The thickness dependence of the crystallization behavior in sandwiched amorphous Ge2Sb2Te5 thin films

The thickness dependent crystallization behavior of thin amorphous Ge₂Sb₂Te₅(GST) films sandwiched between different cladding materials has been investigated based on a thermodynamic model. It is revealed that there is a critical thickness below which the crystallization cannot occur. The critical thickness is determined by the energy difference Δγ between the crystalline GST/substrate interface energy and the amorphous GST/substrate interface energy, the melting enthalpy, and the mole volume. The calculated result is in good agreement with the experiments. Furthermore, the crystallization temperature is also affected by interface energy difference Δγ. Larger Δγ gives rise to a higher crystallization temperature, and vice versa. This impact becomes stronger as the film thickness is decreased.     

Ge/Sb2Te3 nanocomposite multilayer films for high data retention phase-change random access memory application

The amorphous-to-crystalline transition of Ge/Sb₂Te₃ nanocomposite multilayer films with various thickness ratios of Ge to Sb₂Te₃ were investigated by utilizing in situ temperature-dependent film resistance measurements. The crystallization temperature and activation energy for the crystallization of the multilayer films increased with the increase in thickness ratio of Ge to Sb₂Te₃. The difference in sheet resistance between amorphous and crystalline states could reach as high as 10⁴ Ω/□. The crystallization temperature and activation energy for the crystallization of Ge/Sb₂Te₃ nanocomposite multilayer films was proved to be larger than that of conventional Ge₂Sb₂Te₅ film, which ensures a better data retention for phase-change random access memory (PCRAM) use. A data retention temperature for 10 years of the amorphous state [Ge (2 nm)/Sb₂Te₃ (3 nm)]₄₀ film was estimated to be 165 °C. Transmission electron microscopy (TEM) images revealed that Ge/Sb₂Te₃ nanocomposite multilayer films had layered structures with clear interfaces.     

Electrical resistivity of metglas 2826A (Fe32Ni36Cr14P12B6) during crystallization

The electrical resistivity of Metglas 2826A has been measured, during heating at constant rate from room temperature to about 850°C. The ?(T) curves exhibit three “steps”, which are shown to correspond to the crystallization stages of Metglas 2826A. From the resistivity increase at the second “step” we conclude, that crystalline (Ni, Fe, Cr)₃(P, B) is a poorer electric conductor than the amorphous material with the same composition. At temperatures above the third crystallization stage the ?(T) curves suggest atomic rearrangements in the material that are not detected by differential thermal analysis.     

Electrical resistivity of amorphous metallic glasses Fe80B80 and Fe78Mo2B20 between 78 and 1000 K

Electrical resistivity (ρ) of amorphous Fe₈₀B₂₀ and Fe₇₈Mo₂B₂₀ have been studied as a function of temperature (T) between 78 and 1000 K. The ρ vs T curves, obtained with specified warming and cooling rates, show that such curves are sensitive probes of the crystallization process. Within the experimental error, no anomalies in the ρ behavior can be seen at the Curie temperature of each amorphous alloy.     

SEM/TEM characterization of periodical novel amorphous/nano-crystalline micro-composites obtained by laser interference structuring: The system HAlO-Al·Al2O3

Layers of the metastable, amorphous HAlO are synthesized by chemical vapor deposition from the molecular compound tert-butoxyalane ([^tBu-O-AlH₂]₂). At temperatures above 500 °C, these layers transform to biphasic Al·Al₂O₃ due to the elimination of di-hydrogen. The interaction of HAlO films with short laser pulses causes partial transformation of amorphous HAlO into nano-crystalline Al·Al₂O₃. Using an interference pattern of two coherent high-power Nd:YAG laser beams produces local and periodic heating, inducing crystallization at equally distant lines in the HAlO layer. Depending on the laser fluence, different morphologies and different amounts of crystalline phases are obtained. In this study, the surface morphology and the distribution of crystalline phases of the structured samples are analyzed using SEM, FIB and TEM. The two-dimensional structures consist of periodic variations of morphology, chemical composition, and phase identity with a well-defined long-range order. When bio-functionalized, the structured samples may be used as carriers for structurally controlled cell-cultivation.     

