As-deposited Amorphous Phase Research Articles

Evolution of Short-Range Order of Amorphous GeTe Upon Structural Relaxation Obtained by TEM Diffractometry and RMC Methods.

Glasses frequently reveal structural relaxation that leads to changes in their physical properties including enthalpy, specific volume, and resistivity. Analyzing the short-range order (SRO) obtained from electron diffraction by transmission electron microscopy (TEM) in combination with Reverse-Monte-Carlo (RMC) simulations is shown to provide information on the atomic arrangement. The technique elaborated here features several benefits including reliability, accessibility, and allows for obtaining detailed structural data quickly. This is demonstrated with a detailed view of the structural changes in the as-deposited amorphous phase change material (PCM) GeTe. The data show a significant increase in the average bond angle upon thermal treatment. At the same time the fraction of tetrahedrally coordinated Ge atoms decreases due to an increase in octahedrally distorted and pyramidal motifs. This finding provides further evidence for the atomic processes that govern structural relaxation in amorphous GeTe and other PCMs. A thorough literature review finally unveils possible origins of the large discrepancies reported on the structure of amorphousGeTe.

Optical properties of as-deposited, annealed and laser-treated Ge2Sb2Te5 thin films

Knowledge of the correct optical properties of phase change materials (PCM) is important in the development of many devices. Here, we report on the differences in optical properties of as-deposited, annealed, laser-reamorphized and laser-recrystallized G e 2 S b 2 T e 5 thin films. While as-deposited and laser-reamorphized G e 2 S b 2 T e 5 show a similar spectral dependences of refractive indices and extinction coefficients with a small decrease in optical band gap from 0.63 to 0.59 eV, spectral dependences of annealed and laser-recrystallized G e 2 S b 2 T e 5 differ significantly from each other in the entire measured spectral region with values of optical band gap 0.51 and 0.45 eV, respectively. We discuss the main differences in the optical functions between both crystalline phases using X-ray diffraction studies. We further present that the laser-reamorphized phase transforms to the crystalline phase at about 12 °C lower temperature in contrast to the as-deposited amorphous phase.

Comprehensive Study of Kinetics of Processes Competing during PECVD Ultrathin Silicon Layer High-Temperature Annealing

Application of low-temperature PECVD is a very tempting option for formation of ultrathin silicon layers for nanoelectronic and nanophotonic applications, as followed by annealing of this layer, regardless if executed as individual process performed in controlled ambient or during following high-temperature processes, allows for phase and content changes in the silicon layer. Understanding complex changes that can take place during such process, which depend on its temperature, conditions (e.g., oxygen availability), and timeframe, is a fundamental requirement for conscious application of such technology. It is worth realizing that nanodevices with their unprecedented variety of structures and devices require many different fabrication technologies. Hence, depending on the application in mind, different results of ultrathin silicon layer annealing may appear advantageous. During high-temperature processing (e.g., annealing) of PECVD ultrathin silicon layer, three competing effects have to be taken into account. These are amorphous silicon recrystallization and oxidation of amorphous and crystalline (as-deposited or just recrystallized from as-deposited amorphous phase) silicon (both of which by nature exhibit different kinetics). So far, most of attention has been paid to silicon recrystallization, which was justified by the fact that under experimental conditions studied (silicon multilayers) oxidation was certainly of less importance. In certain applications, the required device structure consists of single (and not multiple) ultrathin silicon layer, and thus, oxidation effects certainly have to be included into considerations. Understanding dynamics and very complex relations between these individual effects is thus mandatory for using consciously this technique and achieving needed properties of the layer. It has to be stated clearly that although the achieved results, presented in this study, refer to the silicon layers fabricated under certain conditions (particular type of PECVD reactor and process parameters), they can, however, be easily extrapolated for similar cases too. The presented below results are, to our knowledge, the first successful attempt to address these issues.

Femtosecond laser-induced ultrafast transient snapshots and crystallization dynamics in phase change material.

We report here femtosecond laser-driven transient snapshots of ultrafast crystallization of Ge2Sb2Te5 films from its as-deposited amorphous phase, and the local structural change is validated by micro-Raman spectroscopy and x-ray diffraction. The decay time constant of ∼5 ps in transient spectra with a precise temporal resolution using 400nm (pump) reveals about 68 volumetric percentage crystallization at a remarkably low fluence of 4.78 mJ·cm-2. This is attributed to reiterated excitation after a complete carrier relaxation and formation of a long-lasting transient phase at sub-threshold fluences. Furthermore, Raman spectra of irradiated spots confirm defective-octahedral modes at 110 and 160 cm-1 validating crystallization.

