Phase Change Memory Devices Research Articles

Phase transition behavior of (MoTe2)xSb1-x thin films based on flexible PEEK substrates

Flexible information memory played a key role in flexible electronic devices and smart wearables. This paper was focused on the effect of flexible deformation on the properties of (MoTe2)xSb1-x nano-phase-change films based on PEEK substrates. By placing the films at the finger, wrist, back of the hand and elbow and bending them, the resistance values showed periodic fluctuations with bending but the changes were not significant. After 100,000 bending cycles and 4,000 s of vibration, the films successfully achieved the transformation from a shapeless to a structured state. The stress caused by bending and vibration affects the surface roughness of the flexible film and weakens the adhesion between the film and the substrate. Flexible (MoTe2)0.07Sb0.93 film electronics were prepared, and the phase change memory devices could realize reversible transitions between SET and RESET states with 100 ns pulse widths in the flat state, after 100,000 bending cycles, and after 4,000 s of vibration. These findings indicated that (MoTe2)0.07Sb0.93 films based on flexible PEEK substrates had promising applications in the field of flexible phase change memory.

Thickness dependence and crystallization properties of amorphous GeTe thin films on silicon dioxide

Radio frequency magnetron sputtering was used to prepare the amorphous GeTe thin films on silicon dioxide and the thickness effects on the crystallization behavior were investigated. With the film thickness reducing, the crystallization temperature, crystallization activation energy, amorphous and crystalline resistance increase remarkably, indicating the great improvement in thermal stability and power consumption. Ozawa’s model was used to estimate the crystallization kinetics of GeTe thin films, it shows that nucleation and grain growth occur simultaneously, and grain growth dominates ultimately. XRD analysis demonstrated that the grain size can be reduced and the crystallization process of GeTe thin film can be inhibited with the film thickness decreasing. Furthermore, the thinner film has smaller resistance drift index and surface roughness, which are beneficial to improve the reliability of storage device. T-type phase change memory devices based on 25 nm GeTe thin film were fabricated by 0.13 μm CMOS technology, and the current–voltage and resistance-voltage characteristics demonstrate the excellent electrical performance, including the fast resistance switching between SET and RESET processes, low threshold current and voltage. All the results proved the strong dependency relationships between the crystallization properties and film thickness of GeTe thin film, which paves the way for developing high-density phase change memory in the fields of big data and artificial intelligence.

Influence of HfO2 oxide layer on crystallization properties of In3SbTe2 phase change material

Phase Change Memory (PCM) represents a potential paradigm in the realm of non-volatile memory technologies, and several phase change materials are studied for utilization in PCM devices. This work employs a less explored In3SbTe2 (IST) phase change material (active layer) integrated with an oxide (HfO2) layer and investigates the influence of the oxide layer on the active layer. The oxide layer remains amorphous when annealed at 400 °C, and it doesn’t alter the crystallization temperature (290 °C) of the active layer in IST with HfO2 thin-film. However, after crystallization, the grain size of the active layer is reduced to ∼8.23 nm in IST with HfO2 thin-film, compared to only IST thin-film. XPS core-level spectra (In 3d, Sb 3d, Te 3d) of IST active layer reveal the peak shifting towards higher binding energy at 300 °C and 400 °C annealed films, implying bond energy increase with crystallization and film stability improves. The Hf or O atoms of the oxide layer don’t diffuse into the active layer with annealing, suggesting no interference with the phase switching property of IST. A higher optical bandgap of 1.154 eV in as-deposited IST with HfO2 thin-film compared to the only IST thin-film illustrates better stability of the amorphous state in the film with the oxide layer. In addition, the fabricated PCM device using the IST with the oxide layer demonstrates phase switching at a lower threshold voltage of (2.1 ± 0.1) V, compared to the IST-based device. The findings indicate that the enhanced structural and electrical switching characteristics of IST phase change layer coupled with an oxide layer make it beneficial for data storage PCM applications.

Effect of Ultraviolet Radiation on Properties of Ge2Sb2Te5 Phase Change Films.

