Effects of Si doping on the structural and electrical properties of Ge2Sb2Te5 films for phase change random access memory (original) (raw)
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Si doping in Ge2Sb2Te5 film to reduce the writing current of phase change memory
Applied Physics A, 2007
The characteristics of phase change memory devices in size of several micrometers and with pure Ge 2 Sb 2 Te 5 (GST), N-doped GST, and Si-doped GST films were investigated and compared with each other. The Si-doped GST device can perform SET and RESET cycles, even if the Si dopant is as small as 4.1 at. %. But the GST and N-doped GST device cannot perform the RESET process, though the SET state resistance of N-doped device is almost the same as that of Si-doped device and larger than that of GST device. In order to explain this phenomenon, the electrical and DSC characteristics of three kinds of films were investigated. Phase separation was found in Si-doped GST films. The reason of the RESET ability of Sidoped GST devices is supposed to be the existence of rich Si phases which act as micro-heaters. Thermal conduction simulations confirmed this supposition and indicate that the separated high resistance phase (rich Si phase) can heat the active volume of device efficiently and reduce the writing current largely.
Electrical properties of the Ge2Sb2Te5 thin films for phase change memory application
AIP Conference Proceedings, 2016
In this study I-V characteristic, temperature dependence of resistivity, thermopower, switching and memory effect were investigated for GST225 thin films. Resistivities, ratio of the resistivities of amorphous and crystalline states, activation energies of conductivity, temperature of phase transition, Seebeck coefficient, transition time due to the transformation from OFF to ON states and full recording time were estimated. It was shown that transport mechanism based on the two-channel model has a good correlation with experimental results for Ohmic region of I-V characteristic, while space-charge limited current mechanism for power region.
Journal of Applied Physics, 2008
We present chemical state information on contamination-free Ge 2 Sb 2 Te 5 thin film using high-resolution x-ray photoelectron spectroscopy ͑HRXPS͒ and the corresponding theoretical understanding of the chemical states, on both amorphous and metastable phases, illuminating the phase-change mechanism of the system. HRXPS data revealed that the Sb 4d shallow core level was split into two components having different binding energies and that the spin-orbit splitting feature of the Ge 3d level was enhanced as the system became metastable. Negligible change was observed in the Te 4d shallow core level, and in contrary to the previous report's prediction less change in valance band spectra was observed. The results imply that Sb movement is also involved in the phase-change mechanism and that acquisition of shallow core-level spectra can be a useful measure for understanding phase-change mechanism. Hydrogenated SbTe 6 octahedral-like cluster model was introduced to schematically interpret the generation of the two components in the Sb 4d level in metastable state, having an isotropic six-bonds configuration, and an anistropic six-bonds ͑three-short and three-elongated bonds͒ configuration. The amorphous state was modeled to have three-short bonds configuration. Finally, Stibnite-like building block model was used to show that the existence of the above two configurations for Sb atoms is feasible in the Ge 2 Sb 2 Te 5 system.
Voltage polarity effects in Ge2Sb2Te5-based phase change memory devices
Journal of Applied Physics, 2011
We assess voltage polarity effects in phase-change memory (PCM) devices that contain Ge 2 Sb 2 Te 5 (GST) as the active material through the study of vertically asymmetric pore-cell and laterally symmetric bridge-cell structures. We show that bias polarity can greatly accelerate device failure in such GST-based PCM devices and, through extensive transmission electron microscopy-based failure analysis, trace these effects to a two-stage elemental segregation process. Segregation is initially driven by bias across the molten region of the cell and is then greatly enhanced during the crystallization process at lower temperatures. These results have implications for the design of pulses and PCM cells for maximum endurance, the use of reverse polarity for extending endurance, the requirements for uni-or bi-polar access devices, the need for materials science on active rather than initial stoichiometries, and the need to evaluate new PCM materials under both bias polarities.
Acta Physica Polonica A, 2016
In this work the mechanism and kinetics of crystallization of the Ge2Sb2Te5+Bi thin films were investigated using differential scanning calorimetry. Ge2Sb2Te5 with different amounts of Bi (0, 0.2, 0.5, 0.8, 1, 3, 5 wt.%) was synthesized using quenching technique. Thin films were prepared by thermal evaporation of synthesized materials. X-ray diffraction has shown that synthesized materials had trigonal modification of Ge2Sb2Te5. Introduction of Bi led to the appearance of trigonal modification of Bi2Ge2Te5, which indicates on the replacement of Sb by Bi. Asdeposited thin films were amorphous up to 3% of Bi. Higher concentrations of Bi led to the appearance of crystalline phases. Composition of thin films was verified by Rutherford backscattering, and was found to be close to that of the synthesized materials. The joint application of model-free Ozawa-Flynn-Wall and model-fitting Coates-Redfern methods allowed to estimate kinetic triplet for crystallization process of GST225+Bi thin films, and to predict data processing and storage times of the phase change memory cells. It was shown that GST225+0.5 wt.% Bi thin films have the most promising kinetic characteristics among the investigated materials, due to the predicted smallest data processing and largest storage times.
Journal of Non-Crystalline Solids, 2012
Switching and memory effects in as-deposited amorphous Ge 2 Sb 2 Te 5 films with a considerable concentration of crystalline nuclei have been investigated. Variation of the phase composition of the sample has been confirmed by Raman spectroscopy data. The influence of nuclei on parameters of the current voltage characteristic has been studied. Significant variation of initial resistance and threshold voltage due to a different nuclei configuration has been observed. In some cases the current voltage characteristic was monotonous i.e. the intrinsic S-shape of the current-voltage characteristic disappeared and memory recording occurred without prior switching. The measurements were made in a current controlled mode which allowed conducting a thorough investigation of switching and current filament formation.
