Single Crystal Si Research Articles

Experimental investigations on the laser-assisted machining of single crystal Si for optimal machining

Single crystal Si is a typical infrared optical material, which is widely used in infrared optical systems such as lenses, prisms, windows and filters. But its high brittleness, hardness, and low fracture toughness make it difficult to machine using conventional machining (CM). Laser-assisted machining (LAM) method has become a promising solution for the machining of single crystal Si in recent years. This paper investigated experimentally the effect of spindle speed, feed speed, cutting depth, and laser pulse duty cycle on the surface roughness during the LAM of single crystal Si based on the Taguchi method (TM) and Response surface methodology (RSM). Orthogonal array and Box-Behnken design were used to design the experiment. Analysis of variance, signal-to-noise ratio, main effect plots, 3D response surface, and corresponding contour plots were measured to evaluate the effect of factors on the surface roughness. Based on RSM, a regression model between process parameters and surface roughness was established. The experimental results on LAM demonstrated a considerable improvement in the machinability of single crystal Si through the improved surface quality, high material removal rate, as well as the surface roughness reduction, compared to CM. The contribution rates of spindle speed, feed speed, cutting depth and pulse duty cycle to surface roughness were 21.85%, 39.95%, 10.29% and 14.27%, respectively. According to the 3D response surface, the interaction between different factors had a significant effect on the surface roughness. ANOVA and response optimization were employed to find optimal machining conditions for LAM. Finally, the Optimal parametric combination was as following: a spindle speed of 4000 rpm, a feed speed of 2 mm/min, a cutting depth of 5.86 μm, and a pulse duty cycle of 27.60%. The maximum value of measured roughness was 1.9 nm.

Investigation on the Effect of Charge Carriers in Chemical Shift Anisotropy of Si Single Crystal

The Chemical Shift Anisotropy (CSA) of single crystalline silicon (Si) was investigated using 29Si static NMR for commercially procured p-type and n-type silicon. The observed CSA has been distributed between -80 ppm to -90 ppm for n-type and p-type Si. The NMR parameters such as Isotropic Chemical Shift (5iso), Chemical Shift Anisotropy tensor (ΔS) and the asymmetry parameter (77) of the CSA were extracted by using DMFIT with the CSA-static model. The ΔS and rj are found to be -1.73 ppm and 0.3 ppm respectively for the n-type and the asymmetry parameter of the CSA was found one order different from p-type material. The changes observed in CSA pattern and spin-lattice relaxation (T 1) time are ascribed to strength (mobility) of the shallow donor level and shallow acceptor level effects in the Fermi contact term.

Factors affecting Confidence Index in EBSD analysis

Automated EBSD analysis systems have been used for over two decades relying on the fact that the correctness of a particular orientation solution could be assessed through the calculation of a Confidence Index. However, such an index only reports whether a particular solution stands out among any other possible solutions, by receiving a larger number of votes than others, and does not necessarily imply that the corresponding orientation is correct. This issue is addressed in the present paper, where the correctness of solutions and the factors that might affect it have been studied for single crystal Si and polycrystalline Zn. The results were compared to those presented in previous papers where this matter was studied in particular for an FCC material. It was observed that about 90% of the solutions were correct if they were obtained using a confidence index of 0.1 using at least 8 Hough bands regardless of the crystallographic structure or orientation.

Modified Stoney’s equation with anisotropic substrates undergoing large deformations

Thin film deposition is a key fabrication step in several opto-mechanical and electronics applications. For instance, thin film coatings can act as reflective interfaces, protective coatings or they can be used to modify the thermal behavior of the film-substrate system. For effective design, it is important to understand the deformation of the system in response to residual stresses during thin film deposition. The widely used Stoney’s equation to measure this deformation assumes isotropic elasticity of the substrate together with infinitesimal strains and rotations. In this work we relax both constraints, where we study the deformation of commonly used substrates made of single crystal Si(001) and Si(111) wafers that undergo large rotations. We derive relations between normalized substrate curvature and thin film mismatch strain and validate our analysis with numerical results. The methodology presented can be used for more accurate understanding of a broad range of film-substrate systems in semiconductors.

