Phase Transformations In Thin 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.

Analysis of transformation of a thin graphite-like carbon layer on the detonation nanodiamond surface into diamond

The article considers the process of obtaining nanostructured diamond materials due to phase transformation of thin films based on carbon non-diamond forms on the nanodiamond surface into diamond. For the synthesis of nanostructured diamond polycrystalline superhard material, various variants of the initial mixture based on nanodiamond with non-diamond carbon on the surface are proposed (surface-graphitized nanodiamond, surface-graphitized nanodiamond with addition of purified nanodiamond, detonation diamond-containing charge with a surface layer of “amorphous” carbon, including purified nanodiamond additives). The influence of the structure of a thin non-diamond layer on the parameters of graphite to diamond transition is revealed. For a carbon film with a disordered structure (the so-called “amorphous” carbon), the transition pressure to diamond will be about 10–15 GPa, which is significantly higher than the phase transformation pressure for thin graphite films in this temperature range. It is shown that the increase in the pressure of transformation of a thin layer of “amorphous” carbon is caused by its lower surface energy compared to the surface energy of graphite. It has been established that the region of transformation of a thin graphite-like film with a thickness of about 1 nm, formed on the 2–10 nm nanodiamond surface into diamond will be below the graphite-diamond equilibrium line in the temperature range of 1000– 2500 °C. Additional introduction of purified nanodiamond particles leads to a decrease in the pressure of transformation of thin layers of non-diamond carbon into a diamond structure from 10–15 to 2–7 GPa, which is due to the effect of the surface of catalytically active diamond nanoparticles on the thermodynamic stimulus of phase transformation.

High-Temperature Piezoelectric Resonators: Electroacoustic Properties and Applications of Langasite-Type Crystals

Piezoelectric devices based on high-temperature stable single crystals are of increasing interest due to a wide range of potential applications, such as actuators and resonant gravimetric sensors. The latter require highly accurate resonant-frequency measurements at temperatures as high as 1000 °C, and this can only be achieved with narrow resonance peaks and correspondingly low acoustic loss at these temperatures. Relevant atomistic defects in langasite (La3Ga5SiO14, LGS) and catangasite (Ca3TaGa3Si2O14, CTGS) are reviewed with respect to their impact on electrical conductivity and acoustic loss. The focus on these crystals also enables a comparison of electroacoustic properties of partially disordered (LGS) and ordered (CTGS) crystals and the formulation of approaches for reducing acoustic loss and enhancing device performance.Starting with defect chemistry models for LGS and CTGS, electrical conductivities as a function of temperature and oxygen partial pressure are considered. One result of this analysis is that contributions of oxygen ions to the total conductivity are determined to be relatively small in the studied crystals. Temperature-dependent acoustic loss of LGS and CTGS crystals is also studied. Two independent techniques, contact resonant piezoelectric spectroscopy, and a contactless tone-burst excitation technique, are applied. Contributions to the measured acoustic loss are determined by fitting loss-related functions for point defects and conductivity-related relaxations. The extracted fit parameters, in particular the activation energies of the processes, are discussed and compared with that of the electrical conductivity. The losses are caused by a superposition of several mechanisms including conductivity-related relaxations, point-defect relaxations and intrinsic phonon-phonon loss.The long-term stability of the crystals is also evaluated at 1000 °C with respect to fundamental materials properties including electrical conductivity and resonant frequency.After one year of thermal treatment, the resonance frequency of CTGS resonators from different crystal sources is found to decrease by no more than 0.4 %. Thus, the applicability of CTGS as a piezoelectric resonator that operates for long periods at extreme temperatures, such as a resonant nanobalance, is demonstrated.Application examples to be presented include resonant nanobalances for in-situ monitoring of non-stoichiometry and phase transformations in thin films during growth at high temperatures. In this context, the key application properties of the sensors at high temperatures, such as mass sensitivity and resolution, are discussed. As an application example, the attached figure shows the oxygen non-stoichiometry x of thin praseodymium-cerium oxide films (Pr0.1Ce0.9O2-x) at 700 °C as a function of oxygen partial pressure determined by LGS and CTGS nanobalances, which agrees very well with literature data (chemical capacitance: D. Chen et al., Chem. Mater. 2014, 26, 22, 6622–6627). Figure 1

