Thick Polycrystalline Films Research Articles

Accelerated recrystallization of nanocrystalline films as a manifestation of the inner size effect of the diffusion coefficient

The work is devoted to the study of recrystallization occurring during short-term annealing of 100 nm thick polycrystalline films of copper and silver. It is found that in copper films deposited by the method of thermal evaporation onto a substrate at room temperature, a bimodal crystallite size distribution with maxima at 15 and 35 nm is observed. The bimodal distribution in copper films is preserved during annealing, which leads to a shift of both peaks of the crystallite size distribution histograms to the larger sizes region. In contrast to Cu, even in as-deposited Ag films, besides the nanosized fraction, micron-sized crystallites are present. Apparently, these grains are formed due to the phenomenon of self-annealing and weakly evolve during heating owing to grain growth stagnation. The nanosized fraction in as-deposited Ag films is represented by crystallites with the most probable size of 25 nm, which increases to 50 nm as a result of short-term annealing at the temperature of 250 °C. The grain-boundary diffusion coefficient was determined, which is more than 10−18 m2/s for both films of metals. The obtained value indicates a multiple intensification of self-diffusion processes in films, the thickness of which allows us to refer them to macroscopic sample.

One-Dimensional orthorhombic CsPbI3 polycrystalline thick film for efficient and highly stable direct X-ray detection and imaging

An ideal direct X-ray detector should convert a low dose of X-ray photons into large quantity of electrical signals, coupled with sustained long-term stability. Nevertheless, a notable conflict arises between X-ray absorption efficiency and carrier transport in thick polycrystalline perovskite films. Moreover, conventional perovskite materials exhibit inherent challenges regarding the stability of their crystal phase and the consistency of the photocurrent. To circumvent these limitations, a polycrystalline perovskite thick film is proposed using orthorhombic CsPbI3 (δ-CsPbI3) as a highly stable active material. Combined with the liquid epitaxy process, δ-CsPbI3 prefers one-dimensional growth along the carrier transportation direction, which suppresses the formation of grain boundaries, enabling high carrier mobility while maintaining X-ray absorption for a thick polycrystalline film. Consequently, the polycrystalline δ-CsPbI3 based detector exhibits a highest sensitivity of 1768.46 μC Gyair-1 cm−2 at an electric field of 432 V mm−1 which is 88.42-fold higher than α-Se based detectors and 190-fold higher than the single crystalline δ-CsPbI3 based detector, respectively. The sensitivity maintains 91.34 % of its initial state after 7 days exposure in the air. Combing with a thin-film transistor array, the detector achieves 8-bit imaging within a 64 × 64 matrix. This work provides a feasible method for commercialized production of high-performance direct X-ray detectors.

Homogeneous Bridging Induces Compact and Scalable Perovskite Thick Films for X-Ray Flat-Panel Detectors.

Solution-processed organic-inorganic hybrid perovskite polycrystalline thick films have shown great potential in X-ray detection. However, the preparation of compact perovskite thick films with large area is still challenging due to the limitation of feasible ink formulation and pinholes caused by solvent volatilization. Post-treatment and hot-pressing are usually involved to improve the film quality, which is however unsuitable for subsequent integration. In this work, a homogeneous bridging strategy is developed to prepare compact perovskite films directly. A stable perovskite slurry with suitable viscosity consisting of undissolved grains and supersaturated solution is formed by adding a weak coordination solvent to the pre-synthesized microcrystalline powders. Small perovskite grains in situ grow from the saturated solution during the annealing, filling the pinholes and connecting the surrounding original grains. As a result, large-area perovskite thick film with tight grain arrangement and ultralow current drift is blade-coated to achieve X-ray imaging. The optimal device displays an impressive mobility-lifetime product of 2.2×10-3 cm2 V-1 and a champion ratio of sensitivity to the dark current density of 2.23×1011 µCGyair -1 A-1 . This work provides a simple and effective route to prepare high-quality perovskite thick films, which is instructive for the development of perovskite-based X-ray flat-panel detectors.

