorphous silicon (Si) structure. The multipole decomposition followed by the electromagnetic field distribution analysis explained the chirality-selective response of the metasurface with the chirality-selective excitation of magnetic dipole (MD) and electric quadrupole (EQ). We then analyzed the structural dependence of MD and EQ, finding a group of metasurfaces which can span the entire visible spectrum under left circularly polarized (LCP) light and show dark, faded colors under right circularly polarized (RCP) light. Our result provides design criteria for chirality-selective all-dielectric structural color displays, applicable to energy and time efficient real-time color switching displays.

Hydrogen storage capability of porous silicon powder fabricated from Al–Si alloy

Porous silicon powder has been fabricated from an Al–Si alloy by etching in strong acids, and its hydrogen storage capacity has been studied. Double acid etching in the fabrication and nickel blending have been proposed to further enhance the hydrogen storage capacity of the etched powder. The etching effectively removed the aluminum from the Al–Si alloy to form a porous Si structure, with a large increase in surface area. After charging under a hydrogen pressure of 3.7 MPa at 150 °C, the hydrogen uptake of double-etched and Ni-blend porous silicon samples was 0.81 wt%, which was >13 times higher than that of the as-received Al–Si alloy (0.06 wt%). The percentage of retained hydrogen after desorption was also largely decreased. The results of this study provide strong evidence on the potential of using porous silicon powder from Al–Si alloy as a cost-effective solid-state storage material for hydrogen.

Lithiation and Sodiation of Hydrogenated Silicene: A Density Functional Theory Investigation.

The next-generation renewable energy machineries necessitate the electrodes with appropriate electrochemical performance. Here, the anodic properties of silicane for Li- and Na-ion batteries were scrutinized employing first-principle calculations. The projected single-layer hydrogen-functionalized Si (Si2 H2 ) structure was energetically, mechanically, dynamically, and thermally stable based on theoretical simulations, confirming its experimental feasibility. The electronic properties revealed the semiconducting nature of silicane on the basis of PBE and HSE06 schemes with an indirect bandgap. As anode material for Li- and Na-ion batteries, hydrogenated silicene showed promising electrochemical performance because of the proper adsorption strength between Si2 H2 and the adsorbed Li and Na. The average open circuit voltages for Li2x Si2 H2 and Na2x Si2 H2 were as low as 0.42 and 0. 64 V, while its specific capacity was as high as 921 and 1842 mAh g-1 for Li and Na, respectively. It also showed ultra-fast diffusion channels for Li and Na ions. The diffusion barriers for Li and Na migrations were as low as 0.18 and 0.14 eV, respectively, which revealed rapid charge/discharge processes using hydrogenated silicene as anode. These important features facilitate silicane as favorable anode material for Li/Na-ion batteries.

Получение многослойных периодических структур CaF2/Si/CaF2/Si(111), обладающей фотолюминесценцией в видимой области спектра

The paper presents a method for obtaining some multilayer periodic CaF2/Si/CaF2/Si (111) structures with an emissivity in the visible region of the spectrum. A feature of this technique is that the deposition of Si and CaF2 layers was carried out at room temperature, followed by pulsed an-nealing. The photoluminescence spectra of the grown sample were obtained and measured at excitation wavelengths of 325 nm and 405 nm. The study of low-temperature photoluminescence was carried out with a laser with a wave-length of 405 nm, since no radiation in the structure was observed upon excitation with a laser with a wavelength of 325 nm. The photoluminescence spectra measured at room temperature and liquid nitrogen temperature are presented, according to which the shift of the photoluminescence maximum by 20 nm was established. The FWHM decreased from 284 nm to 210 nm with decreasing tem-perature.

