Ge2Sb2Te5 Layer Research Articles

Mechanical properties of amorphous Ge2Sb2Te5 thin layers

The working principle of a Phase Change Memory (PCM) cell exploits the repeated reversible transition between a crystalline and an amorphous phase of chalcogenide alloys typically Ge2Sb2Te5, that are characterized respectively by a high (SET) and a low (RESET) conductive state. The change in density between the two phases (6%) induces a very high compressive stress to the active amorphous region by the surrounding crystalline materials. Moreover, the physical iterative transformation between crystalline and amorphous phase transformation introduces a swelling and deswelling effect. This is one of the key failure mechanisms that are limiting the reliability of the final integration of the PCM system. Knowledge of the mechanical properties of the amorphous phase is then an important factor. Amorphous structure, i. e. its short-range order, depends on the adopted formation procedure. In this paper we analyze the mechanical characteristics of sputtered amorphous Ge2Sb2Te5 thin layers and the modification introduced by ion irradiation, a procedure adopted to simulate the amorphous state produced by melt quenching. Measurements of Young's Modulus and Hardness were performed using Ultra High-Nano Indentation on plane samples. The values of both quantities increase of about 10–20% in the 30 keV Ge+ irradiated samples. This trend is due to the reduction of homopolar wrong bonds (GeGe and TeTe) present in the as deposited film. Thermal spikes associated to the impinging ion cause a local atomic rearrangement that results in a structure similar to that of the crystalline phase. The investigation was extended to cantilevers of length in the range 10–200 μm, with a layer of 100 nm Ge2Sb2Te5 deposited on 280 nm thick SiN. The cantilever modal analysis and the out of plane deflection measurements were correlated using a Finite Element modeling, that makes use of the mechanical values measured by Ultra high Nano Indentation. After deposition the amorphous Ge2Sb2Te5 layer is subject to a compressive mechanical shrinkage, this internal stress is released after ion implantation.

Mechanical and electrical characterization of CVD-grown graphene transferred on chalcogenide Ge2Sb2Te5 layers

In the present work, we investigate some mechanical and electrical properties of a Chemical Vapor Deposition (CVD)-grown graphene layer transferred on 50 nm-thick Ge2Sb2Te5 chalcogenide and discuss the possible application of graphene as a scaled contact replacing metal lines in phase-change memory devices. At first, the graphene-chalcogenide interface was extensively investigated. The expected chemical composition was confirmed by means of Electron Energy Loss Spectrometry (EELS), and the absence of bond distortions in the chalcogenide layer after graphene transfer was proved through Raman spectroscopy. The latter evidenced also the presence of defects, further confirmed by Transmission Electron Microscopy (TEM) investigations. The quality of the contact stack was evaluated by means of the adhesion among layers and by the sheet resistance of the layers themselves. Scratch tests and numerical simulations revealed a stress distribution compatible with a failure of the interface where the graphene layer features a 2 %-void. Next, the contact resistance proved an effective good Ohmic quality with a high graphene sheet resistance, about 1400 Ohm/square, which suggests that a better optimized transfer process should be applied to reduce it more than one order of magnitude.

Self-Limited CBRAM With Threshold Selector for 1S1R Crossbar Array Applications

In this letter, we demonstrate a self-limited conductive-bridging random access memory (CBRAM) that removes the necessity for external current compliance in a one selector–one resistor (1S1R) architecture. The standard Ge2Sb2Te5 (GST) is used as a CBRAM switching layer. In addition, Te-rich GST is also considered. Their performance is then compared. Both samples exhibit self-limited on-current characteristics, and the on-currents of the standard GST and Te-rich GST are ~300 and $\sim 20~\mu \text{A}$ , respectively. The observed self-limited characteristics are caused by the Te in the GST layer because in the presence of Te, Cu tends to form a more stable CuTe phase that restrict Cu filament growth. Furthermore, to confirm the feasibility of crossbar array applications, the 1S1R device is evaluated using a Ag/TiO2-based threshold selector device reported in our previous work. Hence, we confirm leakage current reduction, a uniform resistance distribution, and stable retention characteristics in the 1S1R configuration with no external current compliance.

