Silicon Crystals Research Articles

Experimental setup for high-resolution characterization of crystal optics for coherent X-ray beam applications.

Stanford Synchrotron Radiation Lightsource serves a wide scientific community with its variety of X-ray capabilities. Recently, a wiggler X-ray source located at beamline 10-2 has been employed to perform high-resolution rocking curve imaging (RCI) of diamond and silicon crystals. X-ray RCI is invaluable for the development of upcoming cavity-based X-ray sources at SLAC, including the cavity-based X-ray free-electron laser and X-ray laser oscillator. In this paper, the RCI apparatus is described and experimental results are provided to validate its design. Future improvements of the setup are also discussed.

Atomistic modeling and characterizaion of light sources based on small-amplitude short-period periodically bent crystals

The feasibility of gamma-ray light sources based on the channeling phenomenon of ultrarelativistic electrons and positrons in oriented crystals that are periodically bent with Small Amplitude and Short Period (SASP) is demonstrated by means of rigorous numerical modeling that accounts for the interaction of a projectile with all atoms of the crystalline environment. Numerical data on the spectral distribution, brilliance, number of photons and power of radiation emitted by 10 GeV electron and positron beams passing through diamond, silicon and germanium crystals are presented and analyzed. The case studies presented in the paper refer to the FACET-II beams available at the SLAC facility. It is shown that the SASP bending gives rise to the radiation enhancement in the GeV photon energy range where the peak brilliance of radiation can be as high as on the 1024 photons/s/mrad2/mm2/0.1%BW. The parameters of radiation can be tuned by varying the amplitude and period of bending.

Structural Characterization of Nanoparticle-Supported Lipid Bilayer Arrays by Grazing Incidence X-ray and Neutron Scattering

Arrays of nanoparticle-supported lipid bilayers (nanoSLB) are lipid-coated nanopatterned interfaces that provide a platform to study curved model biological membranes using surface-sensitive techniques. We combined scattering techniques with direct imaging, to gain access to sub-nanometer scale structural information on stable nanoparticle monolayers assembled on silicon crystals in a noncovalent manner using a Langmuir-Schaefer deposition. The structure of supported lipid bilayers formed on the nanoparticle arrays via vesicle fusion was investigated using a combination of grazing incidence X-ray and neutron scattering techniques complemented by fluorescence microscopy imaging. Ordered nanoparticle assemblies were shown to be suitable and stable substrates for the formation of curved and fluid lipid bilayers that retained lateral mobility, as shown by fluorescence recovery after photobleaching and quartz crystal microbalance measurements. Neutron reflectometry revealed the formation of high-coverage lipid bilayers around the spherical particles together with a flat lipid bilayer on the substrate below the nanoparticles. The presence of coexisting flat and curved supported lipid bilayers on the same substrate, combined with the sub-nanometer accuracy and isotopic sensitivity of grazing incidence neutron scattering, provides a promising novel approach to investigate curvature-dependent membrane phenomena on supported lipid bilayers.

Some Features of Cleavage Cracks in Rocks and Metals

The cracking of some rocks, namely granite, serpentinite and sandstone under tensile stress is examined in details. Brazilian testing or diametral compression, three points bending and explosion testing are used as the loading schemes in air at room temperature. Morphology of cracks in the model rocks are compared with cracks in silicon crystals, as the standard of a brittle crack, with cleavage cracks in iridium single crystals and with cracks in gallium-covered aluminum single crystals. The comparison of cracks between themselves has shown that there is additional channel for stress accommodation in the model rocks. This channel does not lead to transformation of a rock into a macroscopically ductile material, but it causes the arrest of the dangerous crack in it under tensile stress. Its influence causes transition from the brittle crack to the pore-like crack on the microscopic scale. The most probable mechanism of this transition is the dislocation emission from crack, which becomes possible in such a natural covalent solid as a rock due to Rehbinder's effect.

A magnetically controlled chemical-mechanical polishing (MC-CMP) approach for fabricating channel-cut silicon crystal optics for the High Energy Photon Source.

