Silicon Crystal Structure Research Articles

Daily and annual modulation rate of low mass dark matter in silicon detectors

Low-threshold solid-state detectors with single electron excitation sensitivity can probe nuclear recoil energies in the sub-100 eV range, coinciding with the typical threshold displacement energies in the detector material. We investigate the daily and annual modulation of the observable event rate for dark matter mass ranging from 0.2 to 5 GeV/c2 in a silicon detector, considering the energy threshold and the direction of the nuclear recoil. The data for the energy threshold is obtained from a molecular dynamics simulation. It is shown that the directional dependence of the threshold energy and the motion of the laboratory result in the modulation of the interaction event rate. We demonstrate silicon’s average annual interaction rate is more considerable than germanium for low-mass dark matter. However, their event rates take a similar trend in large dark matter masses. Thus, silicon can be a reliable target to discriminate low-mass dark matter from backgrounds. We also find 8 h and 12h periodicities in the time series of event rates for silicon detectors due to the 45-degree symmetry in the silicon crystal structure.

Evaluating approaches for on-the-fly machine learning interatomic potentials for activated mechanisms sampling with the activation-relaxation technique nouveau.

In the last few years, much effort has gone into developing general machine-learning potentials capable of describing interactions for a wide range of structures and phases. Yet, as attention turns to more complex materials, including alloys and disordered and heterogeneous systems, the challenge of providing reliable descriptions for all possible environments becomes ever more costly. In this work, we evaluate the benefits of using specific vs general potentials for the study of activated mechanisms in solid-state materials. More specifically, we test three machine-learning fitting approaches using the moment-tensor potential to reproduce a reference potential when exploring the energy landscape around a vacancy in Stillinger-Weber silicon crystal and silicon-germanium zincblende structures using the activation-relaxation technique nouveau (ARTn). We find that a targeted on-the-fly approach specific to and integrated into ARTn generates the highest precision on the energetics and geometry of activated barriers while remaining cost-effective. This approach expands the types of problems that can be addressed with high-accuracy ML potential.

X-Ray Structural Investigations Of n-Si<Pt> Irradiated with Protons

In this work, the effect of proton irradiation on the change in the structure of silicon samples doped with platinum was studied. The samples were irradiated with protons at a dose of 9×1014 cm-2 with an energy of 600 keV and a current of 1÷1.5 μA. To determine the change in the structure after irradiation, the methods of X-ray diffraction and atomic force microscopy were used. The obtained results indicate that doping with platinum does not lead to a modification of the cubic crystal structure of silicon, but only to minor changes in the structural characteristics and surface morphology. In this case, proton irradiation of a silicon single crystal with a dose of 9.0×1014 cm–2 with an energy of 600 keV leads to the formation of defects without the formation of an amorphous near-surface layer.

Experimental and molecular dynamic studies of phase transformation and amorphization in silicon during high-energy ball milling

Nano-sized and amorphous silicon (α-Si) have a wide range of properties and applications, but their atomistic structure and the process of crystal evolution are still unclear. In this study, a combined action is used to present a mechanism for the crystal-to-amorphous solid–solid phase transition in silicon during mechanical alloying (MA) at room temperature. For this purpose, the ball milling of silicon was performed and changes in crystallographic structure and amorphous phase were studied by scanning electron microscopy (SEM), X-ray diffraction patterns (XRD), and energy dispersive X-ray spectroscopy (EDX). The process of changing phase was determined by quantitative phase analysis (MAUD/Rietveld refinement). In the simulation part, a crystal structure of silicon consisting of 64 atoms was created and uniaxial and biaxial external pressures were applied randomly by BIOVIA Materials Studio. The time step and simulation time were 1 and 300 Ps, respectively. Important atomic properties including density, mean square displacement (MSD), angle length and bonds distribution, and the fraction of broken bands were measured after applying per compression. Atomic structure and changes in crystalline phase to nano and amorphous phases were studied and these results had a considerable agreement with the experimental part and well-known previous studies. The diamond cubic structure shows a remarkable toughness during the modeling and so amorphization was performed at an exponential evolution. The Results of the simulations showed that the bond angle and length distribution and MSD, were increased significantly, and in contrast, density was decreased to 1.254 g/cm3. The percentage of the broken Si-Si bonds was 58.12 % and a new definition of Si amorphization based on the fraction of covalent bonds was proposed. The amorphization started from the surface of Si and spread by the partial distribution toward the crystal structure. These findings show that the proposed mechanism based on the fraction of breaking bonds and pressure-induced amorphization could be used for the accurate simulation of the solid–solid phase transformation.