Grain engineering of high energy density BaTiO3 thick films integrated on Si

Ferroelectric (FE) ceramics with a large relative dielectric permittivity and a high dielectric strength have the potential to store or supply electricity of very high energy and power densities, which is desirable in many modern electronic and electrical systems. For a given FE material, such as the commonly-used BaTiO3, a close interplay between defect chemistry, misfit strain, and grain characteristics must be carefully manipulated for engineering its film capacitors. In this work, the effects of grain orientation and morphology on the energy storage properties of BaTiO3 thick films were systematically investigated. These films were all deposited on Si at 500 °C in an oxygen-rich atmosphere, and their thicknesses varied between ~500 nm and ~2.6 μm. While a columnar nanograined BaTiO3 film with a (001) texture showed a higher recyclable energy density Wrec (81.0 J/cm3vs. 57.1 J/cm3 @3.2 MV/cm, ~40% increase) than that of a randomly-oriented BaTiO3 film of about the same thickness (~500 nm), the latter showed an improved energy density at a reduced electric field with an increasing film thickness. Specifically, for the 1.3 μm and 2.6 μm thick polycrystalline films, their energy storage densities Wrec reached 46.6 J/cm3 and 48.8 J/cm3 at an applied electric field of 2.31 MV/cm (300 V on 1.3 μm film) and 1.77 MV/cm (460 V on 2.6 μm film), respectively. This ramp-up in energy density can be attributed to increased polarizability with a growing grain size in thicker polycrystalline films and is desirable in high pulse power applications.

Effect of surface treatment on photo-electric properties of CZT thick film for radiation detector

The rough surface of CZT film is detrimental to both the surface leakage current and photoelectric response sensitivity. Here, the effect of different surface treatment techniques, such as manual mechanical polishing, chemical etching and passivation, on surface quality and photoelectric properties of the CZT thick film was investigated. The results demonstrated that a combination of surface treatments including mechanical polishing, chemical etching, and passivation can effectively improve the surface quality of CZT polycrystalline thick film, thus reducing the surface leakage current. The dark current of MPEP treated by mechanical polishing, chemical etching and passivation in turn is one order of magnitude smaller than that of as-grown CZT film. The resistivity of MPEP is 1.20 × 1010 Ω cm, which is 8.1 times that of as-grown film. The combination of surface treatment process greatly reduced surface leakage current, improved the resistivity and X-ray photoconductivity response performance of CZT coplanar detector. Our findings provide a guide for surface treatment to further improve both the photoelectric properties of photodevice based on the CZT thick film.

Synthesis, structural, optical, and electrical properties of continuous wave and pulse laser sintered semiconductor Ge films

The crystallization process of Ge films by a continuous wave (CW) and a pulsed laser is very effective for producing smooth, homogeneous, and crack-free polycrystalline films to use in transistors, photodetectors, and photovoltaic applications. However, little progress has been made to directly crystallize Ge films based on micro/nanoparticles (NPs) using the laser sintering (LS) process. In this paper, a simultaneous LS and crystallization process of Ge micro/NPs to develop thick polycrystalline films on silicon substrates is demonstrated. Silicon substrates with a SiO2 insulating layer on top were considered for compatibility with complementary metal–oxide–semiconductor (CMOS) technology. The LS process was applied to solution deposited micro/NPs, 5 µm thick Ge films using both CW mode (infrared laser of wavelength 1070 nm) and pulse mode (UV laser of wavelength 355 nm) laser. After the LS process, around 2–2.5 µm thick film of polycrystalline Ge (pc-Ge) was achieved with optical and electrical properties comparable to traditionally developed chemical vapor deposited films. The crystallinity of the pc-Ge films was evaluated by Raman spectroscopy and x-ray diffraction (XRD). The laser-sintered films exhibited a Raman peak at 300 cm−1 and XRD 2θ peak at 27.35, which indicated the poly-crystalline structure. The fabricated film showed high hole mobility of 203 cm2 V−1 s−1, without any doping and film electrical resistivity value of 6.24 × 105 Ω-cm. The developed LS process allows the quick deposition of polycrystalline thick films, removing surface porosity and voids, increasing films adhesion with the substrate, and faster thermal annealing.