Influence of Mo Doping on the Microstructure, Friction, and Wear Properties of CrAlN Films

In this study, closed-field unbalanced magnetron sputtering ion-plating technology is used to deposit CrAlMoN and CrAlN nanomultilayer films on the surfaces of M2 tool steel and single-crystal Si. Energy-dispersive spectroscopy, scanning electron microscopy, x-ray photoelectron spectroscopy, transmission electron microscopy, x-ray diffraction, scratch tester, nanoindenter, and pin-on-disk wear tester are used to study the influence of Mo doping on the composition, microstructure, mechanical properties, and friction properties of CrAlN films. Results show that CrAlMoN films have a nanomultilayer structure, CrN/Mo2N/CrN/AlN, with a modulation period of 10.5 nm. After Mo doping, the face-centered cubic structure of CrAlMoN films remains the same as that of CrAlN films. The chemical valence of each element in CrAlN films does not change with Mo doping. At room temperature, the wear resistance of CrAlMoN films is significantly improved compared with that of CrAlN films. This is attributed to the significant antifriction effect of MoO3 that forms on CrAlMoN films doped with Mo during the friction process.

Resonant Sensing Element Realized as a Single Crystal Si Cantilever Actuated by Fringing Electrostatic Fields

We report on the fabrication and characterization of a generic, manufacturable, cantilever-type resonant sensing element, actuated by fringing electrostatic fields and fabricated from a silicon on insulator (SOI) wafer. The architecture of the electrode, designed to be thicker than the cantilever, was tailored to allow efficient electrostatic up-tuning of the beam’s frequency and enhancement of the device frequency to voltage/displacement sensitivity in the initial “as fabricated” configuration. The device is not prone to undesired pull-in instability and can be operated at large deflections, which increases the dynamic range. The multilevel structure was fabricated using a two-stage critically timed deep reactive ion etching (DRIE). The device was operated statically and dynamically by a combination of dc and ac voltages, and an increase of the resonant frequency with the increasing voltage/deflection was registered. Consistently with the model predictions, the experimental results suggest that this type of sensing element can be promising for implementation in a large variety of resonant, inertial, force or pressure sensors.

Quantitative agreement between dynamical rocking curves in ultrafast electron diffraction for x-ray lasers

Electron diffraction through a thin patterned silicon membrane can be used to create complex spatial modulations in electron distributions. By precisely varying parameters such as crystallographic orientation and wafer thickness, the intensity of reflections in the diffraction plane can be controlled and by placing an aperture to block all but one spot, we can form an image with different parts of the patterned membrane, as is done for bright-field imaging in microscopy. The patterned electron beams can then be used to control phase and amplitude of subsequent x-ray emission, enabling novel coherent x-ray methods. The electrons themselves can also be used for femtosecond time resolved diffraction and microscopy. As a first step toward patterned beams, we demonstrate experimentally and through simulation the ability to accurately predict and control diffraction spot intensities. We simulate MeV transmission electron diffraction patterns using the multislice method for various crystallographic orientations of a single crystal Si(001) membrane near beam normal. The resulting intensity maps of the Bragg reflections are compared to experimental results obtained at the Accelerator Structure Test Area Ultrafast Electron Diffraction (ASTA UED) facility at SLAC. Furthermore, the fraction of inelastic and elastic scattering of the initial charge is estimated along with the absorption of the membrane to determine the contrast that would be seen in a patterned version of the Si(001) membrane.

Magnetic Properties of Thin Epitaxial SiC Layers Grown by the Atom-Substitution Method on Single-Crystal Silicon Surfaces

A cycle of experimental investigations is carried out, specifically, the measurements and analysis of field dependences of the static magnetic susceptibility in samples of single-crystal SiC thin films grown on the (100), (110), and (111) surfaces of a Si single crystal by the method of the self-consistent substitution of atoms due to the chemical reaction of silicon with carbon-monoxide (CO) gas. As a result of investigations in SiC structures grown on Si(110) and Si(111), the appearance of two quantum effects is found in weak magnetic fields at room temperature. These effects are, first, the formation of hysteresis of the static magnetic susceptibility and, second, the generation of Aharonov–Bohm oscillations in the field dependences of the static magnetic susceptibility. The first effect is associated with the Meissner–Ochsenfeld effect and the second effect, with the presence of microdefects in the form of nanotubes and micropores formed during the synthesis of structures in them under the SiC layer. In the SiC structures grown on Si(100), these effects are not detected, which is related to a different mechanism of SiC formation on the Si(100) surface.

Three-Component Zone-Melting Method: Modeling of the Concentration-Component Distribution in Single Crystals of Ge–Si Solid Solutions

The concept and theoretical basis of the three-component zone melting method is given for growing single crystals of semiconductor solid solutions using seeds from constituent components. In the Pfann approximation, the problem of the axial concentration distribution of components in Ge–Si single crystals grown at different values of the operating parameters such as the length of the melted zone and the composition of the initial macrohomogeneous rods of solid solutions is solved. Analysis of the obtained results determines the potentialities of the method and the optimal conditions for growing single crystals with given homogeneous and graded compositions in the entire continuous series of Ge–Si solid solutions. It is shown that the three-component zone-melting method is promising for the growth of single crystals of semiconductor solid solutions.