Substrate effects in hysteresis of microscale Ni–Mn–Sn Heusler alloy films

During martensitic phase transformations in thin films, substrates impact hysteresis by introducing an additional interface, which can inhibit martensite/austenite interface motion. In order to reduce hysteresis, we examine 2.9–14.5 μm thick Ni–Mn–Sn films, which in some cases have been delaminated from the substrates before or after annealing. We compare thermal hysteresis and defect densities at the interface. Delaminating films prior to annealing decreases hysteresis, whereas delaminating films after annealing does not significantly impact hysteresis. Substrate effects are attributed to the thermal expansion mismatch between the film and substrate, resulting in the formation of dislocations at the interface and, consequentially, an increase in frictional resistance to martensite/austenite interface motion.

Structure and Phase Transformations in Thin Films

The field of thin films has gone through a great development in recent decades [...]

Substrate Effects in Hysteresis of Microscale Ni-Mn-Sn Heusler Alloy Films

During martensitic phase transformations in thin films, substrates impact hysteresis by introducing an additional interface, which can inhibit martensite/austenite interface motion. In order to reduce hysteresis, we examine 2.9-14.5 μm thick Ni-Mn-Sn films which in some cases have been delaminated from the substrates before or after annealing. We compare thermal hysteresis and defect densities at the interface. Delaminating films prior to annealing decreases hysteresis, whereas delaminating films after annealing does not significantly impact hysteresis. Substrate effects are attributed to the thermal expansion mismatch between the film and substrate, resulting in dislocations at the interface and consequentially, an increase in hysteresis.

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.

Phonon renormalization and Raman spectral evolution through amorphous to crystalline transitions in Sb2Te3 thin films

A symmetry specific phonon mode renormalization is observed across an amorphous to crystalline phase transformation in thin films of the topological material Sb2Te3 using Raman spectroscopy. We present evidence for local crystalline symmetry in the amorphous state, eventhough, the q = 0 Raman selection rule is broken due to strong structural disorder. At crystallization, the in-plane polarized (Eg2) mode abruptly sharpens while the out-of-plane polarized (A1g) modes are only weakly effected. This effect unique to the Eg symmetry is exceptional considering that polarized spectra and comparison of the single phonon density of states between the amorphous and crystalline phases suggest that short range order of the amorphous phase is, on the average, similar to that of the crystalline material while electrical transport measurements reveal a sharp insulator-to-metal transition. Our findings point to the important role of anisotropic disorder affecting potential applications of topological and phase-change based electronics.

Ion irradiation-induced phase transformation mechanisms in Y2O3 thin films

This work studies the ion irradiation-induced phase transformations in thin films of yttrium oxide. The films were deposited using an ion beam sputtering technique that allows the control of the order/disorder of the microstructure. As-deposited (disordered) and annealed (ordered) samples were irradiated by xenon ions at different energies and doses. Two different structural phase transformations were observed: cubic/amorphous-like and cubic/monoclinic. In both cases, it is shown that the pristine structure is of prime importance in the transformation process. The phase transformations are analysed in terms of structural extended defect nucleation originating from the oxygen network behaviour, which leads to polygonisation up to a nanocrystallite-like structure, followed by a phase transformation into a monoclinic structure.

Achieving coherent phase transition in palladium–hydrogen thin films

The thermodynamics of structural phase transformations in thin films depends on the mechanical stress that can be released by plastic deformation. For thin films below a critical film thickness, plastic deformation is energetically unfavourable: thus, the system stays coherent and stress remains. For PdHc films less than 22 nm thick, a new situation emerges: while the interfaces between matrix and hydride precipitates remain coherent throughout the complete phase transition, misfit dislocations form between the hydride phase and the substrate.

