Nonvolatile Random Access Memory Devices Research Articles

A theoretical approach to study oxide-based perovskite materials XTiO3 (X = Be, Mg, Ca, Sr and Ba) for photovoltaic applications

Oxide-based perovskite materials have a large application in fuel and hydrogen sensors, non-volatile random access memory devices, semiconductor fabrications, optoelectronic, thermoelectric and photovoltaic devices. In this report, equilibrium geometries, and optoelectronic properties of oxide-perovskite materials XTiO3 (X = Be, Mg, Ca, Sr and Ba) are investigated through Conceptual Density Functional Theory (CDFT) technique. The HOMO–LUMO energy gap obtained from functional B3LYP/LANL2DZ and B3PW91/LANL2DZ are observed in the range of 1.201 eV–4.647 eV and 1.519 eV–4.903 eV respectively, which justifies their applications in solar cells and optoelectronic devices. HOMO–LUMO energy gap shows a downward trend when materials travel from Be to Mg to Ca to Sr to Ba, except for BaTiO3 in B3PW91/LANL2DZ. BeTiO3 displays the maximum value of HOMO–LUMO gap, hardness and electronegativity value. Hardness and softness of these substances are found between 0.600–2.452 eV and 0.204–0.788 eV respectively whereas refractive index and dielectric constant of XTiO3 are observed in the range of 2.017–3.684 and 4.067–13.574 respectively. Across all relationships, XTiO3’s dielectric constant and refractive index show a rising pattern from Be to Mg to Ca to Sr to Ba, except for BaTiO3 computed using B3PW91/LANL2DZ. The lowest refractive index and dielectric constant are displayed by the BeTiO3. TD-DFT calculation is performed to understand the absorption spectra of these materials. Optical transition energy and wavelength of XTiO3 are found between 0.339–3.535 eV and 350.68–3656.15 nm respectively. An interesting relationship is established between HOMO–LUMO energy gap, optical transition energy and wavelength of XTiO3 materials. The investigated compounds exhibit a linear pattern between HOMO–LUMO energy gap and optical transition energy whereas wavelength shows an inverse trend. MEP of these compounds are also discussed.

Effect of Hf alloying on magnetic, structural, and magnetostrictive properties in FeCo films for magnetoelectric heterostructure devices

Materials with high magnetoelectric coupling are attractive for use in engineered multiferroic heterostructures with applications such as ultra-low power magnetic sensors, parametric inductors, and non-volatile random-access memory devices. Iron–cobalt alloys exhibit both high magnetostriction and high saturation magnetization that are required for achieving significantly higher magnetoelectric coupling. We report on sputter-deposited (Fe0.5Co0.5)1−xHfx (x = 0 – 0.14) alloy thin films and the beneficial influence of Hafnium alloying on the magnetic and magnetostrictive properties. We found that co-sputtering Hf results in the realization of the peening mechanism that drives film stress from highly tensile to slightly compressive. Scanning electron microscopy and x-ray diffraction along with vibrating sample magnetometry show reduction in coercivity with Hf alloying that is correlated with reduced grain size and low film stress. We demonstrate a crossover from tensile to compressive stress at x ∼ 0.09 while maintaining a high magnetostriction of 50 ppm and a low coercive field of 1.1 Oe. These characteristics appear to be related to the amorphous nature of the film at higher Hf alloying.

Energy efficient IPC based dual compression for endurance enhancement of NVRAM as main memory in embedded devices