Crystal monochromators are indispensable optical components for the majority of beamlines at synchrotron radiation facilities. Channel-cut monochromators are sometimes chosen to filter monochromatic X-ray beams by virtue of their ultrahigh angular stability. Nevertheless, high-accuracy polishing on the inner diffracting surfaces remains challenging, thus hampering their performance in preserving the coherence or wavefront of the photon beam. Herein, a magnetically controlled chemical-mechanical polishing (MC-CMP) approach has been successfully developed for fine polishing of the inner surfaces of channel-cut crystals. This MC-CMP process relieves the constraints of narrow working space dictated by small offset requirements and achieves near-perfect polishing on the surface of the crystals. Using this method, a high-quality surface with roughness of 0.614 nm (root mean square, r.m.s.) is obtained in a channel-cut crystal with 7 mm gap designed for beamlines at the High Energy Photon Source, a fourth-generation synchrotron radiation source under construction. On-line X-ray topography and rocking-curve measurements indicate that the stress residual layer on the crystal surface was removed. Firstly, the measured rocking-curve width is in good agreement with the theoretical value. Secondly, the peak reflectivity is very close to theoretical values. Thirdly, topographic images of the optics after polishing were uniform without any speckle or scratches. Only a nearly 2.5 nm-thick SiO2 layer was observed on the perfect crystalline matrix from high-resolution transmission electron microscopy photographs, indicating that the structure of the bulk material is defect- and dislocation-free. Future development of MC-CMP is promising for fabricating wavefront-preserving and ultra-stable channel-cut monochromators, which are crucial to exploit the merits of fourth-generation synchrotron radiation sources or hard X-ray free-electron lasers.

THE ROLE OF ELASTIC STRESS DURING THERMOMIGRATION OF LIQUID ZONES BASED ON ALUMINUM IN SILICON

The method of thermomigration of liquid zones based on aluminum makes it possible to create complex structures of closed channels with boundaries formed from p-n junctions in single-crystal silicon wafers. The channels are characterized by the uniformity of their properties, and the pn junctions are characterized by their sharpness. Such structures are used in high-current electronics, photovoltaics, and microelectromechanical converters. Volumetric deformation inside and outside the channel due to alloying of silicon with aluminum leads to the formation of mechanical stresses. The equilibrium shape of the channels formed at high temperatures is determined by the minimum elastic energy and depends on the material parameters and geometry of the structure. Within the framework of the linear theory of elasticity, the behavior of elastic energy during the formation of the structure of thermomigration channels at high temperatures doped with aluminum in a single-crystalline (001) cut disk was studied. The simulation was performed using the finite element method in the COMSOL Multiphysics mathematical package. The study was carried out for practically important structures in which the direction of the edges is oriented along the diagonal of the square. Only such structures do not have breaks during the process of thermomigration. Based on the calculation results, it was revealed that the minimum elastic energy corresponds to structures with different crystalline orientations inside the channel and outside it – in the main silicon matrix. The direction of the crystalline axes inside the channel, corresponding to the minimum elastic energy, is rotated by 45 degrees in the plane of the disk relative to the direction of the axes of the main matrix of the silicon crystal. In addition, calculations showed that with such a turn, the shape of the channels changes. The minimum elastic energy corresponds not to vertical structures, but to inclined ones. The angle of inclination of the pyramids depends on the width of the channels and the distance between them.

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

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

Recent studies of highly oriented pyrolytic graphite and hybrid graphite-silicon monochromator systems

The excellent performance of highly oriented pyrolytic graphite (HOPG) as monochromator, analyzer and filter material for neutron instrumentation was discovered more than 50 years ago. Even though HOPG has been in use for a long time, there is still room for improvements regarding its optimized use in neutron beam conditioning. The features of a novel concept called POSI based on careful x-ray characterization of the HOPG crystals and on proven mounting and focusing technologies show that the efficiency of HOPG monochromators can be increased while decreasing complexity and cost. Moreover, by combining HOPG and bent perfect silicon crystals the functionality of separate single monochromators and analyzers can be expanded to a wider range of applications provided by dual systems or bichromators.

