Chalcogenide-based Phase Change Research Articles

Electrically Reconfigurable Phase-Change Transmissive Metasurface.

Programmable and reconfigurable optics hold significant potential for transforming a broad spectrum of applications, spanning space explorations to biomedical imaging, gas sensing, and optical cloaking. The ability to adjust the optical properties of components like filters, lenses, and beam steering devices could result in dramatic reductions in size, weight, and power consumption in future optoelectronic devices. Among the potential candidates for reconfigurable optics, chalcogenide-based phase change materials (PCMs) offer great promise due to their non-volatile and analogue switching characteristics. Although PCM have found widespread use in electronic data storage, these memory devices are deeply sub-micron-sized. To incorporate phase change materials into free-space optical components, it is essential to scale them up to beyond several hundreds of microns while maintaining reliable switching characteristics. This study demonstrated a non-mechanical, non-volatile transmissive filter based on low-loss PCMs with a 200 × 200 µm2 switching area. The device/metafilter can be consistently switched between low- and high-transmission states using electrical pulses with a switching contrast ratio of 5.5 dB. The device wasreversibly switched for 1250 cycles before accelerated degradation took place. The work represents an important step toward realizing free-space reconfigurable optics based onPCMs.

Altered the structural, morphological and optical properties of SbSe thin films through swift heavy ion irradiation

Chalcogenide based phase change materials are gaining attention due to their ability to exhibit expeditious and reversible structural transition from amorphous to crystalline phase. This work included the effect of swift heavy silver (Ag9+) ion-irradiation (120 MeV), at various fluences (5E11, 1E12, 5E12 and 1E13 ions/cm2) on the structural, optical and morphological properties of pristine and annealed (250 ° C) SbSe thin films. The pristine films undergo a structural transition from amorphous to crystalline upon annealing and from crystalline to amorphous upon irradiation of annealed films. Structural transition caused by annealing and ion-irradiation resulted in a drastic change in morphology and optical properties. The annealed films exhibited less transmission than the pristine and irradiated films, which increased with increase in ion-irradiation fluences because of phase transition. After irradiation, the optical band decreased for pristine thin films, because the forbidden gap defect concentration has increased, but increased after irradiating the annealed thin films that may be due to annealing out of dense localized defect states. The significant optical contrast upon phase transition in near infrared region can be utilized for different optoelectronic applications.

Recent advances in doped Ge2Sb2Te5 thin film based phase change memories

Nowadays, chalcogenide-based phase change materials (PCMs) are proving themselves superior in the category of memory devices due to a combination of their unique set of properties.

Ge1−xSx chalcogenide alloys for OTS applications using magnetron sputtering

3D cross (X)–point memory arrays have attracted attention for future memory architecture due to their cost efficiency and high density. Chalcogenide–based materials are suitable candidates for 3D X–point memory because they can be stacked to form both memory and selector with a simple process. Conventional binary chalcogenide ovonic threshold switching (OTS) selectors such as GeXSe1−x or GeTe suffer from thermal instability. Furthermore, ternary OTS selectors are being investigated by doping various materials to overcome thermal instability. Here, we introduce a wide range of Ge1−xSx films to analyze and confirm the selector behavior for future applications. S–rich region (0.5 ≤ x ≤ 0.67) devices behaved as OTS selector with low leakage current (1.3 nA) and high thermal stability (amorphous up to 600 °C). To study the mechanism of devices with different compositions, the modified PF–model was used to calculate trap density, activation energy, subthreshold swing, and distance between traps. Additionally, extracted optical bandgap (1.65 – 3.45 eV) and urbach energy (166 – 103 meV) had good accordance with electrical characteristics of each device. This innovative research has potential to enable 3D X–point memory by combining conformal OTS selector and chalcogenide–based phase change memory.

Amorphous to Crystal Phase Change Memory Effect with Two-Fold Bandgap Difference in Semiconducting K2Bi8Se13.

