Nanocrystalline Si Research Articles

Microstructure, Particle Size, and Magnetic Property of Fe-6.5 wt.% Si Nanocrystalline Alloys Prepared by Mechanical Alloying

Microstructure, Particle Size, and Magnetic Property of Fe-6.5 wt.% Si Nanocrystalline Alloys Prepared by Mechanical Alloying

Electrochemical Evolution of Ru-Based Polyoxometalates into Si,W-Codoped RuOx for Acidic Overall Water Splitting.

Despite intensive studies over decades, the development of electrocatalysts for acidic water splitting still relies on platinum group metals, especially Pt and Ir, which are scarce, expensive, and poorly sustainable. Because such problems can be alleviated, Ru-based bifunctional catalysts such as rutile RuO2 have recently emerged. However, RuO2 has a relatively low activity for hydrogen evolution reactions (HER) and low stability for oxygen evolution reactions (OER) under acidic conditions. In this study, the synthesis of a RuOx -based bifunctional catalyst (RuSiW) for acidic water splitting via the electrochemical evolution from Ru-based polyoxometalates at cathodic potentials is reported. RuSiW consists of the nanocrystalline RuO2 core and Si,W-codoped RuOx shell. RuSiW exhibits outstanding HER and OER activity comparable to Pt/C and RuO2 , respectively, with high stability. Computational analysis suggests that the codoping of RuOx with W and Si synergistically improves the HER activity of otherwise poor RuO2 by shifting the d-band center and optimizing atomic configurations beneficial for proper hydrogen adsorption. This study provides insights into the design and synthesis of unprecedented bifunctional electrocatalysts using catalytically inactive and less explored elements, such as Si and W.

Electronic structure of P-type amorphous silicon nanowires

Silicon nanowires can improve broadband optical absorption and reduce the radial carrier collection distance in solar cell devices. The disordered nanowire arrays grown by the Plasma-Enhanced Chemical Vapor Deposition method are attractive because they can be embedded on low-cost substrates such as glass, and are compatible with large areas. Here, we experimentally demonstrate that reactive Hydrogen ions with increasing concentrations are doped to construct nanowire architectures in amorphous silicon solar cells. Similar to our investigated planar a-Si: H layers, the amorphous silicon nanowires exhibit a loss function coefficient of about 105/cm. From the reflectivity function, it can be shown that the nanostructures can offer a reliable carrier pool. Our results show that the addition of nanowires can increase the efficiency of a-Si solar cells from 1.11% to 1.57%. The input-photon-to-current conversion efficiency spectrum shows effective carrier collection from 1.2 to 2.2 eV of incident light and the nanowire devices show an increase in short-circuit current of 15% with amorphous Si and 26% with nanocrystalline Si compared to planar devices appropriate.

Numerical modeling based on the analytical solution of the steady-state small signal photocarrier grating equations and analysis of field-dependent measurements for noncrystalline Si: H samples

The available experimental data of field-dependent steady-state photocarrier grating (SSPG) are numerically modeled employing the derived formula of [Formula: see text] (where [Formula: see text] represents the ratio of coherent to incoherent currents in SSPG technique) obtained from the analytical solution of the small signal steady-state grating problem by applying Galerkin’s method. This numerical analysis shows the extent of validity and success of using the derived formula of [Formula: see text] [R. I. Badran, Results Phys. 17, 103079 (2020)] in reproducing the field-dependent data for noncrystalline Si: H samples. The best quality of fittings is reproduced when [Formula: see text] value is minimum. This analysis provides us with approximate and corrected values for each of transport quantities, namely, the small-signal response lifetime, electron and hole mobilities that are found in agreement with the values obtained from the applications of other methods. For two polymorphous (pm) Si: H samples, the average estimated, corrected values for the carrier density and mobility-lifetime product, namely, [Formula: see text][Formula: see text]cm[Formula: see text] and [Formula: see text][Formula: see text]cm[Formula: see text] and ([Formula: see text][Formula: see text]cm2V[Formula: see text] and [Formula: see text][Formula: see text]cm2V[Formula: see text], respectively, are obtained. The analysis is also extended to analyze the field-dependent data for nanocrystalline (nc) Si: H samples at low temperatures. The extracted values of [Formula: see text] and ([Formula: see text] are [Formula: see text][Formula: see text]cm[Formula: see text] and [Formula: see text][Formula: see text]cm2V[Formula: see text].