Effect of high temperature solution heat treatment time on quality index and morphology of A356 DC alloy

Al-Si die casting Aluminum Alloys, such as A380 and A356 are extensively used in automotive die casting applications due to low thermal expansion coefficient and excellent mechanical properties. The relatively high Silicon (Si) content is inevitable in these alloys due to its role in increasing fluidity and reducing shrinkage. However, Si-phase morphology often have an impact on the mechanical properties. This study aims to highlight how altering the morphology of Si phase in A356 can influence the mechanical properties by the mean of solution heat treatment. High temperature solution heat treatment was applied for 1, 4, 6, and 8 h to investigate the effect on mechanical properties and morphology of A356. The solutionization results in significant improvement to the alloy Quality Index. The solution treatment also results in lowering ultimate tensile strength (UTS) while increasing the elongation of the alloy. The microscopic observation reveals that lamellar eutectic Si structure is broken upon solution heat treatment and subsequently spheroidizes into smaller granular structure, which results in improvement of the elongation and Quality Index.

Diffusion‐Triggered Synthesis of Mg2Si Based on Infiltrated Silicon Structures in Molten Mg

Magnesium silicide (Mg2Si) has many good properties and therefore shows a high application potential as thermoelectric material. Herein, the intermetallic layer growth behavior in the Mg–Si system is investigated. A process for synthesizing Mg2Si on the surface of open‐pore cellular Si structures is developed. The structure is fabricated by investment casting and followed by infiltration of magnesium. Subsequently, magnesium silicide is obtained by a two‐step heat treatment. Complete synthesis of Si to Mg2Si is achieved using a liquid–solid reaction between the liquid Mg and solid Si. The optimal heat treatment for the first step is at the eutectic temperature of 950 °C and for a duration of 120 min. The second treatment is at 1150 °C, slightly above the melting point of Mg2Si, and the remaining Si is synthesized. The resulting microstructures are analyzed using a scanning electron microscope (SEM) and energy‐dispersive spectroscopy (EDS).

Effect of B2O3 on the Melt Structure and Viscosity of CaO–SiO2 System

Molecular dynamics (MD) simulations have been used to study the structure properties of CaO–SiO2 slags and CaO–SiO2–B2O3 slags with different basicity. The results show that the addition of B2O3 to the higher basicity system of CaO–SiO2 (basicity >1.12) increases the degree of polymerization (DOP) of the system and the complexity of network structures, theoretically leading an increase in the viscosity of the slag. In the CaO–SiO2–B2O3 ternary system, the structure of Si–BO (bridging oxygen)–B is more dominant than B–BO–B, indicating that the newly introduced B–O network structure will be randomly mixed with the initial Si–O network structure instead of forming enrichment zones. Moreover, with the addition of B2O3, the viscosities of CaO–SiO2 increase in higher basicity systems, but decrease in the lower basicity systems based on the viscosity measurement. The different effect of adding B2O3 on the viscosity of the CaO–SiO2 slag is consistent with the changing trend of the melt structure.

A theoretical study of aluminium doping in silicon anode based lithium‐ion batteries using ReaxFF molecular dynamics simulation

Silicon and Aluminium are abundant elements in nature. Silicon materials have higher capacity that can be used as a replacement for the graphite anode. However, due to mechanical failure and self-degradation during battery operation, Si is not suitable to be used as an anode in LIBs. The cycle performance of LIBs depends on the diffusion of Li within anode material. The high energy barrier of the Si structure results in lesser kinetic transport of Li, hence Si anode is not feasible to be used in LIBs. In the past, many approaches were proposed to improve the rate performance of Si anode that includes doping, porosity, amorphicity, and geometry tailoring. In this paper, the ReaxFF interatomic potential and MD simulations were conducted to study the kinetic behavior of aluminium doped amorphous lithiated silicon (a-LixSiAly) compounds. The results suggest that by doping of aluminium, the coefficient of diffusion improves, although there is no relation found between the amount of dopant and diffusivity. We replaced dopant aluminium with alumina and studied the structural and kinetic behavior of silicon anode based lithium-ion batteries (LiSi). Our results show that the specific amount of alumina can improve diffusivity but is not as significant as in the case of aluminium. We used hybrid GCMC/MD simulations to compute the open-voltage profiles during the cell discharge operation. These simulation results provide an interesting relation between Li and Al content in the compound for voltage discharge profile. Doping of aluminium and its amphoteric oxide can increase the diffusion coefficient of Li in LiSi batteries. A specific amount of doping improves the battery discharge voltage profile. To the best of the authors understanding, not much research has been carried out on this idea. The present research results suggest the improved capacity and cycle performance of batteries. The voltage profile diagrams suggest an exponential relation between aluminium and lithium content while doping, and this can be used in the industrial manufacture of LiSi batteries. The Al2O3 doping results in a parabolic curve, which clarifies the specific amount of dopant that can lead to the highest performance.

