Silicon Crystal Structure Research Articles

Использование атомной структуры кристаллов кремния для получения многоострийных полевых источников электронов

AbstractThe 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.

In-Plane Longitudinal Cutting in Single-Crystal Silicon Wafer Surface Micromachining

The current concept of grinding or abrasive machining involves the formation and removal of segmented strips of material termed chips from the surface of the solid. A novel cutting mechanism is hereby presented in this research study that suggests that the generation of chips from the surface does not occur but only a shearing process that splits material creating added surface features and textures in the silicon surface. This arises from the unique set of factors of abrasive grit size, thrust force, polishing speed, and polishing time that lead to phase transformations in the surface layers of the silicon wafers. Statistical analysis of the factor effects yielded results that show the surface roughness values, Ra and Rz, increasing without any appreciable change in the thickness of the silicon wafers. This can be attributed to the proposed cutting mechanism indicating that only in-plane surface shearing occurred due to the change of the silicon crystal structure from exhibiting brittle behavior to that of ductile mode of deformation. Moreover, experimental quantities of the specific energy for surface machining of silicon was calculated with an overall mean of 50.5 GPa. This is about 33% less than the currently accepted value and can be considered further evidence that polymorphic transitions to a softer material occurred rendering the surface layers more susceptible to longitudinal cutting deformation and fracture. A model based on the inverted spherical cap or spherical bottom geometry for the individual abrasive particle is also proposed, verified by a finite element method analysis simulation, that can mathematically describe this particular micromachining process.

Mechanical activation-induced structural changes in a 5Ti + 3Si mixture

A 5Ti + 3Si powder mixture has been studied by X-ray diffraction in order to assess the effect of mechanical activation (MA) time on its structural state. MA for 11 min or a shorter time leads to the degradation of the crystal structure and partial amorphization of titanium and silicon. MA has been shown to produce different structural changes in silicon and titanium. MA reduces the crystallite size of silicon to 300 nm and increases microstress to 700 MPa, whereas the only consequence of the MA of Ti is an increase in microstress to 550 MPa. MA for more than 11 min initiates a solid-state reaction which results in the mechanochemical synthesis of Ti5Si3.

Mechanism of the emergence of the photo-EMF upon silicon liquid crystal–single crystal contact

The effect light has on a silicon liquid crystal–single crystal contact at different temperatures of the surface doping of silicon, and when BaTiO3 nanoparticles are added to the composition of a liquid crystal, is studied. The mechanism of the emergence of the photo-EMF in the liquid crystal–silicon structure is explained.

Effect of grit shape and crystal structure on damage in diamond wire scribing of silicon

AbstractFundamental understanding of the fixed abrasive slicing of photovoltaic silicon wafers is crucial for producing low‐cost wafers with superior surface quality and mechanical strength. With the goal of understanding the diamond wire sawing process, this paper investigates the scribing of mono‐ and multi‐crystalline silicon by the abrasive grits on an actual diamond wire. Specifically, the effects of grit shape and silicon crystal structure on the resulting surface morphology, subsurface damage, and the critical depth of cut at which ductile‐to‐brittle transition occurs are investigated. Results show that surface cracking depends on the grit shape. Scribing across the grain and twin boundaries in multi‐crystalline silicon impacts the resulting surface morphology, with grit shape producing a greater effect than crystallographic orientation in the grain interior relative to the grain boundary. Subsurface damage depends on the grit shape and crystal structure. Differences in the critical depth of cut for ductile‐to‐brittle transition in scribing of mono‐crystalline silicon are explained via analysis of the stress state produced by idealized grit shapes.

Algorithm for Calculating the Initial Defect Structure of Semiconductor Silicon

An algorithm for calculating the defect structure of semiconductor silicon crystals was proposed. The proposed approach makes it possible to calculate the sizes, distribution densities of grown-in microdefects at any point of the crystal. Calculations are performed by using and analyzing the thermal conditions of crystal growth in the temperature range from 1683 K to 300 K. The algorithm flowchart is given.