With the expanding utilization of space technology, the stability of electronic components' performance in radiation environments has garnered significant attention. In this study, we prepared Ge2Sb2Te5 phase change films and memory units on silicon substrates to explore the influence of ultraviolet (UV) radiation on their characteristics. The experimental findings revealed that UV irradiation at a power density of 450 mW/cm2 decreased the amorphous resistance and thermal stability of Ge2Sb2Te5 films, impeding their multistage storage performance. Nevertheless, the amorphous state could still undergo effective transformation into a crystalline state. Furthermore, UV irradiation triggered the photoelectric effect, narrowing the band gap and causing a redshift of the Raman peak in amorphous films. Remarkably, the surface properties of Ge2Sb2Te5 films remained unchanged under irradiation. The phase change memory device based on Ge2Sb2Te5 film retained its SET-RESET conversion capability at a pulse width of 100 ns post-UV irradiation, demonstrating resilience against UV radiation. This study offers the practical insights for the application of phase change memory in space radiation environments.

Multilevel phase transition behavior of In2Se3-doped with Sb materials based on flexible polyimide substrate

The In2Se3-doped Sb phase change material was deposited onto a polyimide substrate using magnetron sputtering technique. A comprehensive investigation was conducted on the thermal stability, bending resistance, crystal structure, and electrical properties of this material. The findings revealed its remarkable thermal stability, data retention, and bending resilience, particularly in the In0.05Se0.19Sb0.76 film, which exhibited minimal resistance drift. The aging test confirmed the stability of the electronic structure in In2Se3-doped Sb, accompanied by reduced structural relaxation. Notably, no new crystalline phase emerged in In0.05Se0.19Sb0.76 film, but stronger In-Sb bonds were formed. Raman spectroscopy and bending tests further indicated that deformation had minimal impact on the film’s structure and surface. Utilizing the In0.05Se0.19Sb0.76 material and polyimide substrate, a flexible phase change memory device demonstrated reliable SET/RESET operations even after bending, offering three stable resistance states for multilevel storage. This enhanced the storage density, making In2Se3-doped Sb material a promising flexible phase change material with excellent performance and vast potential for applications in phase change storage, such as electronic fabrics and flexible smart wearable electronics.

TEM Preparation and Characterization of a GeTe-based Phase Change Memory Device at Partial SET Mode

TEM Preparation and Characterization of a GeTe-based Phase Change Memory Device at Partial SET Mode

3D Confinement Stabilizes the Metastable Amorphous State of Antimony Nanoparticles - A New Material for Miniaturized Phase Change Memories?

The wet-chemical synthesis of 3D confined antimony nanoparticles (Sb-NP) at low and high temperatures is described. Using reaction conditions that are mild in temperature and strong in reducing power allows the synthesis of amorphous Sb-NP stabilized with organic ligands. Exchanging the organic ligand 1-octanethiol by iodide enabled to investigate the unusual strong stability of this metastable material through simultaneous thermal analysis combining differential scanning calorimetry and thermogravimetric analysis. Additionally, in situ high temperature powder x-ray diffraction (p-XRD) shows a significant increase in stabilization of the amorphous phase in comparison to thin layered, 1D confined Sb or bulk material. Further, it is shown with scattering-type scanning near-field optical microscopy (s-SNOM) experiments that the optical response of the different phases in Sb-NP make the distinctness of each phase possible. It is proposed that the Sb-NP introduced here can serve as a 3D-confined optically addressable nanomaterial of miniaturized phase change memory devices.

Phase-Controlled Synthesis and Phase-Change Properties of Colloidal Cu-Ge-Te Nanoparticles.

Phase-change memory (PCM) technology has recently attracted a vivid interest for neuromorphic applications, in-memory computing, and photonic integration due to the tunable refractive index and electrical conductivity between the amorphous and crystalline material states. Despite this, it is increasingly challenging to scale down the device dimensions of conventionally sputtered PCM memory arrays, restricting the implementation of PCM technology in mass applications such as consumer electronics. Here, we report the synthesis and structural study of sub-10 nm Cu-Ge-Te (CGT) nanoparticles as suitable candidates for low-cost and ultrasmall PCM devices. We show that our synthesis approach can accurately control the structure of the CGT colloids, such as composition-tuned CGT amorphous nanoparticles as well as crystalline CGT nanoparticles with trigonal α-GeTe and tetragonal Cu2GeTe3 phases. In situ characterization techniques such as high-temperature X-ray diffraction and X-ray absorption spectroscopy reveal that Cu doping in GeTe improves the thermal properties and amorphous phase stability of the nanoparticles, in addition to nanoscale effects, which enhance the nonvolatility characteristics of CGT nanoparticles even further. Moreover, we demonstrate the thin-film fabrication of CGT nanoparticles and characterize their optical properties with spectroscopic ellipsometry measurements. We reveal that CGT nanoparticle thin films exhibit a negative reflectivity change and have good reflectivity contrast in the near-IR spectrum. Our work promotes the possibility to use PCM in nanoparticle form for applications such as electro-optical switching devices, metalenses, reflectivity displays, and phase-change IR devices.

