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.     

Tailoring resistive switching characteristics in WOx films using different metal electrodes

We have investigated the role of the metal/oxide junction interface on the resistive switching (RS) characteristics in WO_3+x films. The WO_x films are fabricated on Pt substrates by magnetron sputtering at room temperature. Top metal contact (Au or Al) is fabricated by using thermal evaporator. The thicknesses of WO_x films and top electrodes are 1 μm and 200 nm, respectively. It has been found that the bi-polar RS direction is dependent on the choice of top metal electrode, Au or Al. The sample with a Au top electrode shows clockwise (CW) RS mode whilst the sample with a Al top electrode shows counter-clockwise (CCW) RS mode. The on/off ratio is 10 times for Au/WO_x/Pt and 100 times for Al/WO_x/Pt. The bi-polar RS modes are modeled in terms of the difference in the electronegativity of the top and bottom electrodes.     

Role of sweep direction and substrate work function on resistive switching of titanium-di-oxide [TiO2] nanoparticles

The resistive switching mechanism in titanium-dioxide nanoparticles (TiO₂ NP) is studied using the current-voltage (I–V) measurements. The TiO₂ NP are spin-coated on different substrates like FTO, ITO, Gold, and p-Silicon. The I–V measurements are carried out by changing the initial potential of the substrates to either 0 V (sweep1) or −1 V (sweep2). Resistive switching (RS) was observed only for FTO/TiO₂ NP and ITO/TiO₂NP devices in sweep1 direction. Whereas, in sweep2 direction, no such RS was observed in any of the devices. The detailed I–V analysis infers the Ohmic conduction followed by space charge limited conduction (SCLC) during the RS forming process for FTO/TiO₂ NP and ITO/TiO₂NP devices. The Au and p-Si substrates act as blocking contact for TiO₂ and exhibit Schottky/thermionic emission at lower voltages and SCLC at higher voltages. The TiO₂ NP coated on p-Si substrate exhibits rectifying behaviour with a current ratio of 3 orders of magnitude.     

Resistive switching effect in metal–oxide–metal structures with ZnO:Li oxide layer

The resistive switching effect in metal–oxide–metal (MOM) structures has been investigated, where the 10% Li-doped ZnO layer was used as an oxide layer, as well as Pt and 20% fluorine doped SnO₂ (SnO₂:F) were used as a bottom electrodes. The current–voltage (I–V) and switching (I–t) characteristics of Ag/ZnO:Li/Pt and Ag/ZnO:Li/SnO₂:F structures were investigated. The unipolar resistive switching is detected in the structures with the Pt, while the use of transparent conductive SnO₂:F electrode instead of Pt, results to the bipolar memory effect.     

Effect of cation and anion defects on the resistive switching polarity of ZnO x thin films

In this paper, we achieve the resistive switching (RS) polarity from unipolar to bipolar in a simple Al/ZnO _x /Al structure by moderating the oxygen content in the ZnO sputtering process. In a pure Ar sputtering, Al/ZnO _x /Al shows unipolar behavior, as oxygen partial pressure increases, the RS polarity changes to bipolar, and the switch current decreases by about five orders of magnitude. The current transport properties of unipolar device show ohmic behavior under both high resistance (HRS) and low resistance states (LRS), but the bipolar device shows Schottky barrier modulated current transport properties. We study the defect types in the unipolar and bipolar devices through photoluminescence (PL) spectra. The PL results show that the interstitial zinc (Zn_i) and interstitial oxygen (O_i) are dominant in unipolar and bipolar devices, respectively. We attribute this phenomenon to Zn_i and O_i playing important role in unipolar (URS) and bipolar resistive switching (BRS), respectively.     

Characterization of SnO2/Pb2CrO5 thin film/metal photovoltaic device

The photovoltaic behavior in a design of SnO₂/Pb₂CrO₅ thin film/metal (Au or Al) is described as functions of light intensity and wavelength for two different illumination directions. The Pb₂CrO₅ thin film in each device is formed over the SnO₂ coated glass substrate by an electron-beam evaporation technique. When illuminated from the metal side, the device with Au or Al shows the positive or the negative photovoltage, respectively. For the illumination from the SnO₂ side, the device with Au has the negative photovoltage at the short wavelength, and the positive one at the longer, while the device with Al has the negative photovoltage at both wavelength regions. These photovoltaic devices have two junctions at both sides of the Pb₂CrO₅ thin film. The band model of each device is also discussed.     

