Applications In Nonvolatile Memory Devices Research Articles

Resistive switching behavior in Pt/YSZ/Nb:SrTiO3 heterostructure for nonvolatile multilevel memories

The yittra-stablized zirconia (YSZ) thin film was deposited on a Nb:SrTiO3 (NSTO) substrate by pulsed laser deposition to form a Pt/YSZ/NSTO heterostructure device. This device exhibits high resistive switching ratio of 105 at a read voltage of −0.1V after applied +3V/−4V pulse voltages. Moreover, the resistance states could be reversibly switched among multilevel resistance states by changing the magnitude of Set or Reset pulse voltages, which shows potential application in multilevel nonvolatile memory devices. The RS mechanism of the device could be attributed to the modulation of YSZ/NSTO Schottky-like junction depletion, which is caused by the carrier injection-trapped/detrapped process under the applied electric field.

Superior resistive switching behaviors of FeWO4 single-crystalline nanowires array

The resistive switching behaviors of metal–oxide–metal nanoscale devices are a fascinating phenomenon for next generation nonvolatile memories. Herein for the first time we demonstrate nanoscale memristive devices made from a FeWO4 nanowires array. The device shows superior bipolar resistive switching behaviors. This study is useful for exploring the multifunctional materials and their applications in nonvolatile multistate memory devices.

White-light-controlled resistance switching in TiO2/α-Fe2O3 composite nanorods array

White-light-controlled resistance switching in TiO2/α-Fe2O3 composite nanorods array grown on fluorine-doped tin oxide substrate by hydrothermal process is investigated. The average length of TiO2/α-Fe2O3 nanorods is about 3.5 μm, and the average diameter is about 250 nm. The sizes of the α-Fe2O3 particles are in the range of 30 ~ 70 nm. The current–voltage characteristics of the composite nanorods array show a good rectifying property and bipolar resistive-switching behavior, and the resistive-switching behavior can be regulated by white-light illumination at room temperature. This study is helpful for exploring the multifunctional materials and their applications in nonvolatile multistate memory devices.

Enhanced resistive switching effect upon illumination in self-assembled NiWO4 nano-nests.

The resistive switching effect of devices with metal-oxide-metal structure is a fascinating candidate for next generation nonvolatile memory devices. Here, self-assembled NiWO4 nano-nests on a Ti substrate were synthesized by a hydrothermal process. Moreover, a resistive switching memory device with Ag/NiWO4/Ti structure is demonstrated. The device shows an enhanced bipolar resistive switching effect under white-light illumination. This study is useful for exploring multifunctional materials and their applications in light-controlled nonvolatile memory devices.

Bipolar Resistive Switching Characteristics of TiN/HfOx/ITO Devices for Resistive Random Access Memory Applications

Resistive switching characteristics of hafnium oxide are studied for possible nonvolatile memory device applications. The HfOx films with different oxygen contents are deposited by rf magnetron sputtering under different O2 flow rates. The films are amorphous, and the stoichiometric of the film is improved by increasing the O2 flow rate. Current-voltage characteristics of the TiN/HfOx/ITO device are investigated with 1 mA compliance. The bipolar resistive switching behavior is observed for the TiN/HfOx/ITO structure, and the resistive switching mechanism of the TiN/HfOx/ITO structure is explained by trap-controlled space charge limit current conduction.

In Situ Synthesis of High Density sub-50 nm ZnO Nanopatterned Arrays Using Diblock Copolymer Templates