Sputtering Codeposition and Metal-Induced Crystallization to Enhance the Power Factor of Nanocrystalline Silicon.

The power factor of highly boron-doped nanocrystalline Si thin films with controlled doping concentration is investigated. We achieve a high degree of tuning of boron content with a charge carrier concentration from 1018 to 1021/cm3 and with the electrical conductivity by varying the boron magnetron power from 10 to 60 W while maintaining the power of a SiB source constant during codeposition from two independent sputtering sources. Along with the increase in the electrical conductivity with increased boron doping, we observe a steady decrease in the Seebeck coefficient from 500 to 100 μV/K. These values result in power factors that exhibit a marked maximum of 5 mW/K2m for a carrier concentration of around 1021/cm3 at room temperature. Temperature-dependent measurements up to 650 °C show, with increasing doping concentration, a change of the resistivity from a semiconducting to a metallic behavior and an increase of both Seebeck coefficient and power factor, with this last one peaking at 9.8 mW/K2m in the 350-550 °C temperature range. For higher concentrations, scanning electron microscopy and energy-dispersive X-ray spectroscopy show a partial segregation of boron on particles on the surface. These results exemplify the great advantage of sputtering codeposition methods to easily tune and optimize the thermoelectric performance in thin films, obtaining in our specific case highly competitive power factors in a simple and reliable manner.

Controlling the Carrier Injection Efficiency in 3D Nanocrystalline Silicon Floating Gate Memory by Novel Design of Control Layer.

Three-dimensional NAND flash memory with high carrier injection efficiency has been of great interest to computing in memory for its stronger capability to deal with big data than that of conventional von Neumann architecture. Here, we first report the carrier injection efficiency of 3D NAND flash memory based on a nanocrystalline silicon floating gate, which can be controlled by a novel design of the control layer. The carrier injection efficiency in nanocrystalline Si can be monitored by the capacitance-voltage (C-V) hysteresis direction of an nc-Si floating-gate MOS structure. When the control layer thickness of the nanocrystalline silicon floating gate is 25 nm, the C-V hysteresis always maintains the counterclockwise direction under different step sizes of scanning bias. In contrast, the direction of the C-V hysteresis can be changed from counterclockwise to clockwise when the thickness of the control barrier is reduced to 22 nm. The clockwise direction of the C-V curve is due to the carrier injection from the top electrode into the defect state of the SiNx control layer. Our discovery illustrates that the thicker SiNx control layer can block the transfer of carriers from the top electrode to the SiNx, thereby improving the carrier injection efficiency from the Si substrate to the nc-Si layer. The relationship between the carrier injection and the C-V hysteresis direction is further revealed by using the energy band model, thus explaining the transition mechanism of the C-V hysteresis direction. Our report is conducive to optimizing the performance of 3D NAND flash memory based on an nc-Si floating gate, which will be better used in the field of in-memory computing.

Effects of the Gas-Atomization Pressure and Annealing Temperature on the Microstructure and Performance of FeSiBCuNb Nanocrystalline Soft Magnetic Composites.

