Ge2Sb2Te5 Thin Films Research Articles

Dual-Functional Nanoscale Devices Using Phase-Change Materials: A Reconfigurable Perfect Absorber with Nonvolatile Resistance-Change Memory Characteristics

Integration of metamaterial and nonvolatile memory devices with tunable characteristics is an enthusing area of research. Designing a unique nanoscale prototype to achieve a metasurface with reliable resistive switching properties is an elusive goal. We demonstrate a method to exploit the advantages of a phase-change material (PCM) as a metamaterial light absorber and a nanoscale data storage device. We designed and simulated a metamaterial perfect absorber (MPA) that can be reconfigured by adjusting the visible light properties of a chalcogenide-based PCM. The suggested perfect absorber is based on a Ge2Sb2Te5 (GST) film, and is tuned between two distinct states by heat treatment. Furthermore, we fabricated and characterized a resistive switching memory (ReRAM) device with the same features. The MPA/ReRAM device with a conventional metal/dielectric/metal structure (Ag/GST/Al2O3/Pt) consisted of arrays of Ag squares patterned on a GST thin film and an alumina-coated Pt mirror on a glass substrate. Based on the numerical data, amorphous GST showed perfect absorbance in the visible spectrum, whereas, crystalline GST showed broadband perfect absorbance. The fabricated ReRAM device exhibited uniform, bidirectional, and programmable memory characteristics with a high ON/OFF ratio for nonvolatile memory applications. The elucidated origin of the bipolar resistive switching behavior is assigned to the formation and rupture of conductive filaments.

Optical Study of Ge1Sb2Te4 and GeSbTe thin films

Extensive optical study has been carried out on as-deposited and annealed thin films of Ge1Sb2Te4 and GeSbTe chalcogenides prepared through vacuum thermal evaporation on glass substrates. These films are found to be amorphous in nature for as-dep samples. The modifications that manifest in these films upon annealing are thoroughly investigated in view of the optical parameters. UV–vis spectroscopy has been utilized as the major tool for computation of optical constants, optical bandgap energy, urbach energy, skin depth, optical conductivity etc. Theoretical models for optical analysis like Wemple Di-Domenico model and Single Sellmier model have been applied to evaluate the dispersion energy parameters and oscillator parameters. Comparative analysis has been carried out which highlights the phase change characteristics of Ge1Sb2Te4 and non-phase change behavior of GeSbTe.

Femtosecond-laser-induced modifications of Ge2Sb2Te5 thin films: Permanent optical change without amorphization

Modifications induced by ultrashort laser pulses have been investigated in crystalline Ge2Sb2Te5 thin films. The observations comprise optical and structural changes, amorphization, and ablation. By combining optical microscopy and cross-sectional scanning- and transmission electron microscopy, it is found that the threshold for permanent change of the optical properties of Ge2Sb2Te5 – responsible for the reduced optical reflectivity – is slightly lower than that for amorphization; no further change in reflectivity is seen upon amorphization. The laser-fluence thresholds for amorphization and change of the optical properties both show a strong dependence on film thickness that can be explained thermally by two-temperature simulations. In the case of sufficiently thick films, two distinct low- and high-fluence ablation thresholds are found. The mechanisms of the ablation thresholds are discussed, and it is concluded that ablation in the low-fluence regime proceeds by photomechanical spallation.

Thermal conductivity measurement of Ge2Sb2Te5 thin film using improved 3ω method

Thermal conductivity measurement of Ge2Sb2Te5 thin film using improved 3ω method

Preparation of Ge-Sb-Te Thin Films by Tellurization of Ge-Sb Thin Film for Phase-Change Random-Access Memory Application

We report the preparation of crystalline Ge-Sb-Te (GST) compounds by pulsed tellurization annealing of amorphous sputtered Ge-Sb films in a di(tert-butyl)tellurium [(tBu)2Te] atmosphere. The cycle of feeding (tBu)2Te into the annealing chamber for 1 s and then pumping down for 60 s was repeated, which was to avoid tellurium deposition. We investigated the effect of annealing temperature and annealing cycle number on the composition and crystal structure of GST thin films. The thin film was not wholly tellurized at 160 or 190°C, while the entire film was uniformly tellurized and crystallized at 220°C to form a GST film having a composition ratio close to that of Ge1Sb2Te4. The phase change temperature gradually decreased as tellurization proceeded. In the case of 30 cycles at 220°C, in which the entire Ge-Sb film was tellurized, the resistance reduction was observed at 150°C. The diffusion of germanium from GST into the substrate was also discussed.