Characteristics of 21H-SiC Thin Film-Based Schottky Barrier Diodes Using TiN Contacts

The fabrication of Schottky barrier diodes based on thin films of 21H polytype of SiC is reported. The films were deposited using a single composite target of Si and graphite by magnetron sputtering. The formation of the 21H polytype of SiC was confirmed by x-ray diffraction. The devices were fabricated on single-crystal Si substrates, and electrical properties with TiN and Au/TiN as top lateral contacts were investigated. The rectifying nature, temperature (in the range 300–423 K), and top electrode work function dependence of Schottky parameters were investigated. The room-temperature turn-on voltages for the TiN and Au/TiN top contacts were 8.9 V and 12.8 V, respectively. The ideality factor decreased while the barrier height increased with an increase in temperature. The barrier height for different temperatures was in the range of 1.16–0.82 eV and 0.9–0.77 eV for TiN and Au/TiN interfaces with SiC, respectively. The presence of TiCxNy on the surface of the TiN-SiC-Si-Au device at high temperatures observed using Raman spectroscopy revealed the inhomogeneity due to variation in local interfacial structure. It was demonstrated that the 21H-SiC-based thin film Schottky barrier diodes are a promising alternative for many applications.

Point defect detection and strain mapping in Si single crystal by two-dimensional multiplication moiré method.

Although defect detection is critical for evaluating the manufacturing processes of semiconductor materials and metals, the detection of crystal defects, especially point defects, over a large field of view still faces considerable challenges. Herein, we report on the development of a two-dimensional (2D) multiplication moiré method using digital image processing to simultaneously detect point and line defects in a wide field of view. Defect locations were automatically detected by employing the concept of a hybrid strain, that is, the absolute value of the product of the strain distributions in different principal directions. To demonstrate a typical application of the proposed method, the hybrid strain distribution in a Si single crystal was measured, and point defects were successfully detected by transmission electron microscopy. The effectiveness of the proposed method was experimentally verified based on the enlarged views of atomic structures at several detected defect locations. This method is capable of visualizing defects by magnifying the lattice distortion in situ, which is a good solution to the problem faced by traditional methods in detecting point defects. This study paves the way for the detection of vacancies, interstitial atoms, substitutional atoms, dislocations, slips, and interfaces in various crystal structures and 2D materials.

Experimental and theoretical study of the effect of bombardment with Ar+ ions on the spectrum of valence electrons of a Si (111) single crystal

The paper studies the effect of disordering of the surface layers on the electronic and optical properties of single-crystal silicon.An analysis of the photoelectron spectra shows that with complete amorphization of the surface density, the condition of Si valence electrons of changes significantly. In particular, the positions of the main maximum of the electrons of the valence band of Si (111) shift by ~ 0.4 eV towards higher binding energies and the band gap Eg increases by 0.1-0.15 eV. The energy of a valence electron in amorphized silicon, which at low impurity concentrations, i.e. at low bombardment doses (D <1015 cm–2), the potential Mkk and, therefore, the shift of the maximum of the density of states vary linearly with respect to concentration. At high impurity concentrations (at doses D> 1015 cm–2), corresponding to the transition to amorphous silicon, the concentration dependence of Mkk is very weak. Therefore, upon amorphization, the peak A of the density of states of the silicon valence band under consideration is shifted to the region of lower electron binding energies. The theoretical substantiation of the obtained experimental results is given.

HRTEM image-stitching for measurement of distances

AbstractVery accurate measurement of distances in the order of several µm is demonstrated on a single crystal Si sample by counting the lattice fringes on stitched high resolution TEM/STEM images. Stitching of TEM images commonly relies on correspondence points found in the image, however, the nearly perfect periodic nature of a lattice image renders such a procedure very unreliable. To overcome this difficulty artificial correspondence points are created on the sample using the electron beam. An accuracy better than 1% can be reached while measuring distances in the order of 1 µm. A detailed description of the process is provided, and its usability for accurately measuring large distances is discussed in detail.

