Ge2Sb2Te5 Films Research Articles

Elastic properties and lattice thermal conductivity of amorphous Ge2Sb2Te5 and GeTe thin films

This study reports on the elastic properties and the lattice thermal conductivity of amorphous Ge2Sb2Te5 and GeTe thin films using surface Brillouin scattering. It is demonstrated that this method allows for the determination of the isotropic elastic constants from the measured surface acoustic phonon frequencies. We found elastic constants of C11=48.7/43.6 and C44=14.3/9.4GPa for GeTe and Ge2Sb2Te5, respectively. These results suggest acoustic hardening in GeTe compared to Ge2Sb2Te5 films, and this was supported by the derived, shear, and Young's moduli. The measured longitudinal and transverse velocities were used to determine the lower limit of the lattice thermal conductivity. In general, both chalcogenides alloys exhibit low lattice thermal conductivities of κmin<0.50Wm−1K−1. This could be beneficial for thermal management in phase-change memory devices and for thermoelectric application.

Evolution of Low‐Frequency Vibrational Modes in Ultrathin GeSbTe Films

In article number 2000434, Eugenio Zallo and co-workers report a methodology based on the Raman technique for the measurement of the layer number in ultrathin GeSbTe films obtained by molecular beam epitaxy. The experimental and theoretical evolution of the in-plane low-frequency vibrational modes of Ge2Sb2Te5 from bulk down to a single lamella is demonstrated. The energy shift is mainly related to the weak van der Waals interactions between the layers.

Mössbauer Studies of the Local Surrounding of Atoms in Amorphous and Crystalline Ge2Sb2Te5 Films

Mössbauer Studies of the Local Surrounding of Atoms in Amorphous and Crystalline Ge2Sb2Te5 Films

Phase-change reconfigurable metasurface for broadband, wide-angle, continuously tunable and switchable cloaking.

Being invisible at will has fascinated humanity for centuries and it has become more tangible with the development of metasurfaces, which have demonstrated the extraordinary ability of wavefront manipulation. However, state-of-the-art invisibility cloaks typically work in a deterministic system with a limited bandwidth and small incident angle ranges. Here, by integrating the phase-change material of Ge2Sb2Te5 and the wavefront tailoring functionality of a reflective metasurface, we have achieved a unique carpet cloak that is endowed with broadband invisibility from 6920 to 8220 nm, fully concealing objects over a wide angular span of ±25° and a prominent radar cross-section reduction. Furthermore, the central cloaking wavelength can be continuously tuned with Ge2Sb2Te5 film under different intermediate phases by precisely controlling external stimuli, which will provide a flexible and encouraging way to achieve active features once fabricated. Simulation results also show that the cloaking bandwidth can be significantly extended by triggering Ge2Sb2Te5 from the amorphous to crystalline states. Importantly, the hybrid metasurface can realize switching of "ON" and "OFF" states in terms of cloaking features by converting Ge2Sb2Te5 from the amorphous to the crystalline state. To the best of our knowledge, this is the first metasurface carpet cloak that utilizes the phase-change material of Ge2Sb2Te5 to achieve ultra-broadband, wide-angle, continuously tunable and switchable cloaking with low profiles, light weights, and easy access. This design of a reconfigurable cloak is expected to find potential applications in various areas such as vehicle cloaking, illusions and so on.

Effect of Mg35Sb65 interlayer on the thermal stability and scaling of Ge2Sb2Te5 phase change thin film

The effects of Mg35Sb65 layer on the thermal stability and thickness change rate of Ge2Sb2Te5 in superlattice films were studied. Compared with monolayer Ge2Sb2Te5 films, Mg35Sb65/Ge2Sb2Te5 films had higher resistance and crystallization temperature. The thermal stability and data retention capability were improved significantly. The fundamental reason of the change of film resistance was explained from the change of energy band gap. According to the reliability of phase change memory devices, the thickness change rate of thin films was analyzed in detail.

Nonvolatile and reconfigurable tuning of surface lattice resonances using phase-change Ge2Sb2Te5 thin films

We propose a novel configuration for nonvolatile and reconfigurable tuning of surface lattice resonances (SLRs) based on thin films of phase-change material Ge2Sb2Te5 (GST). The configuration is composed of gold nanorod array embedded in a thin GST film. Results show that, an extremely wide SLR wavelength tuning range reaching 1600 nm, large tuning figure of merit of 21, and high SLR quality factors of 47 and 17 can be achieved in the mid-infrared regime when GST transits from the amorphous state into the crystalline state. The effects of the GST layer thicknesses on the SLR tuning performance are discussed, and the performances of several modified configurations are compared. We expect this work will advance the engineering of SLR tuning based on phase-change materials and promote its applications especially in nanolasing and narrowband filtering.