Optimization of Metal-Assisted Chemical Etching for Deep Silicon Nanostructures.

High-aspect ratio silicon (Si) nanostructures are important for many applications. Metal-assisted chemical etching (MACE) is a wet-chemical method used for the fabrication of nanostructured Si. Two main challenges exist with etching Si structures in the nanometer range with MACE: keeping mechanical stability at high aspect ratios and maintaining a vertical etching profile. In this work, we investigated the etching behavior of two zone plate catalyst designs in a systematic manner at four different MACE conditions as a function of mechanical stability and etching verticality. The zone plate catalyst designs served as models for Si nanostructures over a wide range of feature sizes ranging from 850 nm to 30 nm at 1:1 line-to-space ratio. The first design was a grid-like, interconnected catalyst (brick wall) and the second design was a hybrid catalyst that was partly isolated, partly interconnected (fishbone). Results showed that the brick wall design was mechanically stable up to an aspect ratio of 30:1 with vertical Si structures at most investigated conditions. The fishbone design showed higher mechanical stability thanks to the Si backbone in the design, but on the other hand required careful control of the reaction kinetics for etching verticality. The influence of MACE reaction kinetics was identified by lowering the oxidant concentration, lowering the processing temperature and by isopropanol addition. We report an optimized MACE condition to achieve an aspect ratio of at least 100:1 at room temperature processing by incorporating isopropanol in the etching solution.

Excellent Bipolar Resistive Switching Characteristics of Bi4Ti3O12 Thin Films Prepared via Sol-Gel Process.

Herein, Bi4Ti3O12 (BIT) ferroelectric thin films were fabricated into Au/BIT/LaNiO3/Si structures to demonstrate their memristor properties. Repeatable and stable bipolar resistive switching (RS) characteristics of the device are first reported in this work. The switching ratio of the device annealed in air reached approximately 102 at 0.1 and −0.1 V. The RS performance was not significantly degraded after 100 consecutive cycles of testing. We also explored the factors affecting the RS behavior of the device. By investigating the RS characteristics of the devices annealed in O2, and in combination with XPS analysis, we found that the RS properties were closely related to the presence of oxygen vacancies. The devices annealed in air exhibited a markedly improved RS effect over those annealed in O2. According to the slope fitting, the conduction mechanism of the device was the ohmic conduction and space charge limited current (SCLC). This study is the first to successfully apply BIT ferroelectric films to the RS layers of memristors. Additionally, a theory of conductive filaments is proposed to adequately explain the relationship between RS behavior and oxygen vacancies, providing meaningful inspiration for designing high-quality resistive random access memory devices.

Investigation of Electrical Properties of the Al/SiO2/n++-Si Resistive Switching Structures by Means of Static, Admittance, and Impedance Spectroscopy Measurements.

In this study, the resistive switching phenomenon in Al/SiO2/n++-Si structures is observed and studied by means of DC, small-signal admittance, and complex impedance spectroscopy measurements. Possible transport mechanisms in the high and low resistance states are identified. Based on the results of the applied measurement techniques, an electrical equivalent circuit of the structure is proposed. We discuss the effect of parasitic elements influencing the measurement results and show that a proper model can give useful information about the electrical properties of the device. A good agreement between the characteristics of the proposed equivalent circuit and the experimental data, based on different measurement procedures, confirms the validity of the used methodology and its applicability to the electrical characterization of RRAMs.