Periodic Coupled-Cluster Green's Function for Photoemission Spectra of Realistic Solids.

We present an efficient implementation of the coupled-cluster Green's function (CCGF) method for simulating photoemission spectra of periodic systems. We formulate the periodic CCGF approach with Brillouin zone sampling in the Gaussian basis at the coupled-cluster singles and doubles (CCSD) level. To enable CCGF calculations of realistic solids, we propose an active-space self-energy correction scheme by combining CCGF with the cheaper many-body perturbation theory (GW) and implement the model order reduction (MOR) frequency interpolation technique. We find that the active-space self-energy correction and MOR techniques significantly reduce the computational cost of CCGF while maintaining the high accuracy. We apply the developed CCGF approaches to compute spectral properties and band structure of silicon (Si) and zinc oxide (ZnO) crystals using triple-ζ Gaussian basis sets and medium-size k-point sampling and find good agreement with experimental measurements.

The Effect of Oxidation Temperature on the (1000–1300) cm−1 Band in FT-IR Spectra of Silicon Oxide Synthesized by Thermal Oxidation of Silicon Wafers

In this work, silicon wafers were thermal treated in air at temperatures varied in the range 800—1200 °C. The annealed samples were investigated using FTIR, X-ray diffraction, FTIR and optical reflection spectroscopy. The effect of annealing temperature on the (1000–1300) cm−1 band in FT-IR spectra of the prepared samples was investigated. The results showed that thermal oxidation at temperatures less than 1200 °C leads to enhance the phenomenon of the splitting of longitudinal optical and transverse optical stretching motions. This phenomenon was verified by observing the appearance of two overlapping peaks in the region (1000–1300) cm−1 in FT-IR spectra. The results also showed that the splitting process leads to the formation of defects in the crystal structure of silicon, which in turn leads to the formation of silicon nanoparticles.

Molecular dynamics study of crystal orientation effect on surface generation mechanism of single-crystal silicon during the nano-grinding process

Crystal orientation of the crystalline materials surfaces significantly affects the surface generation and subsurface damage of the materials during the ultra-precision machining. This work is based on the molecular dynamics (MD) simulation method to study the nano-grinding process of the single-crystal silicon workpiece with three different types of surface crystal orientations. And the MD simulation results are compared with the experimental results. The results show that the machined surface quality of workpieces with different surface crystal orientations is similar, but the degree of subsurface damage, processing force, phase transition, and internal residual stress is quite different. By comparing all the three types of the workpiece, the surface and subsurface quality of the single-crystal silicon workpiece with {110} surface crystal orientation has a better processing effect. This work reveals the evolution of the crystal structure of single-crystal silicon with different surface crystal orientations and the mechanism of subsurface damage from the atomic point of view.

The effects of ion implantation damage to photonic crystal optomechanical resonators in silicon

Optomechanical resonators were fabricated on a silicon-on-insulator substrate that had been implanted with phosphorus donors. The resonators’ mechanical and optical properties were then measured (at 6 K and room temperature) before and after the substrate was annealed. All measured resonators survived the annealing and their mechanical linewidths decreased while their optical and mechanical frequencies increased. This is consistent with crystal lattice damage from the ion implantation causing the optical and mechanical properties to degrade and then subsequently being repaired by the annealing. We explain these effects qualitatively with changes in the silicon crystal lattice structure. We also report on some unexplained features in the pre-anneal samples. In addition, we report partial fabrication of optomechanical resonators with neon ion milling.

