Crystalline Ingots Research Articles

Assessment of HVAF thermally sprayed coatings: Unraveling microstructural, electrochemical, and tribological performance using glass former Fe-Cr-Mo-Nb-B feedstock powder

In this paper the microstructural features of the glass former Fe68Cr8Mo4Nb4B16 coatings are unveiled and related to their electrochemical and tribological responses. The coating was mostly glassy with some embedded borides (M3B2, M2B-tetragonal; M being the metallic elements of the alloy) and ferrite. The tribological behavior of the HVAF coated sample, characterized by a thickness of about 200 µm, ∼6% porosity and a Vickers hardness of 357 HV0.5, was assessed in a sphere-on-plate configuration, revealing a specific wear rate of approximately 5 ×10−4 mm3∙N−1m−1. The wear mechanism was dominated by delamination caused by fragile intersplats. The corrosion resistance of HVAF coatings was evaluated in 0.6 M NaCl solution and compared with the results obtained for the crystalline Fe68Cr8Mo4Nb4B16 ingot, produced by melting in an induction furnace, and for the AISI 1020 steel substrate. The HVAF coating showed satisfactory corrosion resistance compared to the carbon steel substrate and the crystalline ingot, with the highest corrosion potential, Ecorr, values (−533 mVSCE) and the lowest corrosion current density, icorr, (10−6 A∙cm−2) followed by a clear passivation window upon anodic polarization in 0.6 M NaCl solution. Evaluations of HVAF coating showed a higher glassy content compared to the gas-atomized feedstock powders. This suggests that during spraying, certain particles were molten and experienced cooling rates adequate to inhibit crystallization, resulting in the freezing of the supercooled liquid. This phenomenon contributes to the good corrosion resistance observed in the present work and offers an opportunity to enhance the electrochemical behavior of HVAF coatings.

Production of detector grade CdZnTe crystal with VGF furnace by analyzing segregation of Zn and In

Production of CdZnTe crystals for radiation detector applications through Bridgman and Vertical Gradient Freeze (VGF) techniques is challenging in many aspects. Due to the structural characteristics of CdZnTe, it is not possible to obtain a large single crystalline ingots from the melt growth technique. In addition, the segregation of dopants and Zn decrease the yield significantly. In order to achieve detector-grade CdZnTe crystals, zinc segregation behavior should be fully understood, and optimum dopant concentration (In) should be determined. In this work, indium-doped Cd0.9Zn0.1Te single crystals grown via the Vertical Gradient Freeze (VGF) technique were compared in terms of dopant segregation, defect concentration, electron mobility, and electrical resistivity. It is observed that Zn segregates towards the tip of the ingot while In segregates towards the heel. Zn segregation coefficient is calculated as 1.35 while In segregation coefficient is found to be 0.4 for the VGF method. The variation of Zn throughout the ingot and excessive In dopant concentration degraded the detector performance. CdZnTe crystals having In doping concentrations of more than 5 ppm were unresponsive to gamma radiation. (µτ)e of VGF-grown crystals in METU Crystal Growth Laboratory is calculated as 4–5 × 10−4 cm2/V.

Crystalline-face-dependent photoelectrochemical properties of single crystalline CuGaSe2 photocathodes for hydrogen evolution under sunlight radiation

Photoelectrochemical (PEC) water splitting by using a electrode based on a single crystalline compound provide a good opportunity for studying on fundamental aspects of net semiconductor bulk properties related to PEC performance. In this study, single crystalline CuGaSe2 wafers exposed (321), (312), and (112) facets were used as sources of photocathodes for investigating PEC properties for water reduction into hydrogen under sunlight radiation related to their surface properties. A CuGaSe2 single crystalline ingot grown by using the zone melting method was cut along with different directions to obtain wafers with different crystal orientations. The electrical and electron-energy properties of wafers after annealing in an Se atmosphere indicated the formation of a Cu-deficient component on their surfaces when the wafers oriented (321) and (312) directions were used, whereas formation of a component was not observed on the wafer oriented in the (112) direction. Since the Cu-deficient component acted as a hole blocking layer to suppress the recombination of photoexcited carries at the heterojunction, photocathodes based on wafers having (321) and (312) orientations showed enhanced PEC performance for water reduction compared to the PEC performance of a photocathode based on a wafer having the (112) orientation. Highly active photocathodes for water reduction based on polycrystalline thin films usually have preferential (112) orientation, whereas those of densely packed powder forms would not be probable for controlling crystalline orientations. Therefore, present results would be a guide to achieve further improvements of PEC water reduction using such conventional systems.

