Effect of defect content on the unipolar resistive switching characteristics of ZnO thin film memory devices

In this study, unipolar resistive switching (URS) characteristics in ZnO thin film memory devices were systematically investigated with variable defect content. ZnO films displayed typically URS behavior while oxygen-deficient ZnO_1?x films did not show resistive switching effects. The devices with two intentional Ohmic interfaces still show URS. These results show that appearance of URS behavior can be dominated by initial oxygen vacancy content in ZnO thin films. Modest increase in oxygen vacancy content in ZnO films will lead to forming-free and narrower distributions of switching parameters (set and reset voltage, high and low resistance states). It indicates that controlling the initial oxygen vacancy content was an effective method to enhance the URS performance.     

Fabrication of solution-processed SnO2–Based flexible ReRAM using laser-induced graphene transferred onto PDMS

In this paper, we are reporting the fabrication of a solution-processed SnO₂-based flexible ReRAM using laser-induced graphene (LIG) transferred onto polydimethylsiloxane (PDMS). The fabricated ReRAM showed forming-free and self-compliance bipolar resistive switching characteristics when the applied voltage was swept from 0 V to 4.5 V for SET and from 0 V to - 4.5 V for RESET. The device operates as a filamentary type ReRAM and its conduction mechanism analysis indicates that the space charge limited conduction (SCLC) is dominant mechanism in the analog resistive switching of the fabricated device. For the reliability analysis, 100 cycles of endurance test and 1.8 × 10³ s of retention test were performed. The flexibility of the fabricated ReRAM device was demonstrated by showing that the resistive switching characteristics were still obtained after bending 200 times repeatedly down to 1 mm radius. Our study suggests the new fabrication process of a solution-processed flexible ReRAM and proves its potential applications to flexible electronics.     

下电极对ZnO薄膜电阻开关特性的影响

本文采用直流磁控溅射法在三种不同的下电极(BEs)上制备了ZnO薄膜, 获得了W/ZnO/BEs存储器结构. 研究了不同的下电极材料对器件电阻开关特性的影响. 研究结果表明, 以不同下电极所制备的器件都具有单极性电阻开关特性. 在低阻态时, ZnO薄膜的导电机理为欧姆传导, 而高阻态时薄膜的导电机理为空间电荷限制电流. 不同下电极与ZnO薄膜之间的肖特基势垒高度对电阻开关过程中的操作电压有较大的影响, 并基于导电细丝模型对不同下电极上ZnO薄膜的低阻态阻值及reset电流的变化进行了解释. 关键词： ZnO薄膜 电阻开关 下电极     

Characterization of the interface between the Hf-based high-k thin film and the Si using spatially resolved electron energy-loss spectroscopy

The interfacial structures of HfO₂ and HfAlO thin films on Si have been investigated using spatially resolved electron energy-loss spectroscopy. We have found that interfaces are not atomically sharp, and variation in the symmetry of the local atomic coordination lasts for a couple of monolayers for both the as-deposited HfO₂ and the HfAlO samples. Annealing of the HfO₂ film in the oxygen environment leads to the formation of a thick SiO₂/SiO_x stack layer in-between the original HfO₂ and the Si substrate. As a comparison, the interfacial stability is significantly improved by Al incorporation into the HfO₂ film (forming HfAlO), which effectively reduced/eliminated the interfacial silicon oxide formation during the oxygen annealing process. The mechanism of the high-k film/substrate stabilization by Al incorporation is discussed based on the experimental results.     

Improved resistive switching performance in Cu-cation migrated MoS2 based ReRAM device incorporated with tungsten nitride bottom electrode

The resistive switching characteristics of sputtered deposited molybdenum disulphide (MoS₂) thin film has been investigated in Cu/MoS₂/W₂N stack configuration for Resistive Random Access Memory (ReRAM) application. The benefits of incorporating tungsten nitride (W₂N) as a bottom electrode material were demonstrate by stability in operating voltages, good endurance (10³ cycles) and long non-volatile retention (10³?s) characteristics. Resistive switching properties in Cu/MoS₂/W₂N structure are induced by the formation/disruption of Cu conducting filaments in MoS₂ thin film. Ohmic law and space charge limited current (SCLC) are observed as dominant conduction mechanism in low resistance state (LRS) and high resistance state (HRS) respectively. This study suggests the application of MoS₂ thin films with W₂N bottom electrode for next generation non-volatile ReRAM application.     

