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).     

Zinc Tin Oxide (ZTO) electron transporting buffer layer in inverted organic solar cell

Solution processed, high electron mobility and highly transparent Zinc Tin Oxide (ZTO) was successfully exploited as electron transporting buffer layer in an inverted organic solar cell. The device configuration of FTO/ZTO/P3HT:PCBM/WO₃/Ag was employed. For comparison, an identical device using a sol–gel derived TiO_x electron extracting layer was also fabricated. Increased short-circuit density (J_sc) and open-circuit voltage (V_oc) were generated in the devices with ZTO layer in comparison to the ones with TiO_x layer. It is attributed to a better electron transporting, hole blocking capacities and reduced recombination probabilities at electron collecting electrode with ZTO layer. A power conversion efficiency of 3.05% was achieved with ZTO devices.     

Development of Ag/ZnO/FTO thin film memristor using aqueous chemical route

The present paper report the development of the Ag/ZnO/FTO memristor device using a simple aqueous chemical route. The structural, electrical, morphological and optical properties of Ag/ZnO/FTO memristor device are characterized using X-Ray diffraction (XRD), semiconductor characterization unit, field emission scanning electron microscopy (FESEM), and UV-Vis-NIR spectrophotometer respectively. The fabricated memristor device shows bipolar resistive switching behavior within low operating voltage (±0.88 V). The hysteresis loop in the I–V plane is the fingerprint characteristics of memristor and same has been seen in developed device. Furthermore, the effect of temperature on ZnO based memristor device is investigated using the thermal reaction model. The results suggested that, temperature of conductive filaments increases rapidly and affected the memory window of memristor.     

Oxygen Ion Drift‐Induced Complementary Resistive Switching in Homo TiOx/TiOy/TiOx and Hetero TiOx/TiON/TiOx Triple Multilayer Frameworks

Developing a means by which to compete with commonly used Si‐based memory devices represents an important challenge for the realization of future three‐dimensionally stacked crossbar‐array memory devices with multifunctionality. Therefore, oxide‐based resistance switching memory (ReRAM), with its associated phenomena of oxygen ion drifts under a bias, is becoming increasingly important for use in nanoscalable crossbar arrays with an ideal memory cell size due to its simple metal–insulator–metal structure and low switching current of 10–100 μA. However, in a crossbar array geometry, one single memory element defined by the cross‐point of word and bit lines is highly susceptible to unintended leakage current due to parasitic paths around neighboring cells when no selective devices such as diodes or transistors are used. Therefore, the effective complementary resistive switching (CRS) features in all Ti‐oxide‐based triple layered homo Pt/TiO_x/TiO_y/TiO_x/Pt and hetero Pt/TiO_x/TiON/TiO_x/Pt geometries as alternative resistive switching matrices are reported. The possible resistive switching nature of the novel triple matrices is also discussed together with their electrical and structural properties. The ability to eliminate both an external resistor for efficient CRS operation and a metallic Pt middle electrode for further cost‐effective scalability will accelerate progress toward the realization of cross‐bar ReRAM in this framework.     

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.     

Resistive Switching Properties of Sol–Gel-Derived V-Doped SrTiO3 Thin Films

V-doped and undoped SrTiO₃ (V:STO and STO) thin films on Pt/Ti/SiO₂/Si substrates were synthesized using a sol–gel method to form metal–insulator–metal (MIM) structures. Coexistence of the bipolar and unipolar resistive switching (BRS and URS) modes in Pt/STO/Pt and Pt/V:STO/Pt structures was observed as a irreversible transition from BRS to URS on adjustment of the compliance current (I _comp). Both states were stable and reproducible over 60 cycles, and the maximum operating voltage of the Pt/STO/Pt was reduced from 10 V to 2 V by doping with V. Linear fitting of current–voltage curves suggests that space-charge-limited leakage was the limiting leakage mechanism for these two devices. Based on these results, a switching mechanism based on filament theory is proposed to explain both resistive switching modes.     

Reliability/Uniformity improvement induced by an ultrathin TiO2 insertion in Ti/HfO2/Pt resistive switching memories

Reliability/Uniformity of resistance switching in Ti/HfO₂/Pt memory structure was improved by inserting an interfacial layer of 5 nm-thick TiO₂ between Ti and 45 nm-thick HfO₂. As a native oxide of Ti, TiO₂, effectively limits the disorder migration of oxygen from HfO₂ to Ti layer, and provides the more chemically-stable and morphologically-uniform interfaces with both the Ti electrode and the HfO₂ layer. Meanwhile, more stable resistive switching was observed in Ti/TiO₂/HfO₂/Pt than that in Ti/HfO₂/Pt memory, and the random variation during endurance test observed in Ti/HfO₂/Pt was also greatly limited in Ti/TiO₂/HfO₂/Pt memory. From these results, a thin TiO₂ insertion between the Ti electrode with the HfO₂ active layer, could greatly improve the reliability/uniformity of the Ti/HfO₂/Pt memory devices.     

