Single Crystal Si Research Articles

Link between anisotropic electrochemistry and surface transformations at single‐crystal silicon electrodes: Implications for lithium‐ion batteries

AbstractSilicon is a promising negative electrode material for high‐energy‐density Li‐ion batteries (LiBs) but suffers from significant degradation due to the mechanical stress induced by lithiation. Volume expansion and lithiation in Si are strongly anisotropic but associated early interfacial transformations linked to these phenomena and their implications for electrode performance remain poorly understood. Here we develop a novel correlative electrochemical multi‐microscopy approach to study local interfacial degradation at the early stages for three different surface orientations of Si single crystals: Si(1 0 0), Si(1 1 0) and Si(3 1 1), after Li‐ion electrochemical cycling. The experimental strategy combines scanning electrochemical cell microscopy (SECCM) measurements with subsequently recorded scanning transmission electron microscopy images of high‐quality cross sections of Si electrodes, extracted at selected SECCM regions, using a novel Xe+ plasma‐focused ion beam procedure. These studies reveal significant surface orientation–dependent nanoscale degradation mechanisms that strongly control electrode performance. Si(1 0 0) was immune to interfacial degradation showing the best lithiation reversibility, whereas local nanoscale delamination was observed in Si(1 1 0) leading to a lower Coulombic efficiency. Continuous electrochemical deactivation of Si(3 1 1) was associated with delamination across the whole interface, Li trapping and formation of thick (ca. 60 nm) SiO2 structures. These results demonstrate surface crystallography to be a critical factor when designing Si‐based battery materials and strongly suggest that promoting Si(1 0 0) facets could potentially provide longer cycling life and performance due to a higher resistance to degradation.

Surface Morphology and Exchange Bias Anisotropy Studies in Large Area Deposited Co2FeSi Ir50Mn50 Multi Layers For Spintronic Applications

Surface morphology and magnetic properties of ferromagnetic Heusler alloy Co2FeSi thin films and their multi-layers with anti-ferromagnetic Ir50Mn50, which find applications in spintronic devices were investigated. The sputtering process flow for large area deposition of thin films on 3 inch size thermally oxidized single crystal Si(100)/SiO2 substrates have been developed by optimizing the sputtering geometry and other process parameters. A uniform film composition, thickness, smooth surface, good crystallinity and magnetic properties have been achieved in the films over 3-inch size wafers. The isotropic magnetic properties such as saturation/remanent magnetizations, coercivity were achieved in Co2FeSi films deposited on 3-inch size Si(100)/SiO2 wafers with 15 nm Cr buffer layer. An exchange bias anisotropy has been established in Co2FeSi/IrMn multilayer by magnetic annealing process using in-house made magnetic annealing set up. A maximum exchange bias anisotropy field, Hex of 178 Oe and low coercivity, Hc of 85 Oe has been achieved in the Co2FeSi/IrMn multilayer stacks suitable for magnetic tunnel junctions for spintronic applications.

Substrate and Excitation Intensity Dependence of Saturable Absorption in Perovskite Quantum Dot Films.

Saturable absorption in perovskite quantum dot (PQD) films, leading to saturation in photoluminescence (PL), is reported. PL of drop-casting films was used to probe how excitation intensity and host-substrate influence the growth of PL intensity. The PQD films were deposited on single-crystal GaAs, InP, Si wafers and glass. Saturable absorption was confirmed through PL saturation in all films, with different excitation intensity thresholds, suggesting strong substrate-dependent optical properties, resulting from absorption nonlinearities in the system. The observations extend our former studies (Appl. Phys. Lett., 2021, 119, 19, 192103), wherein we pointed out that the PL saturation in QDs can be used to create all-optical switches in combination with a bulk semiconductor host.

Thermally activated plastic deformation of Si single crystals at temperatures above 1173 K

The effect of high-temperature heat treatment during production on the mechanical property of Si wafer, namely yield stress, was studied through tensile tests of Si single crystals at 1123–1373 K, where stress–strain curves were obtained along the and directions. The critical resolved shear stress showed a remarkable temperature dependence, and it decreased with increasing test temperature. Below 1348 K, the activation enthalpy determined through strain rate jump tests showed no significant difference between crystals made by the Chokralski and the floating zone methods. This indicated that the solute oxygen does not influence the thermally activated dislocation glides. The activation enthalpy of 4.9 eV for a dislocation glide was much higher than that reported by previous studies (2.2 eV at temperatures below 1100 K), indicating that the thermal activation of dislocation glide is different from kink-pair nucleation. It was found that self-diffusion can assist the dislocation glide at temperatures above 1130 K.

