Crystalline Volume Fraction Research Articles

Laser‐induced heating of porous Ge layers implanted with Ag+ and Cu+ ions

AbstractPorous Ge layers consisting of nanowires were formed by low‐energy high‐dose implantation of monocrystalline c‐Ge substrate with Ag+ and Cu+ ions. The obtained layers were studied by Raman spectroscopy using solid‐state and He‐Ne exciting lasers with wavelength of 532 and 633 nm, respectively. The crystalline volume fraction and local temperature of porous layers at the surface sites of laser probing were determined. To interpret the change in the shape of the spectra under the action of laser radiation, deconvolution into amorphous and nanocrystalline components was carried out. Analysis of the spectral line shape made it possible to estimate the crystalline and amorphous volume fractions. The quantum confinement model for Ge nanocrystallites was taken into account for accurate estimations of volume fractions. These estimates showed that the implanted amorphous layers of all samples were locally crystallized with a solid‐state laser. He‐Ne laser excitation resulted in partial crystallization only for layers obtained by Cu+ ion implantation. The measured ratio of the Stokes to anti‐Stokes components demonstrated that the porous layers obtained by implanting Ag+ ion were not heated by a He‐Ne laser and according to the analysis of spectral lines, the crystalline volume fraction did not appear in these layers during probing. The results obtained are explained by the difference in the penetration depth for various samples of the exciting radiation.

Local Structure Analysis and Modelling of Lignin-Based Carbon Composites through the Hierarchical Decomposition of the Radial Distribution Function.

Carbonized lignin has been proposed as a sustainable and domestic source of activated, amorphous, graphitic, and nanostructured carbon for many industrial applications as the structure can be tuned through processing conditions. However, the inherent variability of lignin and its complex physicochemical structure resulting from feedstock and pulping selection make the Process‐Structure‐Property‐Performance (PSPP) relationships hard to define. In this work, radial distribution functions (RDFs) from synchrotron X‐ray and neutron scattering of lignin‐based carbon composites (LBCCs) are investigated using the Hierarchical Decomposition of the Radial Distribution Function (HDRDF) modelling method to characterize the local atomic environment and develop quantitative PSPP relationships. PSPP relationships for LBCCs defined by this work include crystallite size dependence on lignin feedstock as well as increasing crystalline volume fraction, nanoscale composite density, and crystallite size with increasing reduction temperature.

Virtual Development of Process Parameters for Bulk Metallic Glass Formation in Laser-Based Powder Bed Fusion.

The development of process parameters and scanning strategies for bulk metallic glass formation during additive manufacturing is time-consuming and costly. It typically involves trials with varying settings and destructive testing to evaluate the final phase structure of the experimental samples. In this study, we present an alternative method by modelling to predict the influence of the process parameters on the crystalline phase evolution during laser-based powder bed fusion (PBF-LB). The methodology is demonstrated by performing simulations, varying the following parameters: laser power, hatch spacing and hatch length. The results are compared in terms of crystalline volume fraction, crystal number density and mean crystal radius after scanning five consecutive layers. The result from the simulation shows an identical trend for the predicted crystalline phase fraction compared to the experimental estimates. It is shown that a low laser power, large hatch spacing and long hatch lengths are beneficial for glass formation during PBF-LB. The absolute values show an offset though, over-predicted by the numerical model. The method can indicate favourable parameter settings and be a complementary tool in the development of scanning strategies and processing parameters for additive manufacturing of bulk metallic glass.

Simulation of phase evolution in a Zr-based glass forming alloy during multiple laser remelting

Additive manufacturing by laser-based powder bed fusion is a promising technique for bulk metallic glass production. But, reheating by deposition of subsequent layers may cause local crystallisation of the alloy. To investigate the crystalline phase evolution during laser scanning of a Zr-based metallic glass-forming alloy, a simulation strategy based on the finite element method and the classical nucleation theory has been developed and compared with experimental results from multiple laser remelting of a single-track. Multiple laser remelting of a single-track demonstrates the crystallisation behaviour by the influence of thermal history in the reheated material. Scanning electron microscopy and transmission electron microscopy reveals the crystalline phase evolution in the heat affected zone after each laser scan. A trend can be observed where repeated remelting results in an increased crystalline volume fraction with larger crystals in the heat affected zone, both in simulation and experiment. A gradient of cluster number density and mean radius can also be predicted by the model, with good correlation to the experiments. Prediction of crystallisation, as presented in this work, can be a useful tool to aid the development of process parameters during additive manufacturing for bulk metallic glass formation.