EVOLUTION OF THE FIELD OF DISTRIBUTED DEFECTS IN A CRYSTAL DURING CONTACT INTERACTION WITH A SYSTEM OF RIGID STAMPS

The article discusses the mathematical modeling for the evolution of the stress-strain state and fields of defects in crystals during their contact interaction with a system of rigid punches. From a macroscopic point of view, the redistribution of defects is characterized by inelastic (viscoplastic) deformation, and therefore the processes under study can be classified as elastic-viscoplastic. Elastic and inelastic deformations are assumed to be finite. To take into account inelastic deformations, it is proposed to use a differential-geometric approach, in which the evolution of the fields of distributed defects is completely characterized by measures of incompatible deformations and quantified by material connection invariants. This connection is generated by a non-Euclidean metric, which, in turn, is given by a field of symmetric linear mappings that define (inconsistent) deformations of the crystal. Since the development of local deformations depends both on thecontact interaction at the boundary and on the distribution of defects in the bulk of the crystal, the simulation problem turns out to be coupled. It is assumed that the local change in the defect density is determined by the first-order Alexander Haasen Sumino evolutionary law, which takes into account the deviatoric part of the stress field. An iterative algorithm has been developed to find coupled fields of local deformations and defects density. The numerical analysis for the model problem was provided for a silicon crystal in the form of a parallelepiped, one face of which is rigidly fixed, and a system of rigid stamps acts on the opposite face. The three-constant Mooney Rivlin potential was used to model the local elastic response.

DC hopping photoconductivity via three-charge-state point defects in partially disordered semiconductors

The stationary (DC) hopping photoconductivity caused by the migration of electrons via intrinsic point t-defects of the same type with three charge states (−1, 0, and +1 in units of elementary charge) is theoretically studied. It is assumed that t-defects are randomly (Poissonian) distributed over a crystal and hops of single electrons occur only via t-defects in the charge states (−1), (0) and (0), (+1). Under the influence of intercenter illumination nonequilibrium charge states (−1) and (+1) of defects are generated due to photostimulated electron transitions between pairs of defects in the charge states (0). During the recombination of nonequilibrium charge states (−1) and (+1) of defects, pairs of defects in the charge states (0) are formed. It is assumed that illumination does not heat the crystal, i.e. does not increase the coefficient of thermal ionization of t-defects. The dependence of the ratio of photoconductivity to dark hopping electrical conductivity on the ratio of photoionization coefficient (γ) of neutral t-defects to coefficient of ‘capture’ (α) of an electron from a negatively charged to a positively charged t-defect is calculated. The calculations of hopping photoconductivity were carried out for the partially disordered silicon crystal with total concentration of t-defects of 3·1019 cm−3, compensated by shallow hydrogen-like donors. The ratios of donor concentration to t-defect concentration (compensation ratios) are 0.25, 0.5, and 0.75. It is taken into account that an electron localization radius on t-defect in the charge state (−1) is greater that on t-defect in the charge state (0). The calculated value of the dark hopping electrical conductivity is consistent with the known experimental data. A negative DC photoconduction at γ > α is predicted, due to a decrease in the concentration of electrons hopping via states (−1), (0) and (0), (+1).

Postproduction Approach to Enhance the External Quantum Efficiency for Red Light-Emitting Diodes Based on Silicon Nanocrystals.

Despite bulk crystals of silicon (Si) being indirect bandgap semiconductors, their quantum dots (QDs) exhibit the superior photoluminescence (PL) properties including high quantum yield (PLQY > 50%) and spectral tunability in a broad wavelength range. Nevertheless, their low optical absorbance character inhibits the bright emission from the SiQDs for phosphor-type light emitting diodes (LEDs). In contrast, a strong electroluminescence is potentially given by serving SiQDs as an emissive layer of current-driven LEDs with (Si-QLEDs) because the charged carriers are supplied from electrodes unlike absorption of light. Herein, we report that the external quantum efficiency (EQE) of Si-QLED was enhanced up to 12.2% by postproduction effect which induced by continuously applied voltage at 5 V for 9 h. The active layer consisted of SiQDs with a diameter of 2.0 nm. Observation of the cross-section of the multilayer QLEDs device revealed that the interparticle distance between adjacent SiQDs in the emissive layer is reduced to 0.95 nm from 1.54 nm by "post-electric-annealing". The shortened distance was effective in promoting charge injection into the emission layer, leading improvement of the EQE.