Chalcogenide-based phase change memory (PCM) is a key enabling technology for optical data storage and electrical nonvolatile memory. Here, we report a new phase change chalcogenide consisting of a 3D network of ionic (K···Se) and covalent bonds (Bi-Se), K2Bi8Se13 (KBS). Thin films of amorphous KBS deposited by DC sputtering are structurally and chemically homogeneous and exhibit a surface roughness of 5 nm. The KBS film crystallizes upon heating at ∼483 K. The optical bandgap of the amorphous film is about 1.25 eV, while its crystalline phase has a bandgap of ∼0.65 eV shows 2-fold difference between the two states. The bulk electrical conductivity of the amorphous and crystalline film is ∼7.5 × 10-4 and ∼2.7 × 10-2 S/cm, respectively. We have demonstrated a phase change memory effect in KBS by Joule heating in a technologically relevant vertical memory cell architecture. Upon Joule heating, the vertical device undergoes switching from its amorphous to crystalline state of KBS at 1-1.5 V (∼50 kV/cm), increasing conductivity by a factor of ∼40. Besides the large electrical and optical contrast in the crystalline and amorphous KBS, its elemental cost-effectiveness, stoichiometry, fast crystallization kinetics, as determined by the ratio of the glass transition and melting temperature, Tg/Tm ∼ 0.5, as well as the scalable synthesis of the thin film determine that KBS is a promising PC material for next general phase change memory.

Resistance modulation in Ge2Sb2Te5

Chalcogenide based phase change random access memory (PCRAM) holds great promise for high speed and large data storage applications. This memory is scalable, requires a low switching energy, has a high endurance, has fast switching speed, and is nonvolatile. However, decreasing the switching time whilst increasing the cycle endurance is a key challenge for this technology to replace dynamic random access memory. Here we demonstrate high speed and high endurance ultrafast transient switching in the SET state of a prototypical phase change memory cell. Volatile switching is modeled by electron-phonon and lattice scattering on short timescales and charge carrier excitation on long timescales. This volatile switching in phase change materials enables the design of hybrid memory modulators and ultrafast logic circuits.

Optical and photoluminescence performance of electrodeposited arsenic selenide thin film doped with erbium ion

The solid state performance of Erbium ion (Er3+), a rare-earth ion, incorporated into the lattice of arsenic selenide (As2Se3) film is here presented. By varying the concentration of the dopant from 1 to 5% (wt.%), the films were deposited on a fluorine-doped SnO2 glass via the electrodeposition method. The structural, optical and morphological characteristics of the samples were obtained using the XRD and Raman spectroscopy, UV-VIS-NIR and photoluminescence spectroscopy and scanning electron microscopy respectively. Polycrystalline films having monoclinic and cubic crystal structures for the undoped and doped samples respectively were observed from the XRD analysis. 34.5 nm and 44.2 nm were estimated for the mean grain size of the undoped and doped films respectively. The Raman spectra revealed the diffusion of the Erbium ion (Er3+) into the As2Se3 lattice by Raman shift. The films revealed least absorbance and high transmittance up to 99% in the IR region with increasing doping. A high refractive index value of 2.66 was observed for all the films with a broadening towards the NIR region. The energy bandgap was estimated and lies in the range of 3.61–3.85 eV which slightly decreased with increasing doping concentration. The PL spectra showed intense excitonic (near-edge) emission bands at 348 nm with increasing doping. SEM micrographs revealed fairly homogeneous nano-ball morphology at lower doping concentration and nanoclusters at higher doping concentration. Annealing of the films above the glass-transition temperature (Tg) allowed the transition of the film to the crystalline phase which is an interesting feature in binary chalcogenide-based phase change memory (PCM) photonics devices.