In situ TEM study of deformation-induced crystalline-to-amorphous transition in silicon

The mechanism responsible for deformation-induced crystalline-to-amorphous transition (CAT) in silicon is still under considerable debate, owing to the absence of direct experimental evidence. Here we have devised a novel core/shell configuration to impose confinement on the sample to circumvent early cracking during uniaxial compression of submicron-sized Si pillars. This has enabled large plastic deformation and in situ monitoring of the CAT process inside a transmission electron microscope. We demonstrate that diamond cubic Si transforms into amorphous silicon through slip-mediated generation and storage of stacking faults (SFs), without involving any intermediate crystalline phases. By employing density functional theory simulations, we find that energetically unfavorable single-layer SFs create very strong antibonding interactions, which trigger the subsequent structural rearrangements. Our findings thus resolve the interrelationship between plastic deformation and amorphization in silicon, and shed light on the mechanism underlying deformation-induced CAT in general. Sharper insights into stress-induced, crystalline-to-amorphous transitions in silicon can be realized with crack-resistant micropillars. Mechanical processes such as polishing during the manufacture of photovoltaic cell alter silicon's crystal structure and degrade device performance. Wei Zhang and Zhi-Wei Shan from Xi'an Jiaotong University in China and co-workers have now used high-resolution transmission electron microscopy to capture in situ images of these transitions in progress. The team designed a system of submicrometre pillars consisting of a crystalline silicon core encased in a nanoscale shell of amorphous silicon. The flexible shell helped prevent brittle fracture from occurring while the pillars were gradually compressed into mushroom shapes by an indentation tool. Real-time imaging revealed that mechanical force deformed the silicon framework through plastic motions only – a finding that upsets previous concepts of intermediate crystalline phases taking part in the amorphization process. A novel and effective crystalline-Si/amorphous-Si core/shell sample configuration has been devised. The malleable amorphous Si shell helped inhibit brittle fracture, provide the confinement to significantly raise the stress level, and extend the plastic flow in crystalline Si core. This enabled a real-time observation of the stress-induced crystalline-to-amorphous transition (CAT) process in Si. In situ TEM compression experiment demonstrated a direct amorphization process from the single crystalline diamond cubic Si phase, owing to the accumulation of plastic strain and profuse stacking faults. Deep insights of the CAT process have been achieved from density functional theory simulations.

Engineering inverse woodpile and woodpile photonic crystal solar cells for light trapping

We demonstrate that inverse woodpile and woodpile photonic crystal nanocrystalline silicon structures may be engineered for light trapping in solar cells. We use finite-difference tim-domain simulations to show that the geometry of these photonic crystals may be varied such that absorption in the infrared, visible, and ultraviolet parts of the spectrum may all be improved. The short-circuit current density and ultimate efficiency are also improved. We found a 77.1% and 106% absorption enhancement in the optimized inverse woodpile and woodpile structures respectively, compared to a nanocrystalline silicon thin film of the equivalent thickness. The inverse woodpile structures may be approximated as a thin film with effective index of refraction, whereas the woodpile structures exhibit resonances from the coupling of TE and TM leaky modes in the stacked cylinders. Woodpile photonic crystal structures exhibit improved performance compared to inverse woodpile structures over a range of equivalent thicknesses and incidence angles. The performance of woodpile structures is also generally insensitive to the diameter, pitch and number of layers, whereas inverse woodpile structures are much more sensitive to morphology.

Mathematical modeling of point defect cluster formation in silicon based on molecular dynamic approach

A very important task on the way of improving the technologies of synthesizing highly effective light-emitting diodes on the basis of silicon is theoretical research into the formation of point defect clusters. One method of obtaining silicon with photoluminescent properties is radiation impact. It causes the formation of various defects in its structure, including point and linear defects, their clusters and complexes. In this paper a mathematical model was used to determine the coordinates and velocities of all particles in the system. The model was used for describing point defect formation processes and studying their evolution with time and temperature. The multi-parametrical Tersoff potential was used for the description of interactions between particles. The values of the Tersoff potential were selected by solving the parametric identification problem for silicon. For developing the models we used the system cohesive energy values obtained by an ab initio calculation based on the density functional theory (DFT). The resultant computer model allows MD simulation of silicon crystal structure with point defects and their cluster with possible visualization and animation of simulation results.

Mathematical Modeling of Point Defect Cluster Formation in Silicon Based on Molecular Dynamic Approach

A very important task on the way of improving the technologies of synthesizing highly effective light − emitting diodes on the basis of silicon is theoretical research into the formation of point defect clusters. One method of obtaining silicon with photoluminescent properties is radiation impact. It causes the formation of various defects in its structure, including point and linear defects, their clusters and complexes. In this paper a mathematical model was used to determine the coordinates and velocities of all particles in the system. The model was used for describing of point defect formation processes and studying their evolution with time and temperature. The multi − parametrical Tersoff potential was used for the description of interactions between particles. The values of the Tersoff potential were selected by solving the parametrical identification problem for silicon. For developing the models we used the system cohesive energy values obtained by an ad initio calculation based on the density functional theory (DFT). The resultant computer model allows MD simulation of silicon crystal structure with point defects and their cluster with possible visualization and animation of simulation results.

