Amorphous Ge2Sb2Te5 Thin Films Research Articles

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.

Density of amorphous sputtered Ge2Sb2Te5 thin films

The density, crystallinity, and microstructure of reactively sputtered amorphous Ge2Sb2Te5 thin films have been assessed as a function of deposition temperature. The continuous density increase was observed with increasing substrate temperature between room temperature and 200 °C. The films deposited at room temperature are amorphous and exhibit a columnar structure with a lateral size of cells in the 10–15 nm range. Cells consist of high-density interior with boundaries with the density lower by ∼9% due to incorporation of pores. The pores and the columnar microstructure can be eliminated by deposition at 80 °C while still preserving the amorphous phase. The density of pore- and stress-free amorphous Ge2Sb2Te5 is 6.16 g/cm3 and is only 1.5% lower than the crystalline Ge2Sb2Te5 with NaCl structure.

Кинетика обратимых фазовых переходов в тонких пленках Ge-=SUB=-2-=/SUB=-Sb-=SUB=-2-=/SUB=-Te-=SUB=-5-=/SUB=- при фемтосекундном лазерном облучении

Femtosecond laser irradiation of amorphous Ge2Sb2Te5 thin films initiates reversible phase transitions. The amorphization and crystallization of Ge2Sb2Te5 thin films were experimentally and theoretically confirmed. Electron and lattice temperatures kinetics during laser pulse duration were evaluated by two-temperature models calculations and experimental data. The dynamical changing of optical properties have been taking into account. Temperatures and cooling rates, which are necessary to initiate phase transitions by IR laser pulses with subpicosecond duration. The observed results open perspectives for improvement of Ge2Sb2Te5 nanophotonical devices.

Laser-induced periodic surface structures formation and reversible crystallization in amorphous Ge2Sb2Te5 thin films as a result of femtosecond irradiation

Laser-induced periodic surface structures formation and reversible crystallization in amorphous Ge2Sb2Te5 thin films as a result of femtosecond irradiation

Multi-functional platform based on amorphous Ge2Sb2Te5 thin films for photo/thermodetection and non-volatile memory applications

As a result of the copious interest and development in multifunctional electronics, the need for a multifunctional platform with a stable and unique performance has become one of the most important research concerns. In this embodiment, the amorphous Ge2Sb2Te5 (a-GST) thermally evaporated phase-change thin films are utilized for photo/thermodetection applications. The structural, compositional, and topographical methodologies confirm the amorphous nature with smooth surface topology and average particle size of about 31 ± 10 nm. Furthermore, the optical properties of the deposited a-GST revealed the broadband absorption spectrum with indirect energy gap and Urbach energy of about 0.54 eV and 380 meV, respectively. Furthermore, the dispersion and electronic parameters of the deposited films are determined. The switchable resistivity properties of Au/a-GST/Au are evaluated, and the phase change transition is observed in the temperature range (413–463) K. Moreover, the microelectronic parameters and interface features, in addition to the charge carriers' dynamics in the fabricated Ag/a-GTS/n-Si/Au–Sb heterojunction, are interpreted and analyzed in detail. Besides, the thermodetection capability of the fabricated heterojunction is evaluated in the temperature range (303–573) K. The estimated values of the thermosensitivity of about – (28.02 ± 1) and – (3.60 ± 0.13) mV/K at driving activation current of 600 μA for the two phases regions, respectively. Additionally, the photodetection performance of the implemented architecture is tested under halogen illumination of intensity (20–100) mW/cm2. The fabricated device shows a remarkably stable light signal response with responsivity, specific detectivity, and trise/tfall of about 203.1 mA/W, 2.84 × 109Jones, and 267 ms/84 ms, respectively. Ultimately, based on the recorded results, the Au/a-GST/n-Si/Au–Sb heterojunction is suggested for multifunctional applications with promising performance.

