Nonvolatile Random Access Memory Applications Research Articles

Exploration of the Structural, Dielectric, Ferroelectric, and Piezoelectric Properties in Non-stoichiometric Sr1−XBi2+2X/3Nb2O9 Ceramics

In this study, the authors focused on the preparation and characterization of Lead-free ferroelectric ceramics known as Sr1−XBi2+2X/3Nb2O9: SBBNX, where 0.0≤ × ≤0.4. The ceramics were synthesized using a conventional solid-state reaction route. The researchers performed X-ray diffraction (XRD) analysis to confirm the phase formation and employed the Rietveld refinement technique. The results indicated that the prepared SBBNX ceramics exhibited a single phase with an orthorhombic structure, specifically belonging to the A21am space group. All samples were subjected to analysis using field emission scanning electron microscopy (FESEM) to investigate their surface morphology. The micrographs revealed a uniform distribution of grains (of size 2–4 μm ) on the surface, with distinct grain boundaries. An increasing trend in both the dielectric constant (from 99 to 220 at room temperature) and transition temperature (from 396 °C to 490 °C) was noted with an increase of Bi content in SBN. The researchers estimated the activation energies by analyzing the Arrhenius plots of the ac conductivity. The obtained values ranged from 0.62 to 1.25 eV, indicating the presence of motion of oxygen vacancies in the SBBNX ceramics. The primary objective of this study was to optimize the Sr/Bi ratio in SBBNX ceramics to enhance their potential use in non-volatile random access memory (NVRAM) applications and high-temperature piezoelectric devices.

Magnetic field control and swift heavy ion beam assisted tuning of resistive switching properties of BSFO/CFO/LNO heterostructures

Resistive switching (RS) behavior in mixed oxide insulators has shown a great promise as memristors or non-volatile resistive random-access memory (RRAM) applications. For dilute magnetic oxide multilayers, a novel approach of controlled defects induced and the magnetic field control of RS behavior is proposed. Resistive switching in Bi0.6Sr0.4FeO3 /CoFe2O4 /LaNiO3 (BSFO/CFO/LNO) multilayer heterostructures has been investigated as a case study. All oxide junctions consisting of conducting LaNiO3 (LNO) bottom electrode and BSFO-CFO active layers were fabricated by using chemical solution deposition. A set of samples were irradiated with 150 MeV Ag11+ ions for three different ion fluence of ∼1 × 10+11 ions cm−2, 1 × 10+12 ions cm−2 and 5 × 10+12 ions cm−2. Polycrystalline phase pure films with smooth, crack free surfaces were observed for pristine and irradiated samples. Optical spectroscopy revealed a decrease in the transmittance upon increasing ion fluence due to increase in the light scattering centres. The optical band gap showed a systematic decrease from 2.09 eV to 1.65 eV with increasing ion fluence. Room temperature I-V characteristics showed consistent and pronounced bipolar switching for all samples below ± 5 V. Upon applied magnetic fields of 0.58 T, the resistive switching ratios were found to increase significantly and were further tuned by 150 MeV Ag11+ ion beam irradiations. The magnetic field control of electrical transport properties in the controlled defect assisted oxide heterojunctions offers new insights to the existing understanding of oxide-based RS mechanism.

Formation of ferroelectric multi-domains and electrical conduction in epitaxial BiFeO3 nanodots fabricated with AAO nanotemplates

The electrical conduction property of ferroelectric domains formed in nanoscale dots is an important issue for miniaturized non-volatile random access memory applications. Herein, the conduction characteristics of ferroelectric domain structures formed in epitaxial BiFeO3 (BFO) nanodots are evaluated. Nanoscale BFO dots are fabricated on Nb-doped SrTiO3 (Nb:STO) substrates using an anodic aluminum oxide (AAO) nanotemplate, spin-coating, and etching processes. Using various sizes, such as 65 nm, 50 nm, and 35 nm, of the BFO nanodots, the domain wall current and the current that is dependent on the direction of polarization are measured. A piezoresponse force microscope study reveals that multiple ferroelectric domains coexist in one BFO nanodot. A large current is measured using a conducting atomic force microscope at the domain wall location in the ferroelectric domains. After ferroelectric polarization switching of the domains, the current is slightly dependent on the polarization direction. The electrical resistance state is dependent on the presence or absence of domain walls within the ferroelectric domains of the BFO nanodots.