Effects of excess electrons/holes on fracture toughness of single-crystal Si

This study demonstrates that bond strength can be enhanced by injecting excess electrons or holes into a material by electron beam irradiation. To determine the effect of excess electrons/holes on the interatomic bond strength, fracture toughness tests were performed on single-crystal Si micropillars under various electron-beam irradiation conditions. The fracture toughness under electron beam irradiation was 4%–11% higher than that under non-irradiated conditions. In particular, an increase in strength was large in tests performed under hole-injection conditions. Furthermore, in first-principles calculations of the tensile strength of excess electrons/hole-doped Si, the ideal tensile strength monotonically decreased with an injection in excess electrons and increased monotonically with the injection of holes. This is qualitatively consistent with the experimental result that the fracture toughness increases under hole-injection conditions.

Monodisperse Micro-Droplet Generation in Microfluidic Channel with Asymmetric Cross-Sectional Shape.

Micro-droplets are widely used in the fields of chemical and biological research, such as drug delivery, material synthesis, point-of-care diagnostics, and digital PCR. Droplet-based microfluidics has many advantages, such as small reagent consumption, fast reaction time, and independent control of each droplet. Therefore, various micro-droplet generation methods have been proposed, including T-junction breakup, capillary flow-focusing, planar flow-focusing, step emulsification, and high aspect (height-to-width) ratio confinement. In this study, we propose a microfluidic device for generating monodisperse micro-droplets, the microfluidic channel of which has an asymmetric cross-sectional shape and high hypotenuse-to-width ratio (HTWR). It was fabricated using basic MEMS processes, such as photolithography, anisotropic wet etching of Si, and polydimethylsiloxane (PDMS) molding. Due to the geometric similarity of a Si channel and a PDMS mold, both of which were created through the anisotropic etching process of a single crystal Si, the microfluidic channel with the asymmetric cross-sectional shape and high HTWR was easily realized. The effects of HTWR of channels on the size and uniformity of generated micro-droplets were investigated. The monodisperse micro-droplets were generated as the HTWR of the asymmetric channel was over 3.5. In addition, it was found that the flow direction of the oil solution (continuous phase) affected the size of micro-droplets due to the asymmetric channel structures. Two kinds of monodisperse droplets with different sizes were successfully generated for a wider range of flow rates using the asymmetric channel structure in the developed microfluidic device.

Raman spectroscopy of silicon, doped with platinum and irradiated by protons

In this work, the influence of proton irradiation and platinum impurities on the crystal structure of silicon samples was studied by Raman spectroscopy. It has been established that the doping of single crystals of Si with platinum leads to minor changes and the appearance of new vibrations in the Raman spectra. The intensity of the main silicon peak at 521 cm–1 decreases by a factor of 1.6, while its FWHM practically does not change and is about 4.0 cm–1. Such a decrease in the intensity of the peak is probably due to the weakening and breaking of bonds in the structure of the silicon crystal lattice due to the diffusion of Pt. It is shown that the appearance of new vibrations in the range 60–280 cm–1 in the spectra of Si<Pt> is associated with the presence of elemental Pt and the formation of PtSi. It has been found that irradiation of Si<Pt> samples with 600 keV protons leads to a change in the Raman spectra, and the peaks from Pt and/or PtSi disappear.

Effect of annealing temperature on resistive switching behavior of Al/ La0.7Sr0.3MnO3 /LaNiO3 devices

La0.7Sr0.3MnO3 (LSMO)/LaNiO3 (LNO) thin films were deposited successively on Si single crystal by the sol-gel method. The bipolar resistive switching (RS) behavior was studied in Al/LSMO/LNO devices. Obvious current hysteresis was observed, the set (reset) voltage and resistance ratio were modulated by the annealing temperature of LSMO thin films. At the annealing temperature of 750 °C, good endurance and retention performance were obtained in the memory device. According to the fitting results of I–V curves, based on Schottky emission model, the conduction mechanisms of Al/LSMO/LNO devices were discussed. Our results may give a valuable insight on developing perovskite-type rare earth manganese oxide.