Minority-carrier transport through an isotype amorphous-crystalline Ge2Sb2Te5 heterojunction under forward bias

To predict possible minority-carrier effects in multi-level phase change memory devices, minority-carrier transport through an isotype amorphous-crystalline Ge2Sb2Te5 heterojunction under forward bias is studied for the first time. Electron thermionic emission, thermal generation, drift, diffusion, radiative recombination, Auger recombination, Schockley–Read–Hall recombination via conduction band tails, valence band tails, acceptor-type mid-gap, donor-type mid-gap and multivalent defect distributions, as well as surface recombination are considered in the construction of the steady-state Continuity Equation relevant to the representative amorphous-crystalline Ge2Sb2Te5 heterojunction, which is then numerically solved at 0.15 V and 0.40 V using solar cell capacitance simulations. Provided that radiative recombination is negligible and defect distributions within the band gap of either layer are energetically localised, the simulated electron concentration, electron current density and electron quasi-Fermi level distributions across the heterojunction reveal that transport through the amorphous layer limits electron flow through the device. At low applied bias, net recombination and diffusion within the quasi-neutral region (QNR) of the amorphous layer dominate, whereas at larger applied bias, drift across the QNR, due to the electric field induced by the significant majority-carrier current density, as well as surface recombination at the amorphous layer contact contribute significantly. Within the crystalline layer, net generation of electrons supplies the amorphous layer at all biases, assuming that the crystalline layer contact does not limit electron transport. Thus, the effect of forward bias on the dominant transport mechanisms through the amorphous-crystalline Ge2Sb2Te5 heterojunction demonstrated herein represents the key contribution of this paper.

Reduction of write current with improved thermal stability in GeSe2 doped Sb2Te3 films for phase change memory applications

Chalcogenide alloy-based semiconductors have gained significant attention in recent decades due to its applications in phase change memory (PCM). Sb2Te3 has proven to be an alternative to Static and Dynamic Random Access Memory and can be a suitable candidate for commercial memory devices due to their fast switching speed. However, Sb2Te3 suffers from low amorphous phase stability and high RESET current, which needs further improvement for high power efficiency. In this work, we have prepared (Sb2Te3)1−x (GeSe2) x (x = 0.06, 0.12, 0.18, 0.24, 0.3) films to investigate their PCM properties. The films showed a rise in transition temperature to transform from high resistive amorphous (RESET) to low resistive crystalline (SET) states with doping that leads to significant enhancement in amorphous phase stability. For 30% doping of GeSe2 in Sb2Te3, the data retention temperature increases from 20.2 °C to 84.6 °C, and the resistance contrast also increases from 102 to 105. The rise in electrical resistance with doping in the amorphous as well as crystalline states leads to a drop in threshold current (I th) from 3.5 to 0.8 mA. This also reduces the RESET and SET currents. By analyzing the samples using finite element method, it was found out that the high resistance materials produce more heat, resulting in a lower write current in an energy efficient PCM device.