Detection of N-Te bonds in the as-deposited amorphous nitrogen-doped GeTe-based phase change alloys using N K-edge XANES spectroscopy and their impact on crystallization

Abstract Using N K-edge XANES studies, we demonstrate a noticeable difference in local structure around the nitrogen atoms in as-deposited amorphous and annealed N-doped GeTe-based phase change alloys. The pronounced changes appear as a ≈ 2 eV shift in the absorption edge to higher photon energies and the overall shape of the XANES spectrum. Comparison of the experimental XANES spectrum of the as-deposited amorphous phase with ab-initio XANES simulations discloses that the as-deposited phase mainly consists of the NGe3 and the NTe3 pyramidal units in approximately equal concentration. When annealed, NTe3 units gradually rebond to the NGe3 units and at the same time N atoms diffuse through the amorphous phase to form the GexNy aggregates. Upon long-standing annealing at 400 ∘ C a compact interlayer of Ge3N4 is formed in the crystalline phase.

Post annealing induced magnetic anisotropy in CoFeSi thin films on MgO(0 0 1)

Co61Fe26Si13 (CoFeSi) thin films were deposited on MgO(0 0 1) substrates by magnetron sputtering and were annealed at different temperatures (Ta). X-ray diffraction revealed that the crystal structure varies from the as-deposited amorphous phase to the cubic phase when Ta = 800 °C, indicating the re-crystallization process after annealing treatment. The static magnetic property measurements show a gradually four-fold cubic anisotropy dominating the magnetic anisotropy with the increasing Ta. The ferromagnetic resonance technique based on vector network analyzer was utilized to analyze the dynamic magnetic properties. CoFeSi film annealed at 600 °C presents an obvious in-plane uniaxial magnetic anisotropy, and it presents a low effective damping constant of around 0.011 with a natural resonance frequency of around 3.4 GHz. However, the CoFeSi film annealed at 800 °C showed a strong cubic magnetocrystalline anisotropy and full-in-plane-direction workable natural resonance frequency at around 2.5 GHz.

High-field electrical transport in amorphous phase-change materials

Electrical transport in chalcogenide-based phase change materials is an active area of research owing to the prominent role played by these materials in the field of information technology. Here, we present transport measurements (IV curves) obtained on line-cells of as-deposited amorphous phase change materials (Ge2Sb2Te5, GeTe, Ag4In3Sb66Te27) over a wide voltage and temperature range (300 K to 160 K). The well defined geometry of our devices enables a description of the transport behavior in terms of conductivity vs. electric field. At higher temperatures (300 K ≥ T ≥ 220 K) and low to intermediate fields (F < 20 V/μm), the data can be described within the framework of a previously developed model, which is based on multiple trapping transport together with 3D Poole-Frenkel emission from a two-center Coulomb potential. Based on this model, we observe a temperature dependence of the inter-trap distance, which we can relate to a temperature dependence in the occupation of the defect creating the Coulomb potential governing Poole-Frenkel emission. At higher fields and lower temperatures, the dependency of the IV curve on the electric field can be described by ln(I/I0) = (F/Fc)2. By combining this contribution with that of the Poole-Frenkel emission, we can show that the slope at high fields, Fc, is independent of temperature. We argue that models based on direct tunneling or thermally assisted tunneling from a single defect into the valence band cannot explain the observed behavior quantitatively.

High-pressure Raman spectroscopy of phase change materials

We used high-pressure Raman spectroscopy to study the evolution of vibrational frequencies of the phase change materials (PCMs) Ge2Sb2Te5, GeSb2Te4, and SnSb2Te4. We found that the critical pressure for triggering amorphization in the PCMs decreases with increasing vacancy concentration, demonstrating that the presence of vacancies, rather than differences in the atomic covalent radii, is crucial for pressure-induced amorphization in PCMs. Compared to the as-deposited amorphous phase, the pressure-induced amorphous phase has a similar vibrational spectrum but requires much lower laser power to transform into the crystalline phase, suggesting different kinetics of crystallization, which may have implications for applications of PCMs in non-volatile data storage.