Laser-induced modification and formation of periodic surface structures (ripples) of amorphous GST225 phase change materials

In this paper, we have studied the crystallization behavior of amorphous Ge2Sb2Te5 thin films upon irradiation with femtosecond laser pulses. Crystalline and melt-quenched amorphous regions were produced by exposure to laser multipulses, and were characterized by the optical microscopy and by the micro-Raman spectroscopy. Using irradiation with single laser pulses of varying fluence it was verified that crystallization was not possible, requiring minimum ten pulses for triggering partial crystallization. We obtained laser-induced periodic surface structures in the form of near-subwavelength ripples of two types with different periods. It was found that the fraction of the crystalline phase is higher in the ridges of the ripples compared to the valleys. We used the model of heterogeneous crystallization for bodies with different curvatures of the boundary surface to explain the observed effect of the phase transformation in laser-induced periodic surface structures.

A phase-change thin film-tuned photonic crystal device

This paper reports a tunable photonic device that incorporates a thin layer of phase-change material, Ge2Sb2Te5 (GST), in a photonic crystal (PC) structure. The PC structure is based on a one-dimensional grating waveguide with a metal cladding. The metal-cladded PC structure supports a guided-mode resonance (GMR) that selectively absorbs light at a particular wavelength. Inserting the GST material into the gating waveguide makes it possible to control the GMR mode. Here, the GST-PC device was numerically designed and optimized to obtain significant tuning of the GMR mode around 1550 nm. The tuning phenomena were experimentally demonstrated by the heat-induced phase change between crystalline and amorphous phases of the GST thin film. A spectral shift of the resonant wavelength from 1440 to 1610 nm was achieved via the crystallization process. The phase tuning of GST exhibits good repeatability as demonstrated by switching between amorphous and crystalline phases of GST for multiple cycles. The GST-PC device represents a new approach for tuning optical resonances with potential applications including but not limited to integrated photonic circuits, optical communications, and high-performance optical filters.

Effect of Ag mixing in thermoelectric Ge2Sb2Te5 thin films

Ag-added Ge2Sb2Te5 thin films were prepared by a pulsed-DC co-magnetron sputtering process using Ge2Sb2Te5 and Ag targets. The effect of variation in Ag content through variable power on Ag target was studied on the microstructural and thermoelectric properties of as-deposited and fast annealed thin films at 400 °C in a vacuum. It is found that Ag addition induces enhancement in conductivity. When the power on Ag target is exceeded to a critical power, Ag8GeTe6 phase formation occurs mixed with Ge2Sb2Te5 cubic structure which limits the thermoelectric performance. Best conditions provide the films having a lowest electrical resistivity as 0.98 × 10−4 Ω cm and the maximum power factor as 5.83 × 10−3 W m−1 K−2.

Unique interface-driven crystallization mechanism and element-resolved structure imaging of ZnO-Ge2Sb2Te5 nanocomposites

Amorphous Ge2Sb2Te5 thin films doped with ZnO were prepared and their crystallization behaviors were investigated. Our results showed that thermal annealing of amorphous ZnO-Ge2Sb2Te5 nanocomposites produced face centered-cubic Ge2Sb2Te5-nanocrystals embedded between interfaces. An increase in crystallization temperature and electrical resistance ratio were attributed to the increase of specific interfacial energies and inhomogeneous strain at the oxide/Ge2Sb2Te5 interfaces. The formation of the interfaces not only accelerated crystallization, but also limited the grain growth due to the one-dimensional growth mode. We proposed a crystallization model to elucidate the derived kinetic mechanism of the nanocrystal growth in the Ge2Sb2Te5 matrix. These experimental observations suggest that ZnO-Ge2Sb2Te5 nanocomposites are a good candidate for the applications in phase change memory.

Enhanced optical properties of Sn-doped Ge2Sb2Te5 thin film with structural evolution

The microstructure, texture and optical properties of Sn doped Ge2Sb2Te5 thin films were investigated in this work. The correlation between grain orientation and optical reflectivity of Sn doped Ge2Sb2Te5 thin films has been obtained. Thin films with different compositions were deposited on SiO2/Si (100) wafer using magnetron sputtering method. Both the composition and thickness of thin films were determined using Rutherford backscattering spectrometry (RBS). It appears that face-centered-cubic (fcc) and close-packed-hexagonal (hcp) structure coexisted in Sn doped Ge2Sb2Te5 thin films. Rietveld structure refinement indicates that the lattice constant of the fcc structure increased with increasing Sn content, while the lattice constant of the hcp structure remains nearly unchanged. Sn doping also leads to the appearance of defects and disordered local arrangement in Ge2Sb2Te5 films. Furthermore, the crystalline optical reflectivity of Sn doped Ge2Sb2Te5 thin films increased, which is attributed to texture formation. The texture component of the hcp structure Ge2Sb2Te5 thin films transformed from cylindrical {01-10} to basal {0001} during the addition of Sn atoms. Accordingly, we proposed a model of texture and optical reflectivity, that is, the basal texture facilitates the high crystalline optical reflectivity of Ge2Sb2Te5 thin films thanks to the six-fold rotational symmetry in the hcp structure.