Investigation of structural aspect in terms of atypical phases within material deposited for a-Si:H solar cell fabrication

The structural investigation of the a-Si:H material, deposited at different pressures by PECVD process, has been carried out to analyze the signatures of diffused intermediate sort of crystalline phases within the amorphous silicon matrix. Raman characterization along with the Photoluminescence (PL) and spectroscopic ellipsometry studies were carried out to understand the microstructuree of these films. From Raman analysis the material was found to have indistinguishable crystalline phase, which can also be named as “intermediate amorphous phase” (a phase defined between amorphous and ultra nano-crystalline silicon) with crystalline volume fractions as 56 % and 62 % for 0.23 Torr and 0.53 Torr respectively. Here the contribution of ultra nano-crystallites results in higher crystalline fraction, which is not visibly revealed from the Raman spectra due to its sub nano-crystallite characteristics. For the film deposited at 0.53 Torr stable photo-conductance in conjunction with high photo-response under 10 hour light soaking has been observed, which is as expected due to high crystalline volume fraction. The presence of these phases might be the possible reason for the distinct device characteristics though having nearly the similar electrical properties (photo-response ~104). These studies will help to make improvement in the individual layer properties, other than the interface effect, in the fabrication of efficient p-i-n solar cells.

Comparative Study on the Quality of Microcrystalline and Epitaxial Silicon Films Produced by PECVD Using Identical SiF4 Based Process Conditions.

Hydrogenated microcrystalline silicon (µc-Si:H) and epitaxial silicon (epi-Si) films have been produced from SiF4, H2 and Ar mixtures by plasma enhanced chemical vapor deposition (PECVD) at 200 °C. Here, both films were produced using identical deposition conditions, to determine if the conditions for producing µc-Si with the largest crystalline fraction (XC), will also result in epi-Si films that encompass the best quality and largest crystalline silicon (c-Si) fraction. Both characteristics are of importance for the development of thin film transistors (TFTs), thin film solar cells and novel 3D devices since epi-Si films can be grown or etched in a selective manner. Therefore, we have distinguished that the H2/SiF4 ratio affects the XC of µc-Si, the c-Si fraction in epi-Si films, and the structure of the epi-Si/c-Si interface. Raman and UV-Vis ellipsometry were used to evaluate the crystalline volume fraction (Xc) and composition of the deposited layers, while the structure of the films were inspected by high resolution transmission electron microscopy (HRTEM). Notably, the conditions for producing µc-Si with the largest XC are different in comparison to the fabrication conditions of epi-Si films with the best quality and largest c-Si fraction.

Dry annealing of radiation-damaged zircon: Single-crystal X-ray and Raman spectroscopy study

Structural reconstitution upon dry thermal annealing of mildly to strongly radiation-damaged, gem-quality zircon from Sri Lanka has been studied by single-crystal X-ray diffraction and Raman spectroscopy. Results of structure refinement of a strongly radiation-damaged zircon (sample GZ5, calculated alpha dose ~4 × 1018 g−1) indicate the existence of an interstitial oxygen site that is sparsely occupied (about 4% of all O atoms). Annealing of this sample at Ta (annealing temperature) = 700 °C has resulted in nearly complete recrystallization of its amorphous volume fraction and significant decrease in the occupation of O-interstitial sites. For all samples studied, annealing up to Ta ≤ 650–700 °C is characterised by preferred recovery of Raman shifts (compared to Raman FWHMs; full width at half band maximum) and extensive contraction of the unit-cell volume, in particular along unit-cell dimension a. This low-T annealing is dominated by epitaxial growth of the crystalline volume fraction at the expense of the amorphous volume fraction, and general recovery of low-energy defects. During annealing at Ta = 700–1400 °C there is preferred recovery of Raman FWHMs (compared to Raman shifts) and only mild unit-cell contraction. High-T annealing is dominated by the recovery of high-energy defects such as recombination of cation Frenkel pairs. Here, unit-cell parameter a shows a remarkable behaviour (namely, mild re-increase at Ta = 700–1150 °C and mild final shrinking at Ta = 1000–1400 °C), which is attributed to enhanced contortion of ZrO8 polyhedrons due to cation repulsion. The combined data set of Raman band and unit-cell parameter presented herein will help analysts to assign Raman spectra of annealed unknowns to certain recovery stages.