Tunable broadband near-infrared absorber based on ultrathin phase-change material

In this work, a tunable broadband near-infrared light absorber was designed and fabricated with a simple and lithography free approach by introducing an ultrathin phase-change material Ge2Sb2Te5 (GST) layer into the metal–dielectric multilayered film structure with the structure parameters as that: SiO2 (72.7 nm)/Ge2Sb2Te5 (6.0 nm)/SiO2 (70.2 nm)/Cu (>100.0 nm). The film structure exhibits a modulation depth of ∼72.6% and an extinction ratio of ∼8.8 dB at the wavelength of 1410 nm. The high light absorption (95%) of the proposed film structure at the wavelength of 450 nm in both of the amorphous and crystalline phase of GST, indicates that the intensity of the reflectance in the infrared region can be rapidly tuned by the blue laser pulses. The proposed planar layered film structure with layer thickness as the only controllable parameter and large reflectivity tuning range shows the potential for practical applications in near-infrared light modulation and absorption.

Mixed‐Mode Electro‐Optical Operation of Ge2Sb2Te5 Nanoscale Crossbar Devices

The use of phase‐change materials for a range of exciting new optoelectronic applications from artificial retinas to ultrahigh‐resolution displays requires a thorough understanding of how these materials perform under a combination of optical and electrical stimuli. This study reports for the first time the complex link between the electronic and optical properties in real‐world crossbar nanoscale devices constructed by confining a thin layer of Ge2Sb2Te5 between transparent indium tin oxide electrodes, forming an optical nanocavity. A novel proof‐of‐concept device that can be operated by a combination of optical and electrical stimuli is presented, leading the way for the development of further applications based on mixed‐mode electro‐optical operation.

Dynamic reconfiguration of van der Waals gaps within GeTe-Sb2Te3 based superlattices.

Phase-change materials based on GeSbTe show unique switchable optoelectronic properties and are an important contender for next-generation non-volatile memories. Moreover, they recently received considerable scientific interest, because it is found that a vacancy ordering process is responsible for both an electronic metal-insulator transition and a structural cubic-to-trigonal transition. GeTe-Sb2Te3 based superlattices, or specifically their interfaces, provide an interesting platform for the study of GeSbTe alloys. In this work such superlattices have been grown with molecular beam epitaxy and they have been characterized extensively with transmission electron microscopy and X-ray diffraction. It is shown that the van der Waals gaps in these superlattices, which result from vacancy ordering, are mobile and reconfigure through the film using bi-layer defects and Ge diffusion upon annealing. Moreover, it is shown that for an average composition that is close to GeSb2Te4 a large portion of 9-layered van der Waals systems is formed, suggesting that still a substantial amount of random vacancies must be present within the trigonal GeSbTe layers. Overall these results illuminate the structural organization of van der Waals gaps commonly encountered in GeSbTe alloys, which are intimately related to their electronic properties and the metal-insulator transition.

All-optical tuning of EIT-like dielectric metasurfaces by means of chalcogenide phase change materials.

Electromagnetically induced transparency (EIT) is a pump-induced narrowband transparency window within an absorption line of the probe beam spectrum in an atomic system. In this paper we propose a way to bring together the all-dielectric metamaterials to have EIT-like effects and to optically tune the response by hybridizing them with a layer of a phase change material. We propose a design of the metamaterial based on Si nanoresonators that can support an EIT-like resonant response. On the top of the resonators we consider a thin layer of a chalcogenide phase change material, which we will use to tune the optical response. Our choice is Ge2Sb2Te5 (GST), since it has two stable phases at room temperature, namely amorphous and crystalline, between which it can be switched quickly, nonvolatively and reversibly, sustaining a large number of switching cycles. They differ in optical properties, while still having moderately low losses in telecom range. Since such dielectric resonators do not have non-radiative losses of metals around 1550nm, they can lead to a high-Q factor of the EIT-like response in this range. Firstly, we optimize the starting structure so that it gives an EIT-like response at 1550 nm when the GST layer is in the amorphous state. Our starting design uses glass as a substrate, but we also consider implementation in SOI technology. If we then switch the thin layer of GST to its crystalline phase, which has higher losses, the EIT-like response is red shifted, providing around 10:1 contrast at 1550nm. This reversible tuning can be done with an ns visible pulsed laser. We discuss the results of the simulation of the dielectric metasurface for different configurations and the tuning possibility.

