Phase Transformation Of Films Research Articles

Detection of thin film phase transformations at high-pressure and high-temperature in a diamond anvil cell

Quantifying how grain size and/or deviatoric stress impact (Mg,Fe)2SiO4 phase stability is critical for advancing our understanding of subduction processes and deep-focus earthquakes. Here, we demonstrate that well-resolved X-ray diffraction patterns can be obtained on nano-grained thin films within laser-heated diamond anvil cells (DACs) at hydrostatic pressures up to 24 GPa and temperatures up to 2300 K. Combined with well-established literature processes for tuning thin film grain size, biaxial stress, and substrate identity, these results suggest that DAC-loaded thin films can be useful for determining how grain size, deviatoric stress, and/or the coexistence of other phases influence high-pressure phase stability. As such, this novel DAC-loaded thin film approach may find use in a variety of earth science, planetary science, solid-state physics, and materials science applications.

Thin film phase transformation kinetics: From theory to experiment

The Level-set method simulation is used to address the effect of finite size on kinetics of thin film phase transformations. The results are first interpreted using the classic Johnson-Mehl-Avrami-Kolmogorov (JMAK) description of a nucleation and growth phase transformation that yields the average Avrami exponent and rate constant as a function of film thickness. The analysis reveals that the JMAK framework can yield a spurious thickness dependent activation energy for the transformation. To overcome this problem, we propose an analysis that allows all the kinetic parameters, including the nucleation rate and interface growth velocity in films to be determined from experiment.

Simulation of phase transformation kinetics in thin films under a constant nucleation rate

The classic framework of Johnson-Mehl-Avrami-Kolmogorov (JMAK) has been commonly used in studies of thin film phase transformation kinetics despite its inherent limitation to transformations that occur in finite size domains or via heterogeneous nucleation on surfaces. To address the effects of finite size and heterogeneous nucleation on a JMAK analysis, we employ the level-set method to simulate phase evolution in thin film systems. Isothermal transformations under a constant nucleation rate and isotropic interface growth, with both bulk and surface nucleation cases are considered for broad range of film thicknesses. In agreement with past work, we find that when the thickness of the film is sufficiently small or heterogeneous nucleation on surface is present, it is possible to have a non-constant Avrami exponent over the course of phase transformation. Our results also show that the rate constant varies with the film thickness in contrast to bulk phase transformations. Furthermore, we obtain the grain size distributions at the end of the transformation for various film thicknesses that vary strongly in shape for small changes in film thickness when the film thickness is on the order of the characteristic length. By analyzing this information and determining the change of the average grain size with film thickness, we find that, the film thickness relative to the characteristic length is a reliable indicator of the dominant growth dimensionality in thin film phase transformations.

Phase transformations in films deposited by laser ablation of Hf in an oxygen Atmosphere

The structure of the hafnium dioxide films grown during pulsed laser sputtering of an Hf target in an oxygen atmosphere is examined, and the phase transformations that occur in them at annealing are studied by transmission electron microscopy and electron diffraction. Amorphous, tetragonal, orthorhombic, and monoclinic HfO2 phases are detected in the films. The tetragonal modification of HfO2 exhibits an epitaxy effect on a KCl(001) substrate. When the amorphous film is annealed in vacuum or air, it crystallizes to form the monoclinic modification of HfO2. The action of an electron beam on the amorphous film in vacuum leads to the formation of the orthorhombic and monoclinic modifications of HfO2. The transformation from the orthorhombic to the monoclinic modification of HfO2 is accompanied by a phase size effect. At the final stage of crystallization, the monoclinic modification represented by HfO2 dendrite crystals is the predominant structural constituent.