Effect of annealing temperature on magnetic properties and corrosion resistance of Fe75.8Si12B8Nb2.6Cu0.6P1 alloy

Fe-based nanocrystalline alloys obtained through isothermal annealing of their amorphous counterparts have emerged as an indispensable soft magnetic material. The development of Fe-based nanocrystalline alloys with excellent soft magnetic properties and good corrosion resistance is highly desirable. In this study, the magnetic properties and corrosion resistance of Fe75.8Si12B8Nb2.6Cu0.6P1 alloys are investigated a function of the annealing temperature. Upon crystallization, the permeability (μ) and coercivity (Hc) of the alloy gradually improve with an increase in the annealing temperature owing to the formation of homogeneous and fine α-Fe(Si) grains. However, excessively high temperatures promote the precipitation of Fe2B and Fe23B6 phases, significantly deteriorating the soft magnetic properties. The alloy annealed at 843 K exhibits excellent magnetic softness with a high saturation magnetization of 1.41 T, a good μ of 2.7 × 104 at 1 kHz and a low Hc of 0.8 A/m. On the other hand, the corrosion resistance of the alloy is sensitive to the annealing temperature, and the higher the annealing temperature, the better the corrosion resistance will be. The improved corrosion resistance is due to the increase in the crystalline volume fraction resulting in the formation of an insoluble protective layer rich in Si and Nb oxides. This study further reveals the relationship between the annealing temperature and alloy properties and can facilitate the development of soft magnetic nanocrystalline alloys with good corrosion resistance.

Effects of Chamber Pressures on the Passivation Layer of Hydrogenated Nano-Crystalline Silicon Mixed-Phase Thin Film by Using Microwave Annealing

This paper proposes the effects of chamber pressures on the passivation layer of hydrogenated nano-crystalline silicon (nc-Si:H) mixed-phase thin film using microwave annealing (MWA) to achieve a high-quality thin film. The use of 40.68 MHz very-high-frequency plasma-enhanced chemical vapor deposition (VHFPECVD) deposited the nc-Si:H mixed-phase thin film on the top and bottom of the n-type crystalline silicon substrate. The chamber pressures (0.2, 0.4, 0.6, and 0.8 Torr) of the VHFPECVD were critical factors in controlling the carrier lifetime of the symmetric structure. By using the VHFPECVD to deposit the nc-Si:H and using the MWA to enhance the quality of the symmetric structure, the deposited nc-Si:H’s properties of a crystalline volume fraction of 29.6%, an optical bandgap of 1.744 eV, and a carrier lifetime of 2942.36 μs were well achieved, and could be valuable in thin-film solar-cell applications.

Lithium and Sodium Ion Binding Mechanisms and Diffusion Rates in Lignin-Based Hard Carbon Models.

Hard carbons are the primary candidate for the anode of next-generation sodium-ion batteries for large-scale energy storage, as they are sustainable and can possess high charge capacity and long cycle life. These properties along with diffusion rates and ion storage mechanisms are highly dependent on nanostructures. This work uses reactive molecular dynamics simulations to examine lithium and sodium ion storage mechanisms and diffusion in lignin-based hard carbon model systems with varying nanostructures. It was found that sodium will preferentially localize on the surface of curved graphene fragments, while lithium will preferentially bind to the hydrogen dense interfaces of crystalline and amorphous carbon domains. The ion storage mechanisms are explained through ion charge and energy distributions in coordination with snapshots of the simulated systems. It was also revealed that hard carbons with small crystalline volume fractions and moderately sized sheets of curved graphene will yield the highest sodium-ion diffusion rates at ∼10–7 cm2/s. Self-diffusion coefficients were determined by mean square displacement of ions in the models with extension through a confined random walk theory.