Improved thermal stability and direct hexagonal transition accompanied by metal-insulator transition in Arsenic substituted Ge2Sb2Te5

Amorphous chalcogenides, particularly Ge2Sb2Te5 (GST) based alloys, are well known for their non-volatile phase-change random access memory applications (PCRAM). In this work, the phase change properties of Ge2Sb2−xAsxTe5 (x = 0, 0.5, 1.0, 2.0) thin films deposited by thermal evaporation are reported. The As substituted samples crystallize at higher temperatures compared to parent GST. During the phase change for x > 1.0, a direct transition from amorphous to the stable hexagonal structure has been observed. A distinct two-step transition in Sb rich samples and a single step transition for As rich samples are observed in R-T measurements with a high contrast in electrical resistivity. The transition is becoming sharper and sharper with increasing As substitution. A composition-dependent Metal-Insulator Transition (MIT) is also observed in these samples. Compared to GST, As substituted samples show an increase in crystallization temperature and activation energy for crystallization. For GST, the 10-year data retention temperature is 67 °C, and with complete As substitution, it increases to 169 °C, with a significant rise of 102 °C in the data retention temperature. High thermal stability, sharp transition and increased data retention of As substituted Ge-Sb-Te suggest that they are promising candidates for PCRAM applications.

The technique to symmetrize domain switching hysteresis loops in LiNbO3 domain-wall nanodevices with improved polarization retention

Ferroelectric devices have wide applications in nonvolatile random-access memories, sensors, actuators, and transducers. The built-in potential at the interfaces could result in poor polarization retention. Here, we found an effective way to independently adjust two coercive fields of LiNbO3 mesa-like domain wall devices in contact to two side electrodes at the surface. Taking advantage of the electrode shielding effect on the depolarization field across an interfacial layer, the near-zero coercive field increases almost linearly with respect to the extending length of one side electrode covering the tail of the reversed domain, enabling nonvolatile ferroelectric domain wall devices in good polarization retention and fast operation speeds.

Resistive switching effect in RE-Doped cobalt ferrite nanoparticles

Spinel ferrite along with numerous important properties, also exhibit resistive switching effect. Cerium (Ce) doped Co-ferrite nanoparticles having composition CoFe2-xCexO4 (where x = 0.0, 0.05, 0.15, 0.20) were synthesized by co-precipitation synthesis route. X-ray Diffraction analysis confirmed the formation of cubic spinel phase with minor amount of impurities. Frequency dependent dielectric and electrical properties were studied at room temperature within frequency range of 20 Hz to 3 MHz. Dielectric constant, dielectric losses and AC conductivity decreases with cerium substitutions which is usual trend observed in ferrite materials. Modified Debye function was employed to fit Dielectric constant (ε′)- frequency (f) data. Jonscher's power law was employed to study AC conductivity. DC resistivity decreased with increasing temperature. DC resistivity and drift mobility decreased with cerium substitution while activation energy increased. Current–Voltage (I–V) curves of the synthesized composition were studied at room temperature. Current–Voltage (I–V) curves showed nonlinear hysteresis loop like behavior. Current follow different values for forward and reversed applied DC voltage which confirmed that resistive switching effect exit in studied samples. Sample with Ce composition x = 0.25 exhibit larger hysteresis loop than that of other compositions. This shows that sample for Ce composition x = 0.25, can be employed for nonvolatile Resistive Random-Access Memory (ReRAM) applications. Resistive switching effect was explained by utilizing Space-charge-limited current (SCLC) conduction model and by formation of a Schottky barrier at the metal-semiconductor interface.

Micromagnetic simulation of vortex core polarity in bi-component magnetic nanodisks

The nucleation and management of the vortex state in magnetic nanostructures has received a lot of interest in recent decades, with potential applications in logic networks and non-volatile magnetic random-access memory. Because of their potential use in magnetic memory, magnetic vortex structures are of considerable technical importance. Investigation of bi-component magnetic nanodisks as prospective storage systems, in both single and coupled configurations, has been performed, where the information unit is represented by vortex core polarity. The work mainly engages in magnetic nanodisks of permalloy and iron having a diameter of 200 nm and a thickness of 40 nm. The potential of adjusting the polarity of the vortex core by the application of a magnetic field using micromagnetic simulations has been illustrated. Micromagnetic simulation studies show that bi-material structures are important not just for fundamental research but also for data storage.