Phase Transformations in PZT Thin Films Prepared by Polymeric Chemical Method

Lead zirconate titanate Pb(Zr0.50Ti0.50)O3 (PZT) thin films were deposited by a polymeric chemical method on Pt(111)/ Ti/SiO2/Si substrates to understand the mechanisms of phase transformations in these films. PZT films pyrolyzed at temperatures higher than 350℃ present a coexistence of pyrochlore and perovskite phases, while only perovskite phase is present in films pyrolyzed at temperatures lower than 300℃. For films where the pyrochlore and perovskite phase coexists the amount of pyrochlore phase decreases from top surface to the bottom film-electrode interface and the PZT structure near top surface are Ti-rich compositions while near the bottom film-electrode interface the compositions are Zr-rich. For pyrochlore-free PZT thin film, a small (100) orientation tendency near the film-electrode interface was observed.

Evidence of pentacene bulk and thin film phase transformation into an orthorhombic phase by iodine diffusion

We present evidence for 130 nm thick pentacene films ordered in the orthorhombic phase obtained by iodine diffusion into the film deposited by classical thermal evaporation. After desorption of iodine by vacuum annealing the crystallographic properties of the investigated pentacene films resemble those of very thin thermal evaporated films that exhibit an orthorhombic phase with high electrical performance. Therefore, the results presented in this work are believed to be important for the development of organic field-effect transistors.

Preparation and ferroelectric properties of barium-ion-doped strontium bismuth tantalate thin films

(Sr 0.5Ba 0.5) x Bi y Ta 2O z (SBBT) films were prepared on Pt/Ti/SiO 2/Si substrates by a metal-organic decomposition method. At low temperatures, the phase transformation of films from fluorite-type structure to SBBT structure was suppressed with increasing the amounts of bismuth ions. As temperature increased, the phase transformation was improved by excess bismuth ions. Adding excess bismuth ions also resulted in an increase in the grain size of the prepared films. The value of the remanent polarization of thin film was also increased by adding excess bismuth contents. It was found that the ferroelectric characteristics of these films were significantly affected by the composition in the films.

Phase transformation in chromium nitride films

The phase transformation of CrN films prepared on (100) Si substrates by cathodic arc plasma deposition was investigated at various temperatures using x-ray diffraction and stress measurements. The films were annealed in a N2/H2=9 reducing atmosphere over the temperature range of 300–1200 °C. X-ray diffraction results showed that an additional Cr2N phase appeared both at temperatures above 1100 °C and over temperatures between 500 and 650 °C. Thermodynamics can explain the formation of Cr2N at temperatures above 1100 °C but not that in the low temperature range. Nevertheless, the residual stresses in the films were determined and found to be relaxed largely over such a low temperature range. The stress states of the films were strongly correlated to the phase transformation of the films. It is concluded that the formation of Cr2N at such low temperatures is mainly due to a nonthermodynamic factor—large stress relaxation occurring in the films. More experimental proof of the stress relaxation-induced phase transformation is presented.

Coherent phase equilibria in a bending film

The theory of coherent phase diagrams is expanded to phase transformations in films. The misfit between the phases dictates their layer arrangement with an interface parallel to film surfaces. Then, the transformational self-strain leads to the film bending. The phase equilibrium analysis shows that the polymorphic transformation in a single component system proceeds with the thermodynamic hysteresis. However, the reversible change of the two-phase equilibrium with temperature is possible, if the phase fractions are maintained within a range (1/2, 2/3). The character of polymorphic phase transformations in a binary system depends on the concentration of a second component. The quasi-single component transformation takes place at small concentrations. The reversible coherent phase transformation should be observed at the concentration larger than the critical one. The irreversible transformations take place at intermediate concentrations. The irreversible transformations begin with the formation of a finite fraction of the second phase. Then, the two-phase system evaluates with temperature to the final two-phase state that transfers discontinuously to a single-phase state. The concentration diagram does not coincide with the phase diagram. The concentrations in the transforming phases change non-monotonously with temperature.

Growth and Structure of In2Se3 and CuInSe2 Thin Films

The investigation of phase transformations in indium selenide films during step selenization processes at 200, 350, 400 and 580°C was carried out. It was found that the phase transformations in films are accompanied by recrystallization processes which promote the growth of large crystalline films. Large-crystalline In2Se3 films are formed at last stage of selenization. Superstructure reflections which can be explained by odered vacancy arrays and therefore indicated a Se deficiency in In2Se3 grains were observed. Large crystalline CuInSe2 films have been grown on the base of the In2Se3 films.