Perovskite solar cells: Thermal and chemical stability improvement, and economic analysis

Perovskite solar cells (PSCs) are highly efficient and are comparatively cheaper than the large silicon crystals primarily used in solar cells. Their outstanding photovoltaic performance makes them a potential alternative to silicon solar cells. While efficiency and photovoltaic performance have been investigated in recent decades, a knowledge gap on the degradation, economic feasibility and stability of PSCs exists, and their poor stability remains a barrier to commercialization. Thus, this review aims to fill this knowledge gap by focusing on approaches to improve PSCs’ thermal and chemical stability, and their economic viability under different conditions. The structure and manufacture of PSCs are also discussed along with an economic analysis of different perovskite devices. Improvements in thermal stability can be reached by incorporating inorganic materials into the PSC. A PSC model optimized with ZnO improves chemical stability by 8% and works well under low temperatures. To make PSCs more economically feasible, certain parts like counter electrodes (CE) and hole transport materials (HTMs) can be replaced with alternative elements like carbon and inorganic HTMs, respectively. PSCs with long durability and high conversion efficiency will expand the commercial prospects for this material. To bridge the lack of knowledge, further investigation is required on the sustainability and longevity of PSCs.

Development of multi-beam optics for time-resolved X-ray tomography: from π-polarization to σ-polarization

A multi-beam X-ray optics that can image a sample from different directions in a large angular range simultaneously without sample rotation is reported. It consists of 28 thin silicon crystals that are arranged in a σ-polarization diffraction geometry so that the diffracted X-rays from an incident white synchrotron radiation beam pass through the sample. A tomogram of a simple test sample was calculated from projection images recorded with the optics, showing that an alignment accuracy sufficient for X-ray tomography can be achieved.

Controlling the heat, flow, and oxygen transport by double-partitions during continuous Czochralski (CCz) silicon crystal growth

The effect of the depths of two partitions on the heat, flow, and oxygen transport during the CCz growth of an 8-inch diameter silicon crystal is numerically investigated. A crucible in which two partitions are immersed in the silicon melt is so-called a triple-crucible. The different depths of two partitions change the effective wall surface area, which is the main source of oxygen at the growth interface, melt motion, and heat transfer within the crucible. The diffusion and convection of oxygen from the partitions and crucible wall towards the free surface and the crystal-melt (c-m) interface are also affected. The amount of oxygen that dissolves from the effective wall surface into the melt is proportional to the surface temperature and the surface area. The choice of at least one long partition will prevent the entry of unmelted granular silicon into the melt region under the c-m interface. Comparison of various cases shows that when the partition near the c-m interface is longer (90 mm) and the partition near the crucible sidewall is shorter (40 mm), the power consumption and the content of oxygen along the growth interface are lower.

The Use of Exchange Coupled Atom Qubits as Atomic-Scale Magnetic Field Sensors.

Phosphorus atoms in silicon offer a rich quantum computing platform where both nuclear and electron spins can be used to store and process quantum information. While individual control of electron and nuclear spins has been demonstrated, the interplay between them during qubit operations has been largely unexplored. This study investigates the use of exchange-based operation between donor bound electron spins to probe the local magnetic fields experienced by the qubits with exquisite precision at the atomic scale. To achieve this, coherent exchange oscillations are performed between two electron spin qubits, where the left and right qubits are hosted by three and two phosphorus donors, respectively. The frequency spectrum of exchange oscillations shows quantized changes in the local magnetic fields at the qubit sites, corresponding to the different hyperfine coupling between the electron and each of the qubit-hosting nuclear spins. This ability to sense the hyperfine fields of individual nuclear spins using the exchange interaction constitutes a unique metrology technique, which reveals the exact crystallographic arrangements of the phosphorus atoms in the silicon crystal for each qubit. The detailed knowledge obtained of the local magnetic environment can then be used to engineer hyperfine fields in multi-donor qubits for high-fidelity two-qubitgates.

Numerical Simulation of Species Segregation and 2D Distribution in the Floating Zone Silicon Crystals

The distribution of dopants and impurities in silicon grown with the floating zone method determines the electrical resistivity and other important properties of the crystals. A crucial process that defines the transport of these species is the segregation at the crystallization interface. To investigate the influence of the melt flow on the effective segregation coefficient as well as on the global species transport and the resulting distribution in the grown crystal, we developed a new coupled numerical model. Our simulation results include the shape of phase boundaries, melt flow velocity and temperature, species distribution in the melt and, finally, the radial and axial distributions in the grown crystal. We concluded that the effective segregation coefficient is not constant during the growth process but rather increases for larger melt diameters due to less intensive melt mixing.

