Metal-insulator-metal Devices Research Articles

Nonvolatile Memory Based on Polymer-Suspended Graphene Nanoplatelets with Fractional and Integer Quantum Conductance at 300K and Zero Magnetic Field

Metal-insulator-metal (MIM) structures with graphene submicron-sized nanoplatelets embedded in 3-hexylthiophene (P3HT) polymer were fabricated. Such organic memory devices exhibit reliable memory operation with an ON/OFF ratio exceeding 10. Surprisingly, quantized conductance is observed at zero magnetic field and 300K, in most devices at integer multiples of Go=2e2/h(=12.9 kΩ)-1, and in some devices also at fractional-quantized with v=m/7 filling factor. For memory applications, the quantum conductance is attractive as no heat is dissipated in the cells proper even at 300K. At high GNP concentrations the MIM cell behaves as an excellent conductor, at low GNP concentrations as a stable insulator, and at medium concentrations displays a significant memory hysteresis effect. Graphene-based MIM devices were fabricated on thermally-oxidized silicon wafers using standard semiconductor processes. Bottom Au and top Cu electrodes have been deposited using electron-beam physical vapor deposition. The graphene nanoplatelet (GNP) powder was dispersed into the toluene and further exfoliated by ultrasonication. GNP platelets comprise one or more layers of one-atom-thick planar sheets (5-30) of sp2-bonded carbon atoms densely packed in a honeycomb crystal lattice. The lateral dimension of the flakes ranges 100 nm-10 mm. Ultrasonication is effective method in overcoming the van der Waals forces between the individual carbon sheets leading to uniform dispersions of single flakes or very thin stacks of flakes. The resulting P3HT(GNP) dispersion was drop-deposited onto the bottom Au electrode and covered by islands of Cu electrodes. The graphene concentrations in the P3HT(GNP) solutions vary from 0.05mg/ml to 0.2mg/ml. The P3HT solution is 10 ml. The thicknesses of the layers are 60 nm, 700 nm, 150 nm, for Au, P3HT(GNP), and Cu, respectively. For each concentration, ten devices with the same structure and the same active area were fabricated. For comparison and assessment of the role of carbon nanoplatelets, six Au/P3HT/Cu structures have been fabricated with no graphene nanoplatelets. All of the latter devices show neither integer nor fractional quantum conductance effects. Four types of GNP polymer samples have been manufactured: i) no GNP, ii) 0.05 mg/ml, iii) 0.1 mg/ml, and iv) 0.2 mg/ml of GNP concentration. From these four types of devices, only devices with 0.1 mg/ml show integer and fractional quantum conductance. Roughly 25% of the 0.1 mg/ml devices display integer quantum conductance, 25% fractional quantum conductance, and remaining 50% no quantum conductance. For electrical characterization the bias voltage is first swept along the positive x-axis from 0V to 2V. The current is observed to increase ohmically in the low-voltage region, and then abruptly to increase at three voltages, specifically at 0.75V, 1.2V and 1.87V, to 10mA, 162mA and 409mA. The initially high-resistance state of about 100kΩ is reduced during the voltage sweep to a final low-resistance state, about 8kΩ. When the voltage is swept back from 2V to 0V, a clear hysteresis in the I-V characteristics is observed lending itself to memory applications. The extracted conductance is quantized in multiples of Go. This behavior has been observed repeatedly on several devices and for several voltage sweeps on the same device. The step-like conductance is sloped by the thermal broadening owing to ambient temperature, T=300K. This behavior is modeled consistently with one single suspended graphene ribbon bridging the gap between the Cu and Au electrodes. The conductance changes in quantum units of Go (Go=2e2/h=77.5µS), from 1Go to 2Go and from 2Go to 3Go. In addition, a sequence of smaller conductance steps within the 1Go interval is observed indicating that the fractional quantum conductance coexists with integer quantum conductance. The best fit to the fractional quantum conductance within the first Go step can be obtained by assuming a filling factor of v=1/7, 2/7, 3/7, 4/7, 5/7, and 6/7, although 3/7 step is slightly off the experimental value. The resolution of the fractional conductance between 1Go and 2Go is always much less pronounced and appears to follow the same filling factor of multiples of 1/7. Steps with 8/7, 9/7, 11/7, and 13/7 can be clearly identified. In the third conductance step unit from 2Go to 3Go only a rudimentary fractional quantum conductance can be discerned. This behavior has been observed for several devices and can be repeated on the same device with a slightly changed pattern and length of the steps. The highly conductive devices with GNP concentration of 0.2 mg/ml do not display any quantum conductance effects and are a good material candidate for stretchable transparent electrodes. The broad range of electric tunability of GNP films points to the universal potential of GNP depending on its concentration in a polymer for a wide spectrum of applications.

