Silicon Crystal Structure Research Articles

Evolution of porous silicon crystal structure during storage in ambient air

Both double- and triple- crystal X-ray diffraction techniques (X-ray DCD and TCD techniques) together with transmission electron microscopy (TEM) were employed for the investigation of structural changes in porous silicon (PS) layers during prolonged periods (up to 6800 hours) of their storage in ambient air. Apart from the Bragg reflection from the Si substrate, the diffraction pattern contains an additional maximum caused by the presence of the PS layer with an increased lattice parameter. The position of this peak shifts to smaller Bragg angles and its intensity decreases as the time of storage in air increases. In addition, the profiles of such peaks become clearly asymmetric. In this case, Gaussian curves were used to reach a fit to the experimental X-ray rocking curves. All samples were biaxially bent due to compressive stresses that arise as soon as 10 min after electrochemical process. The values of lattice strains along the surface normal (Δd/d)⊥ and lateral deformation (Δd/d)|| were estimated to be ~ +10−3, ~ −10−5 respectively. The analysis of diffraction curve evolution shows a gradual destruction of the crystal lattice caused by the air oxidation process.

Heat transfer in trapezoidal microchannels of various aspect ratios

Heat transfer in the thermal entrance region of trapezoidal microchannels is investigated for hydrodynamically fully developed, single-phase, laminar flow with no-slip conditions. Three-dimensional numerical simulations were performed using a finite-volume approach for trapezoidal channels with a wide range of aspect ratios. The sidewall angles of 54.7° and 45° are chosen to correspond to etch-resistant planes in the crystal structure of silicon. Local and average Nusselt numbers are reported as a function of dimensionless length and aspect ratio. The effect of Prandtl number upon the thermal entrance condition is explored. The fully developed friction factors are computed and correlated as a function of channel aspect ratio. Correlations are also developed for the local and average Nusselt numbers in the thermal entrance region as a function of a dimensionless axial length variable.

Effect of the plasma composition on the structural and electronic properties of as-grown SiOx/Si heterolayers deposited by reactive sputtering

We report the effects of varying the oxygen partial pressure (OPP) on the structural and electronic properties of SiOx/Si heterolayers grown by RF reactive sputtering. The produced samples present silicon poly-crystalline characteristics for low values of OPP. The crystallinity decreases as the OPP increases due to oxygen interdiffusion until the silicon crystal structure becomes amorphous. The results of infrared and Raman spectroscopies show higher deviation from stoichiometry and an increment of structural disorder for samples grown with higher values of OPP. Room temperature photoluminescence (PL) is present in all as-grown samples. The PL spectra show two bands, around 1.87 and 2.16 eV, for all the samples, while a third broad band at lower energy shows up and shifts to the red as OPP increases. Our results indicate that silicon-related room temperature PL emission is correlated with the stoichiometry of the SiOx and to the formation of silicon crystals embedded in a silicon dioxide matrix.

An X band RF MEMS switch based on silicon-on-glass architecture

Communication systems such as those used on satellite platforms demand high performance from individual components that make up the various systems and sub-systems. Switching and routing of RF signals between various modules is a routine and critical operation that determines the overall efficiency of the entire system. In this paper, we present the design and fabrication aspects of a direct contact RF MEMS switch designed to operate in the X band (8–12 GHz) with a target insertion of about 0·5 dB and isolation better than 30 dB. The actuation voltage is expected to be around 50 V. The die size is designed to be 3 mm (H) × 3 mm(W) × 2 mm(H). The switch is built from a low residual stress device layer of a highly conducting (0·005 Ohms-cm) silicon on insulator (SOI) wafer. After subsequent lithographic steps, the wafer is bonded to a Pyrex glass wafer which has been previously patterned with gold transmission lines and pull in electrodes. Being built from a single crystal silicon structure, the mechanical robustness of the actuator is much greater than the those in similar membrane-based devices. A 6 mask fabrication process utilizing Deep Reactive Ion Etching to achieve high aspect ratio stiction free structures was developed and implemented. Devices from the first fabrication run are being analysed in our laboratory.