Temporal Profile of Nonresonant Sum-Frequency Signal from Single-Crystal Silicon Depends on Crystal Orientation.

Proper analysis of vibrational sum-frequency generation (VSFG) spectra requires that the nonresonant contribution be dealt with correctly. This work shows that the temporal profile of the nonresonant SFG response varies with crystal facing and sample orientation for single-crystal Si and is significantly different than what is observed with polycrystalline Au. These considerations will affect the use of time-delay methods to experimentally suppress the nonresonant signal in broadband SFG measurements. Time-resolved or phase-sensitive SFG measurements will also need to properly account for these effects in post-processing.

Friction properties of the single-crystal Si in collision sliding contacts under different lubrication conditions

With the development of nanotechnology, micro-nano electromechanical systems are widely used in various high-precision instruments, e.g., micro-nano satellites, pico-satellites, in which single-crystal Si is always employed. In outer space, there will be random collisions in space mechanisms caused by unexpected and irregular small disturbances especially under the microgravity environment. In this paper, a molecular dynamics model is used to simulate the motion of the mechanism based on the motion characteristics of random collisions happened within space mechanisms, and irregular collisions are represented by sinusoidal vibrations of the indenter in Y and Z directions. The friction properties of the collision sliding contacts under the lubrication of gold films with different layers are investigated. The results show that a nine-layer gold film presents good vibration absorption and lubrication performance. Then, rectangular textures are designed and made on the gold film. The textured surface can significantly reduce the friction forces at high vibration frequencies. In addition, the friction properties of a molybdenum disulfide (MoS2) film and the combined lubrication behaviors of molybdenum disulfide films and gold films are studied. The molybdenum disulfide film shows excellent friction properties at different vibration frequencies, and the combination of molybdenum disulfide and gold films can significantly reduce the average friction force within a specific vibration frequency range.

Glancing incidence x-ray fluorescence spectrometry based on a single-bounce parabolic capillary

Glancing incidence x-ray fluorescence spectrometry using a single-bounce parabolic capillary is proposed for the analysis of layered samples. The divergence of the x-ray beam was 0.33 mrad. In this paper, we used this instrumental setup to analyze a Si single crystal and a 50 nm HfO2 single-layer film deposited on a Si substrate.

The Search for the Materials That Are Attractive to "Natural" Li Diffusion

"Natural" Li diffusion is defined as the process of Li atom/ions migration under a concentration gradient and activated by thermal energy from atomic vibrations of the host structure at room temperature. This process of passive Li diffusion is important for a better understanding of the active diffusion processes that are happening in lithium-ion batteries (LIBs), where external energy component such as electrical potential is applied. It is expected that materials that exhibit good “natural” Li diffusion properties will perform much better under the external electrical potential. This approach offers a unique opportunity to observe the free movement of lithium atoms/ions into the solid structure and simplify the understanding of diffusion processes especially if single-crystal structures are used. The single-crystal structures are free from grain boundaries and the lithium diffusion process is limited to lattice diffusions such as interstitial, vacancies, and dislocations. This approach allows for categorizing materials that are attractive to lithium diffusion based on the pure lattice component. The characterizing techniques are Auger electron spectroscopy (AES) for tracing Lithium concentration on the surface (Li-KVV peak at 52 eV) and Low Energy Electron Diffraction (LEED) for surface crystallography changes. The lithium concentration gradient is created on the surface of the host material by the evaporation of ultra-thin film of lithium with an effective thickness of 10 Angstrom under ultra-high vacuum conditions.The data obtained from these experiments are showing different lithium diffusion behavior on the selected materials and there is an indication of three categories of the studied materials. 1-Rapid lattice diffusion of Li into HOPG and no change in the surface crystalline structure. 2-Moderate lattice diffusion of Li into CVD Diamond, SiC-6H, LiNbO3, and TiO2 and some change in the surface crystalline structure. 3-No lattice diffusion of Li into Si single crystals, Ga2O3 and SrTiO3, and no long-range order in the surface crystalline structure.Most likely rapid diffusion occurs only in graphite but there are several materials with moderate diffusion properties, and these are potential for novel LIBs electrodes or chemically stable interfaces with enhanced performance. The materials with passive no-diffusion properties present challenges in application to LIBs as there is a special need to create active diffusion paths because there is no lattice diffusion contribution.In addition, this method of tracing lithium passive diffusion using AES is suitable for comparing fabricated polycrystalline LIBs electrodes as a metrology tool for quality control.