Effect of rapid thermal annealing on damage of silicon matrix implanted by low-energy rhenium ions

The structural, electronic, and magnetic properties of low-energy rhenium implanted c-Si are examined for the first time. The damage created by rhenium ions and the following partial reconstruction of the silicon host matrix after rapid thermal annealing (RTA) are investigated as a function of the fluence. Rutherford backscattering spectrometry (RBS) results reveal that the implanted ions are located in the near-surface region with the distribution maximum at about 23 nm below the surface. The analysis of rhenium-depth distribution using the McChasy code shows that the implanted Re-ions are located in the interstitial lattice positions. The RTA leads to a partial recovery of the silicon crystal structure. According to the RBS results, the formed inclusions are not coherent with the silicon host matrix causing an increase of the lattice distortion. Analysis of channeled RBS/c spectra carried out by the McChasy code revealed different levels of bent channels in damaged regions suggesting bimodal distribution of inclusions in the silicon. Studies of high-resolution X-ray photoelectron spectroscopy (XPS) conducted after the RTA showed the shift of Re 4f7/2 binding energy (BE) by +0.68 and + 0.85 eV with respect to metallic rhenium for the samples with lower/higher fluencies, respectively. Complex XPS, density functional theory (DFT) simulations, and transmission electron microscopy (TEM) data analysis allowed us to conclude that the near-surface layer of the sample (∼10 nm) consists of nanoinclusions with cubic and/or hexagonal ReSi. In the middle area of the samples, much larger nanoinclusions (>10/20 nm for higher/lower fluencies, respectively) containing pure metallic rhenium inside are formed. The RTA increases the magnetic moment of the sample with the lower dose nearly 20-fold, whereas in the sample with the higher dose a 3-fold increment is observed only. The magnetic response of the examined systems after the RTA indicates a presence of magnetic interactions between the nanoinclusions resulting in the system exhibiting super-spin glass or super-ferromagnetism.

The Influence of Xenon and Argon Ion Irradiation Parameters on Defect Formation in Silicon

Single-crystal silicon has been irradiated with xenon ions at energies of 100 and 200 keV and argon ions at 110 keV. The irradiation fluence varied in the range of the displacement per atom (dpa) from 0.1 to 1 for both types of ions and selected energies. The influence of irradiation on the destruction of the silicon structure was studied using Rutherford backscattering (RBS) combined with channeling and Raman scattering (RS). The stages of silicon crystal structure destruction based on RBS and RS for different irradiation fluences are demonstrated. It is shown that defects accumulate in the modified layer as the fluence increases to a value corresponding to 0.5 dpa; then highly defective regions merge into amorphous layers. At a dpa of 1, the structure of a single crystal does not become disordered.

INFLUENCE OF SILICON WAFER CRYSTALLOGRAPHIC ORIENTATION ON ANODIZATION MECHANISM

The influence of silicon wafer crystallographic orientation on the formation of porous silicon during anodization in an HF solution is studied. Cross-section SEM image comparison of samples with different crystallographic orientations has shown that (111) Si samples exhibit a more branching, tree-like pore structure with a higher porosity value compared to (100) Si samples. This phenomenon is explained by pointing out differences in crystal structure and numbers of Si-Si chemical bonds in different crystallographic directions. Namely, in (100)-oriented silicon crystals every surface Si atom has two bonds connecting it to atoms underneath it, as well as two broken bonds able to interact with Fions. Through electron injection into silicon, enough energy is applied to break the underlying bonds, forming SiF as a result. The presence of two Fions bonded with every surface silicon atom leads to weakening the bonds of surface silicon atoms with the underlying atoms, thus making the process of breaking the Si-Si bonds more energy efficient. As for (111)-oriented crystals, silicon atoms only have one broken surface bond, and breaking backbonds with underlying silicon atoms requires a higher value of activation energy due to their larger amount (three as opposed to two for (100) silicon). It is concluded that this very reason leads to slower etching speeds of (111)-oriented silicon wafers. The results help evaluate the way the silicon crystal structure affects the etching process, including its speed and direction, which is an especially important factor to consider when forming (111)-oriented porous silicon.