Top electrode effects on resistive switching behavior in CuO thin films

Top electrode (TE) material on the resistive switching behavior of (TE)/CuO/SnO₂:F/Si substrate has been studied. We investigated the switching properties of CuO films deposited by sol-gel process. Two kinds of top electrode (TE) material on the resistive switching behaviors have been studied. The nonpolar and bipolar resistive switching phenomenon was observed in CuO thin films with different top electrodes. The filamentary mechanism was used to explain the two kinds of resistive switching behaviors. For the Pt/CuO/ATO device, it showed the nonpolar resistive switching where conducting path is formed and disappear due to the oxygen vacancy. For the Cu/CuO/ATO device, the resistance reduction is due to the existing Cu to form conduction Cu-rich pathways. An opposite bias takes the existing Cu back to the Cu electrode to its high-resistance state. CuO thin films are also observed by XRD, AFM and XPS.     

Resistive switching mechanism of Ag/ZrO2:Cu/Pt memory cell

Resistive switching mechanism of zirconium oxide-based resistive random access memory (RRAM) devices composed of Cu-doped ZrO₂ film sandwiched between an oxidizable electrode and an inert electrode was investigated. The Ag/ZrO₂:Cu/Pt RRAM devices with crosspoint structure fabricated by e-beam evaporation and e-beam lithography show reproducible bipolar resistive switching. The linear I?CV relationship of low resistance state (LRS) and the dependence of LRS resistance (R _ON) and reset current (I _reset) on the set current compliance (I _comp) indicate that the observed resistive switching characteristics of the Ag/ZrO₂:Cu/Pt device should be ascribed to the formation and annihilation of localized conductive filaments (CFs). The physical origin of CF was further analyzed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). CFs were directly observed by cross-sectional TEM. According to EDS and elemental mapping analysis, the main chemical composition of CF is determined by Ag atoms, coming from the Ag top electrode. On the basis of these experiments, we propose that the set and reset process of the device stem from the electrochemical reactions in the zirconium oxide under different external electrical stimuli.     

基于Au/TiO_2/FTO结构忆阻器的开关特性与机理研究

采用简单的一步水热法在FTO导电玻璃上外延生长了锐钛矿TiO_2纳米线,制备了具有Au/TiO_2/FTO器件结构的锐钛矿TiO_2纳米线忆阻器,系统研究了器件的阻变开关特性和开关机理.结果表明,Au/TiO_2/FTO忆阻器具有非易失的双极性阻变开关特性.同时,在103s的时间内,器件在0.1 V的电阻开关比始终保持在20以上,表明器件具有良好的非易失性.此外,器件在低阻态时遵循欧姆导电特性,而在高阻态时则满足陷阱控制的空间电荷限制电流传导机制,同时提出了基于氧空位导电细丝形成与断开机制的阻变开关模型.研究结果表明Au/TiO_2/FTO忆阻器将是一种很有发展潜力的下一代非易失性存储器.     

Mechanism analysis of switching direction transformation in an Er2O3 based RRAM device

The resistive random access memory (RRAM) based on resistive switching effect has considered to be the most advanced next generation memory, in which the switching direction determines the order of reading-writing. In this work, the rare-earth metal Er₂O₃ was used as functional layer, and Ag and indium-tin-oxide (ITO) are selected as top and bottom electrode to fabricate resistive switching device. Further, it is observed that the switching direction and memory window of resistive switching device can be regulated by exchanging top and bottom electrode. Moreover, the complementary switching memory behavior in Ag/Er₂O₃/ITO/Er₂O₃/Ag structure was also observed. Through mechanism analysis, it is expected that the barrier changes and metal-ions oxidation-reduction should be responsible for the conversion of switching direction and regulation of memory window. This work opens up a way to the development of next generation new concept memory.     