Phase Transformations in Ferromagnetic NiMnGa Shape Memory Films

The paper presents recent work on ferromagnetic NiMnGa films sputter deposited on silicon cantilever substrates. The films are partially crystalline if the substrate is held at room temperature (RT) during the deposition. They become fully crystalline after annealing at 400°C. The internal friction (IF) of the RT deposition shows a peak that is attributed to the nanostructure of the film. Films deposited at RT and annealed and films deposited on substrates held above 385°C show a transformation comparable to NiTi shape memory films. However, the damping is one order of magnitude lower. Annealing does not significantly change the phase transformation of films that were deposited at 385°C or above. Measurements of the temperature dependence of the change of the elastic modulus as a function of the magnetic field of a nominally Ni 50 Mn 25 Ga 25 film suggest that the magnetic anisotropy field increases proportional to the transformation strain, (c/a - 1), from 0.3 T at 50°C to 0.8 T at -150°C.

Use of the effective heat of formation model to determine phase formation sequences of In-Se, Ga-Se, Cu-Se, and Ga-In multilayer thin films

Multilayer thin films of indium-selenium, gallium-selenium, copper-selenium, and gallium-indium have been analyzed for the purpose of understanding thin film phase transformations that are relevant to the production of Cu(In,Ga)Se2 photovoltaic solar cells. For example, the intermetallic phases that are present during selenization of precursor films will affect film microstructure and the resulting film properties. Throughout this research, the effective heat of formation model1 has been used to predict phase formation sequences. The accuracy of these predictions was explored experimentally. By using differential thermal analysis, the reactions of product film formation were examined. X-ray diffraction was used to determine reactant and product phases.

Quantitative investigation of copper/indium multilayer thin film reactions

Solid state devices based on copper indium diselenide, (CIS), have produced world record efficiencies for polycrystalline photovoltaic solar cells. In order to understand thin film phase transformations that are relevant to the production of CIS, multilayer thin films of copper and indium have been analyzed. Using differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy, the reaction pathways for equal atomic concentration, copper rich, and indium rich films were established. Indium was found to react during deposition, so that the as-deposited films consisted of CuIn 2 and copper. During constant heat rate experiments, the excess concentration of copper within the film provided the driving force in the formation of phases richer in copper. The Cu 11In 9 phase was observed to form from CuIn 2 and Cu at 130 °C, and Cu 7In 3 formed from Cu 11In 9 and Cu at 310 °C. In the indium rich samples, a given phase was found to be stable until melting. In films with an overall indium concentration exceeding the concentration of Cu 11In 9, the excess indium was shown to move primarily to the film surface, and in indium rich films, to grain boundaries between the Cu 11In 9 phase.

On the nature of ion bombardment-induced phase transformations in films of transition metals

Abstract Electron diffraction studies have been made of polycrystalline Ni films irradiated with well separated beams of ions of different nature, namely ions of inert (He+, Ne+, Ar+, Kr+, Xe+) and reactive (N+ and O+) gases. The Ni films were prepared under vacuum conditions (P≳ 3·10−6Pa during evaporation) preventing an appreciable contamination of the films with impurities. The samples were irradiated at T≍ 300 K with ion beams of energies from 10 to 100 keV in the dose range between 5·1016 cm−2 and the value leading to sample destruction. Irradiation with noble gas ions revealed no phase transitions in the Ni films. A similar result was obtained in irradiation of Fe and Cr films with He+ ions. The bombardment of Ni films with reactive gas ions does cause changes in the lattice structure of the samples under study, depending on the nature of the bombarding ions. The N+ ion bombardment gives rise to the hcp phase with the lattice parameters typical of the Ni3N compound, and the O+ ion bombardment result...

Phase transformations in thin films

Several aspects are reviewed which cause phase transformations in films to differ from those in the bulk material. Amorphous to crystalline transformations and precipitation effects in FeAu films are described. The influence of film surfaces on precipitation is discussed.