X-ray Absorption Spectroscopy Study of TiO2–x Thin Films for Memory Applications

Metal–insulator–metal (MIM) devices based on titanium dioxide thin films exhibit resistive switching behavior (RS); i.e., they have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage. This behavior makes titanium dioxide thin films extremely valuable for memory applications. The physical mechanism behind RS remains a controversial subject but it has been suggested that it could be interface-type, without accompanying structural changes of the oxide, or filament-type with formation of reduced titanium oxide phases in the film. In this work, X-ray absorption spectroscopy (XAS) at the Ti K-edge (4966 eV) was used to characterize the atomic-scale structure of a nonstoichiometric TiO2–x thin film before and after annealing and for the first time after inclusion in a MIM device based on a Cr/Pt/TiO2–x/Pt stack developed on an oxidized silicon wafer. The advantage of the XAS technique is that is element-speci...

Multiple Diode-Like Conduction in Resistive Switching SiO<italic><sub>x</sub></italic>-Based MIM Devices

Filamentary conduction in resistive switching metal- insulator-metal devices is often modeled from the circuital viewpoint using diode-like structures with series resistances. We show in this letter which arrangement of diodes and resistances is compatible with experimental multilevel set and reset I-V characteristics in electroformed TiN/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /TiN structures. The proposed model is based on the solution of the generalized diode equation corresponding to N diodes arranged in parallel with a single series resistance. The model is simple yet accurate and it is able to capture the essential features exhibited by the I-V curves in the low and high bias regimes, revealing that a single equation can deal with both the low and high resistance states. An exact expression for the differential conductance suitable for small-signal analysis and circuit simulators is also provided.

Infra-red curing methodology for Roll-to-Roll (R2R) manufacturing of conductive electrodes through inkjet technology applicable for devices in the field of flexible electronics

ABSTRACTThe Inkjet printing technology is a direct patterning technique to deposit functional materials with high precision and accuracy. This deposition technology is often used to manufacture conductive electrodes for different active and passive electronic devices on flexible foils. It is an up-scalable process in terms of printing devices from low (via. Sheet-to-Sheet, S2S platform) to high (via. Roll-to-Roll platform) quantities. For manufacturing of these conductive electrodes and hence electronic devices through the R2R platform, a suitable post-treatment/curing methodology is very much desired. In this work, the focus is concentrated on the curing methodology using the Infra-red radiation for both the inkjet-printed conductive electrodes and insulator layer, for completing a “proof of concept” Metal-Insulator-Metal (MIM) electronic device structure over the R2R platform. A conductive silver nano-particle and a polymeric dielectric ink are used to print the top and bottom conductive electrodes, with a middle insulator layer for the MIM structure respectively. It is observed that not only the printed silver electrode layers (both top and bottom) can be cured with the help of the Infra-red radiation, but also the insulator layer. Additionally, the layers constituting the MIM device structure is cured with the conventional curing methodology which in this case is thermal curing using a convection oven. This curing procedure for the printed functional layers is generally performed for the S2S manufacturing process. The conductive electrodes are then electrically characterized by measuring the sheet resistance (on the foil and dielectric layer) as a function of the un-conventional Infra-red radiation and conventional oven curing methodologies. The cured layers for both the conductive electrodes and insulator layers are morphologically analyzed for the layer thickness and homogeneity. The electrical performance of the cured insulator in form of the obtained capacitance from the MIM passive device is compared for the two mentioned curing methodologies.

