Dongbok Lee

Separate domain formation in Ge2Sb2Te5–SiOx mixed layer

We report separate domain formation in cosputtered Ge2Sb2Te5–SiOx mixed layer, with SiOx amount less than 10mol%. As-prepared Ge2Sb2Te5–SiOx layer exhibits amorphous phase with separate domains smaller than 20nm. The separation maintains after thermal annealing, which results in crystallization into fcc phase. The crystallization activation energies of Ge2Sb2Te5–SiOx are obtained as 4.99 and 6.44eV for mixed layers containing 5.3 and 8.4mol% SiOx, respectively. Those are larger than 2.75eV of pure Ge2Sb2Te5. Furthermore, the mixed layer exhibits sublimation at increased temperature. These are interpreted as formation of Ge2Sb2Te5-rich domains separated from each other by SiOx-rich domains.

Low thermal conductivity in Ge2Sb2Te5–SiOx for phase change memory devices

Nanometer scale Ge2Sb2Te5 (GST) domains formed by immiscible mixture of GST-SiOx at room temperature and 180 °C show remarkable suppression in electrical and thermal conductivity. Thermal boundary resistance with increased GST-SiOx interface becomes crucial to the reduction in thermal conductivity. These conductivity reductions concurrently result in the reduction in programming current and power consumption in phase change memory devices.

An experimental investigation on the switching reliability of a phase change memory device with an oxidized TiN electrode

Fluctuations (or drifts) in switching voltages such as programming set/reset voltages and threshold voltage pose serious obstacles to the reliable operation of electrical phase change memory devices. Using a phase change memory device having a GeSb2Te4 phase change material and TiN electrode, these fluctuations are demonstrated to result from device resistances varying with programming cycles. Fluctuating resistances appear to stem primarily from large contact resistances at the interface between the phase change material and the TiN electrode and from inhomogeneous phase distribution across the GeSb2Te4 layer due to unsuccessful heat confinement near the interface with TiN. Oxidation of a TiN electrode surface (via thermal annealing at 350°C under an atmospheric gas mixture of 97.9vol% N2 and 2.1vol% O2) is very effective in the reduction of fluctuations in device resistances and switching voltages hence the resulting increase in the programming cycles by two orders of magnitude. From a high resolution t...

Phase separation behavior of Ge2Sb2Te5 line structure during electrical stress biasing

We report the breakdown behavior of a patterned Ge2Sb2Te5 multiline structure during the voltage-driven electric stress biasing. Scanning Auger microscope analysis shows that the breakdown process accompanies with a phase separation of Ge2Sb2Te5 into an Sb, Te-rich phase and a Ge-rich phase. The phase separation is explained by the incongruent melting of Ge2Sb2Te5 based on the pseudobinary phase diagram between Sb2Te3 and GeTe. It is claimed that this phase separation behavior by incongruent melting provides one of the plausible mechanisms of the device failure in a phase change memory.

Electric-Field-Induced Mass Movement of Ge2Sb2Te5 in Bottleneck Geometry Line Structures

We report an electric-field-induced directional mass movement of Ge 2 Sb 2 Te 5 in bottleneck geometry. Under high-electric-stress circumstances (>10 6 A cm -2 ), a mass of Ge 2 Sb 2 Te 5 tends to move toward the cathode (-) by the remaining mass depletion at the anode (+). The high electric stress induces an asymmetric compositional separation such that Sb is distributed toward the cathode (-) whereas Te is distributed toward the anode (+). Ionicity in Ge 2 Sb 2 Te 5 at high temperature and high electric stress can be one of the origins of the asymmetric behavior. The electric-field-induced mass movement may provide insight on the device reliability of phase-change random access memory.

Analysis of the electric field induced elemental separation of Ge2Sb2Te5 by transmission electron microscopy

The chemical instability of line patterned Ge2Sb2Te5 was studied by transmission electron microscopy after electrically inducing melt and solidification. Compositional analysis showed elemental separation of Te to the anode side, while Ge and Sb mutually separated at the cathode side. Such elemental separation of Ge2Sb2Te5 is explained by the electric field effects and thermodynamic driving forces.

Formation of Ge2Sb2Te5 – TiO x Nanostructures for Phase Change Random Access Memory Applications

Amorphous Ge 2 Sb 2 Te 5 clusters with a size of 20 nm, self-enclosed by a thin layer of TiO x , were obtained by cosputtering Ge 2 Sb 2 Te 5 and TiO 2 targets at room temperature with the aim of reducing the reset current for phase change random access memory applications. Eutectic decomposition during the deposition caused a phase separation of Ge 2 Sb 2 Te 5 and TiO x . The temperature-dependent resistance change results showed that the activation energy for crystallization increased from 2.44 ± 0.76 to 3.84 ± 1.43 eV in the Ge 2 Sb 2 Te 5 film. The set resistance can be tuned within an acceptable range, and the reliability of this microstructure during repetitive laser melt-quenching cycles was tested.

Electron-Beam Lithography Patterning of Ge2Sb2Te5 Nanostructures Using Hydrogen Silsesquioxane and Amorphous Si Intermediate Layer

We report the successful electron-beam patterning of Ge 2 Sb 2 Te 5 nanostructures, in the form of both lines and dots, using hydrogen silsesquioxane (HSQ) resist. Although HSQ has proven to be a good resolution electron-beam resist, the adhesion between Ge 2 Sb 2 Te 5 and the HSQ resist is problematic when trying to form fine patterns. To promote their adhesion, we introduced an amorphous Si (a-Si) layer between the Ge 2 Sb 2 Te 5 and HSQ layers, and the layers were then sequentially removed using Cl 2 reactive ion etching and Ar etching with good selectivity of each layer. The selectivities for HSQ:a-Si etched by Cl 2 reactive gas and a-Si:Ge 2 Sb 2 Te 5 etched by Ar gas were 1:2.7 and 1:17, respectively. On the basis of this multilayer structure and a two-step dry etching process, various Ge 2 Sb 2 Te 5 nanostructures as small as 40 nm were successfully fabricated.

New Method of Evaluating the Crystallization Activation Energy of Ge2Sb2Te5 by In situ Resistance Measurement

A new method of evaluating crystallization activation energy of Ge2Sb2Te5 is proposed by in situ resistance measurement under isothermal annealing conditions. Linear relationship between logarithmic time and reciprocal temperature in modified Johnson–Mehl–Avrami–Kolmogorov equation is derived under the assumption that proportion of resistance drop from the initial value is closely related to crystal fraction. Crystallization activation energy thus obtained is 2.67 eV. Numerical calculation was conducted to manifest the validity of this analysis based on percolation model. Moreover, crystallization behavior of patterned single-line structure of Ge2Sb2Te5 was evaluated, and the scaling effect of increasing activation energy with decreasing line width was observed.

High Speed Phase Change Random Access Memory with (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 Complete Solid Solution

We investigated structures and phase transformation kinetics of (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 alloy mixture and its application for the phase change random access memory device. As-sputtered (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 thin film forms crystalline fcc phase. Meanwhile, we could obtain amorphous RESET state and crystalline SET state reproducibly by using appropriate voltage pulse conditions in device structure. We demonstrate that the minimum time for SET operation of phase change random access memory device with (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 goes down to 20 ns, which is much smaller than 100 ns for device with Ge1Sb2Te4. The accelerated SET operation of the device with (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 is interpreted to originate from reduced bond strength in comparison to pure Ge1Sb2Te4.