Phase change thin films for non-volatile memory applications

The rapid development of Internet of Things devices requires real time processing of a huge amount of digital data, creating a new demand for computing technology. Phase change memory technology based on chalcogenide phase change materials meets many requirements of the emerging memory applications since it is fast, scalable and non-volatile. In addition, phase change memory offers multilevel data storage and can be applied both in neuro-inspired and all-photonic in-memory computing. Furthermore, phase change alloys represent an outstanding class of functional materials having a tremendous variety of industrially relevant characteristics and exceptional material properties. Many efforts have been devoted to understanding these properties with the particular aim to design universal memory. This paper reviews materials science aspects of chalcogenide-based phase change thin films relevant for non-volatile memory applications. Particular emphasis is put on local structure, control of disorder and its impact on material properties, order–disorder transitions and interfacial transformations.

A fully automated temperature-dependent resistance measurement setup using van der Pauw method.

The van der Pauw (VDP) method is widely used to identify the resistance of planar homogeneous samples with four contacts placed on its periphery. We have developed a fully automated thin film resistance measurement setup using the VDP method with the capability of precisely measuring a wide range of thin film resistances from few mΩ up to 10 GΩ under controlled temperatures from room-temperature up to 600 °C. The setup utilizes a robust, custom-designed switching network board (SNB) for measuring current-voltage characteristics automatically at four different source-measure configurations based on the VDP method. Moreover, SNB is connected with low noise shielded coaxial cables that reduce the effect of leakage current as well as the capacitance in the circuit thereby enhancing the accuracy of measurement. In order to enable precise and accurate resistance measurement of the sample, wide range of sourcing currents/voltages are pre-determined with the capability of auto-tuning for ∼12 orders of variation in the resistances. Furthermore, the setup has been calibrated with standard samples and also employed to investigate temperature dependent resistance (few Ω-10 GΩ) measurements for various chalcogenide based phase change thin films (Ge2Sb2Te5, Ag5In5Sb60Te30, and In3SbTe2). This setup would be highly helpful for measurement of temperature-dependent resistance of wide range of materials, i.e., metals, semiconductors, and insulators illuminating information about structural change upon temperature as reflected by change in resistances, which are useful for numerous applications.

Resolving Crystallization Kinetics of GeTe Phase-Change Nanoparticles by Ultrafast Calorimetry.

Chalcogenide-based phase change materials (PCMs) are promising candidates for the active element in novel electrical nonvolatile memories and have been applied successfully in rewritable optical disks. Nanostructured PCMs are considered as the next generation building blocks for their low power consumption, high storage density, and fast switching speed. Yet their crystallization kinetics at high temperature, the rate-limiting property upon switching, faces great challenges due to the short time and length scales involved. Here we present a facile method to synthesize highly controlled, ligand-free GeTe nanoparticles, an important PCM, with an average diameter under 10 nm. Subsequent crystallization by slow and ultrafast rates allows unravelling of the crystallization kinetics, demonstrating the breakdown of Arrhenius behavior for the crystallization rate and a fragile-to-strong transition in the viscosity as well as the overall crystal growth rate for the as-deposited GeTe nanoparticles. The obtained results pave the way for further development of phase-change memory based on GeTe with sub-lithographic sizes.

Local structure of amorphous Ag5In5Sb60Te30 and In3SbTe2 phase change materials revealed by X-ray photoelectron and Raman spectroscopic studies

Reversible switching between highly resistive (binary “0”) amorphous phase and low resistive (binary “1”) crystalline phase of chalcogenide-based Phase Change Materials is accredited for the development of next generation high-speed, non-volatile, data storage applications. The doped Sb-Te based materials have shown enhanced electrical/optical properties, compared to Ge-Sb-Te family for high-speed memory devices. We report here the local atomic structure of as-deposited amorphous Ag5In5Sb60Te30 (AIST) and In3SbTe2 (IST) phase change materials using X-ray photoelectron and Raman spectroscopic studies. Although AIST and IST materials show identical crystallization behavior, they differ distinctly in their crystallization temperatures. Our experimental results demonstrate that the local environment of In remains identical in the amorphous phase of both AIST and IST material, irrespective of its atomic fraction. In bonds with Sb (∼44%) and Te (∼56%), thereby forming the primary matrix in IST with a very few Sb-Te bonds. Sb2Te constructs the base matrix for AIST (∼63%) along with few Sb-Sb bonds. Furthermore, an interesting assimilation of the role of small-scale dopants such as Ag and In in AIST, reveals rare bonds between themselves, while showing selective substitution in the vicinity of Sb and Te. This results in increased electronegativity difference, and consequently, the bond strength is recognized as the factor rendering stability in amorphous AIST.

