Temperature Dependent Characterization and Crystallization Dynamics of Ge2Sb2Te5 Thin Films and Nanoscale Structures

Phase change memory (PCM) is currently seen as the most promising candidate for a future storage-class memory with the potential to be as fast as Dynamic Random-Access Memory but with much longer retention times, and as dense as flash memory but significantly faster due to unique material properties which include strong electrical resistivity contrast, fast crystallization and high crystallization temperature. PCM devices utilize chalcogenide materials (most commonly Ge2Sb2Te5, or GST) that can be reversibly and rapidly switched between amorphous and crystalline phases (enabling storage of information) with orders of magnitude difference in electrical resistivity. Crystallization dynamics, transition temperatures, and grain size distributions depend on the material composition, interfaces and cell geometry and determine the electrical pulses required for operation, cell performance, power consumption and reliability. This work focused on temperature dependent characterization of GeSbTe thin films and nanoscale structures with the goal of contributing to a better understanding of phase-change memory materials and devices. The electrical resistivity of liquid GST (ρGST-Lq )was extracted from measurements on large number of individual GST nanostructures self-heated to melt by single microsecond voltage pulses, as well as on thin film samples.The crystallization behavior of GST films on silicon nitride and on silicon dioxide through slow resistance versus temperature measurements was also characterized. Silicon nitride appears to facilitate the fcc-hcp phase-transition of GST and we speculate this may be due to the hexagonal symmetry of silicon nitride. Our results also show the importance of the heating rate in determining phase transition temperatures. Understanding the crystallization dynamics is critically important for PCM device operation. Above a certain amplitude, a baseline (or offset) voltage after a melting pulse can play an important role in the set operation by leading to retention of a molten filament and growth-from-melt templated from the surrounding crystalline regions. The effect of different baseline voltages on the set dynamics of phase change memory devices was studied by applying melting voltage pulses with varying baseline voltages. Simulations of the effect of different baseline voltages were also performed to compare to and help interpret the experimental results. Lastly, in-situ X-Ray Diffraction (XRD) measurements up to high temperatures and ex-situ XRD measurements on pre-annealed samples were performed to characterize grain size distributions as a function of anneal temperature. The material crystallizes over time as the chuck temperature is increased and the crystallization process is monitored by the evolution of different peaks in the XRD measurement which are related to the grain sizes. These results will be used to improve and calibrate our electrothermal and crystallization models for PCM materials and devices. Kadir Cil - University of Connecticut, 201

Assisted cubic to hexagonal phase transition in GeSbTe thin films on silicon nitride

The amorphous to face-centered cubic (fcc) and fcc to hexagonal close-packed (hcp) crystallization temperatures of GeSbTe thin films on underlying silicon nitride and silicon dioxide films were studied through slow(1 K/min) resistance versus temperature measurements. The amorphous to fcc phase transition is observed at similar to 170 degrees C for both cases but the fcc to hcp phase transition temperature for GeSbTe films on silicon nitride is observed to be similar to 80 degrees C lower than for GeSbTe films on silicon dioxide, possibly due to the hexagonal symmetry of silicon nitride. (C) 2013 Elsevier B.V. All rights reserved

Operation dynamics in phase-change memory cells and the role of access devices

A detailed physical model of the heating and amorphization profiles in phase-change memory elements is applied to illustrate the effects of loads and pulse rise times on the reset operation of phase-change memory cells. Finite element modeling of the electrical and thermal transport is used for a mushroom phase-change memory element - including temperature dependent materials parameters, thermoelectric terms and thermal boundary resistance between different materials - and integrated idealized circuit models are used for the access devices (MOSFET and diode, with a separate series resistance). The results show certain windows of loads and transient times that lead to successful reset operation without excessive wasted power, for the particular PCM cells and programming conditions simulated

Effect of Mn Doping on the Properties of Sol-gel Derived Pb0.3Sr0.7TiO3 Thin Films

Thin films of Pb0.3Sr0.7TiO3 (PST) and 2mol% Mn-doped PST (Pb0.3Sr0.7Ti0.98Mn0.02O3 or PSMT2) were fabricated on (001)-oriented LaAlO3 substrates using sol-gel and spin-coating techniques. The ferroelectric transition temperature did not change with Mn doping. However, dielectric constant and figure of merit (K) were found to increase with Mn doping in PST film. At room temperature and 20kV/cm applied field, a maximum K value of pure PST film was found to be merely 0.887, which improved to 22.36 and 29.85 at 20kV/cm and 40kV/cm, respectively for the 2% Mn doped PST film