Magnetic and structural properties of (Fe-Co)83(Sn-P)5B12 alloys with high saturation

Alloys of composition (Fe-Co)83(Sn-P)5B12 have been prepared by planar flow casting method in air with substitution Co = 0; 5; 10 at. % for Fe and substitution P = 2 at. % for Sn. All as-cast ribbons were amorphous and submitted to chemical and thermal analyses, scanning and transmission electron microscopy and magnetic measurements. Hysteresis loops of strips after nanocrystallization at 380 and 400 °C for 30 min changed their shape – the saturation magnetization increased for Co containing alloys Js = 1.74–1.80 T. The off-axis component of magnetic anisotropy decreased significantly due to the annealing. Coercivity in this case considerably increased due to the growth of grain size and crystalline volume fraction. The samples annealed at 350 °C disclosed a relaxed state with changes in microstructure and achieved very good soft-magnetic properties namely for P substituted alloys.

Effect of magnetic field annealing on soft magnetic properties for (Fe0.5Co0.5)73.5Cu1Nb3Si13.5B9 alloy

The effect of magnetic field annealing on crystallization and high-frequency/temperature soft magnetic properties is investigated for (Fe0.5Co0.5)73.5Cu1Nb3Si13.5B9 alloy. The applied magnetic field in the annealing process effectively facilitates the precipitation of ferromagnetic α-FeCo(Si) grains from the amorphous precursor and softens the magnetic properties, which are verified by the increase of crystalline volume fractions (Vcry) and the decrease of coercivity (HC), respectively. The initial permeability (μi) of sample annealed with applied field exhibits relatively higher value at elevated temperature, which is attributed to the intensive magnetic exchange coupling between the α-FeCo(Si) grains through the intergranular phase.

Amorphous-to-Nanocrystalline Transition in Silicon Thin Films by Hydrogen Diluted Silane Using PE-CVD Method

Objective: Herein, we report the effect of variation of hydrogen flow rate on the properties of Si:H films synthesized using PE-CVD method. Raman spectroscopy analysis show an increase in crystalline volume fraction and crystallite size implying that hydrogen flow in PE-CVD promotes the growth of crystallinity in nc-Si:H films with an expense of a reduction in deposition rate. Methods: FTIR spectroscopy analysis indicates that hydrogen content in the film increases with an increase in hydrogen flow rate and hydrogen is predominantly incorporated in Si-H2 and (Si-H2)n bonding configuration. The optical band gap determined using E04 method and Tauc method (ETauc) show an increasing trend with an increase in hydrogen flow rate and E04 is found higher than ETauc over the entire range of hydrogen flow rate studied. Results and Conclusion: We found that the defect density and Urbach energy increases with an increase in hydrogen flow rate. Photosensitivity (σPhoto /σDark) decreases from ~103 to ~1 when hydrogen flow rate is increased from 30 sccm to 100 sccm and can be attributed to amorphous-to-nanocrystallization transition in Si:H films. The results obtained from the present study demonstrated that hydrogen flow rate is an important deposition parameter in PECVD to synthesize nc-Si:H films.

Role of Deposition Pressure on Properties of Phosphorus Doped Hydrogenated Nano-Crystalline Silicon (nc-Si:H) Thin Films Prepared by the Cat-CVD Method

Objective: Phosphorus doped hydrogenated nano-crystalline silicon (nc-Si:H) thin films were synthesized by catalytic chemical vapor deposition (Cat-CVD) method. Methods: The effect of deposition pressure on opto-electronic and structural properties was studied using various analysis techniques such as low angle XRD analysis, FTIR spectroscopy, Raman spectroscopy, UV-Visible spectroscopy, dark conductivity, etc. Results: From low angle XRD and Raman spectroscopy analysis, it is observed that an increase in deposition pressure causes Si:H films to transform and transit from amorphous to the crystalline phase. At optimized deposition pressure (300 mTorr), phosphorous doped nc- Si:H films having a crystallite size of ∼29 nm and crystalline volume fraction of ∼58% along with high deposition rate (∼29.7 Å/s) have been obtained. The band gap was found to be ∼1.98 eV and hydrogen content was as low as (∼1.72 at. %) for these films. Conclusion: The deposited films can be useful as an n-type layer for Si:H based p-i-n, tandem and c-Si hetero-junction solar cells.

