Crystallization Of Ge2Sb2Te5 Research Articles

Design of a wide bandwidth achromatic metalens based on phase change material GST with aperture sharing

This paper proposes the use of aperture sharing synergistic operation in the structure of the metalens, together with the rational selection of the scattering unit size according to the linear relationship between the output phase and frequency of the scattering unit, and the selection of the slope of the phase variation with frequency, combined with the modulation of the phase change material Ge2Sb2Te5 crystallization rate m value, in the set wavelength band, for different wavelengths of incident light through the metasurface can produce a uniform phase distribution. The method is used to achieve a polarization-insensitive achromatic metalens in the 9.5-13 μm continuous wavelength band by adjusting the m value of the phase transition material Ge2Sb2Te5 crystallization rate and producing a phase distribution with a uniform focal length for different wavelengths of incident light passing through the metasurface. The simulation results show that the focal length of the achromatic metasurface varies by 3.57 μm in the working band, with an error of about 4.3% from the set focal length, and the full width of the half-peak of the focal point at all wavelengths of incidence reaches the diffraction limit, and the focusing efficiency exceeds 60%. The achromatic metalens proposed in this paper provides a new idea for the design of achromatic metasurfaces and promotes the research of phase change materials in broadband achromatic metasurfaces.

Thermo-desorption measurements during N-doped Ge-rich Ge2Sb2Te5 crystallization

Ge-rich Ge2Sb2Te5 (GGST) is considered as one of the best candidates for industrial phase change memory production. GGST memory cells are generally embedded with Si or Ti nitride layers to prevent oxidation, as it leads to an undesired decrease of the GGST crystallization temperature. Furthermore, GGST films are usually doped with elements such as N, C, O, or Bi, aiming to delay GGST crystallization during the fabrication process as well as during memory cell operation. In this work, ultrahigh vacuum thermal desorption spectroscopy (TDS) was performed during isochronal annealing of a N-doped GGST film covered by a 10 nm-thick TiN x layer. Desorption is observed before GGST crystallization, but the comparison between TDS and in situ x-ray diffraction measurements shows that the main desorption peak, observed between 653 K and 703 K, occurs after GGST full crystallization. The most prominent desorbing species are Ar, N2, H2, and H. These results show that the TiN x polycrystalline layer cannot prevent N atoms from leaving the GGST layer during annealing, suggesting a progressive change of the N-doped GGST chemical composition during thermal annealing and crystallization.

Exploring “No Man's Land”—Arrhenius Crystallization of Thin‐Film Phase Change Material at 1 000 000 K s−1 via Nanocalorimetry

AbstractNon‐volatile phase‐change memory (PCM) devices are based on phase‐change materials such as Ge2Sb2Te5 (GST). PCM requires critically high crystallization growth velocity (CGV) for nanosecond switching speeds, which makes its material‐level kinetics investigation inaccessible for most characterization methods and remains ambiguous. In this work, nanocalorimetry enters this “no‐man's land” with scanning rate up to 1 000 000 K s−1 (fastest heating rate among all reported calorimetric studies on GST) and smaller sample‐size (10–40 nm thick) typical of PCM devices. Viscosity of supercooled liquid GST (inferred from the crystallization kinetic) exhibits Arrhenius behavior up to 290 °C, indicating its low fragility nature and thus a fragile‐to‐strong crossover at ≈410 °C. Thin‐film GST crystallization is found to be a single‐step Arrhenius process dominated by growth of interfacial nuclei with activation energy of 2.36 ± 0.14 eV. Calculated CGV is consistent with that of actual PCM cells. This addresses a 10‐year‐debate originated from the unexpected non‐Arrhenius kinetics measured by commercialized chip‐based calorimetry, which reports CGV 103−105 higher than those measured using PCM cells. Negligible thermal lag (<1.5 K) and no delamination is observed in this work. Melting, solidification, and specific heat of GST are also measured and agree with conventional calorimetry of bulk samples.

