Crystallization Of Thin Films Research Articles

Molten pool free surface solidification behavior in laser remelted near-β titanium alloy

Free surface solidification (FSS) is a common physical phenomenon that has been found by various studies in metallurgy field while targeted studies are rare. This work investigated special structure and formation mechanism of surface grains generated by free surface solidification through laser remelting a near-β titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55,511). A thin layer crystal film covers molten pool surface through two-dimensional constrained solidification, which can be classified into flake-like crystal film located at molten pool edge and lotusleaf-like crystal film at molten pool center. Numerous cells grow from overflowing melt chill zone around molten pool edge and undergo two-dimensional constrained growth on surface, forming flake-like crystal films. TiNx particles formed from surface melt and N2 reaction can act as heterogeneous crystal nuclei, forming lotusleaf-like crystal films, whose secondary dendrite arm growth direction transforms from 〈100〉 of primary dendrite arm to 〈110〉. Equiaxed lotusleaf-like crystal films and columnar flake-like crystal films have a competitive growth relationship. Inward growth cells growing from two kinds crystal films form flake-like surface grains and central surface grains, respectively. This study can provide FSS theoretical guidance for laser additive manufacturing and surface modification.

Improved resistive switching performance of amorphous InGaZnO-based memristor with the TiO2 insertion layer

A stable I–V repeatability memristor based on amorphous InGaZnO (a-IGZO) with a TiO2 insertion layer was investigated. Two memristive structures composed of Pt/a-IGZO/Pt and Pt/TiO2/a-IGZO/Pt were manufactured on SiO2/Si substrate by the magnetron sputtering method. The crystallization of IGZO thin films was analyzed by the XRD process under different annealing temperatures, and the results show that the thin films remain in a stable amorphous state. The resistive switching performance of the two structures was tested by the semiconductor parameter analyzer 4200-SCS at room temperature. After 30 cyclic scans, the Pt/a-IGZO/Pt structure shows signs of degeneration in its I–V cyclic characteristics. However, in the Pt/TiO2/a-IGZO/Pt structure, the cyclic characteristics of I–V are highly stable (cycles >105 sweeps) with a limiting operating current of 0.01 A. Its rectification ratio is approximately 20 at low operating voltages (±1.0 V), and conductance increases (or decreases) progressively with continuous positive (or negative) voltage scans. Further analysis of the experimental energy band diagram was measured by UV–vis spectra and ultraviolet photoemission spectroscopy (UPS). The results demonstrate that the operational mechanism is the modulation of the barrier at the TiO2/a-IGZO interface, which is conducive to the stable I–V cyclic characteristic of Pt/TiO2/a-IGZO/Pt memristor.

Exploring the synthesis, structure, and optoelectronic properties of lead-free halide perovskite Cs3Bi2I9 in single crystal and polycrystalline forms

In recent years, caesium bismuth iodide (Cs3Bi2I9), a lead (Pb)-free halide perovskite, has drawn more attention as a potential material than traditional semiconductor materials due to its lack of Pb toxicity and its outstanding stability against atmospheric air and moisture. Herein, the inverse temperature crystallization method is adopted to grow high-quality hexagonal-phase Cs3Bi2I9 perovskite single crystals. Furthermore, a Cs3Bi2I9 perovskite thin film is fabricated by a solution process using the two-step spin coating technique. A collective analysis of the structural properties, surface morphology, thermal stability, phase transition, and optoelectronic properties of these single crystal and polycrystalline thin films provides a comprehensive understanding and design strategy to develop environmentally stable, Pb-free, and high-performance photovoltaic and optoelectronic devices based on Cs3Bi2I9 perovskite. The findings of this study contribute to the advancement of perovskite-based technologies and pave the way for their successful integration into the renewable energy and optoelectronics industries.

Exploring the Influence of P3HT on PTCA Crystallization and Phase Behavior in Thin Films.