Electron microscopic investigation of the kinetics of the layer and island crystallization of amorphous V<Subscript>2</Subscript>O<Subscript>3</Subscript> films deposited by pulsed laser evaporation

An electron microscopic investigation was performed on the kinetics of the layer and island crystallization of amorphous V₂O₃ films deposited by pulsed laser evaporation of vanadium in an oxygen atmosphere. The crystallization was initiated by the action of an electron beam on an amorphous film in the column of a transmission electron microscope. The kinetic curves were plotted on the basis of a frame-by-frame analysis of the video recorded during the crystallization of the film. It was found that the layer crystallization of amorphous films is characterized by a quadratic dependence of the fraction of the crystalline phase x on the time t, whereas the island crystallization is described by an exponential dependence of x on t. The kinetic curves of island crystallization of amorphous films were analyzed on the basis of the α-version of the Kolmogorov model. For each type of crystallization, there are specific values of the dimensionless relative length unit δ₀, which is equal to the ratio of the characteristic length unit to the parameter characterizing the unit cell of the crystal. It was established that, for the layer crystallization, the relative length unit lies in the range δ₀ ~ 4300–4700, whereas for the fine-grained island crystallization, it amounts to δ₀ ~ 110.     

TEP studies on Fe80B20 under pressure

The amorphous to crystalline transformation in the ferromagnetic metallic glass Fe₈₀B₂₀ has been studied up to 30 kbar pressure and 1000K. A previous study at ambient pressure revealed no change in thermoelectric power (TEP) at the crystallization temperature (T_x) while the resistivity showed a sudden decrease at the same temperature. The present experimental results show a distinct anomaly in TEP at T_x even at ambient pressure. This anomaly gets enhanced under pressure.     

Electron transport properties of Co71−xFexCr7Si8B14 (x=0, 2, 3.2, 4, 6,8 and 12 at%) amorphous alloys during devitrification

The investigation addresses the electron transport properties of Co_71−xFe_xCr₇Si₈B₁₄ (x=0, 2, 3.2, 4, 6, 8 and 12 at%) amorphous alloys. The variation in electrical resistivity of as-cast amorphous materials with thermal scanning from room temperature to 1000 K was measured. The CoFe-based alloys revealed an initial decrease in temperature coefficient of resistivity (TCR), a characteristic of spin-wave phenomena in glassy metallic systems. This behaviour in the present alloys was in a sharp contrast to the Co-based amorphous materials that indicate the drop in resistivity much below room temperature. In the studied alloys, the variation in initial TCR values and the full-width at half-maxima determined from X-ray diffraction of as-quenched materials exhibited a similar trend with increasing Fe content, indicating the compositional effect of near neighbouring atoms. After the initial decrease in resistivity, all the alloys indicated a subsequent increase at T_min. The Curie temperature (T_C), which was measured from thermal variation of ac susceptibility showed non-monotonic change with Fe content. In the temperature range between T_min and T_C the relative scattering by electron-magnon and electron-phonon resulted in the non-monotonic change in Curie temperature. At crystallization onset (T_X1) all the alloys except there with X=6, showed a sharp decrease in electrical resistivity which was attributed to ordering phenomena. In contrast to this resistivity decrease, X=6 alloy exhibited a drastic increase in resistivity around T_X1 observed during amorphous to nanocrystalline transformation. Such nanocrystalline state was observed by Transmission electron microscopy.     