The ice–vapour interface during growth and sublimation

Abstract. We employed environmental scanning electron microscopy (ESEM) in low-humidity atmosphere to study the ice growth, coalescence of crystallites, polycrystalline film morphology, and sublimation, in the temperature range of −10 to −20 ∘C. First, individual ice crystals grow in the shape of micron-sized hexagonal columns with stable basal faces. Their coalescence during further growth results in substantial surface defects and forms thick polycrystalline films, consisting of large grains separated by grain boundaries. The latter are composed of 1 to 3 µm wide pores, which are attributed to the coalescence of defective crystallite surfaces. Sublimation of isolated crystals and of films is defect-driven, and grain boundaries play a decisive role. A scallop-like concave structure forms, limited by sharp ridges, which are terminated by nanoscale asperities. The motivation for this work is also to evaluate ESEM's ability to provide a clean and reproducible environment for future study of nucleation and growth on atmospherically relevant nucleators such as materials of biological origin and inorganic materials. Hence, extensive information regarding potential ESEM beam damage and effect of impurities are discussed.

Quasi‐2D Perovskite Thick Film for X‐Ray Detection with Low Detection Limit

AbstractOrganic‐inorganic hybrid perovskites have demonstrated excellent performance in converting X‐ray photons to electrical signals with high detectivity and sensitivity. Solution‐processed large‐area perovskite polycrystalline thick film is promising for scalable X‐ray flat panel detection imaging. However, ionic migration is severe due to the numerous grain boundaries and pinholes in the thick films, resulting in a large dark current and serious baseline drift. Herein, a new strategy is proposed to suppress the ion migration by inserting 2D Ruddlesden–Popper layer into the 3D perovskite film. The quasi‐2D perovskite thick films exhibit lower defects density, suppressed ion migration, and higher thermal stability compared to their 3D counterpart. Based on these advantages, the quasi‐2D perovskite‐based X‐ray detector shows a sensitivity of 10 860 µC Gyair−1 cm−2 with a stable dark current and photocurrent response. Impressively, the extra‐low detection limit of 69 nGyair s−1 is the lowest in all those reported polycrystalline film‐based detectors. The quasi‐2D film is believed to be a very promising choice for digital flat detectors for sensitive radiation detection and low‐dose dosimeter.

Inhomogeneous Broadening of Photoluminescence Spectra and Kinetics of Nanometer-Thick (Phenethylammonium)2PbI4 Perovskite Thin Films: Implications for Optoelectronics.

An outstanding potentiality of layered two-dimensional (2D) organic–inorganic hybrid perovskites (2DHPs) is in the development of solar cells, photodetectors, and light-emitting diodes. In 2DHPs, an exciton is localized in an atomically thin lead(II) halide inorganic layer of sub-nanometer thickness as in a quantum well sandwiched between organic layers as energetic and dielectric barriers. In previous years, versatile optical characterization of 2DHPs has been carried out mainly for thin flakes of single crystals and ultrathin (of the order of 20 nm) polycrystalline films, whereas there is a lack of optical characterization of thick (hundreds of nanometers) polycrystalline films, fundamentals for fabrication of devices. Here, with the use of photoluminescence (PL) and absorption spectroscopies, we studied the exciton behavior in ∼200 nm polycrystalline thin films of 2D perovskite (PEA)2PbI4, where PEA is phenethylammonium. Contrary to the case of ultrathin films, we have found that peak energies and line width of the excitonic bands in our films demonstrate unusual extremely weak sensitivity to temperature in 20–300 K diapason. The excitonic PL band is characterized by a significant (∼30 meV) Stokes shift with respect to the corresponding absorption band as well as by a full absence of the exciton fine structure at cryogenic temperatures. We suggest that the observed effects are due to the large inhomogeneous broadening of the excitonic PL and absorption bands resulting from the (PEA)2PbI4 band gap energy dependence on the number of lead(II) halide layers of individual crystallites. The characteristic time of the exciton energy funneling from higher- to lower-energy crystallites within (PEA)2PbI4 polycrystalline thin films is about 100 ps.

