Crystalline Silicon Phase Research Articles

Using phosphorus doped hydrogenated silicon oxycarbide film as a window layer on the light entrance side of silicon heterojunction solar cells: The role of phase separation on electron transport

The phosphorus doped hydrogenated nanocrystalline silicon oxycarbide (n-nc-SiCO:H) layer can be used as a window layer on the light incident side of silicon heterojunction solar cells (HJT). The chemical composition, structural organization and properties of the n-nc-SiCO:H layer deposited from plasma enhanced chemical vapor deposition (PECVD) method can be easily manipulated via radio frequency power density tuned. Phase separation is observed in the n-nc-SiCO:H layer in which nanoscale silicon crystallites were embedded in amorphous silicon oxycarbide matrix. The optical properties of the n-nc-SiCO:H layer are depended on oxygen and carbon atoms incorporation ratio. The conductivity of the n-nc-SiCO:H layer is dominated by activated phosphorus concentration and the phase separation. The activated phosphorus atoms which play an important role on electron transportation are distributed in both crystalline silicon phase and amorphous silicon phase. Both activated phosphorus concentration and band gap value for oxygen rich amorphous silicon oxycarbide layer are determined by incorporation ratio of O atoms. The interplay effects of optical and electrical properties of the n-nc-SiCO:H layers on HJT cells electric performance variation are revealed out in this report. An average efficiency (across ∼120 cells) of 26.3 % was achieved in cells with optimized power density for the n-nc-SiCO:H layer. The results demonstrate that phase separation in the n-nc-SiCO:H layer plays a critical role on electron transportation.

A Study on the Kinetics and Structural Changes of Silicon during Ball Milling

Enhancing our understanding of the ball milling process is important to optimize silicon amorphization, a key factor in improving the performance of lithium-ion battery anodes. To improve this understanding, we quantitatively studied the kinetics of silicon amorphization during ball milling using x-ray diffraction (XRD) and electrochemical methods, among other methods. The percentage of amorphous and crystalline silicon was deduced by fitting the XRD data and electrochemical results. The resulting trend versus ball milling time was then fitted using the Avrami model. The material resulting from ball milling was further studied using scanning and transmission electron microscopy (SEM/TEM). Under extended milling times, we found that a maximum of 86% of the silicon became amorphous. Similarly, the grain size of the crystalline silicon phase could not be reduced below 6 nm. Our observations suggest that amorphization results from defects creation during ball milling. Amorphization then stops when the grain size reaches a limit where defects are no longer formed. Figure 1

The amorphization of crystalline silicon by ball milling

The transformation of crystalline silicon to amorphous silicon during ball milling was quantitatively measured by x-ray diffraction and electrochemical methods. Amorphous silicon was found to form rapidly from the very initial stages of ball milling. Simultaneously, the grain size of the crystalline silicon phase decreased. Under extended milling times it was found that a maximum of 86 % of the silicon became amorphous. Similarly, the grain size of the crystalline silicon phase could not be reduced below 6 nm. This transformation followed an Avrami kinetic model, which is consistent with a system which reaches a steady state. These observations suggest a mechanism in which ball milling generates defects, resulting in silicon amorphization and grain size reduction, where the degree of amorphization is limited in extent because there exists a limiting silicon grain size below which defects are no longer formed.

Deep-learning approach for predicting crystalline phase distribution of femtosecond laser-processed silicon

In this study, two deep-learning models are presented to predict the crystalline phases of femtosecond laser-processed silicon. To obtain the datasets, single-crystal silicon was processed by a femtosecond laser using 49 different combinations of laser fluence and scanning speed, and for each specimen, Raman spectra were measured at 22,500 locations inside a square domain. The first model was trained to classify the Raman spectra of silicon into six silicon phases. By applying the model to the entire surface of a silicon specimen in batches, the silicon phase distribution was visualized in RGB color values, with each color representing a particular silicon phase. Using the classification results of the 49 specimens obtained by the first model, the second model was developed to predict the silicon phase distribution image from the inputs of the laser fluence and scanning speed. The average prediction accuracy of the second model was 86.60%.

Isolated alternative crystalline silicon phase could lead to improved solar cell performance

Method to isolate type II silicon clathrate films with exciting optoelectronic properties reported.