AbstractThe Non‐Volatile Random Access Memory (NVRAM) is recently identified as the most upcoming main memory technology in Embedded and Internet of Things (IoT) systems due to its appealing qualities like zero static power consumption and great memory cell density. On the other hand, most NVRAMs have low write endurance due to workload‐induced write variance, leading to more write activity at a few blocks of memory than the other blocks. Improving the endurance of NVRAM on embedded architectures is very critical due to the limited constraints of Embedded architectures. The main goal of this paper is to address the complexity of NVRAM designs, with a focus on embedded boards, and to create a prototype that enhances NVRAM endurance using a compression technique. Furthermore, this framework is not particular to a single NVRAM device. An Instruction Per Cycle based Dynamic Pattern Compression (IPC_DPC) model is developed to address NVRAM's high write latency and low endurance. The primary goal of the proposed IPC_DPC model is to minimize the energy utilization and latency of heavy workloads during the execution by pre‐determining the workload's instruction cycles. The IPC_DPC model initially divides the Low IPC and High IPC workloads and then compresses the workloads dynamically based on their IPC values compared to the threshold factor. As a result, IPC_DPC improves the compression ratio and reduces the write latencies associated with traditional compressors, significantly reducing the write activity and improving the lifetime of NVRAM. We implemented the proposed IPC_DPC model using GEM5 with a Raspberry Pi board and took IoMT, MiBench, and EEMBC benchmarks for evaluation. Compared to FPC, BDI, FNW, and DFPC models, the proposed IPC_DPC model is shown to be more efficient in terms of compression ratio (56.05%) and energy reduction (17%).

Effect of bismuth doping on structural and electrical properties of 0.9(BaZr0.2Ti0.8O3)–0.1(Ba0.7Ca0.3TiO3) ceramic

The (Ba0.97Ca0.03)1−3y/2BiyZr0.18Ti0.82O3 ceramic system was prepared by the conventional solid-state sintering technique. The effect of bismuth substitution has been investigated via X-ray diffraction, Scanning electron microscopy, dielectric, electric and ferroelectric measurements. X-ray diffraction showed that these compounds crystallized, at room temperature, in tetragonal P4mm space group symmetry for BCTZ10 and BCTZ10–0.02Bi compounds and in cubic symmetry with space group Pm3‾m for y = 0.06; 0.08 and 0.1 compositions distorted perovskite structures. SEM images were used to observe the microstructure. The effect of the crystal structure change and microstructure features on the dielectric and ferroelectric properties of our new BCTZ10-yBi ceramic system has been discussed. The dependence of the permittivity on temperature indicated a crossover from a normal ferroelectric, for both BCTZ10 and BCTZ10–0.02 ceramics, to a relaxor state for 0.06 < y ≤ 0.1 composition compounds. The enhancement of the dielectric properties was marked by the shift of Tm to room temperature, the increase of the maxim of permittivity and the improvement of the relaxor behavior that characterizes optimal composition due to Bi3+ substitution. BCTZ10–0.06Bi ceramic exhibit the highest εrm', with a value of 11508 at 1 kHz, at Tm of 270 k near room temperature. Both BCZT10 and BCZT-0.02Bi ceramics exhibit a hysteresis loops signature of ferroelectric behavior at room temperature. While doped BCZT-0.02Bi show just a non linear evolution of P vs. E, the increase of the spontaneous and remnant polarizations: Ps from 0.063 μC/mm2 to 0.073 μC/mm2 and Pr from 0.028 μC/mm2 to 0.040 μC/mm2 for BCZT10 and BCZT10–0.02Bi compositions, respectively, is an evidence of the enhancement of ferroelectric properties as a result of bismuth substitution on BCZT10. The electrical properties of our new BCTZ10-yBi ceramic system may be largely tunable and could be attractive for non-volatile random access memory devices.

Microstructural characterization of ferroelectric oxides thin films based on hafnia