Fabrication of high density (~155 Gbit in(-2)) ZnO nanopatterns through in situ decomposition of Zn precursors inside diblock copolymer templates and their application as charge storage centers in nonvolatile memory devices is described. The fabrication is performed in a highly controlled fashion with the resulting ZnO nanopatterned arrays exhibiting diameters of 38 nm and heights of 14 nm offering sub-50 nm feature resolutions. The ZnO nanopatterns are naturally n-type due to the presence of zinc interstitials and oxygen vacancies that act as defect levels in trapping charge carriers. Test capacitors (metal-oxide-semiconductor, MOS) constructed using nanopatterns formed on p-Si exhibited a large flatband voltage shift of about ~2.2 V for a low operating voltage of 10 V. A high charge trap density of 3.47 × 10(18) cm(-3) combined with a good retention capacity is observed with low tunneling oxide (thermally grown) thickness of 3 nm. This demonstrates the significant promise of the ZnO nanopatterned arrays to act as charge storage centers for potential application in nonvolatile flash memory devices. The charge trapping characteristics, the capacitance-voltage measurements, and the potential of ZnO nanopatterns as charge storage centers in fabricating nonvolatile memory devices are discussed.

Effect of La substitution on structure and magnetic properties of sol-gel prepared BiFeO3

Detailed Rietveld analysis of the room temperature (RT) x-ray diffraction data of LaxBi1−xFeO3 (LBFO) ceramics (0 ≤ x ≤ 0.4) prepared by sol-gel route revealed that the system retains distorted rhombohedral R3c structure of parent compound BiFeO3 (BFO) for x ≤ 0.2 but transforms to orthorhombic Pbnm structure for x > 0.2. The fundamental 12 Raman modes of pure BFO (x = 0.0) are reduced to 10 modes for x = 0.2 and further reduced to 7 modes for x = 0.4, which is a clear indication of structural modification and symmetry changes brought about by La doping. The coercive field, HC, increases remarkably with an increase in La doping and x = 0.4 exhibits the highest HC (19.5 kOe at RT) reported so far in any rare earth doped BFO, which will have a great potential for practical application in non-volatile memory devices. Moreover, high-field magnetization and remanence, Mr, increase linearly with La doping content due to the gradual change in spin cycloid structure. M(T) curve shows an anomaly at 50 K, similar to that expected in a spin glass system. The Néel temperature (TN) of the LBFO increases with x, approaching that of the LaFeO3.

Inorganic nanoparticle multilayers using photo-crosslinking layer-by-layer assembly and their applications in nonvolatile memory devices

We introduce a general and facile method for the preparation of organic/inorganic nanoparticle (NP) nanocomposite multilayer films that allows vertical growth of various NP layers (i.e., metal or transition metal oxide NPs) in a densely packed structure. Our approach is based on the successive photo-crosslinking layer-by-layer (LbL) assembly between hydrophobic ligands onto a NP surface and photoinitiator (PI) molecules. Therefore, our approach requires neither the additional surface modification needed for well-defined NPs synthesized in organic media nor the deposition step that inserts a polymer layer bridge between adjacent inorganic NP layers in the preparation of traditional LbL-assembled NP films. We also demonstrate that photo-crosslinking LbL-assembled (metal oxide NP)n films could be used as a nonvolatile memory layer without a high-temperature thermal treatment, unlike conventional vacuum-deposition- or sol-gel-derived memory devices, which require thermal treatments at temperatures greater than 200 °C. This robust method could open a facile route for the design of functional NP-based electronic devices.

Chemoselective reduction of graphene oxide and its application in nonvolatile organic transistor memory devices

A facile method for the chemoselective reduction of graphene oxide (CrGO) has been developed via silver(I)-catalyzed decarboxylation. CrGO was characterized by X-ray photoelectron spectroscopy and X-ray diffraction. CrGO can be well-dispersed in most polar solvents, facilitating its nanosheet thin film preparation via a spin coating solution process for device fabrication. A proof of concept nonvolatile organic transistor memory device using CrGO as the charge-trapping layer showed a larger memory window of over 60 V and a higher ON/OFF current ratio of up to 104 compared to that of the precursor, graphene oxide (GO).

Thermally stable memory devices using graphene flakes sandwiched polymethyl methacrylate polymer layers

Fabrication and characterization of graphene flakes sandwiched polymethyl methacrylate (PMMA) hybrid bistable devices (HBD) is reported in this paper. Graphene flakes (GFs) sandwiched polymer layers were prepared through a simple solution process. The fabricated memory devices have exhibited remarkable electrical bistable behavior even at 150°C. Bistable memory behavior of the fabricated devices have been described on the basis of current-voltage characteristics. Reasonably good ON/OFF current ratio (∼103) and retention property have been obtained from the constructed memory devices. The device showed excellent stability up to 105 switching cycles without any degradation both in ON and OFF states. The results obtained have indicated that the HBD fabricated by embedding graphene flakes in an insulating polymer matrix have potential applications in organic nonvolatile flash memory devices.

