Effect of the top electrode material on the resistive switching of TiO2 thin film

Effect of the top electrode (TE) metal on the resistive switching of (TE)/TiO₂/Pt structure was investigated. It was confirmed that the potential barrier height between the metal and TiO₂ is an important factor on the resistive switching characteristics. When high Schottky barrier was formed with the TiO₂ film, using Pt or Au as a top electrode, both stable URS (unipolar) and BRS (bipolar resistive switching) characteristics were observed depending on the current compliance level. In the case of Ag, which forms a relatively low Schottky barrier, only BRS characteristics were observed, regardless of the current compliance level. In the case of Ni and Al, which have similar work function as Ag, unstable URS and BRS at very low current compliance levels were observed due to a chemical reaction at the interface. For the Ti electrode, resistive switching was not observed, because the work function of Ti is lower than that of TiO₂ and TiO phase was formed at the interface (Ti/TiO_x contact is ohmic).     

Effect of post annealing on the resistive switching of TiO2 thin film

The effect of annealing on the resistive switching of 35-nm-thick TiO₂ thin film deposited with magnetron sputtering system was studied. Pt and Ag were used as a top electrode (TE), and Pt was used as a bottom electrode (BE). For Pt/as-deposited TiO₂/Pt structure, both unipolar (URS) and bipolar resistive switching (BRS) were observed depending on the current compliance level. For Pt/400 °C annealed TiO₂/Pt structure, only BRS was observed regardless of the current compliance level. The increase in the work function of the TiO₂ film after annealing lowers the potential barrier height and changes the electron transfer process which was also confirmed from Ag/as-deposited TiO₂/Pt structure. Above 600 °C, the film becomes leaky with the increase in grain size and roughness and the resistive switching behavior was not observed.     

C-AFM study on multi - resistive switching modes observed in metal–organic frameworks thin films

Metal-organic framework (MOF) materials have recently attracted much attention for use in resistive random-access memory due to the advantages of having high insulative properties, well-defined structures, a large specific surface area, and an adjustable pore size. In this study, the memory device based on zirconium (IV)-carboxylate MOF (UiO-66) nanoparticles exhibits the low operation voltage (V < 0.5 V), high ON/OFF ratio (~10⁴), excellent endurance (5 × 10² cycles), and longtime retention (10⁴ s). To clarify the resistive switching mechanism of the Ag/PVA-MOF/FTO device, conductive atomic force microscopy (C-AFM) was used. The results indicate that all of the electrode, Zr₆ clusters of UiO-66, PVA, and UiO-66 conjugation have simultaneous contributions to the resistance switching behavior. The resistive switching can be controlled either by the electron hopping process between Ag⁺ ions and Zr₆ nodes in threshold mode or the formation/rupture of the metal filaments in the bipolar switching mode. Interestingly, inherent characteristics of MOF materials, such as high porosity and large size cages (octahedral, tetrahedral), strongly influence the transport properties and switching mechanism of the device which is also discussed in detail. These resistive switching characteristics and mechanisms of UiO-66 could provide a thorough understanding for future research and application not just for UiO-66 but also for the general MOFs materials.     

A journey towards reliability improvement of TiO2 based Resistive Random Access Memory: A review

A Resistive Random Access Memory (RRAM), where the memory performance principally originated from ‘resistive’ change rather than ‘capacitive’ one (the case with conventional CMOS memory devices), has attracted researchers across the globe, owing to its unique features and advantages meeting the demands of future generation high-speed, ultra low power, nano dimensional memory devices. A large family of semiconducting oxides have been investigated as insulator for Resistive Random Access Memory (RRAM), amongst which TiO₂ is one of the potential candidate, principally owing to some of its remarkable advantages e.g. wide band gap, high temperature stability and high dielectric constant with flexibility to offer both unipolar and bipolar switching, which are essential for RRAM device applications. In this review article, we tried to represent the long voyage of TiO₂ based RRAM, towards the improvement of the reliability aspects of the device performance in a comprehensive manner. Starting with the key factors like oxygen vacancies, Ti interstitials and electroforming, which are responsible for resistive switching phenomenon, various material preparation techniques for RRAM development have been discussed with emphasis on relative merits and bottlenecks of the process. The factors like electrode material and geometry, device structuring, doping, compliance current, annealing effect etc., which play the pivotal role in determining the switching performance of the device, have been reviewed critically. Finally, the article concludes with the comparison of different TiO₂ based RRAM devices followed by the prediction of possible future research trends.     