Nanocomposite thick films of CsPbI3: PVDF-HFP on plastics for direct conversion low-dose X-ray sensor

Nanocomposite thick films of PbI2: PVDF-HFP and CsPbI3: PVDF-HFP were coated on the top of interdigitated electrodes and studied for their low-dose X-ray sensing behaviour. The X-ray sensing measurement was performed for different exposure durations at room temperature up to 3.2 s using an intra-oral AC X-ray machine. The difference in the measured photocurrent under the illumination of pulsed X-ray was recorded using a Keithley 2450 source meter interfaced with dedicated computer. Prior to the sensing measurement, the pristine polycrystalline samples and thick films were studied for structural, microstructural and optical properties respectively, using a powder X-ray diffraction, scanning electron microscope, Raman and diffuse reflectance spectrophotometers. The observed results reveal that all samples are single phase without any other impurities in the form of a uniform thick dense film with crystallites in rod morphology (2 μm length and 200 nm diameter). An appreciable sensitivity of CsPbI3: PVDF-HFP thick film for a different dose durations as compared with PbI2: PVDF-HFP thick film explores the possibility of using it for low dose X-ray sensor development.

Impact of Pressure and Temperature on the Compaction Dynamics and Layer Properties of Powder-Pressed Methylammonium Lead Halide Thick Films

While halide perovskite X-ray detectors based on single crystals could achieve extraordinary sensitivities, detectors based on polycrystalline thick films lag behind in efficiency. This is unfortunate since the processing methods for producing polycrystalline thick films, especially by pressure treatment of powders, are suitable for upscaling. Here, we investigate in detail the pressing of readily prepared powders of methylammonium lead halide perovskites MAPbI3 and MAPbBr3 to thick layers. By time-dependent pressure measurements, we monitor the occurring compaction dynamics, identifying two relaxation processes with different timescales. When pressing at elevated temperatures from room temperature (RT) to 100 °C, the pressure relaxations change drastically. While the layer properties such as relative density and surface roughness only improve to a certain degree by increasing the pressure at RT, we observe relative densities >97%, considerable reduction in surface roughness, and a significant increase in grain size with tempered pressing. Analyses regarding time-dependent pressure relaxations of tempered pressing allow attributing the dynamics to a sintering process, where we find the sinter onset to be surprisingly low at about 30 °C, mainly independent of the applied pressure (10–100 MPa). Our results will allow for an improved and more targeted powder processing of halide perovskite thick films as they are promising candidates for efficient X-ray detectors.

The analysis methods of electron-microscopic images for automation of building histograms of nanoparticle size distribution

In that work a describing of automated system is made an introduction, which allow to identify particles, that are look after on electron-microscopic images, and to determine their size, area and length of borders independently. Particle identification is based on a threshold criterion, which has low computational complexity, but has proven to be quite effective when applied to SEM and TEM images. Visual control of correct particle identification is facilitated by colorizing images. The proposed software system has a high performance, and the processing of a typical SEM imagine usually take less than 10 seconds. The proposed approach was tested on different vacuum condensates and shown high efficiency for different SEM and TEM images. It is effective for simple pictures, which are characteristic for samples, formed by melting of sufficiently thick polycrystalline films. This approach can also be used for quantitative processing of images obtained from films condensed by the vapor-liquid mechanism. Usually such images are difficult for automatic processing. Data on the size dependence of the most probable particle radius formed during melting tin films, full width at half maximum of histograms of their size distribution and excess energy, stimulating the process of de-wetting of films were received by using the proposed software. It is shown that particle size distribution for samples obtained by melting initially continuous polycrystalline films is single-mode and has normal character. The ratio of the full width at half the height of the distribution histogram to the most probable particle radius does not depend on the film thickness. Applying offered method for alloys films let to establish the effect of composition on films’ de-wetting and to show, that sequential vacuum condensation and subsequent melting are a easy method of forming arrays both single-component particles and alloys particles.