Resistive switching in a GaOx-NiOx p-n heterojunction

We report a unidirectional bipolar resistive switching in an n-type GaOx/p-type NiOx heterojunction fabricated by magnetron sputtering at room temperature. The resistive switching behavior coincides with the switching between Ohmic conduction (low resistance) and rectifying behavior (high resistance) of the heterojunction diode. Under external electric field, electromigrated intrinsic defects, such as oxygen vacancies and oxygen ions, accumulate at the pn junction interface and modify the interface barrier, forming or rupturing the filamentary paths between n-GaOx and p-NiOx, leading to the switching between Ohmic and diode characteristics of the device. The device shows good endurance, retention performance, and scaling capability, signaling the potential of a diode-structured resistive switching device for non-volatile memory applications.

Opportunity of Spinel Ferrite Materials in Nonvolatile Memory Device Applications Based on Their Resistive Switching Performances

The opportunity of spinel ferrites in nonvolatile memory device applications has been demonstrated by the resistive switching performance characteristics of a Pt/NiFe(2)O(4)/Pt structure, such as low operating voltage, high device yield, long retention time (up to 10(5) s), and good endurance (up to 2.2 × 10(4) cycles). The dominant conduction mechanisms are Ohmic conduction in the low-resistance state and in the lower-voltage region of the high-resistance state and Schottky emission in the higher-voltage region of the high-resistance state. On the basis of measurements of the temperature dependence of the resistances and magnetic properties in different resistance states, we explain the physical mechanism of resistive switching of Pt/NiFe(2)O(4)/Pt devices using the model of formation and rupture of conducting filaments by considering the thermal effect of oxygen vacancies and changes in the valences of cations due to the redox effect.

Unipolar Resistive Switching Effects Based on Al/ZnO/P++-Si Diodes for Nonvolatile Memory Applications

Al/ZnO/P++-Si diodes exhibit typical unipolar resistive switching behaviors. The electroforming-free characteristics are observed after annealing the ZnO thin film at 400°C in air. The ON/OFF ratios of the resistance are in the range of 104–105 at a very low operation voltage of 0.1 V, and the devices show good endurance characteristics of over 400 cycles with negligible reduction. Finally, the memory mechanisms of the diodes are proposed on the basis of the current-voltage and resistance-voltage results. These results indicate that Al/ZnO/P++-Si devices have potential applications in nonvolatile memory devices.

Deterministic conversion between memory and threshold resistive switching via tuning the strong electron correlation.

Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices. It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions. Here, using NiO as an example, we show that rationally adjusting the stoichiometry and the associated defect characteristics enables controlled room temperature conversions between two distinct RS modes, i.e., nonvolatile memory switching and volatile threshold switching, within a single device. Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

Ferroelectric memory based on nanostructures

In the past decades, ferroelectric materials have attracted wide attention due to their applications in nonvolatile memory devices (NVMDs) rendered by the electrically switchable spontaneous polarizations. Furthermore, the combination of ferroelectric and nanomaterials opens a new route to fabricating a nanoscale memory device with ultrahigh memory integration, which greatly eases the ever increasing scaling and economic challenges encountered in the traditional semiconductor industry. In this review, we summarize the recent development of the nonvolatile ferroelectric field effect transistor (FeFET) memory devices based on nanostructures. The operating principles of FeFET are introduced first, followed by the discussion of the real FeFET memory nanodevices based on oxide nanowires, nanoparticles, semiconductor nanotetrapods, carbon nanotubes, and graphene. Finally, we present the opportunities and challenges in nanomemory devices and our views on the future prospects of NVMDs.