The thermal properties and alignment of crystallinity of materials in thin films play crucial roles in the performance and reliability of various devices, especially in the fields of electronics, materials science, and engineering. The slight variations in the molecular packing of the active layer can make considerable differences in the optical and thermal properties. Herein, we aim to investigate the tuning of the physical properties of a blended thin film of n-type small organic molecules of perylene-3,4,9,10-tetracarboxylic acid (PTCA-SMs) with the mixing of the p-type polymer poly(3-hexylthiophene) (P3HT). The resulting thin films exhibit an enhanced surface crystallinity compared to the pristine material, leading to the formation of long crystallites, and these crystallites are thermally stable in the solid state, as confirmed by X-ray diffraction (XRD), atomic force microscopy (AFM), and thermal analysis using variable-temperature spectroscopic ellipsometry (VTSE) and differential scanning calorimetry (DSC). We believe that the crystalline structure of the obtained P3HT/PTCA-SMs blends is a combination of edge-on and face-on orientations, which enable the potential use of this material as an active layer in organic electronics.

Ionic Liquid Crystal Thin Film as Switching Layer in Nonvolatile Resistive Memory.

In this study, we propose the use of an ionic liquid crystal (ILC) as a new resistive switching layer in nonvolatile resistive random-access memory (ReRAM) devices. The high-quality vacuum-deposited ILC films of 1-hexadecyl-3-methylimidazolium hexafluorophosphate ([C16mim][PF6]) enabled to demonstrate the first operation of ReRAM devices with a low set voltage of ∼1 V and stable switching behavior for up to ∼44 cycles. The key to the successful operation is that the ILC layer is in the liquid crystal phase (smectic A), where the electric double layers formed at the electrode-ILC interfaces play a significant role. The results of basic electrical properties and I-V curve fittings suggested the following operation principle: the formation and rupture of charge-composed filaments within the ILC film, where the current conduction is primarily governed by the trap charge limited current (TCLC) mechanism. These achievements will pave the way for advanced studies of ILC-based electronic devices.

The influence of the demagnetizing field on the concentration of spin wave energy in two-dimensional magnonic crystals

We use the Plane Wave Method to theoretically study thin-film magnonic crystals (MCs) composed of two very common magnetic materials: cobalt and permalloy. In both cases, we consider Co inclusions in the Py matrix and Py inclusions in the Co matrix. An external magnetic field is applied in the plane of the structure, leading to the formation of a demagnetizing field at the interface between the inclusions and matrix. Previous studies have shown that this field strongly affects the spectrum of spin waves, including the position and width of bandgaps. In this study, we exploit the in-plane squeezing of the MC structure to enhance the demagnetizing field. This results in the transfer of low-frequency spin waves from Py to Co, affecting the energy distribution (i.e., the spin-wave profile). The change in the concentration of spin-wave profiles leads to certain peculiarities in the spin-wave frequency spectrum. These include modes repulsion caused by hybridization, which in turn leads to the reordering of modes in the spectrum.

Femtosecond laser-induced crystallization in Er/Yb:BaTiO3 perovskite films: Effects on the optical and electrical properties

Femtosecond laser-induced crystallization in Er3+/Yb3+:BaTiO3 (BTEY) perovskite films has sparked significant research interest in materials science and photonics, driven by the need for innovative techniques to control material properties and exploit their potential. Our study delves into this burgeoning area by demonstrating the controlled crystallization of amorphous BTEY thin films under femtosecond laser (fs-laser) irradiation at 1030 nm. The threshold fluence to achieve laser processing was determined as approximately 1.1 J/cm2 at 1 kHz, as well as the irradiation conditions for crystallization (pulse energy, repetition rate, and scanning speed), whose background mechanism is discussed. Our Raman spectroscopy analysis confirmed crystallization when irradiation was carried out at 1 kHz repetition rate, while no evidence of the crystalline phase of BTEY emerged at 1 MHz irradiation. Moreover, our results reveal a remarkable increase in luminescence enhancement and surface potential (∼65 mV) in the film. These outcomes represent a significant breakthrough with relevant implications for advanced photonic devices like modulators and detectors. By harnessing fs-laser pulses for BTEY crystallization, our findings offer new avenues for practical device applications.