FeSiBCuNb powders prepared by the gas atomization method generally exhibit a wide particle size distribution and a high degree of sphericity. In addition, the correspondingly prepared nanocrystalline soft magnetic composites (NSMCs) perform good service stability. In this paper, effects of the gas-atomization pressure and annealing temperature on the microstructure and soft magnetic properties of FeSiBCuNb powders and NSMCs are investigated. The results show that the powders obtained by a higher gas-atomization pressure possess a larger amorphous ratio and a smaller average crystallite size, which contribute to the better soft magnetic performance of the NSMCs. After being annealed at 550 °C for 60 min, the NSMCs show a much better performance than those treated by the stress-relief annealing process under 300 °C, which indicates that the optimization of the soft magnetic properties resulting from the precipitation of the α-Fe(Si) nanocrystalline largely overwhelms the deterioration caused by the grain growth of the pre-existing crystals. In addition, the annealed NSMCs prepared by the powders with the gas-atomization pressure of 4 MPa show the best performance in this work, μe = 33.32 (f = 100 kHz), Hc = 73.08 A/m and Pcv = 33.242 mW/cm3 (f = 100 kHz, Bm = 20 mT, sine wave).

Effects of Si on the microstructure, soft magnetic properties and bendability of rapidly-annealed nanocrystalline Fe–Si–B–P–Cu alloy ribbons

The high Fe-content Fe–Si–B–P–Cu nanocrystalline alloys obtained by annealing the amorphous/nanocrystalline precursors are brittle and their magnetic properties are sensitive to the annealing process, which are disadvantageous for the practical applications of the alloys. In this study, the effects of Si on microstructure, soft magnetic properties and bendability of the rapidly-annealed Fe83SixB14−xP2Cu1 (x = 2–6 at%) nanocrystalline ribbons were investigated, and the properties of the nanocrystalline alloys with different Si contents were discussed in correlation to the structural evolution and element distribution. It was found that the partial substitution B with Si in the Fe–Si–B–P–Cu alloy system could promote the primary crystallization of α-Fe(Si) grains and suppress the precipitation of the hard-magnetic Fe-metalloid phase, thus widening the optimum annealing temperature (Top) ranges for preparing the alloys with high saturation magnetization (Bs) and low coercivity (Hc). The alloys rapidly annealed at Top for 30 s possess fine and uniform amorphous/α-Fe(Si) nanocrystalline structure, and exhibit good soft magnetic properties including high Bs of 1.80–1.85 T and low Hc of 5.2–7.2 A/m. Furthermore, the atom probe tomography (APT) results indicated that the distribution of Si in the nanocrystalline alloys is more uniform than that of B. The substitution of B with Si could decrease the compositional inhomogeneity, contributing to an improvement of bendability for the alloys with higher Si contents.

Theoretical study of High-pressure compression of Nano Sized Germanium and Silicon

High pressures have been employed to examine the pressure-induced structural and mechanical characteristics of nanocrystalline Ge and Si with different grain sizes in the current work. Volume collapse in nano and bulk phases at low and high pressures was determined by authors using the isothermal equation of states Vineet, Birch-Murnaghan, and Suzuki based on a molecular dynamic’s simulation model. The acquired result was found that the high-pressure properties depend meaningfully on the particle sizes, the 4.1-nm Si nanoparticle skilled a higher decrease in volume value than the diameter of 10.3 nm but 13nm Ge particle shows a smaller reduction in volume than 49nm and 100nm sizes nanoparticles. The results of the various EOSs agreed with one another, suggesting that the EOSs of bulk materials may be used in the computations of nanoparticles.

Femtosecond Laser Fabrication of Anisotropic Structures in Phosphorus- and Boron-Doped Amorphous Silicon Films.