A study on low-power, nanosecond operation and multilevel bipolar resistance switching in Ti/ZrO2/Pt nonvolatile memory with 1T1R architecture

Low-power, bipolar resistive switching (RS) characteristics in the Ti/ZrO2/Pt nonvolatile memory with one transistor and one resistor (1T1R) architecture were reported. Multilevel storage behavior was observed by modulating the amplitude of the MOSFET gate voltage, in which the transistor functions as a current limiter. Furthermore, multilevel storage was also executed by controlling the reset voltage, leading the resistive random access memory (RRAM) to the multiple metastable low resistance state (LRS). The experimental results on the measured electrical properties of the various sized devices confirm that the RS mechanism of the Ti/ZrO2/Pt structure obeys the conducting filaments model. In application, the devices exhibit high-speed switching performances (250 ns) with suitable high/low resistance state ratio (HRS/LRS > 10). The LRS of the devices with 10 year retention ability at 80 °C, based on the Arrhenius equation, is also demonstrated in the thermal accelerating test. Furthermore, the ramping gate voltage method with fixed drain voltage is used to switch the 1T1R memory cells for upgrading the memory performances. Our experimental results suggest that the ZrO2-based RRAM is a prospective alternative for nonvolatile multilevel memory device applications.

The influence of interfacial barrier engineering on the resistance switching of In2O3:SnO2/TiO2/In2O3:SnO2 device

The I—V characteristics of In2O3:SnO2/TiO2/In2O3:SnO2 junctions with different interfacial barriers are investigated by comparing experiments. A two-step resistance switching process is found for samples with two interfacial barriers produced by specific thermal treatment on the interfaces. The nonsynchronous occurrence of conducting filament formation through the oxide bulk and the reduction in the interfacial barrier due to the migration of oxygen vacancies under the electric field is supposed to explain the two-step resistive switching process. The unique switching properties of the device, based on interfacial barrier engineering, could be exploited for novel applications in nonvolatile memory devices.

Bipolar Resistive Switching Behavior of a Pt/NiO/TiN Device for Nonvolatile Memory Applications

Bipolar Resistive Switching Behavior of a Pt/NiO/TiN Device for Nonvolatile Memory Applications

Bipolar Resistive Switching Behavior of a Pt/NiO/TiN Device for Nonvolatile Memory Applications

Bipolar resistive switching behavior was observed in a Pt/NiO/TiN device. The device exhibited switching behavior that was stable over 100 cycles and did not degrade after 104 s. An electroforming process was required to obtain these bipolar resistive switching properties, and the conduction behavior of the low resistance state followed Ohm's law, indicating that conductive filaments formed during the electroforming process. The conductive filaments consisted of oxygen vacancies and the Pt electrode behaved as an oxygen reservoir. The bipolar resistive switching of the Pt/NiO/TiN device was explained by the generation and annihilation of oxygen vacancies in the filaments.

Low operation voltage and high thermal stability of a WSi2 nanocrystal memory device using an Al2O3/HfO2/Al2O3 tunnel layer

A WSi2 nanocrystal nonvolatile memory device was fabricated with an Al2O3/HfO2/Al2O3 (AHA) tunnel layer and its electrical characteristics were evaluated at 25, 50, 70, 100, and 125 °C. The program/erase (P/E) speed at 125 °C was approximately 500 μs under threshold voltage shifts of 1 V during voltage sweeping of 8 V/−8 V. When the applied pulse voltage was ±9 V for 1 s for the P/E conditions, the memory window at 125 °C was approximately 1.25 V after 105 s. The activation energies for the charge losses of 5%, 10%, 15%, 20%, 25%, 30%, and 35% were approximately 0.05, 0.11, 0.17, 0.21, 0.23, 0.23, and 0.23 eV, respectively. The charge loss mechanisms were direct tunneling and Pool-Frenkel emission between the WSi2 nanocrystals and the AHA barrier engineered tunneling layer. The WSi2 nanocrystal memory device with multi-stacked high-K tunnel layers showed strong potential for applications in nonvolatile memory devices.