Oriented Perovskite Film from Laser Recrystallization in Magnetic Field.

The orientation of crystals on the substrate and the presence of defects are critical factors in electro-optic performance. However, technical approaches to guide the orientational crystallization of electro-optical thin films remain challenging. Here,a novel physical method called magnetic-field-assisted pulse laser annealing (MAPLA) for controlling the orientation of perovskite crystals on substrates is reported. By inducing laser recrystallization of perovskite crystals under a magnetic field and with magnetic nanoparticles, the optical and magnetic fields are found to guide the orientational gathering of perovskite units into nanoclusters, resulting in perovskite crystals with preferred lattice orientation in (110) and (220) perpendicular to the substrate. The perovskite crystals obtained by MAPLA exhibit significantly larger grain size and fewer defects compared to those from pulsed laser annealing (PLA) and traditional thermal annealing, resulting in improved carrier lifetime and mobility. Furthermore, MAPLA demonstrates enhanced device performance, increasing responsivity and detectivity by two times, and photocurrent by nearly three orders compared with PLA. The introduction of Fe2 O3 nanoparticles during MAPLA not only improves crystal size and orientation but also significantly enhances long-term stability by preventing Pb2+ reduction. The MAPLA method has great potential for fabricating many electro-optical thin films with desired device properties and stability.

Temperature dependent aluminum induced crystallization of amorphous germanium thin films

In this work, we present the sequential growth of metal–semiconductor bilayer structure on HF-treated textured glass substrates by thermal evaporation method. The process of aluminum-induced crystallization of amorphous germanium thin films was critically observed under heat treatment. Temperature assisted structural modifications triggered the nucleation phenomenon for controlled grain growth which transformed the nature of Ge-thin film from amorphous-to-polycrystalline. Interestingly, the electrical resistivity also decreased by subsequent annealing which promoted the conduction mechanism. Optical measurements indicated a dwindling in band tail states and related structural defects, leading to better crystallinity.

Electric-field-enhanced aluminum-induced crystallization of amorphous silicon thin film using decreasing stepwise current method

Electric-field-enhanced aluminum-induced crystallization (AIC) was applied to the crystallization of amorphous Si (a-Si) using a stepwise current method with two to four decreasing steps. In AIC, Si diffuses into an Al layer, which generates a layer exchange from an a-Si/Al layer to an Al/polycrystalline Si (p-Si) layer. This increases the electrical resistivity of the Al/p-Si layer. The stepwise decreasing current was an attempt to overcome the limitations of electric-field-enhanced AIC using a constant current. The samples were fabricated by depositing a 200 nm-thick a-Si layer and sputtering a 300 nm-thick Al layer onto an Eagle XG glass substrate. In-situ resistance and reflectivity were measured to monitor the AIC mechanism and layer exchange. The reflectivity measurements were compared with thin-film optics calculations on the a-Si/Al and Al/p-Si layers to indicate the growth of p-Si. The temperature variations during the heating process were supported by a numerical analysis. The crystallinity of produced polycrystalline silicon (p-Si) was verified by a Raman peak at around 519–520 cm−1. The stepwise current supply increased the crystallization time and improved the crystallinity of the produced p-Si compared to the constant current method.

Growth and Photoelectrochemical Characterization of Epitaxial ZnTe Photocathodes for Carbon Dioxide Reduction