Influence of the surface roughness of the bottom electrode on the resistive-switching characteristics of Al/Al2O3/Al and Al/Al2O3/W structures fabricated on glass at 300 °C

Resistive-switching devices based on Metal–Insulator–Metal (MIM) structures have shown promising memory performance characteristics while enabling higher density of integration. Usually, these MIM devices are fabricated using different processing conditions including high temperature thermal treatments that could lead to undesirable chemical reactions in the insulator material and at its interface with the metals involved. In this work, we compare the electrical characteristics of MIM devices (fabricated on glass at 300°C) that use aluminum or tungsten as bottom electrode (BE) in order to study the influence of a highly reactive (aluminum) or inert (tungsten) metal electrode on the memory characteristics. We found that the switching characteristics of Al2O3 (from a high-resistance state HRS to a low-resistance state LRS and vice versa), are highly dependent on the surface roughness of the BE, the thickness of Al2O3 and the current compliance (CC) which limits the electron density flowing through both top/bottom electrodes.

Impact of dielectric crystallinity on the resistive switching characteristics of ZrTiOx-based metal–insulator-metal devices

Graphical abstractDisplay Omitted Investigate the impact of dielectric crystallinity on the performance of RRAM.Provides more insights into the different conduction mechanisms for RRAM operation.Amorphous ZrTiOx outperforms crystalline ZrTiOx for RRAM applications. Amorphous and crystalline ZrTiOx-based metal-insulator-metal (MIM) devices were used as the platform to investigate the impact of dielectric crystallinity on the electrical performance of resistive memory devices. Without any forming step, bipolar switching behaviors can be found for devices with both types of dielectrics. However, the opposite initial state and current-voltage (I-V) hysteresis direction suggest different conduction mechanisms which are mainly determined by whether grain boundaries exist. Combining grain boundaries with a higher permittivity, devices with crystalline ZrTiOx show undesirable operation voltage fluctuation, higher current and switching voltage, and worse endurance as compared to its counterparts. On the contrary, devices with amorphous ZrTiOx are promising in realizing next-generation nonvolatile memory in terms of low operation voltages of ?1.3V, a tight distribution of operation voltages, high operation speed of 200ns, a large sensing margin of 218 times, robust endurance up to 105 cycles, and satisfactory retention performance.

Analytical model for metal–insulator–metal mesh waveguide architectures

Metal–insulator–metal (MIM) waveguide mesh structures utilize X-junctions as power distribution elements to create interference and feedback effects, thereby providing rich device functionality. We present a generalized analytical model for MIM mesh structures by incorporating a modified characteristic impedance model for MIM junctions into the scattering matrix formalism. The modified impedance model accounts for metal absorption and provides accurate prediction of plasmonic field distribution at X-junctions in terms of both magnitude and phase. Closed-form expressions for 2×1 and 2×2 MIM mesh architectures as well as MIM stub structures are then obtained and are dependent only on waveguide geometry and junction configuration. The model does not require numerically extracted parameters, and results agree, within a few percent, with those obtained from finite-difference time-domain method for both two-dimensional and three-dimensional waveguide geometries. The capability of the model for efficient design and optimization of junction-based MIM devices is demonstrated through the development of various filter and resonant devices.