Femtosecond laser-modified amorphous silicon (a-Si) films with optical and electrical anisotropy have perspective polarization-sensitive applications in optics, photovoltaics, and sensors. We demonstrate the formation of one-dimensional femtosecond laser-induced periodic surface structures (LIPSS) on the surface of phosphorus- (n-a-Si) and boron-doped (p-a-Si) amorphous silicon films. The LIPSS are orthogonal to the laser polarization, and their period decreases from 1.1 ± 0.1 µm to 0.84 ± 0.07 µm for p-a-Si and from 1.06 ± 0.03 to 0.98 ± 0.01 for n-a-Si when the number of laser pulses per unit area increases from 30 to 120. Raman spectra analysis indicates nonuniform nanocrystallization of the irradiated films, with the nanocrystalline Si phase volume fraction decreasing with depth from ~80 to ~40% for p-a-Si and from ~20 to ~10% for n-a-Si. LIPSS' depolarizing effect, excessive ablation of the film between LIPSS ridges, as well as anisotropic crystalline phase distribution within the film lead to the emergence of conductivity anisotropy of up to 1 order for irradiated films. Current-voltage characteristic nonlinearity observed for modified p-a-Si samples may be associated with the presence of both the crystalline and amorphous phases, resulting in the formation of potential barriers for the in-plane carrier transport and Schottky barriers at the electric contacts.

Varactor diodes of hydrogenated nanocrystalline Si with hydrogenated amorphous SiC prepared by PECVD

Hetero-structural barrier varactor diodes (HBVDs) can be widely used as variable capacitors, frequency multipliers, parametric amplifiers, etc. in electronic systems. Herein, n-type hydrogenated nano-crystalline Si (n-nc-Si:H) films with narrow band-gap and un-intentional doped hydrogenated amorphous SiC (a-SiC:H) ones with wide band-gap are adopted to innovatively design the n-nc-Si:H/a-SiC:H/n-nc-Si:H-based HBVDs. Two periodic hetero-structures with different doped nc-Si:H modulation layers and different thickness a-SiC:H barriers are severally fabricated by plasma enhanced chemical vapor deposition (PECVD), the optical band-gaps of n-nc-Si:H and a-SiC:H are derived from the ultra violet-visible (UV–vis) absorption spectra, the prepared devices are demonstrated as the HBVDs by measuring the relations of current density–voltage and capacitance–voltage, respectively. The conduction mechanisms of the fabricated devices in different biased voltage ranges are probed, the variation of capacitance modulation ratio with changing the doped-level in modulation layers and thickness of barriers are analyzed. This paper will be helpful to extend the material combination and preparation method for HBVDs.

Effect of microstructural parameters of ball-milled Si powder on reactivity with water to produce H2: Way forward for on-demand H2 production

Silicon (Si) nanoparticles prepared by ball-milling technique can show many exciting properties including improved reactivity with water to produce hydrogen (H2) compared to that of the bulk. This is attributed to the change in microstructural parameters such as particle size and dislocation density of nano-crystalline Si during the milling process. The understanding of such effects is of great importance for the technological applications of these Si nanoparticles in H2 generation from water for fuel cells, and our present work deals with this crucial aspect. In our present investigation, we found that ball-milling technique can be used to tailor the microstructural parameters of milled Si sample where milling time can greatly influence the particle size and dislocation density. In water-splitting experiment, we noticed that these microstructural parameters as well as state of agglomeration of milled Si samples in water have profound effect on the reactivity of Si with water to evolve H2. Furthermore, selecting the right alkali can speed up such H2 release, making rapid on-demand production of H2 viable, particularly for fuel cell applications.

Improvement of blue response of black Si solar cells due to graded band structure

We have investigated characteristics of energy band structure of a nanocrystalline Si (nc-Si) layer produced by the surface structure chemical transfer (SSCT) method, especially its effects on quantum efficiency of crystalline Si solar cells. In the nc-Si layer, the size of Si nanocrystals decreases toward the surface because Si dissolves nonuniformly from the surface during the SSCT reaction. The valence band maximum of the outermost surface of the nc-Si layer is located at 0.1–0.3 eV lower than that of Si bulk, and the conduction band minimum increases by 0.17–0.25 eV toward the surface. These results indicate that the nc-Si layer has a graded band structure in which the band-gap energy gradually increases toward the surface. By optimizing the SSCT reaction conditions, thinner thickness (∼100 nm) of the nc-Si layer and smaller size of Si nanocrystals can be obtained, resulting in a steep graded band structure. Because of reduction of carrier recombination rate in the surface region by the steep graded band structure, a high internal quantum efficiency over 80% in the 300–400 nm wavelength region is obtained.