The influence of layer thickness and post annealing on magnetism of pulsed laser deposited ZnO/Co multilayers

ZnO/Co multilayers with different layers have been deposited on silicate (100) substrate by the pulsed laser deposition method. The as-deposited multilayers present the structure of amorphous phase and a narrow hysteresis loop with high saturation magnetization and low coercivity, showing the typical soft magnetic behavior. The post annealing at the elevated temperature causes the transformation from as-deposited amorphous phase to crystalline metallic cobalt and ZnO phase, as well as the interdiffusion of different layer forming CoO and ZnCo2O4 phases. The presence of crystalline metallic cobalt phase with ferromagnetic (FM) and cobalt oxide (CoO) phase with antiferromagnetic (AFM) is beneficial to the FM–AFM coupling, resulting in the shift of widening of the hysteresis loop. But an excessively high annealing temperatures leads to the formation of large amount of simple or complex oxides with paramagnetic, antiferromagnetic properties, inducing the decrease of magnetic properties significantly.

Structural and Magnetic Properties of FeCuNbSiB Thin Films Deposited by HiPIMS

Using high-power impulse magnetron sputtering (HiPIMS) technique, Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">73.5</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15.5</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> thin films with thickness between 40 and 700 nm were deposited. The influence of deposition conditions (deposition time and argon pressure) and of the annealing temperature on the structure and coercive magnetic field of the magnetic films was analyzed. In the as-deposited state, the coercive magnetic field presents a minimum at about 10 mtorr argon pressure. The X-ray diffraction studies show that in the as-deposited state, the samples are amorphous, while after annealing at temperatures between 400°C to 525°C, α-Fe(Si) grains start to nucleate, the grain size varying from 2 to 18 nm. The Curie temperature of as-deposited amorphous phase and the onset of the primary crystallization are 355°C and 460°C, respectively. The lowest coercive magnetic field was obtained after annealing at 475°C.

Changes of optical, dielectric, and structural properties of Si 15Sb 85 phase change memory thin films under different initializing laser power

The optical, dielectric, and structural characteristics of Si 15Sb 85 phase change memory thin films under a moving continuous-wave laser initialization are studied by using spectroscopic ellipsometry and high-resolution transmission electron microscopy. The dependence of complex refractive index, dielectric functions, absorption coefficient, and optical band gap of the films on its crystallization extents formed by the different initialization laser power are analyzed in detail. The structural change from as-deposited amorphous phase to distorted rhombohedra-Sb-like crystalline structure with the increase of initialization laser power is clearly observed with sub-nanometer resolution. The optical and dielectric constants, the relationship between them, and the local atomic arrangements of this new phase change material can help explain the phase change mechanism and design the practical phase change memory devices.

Intrinsic complexity of the melt-quenched amorphous Ge2Sb2Te5memory alloy

Through the use of first-principles Ge $K$-edge XANES simulations we demonstrate that the structure of melt-quenched amorphous Ge-Sb-Te is intrinsically complex and is a mixture of Ge(3):Te(3) and Ge(4):Te(2) configurations in comparable concentrations, in contrast to the as-deposited amorphous phase that is dominated by the Ge(4):Te(2) configurations. The reasons for Ge-Te polyvalency are discussed and it is argued that both configurations are compatible with the Mott 8--N rule and the definition of an ideal amorphous solid. The near-perfect Te-Te distance match between the two major configurations accounts for the high cyclability of phase-change materials. Stable compositions in the Ge-Sb-Te system are suggested.

Evidence of Germanium precipitation in phase-change Ge1−xTex thin films by Raman scattering

In situ annealing Raman scattering is used to evidence germanium postcrystallization in the crystalline phase of Ge-rich phase-change GeTe thin films. Both reflectivity and Raman scattering show that the crystallization temperature of the as-deposited amorphous phase increases with increasing Ge content going from Tc=180 °C (Ge0.5Te0.5) to 360 °C for Ge-rich Ge0.76Te0.24. The crystallized phase adopts the rhombohedral α-GeTe phase structure, and whatever the starting composition. For Ge-rich GeTe we observe a second characteristic temperature around 375 °C, which signs the crystallization of a precipitated cubic Ge phase and thus the presence of two distinct phases.