Effect of crystallization on the reliability of unipolar resistive-switching in HfO2-based dielectrics

The electroforming and unipolar resistive switching characteristics of as-deposited and annealed HfO₂ and Hf_0.6Si_0.4O₂ films are reported. The reliability of HfO₂ devices is shown to be significantly degraded by annealing at 600 °C, during which the film is observed to crystallize. In contrast, the characteristics of Hf_0.6Si_0.4O₂ devices subjected to the same annealing conditions are found to be unchanged, consistent with the fact that the films remain amorphous. These differences are attributed to the presence of grain boundaries and can have important implications for the use of HfO₂ in ReRAM applications.     

Structural and optical properties of nitrogen-incorporated HfO2 gate dielectrics deposited by reactive sputtering

High-k HfO_xN_y thin films with different nitrogen-incorporation content have been fabricated on Si (1 0 0) substrate by means of radio-frequency reactive sputtering method. Analyses from X-ray diffraction (XRD) and atomic force microscopic have indicated that the increase of the crystallization temperature of HfO₂ thin films and the decrease of the roughness root-mean-square value of HfO₂ thin films due to the incorporation of nitrogen. Based on a parameterized Tauc-Lorentz (TL) dispersion model, the optical properties of the HfO_xN_y thin films related to different nitrogen-incorporation content are systematically investigated by spectroscopic ellipsometer. Increase in the refractive index and the extinction coefficient and reduction in band gap with increase of nitrogen-incorporation content are discussed in detail.     

Improved resistive switching phenomena observed in SiNx‐based resistive switching memory through oxygen doping process

The improvement of resistive switching (RS) phenomena of silicon‐nitride (SiN_x)‐based resistive random access memory (ReRAM) cells through oxygen doping process was investigated. As a result, compared to un‐doped SiN_x films, the oxygen doped SiN_x (SiN_x:O₂)‐based ReRAM cells show a lower current (～0.3 μA) level at a high resistance state and a smaller variation of operating voltage through the reduction of leakage current in the SiN_x:O₂ film by combining silicon dangling bonds and doped oxygen ions. Therefore, we believe that the oxygen doping process in SiN_x films can effectively improve the RS characteristics of SiN_x‐based ReRAM cells. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced bipolar resistive switching of HfO2 with a Ti interlayer

The characteristics of resistive switching of TiN/HfO₂/Ti/HfO₂/Pt/Ti stacks on SiO₂/Si substrates were investigated and compared to TiN/HfO₂/Pt/Ti stacks in order to study Ti interlayer effects on resistive switching. The Ti interlayers were deposited in situ during the reactive sputtering of HfO₂ films. The current–voltage measurements showed that the Ti interlayers enhanced the memory window but reduced the endurance of SET/RESET operations. The energy filtered images by TEM showed asymmetric oxygen accumulation at the Ti/HfO _x interfaces. Subsequent heat treatment improved the endurance of SET/RESET operation of TiN/HfO₂/Ti/HfO₂/Pt/Ti stacks.     

Resistive switching behaviour of a tantalum oxide nanolayer fabricated by plasma oxidation

We investigate the resistive switching behaviour of a tantalum oxide nanolayer‐based nonvolatile memory with Pt/TaO_5–x/TaN structure, which was prepared at room temperature through a processing compatible with CMOS technology. The tantalum oxide nanolayer with thickness of about 5 nm was fabricated by plasma oxidation of TaN films. The switching mechanism can be explained by the modulation of the local oxygen‐deficient conduction channel resulting from oxygen ions drift. This Letter represents a cost‐efficient method for developing nanoscale restive switching nonvolatile memories. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X-ray absorption and emission spectroscopy at the Hf L1 edge of hafnium-(silicon)-oxide ultra-thin films

X-ray absorption and emission spectroscopic study at the Hf L₁ edge was applied to investigate the local structure around hafnium atoms in Hf(Si)O_x ultra-thin films, which are the most promising candidates for the high-k gate dielectric material of next generation CMOS devices. HfSiO_x showed an extra absorption above the Hf-L₁ threshold, which is not seen in HfO_x. HfSiO_x also had stronger Compton scattering peak in Hf-Lγ emission region, and smaller Hf-Lγ₂/Lγ₃ ratio, compared with those of HfO_x. These differences should be caused by partial replacements of hafnium atoms by silicon atoms as the second nearest neighbors of a hafnium atom.     