Modeling of CW laser diode irradiation of amorphous silicon films

The purpose of this work is to determine the optimal parameters required to crystallize thin amorphous silicon films on glass substrate with a continuous wave (CW) laser diode (λ = 808 nm), using a numerical model developed in COMSOL Multiphysics. The numerical simulation of the laser crystallization process takes into account the solid-liquid phase change and the difference between the melting temperature of amorphous (Tm_a-Si = 1420 K) and that of crystalline silicon (Tm_c-Si = 1690 K). We have varied the main parameters controlling the crystallization process, namely the power and the scan speed of the laser beam. Furthermore the initial temperature as well as the thickness of the a-Si:H layer were also taken as a parameter to optimize the process. We have determined the melting, crystallization and ablation energy threshold versus the different operational parameters.     

Ultra fast melting process in femtosecond laser crystallization of thin a-Si layer

In this paper, we investigated the mechanism of crystallization induced by femtosecond laser irradiation for an amorphous Si (a-Si) thin layer on a crystalline Si (c-Si) substrate. The fundamental, SHG, THG wavelength of a Ti:Sapphire laser was used for the crystallization process. To investigate the processed areas we performed Laser Scanning Microscopy (LSM), Transmission Electron Microscopy (TEM) and Imaging Pump-Probe measurements. Except for 267 nm femtosecond laser irradiation, the crystallization occurred well. The threshold fluences for the crystallization using 800 nm and 400 nm femtosecond laser irradiations were 100 mJ/cm² and 30 mJ/cm², respectively. TEM observation revealed that the crystallization occurred by epitaxial growth from the boundary surface between the a-Si layer and c-Si substrate. The melting depths estimated by Imaging Pump-Probe measurements became shallower when the shorter wavelength was used.     

Characteristics of electron beam evaporated nanocrystalline SnO2 thin films annealed in air

Tin oxide (SnO₂) thin films (about 200 nm thick) have been deposited by electron beam evaporation followed by annealing in air at 350-550 °C for two hours. Optical, electrical and structural properties were studied as a function of annealing temperature. The as-deposited film is amorphous, while all other annealed films are crystalline (having tetragonal structure). XRD suggest that the films are composed of nanoparticles of 5-10 nm. Raman analysis and optical measurements suggest quantum confinement effects that are enhanced with annealing temperature. For instance, Raman peaks of the as-deposited films are blue-shifted as compared to those for bulk SnO₂. Blue shift becomes more pronounced with annealing temperature. Optical band gap energy of amorphous SnO₂ film is 3.61 eV, which increases to about 4.22 eV after crystallization. Two orders of magnitude decrease in resistivity is observed after annealing at 350-400 °C due to structural ordering and crystallization. The resistivity, however, increases slightly with annealing temperature above 400 °C, possibly due to improvement in stoichiometry and associated decrease in charge carrier density.     

非晶态Cu+导体分相和晶化行为的XRD和SEM研究

对非晶态Cu⁺快离子导体0.4CuI-0.3Cu₂O-0.3P₂O₅在等温热处理条件下测量离子电导率的同时,进行了X射线衍射(XRD)与扫描电子显微术(SEM)研究。结果表明:初始的非晶态材料是分相的;随着等温热处理,分离的非晶第二相逐渐消失,并发生非晶态晶化;晶态的γ-CuI与Cu₂P₂O₇先后析出,逐渐长大。此材料的分相和晶化行为同电导率反常性的对应,再一次证实了非晶态快离子导体中的相界效应及其普遍意义。 关键词：     

Specific heat and electrical resistivity of an icosahedral-structure Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy and of its amorphous and crystalline analogs

Binary icosahedral and crystalline phases of the Zr₇₀Pd₃₀ alloy were obtained in crystallization from the amorphous state during heat treatment. The specific heat and electrical resistivity of the icosahedral, amorphous, and crystalline phases were measured and compared. An increase in the electronic density of states on the Fermi surface, lattice softening, and an increase in the electron-phonon coupling constant were observed to occur with decreasing structural order. Despite the high valence electron density in the icosahedral phase, where the electronic densities of states are twice those in the crystal, the electrical resistivity of the icosahedral phase is ～50 times as high. Superconductivity was observed for the first time in the icosahedral phase of a binary system of transition metal atoms, Zr₇₀Pd₃₀.     