Harnessing solar energy to drive photocatalytic carbon dioxide reduction reactions (CO2RR) provides an appealing pathway to generate hydrocarbon and oxygenated fuels without an external power source. Zinc telluride (ZnTe) is a II-VI semiconductor which has been identified as a promising photocathode material due to its suitable band gap alignments to CO2 reduction reaction potentials, chemical stability, and strong p-type character. Using molecular beam epitaxy (MBE), single crystal and epitaxial layers are synthesized to gain a deeper understanding of fundamental charge transport and reactivity mechanisms between the single crystal ZnTe thin film and the CO2-saturated electrolyte. These findings are of fundamental interest and are also critical to the design of efficient tandem solar fuels generators for unassisted photoelectrochemical CO2R. Epitaxy allows for highly controlled doping of the thin films over a large range of carrier concentrations. This work focuses largely on the synthesis and characterization of nitrogen doped p-type ZnTe via MBE. Films grown in the temperature range of 340–360ºC on GaAs of (100) orientation with a 200 nm undoped ZnTe buffer layer and a 100 nm doped ZnTe layer have been characterized by RHEED, XRD, AFM, and Hall effect measurements. Doping concentrations between 1020 cm-3 and 1018 cm-3 have been achieved. Dark current-voltage measurements have been used to indicate stability of the electrode in aqueous conditions in less than -0.5 V vs RHE. Future work will include further investigations into carrier dynamics via transient absorption spectroscopy and continual development of a tandem, ZnTe-based photocathode.

Spin Coating of Silica Nanocolloids on Mica: Self-Assembly of Two-Dimensional Colloid Crystal Structures and Thin Films

The viability of spin-coating methods for the self-assembly of 150 nm diameter silica nanocolloids into large crystal structures on mica was investigated using different colloidal concentrations, accelerations, and rotational speeds. The samples were imaged by atomic force microscopy (AFM) in intermittent contact mode. Low colloidal concentration led to a size-dependent ordering configuration. The largest nanocolloidal particles formed crystalline close-packed structures that were surrounded by increasingly smaller nanocolloids configured into more polycrystalline or amorphous formations. This phenomenon became increasingly suppressed by increasing colloidal concentration. Two dimensional-fast Fourier transform (2D-FFT) radially averaged profiles of the topography images revealed increasing interparticle spacing with increasing rotational acceleration, from close-packed structuring at low accelerations to increasingly spaced packing at high acceleration (>800 rpm/s). This behaviour is attributed to rapid liquid shedding from the increased acceleration. Analysis with radial distribution functions quantified the extent of ordering and revealed an optimum spin speed that caused the formation of large, highly crystalline structures. This optimum spin speed is governed by the relationship between the rotational speed and the liquid film thickness that affect the uniformity of the film and the magnitude of the capillary forces generated.

Solution-Processable Indenofluorenes on Polymer Brush Interlayer: Remarkable N-Channel Field-Effect Transistor Characteristics under Ambient Conditions.

The development of solution-processable n-type molecular semiconductors that exhibit high electron mobility (μe ≥ 0.5 cm2/(V·s)) under ambient conditions, along with high current modulation (Ion/Ioff ≥ 106-107) and near-zero turn on voltage (Von) characteristics, has lagged behind that of other semiconductors in organic field-effect transistors (OFETs). Here, we report the design, synthesis, physicochemical and optoelectronic characterizations, and OFET performances of a library of solution-processable, low-LUMO (-4.20 eV) 2,2'-(2,8-bis(3-alkylthiophen-2-yl)indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile small molecules, β,β'-Cn-TIFDMTs, having varied alkyl chain lengths (n = 8, 12, 16). An intriguing correlation is identified between the solid-isotropic liquid transition enthalpies and the solubilities, indicating that cohesive energetics, which are tuned by alkyl chains, play a pivotal role in determining solubility. The semiconductors were spin-coated under ambient conditions on densely packed (grafting densities of 0.19-0.45 chains/nm2) ultrathin (∼3.6-6.6 nm) polystyrene-brush surfaces. It is demonstrated that, on this polymer interlayer, thermally induced dispersive interactions occurring over a large number of methylene units between flexible alkyl chains (i.e., zipper effect) are critical to achieve a favorable thin-film crystallization with a proper microstructure and morphology for efficient charge transport. While C8 and C16 chains show a minimal zipper effect upon thermal annealing, C12 chains undergo an extended interdigitation involving ∼6 methylene units. This results in the formation of large crystallites having lamellar stacking ((100) coherence length ∼30 nm) in the out-of-plane direction and highly favorable in-plane π-interactions in a slipped-stacked arrangement. Uninterrupted microstructural integrity (i.e., no face-on (010)-oriented crystallites) was found to be critical to achieving high mobilities. The excellent crystallinity of the C12-substituted semiconductor thin film was also evident in the observed crystal lattice vibrations (phonons) at 58 cm-1 in low-frequency Raman scattering. Two-dimensional micrometer-sized (∼1-3 μm), sharp-edged plate-like grains lying parallel with the substrate plane were observed. OFETs fabricated by the current small molecules showed excellent n-channel behavior in ambient with μe values reaching ∼0.9 cm2/(V·s), Ion/Ioff ∼ 107-108, and Von ≈ 0 V. Our study not only demonstrates one of the highest performing n-channel OFET devices reported under ambient conditions via solution processing but also elucidates significant relationships among chemical structures, molecular properties, self-assembly from solution into a thin film, and semiconducting thin-film properties. The design rationales presented herein may open up new avenues for the development of high-electron-mobility novel electron-deficient indenofluorene and short-axis substituted donor-acceptor π-architectures via alkyl chain engineering and interface engineering.