Since the discovery of ferroelectricity in HfO2 thin films, HfO2 based materials have become of great interest for applications such as non-volatile random-access memory devices. Morphological and atomic scale structural investigations of nanomaterials are important in further understanding of their electrical properties. The electrical behavior (C-V, I-V characteristics and polarization hysteresis loop) of multilayer structures is greatly influenced by the quality of the deposited thin films and also the quality of the interfaces between the layers. Transmission Electron Microscopy (TEM) is the most appropriate microstructural characterization technique able to provide morphological and structural information about the thin films and their interfaces. We study the morphological properties of this oxide based on hafnia because the orthorhombic phase exhibits ferroelectric properties. In this work we use a Cs probe-corrected JEM ARM 200F electron microscope, TEM-SAED and HRTEM techniques to investigate the morphological structure of HfO2 based thin films and to identify the crystalline phases of the ferroelectric oxide. The studied samples consist in thin films of HfO2 deposited on a TiN electrode and the latter was deposited on a Si (100) substrate with a native SiO2 layer. The thin films were grown by atomic layer deposition (ALD) using TEMAHf and ZyALD precursors deposited at 300oC. HRTEM technique combined with FFT (Fast Fourier Transform) analysis provides a complete characterization of morphological and structural properties of the thin film.

Fabrication of Pb(Zr,Ti)O3 thin films utilizing unconventional powder magnetron sputtering (PMS)

Pb(Zr,Ti)O3 (PZT) ferroelectric ceramic films exhibit highly superior ferroelectric, pyroelectric and piezoelectric properties which are promising for a number of applications including non-volatile random access memory devices, non-linear optics, motion and thermal sensors, tunable microwave systems and in energy harvesting (EH) use. In this research, a thin layer of PZT was deposited on two different substrates of Strontium Titanate (STO) and Strontium ruthenate (SRO) by powder magnetron sputtering (PMS) system. The preliminary powders, consisting of PbO, ZrO2 and TiO2, were manually mixed and placed into the target holder of the PMS. The deposition was performed at an elevated temperature reaching up to 600 °C via a ceramic heater. This high temperature is required for PZT thin film crystallinity, which is never achieved in conventional physical vapour deposition processes. The phase structure, crystallite size, stress-strain and surface morphology of deposited thin films were characterized using X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). The composition of the PZT thin films were also analysed by X-ray photoelectron spectroscopy (XPS). The mechanical properties of the thin films were evaluated with micro-scratch adhesive strength and micro hardness equipment. FESEM results showed that the PZT thin films were successfully deposited on both SRO and STO substrates. The surfaces of the coated samples were free from cracks, relatively smooth, uniform and dense. The profile of X-ray diffraction confirmed the formation of single-c-domain/single crystal perovskite phase grown on both substrates. The XPS analysis have shown that the PZT thin film grown by this method and that a target of PZT+10% PbO is a proper target for growing nominal PZT thin films. The adhesion strength and micro hardness results have confirmed the stability and durability of the thin film on the substrates, although higher values have been reported for thin film of PZT deposited on SRO surfaces.

WrJFS: A Write-Reduction Journaling File System for Byte-addressable NVRAM

Non-volatile random-access memory (NVRAM) becomes a mainstream storage device in embedded systems due to its favorable features, such as small size, low power consumption, and short read/write latency. Unlike dynamic random access memory (DRAM), NVRAM has asymmetric performance and energy consumption on read/write operations. Generally, on NVRAM, a write operation consumes more energy and time than a read operation. Unfortunately, current mobile/embedded file systems, such as EXT2/3 and EXT4, are very unfriendly for NVRAM devices. The reason is that current mobile/embedded file systems employ a journaling mechanism for increasing its data reliability. Although a journaling mechanism raises the safety of data in a file system, it also repeatedly writes data to a data storage while data is committed and checkpointed. Though several related works have been proposed to reduce the amount of write traffic to NVRAM, they still cannot effectively minimize the write amplification of a journaling mechanism. Such observations motivate us to design a two-phase write reduction journaling file system called wrJFS. In the first phase, wrJFS classified data into two categories: Metadata and user data. As the size of metadata is usually very small (few bytes), byte-enabled journaling strategy will handle metadata during commit and checkpoint stages. In contrast, the size of user data is very large relative to metadata; thus, user data will be processed in the second phase. In the second phase, user data will be compressed by hardware encoder to reduce the write size and managed compressed-enabled journaling strategy to avoid the write amplification on NVRAM. Moreover, we analyze the overhead of wrJFS and show that the overhead is negligible. According to the experimental results, the proposed wrJFS outperforms other journaling file systems even though the experiments include the overhead of data compression.