Organic heterostructured scintillators with a high pulse shape discrimination capability

Recently, we have developed a new type of the scintillation material, namely organic composite scintillators. It is a non-scintillating transparent gel composition (a polysiloxane elastomer) containing grains of organic single crystals. The grains are obtained after directional crystallization of the material by crushing a crystalline ingot under a layer of liquid nitrogen. This approach removes technological restrictions on the area of the entrance window of a scintillator, does not require the growth of structurally perfect single crystals and their subsequent mechanical treatment. In contrast to organic single crystals and liquids, these materials are not continuous media but heterostructured ones.The ability of single crystals and liquids to separate signals from radiations with different specific energy losses dE/dx is well known. Due to the peculiarities of the creation and recombination of triplet-excited (T) states in these objects is also well studied. Such information on heterostructured scintillation materials, for which the grain size can limit migration of T-states, is practically absent.We discuss the results of the investigation of the luminescence spectra upon excitation by light into the T-states absorption region and the relative light yield of composite scintillators containing p-terphenyl grains (both activated and non-activated) and trans-stilbene grains. We used the grain fractions from 0.04 to 1.0 mm. We obtained and studied both single-layer samples (the thickness of a sample practically corresponded to the grain size of the scintillation material) and samples of 5 mm thick. The research results showed that samples with grain fractions of less than 0.04, 0.06 and between 0.06 and 0.1 mm have low intensity of the delayed fluorescence and relative light output values. We obtained that to separate radiations with different dE/dx by the shape of the radioluminescence pulse it is advisable to use grain fractions larger than 0.1 mm. We also discuss the physical processes that can lead to such results.

Strong Antibonding I (p)-Cu (d) States Lead to Intrinsically Low Thermal Conductivity in CuBiI4.

Chemical bonding present in crystalline solids has a significant impact on how heat moves through a lattice, and with the right chemical tuning, one can achieve extremely low thermal conductivity. The desire for intrinsically low lattice thermal conductivity (κlat) has gained widespread attention in thermoelectrics, in refractories, and nowadays in photovoltaics and optoelectronics. Here we have synthesized a high-quality crystalline ingot of cubic metal halide CuBiI4 and explored its chemical bonding and thermal transport properties. It exhibits an intrinsically ultralow κlat of ∼0.34-0.28 W m-1 K-1 in the temperature range 4-423 K with an Umklapp crystalline peak of 1.82 W m-1 K-1 at 20 K, which is surprisingly lower than other copper-based halide or chalcogenide materials. The crystal orbital Hamilton population analysis shows that antibonding states generated just below the Fermi level (Ef), which arise from robust copper 3d and iodine 5p interactions, cause copper-iodide bond weakening, which leads to reduction of the elastic moduli and softens the lattice, finally to produce extremely low κlat in CuBiI4. The chemical bonding hierarchy with mixed covalent and ionic interactions present in the complex crystal structure generates significant lattice anharmonicity and a low participation ratio in low-lying optical phonon modes originating mostly from localized copper-iodide bond vibrations. We have obtained experimental evidence of these low-lying modes by low-temperature specific heat capacity measurement as well as Raman spectroscopy. The presence of strong p-d antibonding interactions between copper and iodine leads to anharmonic soft crystal lattice which gives rise to low-energy localized optical phonon bands, suppressing the heat-carrying acoustic phonons to steer intrinsically ultralow κlat in CuBiI4.