Золото-индуцированная кристаллизация тонких пленок аморфного субоксида кремния

For the first time, polycrystalline silicon (poly-Si) was obtained as a result of the gold-induced crystallization of amorphous silicon suboxide (a-SiOx). It is shown that, during annealing of a sample with a “substrate/gold thin film/a-SiO0.2 thin film” structure at 335 ℃, poly-Si is formed in a bottom layer (on the substrate), while gold diffuses into the upper layer. With an increase in the temperature to 370 ℃, the mechanism of poly-Si formation remains unchanged, however, only the rate of the crystallization process increases. Apparently, the process of poly-Si formation proceeds through the formation of gold silicides, which almost completely decompose into crystalline phases of gold and silicon at 370 ℃ for 10 h.

Simulation-Based Analysis of Structural and Thermodynamic Aspects of the Electrochemical Lithiation of Silicon Nanoparticles

Lithium-ion batteries are among the most predominant energy storage systems for portable to stationary electronic devices. Lithium-ion batteries are indispensable to laptops, mobile phones, and electric vehicles due to their high energy/power density and long cycle life. Silicon has been intensively pursued as one of the most promising anode materials because of its high specific capacity (4200 mAh/g for Li22Si5), in comparison with the conventional graphite (372 mAh/g for LiC6), and its abundance. Despite its high capacity, silicon-based anode materials suffer from fast capacity loss caused by its large volume change (>300%), unstable solid electrolyte interphase and the physical disintegration, such as racking and crumbling, of the electrode structure during lithiation and delithiation processes. Therefore, there are various research activities to control the electrochemical performance of silicon anode materials. The engineering of silicon nanostructures proved to be an effective method for improving capacity and cycling stability, since nano-sized silicon can alleviate mechanical fractures during volume changes. In particular, silicon nanoparticles can be added into the void region in the polycrystalline graphite matrix, resulting in an effective increase in the overall energy density of the anode. In this work, we designed reasonable structural models of silicon nanoparticles in terms of particle size and lithiation ratio through Monte Carlo simulations, and we conducted a series of first-principle simulations to understand their intrinsic electrochemical properties. We reported the theoretical understandings of the detailed structural and thermodynamic mechanism of the actual lithiation process of silicon nanoparticle systems based on atomistic simulation approaches. We found that the rearrangement of the silicon bonding network is the key mechanism of the lithiation process, and that it is less frequently broken by lithiation in the smaller sizes of silicon nanoparticles. The decreased lithiation ability of the silicon nanoparticles results in the lithiation potential being significantly lower than that of crystalline silicon phases, which impedes the full usage of the theoretical maximum capacity. Thus, nanosized slicon could have a negative effect on performance if they become too fine-sized. These findings provide a detailed view of the electrochemical lithiation process of silicon nanoparticles and engineering guidelines for designing new silicon-based nanostructured materials for the anode of high capacity lithium ion batteries.

Mathematical Modeling оf the Silicon Production Process from Pelletized Charge

The paper considers the problem of recycling the dust waste resulting from metallurgical silicon production; such dust contains considerable amounts of valuable silica. The problem is solved by redirecting this byproduct to the silicon smelting process. We herein propose using the dust left in silicon and aluminum production as a component of pelletized charge, used for silicon smelting in ore-thermal furnaces (OTF). Mathematical (physico-chemical) modeling was applied to study the behavior of pelletized-charge components, in order to predict the chemical composition of smelting-produced silicon. We generated a model that simulated the four temperature zones of a furnace, as well as the crystalline-silicon phase (25°С). The model contained 17 elements entering the furnace, due to being contained in raw materials, electrodes, and the air. Modeling produced molten silicon, 91.73 wt% of which was the target product. Modeling showed that, when using the proposed combined charge, silicon extraction factor would amount to 69.25%, which agrees well with practical data. Results of modeling the chemical composition of crystalline silicon agreed well with the chemical analysis of actually produced silicon.

Formation of Nanocrystalline Silicon in Tin-Doped Amorphous Silicon Films

The process of crystalline silicon phase formation in tin-doped amorphous silicon (a-SiSn) films has been studied. The inclusions of metallic tin are shown to play a key role in the crystallization of researched a-SiSn specimens with Sn contents of 1–10 at% at temperatures of 300–500 ∘C. The crystallization process can conditionally be divided into two stages. At the first stage, the formation of metallic tin inclusions occurs in the bulk of as-precipitated films owing to the diffusion of tin atoms in the amorphous silicon matrix. At the second stage, the formation of the nanocrystalline phase of silicon occurs as a result of the motion of silicon atoms from the amorphous phase to the crystalline one through the formed metallic tin inclusions. The presence of the latter ensures the formation of silicon crystallites at a much lower temperature than the solid-phase recrystallization temperature (about 750 ∘C). A possibility for a relation to exist between the sizes of growing silicon nanocrystallites and metallic tin inclusions favoring the formation of nanocrystallites has been analyzed.