Resistive switching characteristics of ultra-thin TiOx

We investigated the resistive switching characteristics of Ir/TiO_x/TiN structure with 50 nm active area. We successfully formed ultra-thin (4 nm) TiO_x active layer using oxidation process of TiN BE, which was confirmed by X-ray Photoelectron Spectroscopy (XPS) depth profiling. Compared to large area device (50 μm), which shows only ohmic behavior, 250 and 50 nm devices show very stable resistive switching characteristics. Due to the formation and rupture of oxygen vacancies induced conductive filament at Ir and TiO_x interface, bipolar resistive switching was occurred. We obtained excellent switching endurance up to 10⁶ times with 100 ns pulse and negligible degradation of each resistance state at 85 °C up to 10⁴ s.     

Interface‐Engineered Amorphous TiO2‐Based Resistive Memory Devices

Crossbar‐type bipolar resistive memory devices based on low‐temperature amorphous TiO₂ (a‐TiO₂) thin films are very promising devices for flexible nonvolatile memory applications. However, stable bipolar resistive switching from amorphous TiO₂ thin films has only been achieved for Al metal electrodes that can have severe problems like electromigration and breakdown in real applications and can be a limiting factor for novel applications like transparent electronics. Here, amorphous TiO₂‐based resistive random access memory devices are presented that universally work for any configuration of metal electrodes via engineering the top and bottom interface domains. Both by inserting an ultrathin metal layer in the top interface region and by incorporating a thin blocking layer in the bottom interface, more enhanced resistance switching and superior endurance performance can be realized. Using high‐resolution transmission electron microscopy, point energy dispersive spectroscopy, and energy‐filtering transmission electron microscopy, it is demonstrated that the stable bipolar resistive switching in metal/a‐TiO₂/metal RRAM devices is attributed to both interface domains: the top interface domain with mobile oxygen ions and the bottom interface domain for its protection against an electrical breakdown.     

Improved switching uniformity of a carbon-based conductive-bridge type ReRAM by controlling the size of conducting filament

We investigated the resistive switching properties of a amorphous carbon-based ReRAM device. In order to minimize the fluctuations of switching parameters, we introduced an external load resistor (R_Load) in series, which indirectly acts as a current limiter. Reduced reset current (I_reset) and improved switching uniformity were obtained when the proper external R_Load was connected. The voltage drop at the ReRAM device during switching was directly monitored using an oscilloscope. We have confirmed that fluctuation of the effective voltage applied across the conducting filament was dramatically reduced by adding R_Load. In contrast, we observed degradation of retention characteristic of sample with R_Load. To meet both switching uniformity and retention characteristics, we need to optimize the resistance of low resistance state.     

Potentiality of trap charge effects and SiON induced interface defects in a-Si3N4/SiON based MIS structure for resistive NVM device

Potential application of amorphous silicon nitride (a-Si₃N₄)/silicon oxy-nitride (SiON) film has been demonstrated as resistive non-volatile memory (NVM) device by studying the Al/Si₃N₄/SiON/p-Si metal–insulator–semiconductor (MIS) structure. The existence of several deep trap states was revealed by the photoluminescence characterizations. The bipolar resistive switching operation of this device was investigated by current–voltage measurements whereas the trap charge effect was studied in detail by hysteresis behavior of frequency dependent capacitance–voltage characteristics. A memory window of 4.6 V was found with the interface trap density being 6.4 × 10¹¹ cm⁻² eV⁻¹. Excellent charge retention characteristics have been observed for the said MIS structure enabling it to be used as a reliable non-volatile resistive memory device.     

Resistance switching in amorphous and crystalline binary oxides grown by electron beam evaporation and atomic layer deposition

Resistance switching random access non-volatile memories (ReRAM) could represent the leading alternative to floating gate technology for post 32 nm technology nodes. Among the currently investigated materials for ReRAM, transition metal binary oxides, such as NiO, Cu_xO, ZrO_x, TiO₂, MgO, and Nb₂O₅ are receiving increasing interest as they offer high potential scalability, low-energy switching, thermal stability, and easy integration in CMOS fabrication. In this work we investigate the resistive switching properties of NiO and Nb₂O₅ films grown by electron beam and atomic layer deposition (ALD) as a function of growth technique and electrode materials. The polycrystalline NiO and amorphous Nb₂O₅ films are initially in the high resistance state and exhibit reproducible unipolar switching after an appropriate forming stage. Beside noble metal electrodes, particular focus is on n⁺-Si, W, and TiN materials which are compatible with CMOS device fabrication process.     