Resistivity models of the phase transformation of amorphous Al78W22 thin films under isothermal and isochronal conditions

The phase transformation and crystallization kinetics of amorphous Al₇₈W₂₂ thin films under isothermal and isochronal conditions was examined by continuous in situ electrical resistance measurements. Sample preparation was performed by magnetron co-deposition. The resistivity of the samples was monitored during isothermal annealing at 803, 813, and 823 K, i.e. at temperatures at which the crystalline phase starts to develop. The temperature dependence of the resistivity (T) was estimated from the isochronal measurements, while the volume fraction of the crystalline phase (x,t) was established by using the isothermal measurements and general two-phase resistivity models for this system. Combinations of these two approaches were used to model the variation of the resistivity during the cooling cycles. The activation energy for crystallization and the reaction order were determined within the framework of the JMA theory. The results indicate that the crystallization mechanism is a diffusion-controlled process with quenched-in nuclei during non-isothermal heating, whereas it is a diffusion-controlled process with steady-state nucleation during isothermal heating. PACS 61.43.-j; 81.15.C     

Electrical transport and crystallization studies of glassy semiconducting Si20Te80 alloy at high pressure

The effect of pressure on the electrical resistivity of bulk Si₂₀Te₈₀ glass is reported. Results of calorimetric, X-ray and transmission electron microscopy investigations at different stages of crystallization of bulk Si₂₀Te₈₀ glass are also presented. A pressure induced glass-to-crystal transition occurs at a pressure of 7 GPa. Pressure and temperature dependence of the electrical resistivity of Si₂₀Te₈₀ glass show the observed transition is a pressure induced glassy semiconductor to crystalline metal transition. The glass also exhibits a double T_g effect and double stage crystallization, under heating. The differences between the temperature induced crystallization (primary crystallization) and pressure induced congruent crystallization are discussed.     

Conductive Atomic Force Microscopy (C-AFM) observation of conducting nanofilaments formation in GeSbTe phase change materials

GST (GeSbTe) thin films were deposited on glass substrates by electron beam evaporation; Ni was used as the top and bottom electrodes. The I–V (current–voltage) characteristic of the phase change memory (PCM) cell was measured; results showed an electrical threshold switching characteristic for the sample with a threshold voltage of 3.08 V. The threshold switching is attributed to the formation of conductive filaments in the amorphous matrix. Current-voltage spectra which were obtained by C-AFM show that the GST thin film switching from amorphous to the crystalline phase occurs at 1.51 V. C-AFM was used to fabricate crystalline nanoarrays on the sample surface and examine the electrical properties of arrays. In the I–V measurements by C-AFM, when the applied voltage is higher than threshold voltage, conducting nanofilaments with average sizes of 15–60 nm were formed and crystallized spots with current signals were observed. Different times of I–V spectroscopies were applied on thin films to investigate the electrical properties of films during the phase change process. C-AFM results show that as the times of I–V spectroscopies increased, the morphology of crystallized spots changed from bump to pit; the sizes of conductive nanofilaments and detected current signals increased. These results can be attributed to the energy induced by Joule heating dissipated to surrounding films increases with the increasing times of I–V spectroscopies.     

Characteristics of Si-doped Sb2Te3 thin films for phase-change random access memory

The characteristics of Si-doped Sb₂Te₃ thin films were investigated using differential scanning calorimetry (DSC), four-point probe technique, X-ray diffraction (XRD) analysis and high resolution transmission electron microscopy (HRTEM). It is found that the as-deposited Sb₂Te₃ film in our study is partly crystallized. Silicon doping increases the crystallization temperature and resistivity of Sb₂Te₃ film significantly. XRD and HRTEM analyses indicated that some of the doped Si atoms substitute for Sb or Te in the lattice, while others exists at the grain boundaries in the form of amorphous phase, which may be responsible for grain size reduction and high crystalline resistivity of Si-doped specimens. Compared with the conventional Ge₂Sb₂Te₅ film, Si-doped Sb₂Te₃ films exhibit lower melting temperature and higher crystalline resistivity, which is beneficial to RESET current reduction of phase-change random access memory (PRAM). These results show the feasibility of Si-doped Sb₂Te₃ films in PRAM application.     