Enhancing Electron-Emission Efficiency of MIM Tunneling Cathodes by Reducing Insulator Trap Density

Metal–insulator–metal (MIM) tunneling cathodes demonstrate electron-emission efficiency $(\alpha)$ of 9 $\%$ to 11 $\%$ . This threefold increase in $\alpha$ from that previously reported is achieved by reducing trap density in the cathodes' insulator, thereby enabling tunneling electrons to accelerate with fewer scatterings in the insulator. Life tests show that the high $\alpha$ is maintained for a 19 000-h operation in sealed display panels containing a ZnS-based phosphor. The reduction of trap density also suppresses long-term drifts in emission and diode currents to a satisfactory level. These emission characteristics are sufficient for realizing a large-sized field-emission display with MIM cathodes. In addition, an effective method for measuring trap density in the insulator of MIM devices is devised. This method adopts low-stress $I$ – $V$ measurement following charge-injection stress. Measurement by this method reveals that trap density is proportional to neodymium concentration in the anodized insulator. The trap density is therefore reduced by lowering the neodymium content in the precursor Al–Nd alloy film. Moreover, the robustness of the MIM cathodes to charge-injection stress is demonstrated.

Electrical characterization of poly(amide-imide) for application in organic field effect devices

The admittance spectra and current–voltage (I–V) characteristics are reported of metal–insulator–metal (MIM) and metal–insulator–semiconductor (MIS) capacitors employing cross-linked poly(amide–imide) (c-PAI) as the insulator and poly(3-hexylthiophene) (P3HT) as the active semiconductor. The capacitance of the MIM devices are constant in the frequency range from 10Hz to 100kHz, with tanδ values as low as 7×10−3 over most of the range. Except at the lowest voltages, the I–V characteristics are well-described by the Schottky equation for thermal emission of electrons from the electrodes into the insulator. The admittance spectra of the MIS devices displayed a classic Maxwell–Wagner frequency response from which the transverse bulk hole mobility was estimated to be ∼2×10−5cm2V−1s−1 or ∼5×10−8cm2V−1s−1 depending on whether or not the surface of the insulator had been treated with hexamethyldisilazane (HMDS) prior to deposition of the P3HT. From the maximum loss observed in admittance-voltage plots, the interface trap density was estimated to be ∼5×1010cm−2eV−1 or ∼9×1010cm−2eV−1 again depending whether or not the insulator was treated with HMDS. We conclude, therefore, that HMDS plays a useful role in promoting order in the P3HT film as well as reducing the density of interface trap states. Although interposing the P3HT layer between the insulator and the gold electrode degrades the insulating properties of the c-PAI, nevertheless, they remain sufficiently good for use in organic electronic devices.

Fabrication and Characterization of MIM Diodes Based on Nb/Nb2O5 Via a Rapid Screening Technique

Metal–insulator–metal (MIM) structures are gaining signifi cant attention due to their applications in varied electronic devices such as rectennas for energy harvesting, [ 1–4 ] high-frequency detectors/infrared photo-detection, [ 5–7 ] high-frequency mixers, [ 8–10 ] as well as applications in static memory and switching devices. [ 11 , 12 ] Ideally, for most of these applications, the MIM structure should exhibit current–voltage ( I–V ) characteristics with high asymmetry ( f ASYM > 1), strong nonlinearity ( f NL > 3), fast responsivity ( f RES > 7 V − 1 ), low hysteresis and low turn-on voltage (close to zero bias). [ 3 ] Despite the widespread utility and simple architecture of MIM devices, there is a signifi cant lack of understanding as to which materials properties produce the desired device performance. Although it is commonly stated that a high work-function difference ( Δ φ ) between the metal electrodes is responsible for high f ASYM and f NL , [ 3 , 5 , 7 ]

Linearity Improvement on MIM Capacitors

A study has been carried out to improve metal-insulator-metal (MIM) capacitor's capacitance density and linearity performance. The scopes of the study included single MiM and stack MIM structures. Different dielectric schemes were evaluated with their corresponding capacitance density, breakdown voltages and linearity coefficient to voltage and temperature variation etc. characterised. Trade-off of capacitance density and linearity observed with respect to the dielectrics thickness. In this paper, a special layout design and a new integration concept from stack MIM device will be introduced, with this, we can achieve both high capacitance density as well as ultra low voltage coefficients of capacitance for the desirbale good analog performance on MIM passive devices.