BEOL Integrated Ferroelectric HfO₂-Based Capacitors for FeRAM: Extrapolation of Reliability Performance to Use Conditions

Si doped HfO2 based ferroelectric capacitors integrated into Back-End-Of-Line (BEOL) 130 nm CMOS technology were investigated in regard to critical reliability parameters for their implementation in non-volatile one-transistor one-capacitor ferroelectric random-access memory applications. The assessed reliability parameters are electric field, capacitor area, and temperature and are evaluated on single and parallel structured capacitors to understand their respective impact on wake-up, fatigue, imprint, and retention.

Comments on the electronic transport mechanisms in the crystalline state of Ge—Sb—Te phase-change materials

It is known that phase-change materials, such as Ge–Sb—Te ternary system, are promising resistive non-volatile random-access memory applications with ultra-rapid reversible transformations between the crystalline and amorphous phases. This class of electronic transition is categorized to be the metal-insulator transition (MIT). The Anderson-type MIT has been discussed extensively in phase-change materials (PCMs) and isothermal annealing of amorphous PCMs (a-PCMs) which, above a certain temperature, leads to the metallic (crystalline) phase. In the insulator regime near the MI transition, Mott-type variable-range hopping (VRH) and/or Efros-Shkolvskii hopping (ESH) at low temperatures below 20 K (and down to 1 K) in Ge1Sb2Te4 (GST124) have been discussed extensively, however, we criticize the above argument through a detailed discussion of physical parameters that support the VRH mechanism. It is not clear whether or not the density-of-states (DOS) near the Fermi level is localized (like the Fermi glass) in the crystalline phase. It is also suggested that grain boundaries are expected to interfere with the electronic transport in the crystalline state. We should take into account the grain boundary effects on the electronic transport in the crystalline phase of Ge—Sb—Te system.

Optically triggered multilevel resistive switching characteristics of Cu/MoS2/AlN/ITO bilayer memory structure

In this work, the tunable resistive switching (RS) functionality of a Cu/MoS2/AlN/ITO nanostructured device is systematically investigated in dark and white light illumination. The device exhibits bi-state RS behavior in the dark ambient, whereas light illumination induces an extra intermediate resistance state and provides controllable tri-state RS characteristics. A conceptual model is proposed and discussed to elucidate the origin of the switching behavior of two resistance states and multiple resistance states of the device. Under the dark ambient condition, the high resistance state and the low resistance state in the device could be ascribed to the formation/rupture of a Cu metallic filamentary path between the electrodes. However, the formation of an additional ionic filament via trapping/detrapping of electrons in nitride-sulfide-related vacancies along with the Cu metallic filament is responsible for the tri-state switching under the light illumination. Interestingly, the variation of SET voltage with applied light intensity has also been demonstrated. The calculated value of the temperature coefficient and temperature dependency of resistance in various resistance states confirms the existence of the proposed model. The device performed a good undispersed endurance up to 1.5 × 103 cycles and stable retention over 103 s at room temperature. This optical activity dependent functionality of the device provides a possibility to extend resistive switching-based nonvolatile random access memory applications to the optical domain such as imaging sensors, photodetectors, and optoelectronic switches.

Aminobenzene stabilized bismuth halide nanoparticles with O-shaped hysteresis behaviour

A single pot synthesis method of aminobenzene stabilized bismuth iodide particles (ABBI) is reported for the nonvolatile random access memory application. The microscopic and surface property of the as-synthesized organic-inorganic hybrid system has been characterized using different techniques. The current-voltage characteristic of the device, made with ABBI, exhibited O-shaped hysteresis behaviour. For the random access memory application, the device was demonstrated the steady endurance and retention for 103 times and 2×103 sec, respectively. The charge transport property of the device was followed by space charge limited current and trapped assisted tunneling mechanism.

Electronic mechanism for resistance drift in phase-change memory materials: link to persistent photoconductivity