Theoretical model for measurement of hemoglobin in human blood using 3D photonic crystal structure

Because the concentration of hemoglobin in the human blood is important, optical measurement using 3D photonic crystal structure is presented in this structure. Here the concentration of oxygenated and deoxygenated hemoglobin in human blood is determined for two different wavelengths of 589nm and 633nm. The principle of measurement is based on the linear variation of photonic band gap with respect to concentration of both oxygenated and deoxygenated hemoglobin. The simulations for photonic band gap of 3D photonic crystal structure having different concentrations of hemoglobin are determined using plane wave expansion method. Again, simulation is also done to find out the absorption loss due to both silicon crystal structure and hemoglobin with respect to different concentration of hemoglobin in blood. An experimental set up is designed to measure the transmitted intensity with respect to these concentration of hemoglobin in the blood.

Supertetrahedral aluminum and silicon structures and their hybrid analogues

New structurally stable compounds-superaluminotetrahedrane (Al104H32), supersilicotetrahedrane (Si104H32) and mixed superaluminosilicotetrahedrane (Al64Si40H32) constructed on the basis of the diamond crystal lattice in which the carbon atoms are replaced by, respectively, Al4 and Si4 tetrahedra and which are supramolecular models of corresponding aluminum and silicon crystal structures—have been studied by the B3LYP/6-31G(d,p) density functional theory method. The small difference between the frontier orbital energies indicates that aluminum compounds can have electroconductive properties and silicon compounds can have semiconducting properties.

Effects of Vacancy Cluster Defects on Electrical and Thermodynamic Properties of Silicon Crystals

A first-principle plane-wave pseudopotential method based on the density function theory (DFT) was employed to investigate the effects of vacancy cluster (VC) defects on the band structure and thermoelectric properties of silicon (Si) crystals. Simulation results showed that various VC defects changed the energy band and localized electron density distribution of Si crystals and caused the band gap to decrease with increasing VC size. The results can be ascribed to the formation of a defect level produced by the dangling bonds, floating bonds, or high-strain atoms surrounding the VC defects. The appearance of imaginary frequencies in the phonon spectrum of defective Si crystals indicates that the defect-region structure is dynamically unstable and demonstrates phase changes. The phonon dispersion relation and phonon density of state were also investigated using density functional perturbation theory. The obtained Debye temperature (θ D) for a perfect Si crystal had a minimum value of 448 K at T = 42 K and a maximum value of 671 K at the high-temperature limit, which is consistent with the experimental results reported by Flubacher. Moreover, the Debye temperature decreased with increases in the VC size. VC defects had minimal effects on the heat capacity (C v) value when temperatures were below 150 K. As the temperature was higher than 150 K, the heat capacity gradually increased with increasing temperature until it achieved a constant value of 11.8 cal/cell·K. The heat capacity significantly decreased as the VC size increased. For a 2 × 2 × 2 superlattice Si crystal containing a hexagonal ring VC (HRVC10), the heat capacity decreased by approximately 17%.

Pyramid array substrates for biomedical studies

Cellular reactions to structured surfaces are the subject of intense studies in biomedical research, e.g., for implant technology. Substrates with well-defined structures in relevant materials such as titanium (Ti) are required for these investigations. The pyramidal arrays presented here offer very sharp edges and tips, which appeared to be the main adhesion spots in our earlier cell-surface-interaction studies. Using an etch process based on anisotropic etching of silicon in alkaline solution, the shape of the pyramids is strictly determined by the crystal structure of silicon, while the height as well as the pitch of the pyramids can be precisely controlled and, therefore, be varied in a systematic fashion. Being made from silicon nitride with a thin cover layer of titanium, they offer mechanical stiffness, inertness to all chemicals used in the cell experiments, and a high degree of biocompatibility.

Design and fabrication of a silicon probe for surface profilers

A micromachining probe for surface profilers is reported in this paper. This probe employs single crystal silicon double-ended tuning forks (DETFs) as the sensitive unit to achieve high resolution and frequency read-out. The frequency shift of the DETFs caused by the induced axial stress is directly proportional to the undulation of the measured surface. Moreover, one-stage and two-stage microleverage amplification mechanisms are respectively implemented to improve the probe performance. The silicon on glass (SOG) process which can provide high aspect ratio single crystal silicon structures has been adopted to enhance the performance of the probes. The testing results indicate that the probe with two-stage mechanical amplifiers has a frequency-displacement sensitivity of 257.1 Hz/μm with a nominal frequency 27.3 kHz at room temperature in atmosphere, which is in close agreement with the simulation results.