Raman study of germanium nanowires formed by low energy Ag+ ion implantation

Porous Ge layers of nanowires were formed by low energy (30 keV) Ag+ ion implantation of crystalline c-Ge substrate. Different radiation doses resulted in the formation of nanowires with various mean diameters. The obtained layers were studied by Raman spectroscopy using helium-neon and argon exciting lasers at wavelength of 633 and 488 nm respectively. It was demonstrated that implanted amorphous layers were locally crystallized by the lasers. The crystallization thresholds were found to be 3 kW/cm2 for helium-neon exciting laser and 1 kW/cm2 for argon exciting laser. The variation of thresholds is suggested to be due to the different light penetration depths at these wavelengths in Ge. The crystalline volume fraction was determined and reached maximum value of 6–7%, the divergence of which can be explained by the difference in the distribution of Ge nanowire diameters.

Effects of annealing on phase structure and magnetic characteristics of sputter deposited Ni2FeGa/Si (100) thin films

This paper investigates the effects of post-deposition annealing on the evolution of phase structure and magnetic properties of magnetron sputtered Ni2FeGa/Si (001) thin films. The results revealed that the as-deposited film was partially crystallized in an fcc structure, i.e. [Formula: see text] phase. Crystallization of the amorphous structure into the [Formula: see text] phase was greatly encouraged following annealing at 723 K for 1 h. Annealing at higher temperatures for the same period triggered the formation of the bcc austenitic phase, which competed with the [Formula: see text] phase simultaneously for crystallization and grain growth. The evolution of phase structure and grain size also influenced the nanomechanical properties of the films according to the nanoindentation measurement. The film annealed at 873 K for 1 h showed high hardness and elastic modulus values of 11.1 GPa and 156 GPa. The [Formula: see text] phase showed stronger ferromagnetic characteristics relative to the bcc austenite due to the richer Fe content. This leads to the saturation magnetization to be maximized at 80 emu/g when annealed at 773 K for 1 h attributed to the enhanced film crystallinity and dominant volume fraction of [Formula: see text] phase in the thin film.

Effect of vacuum annealing and heating–cooling cycle annealing on the soft magnetic properties at room- and high-temperatures for nanocrystalline FeCoAlSiBCuNb alloy

Effect of vacuum annealing and heating–cooling cycle annealing on the soft magnetic properties at room- and high-temperatures for (Fe0.9Co0.1)72.7Al0.8Si13.5B9Cu1Nb3 alloy was investigated by the temperature dependence of initial permeability (μi-T curves). The crystallization behavior and microstructural evolution of alloy with the increase of annealing temperature were studied by means of differential scanning calorimeter (DSC) and X-ray diffraction (XRD). After vacuum annealing at 617 °C for 0.5 h, the µi can reach 30,300 and maintain more than 1000 until 640 °C because of larger crystalline volume fraction (Vcry) of 81.9% and smaller intergranular amorphous layer ( $$\Lambda$$ ) of 0.930 nm, which is more excellent than that of other Fe–Co–Al–Si–B–Cu–Nb alloys annealed at their optimum temperatures. In addition, the crystallized interval temperature (∆Tx) of 158 °C, superior to Fe73.5Si13.5B9Nb3Cu1 and (Fe0.9Co0.1)73.5Si13.5B9Nb3Cu1 alloys ever reported, is conducive to precipitating the bcc α-(Fe,Co,Si) and Fe3(Si,Co,Al) soft magnetic crystalline phases and improving thermostability of alloy.