Kinetically-induced hexagonality in chemically grown silicon nanowires

Various silicon crystal structures with different atomic arrangements from that of diamond have been observed in chemically synthesized nanowires. The structures are typified by mixed stacking mismatches of closely packed Si dimers. Instead of viewing them as defects, we define the concept of hexagonality and describe these structures as Si polymorphs. The small transverse dimensions of a nanowire make this approach meaningful. Unique among the polymorphs are cubic symmetry diamond and hexagonal symmetry wurtzite structures. Electron diffraction studies conducted with Au as an internal reference unambiguously confirm the existence of the hexagonal symmetry Si nanowires. Cohesive energy calculations suggest that the wurtzite polymorph is the least stable and the diamond polymorph is the most stable. Cohesive energies of intermediate polymorphs follow a linear trend with respect to their structural hexagonality. We identify the driving force in the polymorph formations as the growth kinetics. Fast longitudinal elongation during the growth freezes stacking mismatches and thus leads to a variety of Si polymorphs. The results are expected to shed new light on the importance of growth kinetics in nanomaterial syntheses and may open up ways to produce structures that are uncommon in bulk materials.

Free-standing <inline-formula><math display="inline" overflow="scroll"><msub><mi mathvariant="normal">C</mi><mn>60</mn></msub></math></inline-formula> nanowire fabricated using <inline-formula><math display="inline" overflow="scroll"><mrow><mi mathvariant="normal">Xe</mi><msub><mi mathvariant="normal">F</mi><mn>2</mn></msub></mrow></math></inline-formula> sacrificial dry etching

We report a fabrication of doubly supported free-standing buckminsterfullerene (C60) nanowires onto single crystal silicon microstructures in order to demonstrate an integration of carbon nanomaterials to surface-micromachined microelectromechanical devices. By irradiating vacuum-deposited C60 film with an electron beam, polymerized C60 nanowires were patterned. Free-standing structures of the C60 nanowires were obtained by sacrificial etching of underlying silicon structures using XeF2 gas. The supporting 5-µm thick silicon structure was released using vapor hydrofluoric acid. The C60 nanowire of 2-µm width, 25-µm length, and 40-nm thickniss connected to a movable single crystal silicon structure was successfully fabricated.

Monte Carlo geometry optimization of Sin (n ⩽ 71) clusters

Optimum geometries of silicon clusters up to 71 atoms have been found by a recently developed Monte Carlo based global optimization method. Structural properties of these clusters have been investigated and the results have been compared with available results obtained by other methods. Radial distribution of atoms of Si$_{71}$ have been compared with the silicon crystal structure.

Generalized Mathematical Homogenization: From theory to practice

A Generalized Mathematical Homogenization (GMH) theory for coupling atomistic and continuum descriptions at finite temperature has been extended to account for many-body potentials. GMH gives rise to the constitutive law-free coarse scale equations where coupled continuum thermo-mechanical equations are directly derived from molecular dynamics equations. An integrated computational approach, which seamlessly integrates ABAQUS for coarse scale (finite element method) computations with a molecular dynamics code, has been developed. The accuracy and efficiency of the method has been studied for crystal silicon structures including failure predictions of nanowires.

Formation, crystal structure, and properties of silicon with buried iron disilicide nanocrystallites on Si (100) substrates

Methods of low-energy electron diffraction, measurements of the Hall effect in situ, atomic-force microscopy, and high-resolution transmission electron microscopy were used to study the formation of iron silicide islands on the Si(100)–(2×1) surface and overgrowth of these islands with silicon; the electrical properties and structure of silicon with buried iron silicide nanocrystallites were studied. The best crystal quality of the continuous single-crystal silicon layer and the minimum roughness of its surface were observed at the silicon growth temperature of 700°C and the layer thickness of 100 nm. A model of growth of silicon over the ironsilicide nanocrystals is suggested. Two types of formed nanocrysrallites were found: small nanocrystallites (5–6 nm) of β-FeSi2 and large nanocrystallites (30–50 nm) of γ-FeSi2. The good agreement between the electrical parameters of silicon with buried iron disilicide nanocrystallites and those of atomic-clean silicon confirmed that there is minimum scattering of charge carriers at nanocrystallites in the temperature range of 300–540 K.

