Ge2Sb2Te5 Thin Films Research Articles

Application of Photo-Induced Chirality in Covert Authentication

A new technology to write and read covert information in authentication labels is described. This technology uses the phenomenon of photo-induced chirality in Ge2Sb2Te5 thin films to encode the left- or right-circular or linear polarization of the laser beam used to write the label. The written polarization can be revealed by a simple reading device, which is demonstrated to provide the same qualitative information as reading based on cyclotron circular dichroism spectroscopy and imaging. The suggested method, while based on existing manufacturing approaches, offers a balance between technological complexity for writing and simplicity for reading, and may be advantageous as a new authentication technology.

Growth mechanism of Ge2Sb2Te5 thin films by atomic layer deposition supercycles of GeTe and SbTe

The film with a composition close to Ge2Sb2Te5 was fabricated by the supercycle atomic layer deposition (ALD) of GeTe and SbTe, followed by tellurization annealing. Supercycle processes are widely used for thin film deposition of multicomponent materials and often exhibit non-ideal growth behavior. Since only in situ analysis can reveal the substrate-dependent growth behavior, we used in situ quartz crystal microbalance (QCM) to study the growth mechanism during ALD supercycle processes at 85 °C. GeTe grown on SbTe was more Te-deficient than continuously grown GeTe film. As a result, more Te-deficient Ge-Sb-Te films were formed than expected. By annealing in a Te ambient at 250 °C, the Te-deficient Ge-Sb-Te film was converted to the Ge0.23Sb0.23Te0.54 close to Ge2Sb2Te5 film, which had a high density equivalent to 95 % of the FCC structure of Ge2Sb2Te5. The film showed excellent conformality and uniform composition in a trench pattern, suggesting a uniform crystallization temperature of 118 °C at all locations.

Effects of thermal and laser annealing on the structure of Ge2Sb2Te5 thin films

In this study, we used Raman spectroscopy to compare the local structure of Ge2Sb2Te5 (GST) thin films with thicknesses of 90 nm and 271 nm that were crystallized through thermal annealing and laser radiation (laser annealing) during the recording of Raman spectra in situ. We found that for all crystallized films, the position of the main peaks in the Raman spectra was almost the same, and their structure corresponded to a hexagonal close packed state. It is noteworthy that the full width at half maximum (FWHM) of the main peaks varies considerably depending on the crystallization method used.

Correction to: Thermoelectric properties of nitrogen-doped Ge2Sb2Te5 thin films

Correction to: Thermoelectric properties of nitrogen-doped Ge2Sb2Te5 thin films

Operando electrical biasing TEM experiments of Ge-rich GST thin films with FIB sample preparation

Ge-Sb-Te (GST) ternary alloy is the core material for phase-change memory (PCM). Compared with Ge2Sb2Te5 (GST-225), Ge-rich GST (GGST) has a higher crystallization temperature (about 400°C) and thereby an improved thermal stability. In this paper, we propose a methodology dedicated to studying the phase changes by operando transmission electron microscopy (TEM). The methodology combines focused ion beam (FIB) specimen preparation and injection of electrical pulses for in-situ studies. We show how the microstructure is correlated to the electrical switching dynamics in GGST nanobridges. The electrical pulse modification, the electrochemical migration, local current density, void formation and the corresponding switching mechanism are discussed.

Influence of phase state, conducting sublayer material and deposition method on mechanical properties and adhesion of Ge2Sb2Te5 thin films

Ge2Sb2Te5 (GST225) thin films are used as a functional element in multilayer cells of phase change random access memory (PCRAM, PCM) and have good prospects in electrically driven tunable reflective metasurfaces and on-chip waveguide devices, including those implemented on a flexible substrate. Knowledge of the mechanical properties of GST225 thin films, their adhesion to conductive layers, and the correct choice of conductive material is critical to the reliable operation of these devices. The present work focuses on the effect of phase change on mechanical parameters such as hardness, Young's modulus and stiffness, as well as on the adhesion of GST225 thin films to various metal sublayers (Al, Ti, TiN, W, Ni). The formation of GST225 films was carried out by vacuum thermal evaporation and DC magnetron sputtering, which made it possible to study layers with different distributions of elements over the thickness.