Growth of Si2Te3/Si heterostructured nanowire and its photoresponse property

Single-crystalline heterostructured Si2Te3/Si nanowires (H-NWs) were synthesized by using gold nanoclusters as a catalyst. Control of the nanowire structures could be achieved by substrate temperature and growth time. Scanning electron microscopy (SEM) study shows that the as-grown nanowires are composed of two parts: one part has a conical shape with gold catalyst on the top and second part is uniform nanowire. High-resolution transmission microscopy study shows that the H-NWs have a core/shell structure with a silicon nanowire core wrapped by a Si2Te3 shell. The composition of the core-shell structure was confirmed by Energy-dispersive X-ray spectrometry (EDX) measurements. X-ray diffraction (XRD) and Raman spectra confirm the two types of crystal structure of silicon and Si2Te3 in H-NWs. A possible growth mechanism of H-NWs is discussed. The photoresponse property of Si2Te3/Si H-NW is observed under illumination of light. The rise and decay time constants are τr∼590 ms and τd∼800 ms, respectively. The unique structure and properties of these silicon-based nanowires of 2D materials are not only interesting for fundamental understanding but also potential building blocks for applications in optoelectronic devices.

Searching the crystal structure of silicon using the generalized scaled hypersphere search method with the rapid nuclear motion approximation

The scaled hypersphere search method with the rapid nuclear motion approximation was applied for the prediction of the crystal structure of silicon. Five well-known structures and one novel structure were found. The space group of the novel structure found is Imma. By comparing this with the three known silicon structures in the same space group, it is found that one of these well-known Imma structures and the novel Imma structure are similar. According to our ab initio calculations based on the projector augmented wave method with the Perdew–Burke–Ernzerhof functional, the novel structure is a semiconductor, with calculated band gap 0.89 eV.

3D Waveguides for Nano Photonic Application

Silicon and gallium arsenide (GaAs) based three dimensional photonic structure is realized here to envisage at Terahertz mirror device, which is purely belonged to the application of nano photonic devices. Further plane wave expansion technique pertaining to the mathematical formulation is disclosed in this research to evaluate the photonic bandgap of silicon and gallium arsenaide crystal structure. Aside this, lattice spacing and diameter of air holes of both the structure play key role to understand the above said application, which controls the reflection and transmission of signal . Finally it is revealed that with the proper combination of lattice spacing and diameter of air holes of the said 3D structure determines a range of Terahertz frequency. Finally, the outcome of the paper claims that the proposed structure can be a suitable candidate for nano photonic application.

On Change in the Silicon Crystal Structure Implanted with Hydrogen Ions during Annealing Based on Three-Crystal X-Ray Diffractometry Data

In this paper, we present the results of a three-crystal X-ray diffractometry (XRD) study of the state of a disturbed layer formed in silicon crystals by implantation of hydrogen ions with energies of 100 + 200 + 300 keV and a total dose of 2 × 1016 cm–2 during the subsequent heat exposure in the temperature range from 200 to 1100°C. Here, X-ray studies were performed in the three-crystal XRD scheme when the sample under consideration operates as a second stationary crystal with various fixed angular detuning α from the Bragg position while the third (perfect) crystal-analyzer sweeps the angular distribution of radiation diffracted by the second crystal. Based on a comparison of the shape of diffraction and diffuse maxima for the samples under study, a qualitative conclusion about a significant transformation of radiation defects at post-implantation annealing was derived.