Application-oriented use of Laser-induced Periodic Surface Structures

Abstract This paper investigates the phenomenon of Laser-induced Periodic Surface Structures (LIPSS) on Si single crystals. As usual, by variation of parameters, the morphological and physical properties of the resulting surfaces can be tailored, with respect to their use in many applications. One application-oriented use of LIPSS is the preparation of SERS (Surface Enhanced Raman Spectroscopy) substrates, which can be used to detect extremely low concentrations of molecules. In this experimental work, a possible way of manufacturing of SERS substrates, followed by SERS enhancement testing has been shown.

Atomistic deformation mechanism of silicon under laser-driven shock compression

Silicon (Si) is one of the most abundant elements on Earth, and it is the most widely used semiconductor. Despite extensive study, some properties of Si, such as its behaviour under dynamic compression, remain elusive. A detailed understanding of Si deformation is crucial for various fields, ranging from planetary science to materials design. Simulations suggest that in Si the shear stress generated during shock compression is released via a high-pressure phase transition, challenging the classical picture of relaxation via defect-mediated plasticity. However, direct evidence supporting either deformation mechanism remains elusive. Here, we use sub-picosecond, highly-monochromatic x-ray diffraction to study (100)-oriented single-crystal Si under laser-driven shock compression. We provide the first unambiguous, time-resolved picture of Si deformation at ultra-high strain rates, demonstrating the predicted shear release via phase transition. Our results resolve the longstanding controversy on silicon deformation and provide direct proof of strain rate-dependent deformation mechanisms in a non-metallic system.

Versatile Approach of Silicon Nanofabrication without Resists: Helium Ion-Bombardment Enhanced Etching.

Herein, we report a helium ion-bombardment enhanced etching method for silicon nanofabrication without the use of resists; furthermore, we demonstrate its unique advantages for straightforward fabrication on irregular surfaces and prototyping nano-electro-mechanical system devices, such as self-enclosed Si nanofluidic channels and mechanical nano-resonators. This method employs focused helium ions to selectively irradiate single-crystal Si to disrupt the crystal lattice and transform it into an amorphous phase that can be etched at a rate 200 times higher than that of the non-irradiated Si. Due to the unique raindrop shape of the interaction volumes between helium ions and Si, buried Si nanofluidic channels can be constructed using only one dosing step, followed by one step of conventional chemical etching. Moreover, suspended Si nanobeams can be fabricated without an additional undercut step for release owing to the unique raindrop shape. In addition, we demonstrate nanofabrication directly on 3D micro/nano surfaces, such as an atomic force microscopic probe, which is challenging for conventional nanofabrication due to the requirement of photoresist spin coating. Finally, this approach can also be extended to assist in the etching of other materials that are difficult to etch, such as silicon carbide (SiC).

Effects of 120 MeV Ag9+ swift heavy ion irradiation on the structural, optical and electrical properties of pristine and Ni doped BiFeO3 thin films grown by pulsed laser deposition

In this work, the effects of swift heavy ion (SHI) irradiation on the structure, morphology, optical and electrical properties of pristine and Ni-doped BiFeO3 thin films have been explored. The thin films with composition of BiFe1-xNixO3 (x = 0, 0.01, 0.03 and 0.05) were deposited on the n-type single crystal Si(100) substrates through pulsed laser deposition technique. Further, irradiation on the thin films was performed with 120 MeV Ag9+ ions at different ion fluences, i.e., 1 × 1013, 5 × 1013 and 1 × 1014 ions/cm2. To investigate the effects of SHI irradiation on the aforementioned properties, all the films have been characterized by various analytical techniques, such as X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, UV-visible diffuse reflectance spectroscopy and two-probe resistivity setup. The effects of the irradiation on the crystal structure and phase of the thin films were analyzed by the XRD and Raman measurements, and it has been observed that irradiation tends to amorphize the thin films. A decrease in the band gap has been observed for the irradiated thin films with the increase of ion fluence. The current-voltage plots of all the unirradiated and irradiated thin films demonstrate the rectifying diode type characteristics.