The FePt L1 phase transformation in thin films using multiple laser pulsing

A series of ≈12 nm thick FePt thin films deposited onto glass substrates have been annealed with multiple 1064 nm wavelength laser pulses. The fluence was varied using pulse widths of 10.0, 5.0, and 2.5 ms. The peak temperature for each individual pulse was kept near 700 °C. The A1 to L10 phase transformation was confirmed by x-ray diffraction. A single pulse was not sufficient to obtain a fully ordered state. A maximum order parameter of 0.89 and coercivity of 10.6 kOe was obtained after 5×10 ms pulses. This particular annealed film showed the greatest amount of grain growth with a mean size of 55.1 nm. This grain size is 20% smaller than that of a furnace annealed sample which was annealed for 60 s and yielded an approximately equivalent order parameter. Similar order parameters, grain sizes, and coercivity values were observed for films that had equivalent total annealing times regardless of pulse widths.

Investigation of Phase Transformation in Thin Film Using Finite Element Method

Cahn-Hilliard type of phase field model coupled with elasticity is used to derive governing equations for the stress-mediated diffusion and phase transformation in thin films. To solve the resulting equations, a finite element (FE) model is presented. The partial differential equations governing diffusion and mechanical equilibrium are of different orders; Mixed-order finite elements, with C0 interpolation functions for displacement, and C1 interpolation functions for concentration are implemented. To validate this new numerical solver for such coupled problems, we test our implementation on thin film diffusion couples.

Laser and electrical current induced phase transformation of In2Se3 semiconductor thin film on Si(111)

Phase transformation of thin film (∼30 nm)In2Se3/Si(111) (amorphous→crystalline) was performed by resistive annealing and the reverse transformation (crystalline→amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In2Se3 is of interest for non-volatile phase-change memory. Amorphous InxSey was deposited at room temperature on Si(111) after pre-deposition of a crystalline In2Se3 buffer layer (0.64 nm). Upon resistive annealing to 380°C, the film was transformed into a γ-In2Se3 single crystal with its {0001} planes parallel to the Si(111) substrate and \((11\bar{2}0)\) parallel to Si \((1\bar{1}0)\) , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and X-ray diffraction. Laser annealing with 20-ns pulses (0.1 millijoules/pulse, fluence≤50 mJ/cm2) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous InxSey film.

Micro-hotplates for high-throughput thin film processing and in situ phase transformation characterization

This paper presents the use of micro-hotplates (MHPs) as thermal processing and in situ characterization platforms for phase transformations in thin films. MHPs are fabricated by microsystem technology processes and consist of a SiO 2/Si 3N 4 membrane (app. 1 μm) supported by a bulk Si frame. Several embedded Pt thin films serve as heater and electrical measurement electrodes. It is shown that the MHPs have unique properties for the controlled annealing of thin film materials (up to 1270 K), as the annealing temperature and heating/cooling rates can be precisely controlled by in situ measurements. These rates can be extremely high (up to 10 4 K/s), due to the low thermal mass of MHPs. The high cooling rates are especially useful for the fabrication of metastable phases (e.g. Fe 70Pd 30) by quenching. By measuring the resistivity of a thin film under test in situ as a function of the MHP temperature, microstructural changes (e.g. phase transformations) can be detected during heating and cooling cycles. In this paper, examples are presented for the determination of phase transitions in thin films using MHPs: the solid–liquid–gas phase transition (Al), the ferromagnetic–paramagnetic phase transition (Fe–Pt) and martensitic transformations (Ni–Ti–Cu, Fe–Pd). Furthermore, it is demonstrated that crystallization processes from amorphous to crystalline (Ni–Ti–Cu) can be detected with this method. Finally the application of MHPs in thin film combinatorial materials science and high-throughput experimentation is described.

High-throughput analysis of thin-film stresses using arrays of micromachined cantilever beams

We report on a technique for making high-throughput residual stress measurements on thin films by means of micromachined cantilever beams and an array of parallel laser beams. In this technique, the film of interest is deposited onto a silicon substrate with micromachined cantilever beams. The residual stress in the film causes the beams to bend. The curvature of the beams, which is proportional to the residual stress in the film, is measured by scanning an array of parallel laser beams generated with a diffraction grating along the length of the beams. The reflections of the laser beams are captured using a digital camera. A heating stage enables measurement of the residual stress as a function of temperature. As the curvature of each beam is determined by the local stress in the film, the film stress can be mapped across the substrate. This feature makes the technique a useful tool for the combinatorial analysis of phase transformations in thin films, especially when combined with the use of films with lateral composition gradients. As an illustration, we apply the technique to evaluate the thermomechanical behavior of Fe-Pd binary alloys as a function of composition.

