Amorphous-crystalline Phase Transition Research Articles

Effect of the amorphous-to-crystalline transition in Ba0.5Sr0.5TiO3 thin films on optical and microwave dielectric properties

Thin films of Ba0.5Sr0.5TiO3 were deposited at ambient temperature on fused silica substrates by RF magnetron sputtering followed by post-deposition annealing at temperatures between 400 and 1000 °C. The post-deposition annealing resulted in an amorphous–crystalline phase transition in the films. The amorphous–crystalline transition is accompanied by an increase in the refractive index, a decrease in the optical band gap and an increase in the microwave dielectric constant. At 800 °C the films exhibit a refractive index of 2.21 at 550 nm, an optical band gap of 3.49 eV and a microwave dielectric constant of 265, all of which indicate the crystalline nature of the films. The films lose oxygen when annealed at high temperatures as evidenced by the optical and microwave dielectric behaviour. The observed behaviour is explained in terms of defect centres created in the form of oxygen vacancies due to the annealing process and the structural phase transition.

Amorphous–crystalline phase transitions in chalcogenide materials for memory applications

Understanding the phase transition processes in chalcogenide materials is of key importance for the development, optimization, and further scaling of phase-change memories (PCM). In particular, the transition from the amorphous to crystalline phase plays a crucial role in both programming and retention. Depending on the chosen material, either nucleation or growth processes are dominating this crystallization. In the first part of this article, different theoretical models for nucleation and growth are compared, and this for two typical phase-change materials. The second part of the article shows how finite element simulations can provide the direct link between these crystallization theories and operation and retention characteristics of a real PCM cell.

Direct observation of amorphous to crystalline phase transitions in nanoparticle arrays of phase change materials

We have used time-resolved x-ray diffraction to study the amorphous-crystalline phase transition in 20–80nm particles of the phase change materials Ge2Sb2Te5, nitrogen-doped Ge2Sb2Te5, Ge15Sb85, Sb2Te, and Sb2Te doped with Ag and In. We find that all samples undergo the phase transition with crystallization temperatures close to those of similarly prepared blanket films of the same materials with the exception of Sb2Te that shows the transition at a temperature that is about 40°C higher than that of blanket films. Some of the nanoparticles show a difference in crystallographic texture compared to thick films. Large area arrays of these nanoparticles were fabricated using electron-beam lithography, keeping the sample temperatures well below the crystallization temperatures so as to produce particles that were entirely in the amorphous phase. The observation that particles with diameters as small as 20nm can still undergo this phase transition indicates that phase change solid-state memory technology should scale to these dimensions.

Effect of mechanochemical treatment on petroleum coke–CO 2 gasification

The effect of mechanochemical treatment during the grinding of petroleum coke on its gasification by CO 2 was studied. An additive derived by drying the black liquor in papermaking industry is adopted in grinding process. Results show that the gasification reactivity of petroleum coke is effectively improved by grinding, and the activation by wet grinding is more noticeable than that by dry grinding. Besides, by wet grinding petroleum coke and additive together, the active metal species in additive are not easily volatilized in gasification, and retain a high catalytic reactivity to the coke–CO 2 reaction throughout most of the conversion range. Changes in crystal structure of the petroleum coke induced by mechanochemical treatment is related to its gasification reactivity. In general, the crystalline–amorphous phase transition is the tendency of long time mechanical grinding, while a crystal structure re-formation stage is observed after wet grinding of petroleum coke with and without additive for some time. Similar phenomenon has also been found in the reported data, but not given attention. Some discussion is made in the paper, and more work should be undertaken to disclose the mechanism.