Low power reconfigurable multilevel nanophotonic devices based on Sn-doped Ge2Sb2Te5 thin films

In the past years, Ge2Sb2Te5 has been considered a promising functional material for a variety of reconfigurable multilevel devices, including photonic integrated circuits for the post-von Neumann arithmetic processing. However, despite significant advances, it is necessary to reduce the switching energy of Ge2Sb2Te5 for creation of the on-chip low power all-photonic spiking neural networks. The present work focuses on the effect of tin ion implantation on the properties of amorphous Ge2Sb2Te5 thin films, as well as on the performance of Mach-Zehnder interferometers and balanced beam splitters based on them. As a result, Sn-doping accompanied by the formation of weaker bonds in Ge2Sb2Te5 thin films is an efficient approach to significantly reduce the threshold energy of fs-laser initiated phase transitions and change the effective absorption coefficient. The possibility of using the Sn-doped Ge2Sb2Te5 thin films for fully optical multilevel reversible recording between 9 different levels (3 bits) has been demonstrated by experimental measurements of fabricated on-chip balanced beam splitters. The obtained results show that the Sn doping of Ge2Sb2Te5 layer can be used to optimize the properties of the GST225 thin films, in particular to reduce the switching energy. So, it has the potential to improve the characteristics of reconfigurable multilevel nanophotonic devices using the GST225 thin films, including fully non-volatile memory and developed on-chip low power all-photonic circuits for post-von Neumann arithmetic processing.

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.

Formation of periodic surface structures in multilayer amorphous Ge2Sb2Te5 thin films irradiated by femtosecond laser pulses

Phase transitions and periodic surface modification in amorphous Ge2Sb2Te5 thin films on multilayer substrate were revealed as a result of the samples irradiation by femtosecond laser pulses with the wavelength of 1250 nm. Raman spectroscopy revealed partial crystallization in the treated samples. Calculations and analysis of scanning electron and atomic-force microscopy data showed that formation of the periodic surface structures is related to photoinduced surface plasmon-polariton excitation and depends on laser radiation fluence. The obtained results are useful for design and fabrication of new promising data-storage and polarization optics devices.

Structural Transitions in Ge2Sb2Te5 Phase Change Memory Thin Films Induced by Nanosecond UV Optical Pulses.

Ge-Sb-Te-based phase change memory alloys have recently attracted a lot of attention due to their promising applications in the fields of photonics, non-volatile data storage, and neuromorphic computing. Of particular interest is the understanding of the structural changes and underlying mechanisms induced by short optical pulses. This work reports on structural changes induced by single nanosecond UV laser pulses in amorphous and epitaxial Ge2Sb2Te5 (GST) thin films. The phase changes within the thin films are studied by a combined approach using X-ray diffraction and transmission electron microscopy. The results reveal different phase transitions such as crystalline-to-amorphous phase changes, interface assisted crystallization of the cubic GST phase and structural transformations within crystalline phases. In particular, it is found that crystalline interfaces serve as crystallization templates for epitaxial formation of metastable cubic GST phase upon phase transitions. By varying the laser fluence, GST thin films consisting of multiple phases and different amorphous to crystalline volume ratios can be achieved in this approach, offering a possibility of multilevel data storage and realization of memory devices with very low resistance drift. In addition, this work demonstrates amorphization and crystallization of GST thin films by using only one UV laser with one single pulse duration and one wavelength. Overall, the presented results offer new perspectives on switching pathways in Ge-Sb-Te-based materials and show the potential of epitaxial Ge-Sb-Te thin films for applications in advanced phase change memory concepts.

Giant Photoinduced Chirality in Thin Film Ge2Sb2Te5

Induction, tuning, or amplification of chirality in various classes of materials and probing their chiral response are subjects of growing research. Herein, a large chiral signal that is rapidly imprinted in achiral amorphous Ge2Sb2Te5 (GST) thin films measured using synchrotron circular dichroism spectroscopy is reported. The chirality is induced by illuminating the films with pulsed circularly polarized (chiral) laser light for less than 2 μs in total. The effects of laser fluence and film thickness on the chiral response are described. The correlation of the optical results with structural studies by electron diffraction and model simulations suggests that alignment of reamorphized fragments in the crystallized film along the electric field vector of the light forms the centers that are responsible for the observed chirality. These results suggest opportunities for practical applications of this phenomenon and provide avenues for further studies of chirality induction in materials with impact in a wide range of disciplines.

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.

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.

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.

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.

Isothermal and CW laser crystallization of amorphous Ge2Sb2Te5 thin films

Transmission electron microscopy results are presented for as-deposited amorphous Ge2Sb2Te5 thin films after theirs isothermal annealing and CW laser illumination. Obtained microphotographs suggested that the crystallization process was driven solely by heterogeneous nucleation on the film boundary. The simplified model of steady-state crystallization process was developed for the case of heterogeneous nucleation mechanism. The values of nucleation rate and growth rate for isothermal and CW laser crystallization are calculated. The calculated values are in reasonable agreement with both our experimental data and with results previously published by the other authors.