Materials and process aspect of cross-point RRAM (invited)

A survey of non-volatile and highly scalable cross-point memory in nanoscale resistive switching device is introduced. We present the basic operation of bipolar switching memory using combination between switching layer (HfO_x/PCMO) and oxygen reservoir layer (ZrO_x/AlO_x) and discuss the crucial issue for cross-point ReRAM. Based on the results, the applications of cross-point structure without any selection device were introduced. To evaluate the feasibility of cross-point ReRAM, read-out margin was calculated using PSPICE simulation. In addition, by the device scaling, three phenomena can be confirmed: (1) reset current reduction, (2) local heating effect and (3) significant improvement of uniformity.     

Resistive switching effects of HfO2 high-k dielectric

Resistive switching behavior of HfO₂ high-k dielectric has been studied as a promising candidate for emerging non-volatile memory technology. The low resistance ON state and high resistance OFF state can be reversibly altered under a low SET/RESET voltage of ±3 V. The memory device shows stable retention behavior with the resistance ratio between both states maintained greater than 103. The bipolar nature of the voltage-induced hysteretic switching properties suggests changes in film conductivity related to the formation and removal of electronically conducting paths due to the presence of oxygen vacancies induced by the applied electric field. The effect of annealing on the switching behavior was related to changes in compositional and structural properties of the film. A transition from bipolar to unipolar switching behavior was observed upon O₂ annealing which could be related to different natures of defect introduced in the film which changes the film switching parameters. The HfO₂ resistive switching device offers a promising potential for high density and low power memory application with the ease of processing integration.     

High-resolution transmission electron microscopy study on bipolar resistive switching behavior in TiO2 thin films

We fabricated TiO₂ thin films the by sol–gel process. Successful I–V curves can be obtained in the Cu/TiO₂/ATO structure device in which TiO₂ thin film was calcined at 300 °C. The bipolar resistive switching behavior was observed and the ratio of R_off/R_on can be increased to 10⁴. The switching voltage changes from 4.8 to 3.5 V when the current compliance drops from 10 to 0.1 mA. We also investigated the microstructure by HRTEM technology.     

Resistive switching characteristics of MIM structures based on oxygen-variable ultra-thin HfO2 and fabricated at low temperature

We report the resistive switching characteristics of Metal-Insulator-Metal (MIM) structures fabricated at low temperature and having different concentrations of oxygen vacancies in the insulator. The oxygen modulation in HfO₂ is promoted by a very simple variation of standard thermal Atomic-Layer Deposition (ALD), so that different exposure times to H₂O during each half-cycle of the hafnium oxide deposition are used (being Tetrakis Dimethylamino Hafnium–TDMAH the other precursor). We show the correlation of the stoichiometry with the forming voltage, conduction mechanisms and resistance windows of memory devices. All structures present a bipolar operation mode in which the resistive switching mechanism is related to the migration of oxygen vacancies inside the dielectric. These MIM devices have a simple structure, low power consumption and they are fabricated using a very low thermal budget of only 250 °C, thus enabling their integration at the Back-End of Line (BEOL) stage of an integrated circuit in order to increase the density of memory arrays in at least one order of magnitude.     

Effect of the oxygen vacancy gradient in titanium dioxide on the switching direction of bipolar resistive memory

Resistive memory switching behavior depending on voltage sweep direction is studied by intentionally creating oxygen vacancies within titanium dioxide (TiO₂). By inserting a reactive Ti layer on the TiO₂, oxygen deficient TiO_2−x layer is created, which then causes TiO_2−x/TiO₂ which has an oxygen vacancy gradient. This gradient of oxygen vacancy makes it possible to create an insulating TiO₂ layer on the bottom electrode during the first reset with a negative bias at the top electrode. This insulating layer makes counterclockwise directional bipolar switching more stable. On the other hand, under the clockwise directional voltage sweeping, the first set switching is prevented by the insulating TiO₂ layers created during the first and second reset, which leads to a short circuit due to local heating eventually.     