Cycle Stability of All-Solid-State Lithium-Ion Batteries with Nanoporous Si Composite Anodes Prepared from SiO2 Fume

A lot of anode materials have been reported to enhance the storage capacity and energy density of lithium-ion batteries (LIBs). Among such advanced candidates, Si is the most promising, mainly for the following reasons: 1) high theoretical capacity, 2) low operating potential, and 3) abundance in nature. However, the cycle life is short because the decomposition proceeds by the large volume change (> 300%) with the storage and release of Li+ ion. Very recently, we have found that sulfide-based all-solid-state LIBs (ASSLIBs) with Si composite anodes exhibited high capacity retention [1,2]. Here, nanoporous-Si particles, which were prepared through the air-oxidation of Mg2Si, were adapted as the anode active material. In the present study, the reaction of Mg2Si and SiO2 fume under vacuum was used to obtain the nanoporous-Si particles. In PRiME, we will report the cycle stability of ASSLIB comprising new Si composite anode.The nanoporous-Si particles were prepared as follows under Ar atmosphere, unless otherwise specified. Mg2Si (382.9 mg), SiO2 fume (300.0 mg), and Mg (12.1 mg) were mechanically milled at 1800 rpm for 15 min by using a high-speed mixer. The mixture was heated to 700 oC with a ramp rate of 100 oC min-1, and it was held at 700 oC for 12 hours in vacuum. The products were soaked in 1M HCl to remove MgO and unreacted Mg2Si, and then collected by centrifugation. Finally, the precipitates were rinsed by deionized water and ethanol three and two times, respectively. Anode composite material was comprised of the nanoporous-Si particles (36wt%), Li3PS4 electrolyte (55wt%), and acetylene black (9wt%). In an electric insulation tube with 10 mm diameter, the Li3PS4 powder (80 mg) and the anode composite material (2 mg) were pressed under 330 MPa to make two-layered pellet. As the counter electrode, Li-In foil was attached on the electrolyte side. Finally, the three-layered pellet was compressed at ca. 75 MPa using stainless-steel disks to prepare the all-solid-state half cells. By a charge/discharge measurement device (BTS-2004, Nagano), electrochemical tests were conducted in the constant current mode of 0.127 mA cm-2 (0.033 C) for initial 3 cycles and 0.3 mA cm-2 (0.077 C) for the following cycles at 30 oC. The cut-off voltages were 0.88 and -0.58 V vs Li-In, corresponding to 1.50 and 0.04 V vs Li+/Li.All the peaks in XRD pattern of the nanoporous-Si particles were indexed to a diamond cubic Si. The calculated lattice constant of a = 0.544 nm was close to the reported value of a = 0.543 nm (JPCDS 27-1402). By using Scherrer equation, the crystallite size was estimated to be about 39.8 nm. In the Raman spectrum, there was a broad peak at 496 cm-1. Compared with the strong scattering at 520 cm-1 in single crystal Si, assignable to the k~0 transverse optical (TO) mode, the signal shifted to lower frequency with broadening. This is due to the decrease in the crystallite size. As shown in Figure 1, the disordered nanometer pores were observed in TEM image. Based on Brunauer-Emmett-Teller analysis, the specific surface area and pore size were found to be approximately 213 m2 g-1 and 5.9 nm, respectively. These experimental results indicate that the nanoporous-Si particles were successfully prepared by the reaction of Mg2Si and SiO2 fume under vacuum. Figure 2 shows the cycle characteristics of the half cell with the nanoporous-Si composite anode. The measurement result on the cell with the composite anode containing commercially available non-porous Si particles is also exhibited to compare the features. Though the initial discharge capacities were similar in both cells, there were large difference in the cycle stability. Namely, the nanoporous-Si cell maintained the capacity retention of 78% at 50th cycle, while 17% in the non-porous Si cell. These results strongly suggest that the volumetric expansion is buffered by the shrinkage of pores in the Si particles. Our finding provides a novel and unique process to improve the cycle stability, and valuable information about the design of composite anodes for ASSLIBs.

Adjusting the Morphology and Properties of SiC Nanowires by Catalyst Control.

We report on the growth of SiC nanowires on a single crystal Si substrate by pyrolysis of polycarbosilane and using two catalyst (Al2O3 and Ni) films with different thickness (2, 4, and 6 nm). The catalyst films were deposited on the Si substrate, and the SiC nanowires were grown according to two mechanisms, i.e., the oxide-assisted growth mechanism and vapor- liquid-solid mechanism. As a result, pearl-chain-like SiC nanowires and straight SiC nanowires were obtained. The prepared nanowires exhibited excellent photoluminescence properties, emission spectra displaying two emission peaks at 395 and 465 nm, and have good thermal stability below 1000 °C. The experimental results revealed the importance of the catalyst in controlling the morphology and properties of SiC nanowires.