TiO 2 -based metal-insulator-metal selection device for bipolar resistive random access memory cross-point application

We report a simple metal-insulator-metal (MIM)-type selection device that can alleviate the sneak current path in cross-point arrays. By connecting a nanometer-scale Pt/TiO2/TiN selection device to a Pt/TiO2−x/TiO2/W resistive random access memory (RRAM), we could significantly reduce read disturbance from unselected memory cells. This selection device could be easily integrated into an RRAM device, in which it suppressed the sneak current and significantly improved the readout margin compared to that obtained for an RRAM not using a selection device. The introduction of this MIM device can fulfill the requirement for an appropriate selection device for bipolar-type RRAM cross-point applications.

Enhanced leakage current behavior of Sr 2Ta 2O 7−x/SrTiO 3 bilayer dielectrics for metal–insulator–metal capacitors

Metal–Insulator–Metal (MIM) capacitors are one of the most essential components of radio frequency devices and analog/mixed-signal integrated circuits. In order to obtain high capacitance densities in MIM devices, high-k materials have been considered to be promising candidates to replace the traditional insulators. The challenging point is that the dielectric material must demonstrate high capacitance density values with low leakage current densities. In this work, SrTiO 3 based MIM capacitors have been investigated and the electrical performance of the devices have been optimized by using bilayered systems of Sr 2Ta 2O 7−x/SrTiO 3 with different thicknesses of Sr 2Ta 2O 7−x. Sputtering X-Ray photoelectron spectroscopy (XPS) measurements have been applied to investigate the interfaces between the thin film constituents of the MIM stacks. The optimized bilayered system provides a leakage current density of 8∗10 − 8 A/cm 2 at 2 V (bottom electrode injection) and a high capacitance density of 13 fF/μm 2.

Characterization of atom and ion-induced “internal” electron emission by thin film tunnel junctions

Highly excited charge carriers are released when single or multiply charged ions impinge on metal surfaces. While electron emission into the adjacent vacuum phase is well investigated, one has only limited knowledge about the transport of excited electrons or holes into the bulk of a metal. This shortcoming can be reduced by studying the transport of these excited carriers over buried tunnel barriers in thin film metal-insulator–metal devices. The internal barriers can be tuned by a tunnel voltage which makes the device to a balance for excited electrons and holes. With a simple theoretical model we derive the balance function of different tunnel barriers and show their feasibility for the characterization of particle induced electronic excitations on metal surfaces.

Influences of Interface States on Resistive Switching Properties of TiOx with Different Electrodes

Different TiOx thin films prepared by graded or sufficient oxidization of Ti are applied with Pt or Ag electrode in metal-insulator-metal (MIM) structures for studying the properties and mechanisms of resistive switching. The differences on the mobile oxygen vacancies in TiOx films and different work functions of the electrode films result in different insulator-metal interface states, which are displayed as ohmic-like or non-ohmic contact. Based on the interface states, the electrical models for MIM devices are analyzed and extracted. The electrode-limited effect and the bulk-limited effect can be unified to explain the mechanisms for resistive switching behavior as the dominant effect respectively in various conditions. All the current-voltage curves of the four kinds of specimens measured in the experiments can be explained and proved in accordance with the theory.

Study of the effect of thermal annealing on high k hafnium oxide thin film structure and electrical properties of MOS and MIM devices

The effect of rapid thermal annealing on structural and electrical properties of high k HfO2 thin films is investigated. The films were initially deposited at pre-optimized sputtering voltage of 0.8 kV and substrate bias of 80 V in order to get optimized results for oxide charges and leakage current as a MOS device. The film properties were investigated for optimum annealing temperature in oxygen and optimum rapid thermal annealing temperature in nitrogen respectively to get the best electrical results as a MOS device structure. The film thickness, composition and microstructure is studied by Laser Ellipsometry, XRD and AFM and the effect of thermal annealing is shown. The electrical I–V and C–V characteristics of the annealed dielectric film were investigated employing Al-HfO2-Si MOS capacitor structure. The flat-band voltage (Vfb) and oxide-charge density (Qox) were extracted from the high-frequency C–V curve. Dielectric study were further carried out on HfO2 thin films having metal–insulator–metal (MIM) configuration over a wide temperature (300–500 K) and frequency (100 Hz to 1 MHz) range.