Defect structure of silicon crystals implanted with H2+ ions

AbstractSi(001) single crystal was implanted with hydrogen ions with energy 135 keV and dose 5 × 1016 cm–2. After implantation, Si:H samples were annealed at 450 and 650 °C under atmospheric and enhanced hydrostatic pressure (1.1 GPa). Defect structure of as‐implanted and processed Si:H was determined by X‐ray scattering methods. Two types of defect clusters contributing to diffuse scattering were revealed: (i) the defects produced by hydrogen diffusion, characterized by small dimensions, with concentration dependent on annealing temperature; (ii) rather large bubbles and platelets produced due to hydrogen agglomeration at defects with negative dilatation. The samples annealed under high pressure contain large defects with the double force tensor equivalent to the dislocation loops one. It is found that the high pressure heat treatment retards the hydrogen diffusion. Distribution of diffuse scattering around the 004 reciprocal lattice point for each type of defects is simulated and compared with the experimental data. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Wide-band transmittance of one-dimensional photonic crystals carved in Si3N4∕SiO2 channel waveguides

Experimental and theoretical investigations of photonic crystal (PhC) structures in silicon nitride/silicon dioxide (Si3N4∕SiO2) vertical waveguiding geometry are reported. One-dimensional patterns, either periodic or with cavity layers, are carved onto the channel waveguides by using focused ion beam lithography. Broadband transmittance spectroscopy in the visible and near-infrared frequency ranges is employed to show photonic band gap behavior up to the fourth order. For structures with a cavity layer, resonant peaks appear in transmittance spectra within the photonic gaps, in agreement with theory. The results show the interest of Si3N4∕SiO2-based PhC waveguides for photonics applications from the infrared up to the visible range.

Etch stop of silicon surface induced by tribo-nanolithography

Tribo-nanolithography (TNL) can form an affected layer on a silicon surface that isresistant to corrosion by KOH, and a nanostructure can be fabricated by combination withwet chemical etching. Transmission electron microscope (TEM), Auger electronspectroscopy (AES) and secondary ion mass spectrometry (SIMS) analyses were utilized toinvestigate the mechanism of the etch stop effect on the machined area. TEM observationrevealed that the silicon crystal structure was converted to an amorphous structuremeasuring approximately 15–20 nm. AES and SIMS analyses indicated that the amorphouslayer consisted entirely of silicon. Thus, the mechanism of the etch stop effect on themachined area was determined to result from the formation of an amorphous siliconstructure.

3D Micro-Fabrication using Combination Technique of Nano-scale Processing and Chemical Etching (4th Report, Mechanism of Masking Effect)

This study is intended to fabricate 3D microstructures on single crystal silicon by tribo-nano-lithography (TNL) and wet chemical etching. The processed area of single crystal silicon by diamond tip withstands etching in KOH solution, and consequently protruding microstructure can be fabricated. Transmission electron microscope (TEM), Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS) analyses are utilized to study the mechanism of masking effect. As a result, it can be known that crystal silicon structures are converted to amorphous silicon by TNL process, resulting in acting to the etch mask against KOH solution. Comparison of etch rate between amorphous and single crystal silicon is conducted. In addition, mechanism of protuberance, which is generated in processing under lower normal load, is studied with minute observation of processed area.

05/00363 The enhancement of homogeneity in the textured structure of silicon crystal by using ultrasonic wave in the caustic etching process: Kim, J. M. and Kim, Y. K. Solar Energy Materials and Solar Cells, 2004, 81, (2), 239–247

05/00363 The enhancement of homogeneity in the textured structure of silicon crystal by using ultrasonic wave in the caustic etching process: Kim, J. M. and Kim, Y. K. Solar Energy Materials and Solar Cells, 2004, 81, (2), 239–247

Defect nucleation in SOI wafers due to hydrogen ion implantation

Hydrogen ion implantation in the Smart-Cut production process leads to fracture of Si–Si bonds, formation of microcavities and splitting of a single-crystal silicon wafer. In the present paper, an analysis model for defect nucleation induced by hydrogen ion implantation is established based on the continuum mechanics theory accounting for the crystal structure of silicon. Using this model and probability theory, an analytical expression is derived to calculate the defect density as a function of the hydrogen ion implantation dose and the temperature.