A Raman study on nanosecond-laser-induced multi-level switching of Ge2Sb2Te5 thin films

As one of the most successful nonvolatile phase change materials (PCMs), Ge2Sb2Te5 (GST) can be reversibly switched between amorphous and crystalline phases by nanosecond laser pulses, which enables its applications in optical disks and various emerging reconfigurable photonic devices. Instead of simply inducing temperature rising of GST thin film via light absorption, high-energy laser irradiation leads to modifications of molecular bonds, forming different local structures. We employed a nanosecond pulse laser with different pulse duration, energy and pulse numbers to induce multi-level switching of GST thin films. By employing Raman spectroscopy, we illustrated that GeTe4–nGen (n = 0, 1, 2) subsystem plays a decisive role in laser-induced crystallization of GST. Different from isothermal annealing, where SbmTe3 (m = 1, 2) subsystem contributes as well. This is evidenced by the suppression of SbmTe3 structure of an isothermally annealed film treated with additional laser crystallization. In addition, our investigations indicated that in GeTe4–nGen subsystem, Joule heating promotes the formation of Te-rich tetrahedral components. These findings provide more insights into the structural characteristics of GST.

Arrhenius behavior of crystallization at up to 184 000 K/s in Ge2Sb2Te5 thin films

We investigate the glass transition and crystallization of Ge2Sb2Te5 by nanocalorimetry on 32–57 nm layers at temperature scanning rates ranging from 23 to 184 kK/s. Crystallization follows the Arrhenius behavior observed at low heating rates, with an activation energy of 2.7 ± 0.1 eV. The crystal-growth velocity (>500 µm/s at 500 K) confirms fast values found in earlier work and by ab initio molecular dynamics simulations. These experiments also outline the importance of carrying out ultra-fast scanning calorimetry studies with samples in excellent thermal contact with the heating device.

Crystallization of Ge2Sb2Te5 under high hydrostatic pressures: Differences in nanoscale atomic ordering in as-deposited and pressure-induced amorphous phases

The crystallization process acts as a bottleneck to the development of phase-change memory devices. Here, we compare the crystallization of as-deposited and pressure-induced amorphous phases of Ge2Sb2Te5 under hydrostatic pressures up to 8.5 GPa. The as-deposited phase fully converts to a stressed metastable cubic phase (at ca. 135 °C) at pressures below 3 GPa and remains cubic up to the maximum temperature used (240 °C). At higher pressures, the as-deposited phase partially crystallizes directly into the stable hexagonal phase at a significantly lower temperature (110 °C), however a significant volume fraction of the amorphous phase remains even for temperatures as high as 240 °C. The intensities of the Bragg diffraction peaks dramatically decrease with increasing pressure, further underscoring the suppression of crystal growth. In stark contrast, the pressure-induced amorphous phase- due to memory effects originating from the crystalline phase - crystallizes at ambient conditions at a lower temperature than its as-deposited counterpart. Furthermore, the pressure-induced amorphous phase also fully transforms directly into the hexagonal modification at pressures up to ca. 5 GPa. At higher pressure (8.5 GPa), an orthorhombic phase is formed. Different from the as-deposited phase, the crystallization temperature of pressure-induced amorphous Ge2Sb2Te5 increases with pressure. The results reported here demonstrate that differences in nanoscale ordering in as-deposited (statistically ordered) and pressure-induced (chemically ordered) amorphous phases dramatically influence crystallization and will serve as a guideline for insightful development of phase-change devices.

Nanoscale mechanics of thermally crystallized GST thin film by in situ x-ray diffraction

The thermal crystallization of Ge2Sb2Te5 (GST) thin film is investigated by in situ x-ray diffraction (XRD). The combination of several x-ray diffraction techniques (a) in-plane XRD, (b) out of plane XRD, (c) high resolution XRD on the substrate, and (d) 2D high energy XRD allows the characterization of the mechanical behavior of GST upon thermal crystallization. A new method is proposed for the evaluation of experimental stress vs strain dependence in thermally crystallized GST. Nanoscale strain, macroscale stress, and nucleation/crystallization are fully described by the correlation of these techniques. Upon crystallization, a progressive tensile stress build-up is observed. Concomitant stress build-up is also observed both in the in-plane and out of plane directions of the film. The 2D high energy XRD demonstrates a homogeneous nucleation process and a progressive crystallization of the GST composed partially of amorphous and crystalline parts in the film. The GST nanomechanics is then characterized at the nanoscale (crystallites scale) and at the macroscale (film scale). By plotting the stress vs strain and assuming a Poisson ratio of 0.28, the mixture of phase results in a Young modulus between 9 GPa and 37 GPa for amorphous and crystalline matrices, respectively. Intermediate states with a partial amorphous/crystalline ratio results in intermediate values of the Young modulus. Finally, cross correlation between all XRD techniques gives EGST ≈ 34 GPa and υGST ≈ 0.34 for fcc crystalline GST.