‘Phase-change’ memory materials, such as the canonical composition Ge2Sb2Te5, are being actively researched for non-volatile resistive random-access memory applications. In these devices, ultra-rapid reversible transformations between metastable highly electrically conducting (degenerate-semiconducting) crystalline and more electrically resistive (semiconducting) glassy phases are produced by the application of appropriate voltage pulses. Multilevel programming, wherein more than two metastable resistance states can be stored in the memory material as different proportions of partially glassy/crystalline regions, allows more than one bit to be stored per memory cell. However, this route to increasing data density, without recourse to device-size down-scaling, is threatened by the phenomenon of ‘resistance drift’, wherein the electrical resistance of the glassy phase slowly increases with time, following a weak power-law dependence, after being written with a voltage pulse. In this paper, we propose an intrinsic electronic mechanism for the resistance drift, particularly valid at ambient temperature and below, by identifying it with the phenomenon of persistent photoconductivity that is commonly observed in a wide range of disordered semiconductors. We develop a model for it in terms of the long-time, deep-trap release and subsequent recombination of charge carriers, akin to that which is believed to be responsible for the long-time photocurrent decay in amorphous semiconductors, such as hydrogenated amorphous silicon. In this case, the parameters controlling the resistance drift are the widths of the (localized) valence- and conduction-band tails in the vicinity of the bandgap. Hence, there is the potential for mitigating resistance drift in the amorphous state of phase-change memory materials by suitable material engineering (e.g. via compositional or fabricational control) to control the extent of band-tailing, thereby facilitating the future introduction of multistate memory.

Dielectric properties of Na and Pr doped SrBi4Ti4O15 ceramics

Bismuth Layer-Structured Ferroelectrics materials (BLSFs) are widely used for many applications like resonators, filters, sensors, actuators and are appropriate for non-volatile Ferroelectric Random Access Memory (FeRAM) applications. The additional advantage of BLSF materials is that in electrical applications in they work at high temperatures. In the case of resonator applications, piezoelectric components are used as inductors. Usually BLSF materials typically have higher Curie temperature than PZT materials, so instead of PZT materials, BLSFs are used for high temperature piezoelectric sensor applications. SrBi4Ti4O15 (SBT) compound is a member of Aurivillius family Bismuth layered structured Ferroelectrics. The dielectric properties of Sodium and Praseodymium doped SBT, Sr0.2Na0.4Pr0.4Bi4Ti4O15 (SNPBT), prepared by solid state double sintering method using planetary ball mill, and is studied in the present work. Single phase is confirmed from XRD and microstructure is studied from SEM. The temperature dependence of dielectric constant and dielectric loss are studied at different frequencies. The Curie temperature increased by doping SBT with Na1+and Pr3+. Thus this piezoelectric ceramic is suitable for high temperature sensor applications.

White light-modulated bipolar resistive switching characteristics of Cu/MoS2 NRs/Pt MIM structure

The present work explores the white light controlled resistive switching functionality of MoS2 nanorods (NRs) by fabricating a metal-insulator-metal stack configuration. The Cu/MoS2 NRs/Pt/Si device demonstrates the reproducible two-state bipolar resistive switching characteristics under both dark and light environments. In the dark condition, the resistive switching behavior of the NR device could be attributed to the metallic path formation/rupture between top and bottom electrodes. Whereas the applied white light causes the lowering of SET and RESET voltages by inducing conducting path formation/rupture via electron trapping/detrapping in sulfur vacancies across the MoS2 NRs. The formation of a conducting path under dark and light illumination conditions is well explained by proposing a conceptual model and analyzing the resistance vs temperature measurements. It is observed that the white light acts as an external tool to modulate the resistive switching behavior of the fabricated NR device. The correlation between the applied light intensity and the SET voltage is also demonstrated. The NR structure of the MoS2 device provides good endurance of 1500 cycles and a long retention time of 103 s at room temperature under light illumination because of straight conducting path formation through NRs. These results demonstrate that the optically active MoS2 NR based devices have potential for next generation tunable nonvolatile resistive random access memory applications with additional functionality such as photosensors and optoelectronic switches.

Lattice Dynamic and Instability in Pentasilicene: A Light Single-Element Ferroelectric Material With High Curie Temperature

For applications in Si-based electronics, it would be interesting and significant to induce spontaneous polarization in some allotrope of pure Si. However, because of the symmetry protection in conventional Si forms, separating positive and negative charge states is difficult. This study shows that a desired Si structure, $T$ pentasilicene, can be realized by tilting the dimers in the two-dimensional pentasilicene sheet, which not only stabilizes the system but also leads to intrinsic ferroelectricity, with a high Curie temperature of 1190 K. This Si allotrope could have potential applications in nonvolatile random-access memory.