Crystallization of (Hf, Zr)O2 thin films via non-heating process and their application to ferroelectric-gate thin film transistors

HfO2-based ferroelectric materials do not necessarily require high-temperature annealing for crystallization, making them attractive for applications in transparent electronic devices on plastic or glass substrate. In this study, (Hf, Zr)O2 (HZO) films prepared via non-heating sputtering are investigated and their application to ferroelectric-gate thin-film transistors (TFTs) is demonstrated. The internal tensile stress induced by (In, Sn)O x (ITO) top-electrode deposition is found to promote the crystallization of HZO from the amorphous state to the ferroelectric phase. ITO/HZO (15–25 nm)/ITO capacitors prepared via the non-heating process exhibit ferroelectric hysteresis loops with remanent polarizations of 6–9 μC cm−2 and coercive fields of 0.6–1.1 MV cm−1. Ferroelectric-gate TFTs with a 10 nm thick ITO channel are also fabricated via the non-heating process. These TFTs show nonvolatile operation with an on/off ratio of ∼10. These findings demonstrate the potential of HZO for transparent devices on substrates with low thermal resistance prepared via the non-heating process.

Performance of LiTaO3 Crystals and Thin Films and Their Application

Lithium tantalate (LiTaO3, or LT) crystal is widely used in optical applications, infrared detection, and acoustic surface wave devices because of its excellent piezoelectric, pyroelectric, and nonlinear optical properties. In this paper, we discuss the defect structure of LT; the preparation method for LT; the influence of doping on LT; and LT’s application in optical, acoustic, and electrical devices. We mainly analyzed the structure and physical properties of LT crystal, the preparation of LT crystal and LT thin films, the periodic polarization of LT crystal, the reduction of LT wafers, and the application potential of LT crystals in lasers and acoustic surface filters according to the most recent research. We also provide an overview of future research directions for LT in the fields of acoustics, optics, and other fields. The applications of LT in 5G, 6G, SAW filters, nonlinear optical devices, and waveguides are expected to provide additional breakthroughs.

The directed self-assembly of reflective liquid crystalline polymer films to form polarization-independent diffractive optical elements

The directed self-assembly of liquid crystal polymers enables significantly enhanced optical properties compared to conventional isotropic optical materials. The transmissive properties of these thin polymeric films have been extensively studied, but the exploration of reflective liquid crystalline polymeric materials films has received little attention due to challenges in producing large, high-quality self-assembled films. We explore here for the first time stacked polymer cholesteric liquid crystal thin films as a pathway to generate reflective diffractive optics that act on both hands of circularly polarized light independently. We present 3 unique exemplar cases with increasing design complexity to communicate the widespread potential of these unique thin polymer films.

Photoluminescent Thin Films of Room-Temperature Glassy Tris(keto-hydrozone) Discotic Liquid Crystals and Their Nanocomposites with Single-Walled Carbon Nanotubes for Optoelectronics.