High-Sensitivity Temperature Sensor on the Basis of Single-Crystal Si(111) Implanted from Multiple Directions with P+ and B+ Ions

The characteristics of available temperature sensors based on single crystals of silicon are analyzed and the causes of limitation of the upper range limit of the temperature are determined. To increase the sensitivity, expansion of the temperature range, and achievement of the linear output characteristic of the temperature sensor, it is proposed that the step-by-step implantation of P+ and B+ ions, with a gradual decrease of energy, be carried out in multiple directions of the Si (111) crystal with the subsequent brief thermal annealing, laser annealing, and annealing by infrared (IR) radiation, after each step of ion implantation. The profiles of the distribution of P and B atoms implanted in Si with a gradual decrease in the energy are studied. The influence of subsequent thermal annealing and IR radiation on the profiles of the distribution of atoms and characteristics of the temperature sensor is investigated. A p–i–n-diode with a high concentration of electrically active atoms (NP = 1021 cm–3, NB = 2 × 1021 cm–3), with an almost step distribution of P and B atoms and a sharp interface of the p–i and i–n-regions, with high thermal sensitivity (2.3 mV/K) within a wide temperature range of 20–550 K, is manufactured in this work.

Effect of the Disordering of Thin Surface Layers on the Electronic and Optical Properties of Si(111)

The degree of disordering and the thickness d of disordered layers and their effect on the band gap Eg of single-crystal Si(111) under Ar+-ion bombardment are studied for the first time. It is shown that the d value at ion energies of E0 = 1 and 2 keV is ~(100–120) and ~(150–160) A, respectively. In this case, the density of states of electrons in the Si(111) valence band significantly changes, the light transmittance decreases to K = 55–60%, and the Eg value increases by ~10%. Under Ni+-ion bombardment, surface disordering is accompanied by a sharp change in the composition of the surface layers and, as a result, the K value decreases to 5–10%. After heating at T = 900 K, nanocrystals (at doses of D ≤ 1015 cm–2) and NiSi2 nanofilms (at D = 6 × 1016 cm–2) are formed.

An efficient approach for atomic-scale polishing of single-crystal silicon via plasma-based atom-selective etching

To realize the damage-free, highly efficient, and atomic-level polishing of single-crystal Si, plasma-based atom-selective etching (PASE) is proposed in this study as a generic polishing approach for Si wafers. The polishing effect of PASE is realized through the selective removal of Si atoms with more dangling bonds under high temperature. Plasma diagnostics are carried out to investigate the radical composition, the density of plasma, and the etching temperature. The key parameters of PASE are optimized, and the PASE of Si (100) with a material removal rate greater than 0.7 μm/min was realized. A ground Si (100) surface can be quickly smoothed by PASE, with the Sa roughness being reduced from 195 nm to below 1.0 nm within 5 min, and the polished surface is proven to be crystallographically perfect. The PASE of (110)- and (111)-oriented Si wafers is also proven effective, demonstrating that PASE is a generic polishing approach for Si regardless of orientation. The entire surface flattening of a 2-inch Si wafer was carried out by numerically controlled PASE, and the wafer flatness was reduced from 37.29 μm to 4.92 μm through optimized scanning conditions. Overall, this study has shown that PASE is a promising approach for high-efficiency and high-quality polishing of Si.

The temperature induced current transport characteristics in the orthoferrite YbFeO3− δ thin film/p-type Si structure

The perovskite ytterbium ferrite is a new ferroelectric semiconductor material. We presented the temperature induced current–voltage (I–V) characteristics of the Al/YbFeO3−δ /p-Si/Al hetero-junction. The orthoferrite YbFeO3−δ thin films were deposited on a single crystal p-type Si substrate by a radio frequency magnetron sputtering system. The potential barrier height (BH) and ideality factor n of the heterojunction were obtained by thermionic emission current method based on the recommendations in the literature. The fact that the calculated slopes of I–V curves become temperature independent implying that the field emission current mechanism takes place across the device, which has been explained by the presence of the spatial inhomogeneity of BHs or potential fluctuations. Moreover, a tunneling transmission coefficient value of 26.67 was obtained for the ferroelectric YbFeO3−δ layer at the Al/p-Si interface.

Soft modes behavior in bilayer (Ba,Sr)TiO3/(Ba,Sr)TiO3 heterostructures

We studied the bilayer heterostructures with the different consequence of the Ba0.4Sr0.6TiO3 and Ba0.8Sr0.2TiO3 layers grown on the single-crystal MgO and Si substrates. The thermodynamic theory for the heterostructures was developed. Based on the experimental data, we determined the thermal deformation parameters in the Ba0.4Sr0.6TiO3 and Ba0.8Sr0.2TiO3 layers, forming the bilayer structures. These parameters are used to plot the temperature dependence of the soft modes in each layer.