Nucleation and growth of metal-catalyzed silicon nanowires under plasma

We report the results of a microscopic study of the nucleation and early growth stages of metal-catalyzed silicon nanowires in plasma-enhanced chemical vapor deposition. The nucleation of silicon nanowires is investigated as a function of different deposition conditions and metal catalysts (Sn, In and Au) using correlation of atomic force microscopy and scanning electron microscopy. This correlation method enabled us to visualize individual catalytic nanoparticles before and after the nanowire growth and identify the key parameters influencing the nanowire nucleation under plasma. The size and position of catalytic nanoparticles are found to play a significant role in the nucleation. We demonstrate that only small isolated nanoparticles in the range of 10–20 nm contribute to the nanowire growth under plasma, while larger nanoparticles are inactive because they get buried under a layer of a-Si:H before reaching supersaturation. Systematic analysis of different growth parameters reveals that the nanowire growth in plasma contradicts the vapor–liquid–solid mechanism at thermal equilibrium in many ways. The nanowire growth is much faster and proceeds even at negligible silicon solubility and bellow the eutectic temperature of the metal-silicon alloy. Based on the observations, we propose the nanowire growth under plasma to be characterized by the rapid solidification mechanism, where a crystalline silicon phase emerges from a metastable supersaturated liquid metal-silicon phase in local nonequilibrium.

Silicon Powder Properties Produced in a Planetary Ball Mill as a Function of Grinding Time, Grinding Bead Size and Rotational Speed

Mechanical milling is a promising route for production of submicron and nano sized silicon powders, but it is challenging to predict and control the product properties. In this study a metallurgical grade silicon quality was milled in a planetary ball mill and the properties of the powder were investigated as a function of grinding time, grinding bead size (20 mm, 2 mm, 0.25 mm) and rotational speed based on the concepts presented in the stress model. The finest powder was characterized by a d50 of 0.62 μm. This powder was produced with the 2 mm grinding beads and 4 h of grinding (i.e. the highest specific energy input to the mill with this bead size). The largest BET specific surface area, the highest concentration of iron contamination and the smallest amount of crystalline silicon phase were characterized in the powder produced using the 0.25 mm grinding beads and 4 h of grinding (i.e. the highest stress number). Aggregates consisting of silicon and iron were formed. An acid wash to reduce the concentration of iron contaminant revealed that silicon did form an amorphous phase as well as the specific surface area increased. With a constant specific energy input to the mill, a reduction in rotational speed (i.e. reduced stress energy) produced similar powder properties with the exception of the particle size distribution.

Ab Initio-Based Structural and Thermodynamic Aspects of the Electrochemical Lithiation of Silicon Nanoparticles

We reported the theoretical understandings of the detailed structural and thermodynamic mechanism of the actual lithiation process of silicon nanoparticle systems based on atomistic simulation approaches. We found that the rearrangement of the Si bonding network is the key mechanism of the lithiation process, and that it is less frequently broken by lithiation in the smaller sizes of Si nanoparticles. The decreased lithiation ability of the Si nanoparticles results in the lithiation potential being significantly lower than that of crystalline silicon phases, which impedes the full usage of the theoretical maximum capacity. Thus, nanosized Si materials could have a negative effect on performance if they become too fine-sized. These findings provide a detailed view of the electrochemical lithiation process of silicon nanoparticles (Si NPs) and engineering guidelines for designing new Si-based nanostructured materials.

Features of Defect Formation in Nanostructured Silicon under Ion Irradiation

Nanostructured silicon is irradiated by Si+ and He+ ions with energies of 200 and 150 keV, respectively. Destruction of the structure of irradiated samples and the accumulation of defects at different irradiation fluences are investigated by Raman scattering. It is shown that single-crystal silicon films are amorphized under irradiation at 0.7 displacements per atom. However, at 0.5 displacements per atom, porous silicon does not completely amorphize and the Raman spectra contain a weak signal of the amorphous silicon phase along with a pronounced signal of the crystalline silicon phase. The size of nanocrystals in the structure of porous silicon at different irradiation fluences is estimated.

Modelling of Quantum Qubit Behaviour for Future Quantum Computers

This work deals with quantum qubit modelling based on a silicon material with embedded phosphorus atoms because a future quantum computer can be built on the basis of this qubit. The building of atomic models of bulk crystalline silicon and silicene, as well as calculation of their total energies, were performed using the Quantum ESPRESSO software package, using highperformance computing (HPC). For silicon and phosphorus atoms the generalized gradient approximation (GGA) was used in terms of the spin-orbit non-collinear interaction by means of the Quantum ESPRESSO package. The equilibrium orientations of the phosphorus qubit spins and localization of the wave functions in the 2D and bulk crystalline silicon phases were theoretically investigated by means of quantum-mechanical calculations. The existence of an exchange interaction between qubits has been confirmed, which leads to a change in the wave function’s localization and spin orientation, and in the case of silicene, this interaction was stronger.