Bipolar resistive switching effect and mechanism of solution-processed orthorhombic Bi2SiO5 thin films

Orthorhombic Bi₂SiO₅ thin films with dense surface were synthesized by using a chemical solution deposition method. The crystallized films were first utilized to implement resistive memory cells with Pt/Bi₂SiO₅/Pt sandwich architecture. It exhibited outstanding switching parameters including concentrated distributions of low and high resistance states, uniform switching voltages, cycling endurance, and long retention. Furthermore, the model of formation and rupture of conductive filaments consisted of oxygen vacancies was used to well explain resistive switching behavior. The results revealed that the solution-processed Bi₂SiO₅ thin film devices have great potential for forefront application in nonvolatile memory.     

Coexistence of resistive switching and magnetism modulation in sol-gel derived nanocrystalline spinel Co3O4 thin films

We report the coexistence of resistive switching and magnetism modulation in the Pt/Co₃O₄/Pt devices, where the effects of thermal annealing and film thickness on the resistive and magnetization switching were investigated. The sol-gel derived nanocrystalline Co₃O₄ thin films obtained crack-free surface and crystallized cubic spinel structure. The 110 nm Co₃O₄ film based device annealed at 600 °C exhibited optimum resistive switching parameters. From I–V curves fitting and temperature dependent resistance, the conduction mechanism in the high-voltage region of high resistance state was dominated by Schottky emission. Magnetization-magnetic field loops demonstrated the ferromagnetic behaviors of the Co₃O₄ thin films. Multilevel saturation magnetization of the Co₃O₄ thin films can be easily realized by tuning the resistance states. Physical resistive switching mechanism can be attributed to the rejuvenation and annihilation of conductive filament consisting of oxygen vacancies. Results suggest that Pt/Co₃O₄/Pt device shows promising applications in the multifunctional electromagnetic integrated devices.     

Composition dependence of unipolar resistance switching in TaOx thin films

Amorphous TaO_x thin films were deposited at different temperatures, and the resistance switching properties of the Pt/TaO_x/Pt structure were investigated. X‐ray photoelectron spectroscopy showed that the amount of Ta₂O₅ in the film decreased and the content of Ta suboxides increased substantially when the growth temperature was increased. Unipolar resistance switching near the anode was stable only for TaO_x film grown at room temperature. The experimental results revealed the critical effect of the film composition on the resistance switching behavior of TaO_x films. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The use of angle resolved XPS to measure the fractional coverage of high-k dielectric materials on silicon and silicon dioxide surfaces

Angle resolved XPS (ARXPS) is a powerful tool for the determination of the thickness of ultra-thin films. In the case of high-k dielectric layers, the technique is capable of measuring the thickness of both the high-k layer and intermediate layers of silicon dioxide or metal silicate. The values for layer thickness are in close agreement with those generated by a variety of other techniques. As well as knowing the thickness of these layers, it is important to determine whether the layers are continuous or whether the coverage of the high-k layer is only partial. Using ARXPS, a method has been developed to determine whether the coverage of the high-k material is continuous and, if not, to calculate the fraction of the surface that is covered. The method is described with reference to the layers of Al₂O₃ grown on SiO₂ using atomic layer deposition (ALD). The method is then applied to HfO₂ layers produced using ALD on silicon wafers whose surfaces had received three different types of surface treatment. The way in which the layers grow and the nature of the resulting layer were found to depend upon the pre-treatment method. For example, growth on a thermal silicon dioxide surface resulted in complete coverage of HfO₂ after fewer ALD cycles than layers grown on an H-terminated surface. The results from ARXPS are compared with those obtained from ToF SIMS that have been shown earlier to be a valuable alternative to the LEIS analysis [1].     

Microstructure and interfacial properties of HfO2-Al2O3 nanolaminate films

High-k HfO₂-Al₂O₃ composite gate dielectric thin films on Si(1 0 0) have been deposited by means of magnetron sputtering. The microstructure and interfacial characteristics of the HfO₂-Al₂O₃ films have been investigated by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry (SE). Analysis by XRD has confirmed that an amorphous structure of the HfO₂-Al₂O₃ composite films is maintained up to an annealing temperature of 800 °C, which is much higher than that of pure HfO₂ thin films. FTIR characterization indicates that the growth of the interfacial SiO₂ layer is effectively suppressed when the annealing temperature is as low as 800 °C, which is also confirmed by spectroscopy ellipsometry measurement. These results clearly show that the crystallization temperature of the nanolaminate HfO₂-Al₂O₃ composite films has been increased compared to pure HfO₂ films. Al₂O₃ as a passivation barrier for HfO₂ high-k dielectrics prevents oxygen diffusion and the interfacial layer growth effectively.     