(Digital Presentation) Growth of Mn4+-Activated Red-Emitting Photoluminescent Crystals

Compared with Mn4+-activated polycrystalline phosphors in micro powder form, the Mn4+-activated red-emitting crystals with considerable size exhibit improved resistance to deterioration under water attack. Besides, large sized fluorescent crystal can be packaged on LED in the remote configuration. We recently developed a series of methods for growing Mn4+-doped red-emitting crystals which have different solubilities.(1) For the fluoride hosts that have large solubility in acid solution such as (NH4)2SiF6, (NH4)2TiF6, K2TiF6, a series of centimeter-sized crystalline ingots were prepared by a cooling induced crystallization method (cooling from 20 °C to minus 40 °C). During cooling, the solubility decreases and the centimeter-scale Na2SiF6:Mn4+ ingots as well as the fluoride phosphors with similar physicochemical property including (NH4)2SiF6:Mn4+, (NH4)2TiF6:Mn4+ and K2TiF6:Mn4+ were successfully grown through the supersaturated crystallization process by cooling down.(2) For the fluoride hosts that are slightly soluble such as BaTiF6, we firstly prepared BaTiOF4 as precursor, which is then mixed with K2MnF6 and gradually fluorinated in HF acid solution at room temperature. After a continuous feeding process, epitaxial growth of the BaTiF6:Mn4+ ended up with ingots up to a record-breaking length of 200−300 μm, which exhibit better water resistance than commercial K2SiF6:Mn4+ micro phosphor.(3) For the oxide hosts that are insoluble such as MgAl2O4, we use the molten salt method, and by optimizing the type of molten salt to adjust the dissolution and migration of Mg2+/Al3+/Mn4+ ions in it, the MgAl2O4:Mn4+ crystals in tetragonal bipyramid shape with uniform size of 2−4 μm were prepared under relatively mild conditions at 950 °C. The phosphor exhibits a broadband emission peaking at 651 nm under the 450 nm excitation, which is the highest energy of the Mn4+ 2E→4A2 transition energy in oxides. Figure 1

Upcycling of silicon scrap collected from photovoltaic cell manufacturing process for lithium-ion batteries via transferred arc thermal plasma

As the supply of photovoltaic industry products increases rapidly, measures to solve the upcoming related waste problem are urgently required. In particular, the fabrication process of Si wafers leads to significant Si scrap residue, and approximately 40% of crystalline Si ingots are wasted as Si scrap. Here, a feasibility study was conducted to investigate the recycling of Si scrap waste collected during the solar panel manufacturing process using the transferred arc thermal plasma. Si-NP with a particle size of 100 nm or less applicable as an anode material for lithium-ion batteries (LIBs) was successfully upcycled by the transferable arc thermal plasma method, and the developed LIB showed excellent electrochemical performance. Si-NPs electrodes show a discharge capacity of 2920 mAh g−1 at 0.1 A g−1, and 2167 mAh g−1 at 0.4 A g−1. In addition, silicon-graphite (Si-G) composites were also prepared to improve the electrochemical properties of LIBs. Furthermore, the possible pros and cons of the method proposed in this study were discussed including economic evaluation. These results show that the transferred arc thermal plasma is a simple, eco-friendly, and cost-effective method of upcycling Si-NPs anode material from Si scrap waste for high-performance LIBs.

Characterization of Cu2ZnSnS4 thin films deposited by one-step thermal evaporation for a third generation solar cell

In this study, one-step thermal evaporation was employed for the fabrication of phase-pure CZTS thin films on glass substrates from single crystalline Cu-poor ingot grown by Bridgman technique. Structural analyses revealed the formation of a mono-phase CZTS kesterite structure for the thin films annealed at 450 °C under N2 atmosphere. The AFM measurements demonstrated significant morphological changes in films with increasing the annealing temperature from 200 °C to 450 °C, supporting the results obtained via XRD and Raman studies. From the optical measurements, the band gap was estimated to be 1.47 eV for the CZTS film annealed at 450 °C, which is close to the optimum value required for photovoltaic applications. As a device application of the optimized CZTS thin films, an attempt was made to construct a TiO2 NRs based and CdS-free FTO/TiO2 NRs/CZTS/Ag superstrate CZTS solar cells. TiO2 NRs were grown by hydrothermal technique, which offers a simple and a cost-effective route for large-scale production of one-dimensional nanostructures. The solar cell exhibited an open-circuit voltage of 0.35 V, a short-circuit current density of 7.28 mA/cm2, a fill factor of 23.9% and a power conversion efficiency of 0.61%, the highest efficiency recorded so far for a single-dimensional TiO2 nanostructures based superstrate CZTS solar cell.