Oxygen Vacancy Creation,Drift, and Aggregation in TiO2‐Based Resistive Switches at Low Temperature and Voltage

Transmission electron microscopy with in situ biasing has been performed on TiN/single‐crystal rutile TiO₂/Pt resistive switching structures. Three elementary processes essential for switching: i) creation of oxygen vacancies by electrochemical reactions at low temperatures (<150 °C), ii) their drift in the electric field, and iii) their coalescence into planar faults (and dissociation from them) have been documented. The faults have a form of vacancy discs in {110} and {121} planes, are bound by partial dislocation loops, and are identical to Wadsley defects observed in nonstoichiometric TiO₂ annealed at high temperatures. The faults can be regarded as a precursor to the formation of oxygen‐deficient Magnéli phases, but 3D secondary phase inclusions have not been detected. Together, the observations shed light on the behavior of oxygen vacancies in relatively low electric fields and temperatures, suggesting that, in addition to the rather accepted notion of oxygen vacancy motion during the writing processes in resistive switching devices, such motion may occur even during reading, and may be accompanied by significant oxygen vacancy creation at modest device excitation levels.     

Dispersion improvement of unipolar resistive switching Ni/CuxO/Cu device by bipolar operation method

Thermally grown Cu_xO thin films were adopted to fabricate a Ni/Cu_xO/Cu structure and investigate its resistive switching properties. The resistance of the device can reversibly switch between the high-resistance-state (HRS) and the low-resistance-state (LRS) by dc voltages. The device with the unipolar switching behavior can be either operated by dc voltages in the same direction (unipolar operation method) or in the opposite directions (bipolar operation method). The switching dispersions when using the bipolar operation method were smaller than those when using the unipolar operation method. This may be attributed to the compensation of defect migration during switching cycles. The switching behaviors of the bipolar operation method and of the unipolar operation method were similar. The conducting filament model with the thermochemical effect was suggested to explain the resistive switching behaviors.     

Resistive switching characteristics of CMOS embedded HfO2-based 1T1R cells

This work addresses a 1T1R RRAM architecture, which allows for the precise and reliable control of the forming/set current by using an access transistor. The 1T1R devices were fabricated in a modified 0.25 μm CMOS technology. The memory cells show stable resistive switching in dc as well as pulse-induced mode with an endurance of 10³ and 10² cycles, respectively. The variation of pulse widths as a function of amplitudes in 1R devices confirmed the set process distribution over a wide range of pulse widths (300 ns-100 μA), whereas the reset process variation is confined (1-3 μs).     

Effect of sweeping voltage and compliance current on bipolar resistive switching and white-light controlled Schottky behavior in epitaxial BaTiO3 (111) thin films

Bipolar resistive switching (RS) phenomenon without required electroforming has been observed in epitaxial (111)-oriented BaTiO₃ (BTO) thin films deposited by PLD technique on conducting Nb-doped substrate of SrTiO₃ (NSTO). Negative differential resistance (NDR) is observed at about −5 V when the maximum of positive voltage exceeds 7 V and the compliance current is more than 1.5 mA. And bipolar resistive switching has also been observed. In addition, the resistance of LRS decreases with increasing compliance current or the maximum of positive voltage while that of HRS barely changes, and the resistance of HRS increases with increasing the absolute of maximum of negative voltage while that of LRS scarcely changes. A typical rectifying behavior is observed when the maximum of positive voltage is less than 4 V (such as 2 V). In this case, the reverse biased current is strongly enhanced under illumination of white-light, and vice versa. The resistance of LRS and HRS can be controlled by the applied voltage or the compliance current. The rectifying behavior can be controlled by the white-light. The transition from rectifying behavior to bipolar resistive switching can be controlled by the applied voltage. The above results were discussed by considering the oxygen vacancies that can trap or release electrons as a trapping layer at the Pt/BTO interface.     

Resistive Random Access Memory Cells with a Bilayer TiO2/SiOX Insulating Stack for Simultaneous Filamentary and Distributed Resistive Switching

In order to fulfill the information storage needs of modern societies, the performance of electronic nonvolatile memories (NVMs) should be continuously improved. In the past few years, resistive random access memories (RRAM) have raised as one of the most promising technologies for future information storage due to their excellent performance and easy fabrication. In this work, a novel strategy is presented to further extend the performance of RRAMs. By using only cheap and industry friendly materials (Ti, TiO₂, SiO_X, and n⁺⁺Si), memory cells are developed that show both filamentary and distributed resistive switching simultaneously (i.e., in the same I–V curve). The devices exhibit unprecedented hysteretic I–V characteristics, high current on/off ratios up to ≈5 orders of magnitude, ultra low currents in high resistive state and low resistive state (100 pA and 125 nA at –0.1 V, respectively), sharp switching transitions, good cycle‐to‐cycle endurance (>1000 cycles), and low device‐to‐device variability. We are not aware of any other resistive switching memory exhibiting such characteristics, which may open the door for the development of advanced NVMs combining the advantages of filamentary and distributed resistive switching mechanisms.