In situ 3ω techniques for measuring thermal conductivity of phase-change materials

Knowledge of the thermal conductivity of phase-change materials is essential for accurate modeling of nonvolatile memory devices that incorporate them. The "3omega method" is a well-established and sensitive technique for measuring this property. We report two new extensions of the 3omega technique that feature in situ monitoring of the phase-change material as it transitions from the as-deposited amorphous phase to the crystalline phase. One technique crystallizes the entire sample in a vacuum oven, while using the 3omega voltage to monitor the phase transition. The other technique uses the 3omega heater to crystallize only the material in the region of measurement.

SET Characteristics of Phase Change Bridge Devices

ABSTRACTScaling studies have demonstrated that Phase Change Random Access Memory (PCRAM) is one of the most promising candidates for future non-volatile memory applications. The search for suitable phase change materials with optimized properties is therefore actively pursuit. In this paper, SET (crystallization) characteristics of an ultra fast switching material Ge15Sb85 in phase change memory bridge cell devices are presented. It was found that reproducible switching between two stable states with one decade resistance contrast and current pulses as short as 10 ns for SET and RESET (re-amorphization) operation is possible. Particular emphasis was placed on the difference in crystallization kinetics between the as-deposited and melt-quenched amorphous phase. Evidence is given for the existence of an electrical field as the critical parameter for threshold switching rather than a threshold voltage. For Ge15Sb85 a threshold switching field of 9MV/m was measured and it was shown that switching from the melt-quenched amorphous phase to the crystalline phase is about 600 times faster than crystallization from the as-deposited amorphous phase.

Stress, resistance, and phase transitions in NiCr(60 wt %) thin films

The evolution in both stress and resistance has been investigated on sputtered NiCr(60 wt %) resistive films during annealing (temperature cycles to maximum 700 °C). Aiming at the correlation of stress, resistance, and microstructure, samples from measurements to various maximum temperatures were analyzed by x-ray diffraction and transmission electron microscopy including microanalysis. A series of metastable phases was found with increasing temperature: the as-deposited amorphous phase a, the supersaturated body-centered-cubic (bcc) solid solution αss(Cr) (400 °C), and the tetragonal σ phase (500 °C). This was followed by the equilibrium two-phase alloy bcc α(Cr) plus face-centered-cubic γ(Ni) (600 °C). The phase transitions, characterized by differential scanning calorimetry, were found to be exothermic. The transition a→αss(Cr) results in a distinct tensile-stress component due to material densification. The resistivity is sharply decreasing and the temperature coefficient of resistance is changing from negative to positive values during the a→αss(Cr) transition.

Experimental and theoretical investigations of laser-induced crystallization and amorphization in phase-change optical recording media

We describe the numerical procedure for calculating three-dimensional profiles of temperature in a multilayer stack illuminated by a laser beam, and model the crystallization and amorphization kinetics for phase-change rewritable media. Experimental methods have been used to determine indirectly the probabilities of nucleation and growth for Ge2Sb2Te5 alloy. Some of the fundamental behaviors of phase-change erasable media, such as the crystallization of as-deposited amorphous phase, amorphization of supercooled liquid, and recrystallization of quenched amorphous phase, have been illustrated based on our three-dimensional temperature calculations and the model kinetics. The calculated transient reflectance behavior of as-deposited Ge2Sb2Te5 amorphous films in a single layer and in a quadrilayer stack, as well as the erasure behavior of Ge2Sb2Te5 alloy in a quadrilayer disk are in good agreement with experimental observations.

Growth of hexagonal plate crystal in vacuum-deposited amorphous films of antimony

Hexagonal plate crystals (HPC) grow at the expense of the surrounding amorphous regions during isothermal annealing at an arbitrary temperature, Ta, for Sb films deposited onto glass or SiOx coated glass substrates held at -130°C in a vacuum of 10-5 Torr. The growth rate of HPC, Rh, was determined from the measurement of the diagonal of HPC with an optical microscope. A ln Rh versus 1/Ta relationship leads to the conclusion that the growth of HPC is under the control of some activation process whose activation energy changes at a certain temperature, Tc: 0.5 eV at TaTc and 1.8 eV at Ta < Tc. It is proposed that growth is due to a successive or series type reaction in the amorphous phase: One process is similar to self-diffusion in a crystalline-like amorphous phase; the other is associated with some re-ordering process to form a crystalline-like amorphous phase in the as-deposited amorphous phase.