Photoexcitation Induced Ultrafast Nonthermal Amorphization in Sb2Te3

Phase-change materials are of great interest for low-power high-throughput storage devices in next-generation neuromorphic computing technologies. Their operation is based on the contrasting properties of their amorphous and crystalline phases, which can be switched on the nanosecond time scale. Among the archetypal phase change materials based on Ge-Sb-Te alloys, Sb2Te3 displays a fast and energy-efficient crystallization-amorphization cycle due to its growth-dominated crystallization and low melting point. This growth-dominated crystallization contrasts with the nucleation-dominated crystallization of Ge2Sb2Te5. Here, we show that the energy required for and the time associated with the amorphization process can be further reduced by using a photoexcitation-based nonthermal path. We employ nonadiabatic quantum molecular dynamics simulations to investigate the time evolution of Sb2Te3 with 2.6, 5.2, 7.5, 10.3, and 12.5% photoexcited valence electron-hole carriers. Results reveal that the degree of amorphization increases with excitation, saturating at 10.3% excitation. The rapid amorphization originates from an instantaneous charge transfer from Te-p orbitals to Sb-p orbitals upon photoexcitation. Subsequent evolution of the excited state, within the picosecond time scale, indicates an Sb-Te bonding to antibonding transition. Concurrently, Sb-Sb and Te-Te antibonding decreases, leading to formation of wrong bonds. For photoexcitation of 7.5% valence electrons or larger, the electronic changes destabilize the crystal structure, leading to large atomic diffusion and irreversible loss of long-range order. These results highlight an ultrafast energy-efficient amorphization pathway that could be used to enhance the performance of phase change material-based optoelectronic devices.

Experimental observation of two-stage crystallization of Ge2Sb2Te5 amorphous thin films under the influence of a pulsed laser

The nature of crystallization in telluride based phase change memory materials is being actively discussed, and we present new experimental results on the effect of a pulsed laser treatment on the amorphous thin film crystallization of composition Ge2Sb2Te5 (GST225). It is shown that there are two ways of crystalline phase formation under femtosecond pulses excitation. At the moderate value of laser fluence, crystalline fraction forms inside the solid amorphous phase. This process is apparently two-stage, and this leads to formation of fine-grained polycrystalline material. The second way of crystallization proceeds at higher levels of the laser fluence, closer to the beam axis. In this case, larger grains are produced closer to the beam axis, and eventually no new nuclei can be formed due to high cooling velocity leading to inhibition of nucleation. The article also presents the results on the effect of the substrate type (metallic or oxide) and film thickness on crystallization, which is due to the light penetration depth and reflection from the film-substrate interface.

Miniature Multilevel Optical Memristive Switch Using Phase Change Material

Optical memristive switches can be used as optical latching switches in which the switching state is changed only by applying an electrical Write/Erase pulse and maintained without an external power supply. We demonstrate an optical memristive switch based on a silicon multimode interferometer structure covered with nanoscale-sized Ge2Sb2Te5 (GST) material on top. The phase change of GST is triggered by resistive heating of the silicon layer beneath GST with an electrical pulse. Experimental results reveal that the optical transmissivity can be tuned in a controllable and repeatable manner. Partial crystallization of GST is obtained by controlling the width and amplitude of the electrical pulses. Crucially, we demonstrate that both Erase and Write operations, to and from any intermediate level, are possible with accurate control of the electrical pulses. Our work marks a significant step forward toward realizing photonic memristive switches without static power consumption, which are highly demanded in integrated photonics.

In-situ observation of Ge2Sb2Te5 crystallization at the passivated interface

Abstract The unique properties of phase change materials (PCMs) in the amorphous and crystalline phase, and the feasibility of manipulating the phase transition process, have enabled the application of PCMs in optics and electronics systems, for data storage and neuromorphic computing. As the semiconductor device employing PCMs scales down with the complementary metal oxide semiconductor (CMOS) technology node, the impact of interface on the crystallization of PCMs becomes increasingly significant, that is of great importance for the system performance, e.g. phase change memory (PCM). In this work, crystallization of Ge2Sb2Te5 (GST) on passivated SiO2 surface is triggered by thermal pulses, in-situ observed in transmission electron microscope. A distinguishing heterogenous nucleation dominated crystallization behavior of GST has been verified at the GST/SiO2 interface. Meanwhile, in the in-situ prepared reactive GST/SiO2 interface, the interfacial covalent interaction between atoms in GST and SiO2 is believed to hamper the GST nucleation at the interface. Interfaces consist of the same materials but with different reactivity will lead to distinct crystallization behaviors of PCMs. By forming passive interface between PCMs and surrounding dielectric materials, promotion in nucleation and enhancement in operation speed can be expected in PCM with decreasing cell size.