Thermally Tunable Ultrasensitive Infrared Absorption Spectroscopy Platforms Based on Thin Phase-Change Films

The thermal tunability of the optical and electrical properties of phase-change materials has enabled the decades-old rewritable optical data storage and the recently commercialized phase-change memory devices. Recently, phase-change materials, in particular, Ge2Sb2Te5 (GST), have been considered for other thermally configurable photonics applications, such as active plasmonic surfaces. Here, we focus on nonplasmonic field enhancement and demonstrate the use of the phase-change materials in ultrasensitive infrared absorption spectroscopy platforms employing interference-based uniform field enhancement. The studied structures consist of patternless thin GST and metal films, enabling simple and large-area fabrication on rigid and flexible substrates. Crystallization of the as-fabricated amorphous GST layer by annealing tunes (redshifts) the field-enhancement wavelength range. The surfaces are tested with ultrathin chemical and biological probe materials. The measured absorption signals are found to be compa...

Atomic Layering, Intermixing and Switching Mechanism in Ge-Sb-Te based Chalcogenide Superlattices.

GeSbTe-based chalcogenide superlattice (CSLs) phase-change memories consist of GeSbTe layer blocks separated by van der Waals bonding gaps. Recent high resolution electron microscopy found two types of disorder in CSLs, a chemical disorder within individual layers, and SbTe bilayer stacking faults connecting one block to an adjacent block which allows individual block heights to vary. The disorder requires a generalization of the previous switching models developed for CSL systems. Density functional calculations are used to describe the stability of various types of intra-layer disorder, how the block heights can vary by means of SbTe-based stacking faults and using a vacancy-mediated kink motion, and also to understand the nature of the switching process in more chemically disordered CSLs.

Atomic-scale quantification of interdiffusion and dopant localization in GeSbTe-based memory devices

Fabrication of phase-change memory devices at modest or ambient temperatures leads to nanoscale compositional variations in phase-transition layers, where amorphous-polycrystalline phase change takes place via electrical switching, and can alter the device's performances. Here, by transmission electron microscopy and atom probe tomography, we address that thermal annealing at 400 °C for 20 min induces an elemental interdiffusion in the devices consisting of TiN (top electrode), carbon-doped GeSbTe (phase-transition layer), and TiSiN (bottom heater). With respect to the employed annealing process, the Ge atoms of GeSbTe layer have diffused into TiSiN layer at a given sample volume, while the Ti atoms of TiSiN layer into GeSbTe layer. Furthermore, non-random nature of dopant distribution in the GeSbTe materials leads to a Ti-localization including dopants at the GeSbTe/TiSiN interfaces. Our findings have two important implications: First, the annealing-driven interdiffusion of Ge and Ti is a predominant mechanism responsible for nanoscale compositional variations in GeSbTe layer; second, such an interdiffusion and the resultant dopant localization play a crucial role on the driving force for amorphous-polycrystalline transition of GeSbTe-based memory devices.

Ultrathin phase-change coatings on metals for electrothermally tunable colors

Metal surfaces coated with ultrathin lossy dielectrics enable color generation through strong interferences in the visible spectrum. Using a phase-change thin film as the coating layer offers tuning the generated color by crystallization or re-amorphization. Here, we study the optical response of surfaces consisting of thin (5–40 nm) phase-changing Ge2Sb2Te5 (GST) films on metal, primarily Al, layers. A color scale ranging from yellow to red to blue that is obtained using different thicknesses of as-deposited amorphous GST layers turns dim gray upon annealing-induced crystallization of the GST. Moreover, when a relatively thick (>100 nm) and lossless dielectric film is introduced between the GST and Al layers, optical cavity modes are observed, offering a rich color gamut at the expense of the angle independent optical response. Finally, a color pixel structure is proposed for ultrahigh resolution (pixel size: 5 × 5 μm2), non-volatile displays, where the metal layer acting like a mirror is used as a heater element. The electrothermal simulations of such a pixel structure suggest that crystallization and re-amorphization of the GST layer using electrical pulses are possible for electrothermal color tuning.