Investigation of the Effect of Electrolyte Composition on the Performance of Pre-Structured Porous Silicon Anode Materials

Crystalline silicon is an ideal anode candidate for lithium ion batteries as it offers a high Li+ ion storage capacity, direct current pathway and is a stable and earth abundant element. One of the limitations of silicon as electrode material is its mechanical instability during repeated cycling of the battery. The incorporation of lithium ions into the silicon crystal structure causes extreme volume expansion (400 %) of the anode material. The high volume expansion during cycling can lead to crack formation and pulverization of the silicon anode. Consequently, the silicon can react further with the electrolyte, resulting in a build-up of the solid electrolyte interphase (SEI) layer, eventually hindering Li+ ion diffusion. Porous silicon, characterized by bulk silicon crystallites separated by pores, can compensate to some extent for the volume expansion without unwanted mass increase of the electrode (as with paste electrodes), while still maintaining lower costs than with silicon microwires due to the scalability of the porous silicon etching process[1]. In addition, porous silicon offers shorter diffusion pathways for Li+ due to the incorporation of the electrolyte into the pore structure. Porous silicon still possesses some limitations regarding the amount of volume expansion it can undergo, as well as the diffusion of the electrolyte into the pore structure. To further compensate for the mechanical stress during cycling, by creating pre-defined breakage points in the silicon surface, and open up the pore structure to improve the electrolyte diffusion, pre-structuring of the porous silicon was performed via reactive ion etching (RIE) techniques using a flow ratio of 2 SF6/6 Ar (sccm/sccm), power level of 200 W and etch times varying from 30 seconds to 8 minutes. The resulting textured surface possessed microstructures with trench depths ranging from 1-2 µm and widths of around 1 µm. Improved cycling stability was seen for samples subjected to lower etch times (30 seconds to 2 minutes). In order to have long term cycling stability, it is required to have an SEI layer that is thin enough to allow faster uptake of Li+ into the Si structure, and stable enough that it will not continuously react with active silicon. The composition of this SEI layer can be tailored by modifying the electrolyte composition. The type and concentration of lithium salt (LiTFSA, LiPF6, LiBOB etc.) and solvent molecule (ethylene carbonate, dimethyl ether, doluene, propylene carbonate etc.) can be altered in order to improve the SEI layer. In this work, we investigate the effect of varying electrolyte compositions on the performance of porous silicon anodes in a half-cell as well as on the SEI layer. The change in composition of the SEI layer is investigated via ex situ Raman and X-ray photoelectron spectroscopy techniques. Electrolytes with high viscosity (such as LiPF6 containing electrolytes) experience limited infiltration into the silicon pore structure of the anode. The viscosity of the electrolyte can be tailored by modifying the salt concentration to ensure easy infiltration into the pores to allow maximum charging of the porous silicon anode. Improved electrolyte infiltration into the pores was seen with the RIE treated porous silicon samples for all electrolytes, including those with high viscosity. Reduction of the organic solvent molecules after repeated cycling lead to higher carbon content in the SEI after repeated cycling. Electrolytes with flourine containing lithium salts lead to the formation of LiF compounds, reducing the amount of active lithium. [1] Hansen, S., Schütt, A., Carstensen, J. and Adelung, R., J. Electrochem. Soc., 2016, 163(14), A3036-A3045.

Synthesis and Properties of Plasmonic Boron‐Hyperdoped Silicon Nanoparticles

AbstractElectronic properties of silicon, the most important semiconductor material, are controlled through doping. The range of achievable properties can be extended by hyperdoping, i.e., doping to concentrations beyond the nominal equilibrium solubility of the dopant. Here, hyperdoping is achieved in a laser pyrolysis reactor capable of providing nonequilibrium conditions, where doping is governed by kinetics rather than thermodynamics. High resolution scanning transmission electron microscopy (TEM) with energy‐dispersive X‐ray spectroscopy shows that the boron atom distribution in the hyperdoped nanoparticles is relatively uniform. The hyperdoped nanoparticles demonstrate tunable localized surface plasmon resonance (LSPR) and are stable in air for periods of at least one year. The hyperdoped nanoparticles are also stable upon annealing at temperatures up to 600 °C. Furthermore, boron hyperdoping does not change the diamond cubic crystal structure of silicon, as demonstrated in detail by high flux synchrotron X‐ray diffraction and pair distribution function (PDF) analysis, supported by high‐resolution TEM analysis.