Improved thermoelectric properties of Ge2Sb2Te5 films by Bi doping: Experiments and first-principles calculation

The high carrier concentration and low Seebeck coefficient limit the thermoelectric properties of Ge2Sb2Te5 materials. In this paper, Bi-doped Ge2Sb2Te5 thin films with different doping amounts were prepared by magnetron sputtering. The thermoelectric properties of Bi-doped Ge2Sb2Te5 were measured by experiments and predicted by density functional theory and Boltzmann transport theory. The results show that Bi atoms occupy Ge vacancies and bond with Te atoms. The replacement of Ge2+ vacancy with Bi3+ effectively modulates the carrier concentration and improves the Seebeck coefficient. Bi doping increases the degeneracy of the valence band and density of state near the valence band maximum (VBM), and thereby beneficial for the thermoelectric properties of p-type Ge2Sb2Te5, while not desirable for n-type. At 673 K, the power factor of the sample at 20 W doping power reaches 549.56 μWK−2m−1, which is nearly fifteen times that of the intrinsic Ge2Sb2Te5.

Design and demonstration of high-power density infrared nonlinear filtering window with EM shielding.

Directional energy weapons such as high-power microwaves and high-energy lasers pose a huge threat to optoelectronic detection systems. With that in mind, we designed an infrared optical window that has a nonlinear optical response to high-energy lasers and electromagnetic shielding to microwaves. By constructing a periodic metal circular hole array structure at the subwavelength scale, surface plasmons resonance is excited and its local field enhanced characteristics are utilized to form information transmission compatibility in the infrared band. At the same time, after laser etching off the subwavelength structure, the remaining metal forms a continuous conductive structure, forming an ultra-wideband shielding layer to achieve ultra-high and wide protection in the microwave band. Moreover, a layer of Ge2Sb2Te5 thin film was deposited between the transparent substrate and the metal film. Utilizing its nonlinear optical properties of high-temperature phase transition to reduce damage of directed energy weapons to the photoelectric detection system and equipment. Thus, when the photoelectric detection system or device is damaged or interfered by signals of different frequency bands or energies, the filtering window can achieve multi-mode shielding function.

Realization of Selector‐Memory Bi‐Functionality with Self‐Current Regulation Utilizing Poly‐Crystalline Based GST Electrolyte for Memristor Hardware Development

AbstractConductive bridge random‐access memory (CBRAM) are two terminal devices that offer excellent switching performance. In addition, CBRAM shows various switching modes, including volatile threshold switching (TS) and nonvolatile threshold switching (N‐TS). These properties expand its applications to memory, selector, biological synapses, and neurons. However, due to the uncontrollable behavior of stochastic switching between TS and N‐TS in CBRAM devices, a novel approach is needed to improve the switching performance of CBRAM. Moreover, conventional devices that have different stacking between TS and N‐TS increase fabrication cost and worsen the device yield. Here, the selector‐memory bi‐functionality with self‐current regulation effect of Ag‐inserted Ge2Sb2Te5 (GST) thin films is demonstrated. Selector‐memory bi‐functionality, having TS behavior with an adjustable on/off current, with confined conductive filaments (CFs) improves the uniformity and reduces the fabrication cost by implementing TS/N‐TS in a single stack. From the material analysis, it becomes evident that confined Ag‐based CFs within GST films are key factors for realizing selector‐memory bi‐functionality. The selector‐memory bi‐functionality is achieved through the reaction of Ag metal cations with non‐bonded Te atoms in GST film depending on field polarity. These results suggest that the Ag‐inserted GST film contributes to the development of large‐scale nonvolatile memory and neuromorphic application.

In situ atomic-scale observation of transformation from disordered to ordered layered structures in Ge-Sb-Te phase change memory thin films

Ge-Sb-Te (GST) thin films are emerging materials for various non-volatile memory applications. The compounds contain a huge number of vacancies that play important roles in structural and electronic transitions. Control of disorder and understanding the structural transformations in GST thin films are of great importance for the design and reliability of phase change memory devices. In this work, in situ heating atomic-resolution transmission electron microscopy is used to observe structural transformations from vacancy disordered (fcc) to layered ordered (vacancy-ordered (vo)) and van der Waals-bonded trigonal (vdW t)) structures. Starting from 160 °C, randomly distributed vacancies in fcc grains gradually accumulate into vacancy layers. Further heating to 220 and 240 °C results in the formation of vdW t-domains within the vo-GST grains and the formation of large <001>-oriented t-grains, in a good agreement with outcomes on in situ XRD heating of epitaxial vo-GST thin film. Moreover, vo and fcc GST structures can coexist with vdW t-structure in one grain. Full transformation to 〈112¯0〉-oriented t-grains occurs at 280 °C. Unlike other grains, the 〈001〉 t-grains show abnormal grain growth behaviour, while the electron beam irradiation accelerates the growth. Detailed characterizations of vo-grains reveal transient structures that indicate shear transformation mechanism and different defects as well as chemical changes during the transformations. Overall, this study provides new insights to understand the phase transition mechanisms and to optimize the microstructure in thin GST layers.