МАКРОСКОПІЧНА НЕОДНОРІДНІСТЬ ОПТИЧНИХ, ДІЕЛЕКТРИЧНИХ І ФОТОДІЕЛЕКТРИЧНИХ ВЛАСТИВОСТЕЙ КРИСТАЛІВ ZnSe

The subject matter of study in the article is the optical, dielectric and photodielectric properties of ZnSe crystals containing macroscopic structural inhomogeneities. The goal of this work is to study large-scale inhomogeneities in ZnSe crystals, which is important for determining the suitability of their application in technology. The tasks to be solved are: in a crystalline ZnSe ingot of cylindrical shape in the optical range at perpendicular to the light flux orientation of the sample axis to detect macroscopic inhomogeneities and establish their relationship with the features of the dielectric properties of the local regions of the ZnSe ingot; to investigate the influence of nonmonochromatic light flux on the distribution of dielectric parameters along the axis of the ZnSe ingot containing areas of macroscopic inhomogeneities. The problems were solved by the following methods: the homogeneity of the zinc selenide ingot was studied by the polarization-optical photoelasticity method; the photometry method was used to study the intensity distribution of the light scattered by the sample perpendicular to its axis; the dielectric properties of a crystalline ZnSe ingot were investigated by the capacitive method. The following results were obtained. In ZnSe crystals regions of three types were found: isotropic, weakly anisotropic, and anisotropic. Sharp changes in the intensity of scattered light are observed in the region of large-scale optical inhomogeneity, and light scattering in this region occurs anisotropically. The angular dependences of C and tgδ at an electric field frequency of 1 kHz are obtained for the region of large-scale optical anisotropy. The changes in dielectric parameters under the action of nonmonochromatic light along the sample axis are measured. Conclusions. It was found that ZnSe crystals are characterized by large-scale inhomogeneities of the investigated properties. A close relationship is noted between the optical and electrophysical properties, which is probably due to the mutual influence of two-dimensional and point defects on the formation of a defect structure during the growth and further cooling of the crystal.

Laser sintered PbSe semiconductor thin films for Mid-IR applications using nanocrystals

Interests in group IV-VI compound semiconductors for MWIR applications have undergone multiple resurgences over the last 70 years due to the high IR responsivity. Many research groups have shown they can achieve high IR response by making modification of the sensitization process; however, nearly all are using traditional deposition methods (Chemical Bath Deposition or Chemical/Physical Vapor Deposition) involving complex chemical processing or vacuum-based equipment. This work introduces a flexible alternative method for producing ultrapure polycrystalline thin films with exceptional morphological control. The process utilizes a planetary ball-mill to wet-mill ultra-pure crystalline ingot into colloidal nanocrystals (NC), followed by rapid deposition in a centrifuge to form a compact, thin film across a silicon substrate. Laser sintering is then used to rapidly form a continuous denser film with excellent substrate adhesion and unique control of the morphological properties. This work demonstrates a new method using laser sintering that can form a 1.5 μm thin film of PbSe with equivalent physical and electrical (p-type with a carrier concentration of 4.5 × 1017/cm3 and resistivity value of 0.17 Ωcm) properties to CBD/PVD. The results are presented for the process development, film morphology, crystal structure, and electrical properties. Lastly, unlike traditional deposition methods, this alternative process is environmentally responsible by producing zero hazardous waste by allowing recovery of all unused PbSe for reuse. The reported method is applicable for achieving thin films from a variety of high purity bulk materials.