Solution-phase deposition and nanopatterning of GeSbSe phase-change materials

Chalcogenide films with reversible amorphous-crystalline phase transitions have been commercialized as optically rewritable data-storage media, and intensive effort is now focused on integrating them into electrically addressed non-volatile memory devices (phase-change random-access memory or PCRAM). Although optical data storage is accomplished by laser-induced heating of continuous films, electronic memory requires integration of discrete nanoscale phase-change material features with read/write electronics. Currently, phase-change films are most commonly deposited by sputter deposition, and patterned by conventional lithography. Metal chalcogenide films for transistor applications have recently been deposited by a low-temperature, solution-phase route. Here, we extend this methodology to prepare thin films and nanostructures of GeSbSe phase-change materials. We report the ready tuneability of phase-change properties in GeSbSe films through composition variation achieved by combining novel precursors in solution. Rapid, submicrosecond phase switching is observed by laser-pulse annealing. We also demonstrate that prepatterned holes can be filled to fabricate phase-change nanostructures from hundreds down to tens of nanometres in size, offering enhanced flexibility in fabricating PCRAM devices with reduced current requirements.

Crystalline-amorphous phase transition of hyperbranched polyurethane phase change materials for energy storage

Hyperbranched polyurethane solid–solid phase change material (HB-PUPCM) has been prepared via a two-step process. The phase transition behaviors and morphologies of these HB-PUPCM films were investigated using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarizing optical microscopy (POM). PEG soft segment in the polyurethane was found to be crystalline at room temperature. However, when the temperature was raised to PEG’s melting point, polyurethane did not melt into the liquid state as in the case of pure PEG but changed to an amorphous solid state. In HB-PUPCM, PEG’s molecules probably are tied to the hard segment chain so strongly by the chemical bonds that they cannot change to a liquid state but change to the amorphous state in the transition processing.

Synthesis and Characterization of Ge2Sb2Te5 Nanowires with Memory Switching Effect

Ge2Sb2Te5 nanowires (NWs) were synthesized by vaporizing GeTe, Sb, and Te precursors assisted by metal catalysts. Current-voltage measurement of the Ge2Sb2Te5 NW device displays fast and reversible switching between two distinct resistive states, which is due to the crystalline-amorphous phase transition nature of these materials

Application of Bond Constraint Theory to the Switchable Optical Memory MaterialGe2Sb2Te5

A new extended x-ray-absorption fine structure spectroscopy study of local bonding identifies for the first time significant concentrations of Ge-Ge bonds in amorphous Ge2Sb2Te5. The study provides a new understanding of the local molecular structure of this phase-change material. Application of bond constraint theory indicates that the amorphous phase is an ideal network structure in which the average number of constraints per atom equals the network dimensionality. Analysis within this framework imparts new and significant insights concerning the nature of the reversible optically driven amorphous-crystalline phase transition of Ge2Sb2Te5.

Phase transitions in the atomic, electronic, and magnetic subsystems of LaSrMnO films versus the synthesis temperature

The evolution of the cluster structure in amorphous LaSrMnO films as synthesis temperature Ts increases from 20 to 300°C is considered. Two order-disorder phase transitions with different scale parameters are observed. One of them, the aggregation of disordered atoms into small (∼20 A) amorphous clusters at Ts = 100°C, shows up as a sharp increase in the intensity of diffuse X-ray scattering (diffuse halo 1) with a simultaneous suppression of incoherent (background) scattering. At Ts > 150°C, disordering dominates (Iincoh = Imax) until the next stage of ordering sets in at Ts = 250−300°C. At this stage, the crystalline phase forms from large (>100 A) crystalline clusters. This amorphous-crystalline phase transition is characterized by the appearance of Debye lines and a reduction of the halo intensity. The structural phase transition to long-range order is accompanied by a decrease in the LaSrMnO resistivity from 1010 to 10 Ω cm and a change from the tunneling mechanism of conductivity involving metallic clusters (which is typical of granulated systems) to the hopping mechanism with a hop variable length following the Mott law ρ ∼ exp(T−1/4). In the magnetic subsystem, the paramagnetic-ferromagnetic phase transition occurs.