The Effects of Structure Thickness, Air Gap Thickness and Silicon Type on the Performance of a Horizontal Electrothermal MEMS Microgripper

The ongoing development of microelectromechanical systems (MEMS) over the past decades has made possible the achievement of high-precision micromanipulation within the micromanufacturing, microassembly and biomedical fields. This paper presents different design variants of a horizontal electrothermally actuated MEMS microgripper that are developed as microsystems to micromanipulate and study the deformability properties of human red blood cells (RBCs). The presented microgripper design variants are all based on the U-shape ‘hot and cold arm’ actuator configuration, and are fabricated using the commercially available Multi-User MEMS Processes (MUMPs®) that are produced by MEMSCAP, Inc. (Durham, NC, USA) and that include both surface micromachined (PolyMUMPs™) and silicon-on-insulator (SOIMUMPs™) MEMS fabrication technologies. The studied microgripper design variants have the same in-plane geometry, with their main differences arising from the thickness of the fabricated structures, the consequent air gap separation between the structure and the substrate surface, as well as the intrinsic nature of the silicon material used. These factors are all inherent characteristics of the specific fabrication technologies used. PolyMUMPs™ utilises polycrystalline silicon structures that are composed of two free-standing, independently stackable structural layers, enabling the user to achieve structure thicknesses of 1.5 μm, 2 μm and 3.5 μm, respectively, whereas SOIMUMPs™ utilises a 25 μm thick single crystal silicon structure having only one free-standing structural layer. The microgripper design variants are presented and compared in this work to investigate the effect of their differences on the temperature distribution and the achieved end-effector displacement. These design variants were analytically studied, as well as numerically modelled using finite element analysis where coupled electrothermomechanical simulations were carried out in CoventorWare® (Version 10, Coventor, Inc., Cary, NC, USA). Experimental results for the microgrippers’ actuation under atmospheric pressure were obtained via optical microscopy studies for the PolyMUMPs™ structures, and they were found to be conforming with the predictions of the analytical and numerical models. The focus of this work is to identify which one of the studied design variants best optimises the microgripper’s electrothermomechanical performance in terms of a sufficient lateral tip displacement, minimum out-of-plane displacement at the arm tips and good heat transfer to limit the temperature at the cell gripping zone, as required for the deformability study of RBCs.

Field Emission Properties of Nanostructured Silicon Cathode Arrays

The fabrication method of field electron sources using the atomic structure of silicon crystals and the processes of heterophase vacuum-plasma self-organization of island carbon coatings and highly anisotropic plasma-chemical etching under low adsorption is theoretically justified and experimentally implemented. The interrelation of the morphological and field emission characteristics of fabricated field emission cathodic microstructures is established. The experimental results are interpreted using the Fowler-Nordheim representation in connection with the changes of the surface phases of multipoint silicon microstructures.

Use of the Atomic Structure of Silicon Crystals to Obtain Multi-Tip Field-Emission Sources of Electrons

The field-emission properties of field electron sources produced using the atomic structure of silicon crystals, the heterophase vacuum-plasma self-organization of island carbon coatings, and highly anisotropic plasma-chemical etching under weak-adsorption conditions are studied. A correlation between the morphological and field-emission characteristics of field cathode microstructures on silicon crystals with various conductivity types is ascertained. The results of experimental studies are interpreted using the Fowler–Nordheim theory in conjunction with changes in the compositions of the surface phases formed in fabricating emitting silicon projections.