Investigation on sub nano-crystalline silicon thin films grown using pulsed PECVD process

Present work discusses the structural modifications in intrinsic layer of hydrogenated amorphous silicon (a-Si:H) deposited using Pulsed Wave Plasma Enhanced Chemical Vapor Deposition (PW-PECVD) technique which highlights the crystallite formation within the boundaries of nano crystallite silicon thin films. These investigations were carried out for the films deposited under the variation of applied pulsed power from 10 W to 60 W. The resultant film phases were generalized as sub-nano crystalline silicon phases. The evolution of such phases has been effectively probed using various spectroscopic and structural characterization techniques including Raman spectroscopy, Fourier Transform Infrared spectroscopy (FTIR), and Field Emission Scanning Electron Microscopy (FESEM). The observed sub-nano crystalline volume fraction varies from ~ 28–46%. This marks the modification in crystallite growth from the partial nucleation to coalescence phase. From this the importance of pulsed wave PECVD (PW-PECVD) has been discussed in terms of high growth rates as well as the extended transition zones with the formation of sub-nano crystallite structures. The study found to be specific for understanding the sub-nano crystalline phases in the film silicon having high photo-stability and photo-response like in µc/nc-Si:H (micro/nano crystalline silicon) and a-Si:H respectively.

Mathematical model of silicon smelting process basing on pelletized charge from technogenic raw materials

The silicon production process in the electric arc reduction furnaces (EAF) is studied using pelletized charge as an additive to the standard on the basis of the generated mathematical model. The results obtained due to the model will contribute to the analysis of the charge components behavior during melting with the achievement of optimum final parameters of the silicon production process. The authors proposed using technogenic waste as a raw material for the silicon production in a pelletized form using liquid glass and aluminum production dust from the electrostatic precipitators as a binder. The method of mathematical modeling with the help of the ‘Selector’ software package was used as a basis for the theoretical study. A model was simulated with the imitation of four furnace temperature zones and a crystalline silicon phase (25 °C). The main advantage of the created model is the ability to analyze the behavior of all burden materials (including pelletized charge) in the carbothermic process. The behavior analysis is based on the thermodynamic probability data of the burden materials interactions in the carbothermic process. The model accounts for 17 elements entering the furnace with raw materials, electrodes and air. The silicon melt, obtained by the modeling, contained 91.73 % wt. of the target product. The simulation results showed that in the use of the proposed combined charge, the recovery of silicon reached 69.248 %, which is in good agreement with practical data. The results of the crystalline silicon chemical composition modeling are compared with the real silicon samples of chemical analysis data, which showed the results of convergence. The efficiency of the mathematical modeling methods in the studying of the carbothermal silicon obtaining process with complex interphase transformations and the formation of numerous intermediate compounds using a pelletized charge as an additive to the traditional one is shown.

Spectroscopic ellipsometry studies on microstructure evolution of a-Si:H to nc-Si:H films by H2 plasma exposure

This paper presents the influence of hydrogen plasma treatment on the growth of nano crystalline silicon phase in a-SiH films. Series of hydrogenated amorphous silicon films were prepared by PECVD technique with intermittent hydrogen plasma treatment for 5 min each time followed by silane plasma, keeping the total silane plasma time fixed at 60 min. The evolution of nanocrystalline Si phase due to intermittent hydrogen plasma treatment is studied through Spectroscopic Ellipsometry(SE), Raman and FTIR absorption spectroscopy. Dielectric function obtained from the SE measurements was used to determine the optical constant, thickness and band gap of the films. The hydrogen plasma treated films have less void fraction and higher density. The growth of nanocrystalline phase is enhanced due to etching of weak SiSi bonds and reduction in bonding disorder during hydrogen plasma. Hydrogen plasma treatment results in nc-Si:H films at high deposition rate and low substrate temperature.

Effect of the amorphization around spherical nano-pores on the thermal conductivity of nano-porous Silicon

The thermal conductivity of nano-porous Silicon with amorphous shells around the pores is computed by Molecular Dynamics simulations. For the latter property, a systematic investigation of the porosity and the thickness of the amorphous shells has been performed. Sub-amorphous thermal conductivity is reached for systems with large porosity and amorphous shell, while a non-negligible fraction of crystalline Silicon phase is still present. The thermal conductivity of all studied systems can be controlled by a key parameter which is the ratio of crystalline/amorphous or crystalline/void interface to the volume of material.