Temperature-driven topological quantum phase transitions in a phase-change material Ge2Sb2Te5

The Ge2Sb2Te5 is a phase-change material widely used in optical memory devices and is a leading candidate for next generation non-volatile random access memory devices which are key elements of various electronics and portable systems. Despite the compound is under intense investigation its electronic structure is currently not fully understood. The present work sheds new light on the electronic structure of the Ge2Sb2Te5 crystalline phases. We demonstrate by predicting from first-principles calculations that stable crystal structures of Ge2Sb2Te5 possess different topological quantum phases: a topological insulator phase is realized in low-temperature structure and Weyl semimetal phase is a characteristic of the high-temperature structure. Since the structural phase transitions are caused by the temperature the switching between different topologically non-trivial phases can be driven by variation of the temperature. The obtained results reveal the rich physics of the Ge2Sb2Te5 compound and open previously unexplored possibility for spintronics applications of this material, substantially expanding its application potential.

A nonvolatile memory device made of a graphene nanoribbon and a multiferroic BiFeO3 gate dielectric layer

We demonstrate a field-effect nonvolatile random access memory (NVRAM) device made of a graphene nanoribbon (GNR) and a multiferroic epitaxial BiFeO3 thin film. The GNR and the source/drain electrodes were formed by position-controlled dip-pen nanolithography. The NVRAM device exhibited asymmetric hysteresis behavior originating from the combination of the p-type semiconducting behavior of the GNR and the ferroelectric hysteresis of the BiFeO3 layer. The memory window of the NVRAM device was significantly improved by a NH3 annealing process which changed the p-type GNR to n-type.

Low Temperature Method for Enhancing Ferroelectric Thin Films in Non-Volatile Random Access Memory Devices

In this study, the electrical properties of as-deposited Sr0.4Ba0.6Nb2O6 (SBN) ferroelectric thin films on SiO2/Si(100) substrates were improved by low temperature supercritical carbon dioxide fluid (SCF) process treatment. The as-deposited SBN ferroelectric thin films were treated by SCF process which mixed with pure H2O and propyl alcohol. After SCF process treatment, the memory windows increased in C-V curves, and the passivation of oxygen vacancy and defect in leakage current density curves were obtained. In addition, the improvement properties of as-deposited SBN thin films after SCF process treatment were found by XPS, C-V, and J-E measurement. Finally, the mechanism concerning the dependence of electrical properties of the SBN ferroelectric thin films on the SCF process was discussed.

Fast phase transitions induced by picosecond electrical pulses on phase change memory cells

The reversible and fast phase transitions induced by picosecond electrical pulses are observed in the nanostructured GeSbTe materials, which provide opportunities in the application of high speed nonvolatile random access memory devices. The mechanisms for fast phase transition are discussed based on the investigation of the correlation between phase transition speed and material size. With the shrinkage of material dimensions, the size effects play increasingly important roles in enabling the ultrafast phase transition under electrical activation. The understanding of how the size effects contribute to the phase transition speed is of great importance for ultrafast phenomena and applications.

A Low‐Temperature‐Grown Oxide Diode as a New Switch Element for High‐Density, Nonvolatile Memories

