Kadir Cil

Electrical Resistivity of Liquid

The electrical resistivity of liquid Ge2Sb2 Te5 (GST) is obtained from dc-voltage measurements performed on thin GST films as well as from device-level microsecond-pulse voltage and current measurements performed on two arrays (thicknesses: 20 ± 2 nm and 50 ± 5 nm) of lithographically defined encapsulated GST nano-/microwires (length: 315 to 675 nm; width: 60 to 420 nm) with metal contacts. The thin-film measurements yield 1.26 ±0.15 mΩ·cm (thicknesses: 50, 100, and 200 nm); however, there is significant uncertainty regarding the integrity of the film in liquid state. The device-level measurements utilize the melting of the encapsulated structures by a single voltage pulse while monitoring the current through the wire. The melting is verified by the stabilization of the current during the pulse. The resistivity of liquid GST is extracted as 0.31 ± 0.04 and 0.21 ±0.03 mΩ·cm from 20- and 50-nm-thick wire arrays.

\hbox{Ge}_{2} \hbox{Sb}_{2}\hbox{Te}_{5}

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.

Based on Thin-Film and Nanoscale Device Measurements

Nanocrystalline silicon microwires are self-heated through microsecond voltage pulses. Nonlinear changes in current level are observed during the voltage pulse, which end with melting

Operation Dynamics in Phase-Change Memory Cells and the Role of Access Devices

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 ∼1 μs for FTO glass substrate and ∼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 substr...

Measurements of Liquid Silicon Resistivity on Silicon Microwires

High-temperature characterization of the thermoelectric properties of chalcogenide Ge2Sb2Te5 (GST) is critical for phase change memory devices, which utilize self-heating to quickly switch between amorphous and crystalline states and experience significant thermoelectric effects. In this work, the electrical resistivity and Seebeck coefficient are measured simultaneously as a function of temperature, from room temperature to 600 °C, on 50 nm and 200 nm GST thin films deposited on silicon dioxide. Multiple heating and cooling cycles with increasingly maximum temperature allow temperature-dependent characterization of the material at each crystalline state; this is in contrast to continuous measurements which return the combined effects of the temperature dependence and changes in the material. The results show p-type conduction (S > 0), linear S(T), and a positive Thomson coefficient (dS/dT) up to melting temperature. The results also reveal an interesting linearity between dS/dT and the conduction activat...

Atmospheric pressure microplasmas in ZnO nanoforests under high voltage stress

Resistivity of metastable amorphous Ge2Sb2Te5 (GST) measured at device level show an exponential decline with temperature matching with the steady-state thin-film resistivity measured at 858 K (melting temperature). This suggests that the free carrier activation mechanisms form a continuum in a large temperature scale (300 K – 858 K) and the metastable amorphous phase can be treated as a super-cooled liquid. The effective activation energy calculated using the resistivity versus temperature data follow a parabolic behavior, with a room temperature value of 333 meV, peaking to ∼377 meV at ∼465 K and reaching zero at ∼930 K, using a reference activation energy of 111 meV (3kBT/2) at melt. Amorphous GST is expected to behave as a p-type semiconductor at Tmelt ∼ 858 K and transitions from the semiconducting-liquid phase to the metallic-liquid phase at ∼ 930 K at equilibrium. The simultaneous Seebeck (S) and resistivity versus temperature measurements of amorphous-fcc mixed-phase GST thin-films show linear S-T trends that meet S = 0 at 0 K, consistent with degenerate semiconductors, and the dS/dT and room temperature activation energy show a linear correlation. The single-crystal fcc is calculated to have dS/dT = 0.153 μV/K2 for an activation energy of zero and a Fermi level 0.16 eV below the valance band edge.

High temperature electrical resistivity and Seebeck coefficient of Ge2Sb2Te5 thin films

Relaxation oscillations are observed in systems where an element with resistance switching characteristic, such as discharge tubes, tunnel diodes, bistable quantum hall effect devices, phase-change memory (PRAM) devices, is connected in parallel with a capacitor (C) and biased through a load resistor (RL) (Figure 1). The hysteretic behavior in the resistance switching of these elements allows the capacitor to charge when the device is in high-resistance state and discharge when the device switches to low-resistance state in repeated cycles. We have recently reported observations of large amplitude oscillations of this nature due to solid-liquid phase changes in silicon (Si) wires. These oscillations had frequencies on the order of 1–2 MHz (Figure 2).