Structural, thermal and optical studies of mechanical alloyed Ga 40Se 60 mixture

A mixture of elemental Ga and Se with the nominal composition Ga 40Se 60 was submitted to the Mechanical alloying technique and their structural, thermal and optical properties were followed by X-ray diffraction, differential thermal analysis, photoacoustic spectroscopy, UV–VIS‐NIR absorbance spectroscopy and Raman spectroscopy techniques. After 10 h of milling the X-ray pattern showed monoclinic Ga 2Se 3 phase nucleation, which is in the nanometric form, and also a minority amorphous phase. The DSC results showed exothermic reactions between 430 and 720 K attributed to amorphous–crystalline phase transition and structural relaxation of Ga 2Se 3 phase. Based on this a small amount of the as-milled sample was annealed at 723 K. Its XRD pattern showed evidences of grain growth, reduction of the interfacial component, as well as, disappearance of the amorphous phase. The annealing process induced thermal diffusivity increasing, while the optical band gap energy and Raman profile remained practically unchanged.

The nitrogen doping effect on the properties of Ge–In–Sb–Te phase-change recording media investigated by blue-light laser

This work investigates the thermal, optical and recrystallization properties as well as the microstructure of nitrogen-doped Ge–In–Sb–Te (GIST) phase-change material when irradiated by blue-light laser. The experimental results showed that nitrogen doping at the condition of N 2/Ar sputtering gas flow ratio equals to 3% might enhance the recrystallization speed of GIST recording layer up to 1.5 times. However, the disk failed when too much nitrogen (N 2/Ar ≥ 5.0%) was introduced. The data obtained by differential scanning calorimetry, X-ray diffraction and ellipsometry revealed changes of thermal and optical properties due to the nitrogen doping in GIST. When appropriate amount of nitrogen was added, the activation energy ( E a) of amorphous–crystalline phase transition of GIST decreased and the optical constants of amorphous and crystalline phases (except the k value of amorphous phase) gradually reduced with the increase of wavelength in the range of 600–750 nm. Modulation simulation based on the reflectively of doped GIST layers obtained from static test indicated that appropriate nitrogen doping benefited the signal characteristics of optical disks. Transmission electron microscopy observed numerous tiny precipitates uniformly distributed in the doped GIST layers. These were believed to be nitride particles generated by nitrogen doping that might offer the preferential sites for amorphous–crystalline phase transition so that the recrystallization speed was accelerated.

Model-dielectric-function analysis of ion-implanted Si(100) wafers

Optical properties of Si+, P+, and Ar+ ion-implanted Si(100) wafers have been studied using spectroscopic ellipsometry. The Si+, P+, and Ar+ ions are implanted at 150 keV with fluences ranging from 1×1014 to 2×1015 cm−2 at room temperature. A model dielectric function (MDF), which was developed for modeling the optical constants of perfectly crystalline semiconductors, has been applied to investigate the optical response of the ion-implanted Si(100) wafers. The MDF analysis indicates a distinct structural transition from the crystalline to amorphous phase at some ion fluences around 1014–1015cm−2. Since the critical points do not have any validity in amorphous material, the band gaps used in the MDF are not a result of the Bragg gaps at the Brillouin-zone boundaries, but are considered to arise from the short-range order determined by the covalent bonding. Using these results, we obtain an expression, D=(1.6×108/M)2.2 cm−2 which enables us to estimate the crystalline-amorphous phase transition fluence D for silicon wafers implanted with optional ion species of mass number M. No clear change in the morphology of silicon surface after ion implantation has also been confirmed by atomic force microscopy.