Effect of Heat Treatment of a Melt on the Structure and Properties of the Corresponding Crystalline Ingots or Castings

The modern concepts of the structure of liquid metals and alloys are considered. Several types of microinhomogeneity and microheterogeneity are shown to exist in liquid metal solutions. Their structural state changes as a result of variations in composition, history, temperature, and pressure or the influence of various external actions. Upon subsequent cooling at an appropriate rate, these changes can persist up to liquidus and affect the structure and properties of the solidified alloy. The main attention is paid to the influence of the heating temperature of a liquid metal. For aluminum-based alloys, the possibility of developing the optimum heat-treatment conditions for melting using the results of studying the structure and properties of melts has been shown. This optimized heat treatment of melts is shown to be an effective method to improve the quality of alloys.

Spark Plasma Sintering Effect on Thermoelectric Properties of Nanostructured Bismuth Telluride Synthesized by High Energy Ball Milling.

Thermoelectric properties of high energy ball milled nano structured bismuth telluride (Bi₂Te₃) have been reported. By high energy ball milling, alloyed bulk crystalline ingots crush into nanopowder and followed by spark plasma sintering (SPS), we have demonstrate high figure of merit (ZT) in bismuth telluride pellet samples. In this work systematic study carried out on three pellet samples of Bi₂Te₃, synthesized by high ball milling for the time period of 4 hours, 8 hours and 12 hours and followed by SPS at the same processing parameters. A peak value of dimensionless figure of merit of about 1.22 at the temperature of 473 K has been achieved for 8 hours ball milled pellet sample. This enhancement in ZT value is mostly due to decrease in thermal conductivity. Results of this study demonstrate that ball milling and SPS has a major effect in controlling the density of grain boundaries of Bi₂Te₃ nano particles, while the pressure exerted on the powder samples during SPS introduce stress at the boundaries of the crystallites. These disordered crystallite boundary regions exert scattering of thermal energy carriers which reduced the thermal conductivity of the materials.

3D visualization of growth interfaces in cast Si ingot using inclusions distribution

• A method of 3D visualization of growth interface in cast silicon ingots is proposed. • The method utilizes inclusion distribution along with the growth interface. • Inclusion distribution is measured by an infrared brick inspection system. • The interface is extracted by fitting with a simple polynomial expression. • This visualization will contribute to the development of the crystal growth method. We propose a method of three-dimensional (3D) visualization of growth interfaces in crystalline ingots prepared by directional solidification and apply the method to cast silicon ingot for solar cells. The method consists of dispersing inclusions along with the growth interface by insertion of a quartz rod followed by measuring inclusions distribution by an infrared brick inspection system. Then, inclusions distribution can be visualized in 3D, which allows extracting growth interfaces as planes. By fitting the plane with a simple polynomial expression, its macroscopic shape and local gradient can be estimated. The method yields us the local changes in growth interfaces and thus contributes to the development of the crystal growth method for high-quality ingots.

Structural quality in single crystalline CdSe ingots grown by PVT

CdSe is II-VI semiconductor with compact hexagonal structure. It has a band gap of 1.82 eV and a high stopping power for nuclear radiation. Single crystalline CdSe ingots were grown by Physical Vapor Transport (PVT) employing a horizontal reactor. As devices critically depend on material properties its single crystalline quality was determined by chemical etching and transmission electron microscopy. Results were compared to those corresponding to Bridgman High Pressure (HPB) grown material and also to PVT material grown in a vertical reactor. Keywords: Single crystalline CdSe ingots, PVT, HPB, Chemical etching, Optical Microscopy, Transmission and scanning electron microscopy.

Synthesis of Ultrathin Few-Layer 2D Nanoplates of Halide Perovskite Cs3Bi2I9 and Single-Nanoplate Super-Resolved Fluorescence Microscopy.