Active-Tuning and Polarization-Independent Absorber and Sensor in the Infrared Region Based on the Phase Change Material of Ge2Sb2Te5 (GST)

Phase-change materials (PCMs), possessing thermo-optic and thermo-electric properties, have constantly enabled the rewritable optical data storage and the commercialized phase-change memory devices. In particular, Ge2Sb2Te5 (GST) has been considered for configurable photonics applications, such as active dielectric metasurface. In this paper, we report an active absorber with metal-insulator-metal (MIM) scheme with GST in the infrared region. The absorber consists of Al disk and reflective Al film with a GST spacer layer. Extraordinary absorption with peaks of more than 90% can be achieved over a broad bandwidth, attributing to highly confined gap surface plasmon resonance. In addition, the absorption can be tuned via adjusting the proportion of GST crystallization, which is a unique advantage to design active device. Meanwhile, the absorption is polarization-independent owing to its structural symmetry. Furthermore, we introduce the designed absorber to the application of sensing. This nearly perfect absorbing strategy offers great potential in sensing applications due to its flexibility and polarization-independence.

Crystal growth of Ge2Sb2Te5 at high temperatures

Phase-change materials (PCMs) have important applications in optical and electronic storage devices. Ge2Sb2Te5 (GST) is a prototypical phase-change material (PCM) employed in state-of-the-art storage-class memories. In this work, we investigate crystallization of GST at temperatures 600–800 K by ab initio molecular dynamics. We consider large models containing 900 atoms, which enable us to investigate finite-size effects by comparison with smaller models. We use the metadynamics method to accelerate the formation of a large nucleus and then study the growth of the nucleus by unbiased simulations. The calculated crystal growth speed and its temperature-dependent behavior are in line with recent experimental work.

Impact of interfaces on scenario of crystallization of phase change materials

Chalcogenide phase change materials (PCMs), such as Ge-Sb-Te alloys, have outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and, more recently, in Phase Change Random Access Memories. The latter are the most promising candidate to replace the current FLASH memories. The main feature of PCMs are fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties. Controlling their crystallization, however, is a challenge. In the present work, a new picture of the prototypical GeTe and Ge2Sb2Te5 thin film crystallization is provided from optical reflectivity, resistivity and Scanning Transmission Electron Microscopy studies of 100 nm thick films. The crystallization temperature Tx significantly varies as a function of surface oxidation. Whereas Ge2Sb2Te5 has been, and is still, the subject of many publications, the present results reveal that its crystallization mechanism was not fully understood. Heterogeneous crystallization of Ge2Sb2Te5 at the upper film surface, reported so far, results from a surface alteration. Adequately protected Ge2Sb2Te5 and GeTe crystallize through nucleation inside the amorphous phase. Preventing surface alteration due to oxidation as demonstrated in the present study, through suitable capping layer deposition methods, yields a spectacular increase in Tx above the values that have been reported in the literature. The present study demonstrates that interface engineering allows one to select the crystallization mechanism, and hence control the stability of the amorphous phase in PCMs.

Engineering the phase front of light with phase-change material based planar lenses.

A novel hybrid planar lens is proposed to engineer the far-field focusing patterns. It consists of an array of slits which are filled with phase-change material Ge2Sb2Te5 (GST). By varying the crystallization level of GST from 0% to 90%, the Fabry-Pérot resonance supported inside each slit can be spectrally shifted across the working wavelength at 1.55 µm, which results in a transmitted electromagnetic phase modulation as large as 0.56π. Based on this geometrically fixed platform, different phase fronts can be constructed spatially on the lens plane by assigning the designed GST crystallization levels to the corresponding slits, achieving various far-field focusing patterns. The present work offers a promising route to realize tunable nanophotonic components, which can be used in optical circuits and imaging applications.