Amorphous-crystalline phase transition during the growth of thin films: The case of microcrystalline silicon

Thin silicon films of varying thickness were deposited on foreign substrates by electron-cyclotron resonance chemical vapor deposition from ${\mathrm{SiH}}_{4}{\ensuremath{-}\mathrm{H}}_{2}$ mixtures at 600 K. Optical thickness measurements, Rutherford backscattering, and transmission electron microscopy reveal that a thin amorphous interlayer of some 10 nm thickness has formed upon the substrate, before the growth of a microcrystalline layer begins. The amorphous layer is found to be deposited with a higher rate than the crystalline phase. Since similar effects have been observed for a large variety of deposition techniques, the amorphous-crystalline phase transition is considered as an inherent property of the growth of thin silicon films on foreign substrates at low homologous temperatures. The change in growth mode is interpreted in terms of Ostwald's rule of stages, which predicts the evolution of film growth to proceed via a set of phases of descending metastability and nucleation rate. In applying capillarity theory a criterion is derived from the ratio of amorphous-phase and crystalline-phase nucleation rates ${J}_{a}{/J}_{c}.$ This ratio is developed into basic thermodynamic functions and is shown to govern the formation of either the stable or metastable phase. The approach is of general validity for thin-film deposition processes. In the case of microcrystalline silicon, experimental measures can be derived from the developed model to directly design the evolution of film structure.

Ion beam induced amorphous–crystalline phase transition in Si: Quantitative approach

A point defect diffusion model for ion beam induced crystallization and amorphization at the amorphous/crystalline interface in Si is presented and shown to be successful in providing a quantitative explanation for the phase transition kinetics. The model takes into account a minimum set of the most plausible elementary processes of point defects in crystalline Si as well as the formation and dynamic annealing of disordered regions during ion bombardment. Two mechanisms for the phase transition are proposed and shown to lead to nearly identical mathematical formulations. The first mechanism assumes that the recombination of interstitial atoms at the phase boundary leads to crystallization, while the same process for vacancies causes the growth of the amorphous layer. Based on the second mechanism, the recombination of a Frenkel pair at the amorphous/crystalline interface produces the recrystallization of a small volume, whereas excess of the vacancies provokes the process of amorphization. Contribution to amorphization from disordered regions, vacancies, and divacancies at the phase boundary is also taken into account in the model. The defect processes are described by nonlinear differential equations. The defect reaction parameters are identified based on experimental data. The results indicate that the transition from amorphization to crystallization regime with increase in the substrate temperature is mainly due to strong reduction of vacancy concentration and a weak increase in the concentration of interstitial atoms. A number of experiments are simulated with the calculated parameters to test the developed model.

Probing low-temperature water ice phases using electron-stimulated desorption

Low-energy electron-stimulated desorption (ESD) of D + from D 2O has been used to examine the phase and growth behavior of nanoscale vapor-deposited ice films grown on Pt(111) between 90–155 K. The D + yield from porous amorphous solid water (deposited at 90 K) shows evidence for sintering near 120 K, increases between 120 and 140 K, and then drops at the amorphous–crystalline phase transition near 155 K. Ice deposited at 155 K forms an epitaxial crystalline film, with a D + yield nearly one-third larger than the yield from crystalline films prepared by annealing the amorphous phase. This suggests that the film formed by annealing may have a different crystalline ordering or morphology than the epitaxial film deposited between 150 and 155 K. Ice deposited at 90 K on top of the epitaxial film is amorphous, but it crystallizes to a form similar to that of the underlying crystalline ice substrate. This suggests that, in this case, the buried two-dimensional interface nucleates the crystallization.

Electrical conductivities and dielectric constants of some sodium tetraborate glasses containing CuO, irradiated with a low-level fast neutron beam

A number of glass samples were prepared from a mixture of Na2B4O7 and CuO. The prepared glasses were irradiated with a low-level fast neutron beam (flux density 108 n cm-2). The alternating current (a.c.)-electrical conductivity and the dielectric constant at a frequency of 1 kHz of these glasses have been studied as a function of temperature in the range 300–600 K. The measurements showed that an amorphous–crystalline phase transition characterized these samples. This phase transition was affected by neutron irradiation. Also a decrease in the activation energy for the irradiated undoped sample and an increase in the activation energy for the irradiated doped samples were found. After irradiation, the maximum peak, which characterized the dielectric constant of the samples before irradiation, disappeared. © 1998 Chapman & Hall

Preparation and optical studies on flash evaporated Sb2S3 thin films

Thin films of Sb2S3 were prepared by flash evaporation. As-deposited films were found to be amorphous and highly resistive (R>106Ω) in nature. An amorphous-crystalline phase transition occurs at 498 K on subsequent annealing in nitrogen atmosphere. The fundamental optical transition is direct in nature and the optical band gaps (Eg) lie in the region 1.70–2.07 and 1.42–1.65 eV for amorphous and crystalline films, respectively. A near-linear decrease in Eg with decreasing S/Sb ratio was observed suggesting the dependence of the band gap on film composition.