Investigations of ferroelectric polycrystalline bulks and thick films using piezoresponse force microscopy.

In recent years, ferroelectric/piezoelectric polycrystalline bulks and thick films have been extensively studied for different applications, such as sensors, actuators, transducers and caloric devices. In the majority of these applications, the electric field is applied to the working element in order to induce an electromechanical response, which is a complex phenomenon with several origins. Among them is the field-induced movement of domain walls, which is nowadays extensively studied using piezoresponse force microscopy (PFM), a technique derived from atomic force microscopy. PFM is based on the detection of the local converse piezoelectric effect in the sample; it is one of the most frequently applied methods for the characterization of the ferroelectric domain structure due to the simplicity of the sample preparation, its non-destructive nature and its relatively high imaging resolution. In this review, we focus on the PFM analysis of ferroelectric bulk ceramics and thick films. The core of the paper is divided into four sections: (i) introduction; (ii) the preparation of the samples prior to the PFM investigation; (iii) this is followed by reviews of the domain structures in polycrystalline bulks; and (iv) thick films.

Electrical, structural and optical properties of Cd1-xZnxTe thick polycrystalline films

Thick polycrystalline Cd1-xZnxTe films with different Zn concentration were deposited by the close-spaced vacuum sublimation method on ITO-coated glass substrates. In order to evaluate the usefulness of the films as a base layer in X-ray detector, the study of their electrical properties and photoresponse was carried out. Structural, microstructural properties and phase composition of the films were investigated using X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and Raman spectroscopy. It was found that films have cubic zinc-blende crystal lattice; the microstructural analysis showed that crystalline quality (e.g. coherent scattering domains size and microstrains level) of the films is decreasing with increasing of Zn concentration. Raman measurements showed only CdTe- and ZnTe-related vibrational modes, which indicates the absence of tellurium inclusions at the surface.The temporal photoresponse of the detector was described with the help of the analytical model, taking into account the presence of two types of recombination centers. As a result, the parameters of the recombination centers and their possible influence on the current flow in the films were established.

Raman Characterisation of Cd1-xZnx Te Thick Polycrystalline Films Obtained by the Close-Spaced Sublimation

Raman Characterisation of Cd_1-xZn_x Te Thick Polycrystalline Films Obtained by the Close-Spaced Sublimation

PVT growth of exfoliated CdZnTe polycrystalline thick films based on stress mismatch mechanism

The exfoliated CdZnTe (CZT) films were grown on quartz substrates by closed tube physical vapor transport method based on stress mismatch mechanism. The structure and morphology of the exfoliated CZT thick films were characterized by X-ray diffraction and scanning electron microscope. The as-grown CZT films are polycrystalline with (111) preferential orientation and exhibit a dense pyramid surface structure and a thickness of more than 600 μm. The coplanar and planar X-ray detectors were fabricated based on the exfoliated CZT film. The discrepant dark resistivity is 3.17 × 109 and 5.14 × 1010 Ω cm with regard to the coplanar and planar structure detector, respectively. The planar structure detector exhibits higher quantum efficiency at low doses X-ray radiation.