Growth and Optical Properties of Ga2O3 Layers of Different Crystalline Modifications

In the present work, a new method of growing layers of three main crystal modifications of Ga2O3, namely α-phase, ε-phase, and β-phase, with thickness of 1 µm or more was developed. The method is based on the use of two approaches, namely a combination of Ga2O3 growth using the hydride vapor-phase epitaxy (HVPE) method and the use of a silicon crystal with a buffer layer of dislocation-free silicon carbide as a substrate. As a result, Ga2O3 gallium oxide layers of three major Ga2O3 crystal modifications were grown, namely, α-phase, ε-phase, and β-phase. The substrate temperatures and precursor flux values at which it is possible to grow only α-phase, only ε-phase, or only β-phase without a mixture of these phases were established. It was found that the metastable α- and ε-phases change into the stable β-phase when heated above 900 °C. Experimentally obtained Raman and ellipsometric spectra of α-phase, ε-phase, and β-phase of Ga2O3 are presented. The theoretical study of the Raman spectra and the dependences of dielectric function on photon energy for all three phases was carried out. The vibrations of Ga2O3 atoms corresponding to the main lines of the Raman spectrum of the α-phase, ε-phase, and β-phase were simulated by density functional methods.

Toward attojoule switching energy in logic transistors.

Advances in the theory of semiconductors in the 1930s in addition to the purification of germanium and silicon crystals in the 1940s enabled the point-contact junction transistor in 1947 and initiated the era of semiconductor electronics. Gordon Moore postulated 18 years later that the number of components in an integrated circuit would double every 1 to 2 years with associated reductions in cost per transistor. Transistor density doubling through scaling-the decrease of component sizes-with each new process node continues today, albeit at a slower pace compared with historical rates of scaling. Transistor scaling has resulted in exponential gain in performance and energy efficiency of integrated circuits, which transformed computing from mainframes to personal computers and from mobile computing to cloud computing. Innovations in new materials, transistor structures, and lithographic technologies will enable further scaling. Monolithic 3D integration, design technology co-optimization, alternative switching mechanisms, and cryogenic operation could enable further transistor scaling and improved energy efficiency in the foreseeable future.

Investigation of Facetted Growth in Heavily Doped Silicon Crystals Grown in Mirror Furnaces

Herein, facets and related phenomena are studied for silicon crystals grown in the <100> and <111> directions, using the Zone Melting and Floating Zone techniques. Investigating the central facets of dislocation-free <111> crystals as a baseline allowed for the determination of the local temperature gradients. When comparing these results to dislocated <111> crystals, the presence of dislocations caused a clear reduction in the facet size, correlated with a reduction in the required local supercooling to estimated maximum values of around 0.6 K. Furthermore, for crystals grown on the rough {100} interface, attempts to provoke a morphological instability of the local phase boundary succeeded for crystallization velocities in the range of 10–16 mm/min, in good agreement with theory. Contrary to this observation, crystals grown in the <111> direction remained morphologically stable even at higher crystallization velocities due to the stabilizing effect of the atomically smooth interface. Additionally, crystals grown in the <111> direction with an oxygen skin by the Zone Melting technique reproducibly showed a non-periodic fluctuation of the central facet diameter at a certain translation velocity.

Electronic Properties of Light- and Elevated Temperature-Induced Degradation in Float-Zone Silicon

Light- and elevated temperature-induced degradation (LeTID) causes long-term instabilities, especially in passivated emitter and rear cells, leading to severe performance loss of commercial modules. Despite the hundreds of LeTID reports available in the literature, there is little consensus regarding the underlying defects and defect formation mechanism responsible for this degradation and its exact electronic properties. Recently, it has been shown that a form of carrier-induced degradation similar to that of LeTID is also observed in some high-purity silicon crystals grown by the float-zone method. In this work, using deep level transient spectroscopy and lifetime spectroscopy, we study the role of nitrogen on the degradation. Intentional contaminated samples with different and known level of nitrogen have been specially grown and characterized for this study. We detect the appearance of a set of majority carrier traps in the degraded state of the samples with activation energies 0.1 (H85), 0.43 (H270A), 0.39 (H270B), and 0.46 eV (H200) with respect to the valence band, from which H270A appears to correlate with the degradation. The results show that the extent of degradation in the nitrogen-rich samples is at least double that of the nitrogen-lean samples.