Microstructure and crystallization kinetics of Ge2Sb2Te5–Sn phase change materials

In this paper, the effects of Sn doping on the microstructure and electrical properties of amorphous and crystalline Ge2Sb2Te5 thin films are reported. The thin films bonding states are measured through X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy, which prove that the bonding states of Ge2Sb2Te5–Sn thin films regularly change with the additions of various Sn contents. Sn atoms alter the chemical surrounding of Te atoms significantly, having a low impact on the bonding environment of Sb atoms, since the additional Sn atoms bond with Te atoms and lead to the SnTe phase formation. Both the homogeneity of bonding characteristics and crystallinity in Ge2Sb2Te5–Sn thin films are improved. The atomic arrangement of the crystalline states Ge2Sb2Te5–Sn thin films is also obtained. It could be inferred that the Sn addition lead to higher crystalline interplanar spacing and arrangement of many disordered atoms. Furthermore, the corresponding activation energy value (EA) of Ge2Sb2Te5–Sn thin films is calculated. This display that the EA values of Ge2Sb2Te5–Sn increase compared to the pure Ge2Sb2Te5 thin films, since the Ge2Sb2Te5–Sn thin film structures are stabilized due to the compressed bonds and various bonding states.

Improvement in cyclic operation of unit pixel device using Sb-excess Ge2Sb2Te5 thin films for hologram image implementation

The effects of the atomic composition of Ge2Sb2Te5 (GST), especially the Sb-excess phase, on device reliability were investigated for applications of spatial light modulator (SLM) devices using phase-change materials. Sb-excess GST phases were prepared by cosputtering with two target systems. For Sb-excess GST thin films, amorphous phases were found to be more directly crystallized to more conductive hexagonal-close-packing phases at higher crystallization temperatures without the appearance of a less conductive face-centered-cubic phase with increasing amount of incorporated Sb. The marked changes in crystallization behavior and improved thermal stability were suggested as critical benefits for enhancing the device durability during cyclic operations, considering that the interdiffusion within the GST could be the main origin of the final device failure. The number of cyclic operations of the unit pixel device was markedly improved to more than 103 when the amount of excess Sb was modulated from 0 to 21 at. %.

Highly sensitive integrated flexible tactile sensors with piezoresistive Ge2Sb2Te5 thin films

Flexible tactile sensor has been extensively investigated as a key component for emerging electronics applications such as robotics, wearable devices, computer hardware, and security systems. Tactile sensors based on various one-dimensional materials have been widely explored. However, precise control of the direction and distribution of these nanomaterials remains a great challenge, and it has been difficult to scale down the device. Here, we introduce highly sensitive integrated flexible tactile sensors based on uniform phase-change Ge2Sb2Te5 (GST) thin films that can scale device size down, at least, to micrometer range. Significant piezoresistive effect has been observed in GST-based sensors, showing a giant gauge factor of 338. A proof of concept 5 × 5 sensor array functioning as a touch panel has been demonstrated. Also, the flexible GST tactile sensor has been utilized for monitoring of radial artery pulse. In addition to the well-known tunable electrical and optical properties, the piezoresistive GST films provide a versatile platform for the integration of sensing, recording, and displaying functions.

Microstructure evolution of the crystallization of amorphous Ge2Sb2Te5 thin films induced by single picosecond pulsed laser

Phase-change materials (PCMs) have been widely used in optical and electronic applications for their extraordinary properties. However, the procedure of fast transition about PCMs is still not clear for the time limit. Pulse laser, especially ultra-short pulse laser, is an excellent tool to study the crystallization characteristics of PCMs for its high heating rate. In this paper, picosecond pulsed laser was employed to study the crystallization of amorphous Ge2Sb2Te5 (a-GST) thin films via the evolution of morphology and microstructure. The threshold for inducing crystallization was 7.9 mJ/cm2 and amorphization occured at 17.2 mJ/cm2. With the aid of high-resolution transmission electron microscopy (HRTEM), the growth of grains were observed to own a preferable growth direction of [100]. The grain size and laser fluence showed an exponential relationship under the ps laser pulse, and the maximal grain reached to ~5 μm when approaching the melting point. The morphology and grain distribution of GST film under 18.8 mJ/cm2 indicated a solid-state phase transition. Besides, the matrix of a-GST film changed into crystalline state even though the surface melted into amorphous state when irradiated by high laser pulse. Finally, the reason for fast transition of a-GST film under ultra-short laser pulse was discussed in terms of heating rate and temperature. The present study is fundamental for the understanding of the fast phase transition of PCMs.