(Invited) Synaptic Plasticity in a Memristive Device below 500mV

While machine learning algorithms are being successfully employed for a wide variety of high level cognitive tasks, their wide-spread adoption on mobile platforms faces challenges because of the large network sizes and extremely long training times involved in learning underlying features of user-dependent data. In this paper, we first discuss the requirements and target specifications of nanoscale devices that could enable hardware platforms supporting on-chip, real-time learning based on third generation spiking neural networks. We then survey some recent developments from the field of emerging non-volatile memory technologies such as chalcogenide based phase change memory and other memristive devices that could be used to build cognitive learning systems. We will then discuss how we have implemented synaptic plasticity in a Cu/SiO2/W memristive device at voltages below 500 mV using bio-mimetic waveforms. These devices hold promise to herald the next generation of information processing systems that are inspired by the brain.

Redefining the Speed Limit of Phase Change Memory Revealed by Time-resolved Steep Threshold-Switching Dynamics of AgInSbTe Devices.

Although phase-change memory (PCM) offers promising features for a ‘universal memory’ owing to high-speed and non-volatility, achieving fast electrical switching remains a key challenge. In this work, a correlation between the rate of applied voltage and the dynamics of threshold-switching is investigated at picosecond-timescale. A distinct characteristic feature of enabling a rapid threshold-switching at a critical voltage known as the threshold voltage as validated by an instantaneous response of steep current rise from an amorphous off to on state is achieved within 250 picoseconds and this is followed by a slower current rise leading to crystallization. Also, we demonstrate that the extraordinary nature of threshold-switching dynamics in AgInSbTe cells is independent to the rate of applied voltage unlike other chalcogenide-based phase change materials exhibiting the voltage dependent transient switching characteristics. Furthermore, numerical solutions of time-dependent conduction process validate the experimental results, which reveal the electronic nature of threshold-switching. These findings of steep threshold-switching of ‘sub-50 ps delay time’, opens up a new way for achieving high-speed non-volatile memory for mainstream computing.

High-field electrical transport in amorphous phase-change materials

Electrical transport in chalcogenide-based phase change materials is an active area of research owing to the prominent role played by these materials in the field of information technology. Here, we present transport measurements (IV curves) obtained on line-cells of as-deposited amorphous phase change materials (Ge2Sb2Te5, GeTe, Ag4In3Sb66Te27) over a wide voltage and temperature range (300 K to 160 K). The well defined geometry of our devices enables a description of the transport behavior in terms of conductivity vs. electric field. At higher temperatures (300 K ≥ T ≥ 220 K) and low to intermediate fields (F < 20 V/μm), the data can be described within the framework of a previously developed model, which is based on multiple trapping transport together with 3D Poole-Frenkel emission from a two-center Coulomb potential. Based on this model, we observe a temperature dependence of the inter-trap distance, which we can relate to a temperature dependence in the occupation of the defect creating the Coulomb potential governing Poole-Frenkel emission. At higher fields and lower temperatures, the dependency of the IV curve on the electric field can be described by ln(I/I0) = (F/Fc)2. By combining this contribution with that of the Poole-Frenkel emission, we can show that the slope at high fields, Fc, is independent of temperature. We argue that models based on direct tunneling or thermally assisted tunneling from a single defect into the valence band cannot explain the observed behavior quantitatively.