Development of Sb phase change thin films with high thermal stability and low resistance drift by alloying with Se

A single Sb phase demonstrates potential for use in phase change memory devices. However, the rapid crystallization of Sb at room temperature imposes limitations on its practical application. To overcome this issue, Sb is alloyed with Se using a reactive co-sputtering deposition technique, employing both Sb and Sb2Se3 sputter targets. This process results in the formation of Sb-rich Se thin films with varying compositions. Compared to pure Sb, the Sb-rich Se thin films exhibit enhanced thermal stability due to the formation of Sb–Se bonds and reduced resistance drift. In particular, the Sb86.6Se13.4 thin film demonstrates an exceptionally low resistance drift coefficient (0.004), a high crystallization temperature (Tc = 195 °C), a high 10-year data retention temperature (116.3 °C), and a large crystallization activation energy (3.29 eV). Microstructural analysis of the Sb86.6Se13.4 reveals the formation of a trigonal Sb phase with (012) texture at 250 °C, while Sb18Se and Sb2Se3 phases form at 300 °C. Conversely, the Sb98.3Se1.7 thin film shows the formation of the single Sb phase with (001) texture, a Tc of 145 °C, and a low resistance drift coefficient (0.011). Overall, this study demonstrates that the alloying strategy is a viable approach for enhancing thermal stability and reducing resistance drift in Sb-based phase-change materials.

Effect of humidification on antimony-based flexible phase change memory

Ambient humidity has an important impact on the performance of flexible electronic devices, and wearable electronic devices will inevitably involve a high humidity environment. To investigate the impact of humidification on flexible phase-change memory, we prepared Sb phase-change film and memory device unit with polyether ether ketone as substrate, and studied the effect of humidification on the properties of the film and device. The results show that the effective transition from an amorphous to a crystalline state can still be achieved by the film after soaking in water for 55 min, and the phase transition temperature increases with the increase of soaking time. Water molecules refine the film grains, resulting in increased crystalline resistance. Sb is a hydrophilic material, and its hydrophilicity decreases after crystallization. The reversible conversion of SET-RESET can be realized under different pulse widths even after the phase change memory device is soaked in water and bent many times. This study shows that antimony-based flexible phase change memory has certain deformation self-healing ability and moisture resistance, which provides a way to expand its application environment.

Controlled Self-Assembly of Nanoscale Superstructures in Phase-Change Ge-Sb-Te Nanowires.

Controlled growth of semiconductor nanowires with atomic precision offers the potential to tune the material properties for integration into scalable functional devices. Despite significant progress in understanding the nanowire growth mechanism, definitive control over atomic positions of its constituents, structure, and morphology via self-assembly remains challenging. Here, we demonstrate an exquisite control over synthesis of cation-ordered nanoscale superstructures in Ge-Sb-Te nanowires with the ability to deterministically vary the nanowire growth direction, crystal facets, and periodicity of cation ordering by tuning the relative precursor flux during synthesis. Furthermore, the role of anisotropy on material properties in cation-ordered nanowire superstructures is illustrated by fabricating phase-change memory (PCM) devices, which show significantly different growth direction dependent amorphization current density. This level of control in synthesizing chemically ordered nanoscale superstructures holds potential to precisely modulate fundamental material properties such as the electronic and thermal transport, which may have implications for PCM, thermoelectrics, and other nanoelectronic devices.

Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet

Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two topologically distinct phases requires controlled non-volatile switching between two crystalline phases with distinct symmetries. Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe5−δGeTe2. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.

Low‐Power Crystallization Process in In3SbTe2 Phase Change Memory Devices with Thin Oxide Layer

Phase change memory (PCM) technology is a strong contender for the next‐generation memory in today's data‐centric era. In3SbTe2 (IST) phase change material is recently explored due to its good thermal stability, rapid electrical switching, and significant resistance contrast for multibit storage. However, the main concern in PCM devices is the high‐energy usage during the phase switching process. Herein, a thin oxide (HfO2) layer is inserted between the IST active layer and bottom electrode in PCM device. Such a device exhibits amorphous‐to‐crystalline phase switching (crystallization or SET operation) at a lower voltage of (2.1 ± 0.1) V and 66% reduction in energy consumption compared to the device using only IST active layer. Higher temperature of 688 K is acquired in the device with the oxide layer demonstrating that the Joule heat is effectively confined by the oxide layer assisting the crystallization process in the active layer at a less voltage. Such a device also exhibits a significant resistance contrast of ≈2 orders magnitude, allowing for stable data storage. These results reveal that the switching performance of PCM device using IST phase change material is improved by the oxide layer, which is beneficial for energy‐efficient, nonvolatile data storage applications.

Drift of Schottky Barrier Height in Phase Change Materials.