Nanocrystallisation of an Fe44.5Co44.5Zr7B4 amorphous magnetic alloy

The crystallisation of amorphous Fe_44.5Co_44.5Zr₇B₄ was investigated using DSC, electrical resistivity, TEM, HRTEM, CBED and VSM. Melt-spun amorphous Fe_44.5Co_44.5Zr₇B₄ crystallised by the primary crystallisation mode: the DSC results showed two exothermal peaks during heating. The electrical resistivity dropped sharply during the crystallisation event, which was consistent with DSC characterisation. From TEM, HRTEM and CBED results, primary crystallisation products which appeared to be clusters of crystals were found to be single crystal precipitates; these crystals formed in a compact dendritic morphology. Direct measurement of nucleation density and volume fraction was carried out using TEM analysis. The nucleation density was found to be high even in the absence of copper addition. The crystal growth was slow when the average size reached around 30?nm; this resulted in a stable nanocrystalline structure. The soft magnetic properties were improved after nanocrystallisation, the magnetic properties were related to the crystalline volume fraction and the Herzer model.     

Structural transformations in MoO<Subscript>x</Subscript> thin films grown by pulsed laser deposition

In this work, laser-induced crystallization in MoO_x thin films (1.8x2.1) is reported. This transformation involves a MoO_x oxidation and subsequently a crystallization process from amorphous MoO₃ to crystalline MoO₃. For comparison purposes crystallization is induced thermally, in an oven, as well. The crystallization kinetics is monitored by Raman spectroscopy; a threshold in the energy density necessary to induce the phase transformation is determined in the case of photo-crystallization. This threshold depends on the type of substrate on which the film is deposited. For the thin films deposited on glass substrates, the structural transformation is from amorphous MoO_x to the thermodynamically stable MoO₃ crystalline phase. For the thin films deposited on Si(100) the structural transformation is from amorphous MoO_x to a mixture of MoO₃ and the thermodynamically unstable MoO₃ crystalline phases. The structural transformations are also characterized by scanning electron microscopy and light-transmission experiments. PACS 81.15.Fg; 61.80.Ba; 78.30.-j     

Optical and electrical properties of thermally evaporated In49Se48Sn3 films

Nearly stoichiometric thin films of In₄₉Se₄₈Sn₃ were deposited at room temperature, by conventional thermal evaporation of the presynthesized materials, onto precleaned glass substrates. The microstructural studies on the as-deposited and annealed films, using transmission electron microscopy and diffraction (TEMD), revealed that the as-deposited films are amorphous in nature, while those annealed at 498 K are crystalline. The optical properties of the investigated films were determined from the transmittance and reflectance data, in the spectral range 650-2500 nm. An analysis of the optical absorption spectra revealed a non-direct energy gap characterizing the amorphous films, while both allowed and forbidden direct energy gaps characterized the crystalline films. The electrical resistance of the deposited films was carried out during heating and cooling cycles in the temperature range 300-600 K. The results show an irreproducible behavior, while after crystallization the results become reproducible. The analysis of the temperature dependence of the resistance (ln(R) vs. 1000/T) for crystalline films shows two straight lines corresponding to both extrinsic and intrinsic conduction. The room temperature I-V characteristics of the as-deposited films sandwiched between similar Ag metal electrodes shows an ohmic behavior, while non-ohmic behavior attributed to space charge limited conduction has been observed when the films are sandwiched between dissimilar Ag/Al metal electrodes.