Debashis Panda

Resistive switching characteristics of nickel silicide layer embedded HfO2 film

Resistive switching behavior of the Ti/HfO2:NiSi:HfO2/Pt memory structure is investigated. Auger electron spectroscopy analysis indicates no metal diffusion from the electrodes and silicide layer on high-k film. Cross-sectional transmission electron microscopic micrographs revealed the thicknesses of the HfO2 and silicide layer. Significant decrease of forming voltage is observed for the 550 °C, 1 min annealed device embedded with nickel silicide (NiSi) layers. Entire device shows bipolar switching properties with very low set/reset voltage. The optimized annealed device with NiSi embedded layer exhibits improved memory performances such as good on/off ratio (>102), long retention more than 104 s, and reasonable endurance (>103 cycles). A conducting filament model based on two stacks structure is employed to well explain the switching behaviors.

Forming-free bipolar resistive switching in nonstoichiometric ceria films

The mechanism of forming-free bipolar resistive switching in a Zr/CeOx/Pt device was investigated. High-resolution transmission electron microscopy and energy-dispersive spectroscopy analysis indicated the formation of a ZrOy layer at the Zr/CeOx interface. X-ray diffraction studies of CeOx films revealed that they consist of nano-polycrystals embedded in a disordered lattice. The observed resistive switching was suggested to be linked with the formation and rupture of conductive filaments constituted by oxygen vacancies in the CeOx film and in the nonstoichiometric ZrOy interfacial layer. X-ray photoelectron spectroscopy study confirmed the presence of oxygen vacancies in both of the said regions. In the low-resistance ON state, the electrical conduction was found to be of ohmic nature, while the high-resistance OFF state was governed by trap-controlled space charge-limited mechanism. The stable resistive switching behavior and long retention times with an acceptable resistance ratio enable the device for its application in future nonvolatile resistive random access memory (RRAM).

Nonvolatile and unipolar resistive switching characteristics of pulsed laser ablated NiO films

Unipolar nonvolatile resistive switching memory properties of pulse laser ablated nickel oxide films have been studied. Grazing incidence x-ray diffraction and electron diffraction spectra of the oxide films reveal polycrystalline nature of deposited NiO films. Cross-sectional transmission electron micrograph shows a fairly uniform oxide surface. The rms surface roughnesses of deposited oxides have been studied as a function of annealing temperature using atomic force microscopy. By applying a proper voltage bias and compliance, Pt/NiO/Pt structures exhibited unipolar resistive switching having a very low SET and RESET voltages. The OFF state resistance and SET voltage are found to increase with the increase in annealing temperature. The ratio between the two resistance states can be as high as 1000. The current conduction phenomena at two resistance states have been studied. The switching phenomena have been explained using the rupture and formation of conducting filaments. The effect of postdeposition a...

Perovskite Oxides as Resistive Switching Memories: A Review

Numerous metal-insulator-metal systems demonstrate electrically induced resistive switching effects and have therefore been proposed as the basis for future nonvolatile memories. They combine the advantages of flash memories and dynamic random access memories while avoiding their drawbacks, such as operation speed, power consumption and device integration and scalable issues. The RRAM devices primarily operate at different resistance values to store the digital data and can keep the resistance state without any power supply. Recent advances in the understanding of the resistive switching mechanism are explained by a thermal or electrochemical redox reaction near the interface between the oxide and the top active metal electrode. Here we review the ongoing research and development activities on the perovskite based resistive switching memory devices. The possible switching mechanisms for the resistive switching are described. The effects of crystal structure, dopants, doping concentrations, annealing temperature, device structures and thickness of the active oxide layer on the resistive switching characteristics and consequently the memory performances are also discussed. From this insight, we take a brief look into different effect on the switching of the perovskite material systems.

Nonvolatile Memristive Switching Characteristics of TiO

Nickel nanocrystal (Ni-NC)-embedded titanium dioxide films have been deposited for nonvolatile resistive switching memory devices. The polycrystalline behavior of the films has been observed from the X-ray diffraction spectra. Tiny isolated Ni-NCs with an average size of 4 nm are observed for the 1000 °C, 5-min annealed samples. Stable, bipolar, nonvolatile, and bistable resistive switching states are observed for the optimized annealed Ni-NC-embedded devices with a low SET and RESET voltage of 0.8 and -0.2 V, respectively. A high resistance ratio (>;10), good stability, and retention properties are observed for the nanocrystal sample. The role of thin Ni-NC layer on memory switching stability is discussed.