Local structure of amorphous and crystalline Ge2Sb2Te5 films

An effective way to examine structural rearrangement in solids is Mössbauer spectroscopy. A key requirement to Mössbauer probes used for these purposes is the possibility of their localization in a certain site of the crystal lattice or in the structural network of the amorphous material. When absorption spectroscopy is used to examine the local structure of crystalline and amorphous Ge2Sb2Te5 films, this requirement is satisfied for 119Sn isotope. Tin atoms 119Sn isovalently substitute germanium atoms in the structure of both vitreous and crystalline germanium tellurides. The absorption Mössbauer spectroscopy on 119Sn impurity centers shows that germanium atoms in the structure of amorphous and polycrystalline Ge2Sb2Te5 films have different local symmetries (tetrahedral in the amorphous phase and octahedral in the crystalline).

Local Structure and Anti-Structural Defects of Tin in Amorphous and Crystalline Ge2Sb2Te5 Films

The absorption Mossbauer spectroscopy on 119Sn impurity centers shows that germanium atoms in the structure of amorphous and polycrystalline Ge2Sb2Te5 films have different local symmetries (tetrahedral in the amorphous phase and octahedral in the crystalline). By emission Mossbauer spectroscopy on 119mSn impurity centers formed after the radioactive decay of 119Sb or 119mTe parent atoms, tin antisite defects at antimony and tellurium sites of crystalline Ge2Sb2Te5 films are identified.

Nonvolatile, Reconfigurable and Narrowband Mid-Infrared Filter Based on Surface Lattice Resonance in Phase-Change Ge2Sb2Te5.

We propose a nonvolatile, reconfigurable, and narrowband mid-infrared bandpass filter based on surface lattice resonance in phase-change material GeSbTe. The proposed filter is composed of a two-dimensional gold nanorod array embedded in a thick GeSbTe film. Results show that when GeSbTe transits from the amorphous state to the crystalline state, the narrowband reflection spectrum of the proposed filter is tuned from 3.197 m to 4.795 m, covering the majority of the mid-infrared regime, the peak reflectance decreases from 72.6% to 25.8%, and the corresponding quality factor decreases from 19.6 to 10.3. We show that the spectral tuning range can be adjusted by varying the incidence angle or the lattice period. By properly designing the gold nanorod sizes, we also show that the quality factor can be greatly increased to 70 at the cost of relatively smaller peak reflection efficiencies, and that the peak reflection efficiency can be further increased to 80% at the cost of relatively smaller quality factors. We expect that this work will advance the engineering of GeSbTe-based nonvalatile tunable surface lattice resonances and will promote their applications especially in reconfigurable narrowband filters.

Stress-Induced In Situ Modification of Transition Temperature in VO2 Films Capped by Chalcogenide.

We attempted to modify the monoclinic–rutile structural phase transition temperature (Ttr) of a VO2 thin film in situ through stress caused by amorphous–crystalline phase change of a chalcogenide layer on it. VO2 films on C- or R-plane Al2O3 substrates were capped by Ge2Sb2Te5 (GST) films by means of rf magnetron sputtering. Ttr of the VO2 layer was evaluated through temperature-controlled measurements of optical reflection intensity and electrical resistance. Crystallization of the GST capping layer was accompanied by a significant drop in Ttr of the VO2 layer underneath, either with or without a SiNx diffusion barrier layer between the two. The shift of Ttr was by ~30 °C for a GST/VO2 bilayered sample with thicknesses of 200/30 nm, and was by ~6 °C for a GST/SiNx/VO2 trilayered sample of 200/10/6 nm. The lowering of Ttr was most probably caused by the volume reduction in GST during the amorphous–crystalline phase change. The stress-induced in in situ modification of Ttr in VO2 films could pave the way for the application of nonvolatile changes of optical properties in optoelectronic devices.

Peculiarities of the synthesis of GST films by magnetron sputtering for nonvolatile optical memory cells

The structure and optical properties of thin films of the chalcogenide semiconductor Ge2Sb2Te5, deposited by magnetron sputtering, was studied. A significant change in the topology and optical properties of the films is demonstrated with varying deposition conditions. It is shown that an increase in the deposition time leads to an increase in the surface roughness. Doping with nitrogen during deposition leads to a smoothing of the film surface, and also provides a strong change in the band structure and optical properties.