The effect of metal work function on the barrier height of metal/CdS/SnO2/In–Ga structures

In order to interpret the effect of metal work function on the formation of the barrier height at metal/semiconductor (M/S) interface, the CdS/SnO₂/In–Ga structures with several metals (Ag, Au, Al, Te) have been investigated by using I–V characteristics at room temperature. The main electrical parameters such as ideality factor (n), zero-bias barrier height (Φ_Bo), series resistance (R_s) have been determined and compared with each other. The values of n were found to be 3.00, 2.56, 3.83, and 3.31 for Al, Ag, Te, and Au/CdS/SnO₂/In–Ga structures, respectively. The values of Φ_Bo were also found to be 0.489 eV, 0.490 eV, 0.583 eV, 0.591 eV for Al, Ag, Te, and Au/CdS/SnO₂/In–Ga structures, respectively. The Φ_Bo dependence on the metal work function (Φ_m) was found to vary almost linearly as Φ_Bo = 0.106Φ_m + 0.028. The low value of the slope S (dΦ_B/dΦ_M ? 0.106) shows a weak relationship between Φ_Bo and Φ_m due to serious Fermi-level pinning in the conduction band. In addition, the I–V plots have a rectifying behavior. The rectification ratio, defined by the ratio of forward to reverse current (RR = I_F/I_R) measured at the same absolute bias, was found as 11.96, 20.88, 35.82, and 75.61 for Al, Ag, Te, Au/CdS/SnO₂/In–Ga diodes, respectively. In addition, the values of R_s were determined from Ohm's Law and Norde's method. Analysis of I–V characteristics confirm that using of different metal (Al, Ag, Te, Au) has significant effect on electrical parameters of such devices.     

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 interracial barriers are inves- tigated 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 interracial 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 interracial barrier engineering, could be exploited for novel applications in nonvolatile memory devices.     

Impact of amorphous titanium oxide film on the device stability of Al/TiO2/Al resistive memory

We have investigated the role of amorphous titanium oxide film in the reliable bipolar resistive switching of Al/TiO₂/Al resistive random access memory devices. As TiO₂ deposition temperature decreased, a more stable endurance characteristic was obtained. We proposed that the degradation of the bipolar resistive switching property of Al/TiO₂/Al devices is closely related to the imperfect migration of oxygen ions between the top insulating interface layer and the oxygen-deficient titanium oxide during the set and reset operations. In addition, the dependence of the TiO₂ film thickness on the switching property was also studied. As the thickness of the film increased, a reduction in the resistance of the high resistance state rapidly appeared. We attribute the improved endurance performance of thin and low-temperature grown TiO₂ devices to the amorphous state with a low film density.     

Top electrode-dependent resistance switching behaviors of lanthanum-doped ZnO film memory devices

Lanthanum-doped ZnO (Zn_0.99La_0.01O) polycrystalline thin films were deposited on Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. Metal/La-doped ZnO/Pt sandwich structures were constructed by depositing different top electrodes (Ag and Pt). Unipolar switching and bipolar switching characteristics were investigated in Pt/La-doped ZnO/Pt and Ag/La-doped ZnO/Pt structures, respectively. Compared with the undoped devices (Pt/ZnO/Pt and Ag/ZnO/Pt), the La-doped devices exhibits superior resistive switching performances, such as narrow distribution of the resistive switching properties (R _ON, R _OFF, V _Set, and V _Reset), higher R _OFF/R _ON ratio and sharp switching transition.     

The role of metal contacts in the stability of n-type organic field effect transistors

We are presenting a long-time bias stress stability of C₆₀-based n-type organic field effect transistors (OFETs), in bottom gate, top contacts configuration, with aluminium (Al), silver (Ag) and gold (Au) source–drain contacts. The results clearly shows that the bias stress effects in C₆₀-based n-type OFETs is similar to p-type OFETs and it can be reduced by using an appropriate metal for the source–drain contacts. During the bias stress time, the threshold voltage shift and an increase in the contacts resistance have also been measured. On the basis of the stability of the device parameters, it is proposed that the Al source–drain contact-based devices gives better stability as compared to the devices with Ag and Au source–drain contacts. Our results show that the bias stress-induced threshold voltage shift is due to the trapping of charges in the channel region and in the vicinity of the source–drain contacts.     

Redox driven conductance changes for resistive memory

The relationship between bias-induced redox reactions and resistance switching is considered for memory devices containing TiO₂ or a conducting polymer in “molecular heterojunctions” consisting of thin (2–25 nm) films of covalently bonded molecules, polymers, and oxides. Raman spectroscopy was used to monitor changes in the oxidation state of polythiophene in Au/P3HT/SiO₂/Au devices, and it was possible to directly determine the formation and stability of the conducting polaron state of P3HT by applied bias pulses [P3HT = poly(3-hexyl thiophene)]. Polaron formation was strongly dependent on junction composition, particularly on the interfaces between the polymer, oxide, and electrodes. In all cases, trace water was required for polaron formation, leading to the proposal that water reduction acts as a redox counter-reaction to polymer oxidation. Polaron stability was longest for the case of a direct contact between Au and SiO₂, implying that catalytic water reduction at the Au surface generated hydroxide ions which stabilized the cationic polaron. The spectroscopic information about the dependence of polaron stability on device composition will be useful for designing and monitoring resistive switching memory based on conducting polymers, with or without TiO₂ present.     