Ultra-short pulsed laser powder bed fusion of Al-Si alloys: Impact of pulse duration and energy in comparison to continuous wave excitation

Laser assisted powder bed fusion of Al-40 wt%Si using ultra-short laser pulses is presented. The effects of the pulse duration as well as the pulse energy on the melt pool size, solidification morphology and material properties have been investigated. The experiments have been carried out with a mode-locked fiber laser system delivering pulses from 500 fs up to 800 ps at a wavelength of 1030 nm. Comparative investigations have been performed using a continuous wave Yb-fiber laser operating at 1070 nm. The results show that the melt pool width is reduced at shorter pulse durations and higher repetition rates while maintaining the same average power. Additionally, keyhole melting is achieved in pulsed operation after exceeding the threshold fluence for single pulse ablation. In comparison to continuous wave radiation, powder bed fusion using ultra-short laser pulses leads to a more uniform melt pool shape with refined primary Si and eutectic structure that is accompanied with an improvement of the mechanical properties.

Thermodynamic properties of propane and methane hydrates doped with sodium hydroxide

Pure and sodium hydroxide doped with single-component hydrates of methane and propane are studied using molecular and lattice dynamics methods. Vibration density of states and the dependence of Helmholtz free energy on temperature and cell volume are calculated. Dynamic stability of empty and filled by gas sI and sII structures doped with 1 or 2 NaOH molecule is shown in the wide range of temperature values. Comparison of free energy values allows calculating the thermal expansion coefficient and demonstrating the possibility of self-preservation effect in NaOH-doped methane and propane hydrates with ~1.8 and ~1.4 mol% of sodium hydroxide, respectively.

First-principles calculations and experimental study of Mo-doped porous nano-silicon anodes for Li-ion batteries

The poor electronic conductivity and inherent volume expansion pose challenges for Silicon (Si) to be commercialized. Herein, we investigate the influence of Mo-dopant on the electronic structure and crystal structure of Si using first-principle calculations. The results show that Mo-doping can expand the crystal lattice and reduce the bandgap of Si, which improves its intrinsic conductivity. Furthermore, Mo-doped porous nanostructured Si (Mo-PNSi) were first prepared through a hydrothermal method and magnesiothermic reduction. As a result, the obtained Mo-PNSi anodes depict a high specific capacity of 2197.7 mAh g−1 at a current density of 0.84 A g−1 and a high capacity retaining of nearly 100% from 30 to 200 cycles. In particular, Mo-PNSi anodes display superior cycling stability of 911.5 mAh g−1 after 1000 cycles at a current density of 4.2 A g−1. Even at a high rate of 6.72 A g−1, a specific capacity of 840.1 mAh g−1 can be delivered.

Structure-H hydrate of mixed gases: Phase equilibrium modeling and experimental validation

Clathrate hydrates are solid ice-like compounds that entrap the gas or liquid molecules into their cavities of hydrogen-bonded water molecules. As compared to commonly available sI and sII structures of clathrate hydrate, the sH-type is reported to possess more stability and gas uptake capacity. Despite copious implications of the sH-type hydrates of mixed gases in gas storage and separation, the phase equilibrium modeling to predict their formation condition is rarely available in the literature due to inaccurate estimation of the interactions between the guest components. In this view, we develop a novel methodology using the Wong-Sandler mixing rule extended for the three-parameter Patel-Teja equation of state, in association with the statistical thermodynamics based van der Waals and Platteeuw model to define the hydrate phase. This apart, the solubility of the help gases in water is estimated using the Patel-Teja model instead of the empirical correlations. It is observed that the proposed formulation is capable of predicting the experimental phase equilibrium data with a range of 1.389–2.812 %AARD for a variety of sH hydrates (i.e., neohexane, methylcyclopentane, isopentane, 2,2-dimethylpentane and a recently discovered first vapor phase heavy former tetrafluorocyclobutane) having methane, nitrogen or carbon dioxide as their co-guest.