Evaluation of probe lasers employed in optical diagnostics for phase transformation of thin films during excimer laser crystallization

The stability and reliability of probe laser is an important factor affecting the inspection of the phase transformation process of Si thin films during excimer laser crystallization using in-situ time-resolved optical measurements. The changes in 2D intensity profile, peak power density, and beam wander of the commonly used helium–neon (He–Ne) and diode laser are investigated experimentally. It is found that the peak power density of He–Ne laser is higher than that of diode laser, while the total power of He–Ne laser is lower than that of diode laser. Although the instability in the peak power density of He–Ne laser will increase with increasing the operation time, the beam stability of He–Ne laser is better than that of diode laser. For long-time operation (>24 h) of optical measurements, the diode laser is a good candidate of probe laser. Conversely, the diode laser is suitable for the short-time operation (<24 h) of optical measurements because the beam-wander is higher than that of He–Ne laser.

Electron microscopy of γ‐irradiated polyethylene

AbstractIn this paper, we have studied the structure and the phase transformations in thin films (obtained by evaporation from γ‐irradiated polyethylene bulks—ageing polymer) before and after laser irradiation. We made a comparison between the following samples: A—thin films obtained from γ‐irradiated polyethylene bulk and not subjected to laser irradiation; B—thin films obtained from γ‐irradiated polyethylene bulk and subjected to laser irradiation. We have revealed many modifications in the dielectric and structural properties of the thin films, depending on the dose, dose rate and the time of laser irradiation (in the case of samples B). The thickness of all deposited films was 90 nm. The samples (non‐irradiated or irradiated) were analysed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), in combination with selected area electron diffraction (SAED). We have also presented the structure of non‐aged and non‐irradiated thin polyethylene films (control polyethylene film), in which we observed a particularly puzzling phase between an incipient spherulitic phase and a fine structure of the spherulitic phase (specifically for thin PE films structure). After γ‐ and laser irradiations, we observed a strong modification of the structure. The spherulitic phase was transformed. We observed large, amorphous, transparent crystals containing many defects, carbon precipitates, the beginning of crystallisation revealed by electron diffraction, treeing and breakdown figures (usually after laser irradiation). Copyright © 2006 John Wiley &amp; Sons, Ltd.

Isothermal phase transformation kinetics in stoichiometric GeSbTe thin films

The purpose of this work is to investigate the isothermal phase transformation in thin films of Ge2Sb2Te5 and Ge1Sb2Te4 alloys. From experimental and computed simulations it was shown that if the kinetic behavior is interpreted employing the Johnson–Avrami–Mehl–Kolmogorov theory, three slopes can be observed: the first related to a metastable transformation, the second to the step between metastable and stable transformation, and the last to the stable phase nucleation and growth. From X-ray patterns it was found that a metastable phase, which appears during the beginning of crystallization for Ge2Sb2Te5 and Ge1Sb2Te4 thin films, corresponds to the Ge1Sb4Te7 fcc composition.

Au Ion Induced Modification of C60 Thin Film Samples

The present work reports phase transformations of thin films of C60 irradiated with 100 MeV 197Au8+ ions. This work is in continuation with our earlier work using 58Ni10+ and 16O6+ ions, to study the modification in C60 thin films. The study of C60 thin films using 197Au8+ ions, provides us enough additional data to investigate thoroughly the role of Se in causing phase transformations under different ion fluences. The Raman spectra indicate that swift heavy ion (SHI) irradiation results in several transformations of crystalline C60. At low fluences along with the fragmentation of C60 there is dimer/polymer formation. As fluence increases the dimer/polymer content first rises, optimizes, decreases and finally vanishes at very high fluences. At high fluences, all the C60 molecules as well as the polymer C60 break up, possibly resulting in nano-crystalline graphite embedded in amorphous carbon (a-C).