Nonvolatile Resistive Switching and Physical Mechanism in LaCrO_3 Thin Films

Polycrystalline LaCrO_3(LCO) thin films are deposited on Pt/Ti/SiO_2/Si substrates by pulsed laser deposition and used as the switching material to construct resistive random access memory devices. The unipolar resistive switching(RS) behavior in the Au/LCO/Pt devices exhibits a high resistance ratio of ~104 between the high resistance state(HRS) and low resistance state(LRS) and exhibits excellent endurance/retention characteristics.The conduction mechanism of the HRS in the high voltage range is dominated by the Schottky emission, while the Ohmic conduction dictates the LRS and the low voltage range of HRS. The RS behavior in the Au/LCO/Pt devices can be understood by the formation and rupture of conducting filaments consisting of oxygen vacancies,which is validated by the temperature dependence of resistance and x-ray photoelectron spectroscopy results.Further analysis shows that the reset current I_R and reset power P_R in the reset processes exhibit a scaling law with the resistance in LRS(R_0), which indicates that the Joule heating effect plays an essential role in the RS behavior of the Au/LCO/Pt devices.     

Multilevel resistive switching performance of TiO2-based flexible memory prepared by low-temperature sol-gel method with UV irradiation

The flexible Ag/TiO₂/ITO/PET resistive switching memory is prepared by low-temperature sol-gel method with UV irradiation, and the simple method that combined the advantages of sol-gel method and low temperature can be applied to fabricate high-quality film. The flexible Ag/TiO₂/ITO/PET memory device displays good resistive behavior, for instance, the narrow distributions of switching voltages, good cycle endurance, and long retention time. Meanwhile, the multilevel resistance states of the device can be realized by controlling the compliance current or reset voltages, showing the potential of applications in neural networks and high-density storge. In addition, flexibility of the Ag/TiO₂/ITO/PET is studied, which exhibit good endurance and retention properties under bending condition. The I–V curves are replotted and fitted for analyzing the conductive mechanism of the device. The fitting results show that SCLC and Ohmic mechanism are main mechanisms of high resistance state and low resistance state respectively. The electrochemical and thermochemical modes are adopted to explain resistive switching behavior. Our results indicate the Ag/TiO₂/ITO/PET memory has potential application in wearable and foldable electronics.     

Unipolar resistive switching in Au/Cr/Mg0.84Zn0.16O2?δ /p+-Si

Unipolar resistive switching memory cells were fabricated using a Mg_0.84Zn_0.16O_2?δ thin film, sandwiched between p⁺-Si (100) substrate and Cr/Au top electrodes. Electrical measurements showed a large memory window and memory window margin of 10⁷ and 10⁴, respectively. Furthermore, a wide switching voltage distribution gap of 3.6?V between the switching-ON and -OFF processes was obtained for different sweeping cycles. Gas bubbles at four different stages were observed on the top electrodes after electrical stimulus, indicating that conducting filaments consisting of oxygen vacancies are responsible for the resistive switching characteristics. Conductive atomic force microscopy results show that the highly conductive areas are along the edge of the gas bubble or the edge of the device. This phenomenon suggests the potential of scaling down the device area to lower than 32?nm.     

Characteristic modification by inserted metal layer and interface graphene layer in ZnO-based resistive switching structures

ZnO-based resistive switching device Ag/ZnO/TiN, and its modified structure Ag/ZnO/Zn/ZnO/TiN and Ag/graphene/ZnO/TiN, were prepared. The effects of inserted Zn layers in ZnO matrix and an interface graphene layer on resistive switching characteristics were studied. It is found that metal ions, oxygen vacancies, and interface are involved in the RS process. A thin inserted Zn layer can increase the resistance of HRS and enhance the resistance ratio. A graphene interface layer between ZnO layer and top electrode can block the carrier transport and enhance the resistance ratio to several times. The results suggest feasible routes to tailor the resistive switching performance of ZnO-based structure.