Self-heating of silicon microwires: Crystallization and thermoelectric effects

We describe experiments on self-heating and melting of nanocrystalline silicon microwires using single high-amplitude microsecond voltage pulses, which result in growth of large single-crystal domains upon resolidification. Extremely high current densities (>20 MA/cm(2)) and consequent high temperatures (1700 K) and temperature gradients (1 K/nm) along the microwires give rise to strong thermoelectric effects. The thermoelectric effects are characterized through capture and analysis of light emission from the self-heated wires biased with lower magnitude direct current/alternating current voltages. The hottest spot on the wires consistently appears closer to the lower potential end for n-type microwires and to the higher potential end for p-type microwires. The experimental light emission profiles are used to verify the mathematical models and material parameters used for the simulations. Good agreement between experimental and simulated profiles indicates that these models can be used to predict and design optimum geometry and bias conditions for current-induced crystallization of microstructures

Atmospheric pressure microplasmas in ZnO nanoforests under high voltage stress

Atmospheric pressure ZnO microplasmas have been generated by high amplitude single pulses and DC voltages applied using micrometer-separated probes on ZnO nanoforests. The high voltage stress triggers plasma breakdown and breakdown in the surrounding air followed by sublimation of ZnO resulting in strong blue and white light emission with sharp spectral lines and non-linear current-voltage characteristics. The nanoforests are made of ZnO nanorods (NRs) grown on fluorine doped tin oxide (FTO) glass, poly-crystalline silicon and bulk p-type silicon substrates. The characteristics of the microplasmas depend strongly on the substrate and voltage parameters. Plasmas can be obtained with pulse durations as short as similar to 1 mu s for FTO glass substrate and similar to 100 ms for the silicon substrates. Besides enabling plasma generation with shorter pulses, NRs on FTO glass substrate also lead to better tunability of the operating gas temperature. Hot and cold ZnO microplasmas have been observed with these NRs on FTO glass substrate. Sputtering of nanomaterials during plasma generation in the regions surrounding the test area has also been noticed and result in interesting ZnO nanostructures ('nano-flowers' and 'nano-cauliflowers'). A practical way of generating atmospheric pressure ZnO microplasmas may lead to various lighting, biomedical and material processing applications. (C) 2015 Author(s)

Manganese and Zinc Spinel Ferrites Blended with Multi-Walled Carbon Nanotubes as Microwave Absorbing Materials

Magnetic and dielectric materials can be blended to enhance absorption properties at microwave frequencies, although the materials may have relatively weak attenuation capabilities by themselves. The specific goal of this work is to enhance microwave absorption properties of materials with interesting dielectric behavior by blending them with magnetic materials based on transition metals. The synthesized Mn1−xZnxFe2O4 (x = 0.0 and 1.0) spinel ferrite nanoparticles (MZF NPs) were blended with commercial multi-walled carbon nanotubes (MWCNTs) in various proportions with a binder matrix of paraffin. This simple and efficient process did not cause a significant variation in the energy states of MWCNTs. MZF NPs were synthesized with a citric acid assisted sol–gel method. Their electromagnetic characteristics and microwave absorption properties were investigated. These properties were derived from the microwave scattering parameters measured via the transmission line technique by using a vector network analyzer (VNA) in conjunction with an X band waveguide system. The return loss (RL) values of the samples were obtained from the electromagnetic constitutive parameters (permittivity and permeability). The results indicate that the minimum RL value and the bandwidth change significantly with the amount of ferrite material in the blend. These results encourage further development of MWCNTs blended with ferrite nanoparticles for broadband microwave applications

Thickness dependence of the amorphous-cubic and cubic-hexagonal phase transition temperatures of GeSbTe thin films on silicon nitride

The crystallization temperature of GeSbTe thin films with thicknesses between 11 and 87 nm on silicon nitride was studied through resistance versus temperature measurements. The amorphous-cubic phase transition occurs at similar to 150 degrees C for all films thicknesses, whereas the cubic-hexagonal phase transition temperature increases with film thickness, from similar to 200 degrees C for the 20 nm film to similar to 250 degrees C for the 87 nm film. The cubic-hexagonal transition occurs gradually for the 11 nm film. Implications for phase-change memory devices are discussed. (C) 2011 Elsevier B.V. All rights reserved