Improvement of ultralow reflectance Si solar cells by machine-learning-assisted optimization for diffusion process

An improvement in solar cell characteristics of simple structured p-Si based solar cells has been achieved with an optimal diffusion process determined using Bayesian optimization. The simple solar cells possess a nanocrystalline Si (nc-Si) layer on the surfaces, which is formed with the Surface Structure Chemical Transfer method. A pn-junction and a boron doped back-surface-field (B-BSF) are formed by co-diffusion of phosphorus and boron atoms from phosphosilicate glass (PSG) and boron-containing compounds on the front and the rear surfaces, respectively. During the heat treatment of the diffusion process, the surfaces of the nc-Si layer are passivated with PSG. Bayesian optimization (BO) of the diffusion process has been performed to improve implied open-circuit voltage values which mainly depend on the pn-junction, the B-BSF, and PSG passivation qualities. Using BO, three experimental parameters of the diffusion process have been optimized in just 15 experiments. The best cell characteristics have been obtained by an improvement of PSG passivation.

Integration of Si Heterojunction Solar Cells with III-V Solar Cells by the Pd Nanoparticle Array-Mediated "Smart Stack" Approach.

This paper describes the way to fabricate two-terminal tandem solar cells using Si heterojunction (SHJ) bottom cells and GaAs-relevant III-V top cells by "smart stack", an approach enabling the series connection of dissimilar solar cells through Pd nanoparticle (NP) arrays. It was suggested that placing the Pd NP arrays directly on typical SHJ cells results in poor tandem performance because of the insufficient electrical contacts and/or deteriorated passivation quality of the SHJ cells. Therefore, hydrogenated nanocrystalline Si (nc-Si:H) layers were introduced between Pd NPs and SHJ cells to improve the electrical contacts and preserve the passivation quality. Such nc-Si:H-capped SHJ cells were integrated with InGaP/AlGaAs double-junction cells, and a certified efficiency of 27.4% (under AM 1.5 G) was achieved. In addition, this paper addresses detailed analyses of the 27.4% cell. It was revealed that the cell had a relatively large gap at the smart stack interface, which limited the short-circuit current density (thereby the efficiency) of the cell. Therefore, higher efficiency would be expected by reducing the interfacial gap distance, which is governed by the height of the Pd NPs.

Effects of annealing temperature on the structure characteristics of Fe83Si6B6Cu1Nb1P1.5C1.5 amorphous ribbons

As an excellent soft magnetic material with high magnetic permeability and low coercivity, Fe-based amorphous alloys are widely applied in power electronics and other fields, which greatly depend on its microstructure. In the current work, the microstructure and magnetic properties of Fe83Si6B6Cu1Nb1P1.5C1.5 amorphous ribbons annealed at different temperatures were systematically studied by X-ray diffraction (XRD) and atom probe tomography (APT). Results reveal that the α-Fe(Si) nanocrystalline phase preferentially nucleates on the surface of the pre-precipitated Cu clusters, which promotes the nucleation rate of α-Fe(Si) grains, by providing heterogeneous nucleation sites. The amorphous forming elements of B, P, and C elements exist around α-Fe(Si) phase in the forms of residual amorphous, forming a core-shell structure, which confined the growth of nanocrystalline α-Fe(Si) grains at high temperature. The optimum saturation magnetization with the highest value of the ribbons is up to 184.25 emu/g for samples annealed at 550 °C for 30 min.

Phase stability and the interface structure of a nanoscale Si crystallite in Al-based alloys.