A one-bit cell of a general nonvolatile memory consists of a memory element and a switch element. Several memory elements have been tried given that any bistable states, that is, two charging states, two spin states, or two resistance states, can be used for a memory element. On the other hand, silicon-based transistors have been the most popularly used switch element. However, silicon-based transistors do not conform to high-density, nonvolatile memories with three-dimensional (3D) stack structures due to their high processing temperatures and the difficulty of growing high-quality epitaxial silicon over metals. Here, we show a low-temperaturegrown oxide diode, Pt/p-NiOx/n-TiOx/Pt, applied as a switch element for high-density, nonvolatile memories. The diode exhibits good rectifying characteristics at room temperature: a rectifying ratio of 10 at ± 3 V, a forward current density of up to ∼ 5×10 A cm, an ideality factor of 4.3, and a turn-on voltage of 2 V. Furthermore, we verify its ability to allow and deny access to the Pt/NiO/Pt memory element with two stable resistance states. Under the forward-bias condition, we could access the memory element and change the resistance state, although access was denied under the reverse bias condition. This one-diode/one-resistor (1D/1R) structure could be a promising building block for high-density, nonvolatile random-access memories with 3D stack structures. A p–n diode, like a transistor, is a fundamental circuit element for thin-film electronics. Until now, epitaxial silicon was most frequently used to fabricate p–n diodes in electronic devices with planar structures. However, to increase device density further, we require p–n diodes that are applicable to devices with 3D stack structures. Epitaxial silicon-based p–n diodes cannot be fabricated with stack structures as it is difficult to grow on a metal layer and high processing temperatures are required. On the other hand, although amorphous silicon allows for lower processing temperatures, it does not provide the required semiconducting performance. Therefore, to realize high-density electronic devices with 3D stack structures, we need new p–n diodes composed of semiconducting materials with low processing temperatures and high performance. In particular, new p–n diodes with low processing temperatures and high performance are indispensable to high-density, nonvolatile random-access memory devices. By replacing a transistor with a simpler diode as a switch element, there exists the possibility of producing memory cells with cross-point structures composed of bit lines and word lines perpendicular to each other, with a memory element lying between them. Theoretically, by utilizing this cross-point structure, the cell size can be scaled down to 4F (F: feature size used for patterning the cell), which is the smallest cell size attainable in nonvolatile memories with planar structures. Furthermore, by fabricating 3D stacks of the cross-point structure, the effective cell size can be scaled down to 2F, 1F, and so on. A common issue in realizing a cross-point structure is the availability of a thin-film diode with the high rectifying ratio and current density required for the switch element to access the memory element. Oxide based p–n diodes are good candidates to provide solutions to the issues associated with Si-based diodes. Most oxides, such as TiO2, [4] ZrO2, [5] ZnO, and indium tin oxide (ITO), are well-known n-type semiconductors that are characterized by the electron-transport properties of oxygen vacancies. As NiOx is a well-known p-type semiconductor beC O M M U N IC A IO N

Electrical properties of metal-ferroelectric-insulator-semiconductor structures based on ferroelectric polyvinylidene fluoride copolymer film gate for nonvolatile random access memory application

Metal-ferroelectric–insulator-semiconductor device structures with ferroelectric vinylidene fluoride-trifluoroethylene copolymer and SiO2 buffer layer integrated gate stack over n-Si are formed, and their potential for fabricating polymeric nonvolatile random access memory devices is demonstrated. Capacitance-voltage (C–V) studies show that switchable polarization in poled polyvinylidene fluoride PVDF copolymer film changes the Si-surface potential and causes modulation of the Si-surface conductance. The (C–V) hysteresis and bidirectional flatband voltage shift at −10 to +6V, depending on the polarization field direction and remnant polarization at the ferroelectric PVDF copolymer gate, presents a memory window. The space charge at n-Si and switchable polarization both reduce the field across the ferroelectric PVDF. The observed asymmetry of the negative flatband-voltage shifts in the negatively poled ferroelectric polymer state is the result of the depletion layer formation, which reduces the field across the polymeric gate. Internal field due to negative and positive bound charges within PVDF copolymer and SiO2, respectively, influences polarization switching by pinning of dipoles. Higher negative gate bias is needed to overcome the pinning effect and to switch the polarization field. @2004 American institute of Physics.