Influence of the Degree of Crystallinity on the Dispersion of the Optical Parameters of Ge2Sb2Te5 Phase-Change Memory Thin Films

Abstract—Extensive studies of Ge2Sb2Te5 material are associated with the possibility of producing multilevel nonvolatile elements for high-speed integrated optical functional circuits. The principle of multilevel recording in such devices is based on the formation of partially crystallized regions with substantially different optical properties in Ge2Sb2Te5 thin films. To predict the parameters of the effect initiating phase transformations and to reliably provide a reversible transition between many logic states, it is necessary to have reliable data on the optical characteristics of Ge2Sb2Te5 thin films in states with different degrees of crystallinity and on the conditions of attainment of such states. In the study, the influence of the phase state of Ge2Sb2Te5 films on the extinction coefficient and refractive index and variations in the optical band gap in relation to the temperature of heat treatment are investigated. Ge2Sb2Te5 thin-film samples are examined by means of atomic-force microscopy, X-ray phase analysis, and energy-dispersive microanalysis to determine the film thickness, morphology, phase state, and composition. By spectroscopic ellipsometry, the spectra of the ellipsometric angles ψ and Δ (the amplitude and phase components of the light wave) are obtained, and the extinction coefficient and refractive index are determined. The influence of the layer models and mathematical models on calculation of the dispersions of the optical parameters of Ge2Sb2Te5 films is considered. A substantial increase in the extinction coefficient and refractive index at the wavelength 1550 nm on heat treatment at temperatures higher than 200°С is established. It is shown that the optical band gaps of Ge2Sb2Te5 thin films in the amorphous and crystalline states are 0.71 and 0.47 eV, respectively. It is found that the dependences of the extinction coefficient, refractive index, and band gap on the degree of crystallinity of Ge2Sb2Te5 thin films are close to linear.

Evolution of Low‐Frequency Vibrational Modes in Ultrathin GeSbTe Films

GeSbTe (GST) phase‐change alloys feature layered crystalline structures made of lamellae separated by van der Waals (vdW) gaps. This work sheds light on the dependence of interlamellae interactions at the vdW gap on film thickness of GST alloys as probed by vibrational spectroscopy. Molecular beam epitaxy is used for designing GST layers down to a single lamella. By combining density‐functional theory and Raman spectroscopy, a direct and simple method is demonstrated to identify the thickness of the GST film. The shift of the vibrational modes is studied as a function of the layer size, and the low‐frequency range opens up a new route to probe the number of lamellae for different GST compositions. Comparison between experimental and theoretical Raman spectra highlights the precision growth control obtained by the epitaxial technique.

Changes in Resistance and Bandgap of V2O5 and Ge2Sb2Te5 during Phase Transition

V2O5 and Ge2Sb2Te5 (GST) films have been prepared by magnetron sputtering, and their transformation from amorphous to crystalline state studied by in situ resistance temperature measurements. During the heating process, a two-step transition was observed for both films, revealing three resistances corresponding to amorphous, intermediate, and crystalline states. The two phase-transition temperatures were 371.1°C and 394.0°C for V2O5, much higher than the values of ∼ 172.4°C and 240.4°C for GST. Compared with GST, the resistance of V2O5 was two orders of magnitude higher. The changes in the resistance and bandgap of V2O5 and GST during phase transition were investigated in detail. A phase-change memory device based on V2O5 was fabricated. Compared with GST, V2O5 exhibited a lower operating power and shorter phase-transition time. The application potential of V2O5 and GST for multilevel storage was compared.

Reversible phase-change characteristics and structural origin in Cr doped Ge2Sb2Te5 thin films

Phase-change material is the key factor in phase change storage technology, among which Ge2Sb2Te5 (GST) possesses relatively excellent comprehensive performance. In this work, Cr dopants with different concentrations are introduced into GST to further improve the amorphous thermal stability for high-temperature application. The reversible phase-change processes in Cr-doped GST (Cr-GST) films are investigated by femtosecond laser pulses and the structural origin is analyzed. It is found that the thermal stability of Cr-GST film is greatly improved with the increase of Cr-dopants. The 10-year-data retention (T10-year) of Cr-GST film is higher than 120 °C, which is much larger than that of GST and is able to meet the demand of high-temperature applications such as automotive electronics. In SET process, high fluence will probably introduce the competition of melting-quenched amorphization, which may lead to the unexpected decrease of reflectivity. In RESET experiments, the necessary pulse number for amorphization is much fewer and even one pulse is enough at high fluence. The necessary amorphization energy for Cr-GST film with the Cr atom concentration of 10 % is as low as that of GST film, indicating promising application in the fields of high temperature and low power consumption.