A study on the crystallization behavior of Sn-doped amorphous Ge2Sb2Te5 by ultraviolet laser radiation

In this paper, the influence of Sn doping (0%, 8% and 14%) on the crystallization of Ge2Sb2Te5 was studied with the aid of an ultraviolet laser. The XRD analyses revealed that the addition of Sn maintained the NaCl-type structure of Ge2Sb2Te5 after crystallization but expanded the lattice parameter due to the smaller atomic radii of Ge replaced by Sn. Raman peaks (123, 150 and 110cm−1) moved towards lower wavenumbers (118, 137 and 104cm−1), which can be explained by the remarkable decrease of the binding energy from Ge–Te to Sn–Te. A remarkable increase in optical contrast from 15% to 40% was observed in the Sn-doped Ge2Sb2Te5 film after crystallization with both the isothermal annealing and laser radiance. While the optical contrast changed little for a fixed volume fraction of Sn-doped sample with the variation of laser fluence which is attributed to the crystallization mechanism induced by laser under different fluences is the same.

Crystallization process of in situ annealed Ge2Sb2Te5 films

Electron-diffraction radial distribution function (RDF) was used as a structural probe to study the process of crystallization of Ge2Sb2Te5 (GST) films annealed in situ. The GST thin film began to crystallize after a characteristic peak of 0.52nm appeared in the RDF, indicating the formation of third nearest neighbour ordering. The GST films preferentially form uniform nanosized grains. The similarities and differences in the structures of the amorphous phases and the polycrystalline phases are described.

Influence of laser energy on the crystallization of Ge2Sb2Te5 thin film prepared by pulsed laser deposition

AbstractFour Ge2Sb2Te5 thin films were prepared by pulsed laser deposition (PLD) at 58, 100, 140, and 190 mJ/pulse laser energy, respectively. The influence of laser energy on the crystallization of Ge2Sb2Te5 was investigated. The result shows that laser energy has evident effect on the crystallization of Ge2Sb2Te5, including crystallization temperature, incubation time, Avrami coefficient, and grain size. The sample prepared at 140 mJ/pulse laser energy has the lowest crystallization temperature, the shortest incubation time, the largest Avrami coefficient, and the maximum mean grain size. The result of X‐ray diffraction indicates that there are impurity‐phase peaks in the pattern of the sample prepared at 190 mJ/pulse laser energy. A possible reason was given for these phenomenons.

X-ray Stress Evaluation in Phase Change GeSbTe Material and TiW Electrodes

Stress generation and relaxation upon Ge2Sb2Te5 (GST) crystallization were studied by X-ray diffraction technique, which allows the stress in either GST or Ti3W7 (TiW) to be evaluated independently within TiW/GST/TiW multilayer film. The GST crystallization results in tensile stress in the GST film and additional compressive stress in the TiW film, due to volume shrinkage of the GST film. Moreover, the tensile stress in the GST film is significantly increased when it is sandwiched between TiW films. Interfacial effect is proposed to attribute the dependence of stress on the capping layer thickness.

Ce Doped-GeSbTe Thin Films Applied to Phase-change Random Access Memory Devices

AbstractA study on microstructure and electrical property of cerium (Ce)-doped Ge2Sb2Te5 (GST) layers for phase-change memory (PCM) application were presented. Ce doping does not suppress the resistivity of amorphous GST and the resistivity ratio of amorphous and crystalline GST remains at about 105. Further, Ce-doping escalates the recrystallization temperature (Tx) of GST from 159 to 236°C. Such a unique behavior would greatly benefit the preservation of signal contrast as well as the high-density signal storage and will not cause the increase of device writing current. X-ray diffraction (XRD) indicated that Ce doping stabilizes amorphous GST and suppresses the formation of hexagonal phase. Transmission electron microscopy (TEM) revealed Ce doping refines the grain size of GST. Kissinger's analysis found that Tx and activation energy (Ea) of phase transition for doped-GST both increase with the increase of Ce content. Isothermal experiment found the Ce doping increases temperature for 10-yr data retention from 76 and 170°C. This is attributed to the presence of Ce solutes in GST matrix that inhibits the grain growth during recrystallization.Static-mode electrical test on PCM device containing doped GST as the programming layer found that Ce incorporation indeed increases the switching threshold voltage (Vth). This confirmed that Ce doping effectively retards the crystallization of GST and improves the stability of amorphous GST.