This study addresses the photoresponse of liquid-crystalline tris(keto-hydrozone) discotic (TKHD)-a star-shaped molecular structure with three branches. Object of our research interest was also TKHD filled with single-walled carbon nanotubes (SWCNTs) at a concentration of 1 wt %. At room temperature, the discotic liquid crystals in thin films (thickness 3 μm) of both TKHD and nanocomposite SWCNT/TKHD were in a glassy state. Such glassy thin films exhibited photoluminescence ranging from the deep-red to the near-infrared spectral region, being attractive for organic optoelectronics. The addition of SWCNTs to TKHD was found to stabilize the photoluminescence of TKHD, which is of significance for optoelectronic device applications. The photothermoelectrical response of highly conductive SWCNT/TKHD nanocomposite films was characterized by electrical impedance spectroscopy in the frequency range from 1 Hz to 1 MHz of the applied electric field. It was elucidated that the reversible photothermoelectrical effect in SWCNT/TKHD films occurs through SWCNTs and their network.

Exciton dissociation in 2D layered metal-halide perovskites

Layered 2D perovskites are making inroads as materials for photovoltaics and light emitting diodes, but their photophysics is still lively debated. Although their large exciton binding energies should hinder charge separation, significant evidence has been uncovered for an abundance of free carriers among optical excitations. Several explanations have been proposed, like exciton dissociation at grain boundaries or polaron formation, without clarifying yet if excitons form and then dissociate, or if the formation is prevented by competing relaxation processes. Here we address exciton stability in layered Ruddlesden-Popper PEA2PbI4 (PEA stands for phenethylammonium) both in form of thin film and single crystal, by resonant injection of cold excitons, whose dissociation is then probed with femtosecond differential transmission. We show the intrinsic nature of exciton dissociation in 2D layered perovskites, demonstrating that both 2D and 3D perovskites are free carrier semiconductors and their photophysics is described by a unique and universal framework.

Strategic Optimization of Annealing Parameters for Efficient and Low Hysteresis Triple Cation Perovskite Solar Cell

AbstractThe highly efficient perovskite solar cells (PSCs) are often developed with a hybrid metal halide perovskite material as the absorber layer. Particularly, cesium (Cs) containing triple cation perovskites (CsFAMA) have emerged as prospective absorber materials owing to their high reproducibility and superior reliability. Mostly, the perovskite thin films preparation involves thermal annealing as a post‐treatment technique strongly dependent on the perovskite materials. By optimizing the parameters for thermal annealing temperature and annealing time, the fabrication of economical perovskite devices is achievable. Herein, we strategically investigated the influence of annealing time by keeping a constant annealing temperature on the growth of triple‐cation perovskite thin films and solar cells. We observed the formation of highly crystallized perovskite thin films with efficient photovoltaic performance for the heating time of just 10 min at 120 °C. Further, the perovskite devices with optimized annealing temperature and time exhibited low hysteresis index might be attributed to reduced trap states owing to improved grain size and crystallinity of CsFAMA thin films. Reduced heating time is desirable and beneficial in the industrial fabrication of photovoltaic devices.

Influence of current density on constant current electric field enhanced aluminum induced crystallization of amorphous silicon thin films

In this study, electric field-enhanced aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) thin films was investigated considering various current densities. Constant electric currents of 4, 5, and 6 A were applied to assess the Joule heating characteristics during specific durations (40, 15.5, and 9 s). The samples were prepared by depositing a 200-nm thick a-Si layer on a glass substrate and sputtering a 300-nm thick aluminum (Al) layer onto the a-Si layer. During the AIC process, layer exchange occured via the diffusion of silicon (Si) into the Al layer. This phenomenon was verified by the in-situ reflectivity, which was in excellent agreement with thin-film optics calculations. During the annealing, processing temperatures were lower than 470 °C, which is less than the Al-Si eutectic temperature of 577 °C. The Raman peak observed near a wavenumber of 519 cm−1 revealed the formation of polycrystalline silicon films through Joule heating processes. Field-effect scanning electron microscopy images were captured and analyzed to investigate the surface morphology of the samples, specifically focusing on grain growth.