Distinguishing mechanisms of morphological instabilities in phase change materials during switching

The process of dewetting of a thin film from a solid substrate is important for its scientific and technological relevance, but can be difficult to observe experimentally. We report on an experimental method that may be used to investigate morphological changes, including dewetting, during laser heat treatment of alloys used for phase change memory devices. We have used nanosecond-scale time-resolved imaging to differentiate between competing thin film instabilities in GeTe, a chalcogenide-based phase change material. It is shown that in the absence of nucleated dewetting, thin films of phase change alloys may be unstable, but that nucleated dewetting can lead to a more disrupted final state of the thin film.

Ferromagnetism modulation by phase change in Mn-doped GeTe chalcogenide magnetic materials

In this work, an effective method to modulate the ferromagnetic properties of Mn-doped GeTe chalcogenide-based phase change materials is presented. The microstructure of the phase change magnetic material Ge1−x Mn x Te thin films was studied. The X-ray diffraction results demonstrate that the as-deposited films are amorphous, and the crystalline films are formed after annealing at 350 °C for 10 min. Crystallographic structure investigation shows the existence of some secondary magnetic phases. The lattice parameters of Ge1−x Mn x Te (x = 0.04, 0.12 and 0.15) thin films are found to be slightly different with changes of Mn compositions. The structural analysis clearly indicates that all the films have a stable rhombohedral face-centered cubic polycrystalline structure. The magnetic properties of the amorphous and crystalline Ge0.96Mn0.04Te were investigated. The measurements of magnetization (M) as a function of the magnetic field (H) show that both amorphous and crystalline phases of Ge0.96Mn0.04Te thin film are ferromagnetic and there is drastic variation between amorphous and crystalline states. The temperature (T) dependence of magnetizations at zero field cooling (ZFC) and field cooling (FC) conditions of the crystalline Ge0.96Mn0.04Te thin film under different applied magnetic fields were performed. The measured data at 100 and 300 Oe applied magnetic fields show large bifurcations in the ZFC and FC curves while on the 5,000 Oe magnetic field there is no deviation.

Fault model and test procedure for phase change memory

Chalcogenide-based phase change memory (PCM) is a type of non-volatile memory that will most likely replace the currently widespread flash memory. Current research on PCM targets the integration feasibility, as well as the reliability of such memory technology into the currently used complementary metal oxide semiconductor (CMOS) process. Such studies identified special failure modes, known as disturbs, as well as other PCM specific faults. In this study, the authors identify these failures, analyse their behaviours and develop fault primitives/models that describe these faults accurately and effectively. In addition, the authors propose an efficient test algorithm, called March-PCM, to test for these faults and compare its performance to some previously developed test algorithms.

High-resolution mastering using AlNiGd metallic glass thin film as thermal absorption layer

We have succeeded in patterning narrow lines and dots with nano-scale dimensions of ZnS–SiO 2 using Al 90Ni 3Gd 7 metallic glass thin film as the thermal absorption layer. The laser thermal lithography technique was carried out using a semiconductor laser with 405 nm wavelength. The demonstrated resolution of the patterns produced is far beyond the diffraction limit of 200 nm given by our optical setup. The use of metallic glass as the thermal absorption layer has a number of advantages, including providing a cost-effective approach, high performance such as high selectivity and high aspect (height to width) ratio, loose environment requirement, reliable and stable compared with existing thermal absorption chalcogenide-based phase change layer materials, such as GeSbTe and AgInSbTe.

Electrical probe storage using Joule heating in phase change media

We demonstrate the feasibility of ultrahigh-density probe recording in chalcogenide-based phase change media. The multilayer medium has been optimized with respect to electrical, thermal, and tribological requirements. Design of the multilayer takes into account the nonlinear dependence of the electrical properties of the phase change layer with respect to both electrical field and temperature. Memory dots as small as 15nm have been written and read repeatably. Data storage density of greater than Tbit∕in.2 density has been successfully achieved.