Heterogeneous CFs induced unipolar and bipolar resistive switching behaviors in InGaZnO thin films

This paper investigates resistive switching behaviors of amorphous InGaZnO (a-IGZO) thin films for non-volatile resistive random access memory applications. Reversible transition process between bipolar and unipolar modes in Ag/InGaZnO/Pt memory cell takes place through changing the polarities of sweeping voltages, whereas the reset process cannot be operated by applying a positive voltage. The dominant conduction mechanisms in both modes are Ohmic conduction for the low resistance state, but different for the high resistance state, that is, Poole-Frenkel emission in the unipolar mode and trap-controlled space charge limited conduction in the bipolar mode. Temperature dependence of low resistance state and transmission electron microscopy analysis further indicate that the dominant conductive filaments (CFs) are composed of Ag atoms for the bipolar mode and oxygen vacancies for the unipolar mode, respectively. These two kinds of conductive filaments play a coordination role in resistive switching transition between bipolar and unipolar modes in Ag/InGaZnO/Pt memory devices. Based on the conductive filament model, the physical mechanisms for resistive switching are discussed.

Bipolar resistive switching behavior in MoS2 nanosheets fabricated on ferromagnetic shape memory alloy

The present study explores the systematic investigation of resistive switching response of magnetron sputtered MoS2 thin films sandwiched between a Ni-Mn-In ferromagnetic shape memory alloy (bottom) and copper (top) electrodes. The Cu/MoS2/Ni-Mn-In device exhibits stable and reproducible bipolar resistive switching behavior. The current-voltage (I-V) analysis suggests that the device shows ohmic conduction behavior in the low resistance state (LRS) while space charge limited conduction is the dominating conduction mechanism in the high resistance state (HRS). The compliance current vs reset current measurements were also performed which reveals that the power consumption of the device can be suppressed by decreasing the compliance current. To explain the resistive switching behavior in the Cu/MoS2/Ni-Mn-In device, a conceptual model based on copper ion migration, is proposed and well explained. The resistive switching parameters such as the set voltage, LRS and HRS resistances, are also investigated in a temperature range overlapping with the martensite phase transformation temperatures of the bottom ferromagnetic shape memory alloy (Ni-Mn-In) electrode. It provides temperature as an extra degree of freedom to modulate the resistive switching characteristics of the fabricated device. The Cu/MoS2/Ni-Mn-In device shows a decent endurance of 500 and a long retention of 103. These results demonstrate the feasibility of the MoS2 based devices for futuristic tunable nonvolatile resistive random access memory applications.

Observation of AlO x material in electrical resistive switching for nonvolatile random access memory application

We fabricated an Al / AlO x / Al device by using a RF magnetron sputter system. The device showed a unipolar resistive switching process. In this study, the switching mechanism of the device followed the conductive filament model. The conduction mechanisms for the conductive filament model were explained by using Ohmic conduction for the low resistance state (LRS) and Schottky emission for the high resistance state (HRS). The average value of the resistance ratio between the HRS and the LRS was about 3.48 × 107 when the reading voltage (0.1 V) was achieved. The electrical property of the endurance was achieved under 50 switching cycles. A low switching voltage could be obtained for a low power consuming device. These results proved that the AlO x material has various possibilities for use in nonvolatile random access memory applications.

Synthesis, structural and dielectric properties of Ho-doped SrBi2Nb2O9 prepared by Co-precipitation method

This paper reported the synthesis of Ho-doped SBN (SrBi2− x Ho x Nb2O9: x = 0 and 0.4) by a co-precipitation method and investigated the synthesized material with X-ray diffraction analyses, infrared spectroscopy, and electrical measurements. The results show that the Ho-doped SBN has low loss values at higher frequencies and the flattening of dielectric constant which are very desirable in nonvolatile ferroelectric random access memory applications.

Tunneling electroresistance of MgZnO-based tunnel junctions

We investigated the tunneling electroresistance (TER) in metal/wurtzite-MgZnO/metal junctions for applications in nonvolatile random-access memories. A resistive switching was detected utilizing an electric-field cooling at ±1 V and exhibited a TER ratio of 360%–490% at 2 K. The extracted change in the average barrier height between the two resistance states gave an estimation of the MgZnO electric polarization at 2.5 μC/cm2 for the low-temperature limit. In addition, the temperature-dependent TER ratio and the shift of the localized states energies at the barrier interface supported the ferroelectric behavior of the MgZnO tunnel-barrier. From the first-principles calculations, we found a similar effect of the barrier height change coming from the reversal of ZnO electric polarization. The possibility of using metal electrodes and lower growth temperatures, in addition to the ferroelectric property, make the ZnO-based memory devices suitable for CMOS integration.