The resistive switching characteristics of a Ti/Gd2O3/Pt RRAM device

We successfully fabricated the Gd₂O₃ film for the application of resistive random access memory (RRAM). The resistive switching behavior of the Ti/Gd₂O₃/Pt capacitor structure could be both operated under positive or negative bias. However, there was a significant difference on the switching properties. The switching behavior under positive bias operation was more stable, had less voltage and resistance fluctuation, and had longer endurance than that of the negative one. We propose that the anode electrode plays an important role in the switching characteristics and may be the cause of the asymmetry of the I-V curves between positive and negative operation.     

Low‐Dimensional Lead‐Free Inorganic Perovskites for Resistive Switching with Ultralow Bias

3D organic–inorganic and all‐inorganic lead halide perovskites have been intensively pursued for resistive switching memories in recent years. Unfortunately, instability and lead toxicity are two foremost challenges for their large‐scale commercial applications. Dimensional reduction and composition engineering are effective means to overcome these challenges. Herein, low‐dimensional inorganic lead‐free Cs₃Bi₂I₉ and CsBi₃I₁₀ perovskite‐like films are exploited for resistive switching memory applications. Both devices demonstrate stable switching with ultrahigh on/off ratios (≈10⁶), ultralow operation voltages (as low as 0.12 V), and self‐compliance characteristics. 0D Cs₃Bi₂I₉‐based device shows better retention time and larger reset voltage than the 2D CsBi₃I₁₀‐based device. Multilevel resistive switching behavior is also observed by modulating the current compliance, contributing to the device tunability. The resistive switching mechanism is hinged on the formation and rupture of conductive filaments of halide vacancies in the perovskite films, which is correlated with the formation of AgI_x layers at the electrode/perovskite interface. This study enriches the library of switching materials with all‐inorganic lead‐free halide perovskites and offers new insights on tuning the operation of solution‐processed memory devices.     

Organic memristive devices based on silver nanoparticles and DNA

A novel organic memory device ‘Al/silver nanoparticles-deoxyribonucleic acid-cetyltrimethylammonium Bromide/ITO’ (Al/Ag NPs–DNA–CTMA/ITO) was fabricated. The measured I–V curve of the device exhibits unipolar switching. The conductivity and the memristive characteristics are significantly improved by the introduction of Ag nanoparticles, but with a poor stability. Better stability is achieved by annealing the device, which also changes the switching characteristic from unipolar to bipolar. As the annealing temperature is raised, the switching voltage first decreases and then increases, while the current I_RESET first increases and then decreases. The range of the optimal annealing temperature is from 383 K to 403 K and the maximum ON/OFF current ratio (I_ON/I_OFF) can reach 10⁴. The switching voltage, the current, and I_ON/I_OFF all increase with the applied voltage amplitude, and V_SET and I_ON/I_OFF obey a quadratic and Boltzmann relationship, respectively.     

Control of the Ti diffusion in Pt/Ti bottom electrodes for the fabrication of PZT thin film transducers

For the achievement of microactuators based on piezoelectric thin films, a Pt/Ti/Si bottom electrode is widely used. This study presents the experimental results for Ti out-diffusion in Pt and Si for both sputtered Pt/Ti and Pt/TiO_x electrodes. These results have been compared before and after a rapid thermal annealing (RTA). The diffusion has been characterized by secondary ion mass spectroscopy (SIMS) analysis using Cs⁺ as a primary ion source. The Pt orientation has been observed by XRD measurements. Ti thin films (20 nm) have been sputtered in pure Ar whereas TiO_x films have been obtained by reactive sputtering in a mixture of Ar/O₂ (90/10). Finally, the Pt (100 nm) has been sputtered without vacuum breaking. After RTA (400°C, 30 s, in N₂), the Pt film exhibited a (1 1 1) orientation for both Ti and TiO_x adhesion films. The roughness of the Pt film measured by AFM with TiO_x underlayer was 80% less than that of the Pt/Ti bi-layer. The TiO_x film, as shown by SIMS analysis, has drastically reduced the diffusion of Ti in both Pt and Si. This phenomenon is accompanied by a very low Pt roughness. These results are analyzed in terms of diffusion and regrowth mechanisms inside the Pt film.     