Self-separation of freestanding diamond films using graphite interlayers precipitated from C-dissolved Ni substrates

Freestanding diamond films were fabricated by a new self-separation method. Thick poly-crystalline diamond films were grown on poly-crystalline Ni substrates by microwave plasma-enhanced chemical vapor deposition after the substrates were saturated with carbon via a saturation process using a carbon solid solution. This saturation process suppressed the erosion of diamond nuclei on the Ni substrates. During the cooling process after diamond growth, the carbon atoms dissolved in the Ni substrates became supersaturated and precipitated as graphite interlayers at the interfaces between the diamond films and Ni substrates. The graphite interlayers caused the thick diamond films to spontaneously separate from the Ni substrates without cracking, allowing the Ni substrates to be reused. This self-separation method provides a low-cost way to fabricate freestanding diamond films.

The surface morphology, structural properties and chemical composition of Cd1−xZnxTe polycrystalline thick films deposited by close spaced vacuum sublimation

Thick polycrystalline Cd1−xZnxTe films with x ranged from 0.37 to 0.80 were obtained by the close spaced vacuum sublimation method. In order to investigate properties of the films structural, PIXE and Raman studies were carried out. Determination of chemical composition of the films by EDS, PIXE and XRD has shown good correlation of results. Raman spectroscopy reveals the relation between zinc concentration and vibrational properties of the films. Studies of the spatial distribution of the chemical elements on the film surface by micro-PIXE and micro-Raman spectroscopy have shown that films are uniform and free of secondary phases such as CdTe, ZnTe and Te inclusions.

Composite Cu-LaFeO3 Conversion Coatings on a 18Cr Ferritic Stainless Steel for IT-SOFC Interconnects: An Investigation on Structure and Formation Mechanism

In this work, a novel iron chemical conversion coating process has been applied to deposit a composite Cu-LaFeO3 perovskite coating layer on a commercial 18Cr ferritic stainless steel for application as Intermediate-Temperature Solid Oxide Fuel Cell (IT-SOFC) interconnect. The conversion process is carried out at 600°C under a CO2 gas atmosphere in a binary eutectic lithium-sodium molten carbonate bath containing lanthanum, magnesium and copper additions in form of their respective oxides. Multi-layer coatings are obtained, with a surface composite layer formed by a dense perovskite polycrystalline thick film with metallic copper particles encased in the perovskite grains grown on a Fe-rich spinel sublayer. Copper oxide additions significantly promote the formation of the LaFeO3 perovskite layer, and the experimental results suggest that galvanic coupling phenomena between the alloy elements and copper take place during the coating formation process. Chemical stability and Cr diffusion barrier properties of the composite coating have been evaluated in air exposure experiments at 700°C for 200 h. The perovskite phase shows high stability after the high temperature exposure, with no detectable sign of Cr diffusion in the layer. Details on functional properties of these composite conversion coatings for IT-SOFC applications will be reported in a subsequent study.

Electrical properties of UV-irradiated thick film piezo-sensors on superalloy IN718 using photochemical metal organic deposition

Lead zirconate-titanate thick films of perovskite structure Pb(Zr0.52Ti0.48)O3 were fabricated on the curved surface of IN718 superalloy substrate using photochemical metal organic deposition technique. The films were heated at 200°C, irradiated by UV lamp (365nm, 6W) for 10min, pyrolyzed at 400°C, and subsequently annealed at 700°C for 1h after successive multi-step coating with 10wt.% excess PbO. Fairly smooth, dense, and crack-free polycrystalline thick films of Au/PZT/Au structure were deposited on IN718. The films were characterized to investigate the effect of photochemical deposition process and substrate on the crystalline, morphological, dielectric, ferroelectric and piezoelectric properties. Remanent polarization (Pr=20.2μC/cm2), coercive field (Ec=−10kV/cm), permittivity (εr=324@1kHz), dielectric loss (tanδ≈3%), and piezoelectric charge coefficient (d33=113.4pm/V) of the PZT thick film transducers were measured and interpreted as regards domain wall motions. Consequently, the photochemical grown lead zirconate-titanate thick film on the superalloy substrate IN718 was confirmed to be consistent to structural health monitoring due to its acceptable electrical response.