An atomic-scale understanding of the role of strain on the microstructural properties of nanoscale precipitates will be helpful to explore the precipitation behavior as well as the structure-property relationships in crystalline multi-phase systems. Nanoscale Si precipitates are formed in Al-based alloys prepared by selective laser melting. The phase structure and the nature of heterointerface have been characterized using advanced electron microscopy. The nanocrystalline Si mainly contains two polymorphs, diamond-cubic Si (DC-Si) and 4H hexagonal Si (4H-Si). Heteroepitaxy occurs at the DC-Si(111)/Al(100) and 4H-Si(0001)/Al(100) interfaces in terms of a coincidence-site lattice model. The nanocrystalline Si undertakes tensile strain superposed by the matrix through heterointerfaces, facilitating the formation of 4H-Si in the nanoscale crystallite, which provides a strategy for designing Si polymorphic materials by strain engineering.

FROM Si NANOWIRES TO Ge NANOCRYSTALS FOR VIS-NIR-SWIR SENSORS AND NON-VOLATILE MEMORIES: A REVIEW

"Nanocrystalline Si and Ge are ofhigh interestfor integrated Si photonics related to light emission, opticul sensors, photodetectors, solar energy harvesting and conversion devices, and also forfloating gate non-volatile memories (NVMs). In this review, we have focused on nanocrystalline porous Si (nc-PS) with extension to Si nanodots, and Ge nanocrystals (NCs)Zquantum dots (QDs)/nanoparticles (NPs) embedded in oxides (SiCh, TiCE, HfCh, AI2O3). The great asset ofnc-PS is its intense photoluminescence in VIS at room temperature (RT), while Ge NCs/NPs embedded in oxides show high photosensitivity in VISNIR-SWIR in the spectral photocurrent up to 1325 nm at RT. Ge NCs/NPs/QDsfloating gate NVMs present high memory performance, the retention characteristics corresponding to the state of the art for NCs floating gate NVMs. We prove the relevance of controlling the preparation parametersfor obtainingfilms with targetedphotoluminescence, photosensitivity and charge storage properties for applications, e.g. VIS-NIR-SWIR optical sensors and photodetectors, and electronic and photoelectric NVMs. We evidence the correlation of preparation conditions, morphology, composition and crystalline structure with optical, electrical, photoelectrical and charge storage properties and also evidence the contribution ofquantum confinement effect, localized States and trapping centers."

A volume plasmon blueshift in thin silicon films embedded within Be/Si periodic multilayer mirrors.

Microstructural properties of the beryllium (Be) and silicon (Si) in periodic multilayer mirrors Be/Si with the variation of film thickness were comprehensively determined by Raman scattering. For the thinner films, the structure of Be evolved in the amorphous phase, and it was transformed into the polycrystalline phase for thicker films. The Si films in the periodic structure were condensed into the amorphous phase. The small fraction of nanocrystalline Si particles was distributed within the amorphous phase. A shake-up satellite peak of Si 2s photoelectrons was observed in X-ray photoelectron spectroscopy which suggested the excitation of a plasmon in Si films embedded within Be/Si periodic multilayers. The energy of plasmons was sensitive to the film thickness of Si in the periods which directly corresponds to the particle size. The binding energy of the satellite peak of Si 2s photoelectrons was blueshifted (higher energy) with a decrease in the particle size. This was explained by size dependent quantum confinement of particles.

Exceptional thermoelectric power factors in hyperdoped, fully dehydrogenated nanocrystalline silicon thin films

Single-crystalline silicon is well known to be a poor thermoelectric material due to its high thermal conductivity. Most excellent research has focused on ways to decrease its thermal conductivity while retaining acceptably large power factors (PFs). Less effort has been spent to enhance the PF in poly- and nanocrystalline silicon, instead. Here, we show that in boron-hyperdoped nanocrystalline thin films, the PF may be increased up to 33 mW K−2 m−1 at 300 K when hydrogen embedded in the film during deposition is removed. The result makes nanocrystalline Si a realistic competitor of Bi2Te3 for low-temperature heat harvesting, also due to its greater geo-availability and lower cost.