Performance‐Enhancing Selector via Symmetrical Multilayer Design

Two‐terminal selectors with high nonlinearity, based on bidirectional threshold switching (TS) behaviors, are considered as a crucial element of crossbar integration for emerging nonvolatile memory and neuromorphic network. Although great efforts have been made to obtain various selectors, existing selectors cannot fully satisfy the rigorous standard of assorted memristive elements and it is in great demand to enhance the performance. Here, a new type of Ag/TaO_x/TaO_y/TaO_x/Ag (x < y) selector based on homogeneous trilayered oxides is developed to attain the required parameters including bidirectional TS operation, a large selectivity of ≈10¹⁰, a high compliance current up to 1 mA, and ultralow switching voltages under 0.2 V. Tunable operation voltages can be realized by modulating the thickness of inserted TaO_y. All‐TaO_x‐based integrated 1S1R (one selector and one memristor) cells, prepared completely by magnetron sputtering and no need of a middle electrode, exhibit a nonlinear feature, which is quite characteristic for the crossbar devices, avoiding undesired crosstalk current issues. The tantalum‐oxide‐based homojunctions offer high insulation, low ion mobility, and rich interfaces, which is responsible for the modulation of Ag conductive filaments and corresponding high‐performance cation‐based selector. These findings might advance practical implementation of two‐terminal selectors in emerging memories, especially resistive random access memories.     

Study of diffusion barrier properties of ternary alloy (TixAlyNz) in Cu/TixAlyNz/SiO2/Si thin film structure

The effects of aluminum (Al) incorporation on the performance of a titanium nitride (TiN) diffusion barrier were investigated up to the temperature of 1000°C in the Cu/Ti_xAl_yN_z/SiO₂/Si structure. The thermal stability of the structure was evaluated by using four-point probe, X-ray diffraction, and Rutherford Backscattering Spectroscopy. The Cu/Ti_xAl_yN_z/SiO₂/Si system retained its structure up to 1000°C. The incorporation of Al into the Ti_xN_y film modified the microstructure of the Ti_xN_y film, especially the microstructure of grain boundaries in which oxide and nitride compounds of Al and Ti were formed during thermal annealing. As a result, the fast pathways for copper (Cu) diffusion were effectively blocked by these compounds and the stability of the barrier performance was enhanced up to 1000°C.     

Integration of HfxTayN metal gate with SiO2 and HfOxNy gate dielectrics for MOS device applications

Interaction of Hf_xTa_yN metal gate with SiO₂ and HfO_xN_y gate dielectrics has been extensively studied. Metal-oxide-semiconductor (MOS) device formed with SiO₂ gate dielectric and Hf_xTa_yN metal gate shows satisfactory thermal stability. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis results show that the diffusion depths of Hf and Ta are less significant in SiO₂ gate dielectric than that in HfO_xN_y. Compared to HfO_xN_y gate dielectric, SiO₂ shows better electrical properties, such as leakage current, hysteresis, interface trap density and stress-induced flat-band voltage shift. With an increase in post metallization annealing (PMA) temperature, the electrical characteristics of the MOS device with SiO₂ gate dielectric remain almost unchanged, indicating its superior thermal and electrical stability.     

Electrode Materials for Ge2Sb2Te5-Based Memristors

The memristive characteristics of Ge₂Sb₂Te₅ (GST) as a representative chalcogenide material have been verified and show great potential for memory applications. This paper focuses on the influence of different electrode materials on the properties of GST-based memristors. Several electrode materials (Ti₃W₇, Ag, Cu, and Ta) were adopted in devices with a top electrode (TE)/GST/bottom electrode (BE) layer structure. Through different current–voltage (I–V) curves, it was demonstrated that devices with Ag or Cu electrodes are suitable for GST-based memristors, while those utilizing inert electrodes are not. Because of their relatively smaller radius and lower binding energy, it is much easier for Ag and Cu to diffuse into the GST layer and form conductive filaments. The results obtained from memristors annealed at different temperatures further support the conductive filament model. Moreover, an optimized Cu/Ag/GST/Cu device structure different from the traditional TE/GST/BE structure is proposed, showing improved stability with higher R _off/R _on ratio and good endurance.     

