Phase Transformations In Thin Films Research Articles

Calorimetric measurements of phase transformations in thin films of amorphous Te alloys used for optical data storage

Sputtered amorphous Ag0.055 In0.065 Sb0.59 Te0.29, Ge4 Sb1 Te5, and Ge2 Sb2 Te5 thin films were studied by differential scanning calorimetry. The crystallization temperature and the heat of crystallization of the amorphous phases, the melting temperature and the heat of fusion of the crystalline phases, and the heat capacities of crystalline and liquid AgInSbTe were measured. The entropies of fusion are large (⩾2R), which suggests a change of bonding type between liquid and crystal. In contrast to amorphous AgInSbTe and Ge4 Sb1 Te5, which upon heating crystallize to a single phase within a small temperature interval, the crystallization of amorphous Ge2 Sb2 Te5 is complicated by a subsequent cubic-to-hexagonal transformation. No thermal evidence of a glass transition was found below the crystallization temperature. The ratio of the glass transition temperature (approximated as the crystallization temperature) to the liquidus temperature is 0.49–0.56, which identifies the materials as marginal glass formers. The heat capacity measurements on AgInSbTe were used to estimate the temperature dependence of the difference in enthalpy, entropy, and Gibbs free energy between the undercooled liquid and the crystal.

Computer Simulation of Martensitic Transformation in Constrained Films

dimensional phase- field model is d eveloped to describe the cubic to tetragonal martensitic phase transformation in thin film attached to a substrate. Elastic solutions are derived for both elastically anisotropic and isotropic thin films with arbitrary domain structures, subject to the mixed stress -free and constraint boundary conditions. Nucleation process as well as final domain structure strongly depends on the substrate constraint. Lower undercooling results in less volume fraction of martensite, finer domain structure and longer nucleation period.

Thickness dependent phase transformations in implanted iron layers

This article shows that phase transformations in implanted iron may strongly depend on the sample thickness. The effect essentially relies on the lower impurity concentration required for the formation of a given phase in thin films as compared with thick layers or bulk samples. The phase transformation in thin films is also more complete. In this work, the total transformation of pure iron into iron nitrides was observed upon nitrogen implantation of thin iron layers, whereas about 20% of Fe atoms remained in the crystalline α-Fe phase in nitrogen-implanted bulk samples.

Phase transformations in nitrogen-implanted iron layers

Phase transformations in implanted iron may strongly depend on the sample thickness. The effect essentially relies on lower impurity concentration required for given phase formation in thin films when compared with thick layers or bulk samples. The phase transformation in thin films is also more complete, a full transformation of thin Fe layers into iron nitrides was observed, whereas in thick or bulk samples about 20% of Fe atoms remain in crystalline α-Fe phase.

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.

Kinetics of the initial stage in a first-order phase transformation in thin films

The nucleation and growth of islands of a new phase on the surface of solids has been studied both experimentally and theoretically for the particular case of the transition from the pyrochlore to perovskite phase in a thin film of a lead zirconate-titanate ferroelectric. This transformation was chosen because the new-phase islands have a stable circular shape in this case, a relatively large size (10−5–10−4 m) permitting their observation with an optical microscope, and a low growth rate (10−8–10−9 m/s). A theoretical analysis of the process, based on the kinetic theory of first-order phase transitions proposed earlier, has been carried out and the behavior in time of all main characteristics of a phase transformation, namely, nucleation rate, concentration of the new-phase islands, their size distribution, and relative overheating, has been calculated. The same characteristics have been measured experimentally, thus permitting one for the first time to make a thorough comparison of the theoretical with experimental data on the kinetics of first-order phase transitions. They have been found to be in a good agreement.

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.

The use of transmission and analytical electron microscopy in studying reactions and phase transformations in thin film

Generally, processing of thin films involves several annealing steps in addition to the deposition step. During the annealing steps, diffusion, transformations and reactions take place. In this paper, examples of the use of TEM and AEM for ex situ and in situ studies of reactions and phase transformations in thin films will be presented.The ex situ studies were carried out on Nb/Al multilayer thin films annealed to different stages of reaction. Figure 1 shows a multilayer with dNb = 383 and dAl = 117 nm annealed at 750°C for 4 hours. As can be seen in the micrograph, there are four phases, Nb/Nb3-xAl/Nb2-xAl/NbAl3, present in the film at this stage of the reaction. The composition of each of the four regions marked 1-4 was obtained by EDX analysis. The absolute concentration in each region could not be determined due to the lack of thickness and geometry parameters that were required to make the necessary absorption and fluorescence corrections.

Experimental determination of transformation enthalpies of metastable phase transitions in ion-irradiated NiAl 3

A novel experimental technique has been developed by which metastable phase transformations in thin films may be investigated. The technique is demonstrated by the determination of the enthalpies of transformation of two commonly observed metastable phases in the compound NiAl 3; the vapor deposited, fcc extended solid solution phase, and the ion-induced amorphous phase. The measured transformation enthalpies are −4.16±0.35 and −5.02 ±0.49 kcal/mole, respectively. The latter compares with a calculated value of −5.83kcal/mole, determined using the embedded atom model. This technique demonstrates the feasibility of determining phase transformation enthalpies in ion-irradiated and vapor-deposited metastable phase thin films.

7275. Development of multispecimen holder for ion irradiation experiments: N Shigenaka et al,Nucl Instrum Meth Phys Res, B51, 1990, 301–304

A novel experimental technique has been developed by which metastable phase transformations in thin films may be investigated. The technique is demonstrated by the determination of the enthalpies of transformation of two commonly observed metastable phases in the compound NiAl3; the vapor deposited, fcc extended solid solution phase, and the ion-induced amorphous phase. The measured transformation enthalpies are −4.16±0.35 and −5.02 ±0.49 kcal/mole, respectively. The latter compares with a calculated value of −5.83kcal/mole, determined using the embedded atom model. This technique demonstrates the feasibility of determining phase transformation enthalpies in ion-irradiated and vapor-deposited metastable phase thin films.

Phase transformation kinetics—the role of laser power and pulse width in the phase change cycling of Te alloys

The effect of laser pulse width and amplitude on the kinetics of laser-induced phase transformations of thin films of Te alloys has been mapped. This map, called a phase transformation kinetics diagram, shows distinct regions of crystallization and amorphization and allows important material parameters to be determined. The observed regions were correlated with results from temperature modeling. The minimum crystallization time was measured to be 50 ns for pure Te and increases to 550 ns for Te80Sn20 and 80 μs for Te90Ge10. A boundary, determined by the critical quench rate, separates the region of melt followed by amorphous quench, from melt followed by crystallization. Three methods of reversible cycling are demonstrated.

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.

Phase transformations in thin films

Phase transformations in thin films

The monoclinic-tetragonal transformation and associated twinning in thin films of zirconia

A transmission electron microscopy study has been made of the monoclinic to tetragonal phase transformation in thin films of zirconia. The films were prepared by the oxidation and subsequent recrystallization of zirconium foils. The transformation, which is rapid and reversible, is generally accompanied by twinning on (100) and {110} planes. The orientation relations between the two phases and the shape change accompanying transformation have been determined. The twinning shears have also been measured. Stacking faults on several low index planes have been observed in monoclinic zirconia and dislocations are present in both phases. Cracks occur in the films on transformation from the tetragonal to the monoclinic phase.