Fabrication and electrical characterization of a MOS memory device containing self-assembled metallic nanoparticles

Floating gate devices with nanoparticles embedded in dielectrics have recently attracted much attention due to the fact that these devices operate as non-volatile memories with high speed, high density and low power consumption. In this paper, memory devices containing gold (Au) nanoparticles have been fabricated using e-gun evaporation. The Au nanoparticles are deposited on a very thin SiO₂ layer and are then fully covered by a HfO₂ layer. The HfO₂ is a high-k dielectric and gives good scalability to the fabricated devices. We studied the effect of the deposition parameters to the size and the shape of the Au nanoparticles using capacitance–voltage and conductance–voltage measurements, we demonstrated that the fabricated device can indeed operate as a low-voltage memory device.     

Hafnia-based resistive switching devices for non-volatile memory applications and effects of gamma irradiation on device performance

Non-volatile memory (NVM) devices were fabricated as a Metal– Insulator–Metal (MIM) structures by sandwiching Hafnium dioxide (HfO₂) thin film in between two metal electrodes. The top and bottom metal electrodes were deposited by using the thermal evaporation, and the oxide layer was deposited by using the RF magnetron sputtering technique. The Resistive Random Access Memory (RRAM) device structures such as Ag/HfO₂/Au/Si were fabricated and I-V characteristics for the pristine and gamma-irradiated devices with a dose 24?kGy were measured. Further we have studied the thermal annealing effects, in the range of 100°–400°C in a tubular furnace for the HfO₂/Au/Si samples. The X-ray diffraction (XRD), Rutherford Backscattering Spectrometry (RBS), field emission-scanning electron microscopy (FESEM) analysis measurements were performed to determine the thickness, crystallinity and stoichiometry of these films. The electrical characteristics such as resistive switching, endurance, retention time and switching speed were measured by a semiconductor device analyser. The effects of gamma irradiation on the switching properties of these RRAM devices have been studied.     

Band edge electronic structure of transition metal/rare earth oxide dielectrics

This article addresses band edge electronic structure of transition metal/rare earth (TM/RE) non-crystalline and nano-crystalline elemental and complex oxide high-k dielectrics for advanced semiconductor devices. Experimental approaches include X-ray absorption spectroscopy (XAS) from TM, RE and oxygen core states, photoconductivity (PC), and visible/vacuum ultra-violet (UV) spectroscopic ellipsometry (SE) combined with ab initio theory is applied to small clusters. These measurements are complemented by Fourier transform infra-red absorption (FTIR), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). Two issues are highlighted: Jahn-Teller term splittings that remove d-state degeneracies of states at the bottom of the conduction band, and chemical phase separation and crystallinity in Zr and Hf silicates and ternary (Zr(Hf)O₂)_x(Si₃N₄)_y(SiO₂)_1−x−y alloys. Engineering solutions for optimization of both classes of high-k dielectric films, including limits imposed on the continued and ultimate scaling of the equivalent oxide thickness (EOT) are addressed.     

Improving the dielectric constant of Al2O3 by cerium substitution for high-k MIM applications

Process compatible high-k dielectric thin films are one of the key solutions to develop high performance metal–insulator–metal (MIM) structures for future microelectronic devices. Engineered cerium–aluminate (Ce_xAl_2–xO₃) thin films were deposited on titanium nitride metal electrodes by electron-beam co-evaporation of ceria and alumina in a molecular beam deposition chamber. X-ray photoelectron spectroscopy clearly reveals that Ce cations can be stabilized in the 3+ valence state in Ce_xAl_2–xO₃ up to x = 0.7 by accommodation in the alumina host matrix. Higher Ce content was observed to result in cerium dioxide segregation in cerium aluminate matrix, probably due to the chemical tendency of Ce cations to exist rather in the 4+ than in the 3+ state. Electrical characterization of the X-ray amorphous Ce_0.7Al_1.3O₃ films reveals a dielectric constant value of about 11 and leakage current lower than 10^?4 A/cm². No parasitic low-k interface formation between the high-k Ce_0.7Al_1.3O₃ film and the TiN metal electrode is detected.