Effect of Power on Crystallinity and Opto-Electronic Properties of Silicon Thin Films Grown Using VHF PECVD Process

The silicon thin films have been deposited using VHF (60 MHz) plasma enhanced chemical vapor deposition (PECVD) process. The influence of the power (10–50 W), used during the deposition, on the silicon thin films properties was studied. The structural transition (different crystalline fraction) along with their variable grain distribution within amorphous silicon matrix has been discussed with the help of optical, electrical and structural measurements. Raman characterization along with photoluminescence (PL), UV-VIS spectroscopy, micro-ellipsometry spectroscopy and Field emission scanning electron microscopy (FESEM) have been performed to recognize the transitions in deposited silicon films including the microstructure mainly related with crystallite fraction and optical constants. FESEM was used to investigate the characteristics and variation in the microstructure. From Raman results, the structural transition from amorphous to micro/nanocrystalline growth occur as indicated by narrow band at 515 cm−1 observed at 10 W applied power deposited film. From these results, it was found that with increasing power, the crystallinity increases and the films are highly crystalline (Xc > 50%) in nature above 20 W applied power. The contribution of nano-sized crystallites increases the crystalline volume fraction from 32 to 84%. The peak observed nearly at 2.0 eV in photoluminescence emission spectra (PL) confirms the presence of nanocrystallites in the deposited films which is well correlated with the band gap values observed from UV-VIS spectroscopy. However, red shift in PL peak energy and decrease in PL intensity observed and are explained in terms of grain boundaries. The shift in refractive index (n) and extinction coefficient (k) were also observed at different power variation. The investigation of spectroscopic ellipsometry result explains the microstructure of deposited film with changes in crystallite sizes and their volume fractions.

Effect of B/Si ratio on structure and properties of high-entropy glassy Fe25Co25Ni25(BxSi1-x)25 coating prepared by laser cladding

The effects of the B/Si ratio on the glass formation, structure and properties evolutions for the high-entropy Fe25Co25Ni25(BxSi1-x)25 (x = 0.5, 0.6, 0.7, 0.8) coatings were investigated. The crack-free high-entropy glassy coatings were synthesized by using the optimized three-step broadband laser cladding process. The microstructure of the coatings consists of face-centered cubic (FCC), (Fe, Co, Ni)2B and amorphous phases for 0.5 ≤ x ≤ 0.6. Further the increase of the B/Si ratio (0.7 ≤ x ≤ 0.8), it promotes new tiny precipitation of (Fe, Co, Ni)3B phase. The average crystalline size and volume fraction of interdendrite regions are depending on B content. The variations in the mechanical properties of the coatings correspond to its structural evolutions. The hardness increases almost linearly from 5.24 GPa of the Fe25Co25Ni25(B0.5Si0.5)25 coating to 8.39 GPa of the Fe25Co25Ni25(B0.8Si0.2)25 coating. The wear resistance is also proportional to the x content. The mechanisms of the glass formation ability and the relationships between structure and properties of the present coatings were discussed. This combination of these excellent properties gives the new high-entropy glassy coating good prospect for the potential engineering applications.

Direct observation of ultra-rapid solid phase crystallization of amorphous silicon films irradiated by micro-thermal-plasma-jet

This paper presents a new method to directly observe the transient evolution of amorphous silicon temperature and the solid phase crystallization during micro-thermal-plasma-jet irradiation. Using the time-resolved reflectivity analysis, nucleation temperature and characteristic crystallization time were determined. When the heating rate increases from 4.45×105to 2.28×106K/s, the nucleation temperature increases from 985 to 1071 °C. The characteristic crystallization time decreases from 95 to 20 μs with an increase in heating rate. Crystallization time as a function of temperature corresponds to the activation energy of 2.9 eV. This energy relates to both the nucleation and growth process. Micro-Raman spectroscopy results indicate that the crystalline volume fraction of solid–phase crystallized Si film is higher than 65 %, and it increases to 80 % with decreasing scanning speed. Both high resolution transmission electron microscopy results and theoretical calculation indicate that there is a distribution of final crystal size. The average grain size is smaller than 50 nm and depends on the annealing conditions, namely the faster heating rate is, the smaller grain size is.