Advanced Single Particle Measurement Technique for Fundamental Study of First Charging Mechanism of One Silicon Particle

Silicon is very attractive material for the negative electrode of next-generation Li batteries, because of its very large capacity (3580 mAh g-1). However, very large volume change during the lithiation and delithiation causes very rapid electrode degradation. There are many researches for the restriction and accommodation of the volume change of Si based materials by using nano-sized Silicon and functional binders [1]. Also, in-situ TEM observation technique has been utilized to understand the lithiation and delithiation mechanism of nano-sized Si particles [2]. The in-situ TEM technique is excellent technique, however it needs high vacuum condition or no vapor pressure liquid like ionic liquids. It is also indispensable to find the electrochemical characteristics in actual organic electrolytes system. Single particle measurement technique is very useful to reveal the intrinsic electrochemical characteristics of a micro-scale particle in commercial Li-ion battery electrolytes. Furthermore, the effect of binders and conductive agents which are used in standard composite electrode can be removed in this technique. In this study, the single particle measurement technique is used to understand the volume expansion mechanism of a micro-size silicon particle in the first lithiation reaction.The detailed experimental conditions for the single particle measurement were described elsewhere [3]. After the electrochemical characterization, the lithiated silicon particle was picked-up by micro-tweezers in order to transfer the particle to other analytical equipment, for example, SEM, FIB-SEM, TEM, micro-Raman spectroscopy, and Auger electron spectroscopy. In previous study, the volume expansion behavior of a micro-sized silicon particle was focused. Figure 1 shows the drastic volume expansion behavior of a silicon particle during the first lithiation and the charging curve. The measured apparent volume expansion ratio is much larger than the theoretical values [3, 4]. In this study, the further analysis about lithiated silicon particle was conducted. In order to reveal the volume expansion mechanism of a silicon particle, ex-situ SEM, TEM observation was conducted. For TEM observation, the lithiated silicon particle was tailored to be thin slice by FIB treatment. TEM observation revealed that the lithiated silicon particle is mainly amorphous Si-Li phase and only a small parts remain crystalline silicon phase. It is caused by the local electrical disconnection because of very large volume expansion. EDS analysis with SEM and TEM shows surface SEI layer composition, and TEM-EELS analysis reveals chemical species in the SEI layer. The detailed results and the discussion for SEI formation during the initial charging of a silicon particle will be shown in the presentation.[1] M. Wu, et. al., J. Am. Chem. Soc., 135 (2013) 12048.[2] M. T. McDowell, et al., Nano Lett., 13 (2013) 758.[3] K. Nishikawa, et. al., J. Power Sources, 243 (2013) 630.[4] K. Nishikawa, et al., J. Power Sources 302 (2016) 46. Figure 1

Micro-Raman spectroscopy characterization of silicon with different structures irradiated with energetic Bi-ions

Researches of irradiation effects on silicon possess not only fundamental interests but also potential application prospects. Comparison studies about structural modification of silicon materials with different structures under identical irradiation conditions can reveal the irradiation mechanisms for amorphous and crystalline phases of silicon. For this purpose, amorphous silicon (a-Si) and nano-crystalline silicon (nc-Si) films as well as mono-crystalline silicon (c-Si) samples were irradiated with 6.0MeV Bi-ions at room temperature. The ion fluences are 1.0×1013, 5.0×1013 and 1.0×1014ions/cm2. All samples were analyzed by using a Raman spectrometer. The obtained results show that the crystalline fraction of c-Si and nc-Si decrease with increasing fluence, which indicates that the irradiation induces the amorphization of nc-Si and c-Si samples. In addition, the variation in Raman frequency of crystalline peak after irradiation reveals that the irradiation also results in the increased stress in crystalline phase of c-Si and nc-Si samples. As the fluence increases, the bond angle deviation and the ratio of TA to TO mode of amorphous network of a-Si and nc-Si films initially increase and then decrease by a diminishing degree, while the bond angle deviation and the ratio of TA to TO mode of amorphous network of c-Si samples increase continuously. This gives the dependence of short-range structural order of amorphous network of a-Si, nc-Si and c-Si samples on the ion fluence, which is related with the irradiation induced variation of local free energy. It is considered that the irradiation induced structural modification of silicon samples is mainly attributed to the nuclear energy loss. The irradiation effects of energetic heavy-ions on crystalline and amorphous phases of silicon have been discussed, respectively.