Impedance Spectroscopy and XPS Comparative Investigations of the State of Boron Atoms in an Amorphous Metallic Matrix

The purpose of this study is to compare the investigations of amorphous magnetic ribbons combining the usual structural methods with physical impedance spectroscopy (PIS) and X-ray photoelectron spectroscopy (XPS). The samples are commercially available rapidly quenched ribbons for torroidal transformers. Two types of ribbons were thermally treated below the temperature of amorphous–crystalline phase transition. Two binding energies of B 1selectrons are determined by XPS in one of the as-quenched ribbons, which show impedance time constant spectra without dispersion. The other ribbon, possessing only one value of binding energy of the B 1selectron (BIstate) in a rapidly as-quenched state, undergoes transition to a still amorphous state but acquires high electromagnetic losses and show another value of the binding energy of the B 1selectron (BIIstate) after the heat-treatment procedure. This latter type of ribbons show structural impedance spectra containing various time constant values.

Electron-stimulated desorption of D+from D2O ice: Surface structure and electronic excitations

We present a study of the electron-stimulated desorption of deuterium cations (D${}^{+})$ from thin (1--40 ML) D${}_{2}\mathrm{O}$ ice films vapor deposited on a Pt(111) substrate. Measurements of the total yield and velocity distributions as a function of temperature from 90 to 200 K show that the D${}^{+}$ yield changes with film thickness, surface temperature, and ice phase. We observe two energy thresholds for cation emission, near 25 and 40 eV, which are weakly dependent upon the ice temperature and phase. The cation time-of-flight (TOF) distribution is at least bimodal, indicating multiple desorption channels. A decomposition of the TOF distributions into ``fast'' and ``slow'' channels shows structure as a function of excitation energy, film thickness, and temperature. The D${}^{+}$ yield generally increases with temperature, rising near 120 K on amorphous ice, and near 135 K on crystalline ice. The amorphous-crystalline phase transition at $\ensuremath{\sim}160$ K causes a drop in total desorption yield. The temperature dependence of D${}^{\ensuremath{-}}$ desorption via the ${}^{2}{B}_{1}$ dissociative electron attachment resonance is very similar to the slow D${}^{+}$ yield, and likely involves similar restructuring and lifetime effects. The data collectively suggest that a thermally activated reduction of surface hydrogen bonding increases the lifetime of the excited states responsible for ion desorption, and that these lifetime effects are strongest for excited states involving ${a}_{1}$ bands

Temperature dependence of microstructure and magnetic properties of Co/Ti multilayer thin films

The microstructural and magnetic properties of amorphous Co/Ti multilayer films and their variation with temperature are investigated by transmission electron microscopy (TEM) analysis and thermomagnetic measurements. Thermomagnetic curves showed two peaks at about 400 and 520 °C. The evolution of the structure monitored in the hot stage of the TEM was found to be consistent with the magnetic changes. The first peak of the saturation magnetization Ms at 400 °C was associated with the transformation from amorphous ferromagnetism to paramagnetism due to the amorphous Co existing in the film. Ms began to increase corresponding to the crystallization point of the ferromagnetic Co phase, which decreased with increasing amounts of Co in the film. Ms reached its maximum at 520 °C and then decreased because the phase transition occurred at a temperature greater than 520 °C and approached completion at 650 °C. The amorphous phase and crystalline phase formation and phase transition during annealing were observed in Co/Ti multilayer thin films and successfully explained the thermomagnetic properties of the film.