The discovery of new two-dimensional (2D) perovskite halides has created sensation recently because of their structural diversity and intriguing optical properties. The toxicity of Pb-based perovskite halides led to the development of Pb-free halides. Herein, we have demonstrated a one-pot solution-based synthesis of 2D ultrathin (∼1.78 nm) few-layer (2-4 layers) nanoplates (300-600 nm lateral dimension), nanosheets (0.6-1.5 μm), and nanocrystals of layered Cs3Bi2I9 by varying the reaction temperature from 110 to 180 °C. We have established a mechanistic pathway for the variation of morphology of Cs3Bi2I9 with temperature in the presence of organic capping ligands. Further, we have synthesized the bulk powder of Cs3Bi2I9 by mechanochemical synthesis and liquid-assisted grinding and crystalline ingot by vacuum-sealed tube melting. 2D nanoplates and bulk Cs3Bi2I9 demonstrate optical absorption edge along with excitonic transition. Photoluminescence properties of individual nanoplates were studied by super-resolution fluorescence imaging, which indicated the blinking behavior down to the level of an individual Cs3Bi2I9 nanoplate along with its emission at the far-red region and high photostability.

(Invited) Monocrystalline Si Thin Film By High Speed Zone Melting / Heating Crystallization with Controlling Nano-Surface Roughness for Cost-Cutting Solar Cells with High Efficiency

Recent development in monocrystalline Si (c-Si) solar cell technology is significant both for reducing the cell cost andfor increasing the photoelectric conversion efficiency. The recent improved efficiency and cost-cutting of c-Si solar cell is mainly due to technology that (a) improves the carrier lifetime via surface passivation, (b) uses higher quality crystalline Si, and (c) reduces wafer thickness. A challenge in achieving further cost reduction, however, is reducing the thickness of c-Si while maintaining its crystallinity. Fabrication of c-Si is commonly attained by wire-saw slicing of a crystalline ingot or by a layer transfer process (peeled off). The wire-saw slicing can keep high quality of c-Si but, cause kerf loss and the limitation of reducing thickness. In contrast, technical challenges for layer transfer processes are 1) the formation of a high-quality thin film Si, 2) achieving a structure that can easily be peeled off from Si substrate, 3) improving the growth rate and Si raw material yield (necessary equipment costs are determined by the growth rate), and 4) being able to use the substrate after layer transfer without any waste. In the paper, I will introduce the zone melting / heating (re-) crystallization (ZMC/ZHR), shown in Fig.1, to fabricate monocrystalline Si thin film as a key technology in two different kinds of layer transfer process for cost-cutting solar cells with high efficiency. In both layer transfer processes, the control of nano-surface roughness was crucial to improve the crystal quality of Si thin film. Fabrication of quasi-monocrystalline Si thin film without seed layer by high-speed ZMC[1~3] Silicon-on-insulator (SOI) films for solar cells were fabricated by high-speed ZMC. For ZMC, amorphous Si films were deposited on SiO2layers by rf sputtering. After SOI (Si/SiO2) was covered with another SiO2layer (forming a sandwich structure of SiO2/Si/SiO2), the SOI films were crystallized by ZMC. In ZMC, monocrystalline Si thin film was prepared by melting and crystallizing in a line (using a lamp line heater) and then scanning in one direction. Due to the stability at the SiO2/melted Si interface, the result is quasi-monocrystalline Si without requiring any seed for crystal growth. The surface roughness (several ten nanometer) of deposited Si film determined the crystal defect density of the ZMC film. Monocrystalline Si thin film with 10 times faster growth rate on nano smoothing porous Si by ZHR for cost-cutting solar cells with high efficiency[4~6] A high-quality thin film monocrystallinesilicon was developed for solar cells. The crystal defect density of the film could be reduced to the silicon wafer level at a growth rate that is more than 10 times higher than typical CVD. First, double-layer nano-order porous Si was formed from Siwafer using an electrochemical oxidation. Subsequently, the surface was smoothed to a roughness of 0.2 to 0.3 nm by ZHR. This substrate was used for high-speed growth of monocrystalline Si by Rapid vapor deposition (RVD), developed by S. Noda et al.[7]. The grown film can easily be peeled off using the double-layer porous Si layer, and the substrate can be reused or used as an evaporation source for RVD, which significantly reduces material loss. When the surface roughness of the porous Si substrate is reduced by changing the ZHR conditions, the defect density of the Si thin film, measured by ESR, could be reduced to the Si wafer level of about 1/10th. The results showed that a surface roughness in the range of only 0.1-0.2 nm (level of atoms to several tens of layers) has an important impact on the formation of crystal defects, which is also of interest as a crystal growth mechanism. (Figure 1 showed the outline of these results.) Acknowledgement I summarized the results of reference papers [1~6] focusing on zone melting / heating (re-) crystallization and surface roughness control as an invited talk in the 234thMeeting of The Electrochemical society (ECS). I appreciate for Prof. S. Noda, Dr. K. Hasegawa, Dr. A. Lukianov, Ms. C. Takazawa and all authors of these papers. Figure 1