Study on the crystallization mechanism of amorphous Ge2Sb2Te5 thin films induced by a short single pulsed laser

The crystallization mechanism in an amorphous Ge2Sb2Te5 (a-GST) thin film induced by a single pulsed laser was investigated in this paper. The finite element simulation and x-ray diffraction analysis showed that two kinds of crystallization mechanisms performed for the laser-induced phase transition of a-GST, that is, the solid-state phase transition took place at a lower laser fluence while the liquid–solid phase transition occurred at a relatively high laser fluence. Transmission electron microscopy observations showed that the microstructure in the liquid–solid phase transition was more uniform as compared to that in the solid-state phase transition because of poor atom diffusion. Crystallization characteristics at different laser fluences and film thickness were elucidated. It was found that at a lower laser fluence a thinner film had the better crystallinity owing to thermal convection, while at a higher laser fluence a thicker film showed the better crystallinity due to the release of latent heat in the liquid–solid phase transition. These findings enable a deep understanding of ultra-fast phase transition induced by laser irradiation.

Activation energy of metastable amorphous Ge2Sb2Te5 from room temperature to melt

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.

Microstructural control by substrate heating in Pulse-DC sputtering induced thermoelectric Ge2Sb2Te5 thin films

Optimization of substrate heating during sputtering processes is essential to obtain desired microstructures of deposited thin films, as it provides the required energy flux during the nucleation and growth. In this work, Ge2Sb2Te5 thin films were prepared by a pulsed-DC magnetron sputtering process at optimized plasma conditions (pulsed frequency and pulse reversal time). The effect of substrate heating, in temperature range of 250–450 °C, was systematically investigated on the process throughput and various properties i.e. microstructure, morphology, atomic composition, carrier concentration, mobility and Seebeck coefficient of deposited films. The substrate heating was found to be required to obtain films in cubic crystalline phase. Through the optimization of substrate temperature, process throughput and surface properties, electrical properties as well as thermoelectric power factors were enhanced. The maximum power factor value of thin films was achieved as 0.77 mW m–1 K–2 for the substrate temperature as 400 °C.

Laser-Induced Modification of the Surface of Ge2Sb2Te5 Thin Films: Phase Changes and Periodic-Structure Formation

Submicron periodic lattices are formed at the surface of phase-change-memory film materials based on the complex chalcogenide Ge2Sb2Te5 when exposed to nanosecond laser pulses. The geometric characteristics and structural properties of laser-induced lattices are studied by optical and atomic-force microscopies and Raman spectroscopy. It is shown that, at appropriately chosen parameters of exposure to laser radiation, it is possible to implement periodic modulation of the refractive index in the structures formed. Modulation is due to the postexposure solidification of grating ridges and valleys in different phase states, whose dielectric constants widely differ from each other. In the vicinity of the maxima of the wavy structure, the amorphous state is mainly formed, whereas in the region of minima, the Ge2Sb2Te5 structure corresponds mainly to the crystalline phase.

The microstructural changes of Ge2Sb2Te5 thin film during crystallization process

Phase change memory is known as the most promising candidate for the next generation nonvolatile memory technology. In this paper, the microstructural changes of Ge2Sb2Te5 film, which is the most common choice of phase change memory material, has been carefully studied by the combination of several characterization techniques. The combination of resistance measurements, X-ray diffraction, Raman spectroscopy and X-ray reflectivity allows us to simultaneously extract the characteristics of microstructural changes during crystallization process. The existence of surface/interface Ge2Sb2Te5 layer has been proposed here based on X-ray reflectivity measurements. Although the total film thickness decreases, as a result of the phase transition from amorphous to metastable crystalline cubic and then to the stable hexagonal phase, the surface/interface thickness increases after crystallization. Moreover, the increase of average grain size, density and surface roughness has been confirmed during thermal annealing process.

Multi-level coding-recoding by ultrafast phase transition on Ge2Sb2Te5 thin films

Quickly switching among different states (levels) is crucial for reconfigurable metamaterials and devices. In this study, the dynamics of establishment and transformation of five amorphous or near-amorphous intermediate states with obvious optical contrasts on Ge2Sb2Te5 phase-change thin films driven by ultrashort laser pulses were investigated using real-time reflectivity measurements. The reversible coding-recoding among the five optical levels was realized by using single-shot picosecond laser pulses with designed fluences. The optical constants, crystalline states and surface morphologies before and after ultrafast multi-level coding were also compared and analyzed. These results may lay a foundation for the further design and application of dynamically reconfigurable optical/photonic devices.