Local structure effects of carbon-doping on the phase change material Ge2Sb2Te5

In situ X-ray absorption spectroscopy on C-doped Ge2Sb2Te5 thin films reveals that C-content increases the rigidity of the Ge–Te bond in the amorphous phase, but curiously, only at moderate doping levels.

Controlled Optical Contrast Caused by Reversible Laser-Induced Phase Transitions in GeTe and Ge2Sb2Te5 Thin Films in the Spectral Range from 500 to 20,000 nm

Controlled Optical Contrast Caused by Reversible Laser-Induced Phase Transitions in GeTe and Ge2Sb2Te5 Thin Films in the Spectral Range from 500 to 20,000 nm

High thermal stability and optical contrast of Mo-doped Ge8Sb2Te11 films prepared by magnetron co-sputtering

In this work, Ge8Sb2Te11 (GST) films with high melting point element Mo doping were prepared by magnetron co-sputtering, and the influence of Mo content on the phase transformation and optical properties of the films was investigated. The results of GIXRD, XPS, and ellipsometry analysis show that Mo will occupy the lattice vacancy, promoting grain growth and accelerating phase transitions. With an increase in Mo content, the phase transition temperature of the films can reach 360 °C, and the amorphous thermal stability of the films can be enhanced by Mo doping. Meanwhile, the optical properties of the films are improved. The optical band gap of Mo-doped films is reduced, the absorption coefficient rises, and the refractive index contrast of GST films with low Mo-doping in the Vis-NIR band is as high as 2.27. This result will make it possible to apply Ge8Sb2Te11 thin films in structural color fields and broaden their color spectrum.

Thermal stability and high speed for optoelectronic hybrid phase-change memory based on Cr doped Ge2Sb2Te5 thin film

Non-volatile memory, which can simultaneously store data and compute in memory, is a promising candidate to break through “memory bottleneck”. However, its operation speed and thermal stability are still contradictory and difficult to meet the requirement of in-memory computing especially in high-temperature applications. In this work, optoelectronic hybrid phase-change memory based on Cr doped Ge2Sb2Te5 material is proposed, where the picosecond laser is used to perform the information recording/erasing operation based on the reversible phase-change characteristics while resistance signal is detected to realize the information readout. Results imply that the Cr doped Ge2Sb2Te5 film with Cr concentration of 10.7 at. % has high crystallization temperature (289 °C) and ten-year data-retention temperature (221 °C), low density-change rate (2.3%) and small resistance drift coefficients (0.022 at 85 °C, 0.056 at 120 °C, 0.070 at 150 °C, and 0.00131 for the laser induced SET state). The corresponding memory device has the SET/RESET operation speeds of as fast as 1820 ps/13 ps accompanied by the RESET/SET resistance ratio of higher than two orders of magnitude. The structural analyses indicate that Cr doping can suppress the growth of grains and reduce the grain size, improving the thermal stability of amorphous Ge2Sb2Te5 thin film while the octahedral/defective octahedral motif ensures its fast phase-change speed. Therefore, Cr doped Ge2Sb2Te5 based optoelectronic hybrid phase-change memory with superior thermal stability and ultrafast operation speed may be one of the promising solutions for in-memory computing.

Crystallization kinetics from Ge-rich Ge–Sb–Te thin films: Influence of thickness

The phase transition temperature and crystallization kinetics of phase-change materials (PCMs) are crucial characteristics for their performance, data retention, and reliability in memory devices. Herein, the crystallization behavior and kinetics of a compositionally optimized, N-doped Ge-rich Ge–Sb–Te alloy (GGST) in the slow crystallization regime are systematically investigated using synchrotron x-ray diffraction (XRD) in situ during heat treatment. Uniform thin films (50, 25, 10, and 5 nm) of initially amorphous N-doped GGST are investigated. The specimens were heated up to 450 °C at a rate of 2 °C/min to estimate crystallization onsets by quantifiying the crystallized quantity during material transformation from the XRD patterns. Subsequent isothermal anneals have been performed to assess crystallization behavior and activation energies. Nucleation-controlled crystallization that progresses in two steps is observed, together with the emergence of Ge preceding cubic Ge2Sb2Te5, with a mild dependence of crystallization temperature on film thickness that is inverse to what has been observed in other systems. Ge and GST crystallization may be described occurring in three-time stages: (i) an incubation period; (ii) a fast growth period; and (iii) a very slow-growth period. Very high activation energies (between 3.5 and 4.3 eV) for each phase are found for the incubation time t0. The activation energy for Ge in the fast growth regime is close to the one reported for the crystallization of pure Ge films. In the case of Ge, the incubation time is strongly thickness-dependent, which may have important consequences for the scaling of memories fabricated with this class of materials.