Advanced Electronic Materials, 2018
and ease of integration in the back end of a logic process. [7] CBRAM works based on electrochemical reactions of active metallic electrodes in a solid electrolyte (SE). [9] When an electric field is applied to CBRAM, cations are driven through the SE by an electrochemical reaction, and metallic conductive filaments (CFs) between two electrodes are constructed and annihilated by subsequent changes in the voltage polarity. [7] Various types of SEs in PMCs have CBRAM memory characteristics due to having high ionic conductivity for cations. [6] A variety of oxides, [10] halides, [11] polymers, [7,12] chalcogenides, [13,14] and phase-change materials (PCMs) have been used as insulator layers that work as an ionic transport medium in a CBRAM that has metal-insulator-metal (MIM) structure. [15] As an SE, we selected a phase change material (PCM) that can be integrated into electrical and optical circuits with widespread applications including rewritable optical data storages, [16] photonic memories, [17] phase change memories, [18,19] artificial synapses, [20] and conductive bridge random access memories (CBRAMs). [21] PCMs have long-term stability, [22] rapid switching, quick response to stimuli, very short crystallization times, [21] small switching energies, [23] and high endurance [24-26] ; for these reasons, PCMs are perfect materials for ultrafast data processing systems that require low power. [27] We used a stoichiometric composition of Ge 2 Sb 2 Te 5 (GST) alloy in its amorphous state as an insulator and active layer of the CBRAM. GST is a ternary chalcogenide-based PCM (Ch-PCM) and is currently the most important functional PCM. It is a desirable material for realization of eNVMs and CBRAMs owing to its high crystallization speed, [28] fast ionic movement, nanoscale CFs, phase stability, reversible switchability, [29] high data retention for both phases under ambient condition, [30] and low power consumption. [23] GST has been studied as SE, [31] switching layer, [32] thermoelectric heater, and thermal barrier [33] in CBRAMs with a reasonable memory characteristics. Although GST has more beneficial characteristics than other chalcogenides for a reliable CBRAM, its performance must be improved by elevating the crystallization temperature. The most important effect of N-doping on GST thin film is an increase in its crystallization temperature (and therefore, the crystallization activation energy); this change is advantageous for CBRAM technology. Nitrogen-incorporated Ge 2 Sb 2 Te 5 Conductive bridge random access memory (CBRAM) is a possible replacement for static field-programmable gate arrays (FPGAs) based on randomaccess memory. Ge 2 Sb 2 Te 5 (GST) is used in CBRAMs as a solid electrolyte due to its high diffusion properties of active ions and scalability to obtain high-density memory devices. Here, the trade-off between high memory window and uniformity of CBRAM based on GST is solved by introducing N atoms into the SE. Nitrogen-incorporated GST film (N-GST) is proposed as a replacement for the current GST-based CBRAMs with improved performance and better opportunities for conventional FPGA technologies. A bidirectional polarity-dependent characteristic with high I ON /I OFF ratio and satisfactory operation voltage is achieved by using N-GST thin film in a programmable metallization cell (PMC). Integration of N atoms in the GST-based PMC with a simple structure of Ag/ N-GST /Pt increases the resistance ratio more than 100 times compared to an undoped one. Consistent data retention is attained in both resistance states for ≥3.5 × 10 4 s at temperature up to 85 °C.
Solid-State Electronics, 2013
This paper shows that the well-know chalcogenide Ge 2 Sb 2 Te 5 (GST) in its amorphous state may be advantageously used as solid electrolyte material to fabricate Conductive-Bridge Random Access Memory (CBRAM) devices. GST layer was sputtered on preliminary inkjet-printed silver lines acting as active electrode on either silicon or plastic substrates. Whatever the substrate, the resistance switching is unambiguously attested at a nanoscale by means of conductive-atomic force microscopy (C-AFM) using a Pt-Ir coated tip on the GST surface acting as a passive electrode. The resistance change is correlated to the appearance or disappearance of concomitant hillocks and current spots at the surface of the GST layer. This feature is attributed to the formation/dissolution of a silver-rich protrusion beneath the AFM tip during set/reset operation. Beside, this paper constitutes a step toward the elaboration of crossbar memory arrays on flexible substrates since CBRAM operations were demonstrated on W/GST/Ag crossbar memory cells obtained from an heterogeneous fabrication process combining physical deposition and inkjetprinting.
Advances in OptoElectronics, 2012
Germanium antimony (Ge-Sb) thin films with tuneable compositions have been fabricated on SiO2/Si, borosilicate glass, and quartz glass substrates by chemical vapour deposition (CVD). Deposition takes place at atmospheric pressure using metal chloride precursors at reaction temperatures between 750 and 875°C. The compositions and structures of these thin films have been characterized by micro-Raman, scanning electron microscope (SEM) with energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) techniques. A prototype Ge-Sb thin film phase-change memory device has been fabricated and reversible threshold and phase-change switching demonstrated electrically, with a threshold voltage of 2.2–2.5 V. These CVD-grown Ge-Sb films show promise for applications such as phase-change memory and optical, electronic, and plasmonic switching.