Kinetic Monte Carlo simulation of formation of microstructures in liquid droplets

We study the deposition of indium droplets on a glass surface and the subsequent formation of silicon microcrystals inside these droplets. Kinetic Monte Carlo methods are used to analyse the influence of growth temperature, flux of incoming particles, surface coverage, and in particular an energy parameter simulating the surface tension, upon the morphology of growth. According to the experimental conditions of crystallization, a temperature gradient and diffusion in spherical droplets are included. The simulations explain the formation of silicon crystal structures in good agreement with the experiment. The dependence of their shape and the conditions of formation on the growth parameters are investigated in detail.

Mechanics of Smart-Cut ® technology

Smart-Cut ® is a recently established, advanced technology for fabricating high-quality silicon-on-insulator (SOI) systems and has found many other successful applications. It meets almost all the high requirements for processing and manufacturing SOI wafers, which provide the basis of ultra-large-scale integration device structures of modern microelectronic industry. In the present paper, we present a fundamental study on the basic mechanisms in the Smart-Cut technology from the viewpoints of mechanics and physics. First, a model for defect nucleation induced by hydrogen ion implantation is established based on the continuum mechanics theory accounting for the crystal structure of silicon. This model is used to provide an upper bound on the implantation dose of hydrogen ions, one of the most important process parameter in the Smart-Cut technology. An analytical formulation is derived to calculate the defect density as a function of the H-implantation dose and the temperature. Then, the splitting of SOI wafers in the Smart-Cut technology is analyzed using the elastic fracture mechanics theory. Accounting for the embrittlement and diffusion effects of hydrogen, a lower bound of the implantation dose of hydrogen ions is derived, which agrees reasonably with experimental observations. Furthermore, the effects of the handle wafer adopted in the Smart-Cut technique are examined on the splitting process. It is found that the handle wafer leads to uniform crack propagation and higher uniformity in the thickness of the final SOI systems, in comparison with conventional techniques to produce SOI substrates, and prohibits the blistering and flaking failure of an H-implanted wafer. This work provides not only a fundamental understanding to the physical mechanisms associated with the Smart-Cut technology but also a useful reference for determining the process parameters of SOI industrial production.

The enhancement of homogeneity in the textured structure of silicon crystal by using ultrasonic wave in the caustic etching process

The presence of the ultrasonic wave in the caustic etching process enhances the etching rate and results in a finer, and more homogeneous, textured structure. The silicon solar cell, texture etched for 20 min at 60°C in the caustic solution with ultrasonic wave, gives higher cell performance than the cell texture etched for 40 min at 70°C without ultrasonic wave. This comparison indicates a strong possibility of lowering the texturing cost of the silicon crystal by saving time and expensive chemicals normally employed in the texturisation of the crystalline silicon.

マイクロ・ナノ単結晶Siの高サイクル疲労試験と疲労寿命予測パラメータの提案(S05-1 ナノ・マイクロ材料の強度評価,S05 薄膜の強度物性と信頼性)

This paper proposes a new high-cycle fatigue parameter for correlating fatigue lives of micro/nanoscale single crystal silicon (Si) structures under bending and tensile stressing. Fatigue tests of micro/nanoscale Si structures under bending/tensile stressing were conducted in order to reveal the influence of deformation mode on Si fatigue lives. Consequently, the authors enabled to propose a shear stress parameter for predicting fatigue lives of Si structures ranging from micro- to nanoscale, regardless of deformation mode and specimen size. The parameter can be used for reliable design of MEMS/NEMS components subjected to fluctuating stress.

Shape Controlling Synthesis — Formation of Fe3Si by the Reaction of Iron with Silicon Tetrachloride and Crystal Structure Refinement

AbstractPure Fe3Si is formed by the reaction of an iron‐wire with SiCl4‐vapor at a reaction temperature of 1000 °C and a SiCl4‐pressure of 2 bar. Remarkable is the fact that the outer shape of the used iron wire has not been changed during the reaction. The crystal structure of Fe3Si was determined using single crystal methods. Fe3Si crystallizes in the space group Fm3m. The lattice parameters depend on the composition.