Low-energy phase change memory with graphene confined layer

How to reduce the Reset operation energy is the key scientific and technological problem in the field of phase change memory (PCM). Here, we show in the Ge2Sb2Te5 based PCM cell, inserting an additional graphene monolayer in the Ge2Sb2Te5 layer can remarkably decrease both the Reset current and energy. Because of the small out-of-plane electrical and thermal conductivities of such monolayer graphene, the Set resistance and the heat dissipation towards top TiN electrode of the modified PCM cell are significantly increased and decreased, respectively. The mushroom-typed larger active phase transition volume thus can be confined inside the underlying thinner GST layer, resulting in the lower power consumption.

Design of practicable phase-change metadevices for near-infrared absorber and modulator applications.

Phase-change chalcogenide alloys, such as Ge2Sb2Te5 (GST), have very different optical properties in their amorphous and crystalline phases. The fact that such alloys can be switched, optically or electrically, between such phases rapidly and repeatedly means that they have much potential for applications as tunable photonic devices. Here we incorporate chalcogenide phase-change films into a metal-dielectric-metal metamaterial electromagnetic absorber structure and design absorbers and modulators for operation at technologically important near-infrared wavelengths, specifically 1550 nm. Our design not only exhibits excellent performance (e.g. a modulation depth of ~77% and an extinction ratio of ~20 dB) but also includes a suitable means for protecting the GST layer from environmental oxidation and is well-suited, as confirmed by electro-thermal and phase-transformation simulations, to in situ electrical switching. We also present a systematic study of design optimization, including the effects of expected manufacturing tolerances on device performance and, by means of a sensitivity analysis, identify the most critical design parameters.

Phase change memory employing a Ti diffusion barrier for reducing reset current

Phase Change Memory (PCM) has the potential for use as the flash memories for the next generation due to its scalability, long endurance, high speed, and the possibility of random access. To successfully integrate phase change materials of a GeSbTe (GST) alloy in the device fabrications, the presence of a Ti layer is required to obtain proper contact resistance characteristics and also to improve the adhesion property between the GST alloys and the top TiN electrode. The findings reported herein indicate that Ti can readily diffuse into the GST layer and reduce the set resistance when the subsequent device fabrication involves the heating process. As a result, the reset current significantly increases and the endurance cycles of the PCM device are significantly degraded. We proposed the use of a Highly Nitrogen doped GST (HNGST) layer as a barrier to Ti diffusion, in which the presence of the barrier was able to restore the reset current of the PCM devices down to the level of as-etched PCM cell by the use of a 9nm thick HNGST barrier. In addition, endurance cycles were significantly enhanced from 106 to over 108 by blocking the diffusion of Ti into the program volume of the cell with the HNGST barrier layer.

The Effect of Ti-Rich TiN Film on Thermal Stability of Ge2Sb2Te5 for Phase Change Memory

Investigation of the thermal stability of Ge2Sb2Te5 (GST) layer in contact with TiN film is very valuable for phase-change memory applications. In this work, the effect of Ti/N atomic ratio in TiN film on thermal stability of GST film has been thoroughly analyzed. After annealing at 410°C for 30 min, the Ti-rich TiN thin film is found to interact with GST material but there does not exist any mixing of Ti, Sb and Te atomic species at the interface between the stoichiometric TiN film and GST, which indicates the stoichiometric TiN film is stable against GST. A Ti-rich TiN film has Ti phase and other TiN compound phases. The presence of the Ti atoms promotes the mutual diffusion and interaction of Ti, Sb and Te under thermal annealing. To ensure the thermal stability of GST in contact with TiN and the performance of device, the Ti/N atomic ratio should be given more attention in the fabrication of PCM devices. Clear boundary and normal operation character are achieved in the T-shaped phase change memory devices with TiN0.6 film.

Electrically actuated phase-change pixels for transmissive and reflective spatial light modulators in the near and mid infrared.

Transmissive and reflective spatial light modulators have been designed and simulated for the 1.55 to 2.10 μm spectral region. An electrically actuated layer of phase-change material (PCM) was employed as the electro-optical medium for two-state self-holding "light-to-dark" intensity modulation of free-space light beams. The PCM was sandwiched between transparent conductive N-doped Si or indium tin oxide contact layers in a simple planar structure. A 100 to 500 nm PCM layer of Ge2Sb2Te5 (GST) was employed for optimum performance at 1.55 μm where the transmissive-modulator insertion loss was around 4.5 dB. The GST light-dark contrast was found to be 32 dB. For the GST reflection device, an included metal film (Ag) improved the 1.55 μm performance metrics to 0.7 dB of insertion loss with a contrast around 26 dB. The calculated performance for both types of spatial light modulators was robust to changes in the input incidence angle near normal incidence. Applications include infrared scene generation and signal processing.