Phase-change memory (PCM) devices have great potential as multilevel memory cells and artificial synapses for neuromorphic computing hardware. However, their practical use is hampered by resistance drift, a phenomenon commonly attributed to structural relaxation or electronic mechanisms primarily in the context of bulk effects. In this study, we reevaluate the electrical manifestation of resistance drift in sub-100 nm Ge2Sb2Te5 (GST) PCM devices, focusing on the contributions of bulk vs interface effects. We employ a combination of measurement techniques to elucidate the current transport mechanism and the electrical manifestation of resistance drift. Our steady-state temperature-dependent measurements reveal that resistance in these devices is predominantly influenced by their electrical contacts, with conduction occurring through thermionic emission (Schottky) at the contacts. Additionally, temporal current-voltage characterization allows us to link the resistance drift to a time-dependent increase in the Schottky barrier height. These findings provide valuable insights, pinpointing the primary contributor to resistance drift in PCM devices: the Schottky barrier height for hole injection at the interface. This underscores the significance of contacts (interface) in the electrical manifestation of drift in PCM devices.

The application of C/Sb composite multilayer films on fast flexible phase change memory

In this paper, the phase transformation properties of C/Sb composite multilayer films based on flexible substrates were studied. After different times of bending, the flexible films can still achieve amorphous to crystalline phase transition, but the crystalline resistance increases to different degrees. When the bending times are less than 15000 times, the deformation makes the grain fine, the surface of the film tends to be flat, the surface electric field effect is weakened, and the resistance drift is reduced. When the bending times are higher than 15000 times, cracks begin to appear in the film, and the cracks’ depth and width increase with the bending times. The distribution of C and Sb elements at the crack was analyzed by scanning electronic microscope energy spectrum. The phase change memory device of C/Sb composite multilayer film based on flexible PEEK substrate has been made, and the rapid reversible resistance transition (∼10 ns) in various complex states has been successfully achieved. This research provides a new idea for flexible information storage.

Programming Operations Analysis and Statistics in One Selector and One Memory Ovonic Threshold Switching + Phase‐Change Memory Double‐Patterned Self‐Aligned Structure

This study explores the reliability of a phase‐change memory (PCM) cointegrated with an ovonic threshold switching (OTS) selector (one selector and one memory [1S1R] structure) based on an innovative double‐patterned self‐aligned architecture. The variability of the threshold voltage () for both the SET and RESET states is examined, comparing different distribution models to validate the use of mean and standard deviation as viable metrics. The dispersion of is tracked under different programming conditions to provide insight into the evolution of device behavior over SET/RESET, endurance cycles, and read cycles. The PCM device is based on a “wall” structure and on Ge2Sb2Te5 alloy, while the OTS is based on a GeSbSeN alloy. The analysis focuses on the programming characteristics and SET pulse optimization, studying current control and pulse fall times. The results are based on statistical data obtained from a kb‐sized memory array. A memory window of over 2 V is achieved. The research helps understanding the DPSA architecture, and PCM + OTS in general, offering insights into their programming, variability, and reliability targeting crossbar applications.

Unravelling the amorphous structure and crystallization mechanism of GeTe phase change memory materials

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable ‘ideal glass’ state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature.

Resistance Drift in Melt-Quenched Ge2Sb2Te5 Phase Change Memory Line Cells at Cryogenic Temperatures

We characterized resistance drift in phase change memory devices in the 80 K to 300 K temperature range by performing measurements on 20 nm thick, ∼70–100 nm wide lateral Ge2Sb2Te5 (GST) line cells. The cells were amorphized using 1.5–2.5 V pulses with ∼50–100 ns duration leading to ∼0.4–1.1 mA peak reset currents resulting in amorphized lengths between ∼50 and 700 nm. Resistance drift coefficients in the amorphized cells are calculated using constant voltage measurements starting as fast as within a second after amorphization and for 1 h duration. Drift coefficients range between ∼0.02 and 0.1 with significant device-to-device variability and variations during the measurement period. At lower temperatures (higher resistance states) some devices show a complex dynamic behavior, with the resistance repeatedly increasing and decreasing significantly over periods in the order of seconds. These results point to charge trapping and de-trapping events as the cause of resistance drift.