Nanoscale multilevel switching in Ge2Sb2Te5 thin film with conductive atomic force microscopy

We demonstrate three-level data storage in amorphous Ge2Sb2Te5 (GST) thin film by conductive atomic force microscopy (C-AFM). Due to the high resolution and current sensitivity of AFM, the electrical properties of GST are investigated in the nanoscale. By applying an electric field between an AFM probe tip and the GST surface, well-resolved threshold switching and memory switching are obtained successively in a current–voltage sweeping. Correspondingly, three states with high, intermediate and low resistances, which are assigned data values ‘0’, ‘1’ and ‘2’ respectively, are observed in an IV-spectrum. The electrical resistance of GST thin film decreases by over two orders of magnitude in both switching processes, which provides a clear contrast to distinguish the three logical states. We also discuss the threshold electrical field of threshold switching in the amorphous GST thin film. Nanoscale conductive marks in the amorphous ON state and crystalline state are successfully fabricated by applying IV-spectra with different voltage ranges on the GST thin films.

Coherent phonon modes of crystalline and amorphous Ge2Sb2Te5 thin films: A fingerprint of structure and bonding

Femtosecond optical pump-probe measurements have been made upon epitaxial, polycrystalline, and amorphous thin films of Ge2Sb2Te5 (GST). A dominant coherent optical phonon mode of 3.4 THz frequency is observed in time-resolved anisotropic reflectance (AR) measurements of epitaxial films, and is inferred to have 3-dimensional T2-like character based upon the dependence of its amplitude and phase on pump and probe polarization. In contrast, the polycrystalline and amorphous phases exhibit a comparatively weak mode of about 4.5 THz frequency in both reflectivity (R) and AR measurements. Raman microscope measurements confirm the presence of the modes observed in pump-probe measurements, and reveal additional modes. While the Raman spectra are qualitatively similar for all three phases of GST, the mode frequencies are found to be different within experimental error, ranging from 3.2 to 3.6 THz and 4.3 to 4.7 THz, indicating that the detailed crystallographic structure has a significant effect upon the phonon frequency. While the lower frequency (3.6 THz) mode of amorphous GST is most likely associated with GeTe4 tetrahedra, modes in epitaxial (3.4 THz) and polycrystalline (3.2 THz) GST could be associated with either GeTe6 octahedra or Sb-Te bonds within defective octahedra. The more polarizable Sb-Te bonds are the most likely origin of the higher frequency (4.3–4.7 THz) mode, although the influence of Te-Te bonds cannot be excluded. The effect of high pump fluence, which leads to irreversible structural changes, has been explored. New modes with frequency of 3.5/3.6 THz in polycrystalline/amorphous GST may be associated with Sb2Te3 or GeTe4 tetrahedra, while a 4.2 THz mode observed in epitaxial GST may be related to segregation of Sb.

Bipolar resistive switching behavior of an amorphous Ge₂Sb₂Te₅ thin films with a Te layer.

The mechanism of bipolar resistive switching (BRS) of amorphous Ge2Sb2Te5 (GST) thin films sandwiched between inert electrodes (Ti and Pt) was examined. Typical bipolar resistive switching behavior with a high resistance ratio (∼10(3)) and reliable switching characteristics was achieved. High-resolution transmission electron microscopy revealed the presence of a conductive Te-filament bridging between the top and bottom electrodes through an amorphous GST matrix. The conduction mechanism analysis showed that the low-resistance state was semiconducting and dominated by band transport, whereas Poole-Frenkel conduction governed the carrier transport in the high-resistance state. Thus, the BRS behavior can be attributed to the formation and rupture of the semiconducting conductive Te bridge through the migration of the Te ions in the amorphous GST matrix under a high electric field. The Te ions are provided by the thin (∼5 nm) Te-rich layer formed at the bottom electrode interface.

Defect states in amorphous Ge2Sb2Te5 phase change material

Steady-state photoconductivity measurements in the temperature range 100–300 K on amorphous Ge2Sb2Te5 thin film prepared by dc sputtering are analyzed. The dark conductivity is thermally activated with a single activation energy that allocates the position of the Fermi level approximately in the middle of the energy gap relative to the valance band edge. The temperature dependence of the photoconductivity ensures the presence of a maximum normally observed in chalcogenides with low- and high-temperature slopes, which predict the location of discrete sets of localized states (recombination levels) in the gap. The presence of these defect states close to the valence and conduction band edges leaves the quasi Fermi level shifts in a continuous distribution of gap states at high temperatures, as evidenced from the γ values of the lux–ampere characteristics.