Controlling impurity distribution in quasi-mono crystalline Si ingot by seed manipulation for artificially controlled defects technique

We report on our attempt to control impurity distributions by locally introduced high density of dislocations based on seed manipulation for artificially controlled defect technique (SMART). To grow a quasi-mono crystalline Si ingot to demonstrate the impact of SMART, seed arrangement was designed to have SMART and conventional parts in a single ingot. After crystal growth, the ingot was cut in half and the half one was annealed at 600 °C for 1 week. Photoluminescence imaging clarified that impurities are efficiently trapped at functional defects after annealing, which led to improvement of the crystal quality along the functional defects. This result suggests that combining SMART with annealing after crystal growth is the effective way to obtain a higher manufacturing yield of quasi-mono crystalline Si ingot for solar cells.

Electrodeposition of Si Film from Water-Soluble KF-KCl Molten Salt and Feasibility of SiCl4 as a Si Source

1. Introduction Presently, Si substrate used for solar cells are manufactured by a multi-step process of production of polycrystalline Si, pulling up of single crystalline Si ingot or casting into a mold, and slicing by diamond wire saws. However, this process inevitably produces a large amount of kerf loss, which lowers the yield of the product. Thus, a direct formation of high-quality crystalline Si films on desired substrates at a low cost has been desired as a new production process of crystalline Si solar cells. For this purpose, electrodeposition in high temperature molten salts has been studied as a potential method [1,2]. Recently, we proposed a new electrodeposition process of crystalline Si in which molten KF–KCl is used as an electrolyte and SiCl4as a Si source [3]. In the present study, we first explored the optimum condition for electrodepositing compact and smooth Si films in molten KF–KCl, and then investigated the feasibility of SiCl4gas as a Si source. 2. Experimental The electrodeposition was conducted in a dry Ar atmosphere at 923 K, 1023 K or 1073 K. Reagent-grade KF and KCl were mixed to a eutectic composition (KF:KCl = 45:55 mol%, melting point = 878 K) and loaded in a graphite crucible. The crucible was placed at the bottom of a quartz or stainless steel vessel in an air-tight Kanthal container and dried under vacuum at 673 K for 24 h. A Ag plate was used as the working electrode and a glassy carbon rod as the counter electrode. A Pt wire was employed as the quasi-reference electrode. The potential was calibrated and given with reference to K+/K electrode potential. Galvanostatic electrolysis was conducted with various K2SiF6 concentrations and current densities. The electrolyzed samples were washed in hot distilled water at 333 K for 24 h to remove the adhered salt on the deposit, and dried under vacuum for 12 h. The samples were analyzed by SEM, EDX, EBSD, XRD and Raman spectroscopy. An SiCl4 gas was introduced into KF-KCl molten salt containing no K2SiF6by using argon as a carrier gas. After the introduction, cyclic voltammetry and galvanostatic electrolysis were conducted to confirm the electrodeposition of silicon. 3. Results and discussion The galvanostatic electrolysis was carried out in various current densities and K2SiF6 concentrations in a eutectic KF-KCl melt. The ionic state after the introduction of SiCl4 gas in the melt is regarded as the same with the addition of K2SiF6to the melt. SiCl4 + 6F− → SiF6 2− + 4Cl− (1) From cross-sectional SEM/EDX analysis of the samples, the optimum condition for obtaining compact and smooth Si films has been found to be the K2SiF6 concentration range of 2.0–3.5 mol% and the current density range of 50–200 mA cm−2at 923 K [4]. XRD and Raman spectrocscopy confirmed that the obtained film was crystalline Si. According to the EBSD analysis, the typical crystallite size of the deposited Si was <0.1μm at 923 K, 1–10 μm at 1023 K and 5-30 μm at 1073 K. The dependence of the crystallite size on the deposition temperature could be explained by the crystallization rate of deposited Si. In order to investigate the feasibility of SiCl4 as a Si source, 2.88 mol% of SiCl4 gas was introduced into a pure KF–KCl molten salt. After the introduction, cyclic voltammetry was performed on a Ag electrode to check the existence of Si(IV) ions. The obtained cyclic voltammogram shown in Fig. 1 indicates a couple of redox peaks which is indicative for Si(IV) ions. The concentration of Si(IV) ions is estimated to be 2.30 mol% from the height of peak current density. The sample prepared by galvanostatic electrolysis on a Ag wire at 155 mA cm-2for 30 minutes was confirmed to be a crystalline Si film. Acknowledgement This study was partly supported by the Core Research for Evolutionary Science and Technology (CREST) of the Japan Science and Technology Agency (JST). Reference [1] U. Cohen and R.A. Huggins, J. Electrochem. Soc., 123, 381 (1976). [2] D. Elwell and R.S. Feigelson, Sol. Energ. Mat., 6, 123 (1982). [3] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara and T. Homma, J. Electrochem. Soc., 162, D444 (2015). [4] K. Yasuda, K. Maeda, T. Nohira, R. Hagiwara and T. Homma, J. Electrochem. Soc., 163, D95 (2016). Figure 1