Ge2Sb2Te5 Thin Film as a Promising Heat‐Mode Resist for High‐Resolution Direct Laser Writing Lithography

Ge2Sb2Te5 thin film is investigated as a positive heat‐mode resist and environmentally friendly FeCl3 solution is as means an efficient developer. The corrosion selectivity of exposed to as‐deposited thin film reaches 2.3 and the etching selectivity of Si to Ge2Sb2Te5 thin film is as high as 15.75. Moreover, high‐resolution nanostructures with minimum linewidth of 180 nm and period of 400 nm are obtained along with high resolution of 130 nm. In addition, the mechanism of corrosion selectivity between as‐deposited and exposed thin film is further elucidated based on microstructural analysis. Hence, Ge2Sb2Te5 thin film is a promising positive resist for high‐resolution direct laser writing lithography.

Influence of bismuth on optical and electrical properties of thin Ge2Sb2Te5 films prepared by DC ion plasma sputtering

The structure of Ge2Sb2Te5 thin films doped with Bi has been studied using TEM, Raman spectroscopy, and Optical Transmission Spectroscopy. Thin films were obtained by direct current (DC) ion–plasma sputtering with thicknesses from 60 to 200 nm. TEM images revealed an amorphous film structure with bismuth nanocrystallites with a size of ∼1 to ∼20 nm. A shift in the Raman peaks of Bi–doped Ge2Sb2Te5 films compared to undoped samples was observed. A shift of the fundamental absorption edge to the long wavelength region of the spectrum along with an increase in the concentration of bismuth were detected in the optical transmission spectra of the films. A decrease in the optical band gap of bismuth-doped films was found. It has been shown that with increasing bismuth content in the films, their conductivity also increases. It has been found from studies of the current–voltage film characteristics reversible switching effect takes place.

Multilevel optoelectronic hybrid memory based on N-doped Ge2Sb2Te5 film with low resistance drift and ultrafast speed

Multilevel phase-change memory is an attractive technology to increase storage capacity and density owing to its high-speed, scalable and non-volatile characteristics. However, the contradiction between thermal stability and operation speed is one of key factors to restrain the development of phase-change memory. Here, N-doped Ge2Sb2Te5-based optoelectronic hybrid memory is proposed to simultaneously implement high thermal stability and ultrafast operation speed. The picosecond laser is adopted to write/erase information based on reversible phase transition characteristics whereas the resistance is detected to perform information readout. Results show that when N content is 27.4 at.%, N-doped Ge2Sb2Te5 film possesses high ten-year data retention temperature of 175 °C and low resistance drift coefficient of 0.00024 at 85 °C, 0.00170 at 120 °C, and 0.00249 at 150 °C, respectively, owing to the formation of Ge–N, Sb–N, and Te–N bonds. The SET/RESET operation speeds of the film reach 520 ps/13 ps. In parallel, the reversible switching cycle of the corresponding device is realized with the resistance ratio of three orders of magnitude. Four-level reversible resistance states induced by various crystallization degrees are also obtained together with low resistance drift coefficients. Therefore, the N-doped Ge2Sb2Te5 thin film is a promising phase-change material for ultrafast multilevel optoelectronic hybrid storage.

Structure and electronic properties of thin Ge2Sb2Te5 films produced by DC ion-plasma spattering

The optical properties of Ge2Sb2Te5 thin films were studied as a function of thickness. An increase in optical band gap with decreasing film thickness has been observed. The current– voltage characteristics measured in Ge2Sb2Te5 thin films in the current mode are studied. A decrease in switching time and threshold voltage with decreasing film thickness is established.

Controlling optical properties of GST thin films by ultrashort laser pulses series impact

This paper presents optical transmission and reflection coefficients of Ge2Sb2Te5 thin films changing dynamics during the femtosecond laser radiation induced phase transitions studying results. The authors propose a predictive model based on the thermokinetic approach allowing to qualitatively and quantitatively determine the fraction of the crystalline phase and the depth of its occurrence in the GST film. The research demonstrates that in a series of controlling femtosecond laser pulses for 100 nm or more thin films, the most accurate control of the crystalline phase fraction is ensured at energy fluences in the range of 10–20 mJ/cm2. At energy fluences higher than 25 mJ/cm2 in the two-pulse mode, a significant part of the film is reamorphized. For 100–200 nm thin films, the phase transformation begins a few nanoseconds after the onset of exposure and ends in no more than 100 ns under the described control mode.