Numerical study of achiral phase-change metamaterials for ultrafast tuning of giant circular conversion dichroism.

Control of the polarization of light is highly desirable for detection of material’s chirality since biomolecules have vibrational modes in the optical region. Here, we report an ultrafast tuning of pronounced circular conversion dichroism (CCD) in the mid-infrared (M-IR) region, using an achiral phase change metamaterial (PCMM). Our structure consists of an array of Au squares separated from a continuous Au film by a phase change material (Ge2Sb2Te5) dielectric layer, where the Au square patches occupy the sites of a rectangular lattice. The extrinsically giant 2D chirality appears provided that the rectangular array of the Au squares is illuminated at an oblique incidence, and accomplishes a wide tunable wavelength range between 2664 and 3912 nm in the M-IR regime by switching between the amorphous and crystalline states of the Ge2Sb2Te5. A photothermal model is investigated to study the temporal variation of the temperature of the Ge2Sb2Te5 layer, and shows the advantage of fast transiting the phase of Ge2Sb2Te5 of 3.2 ns under an ultralow incident light intensity of 1.9 μW/μm2. Our design is straightforward to fabricate and will be a promising candidate for controlling electromagnetic (EM) wave in the optical region.

Electro-optical phase-change 2 × 2 switching using three- and four-waveguide directional couplers.

Theoretical modeling and numerical simulation have been performed at λ=2100 nm on silicon-on-insulator channel-waveguide directional couplers in which the outer two Si waveguides are passive and the central waveguide(s) are electro-optical (EO) "islands." The EO channel(s) utilize a 10 nm layer of Ge2Sb2Te5 phase-change-material sited at midlevel of a doped Si channel. A voltage-driven phase change produces a large change in the effective index of the TE(o) and TM(o) modes, thereby inducing crossbar 2×2 switching. A mode-matching method is employed to estimate EO switching performance in the limit of strong interguide coupling. Low-loss switching is predicted for cross-to-bar and bar-to-cross coupling lengths. These "self-holding" switches had active lengths of 500-1000 μm, which are shorter than those in couplers relying upon free-carrier injection. The four-waveguide devices had lower cross talk but higher loss than the three-waveguide devices. For the crystalline phase we sometimes used an active length that was smaller than that for the amorphous phase.

Tunable near-infrared plasmonic perfect absorber based on phase-change materials

A tunable plasmonic perfect absorber with a tuning range of similar to 650 nm is realized by introducing a 20 nm thick phase-change material Ge2Sb2Te5 layer into the metal-dielectric-metal configuration. The absorption at the plasmonic resonance is kept above 0.96 across the whole tuning range. In this work we study this extraordinary optical response numerically and reveal the geometric conditions which support this phenomenon. This work shows a promising route to achieve tunable plasmonic devices for multi-band optical modulation, communication, and thermal imaging. (C) 2015 Chinese Laser Press

All-optical switching of localized surface plasmon resonance in single gold nanosandwich using GeSbTe film as an active medium

Localized surface plasmon resonance (LSPR) switching was investigated in a Au/GeSbTe/Au nanosandwich as a key active element for plasmonic integrated circuits and devices. Near-infrared single-particle spectroscopy was conducted to examine the interaction of a Au nanorod (AuNR) and Au film, between which a GeSbTe layer was incorporated as an active phase-change media. Numerical calculation revealed that hybridized modes of the AuNR and Au film exhibit a significant change of scattering intensity with the phase change. In particular, the antisymmetric (magnetic resonance) mode can be modulated effectively by the extinction coefficient of GST, as well as its refractive index. Experimental demonstration of the switching operation was performed by alternate irradiation with a picosecond pulsed laser for amorphization and a continuous wave laser for crystallization. Repeatable modulation was obtained by monitoring the scattering light around the LSPR peak at λ = 1070 nm.