Insight into solidification microstructure control by trace TiCN–TiB2 particles for yielding fine-tuned nanoprecipitates in a hypoeutectic Al–Si–Mg alloy

Solution heat treatment (SHT), immediately followed by water quenching, can significantly improve the strength of precipitation-hardenable metallic materials, but frequently compromises the well-defined dimensional design of the geometrically complex or thin-wall engineering components. Here, to avoid this catastrophic high-temperature scheme, we introduce an alternative conceptual strategy to design a novel hypoeutectic Al–10Si–Mg alloy especially for “as-cast” services, i.e., extraordinarily high Mg addition and trace co-incorporation of innovative in situ TiCN–TiB2 particles coupled with a direct post-solidification isothermal low-temperature stabilization treatment (LTST). The performance was carefully characterized by multi-scale characterization techniques. Solidification kinetics and the resultant microstructural arrangement response to the TiCN–TiB2 particle additions were systematically investigated. Results demonstrated that both α-Al dendrites and eutectic Si structures were substantially refined by the TiCN–TiB2 particles. In the absence of conventional SHT, the precipitation kinetics associated with the customized LTST was also thoroughly studied. Interestingly, an extremely fine dispersion and high number density of perfectly coherent GP zones and pre-β'' nanoprecipitates were found coexisting after LTST. As a result, well-combined strength and ductility were therefore achieved by the synergistic effects of trace TiCN–TiB2 particles and LTST in this Al–10Si-2.0 Mg system.

Molecular-level insights into the structure stability of CH4-C2H6 hydrates

The structure of CH4-C2H6 hydrates changes with gas composition and pressure. This work employs molecular dynamic simulation to explore the stability variation of CH4-C2H6 hydrates in sI and sII structures. The rotational and translational motions of water molecules reveal that the CH4-C2H6 hydrates are more stable in sI structure when the CH4 composition is 20 mol%, whereas sII hydrates are more stabilized with 90 mol% CH4. Cage occupancy greatly influences the motion of gas molecules. The motion of gas molecules is affected by the C2H6 molecules in the neighboring hydrate cages, reflected by the fluctuation amplitude of the rotational and translational curves. The fluctuation frequency in the rotational motion curves of C2H6 molecules is smaller than that of CH4, due to the higher probability of C2H6 getting close to the cage water molecules. The impact of pressure on the structure stability of CH4-C2H6 hydrates varies with gas composition.

The structural and optical properties of GO: Temperature-dependent analysis of the electrical properties of Al/GO/p-type Si semiconductor structures

A spin-coating process has been used to generate graphene oxide (GO) thin films on p-type silicon. The temperature-dependent basic electrical characteristics of Al/GO/p-type Si metal–semiconductor structures have been examined through current–voltage (I–V) measurements in the temperature range 80–420 K by increments of 20 K based on thermionic emission (TE) theory. At each temperature, the main electrical parameters, such as ideality factor (n), rectification ratio (RR), saturation current (I0), barrier height (Φbo), and series resistance (RS), were determined. With increasing temperature, the values of n, I0, RR, and RS decreased, whereas the value of Φbo increased. These unusual temperature fluctuations in Φbo and n may be rationalized in terms of twofold Gaussian distributions at 80–200 K and 220–420 K. For the high- and low-temperature regions, evaluation of these two Gaussian distributions of the I–V curves for the Al/GO/p-type Si structure yielded mean barrier heights of 1.676 and 0.555 eV with standard deviations (σ0) of 166 mV and 76 mV, respectively. Modified ln(I0/T2)−(q2σs2)/(2k2T2) vs. q/(kT) plots, which correspond to these two separate temperature regions, further corroborated these barrier height values. The value of the Richardson constant (A*), 3.567 × 10−6 A K−2 cm−2, is substantially lower than the known value for p-Si. However, for the distribution in the 220–420 K region, the Richardson constant of 48.069 A K−2 cm−2 is near to the known theoretical value of 32 A K−2 cm−2 for p-type Si.