Characterization of CoxNiyO hybrid metal oxide nanoparticles as charge trapping nodes in nonvolatile memory devices

The Co_xNi_yO hybrid metal oxide nanoparticles (HMONs) embedded in the HfO_xN_y high-k dielectric as charge trapping nodes of the nonvolatile memory devices have been formed via the chemical vapor deposition using the Co/Ni acetate calcined and reduced in the Ar/NH₃ ambient. A charge trap density of 8.96 × 10¹¹ cm^?2 and a flatband voltage shift of 500 mV were estimated by the appearance of the hysteresis in the capacitance–voltage (C–V) measurements during the ±5 V sweep. Scanning electron microscopy image displays that the Co_xNi_yO HMONs with a diameter of ～10–20 nm and a surface density of ～1 × 10¹⁰ cm^?2 were obtained. The mechanism related to the writing characteristics are mainly resulted from the holes trapping. Compared with those devices with the Co_xNi_yO HMONs formed by the dip-coated technique, memory devices with the Co_xNi_yO HMONs fabricated by the drop-coated technique show improved surface properties between the Co_xNi_yO HMONs and the HfON as well as electrical characteristics.     

Effect of oxygen concentration on resistive switching behavior in silicon oxynitride film

SiO_xN_y films with different oxygen concentrations were fabricated by reactive magnetron sputtering, and the resistive switching characteristics and conduction mechanism of Cu/SiO_xN_y/ITO devices were investigated. The Cu/SiO_xN_y/ITO device with SiO_xN_y deposited in 0.8-sccm O₂ flow shows a reliable resistive switching behavior, including good endurance and retention properties. As the conductivity of SiO_xN_y increases with the increase of the oxygen content dynamical electron trapping and detrapping is suggested to be the conduction mechanism. The temperature dependent I-V measurement indicates that the carrier transport can be ascribed to the hopping conduction rather than the metallic conductive filament.     

Ferroelectric and leakage current characteristics of (Pb0.72La0.28)Ti0.93O3 thin films prepared by a sol-gel process

(Pb_1 − xLa_x)Ti_1 − x/4O₃(x = 28 mol%, denoted as PLT) thin films were grown on Pt/Ti/SiO₂/Si substrates by using a sol-gel process. The Pt/PLT/Pt film capacitor showed well-saturated hysteresis loops at an applied electric field of 500 kV/cm with spontaneous polarization (P_s), remanent polarization (P_r) and coercive electric field (E_c) values of 9.23 μC/cm², 0.53 μC/cm² and 19.7 kV/cm, respectively. At 100 kHz, the dielectric constant and dissipation factor of the film were 748 and 0.026, respectively. The leakage current density is lower than 1.0 × 10⁻⁷ A/cm²over the electric field range of 0 to 200 kV/cm. And the Pt/PLT interface exist a Schottky emission characteristics.     

Characterization of TiOxNy nanoparticles embedded in HfOxNy as charge trapping nodes for nonvolatile memory device applications

Silicon-oxide–nitride-oxide–silicon devices with nanoparticles (NPs) as charge trapping nodes (CTNs) are important to provide enhanced performance for nonvolatile memory devices. To study these topics, the TiO_xN_y metal oxide NPs embedded in the HfO_xN_y high-k dielectric as CTNs of the nonvolatile memory devices were investigated via the thermal synthesis using Ti thin-film oxidized in the mixed O₂/N₂ ambient. Well-isolated TiO_xN_y NPs with a diameter of 5–20 nm, a surface density of ～3 × 10¹¹ cm^?2, and a charge trap density of around 2.33 × 10¹² cm^?2 were demonstrated. The writing characteristic measurements illustrate that the memory effect is mainly due to the hole trapping.     

Structural and Electrical Properties of Ta ax La(1−a)x O y Thin Films

Effect of annealing temperature on the characteristics of sol–gel-driven Ta _ax La_(1?a)x O _y thin film spin-coated on Si substrate as a high-k gate dielectric was studied. Ta _ax La_(1?a)x O _y thin films with different amounts of a were prepared (as-prepared samples). X-ray diffraction measurements of the as-prepared samples indicated that Ta_0.3x La_0.7x O_y film had an amorphous structure. Therefore, Ta_0.3x La_0.7x O _y film was chosen to continue the present studies. The morphology of Ta_0.3x La_0.7x O _y films was studied using scanning electron microscopy and atomic force microscopy techniques. The obtained results showed that the size of grain boundaries on Ta_0.3x La_0.7x O _y film surfaces was increased with increasing annealing temperature. Electrical and optical characterizations of the as-prepared and annealed films were investigated as a function of annealing temperature using capacitance–voltage (C–V) and current density–voltage (J–V) measurements and the Tauc method. The obtained results demonstrated that Ta_0.3x La_0.7x O _y films had high dielectric constant (≈27), wide band gap (≈4.5 eV), and low leakage current density (≈10^?6 A/cm² at 1 V).