Reconfigurable Flat Optics with Programmable Reflection Amplitude Using Lithography‐Free Phase‐Change Material Ultra‐Thin Films

AbstractA very large dynamic optical reflection modulation from a simple unpatterned layered stack of phase‐change material ultra‐thin films is experimentally demonstrated. Specifically, this work demonstrates that properly designed systems comprising deeply subwavelength GeSbTe (GST) films, a dielectric spacer, and a metallic mirror produce a dynamic modulation of light in the near‐infrared from very strong reflection (up to ) to perfect absorption () by simply controlling the crystalline state of the phase‐change material. While the amplitude of modulation experimentally reaches an optical contrast higher than 104, intermediate levels of reflection in between extreme values can also be actively encoded, corresponding to partial crystallization of the GST layer. Several layered system designs are further explored and guidelines are provided to tailor the efficient wavelength range, the angle of operation, and the degree of crystallization leading to perfect absorption.

Impact of Nitrogen on the Crystallization and Microstructure of Ge‐Rich GeSbTe Alloys

The influence of N concentration on the crystallization kinetics, microstructural evolution, and composition of Ge‐rich GeSbTe (GGST) alloys during thermal annealing, using X‐ray diffraction and scanning and transmission electron microscopy is reported. It is shown that the incorporation of N in GGST tends to slow down the phase separation, crystallization, and growth processes during annealing. This can be attributed to the reduced diffusivity of Ge, which interacts and quickly bonds with N. Technological advantages of N doping are also discussed, considering the increased stability of the amorphous phase with respect to its parent crystalline phase, finer microstructure, flatness of the GeSbTe (GST) films after crystallization, and disappearance of the low‐resistivity hexagonal phase at high temperature.

Observation of the femtosecond laser-induced ultrafast amorphization in Ge2Sb2Te5 films by dynamics spectrum

We demonstrate ultrafast amorphization in a Ge2Sb2Te5 (GST) film through a nonthermal process by femtosecond pump-probe experiments. A strong absorption peak in 532 nm is founded during the amorphization and the energy level corresponded is about 0.65~3.82 eV by calculations. First-Principles Calculations simulation experiments are used to prove the results of experiment and then explain the mechanism of amorphization of GST. Also, results of experiments show that the time to reach the absorption peak is decided by pulse width and wavelength but not the laser influence.

Investigation of the evolution of free carriers during femtosecond laser-induced ultrafast amorphization in nitrogen doped Ge2Sb2Te5 films

The femtosecond laser-driven ultrafast evolution of free carriers during the phase-change process of N-doped Ge2Sb2Te5 (N-GST) film was studied in this paper. And mechanism of initial absorption peak was calculated by two-temperature model. We found that the formation of Ge-N weaken the initial optical absorbance and decreased the concentration of free carriers, then improved the temperature of crystallization in N-GST. The doping(N) can reduce the grain size because of the easing of optical energy absorbance. At last, we inferred that, if the incident laser fluence is enough for phase transformation point, the transformation velocity of N-doped GST is faster than GST.

Multi-color modulation of phase-changing optical coatings

Now, various display screens transmit massive information to people all the time and become an important window of human-computer interaction. Solid state optical coating utilizes optical interference effect and inherent optical properties of materials to achieve multiple colors. In this paper, between Fabry-Perot (F-P) cavity coating with dielectric layer and strong interference effect (SIE) coating without dielectric layer were compared. Effect of film thickness on F-P cavity optical phase change coating was studied. The results show that the F-P cavity coating with a dielectric layer has more reflective states than the coating without a strong interference effect, that is, potential for optical coating display. In addition, in the F-P cavity coating, by controlling the thickness of SiO2 and Ge2Sb2Te5 (GST) film, the optical coating shows rich color due to the change of F-P optical cavity length and strong interference effect of materials. We also carried out a study of the flexible substrate compatibility and substrate color of the two structures, and found that they are well compatible with flexible materials, and F-P is more sensitive to substrate color than SIE structures in terms of substrate color. Due to the far subwavelength layer thickness, most phase shift is accumulated on reflection at the interfaces, which makes the reflectance profile robust under tilted incidence.