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In the advancement of the semiconductor device technology, ZnO could be a prospective alternative than the other metal oxides for its versatility and huge applications in different aspects. In this review, a thorough overview on ZnO for the application of resistive switching memory (RRAM) devices has been conducted. Various efforts that have been made to investigate and modulate the switching characteristics of ZnO-based switching memory devices are discussed. The use of ZnO layer in different structure, the different types of filament formation, and the different types of switching including complementary switching are reported. By considering the huge interest of transparent devices, this review gives the concrete overview of the present status and prospects of transparent RRAM devices based on ZnO. ZnO-based RRAM can be used for flexible memory devices, which is also covered here. Another challenge in ZnO-based RRAM is that the realization of ultra-thin and low power devices. Nevertheless, ZnO not only offers decent memory properties but also has a unique potential to be used as multifunctional nonvolatile memory devices. The impact of electrode materials, metal doping, stack structures, transparency, and flexibility on resistive switching properties and switching parameters of ZnO-based resistive switching memory devices are briefly compared. This review also covers the different nanostructured-based emerging resistive switching memory devices for low power scalable devices. It may give a valuable insight on developing ZnO-based RRAM and also should encourage researchers to overcome the challenges.

Films Embedded With Nickel Nanocrystals

The resistive switching characteristics of indium tin oxide (ITO)/Zn1−xCoxO/ITO transparent resistive memory devices were investigated. An appropriate amount of cobalt dopant in ZnO resistive layer demonstrated sufficient memory window and switching stability. In contrast, pure ZnO devices demonstrated a poor memory window, and using an excessive dopant concentration led to switching instability. To achieve suitable memory performance, relying only on controlling defect concentrations is insufficient; the grain growth orientation of the resistive layer must also be considered. Stable endurance with an ON/OFF ratio of more than one order of magnitude during 5000 cycles confirmed that the Co-doped ZnO device is a suitable candidate for resistive random access memory application. Additionally, fully transparent devices with a high transmittance of up to 90% at wavelength of 550 nm have been fabricated.

Status and Prospects of ZnO-Based Resistive Switching Memory Devices

The influence of single and double forming on the switching stability of AZO/ZnO1−x/ITO transparent resistive memory devices was investigated. Devices that underwent single forming exhibited severe switching instability, where as those that underwent double forming exhibited excellent switching uniformity. The quantity of conducting filaments can be limited by applying the two-step forming process. Consequently, the set/reset process can be controlled, enhancing switching stability. Satisfactory endurance with an acceptable ON/OFF ratio of 102 and satisfactory retention behavior of 104 s at room temperature confirmed the reliability of optimized devices. Furthermore, highly transparent devices (transparency of approximately 85% in visible range) have been fabricated.

Impacts of Co doping on ZnO transparent switching memory device characteristics

An improvement in resistive switching (RS) characteristics of CeO2-based devices has been reported by charge transfer through Al metal as a dopant. Moreover, density functional theory (DFT) calculations have been performed to investigate the role of Al-layer sandwiched between CeO2 layers by the Vienna ab initio simulation package (VASP). Total density of states (TDOS) and partial electron density of states (PDOS) have been calculated and analyzed with respect to resistive switching. It is established that the oxygen vacancy based conductive filaments are formed and ruptured in the upper region of CeO2 layer, because of the fact that maximum transport of charge takes place in this region by Al and Ti (top electrode), while the lower region revealed less capability to generate conductive filaments because minimum charge transfer takes place in this region by Al and/or Pt (bottom electrode). The effect of Al and Al2O3 on both the electronic charge transfer from valence to conduction bands and the formation stability of oxygen vacancies in conductive filament have been discussed in detail. Experimental results demonstrated that the Ti/CeO2:Al/Pt sandwich structure exhibits significantly better switching characteristics including lower forming voltage, improved and stable SET/RESET voltages, enhanced endurance of more than 10(4) repetitive switching cycles and large memory window (ROFF/RON > 10(2)) as compared to undoped Ti/CeOx/Pt device. This improvement in memory switching behavior has been attributed to a significant decrease in the formation energy of oxygen vacancies and to the enhanced oxygen vacancies generation within the CeO2 layers owing to charge transferring and oxygen gettering ability of Al-dopant.

Enhanced switching uniformity in AZO/ZnO1−x/ITO transparent resistive memory devices by bipolar double forming

Metal-oxide-semiconductor capacitors with a tetralayer structure consisting of nickel nanocrystals sandwiched between SiO2 and HfO2 tunnel and Al2O3 cap oxides were fabricated on p-Si substrates. Cross-sectional transmission electron micrographs revealed the formation of nickel nanocrystals having size 5–7 nm. The maximum charge injection capability (ΔVFB ~ 7.8 V; @ ± 15 V) of nanocrystals was observed for the device RTA annealed at 950 °C for a minute. Charge storage and leakage current characteristics of the nanocrystal memory structures were studied through capacitance–voltage and current–voltage measurements, respectively. The improved retention properties with good thermal stability and endurance properties were also studied.