Pulsed laser ablation deposition and characterization of superconductive/ferroelectric bilayers

Superconductive/ferroelectric YBCO/PZT bilayer structures have been deposited by pulsed laser ablation on (100) MgO and substrates. Before the fabrication of the bilayer, optimization of the YBCO deposition parameters was carried out to improve the electrical, morphological and structural characteristics of the films. The results of x-ray diffraction spectroscopy, resistance versus temperature measurements, atomic force and scanning electron microscopies demonstrate the growth of YBCO films suitable as bottom layers for subsequent PZT deposition. The superconductive/ferroelectric bilayers, as characterized by x-ray diffraction and scanning electron microscopy, demonstrate the presence of the correct lattice structures for both YBCO and PZT layers and a good surface morphology (i.e. the absence of cracks and holes and low droplet density). The ferroelectric capacitors realized by using the YBCO layer and evaporated gold dots as bottom and top electrodes, respectively, exhibit excellent ferroelectric behaviour (remanent polarization , coercive field ), suggesting that laser deposited YBCO/PZT bilayers are promising as candidates for the fabrication of non-volatile random access memory devices.

TfC17. Microstructural, compositional and electrical characterization of ferroelectric lead zirconate titanate thin films

Abstract Thin lead zirconate titanate (PZT) ferroelectric films are currently being used in a variety of applications. One of the more novel uses is in non-volatile random access memory devices. In this study, polycrystalline ferroelectric thin films were rf-sputtered on Pt-metallized Si wafers. La-doped as well as undoped PZT films were analyzed. The microstructure was examined by X-ray diffraction and TEM. The electrical characterization was carried out on discrete PZT capacitors by using switching measurements between 20 and 150°C. Nanoprobe analysis of the composition of the films has also been performed and the difficulties of obtaining reliable analytical results are explored in detail. The data obtained from these electrical and microstructural studies were correlated and modeled in order to directly relate switching to the microstructure required for improved device performance.

Structural and chemical composition investigation of thin lead zirconate titanate films

Thin lead zirconate titanate (PZT) ferroelectric films are currently being used in a variety of applications. One of the more novel uses is in nonvolatile random access memory devices. As part of such a memory device, these PZT films undergo a significant amount of thermal cycling during standard semiconductor processing. Therefore, it is of considerable importance to understand the evolution of the PZT film morphology and composition as a function of temperature. The ferroelectric domain and PZT grain boundary evolution in sputtered and sol-gel deposited PZT films has been recorded. Hot stage transmission electron microscopy has been used for this investigation between room temperature and 600 °C. The ferroelectric domains in all the samples are very distinct. The transition from the tetragonal to the cubic paraelectric phase was observed at Curie temperatures (TC) close to those expected for the various compositions analyzed. Zr/Ti ratios in these films ranged from 48/52 to 30/70. Since the films are polycrystalline and some compositional inhomogeneity exists from crystallite to crystallite, a distribution of transition temperatures has been observed throughout a single PZT film. This result fits very well with electrically measured values of TC, which exhibit very broad maxima. Nanoprobe analysis of the composition of these films was performed before and after heating to see if there are any significant changes. Line scans were used to analyze grain and grain boundary composition in both plan view and cross section. The difficulties of obtaining reliable analytical results are explored in detail.

Compositional and microstructural characterization of thin film lead zirconate titanate ferroelectrics

Recent interest in thin film lead zirconate titanate (PZT) ferroelectrics has been intensified by the possible applications of these materials to non-volatile random access memory devices. In general, these thin films have different properties when compared with bulk ceramics of the same composition (Zr:Ti ranging from 48:52 to 30:70). We have begun investigating the composition and microstructure of sputtered submicron PZT films via electron probe microanalysis (EPMA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Bulk analysis of various films (EPMA) shows reasonable agreement with the composition of the target (especially Zr:Ti ratios). SEM illustrates a variety of microstructures which primarily depend on target material, sputtering conditions and substrate morphology. Plan-view TEM has been used to investigate the grain orientation and morphology of the ferroelectric films. Grain size for PZT varies considerably from tens of nanometers to several hundred nanometers. The grain size depends on the composition of the films and the sputtering conditions. Cross-sectional TEM samples are being studied to determine the degree of abruptness of the various heterointerfaces in a device structure.