Combining micro- and nano-texture to fabricate an antireflective layer

We combined photonic crystal (PC) and nanoporous structures in silicon (Si) to decrease the reflectance of the Si surface. Due to gradual increase of effective refractive index in this structure, reflection is reduced drastically over a broad spectral range. A two-dimensional Si PC was fabricated by interference lithography, then a nanoporous structure was made on the PC by using metal-assisted chemical etching. The obtained reflectance is about 3% across a spectral range of 400 to 2000 nm. This indicates an improvement of reflectance up to 90% compared to bare Si.

Measurement of the Anisotropy of Young's Modulus in Single-Crystal Silicon

In (100) silicon wafers, the most commonly used in microelectromechanical systems (MEMS) fabrication, the value of Young's modulus of a MEMS structure can vary by over 20%, depending on the structure's orientation on the wafer surface. This anisotropy originates from the crystal structure of silicon. We have directly measured the anisotropy of Young's modulus in the (100) plane of silicon from the measured resonance frequencies of a “wagon-wheel” test structure comprising an arc of identical microcantilevers fabricated in the structural layer of a (100) silicon-on-insulator wafer. The direction of the principal axis of the cantilevers increased from 0° to 180 ° in 10° steps with respect to the [110] direction, allowing the angular dependence of Young's modulus to be experimentally mapped out. The Young's modulus was measured to have a value of 170 GPa ± 3 GPa at 0° and 90 ° to the [110] direction and a value of 131 GPa ± 3 GPa at ±40° and ±50° to the [110] direction. The measured values of Young's modulus and their angular dependence agree very well with the theoretical values that were recently reported, thereby experimentally verifying the theoretical calculations.

First principles analysis on interaction between vacancy near surface and dimer structure of silicon crystal

We investigated the interactions between the dimer structures in a reconstructed Si (100) surface and a vacancy to understand the behavior of the point defects near the surface using first principles calculations. The calculated results showed that even a vacancy set in the ninth layer from the surface affects the charges around the dimers. This effect intensifies if the vacancy moves to the surface. Similar to the vacancies in a bulk, Jahn-Teller distortion occurred at atoms around the vacancy. When the introduced vacancy approaches to the fourth layer from the bottom, this distortion increased, and thus decreases the total energy. The third layer, in which forming a vacancy requires the highest energy, becomes a singularity layer in a Si (100) surface with dimer structures. The model, in which a vacancy was introduced into the fifth layer from the surface, came to have a vacancy in the fourth layer as an optimized structure due to a Si atom moving from the fourth to the fifth layer. We can see a clear interaction between the vacancy-originated charges and the surface dimer-originated one in this movement. As a consequence, the nature of the interaction between a surface dimer and a vacancy is intermediated by their accompanying charges.

Direct fabrication of two-dimensional photonic crystal structures in silicon using positive and negative Hydrogen Silsesquioxane (HSQ) patterns

The use of Hydrogen Silsesquioxane (HSQ) as a negative-tone electron-beam resist and a hard etching mask to directly fabricate two-dimensional Si-based photonic crystals (PhC) was investigated. We demonstrate a protocol using the HSQ resist, including spin coating processes, low-energy electron-beam exposures, development, post-development treatments, and dry-etching, to achieve sub-micron two-dimensional photonic crystal structures in silicon. An electron-beam area dose of 315@mC/cm^2 at 30kV was sufficient to produce anisotropic 267-nm deep HSQ-based PhC features upon development. Subsequent oxygen plasma treatment and dry etching processes were done to transfer the final patterns to the underlying silicon with a 1:1.23 etch selectivity (Si:HSQ) using a CF4/O2 plasma. These results demonstrate the use of HSQ to simplify the fabrication processes for photonic crystals and their integration with other optical-based on-chip components.

Microfabrication Process for XYZ Stage-Needle Assembly for Cellular Delivery and Surgery

In this paper, we present our ongoing work on developing a microfabricated XYZ stage-needle arrayed single crystal silicon (SCS) structure for cellular delivery and surgery. We discuss the device design and working principle based on electrostatic actuation. We also briefly discuss our microfabrication process flow and show some preliminary results of fabricating arrays of microneedles that are 250 µm long and 5 µm at the tip diameter.