Exploring the applicability of amorphous films of system In-Sb-Te as phase change materials

Hereby, we present In-Sb-Te amorphous films grown by pulsed laser deposition technique using, as targets, crystalline ingots (with corresponding stoichiometry) prepared by traditional melt-quenching method. The explored nominal compositions were In50Sb15Te35, about a ternary compound formed in the quasi-binary InTe-SbTe system, metastable at room temperature, and two eutectics In8Sb8Te84 and In10Sb51Te39. Measurements of electrical sheet resistance evidenced that the amorphous films behave as electrical insulators at room temperature and present a giant jump in resistivity towards a conducting state on crystallization. Differential scanning calorimetry technique complemented the structural information obtained by X-ray diffraction and revealed temperatures of crystallization of the amorphous films as well as their melting points. Due to their temperature characteristics (crystallization temperature≈225°C and melting temperature≈540°C), In10Sb51Te39 film results very attractive from technological point of view. These characteristics could make this eutectic composition a good candidate for using in phase-change memory devices.

Optical Characterizations of TlBr Single Crystals for Radiation Detection Applications

Background: TlBr is of considerable technological importance for radiation detection applications where detecting high-energy photons such as X-rays and γ-rays are of prime importance. However, there were few reports on investigating optical properties of TlBr itself for deeper understandings of this material and for making better radiation detection devices. Thus, in this paper, we report on the optical characterizations of TlBr single crystals. Spectroscopic ellipsometry (SE) and photoluminescence (PL) measurements at RT were performed for this work. Materials and Methods: A 2-inch TlBr single crystalline ingot was grown by using the vertical Bridgman furnace. SE measurements were performed at RT within the photon energy range from 1.1 to 6.5 eV. PL measurements were performed at RT by using a home-made PL system equipped with a 266 nm-laser and a spectrometer. Results and Discussion: Dielectric responses from SE analysis were shown to be slightly different among the different samples possibly due to the different structural/optical properties. Also from the PL measurements, it was observed that the peak intensities of the middle samples were significantly higher than those of the other two samples. With the given values for permittivity of free space (e0 = 8.854x10-12 F·m-1), thickness (d = 1 mm), and area (A = 10x10 mm2) of the TlBr sample, capacitances of TlBr were 6.9 pF (at hν = 3 eV) and 4.4 pF (at hν = 6 eV), respectively. Conclusion: SE and PL measurement and analysis were performed to characterize TlBr samples from the optical perspective. It was observed that dielectric responses of different TlBr samples were slightly different due to the different material properties. PL measurements showed that the middle sample exhibited much stronger PL emission peaks due to the better material quality. From the SE analysis, optical, dielectric constants were extracted, and calculated capacitances were in the few pF range.