Effects of the Ag content on the geometrical and electrical characteristics of the screen-printed etched gate electrodes of OTFTs using Ag ink

During the fabrication of gate electrodes by Ag ink screen-printing combined with a wet-etching process, the effects of the Ag content on the geometrical and electrical characteristics such as the thickness and surface roughness of gate electrode, step coverage with the gate dielectric, leakage current associated with the step coverage, and the electrical performance of organic thin film transistors (OTFTs) were investigated. An increase of Ag content resulted in the thick and densely-packed Ag electrode, which had a stable and excellent conductivity. But, the large thickness of Ag electrode caused the worse step coverage of PVP (polyvinylphenol) dielectric layer on the edge of the Ag gate electrode, therefore, for Ag contents more than 40 wt.%, MIM (metal–insulator–metal) devices and OTFTs with the Ag gate electrodes had very large leakage current (>10 −4 A/cm 2) and off-state current (>∼19 pA/μm) due to the poor step coverage of PVP dielectric layers, respectively. Finally, we found that an Ag content of 20–30 wt.% was suitable for the screen-printed etched gate electrode of OTFTs using Ag ink. This range generated a mobility of 0.18 cm 2/V s, an on/off current of 5 × 10 6, and an off-state current of 0.002 pA/μm, which are suitable to drive e-paper.

Improved Lower Electrode Oxidation of High-k TiCeO Metal-Insulator-Metal Capacitors by Using Dual Plasma Treatment

We have fabricated high-k Ir/TiCeO/TaN metal-insulator-metal (MIM) capacitors using a dual plasma treatment on bottom electrode. The novel plasma treatment suppresses the growth of bottom interface layer to reduce the degradation of capacitor performance under 400oC PDA. A low leakage current of 1.7x10-7 A/cm2 at -1 V and capacitance density of ~18 fF/μm2 at 1 MHz were obtained at 21nm TiCeO MIM devices; moreover, a 37 nm-thick TiCeO film has a capacitance density of 11 fF/um2, which gives a k value of 45. Consequently, the excellent device performance is due to the combined effects of the dual plasma treatment, high-k. TiCeO dielectric and a high work-function Ir metal

Atomic Vapor Deposition of Titanium Nitride as Metal Electrodes for Gate‐last CMOS and MIM Devices

AbstractPure and diluted Ti[N(Et)2]4 precursors are used to grow TiN layers at 400–600 °C by using atomic vapor deposition (AVD®). The composition, microstructure, and electrical properties of TiN films with various thicknesses are investigated. The determined work function of 4.7 eV indicates the possibility of using AVD®‐grown TiN as a metal gate electrode for PMOSFET and metal‐insulator‐metal (MIM) devices. TiN/HfO2/SiO2 stacks are integrated into gate‐last PMOS transistors, and the extracted parameters are compared to poly‐Si/SiO2 reference transistors. The optimized films grown at 400 °C with a thickness of 20 nm exhibit a resistivity of 400 µΩ cm.

Evaluation of Hydrogen in Tantalum Thin Films Using Secondary Ion Mass Spectrometry

The hydrogen distribution in tantalum and tantalum oxide thin films has been examined by secondary ion mass spectrometry (SIMS). Tantalum and tantalum oxide thin films implanted with deuterium were analyzed by conventional SIMS. Metal–insulator–metal (MIM) devices were analyzed by backside SIMS. The results of these measurements show that the hydrogen distribution is concentrated on the front side in tantalum films, while the hydrogen distribution in tantalum oxide films is not affected by SIMS measurement. A model has been proposed to explain the cause of an anomalous hydrogen peak forming at the interface between the tantalum oxide and tantalum films in the MIM devices. In the model, when the tantalum oxide film covering the tantalum film is removed by primary ion beam sputtering, hydrogen atoms in the tantalum film are quickly discharged to a vacuum in a sample chamber and the anomalous hydrogen peak is detected.