Synthesis of rare earth doped Si3N4 nanowires with excellent luminescence properties by plasma-assisted direct nitridation method

Si 3 N 4 nanowires doped with rare earth ions (such as Ce 3+ , Tb 3+ , Eu 2+ and Eu 3+ ) were synthesized by plasma assisted direct nitridation method using Si, rare earth oxides and N 2 as raw materials. The prepared doped Si 3 N 4 nanowires were characterized by XRD, EDS, XPS, SEM and TEM. The obtained single-crystal doped Si 3 N 4 nanowires have uniform diameters of about 50–100 nm and lengths of more than 10 µm. The photoluminescence (PL), PL decay curves as well as thermal quenching behaviors of doped Si 3 N 3 nanowires were systematically investigated. This work provides an effective strategy for doping large-size functional atoms in Si 3 N 4 nanowires. • Rare earth doped Si 3 N 4 nanowires were synthesized by a direct nitridation method. • Rare earth doped Si 3 N 4 nanowires have excellent luminescence properties. • The luminescence peaks of doped Si 3 N 4 nanowires are related to the doping type.

Numerical analysis of constitutional supercooling in heavily doped silicon crystals grown using the Czochralski method

In the growth of silicon single crystal using the Czochralski method, heavy doping using species such as B, P, and As increases the probability of the deterioration of the crystal quality. This is attributed to morphological instability at the growth interface in the crystal because of constitutional supercooling occurring near the growth interface. Therefore, it is necessary to control constitutional supercooling during growth to stabilize the growth of heavily doped Si single crystals. In this paper, we proposed a new approach to predict the formation and distribution of constitutional supercooling based on a three-dimensional numerical model accounting for heat and mass transport, including the segregation effects of the dopants. The dependency of the growth rate, dopant type, and dopant concentration on the formation of constitutional supercooling was evaluated using this approach. Furthermore, the way to suppress constitutional supercooling was discussed through the example of the effect of crystal rotation rate.

Fracture toughness of Fe–Si single crystals in mode I: Effect of loading rate on an edge crack (–110)[110] at macroscopic and atomistic level

This paper is devoted to an experimental and 3D atomistic study of the influence of loading rate on fracture toughness in dilute Fe–Si alloys and in bcc iron. We analyze new and previous experimental results from fracture tests performed at room temperature on bcc iron–silicon single crystals with edge cracks (1¯10)[110] (crack plane/crack front). The specimens of single edge notch-type were loaded in tension mode I under different loading rates. The ductile–brittle behavior at the crack front was monitored online via optical microscopy together with external force and prolongation of the specimens. About 30% decrease in fracture toughness was monitored in the new experiment under the highest loading rate. The nanoscopic processes produced by the crack itself were studied at room temperature via 3D molecular dynamics (MD) simulations in bcc iron under equivalent boundary conditions as in experiments to reveal (explain) the sensitivity of the crack to loading rate. For this purpose, this MD study utilizes the self-similar character of linear fracture mechanics. The results show that the emission of blunting dislocations from the crack is the most difficult under the highest loading rate, which leads to the reduced fracture toughness of the atomistic sample. This is in a qualitative agreement with the experimental (macro) results. Moreover, MD indicates that there may be some synergetic (resonant) effect between the loading rate and thermal activation that promotes dislocation emission.

Robotic Fabrication of High-quality Lamellae for Aberration-corrected Transmission Electron Microscopy

Aberration-corrected scanning transmission electron microscopy (STEM) is widely used for atomic-level imaging of materials. To accelerate the discovery of new materials based on atomic-level investigations, the throughput of aberration-corrected STEM experiments becomes more and more important. However, the throughput of the full workflow of aberration-corrected STEM is still quite low. A fundamental problem is that the preparation of high-quality thin STEM samples (lamellae) depends on manual operation. Here, inspired by the recent successes of robot scientists, we demonstrate robotic fabrication of high-quality lamellae by focused-ion-beam (FIB) with full automation software. First, we show that robotic FIB can prepare lamellae with a high success rate, where the robotic FIB controls rough-milling, lift-out, and final-thinning processes. Then, we optimize the FIB parameters of the final-thinning process for single crystal Si. Aberration-corrected STEM imaging of these Si lamellae shows atomic-level images with 55 pm resolution. We also demonstrate robotic fabrication of high-quality lamellae of SrTiO3 and sapphire. The robotic FIB system will resolve the current bottleneck of the full workflow of aberration-corrected STEM analysis and accelerate materials discovery based on atomic-level imaging.