A number of applications call for the organization of resistive non-volatile memory (NVM) into large, densely-packed crossbar arrays. While resistive-NVM devices often possess some degree of inherent nonlinearity (typically 3-30× contrast), the operation of large (>1000×1000 device) arrays at low power tends to require large (> 1e7) ON-to-OFF ratios between the currents passed at high and at low voltages. Such large nonlinearities can be implemented by including a distinct access device together with each of the state-bearing resistive-NVM elements. While such an access device need not store data, its list of requirements is almost as challenging as the specifications demanded of the memory device.

We review our work on high-performance access devices based on Cu-containing Mixed-Ionic-Electronic Conduction (MIEC) materials [1–7]. (This version focuses only on the MIEC-based access device itself; previously-published longer versions of this work [8–10] also include more extensive surveys of competing devices as well.) These devices require only the low processing temperatures of the Back-End-Of-the-Line (BEOL), making them highly suitable for implementing multi-layer crossbar arrays. MIEC-based access devices offer large ON/OFF ratios (>1e7), a significant voltage margin Vm (over which current < 10nA), and ultra-low leakage (<10pA), while also offering the high current densities needed for PCM and the fully bipolar operation needed for high-performance RRAM. Scalability to critical dimensions (CD) <30nm and thicknesses <15nm, tight distributions and 100% yield in large (512kBit) arrays, long-term stability of the ultra-low leakage states, and sub-50ns turn-ON times have all been demonstrated. Numerical modeling of these MIEC-based access devices shows that their operation depends on Cu+ mediated hole conduction. Circuit simulations reveal that while scaled MIEC devices are suitable for large crossbar arrays of resistive-NVM devices with low (<1.2V) switching voltages, a compact vertical stack of two MIEC devices in series could support large crossbar arrays for switching voltages up to 2.5V.

In this work we examine the electrical characteristics and the memory properties of metal-alumina-nitride-oxide-silicon (MANOS) devices as a function of the post deposition annealing conditions. Post deposition annealing of the samples was performed at 850 or 1050 °C in nitrogen ambient using two different processes: (1) Furnace annealing for 15 min and (2) rapid thermal annealing for 1 or 5 min. The capacitance equivalent thickness as extracted from the capacitance voltage characteristics depends strongly on the annealing process, being smallest for the furnace annealing. Furthermore, the experimental results indicate that the type of the annealing determines the defect state density of the Al2O3 layer, via which the undesired effect of gate electrode electron injection takes place in the negative voltage regime. For inert ambient annealing the furnace process appears more efficient as compared to RTA.

Rare-earth oxides have attracted considerable research interest in resistive random access memories (ReRAMs) due to their compatibility with complementary metal-oxide semiconductor (CMOS) process. To this end we report unipolar resistive switching in a novel ternary rare-earth oxide LaHoO3 (LHO) to accelerate progress and to support advances in this emerging densely scalable research architecture. Amorphous thin films of LHO were fabricated on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition, followed by sputter deposition of platinum top electrode through shadow mask in order to elucidate the resistive switching behavior of the resulting Pt/LHO/Pt metal-insulator-metal (MIM) device structure. Stable unipolar resistive switching characteristics with interesting switching parameters like, high resistance ratio of about 105 between high resistance state (HRS) and low resistance state (LRS), non-overlapping switching voltages with narrow dispersion, and excellent retention and endurance features were observed in Pt/LHO/Pt device structure. The observed resistive switching in LHO was explained by the formation/rupture of conductive filaments formed out of oxygen vacancies and metallic Ho atom. From the current-voltage characteristics of Pt/LHO/Pt structure, the conduction mechanism in LRS and HRS was found to be dominated by Ohm’s law and Poole-Frenkel emission, respectively.

The behavior of the redox-based resistive switching memories is influenced by chemical interactions between the electrode and the solid electrolyte, as well as by local environment. The existence of different chemical potential gradients is resulting in nanobattery effect lowering the stability of the devices. In order to minimize these effects we introduce a graphene layer at the active electrode – solid electrolyte interface. We observe that graphene is acting as an effective diffusion barrier in the SiO2-based electrochemical metallization cells and acts catalytically on the electrochemical processes prior to resistive switching.

The switching speed of a Cu/Ta2O5/Pt atomic switch between a high-resistance (OFF) state and a low-resistance (ON) state was evaluated by transient current measurements under the application of a short voltage pulse. It was found that the SET time from the OFF state to the ON state decreased as low as 1 ns, and the RESET time from the ON state to the OFF state reached a few ns using moderate pulse amplitudes. The switching time depends strongly on the pulse amplitude and the cell resistance before applying a voltage pulse. This observation indicates that oxide-based atomic switches hold potential for fast-switching memory applications. It was also found that Cu nucleation on the Pt electrode is likely to the rate-limiting process determining the SET time and the REST time appears to be preferentially determined by thermochemical reaction.

We studied GeTe structures in topological switching random access memories (TRAMs) with a [GeTe/Sb2Te3] superlattice by using X-ray diffraction (XRD) analysis. We examined the electrical characteristics of the TRAMs deposited at different temperatures. We found that XRD spectra differed between the films deposited at 200 and 240°C and that the differences corresponded to the differences in the GeTe sequences in the films.

We have demonstrated that pulsed laser deposition (PLD) conditions, i.e. O2 partial pressure (pO2) and temperature (T), enable control over the polarity of resistance switching in PCMO (Pr0.7C0.3MnO3) i.e. unipolar resistive switching (URS) vs. bipolar resistive switching (BRS). We observe by detailed physical characterization that BRS occurs in poly-crystalline thin films while URS is seen in amorphous films – indicating the materials origin of URS vis-a-vis BRS. BRS shows attractive lower voltage operation, no forming and lower variability than URS.

Resistive memory materials and devices (often called memristors) are an area of intense research, with metal/metal oxide/metal resistive elements a prominent example of such devices. Electroforming (the formation of a conductive filament in the metal oxide layer) represents one of the often necessary steps of resistive memory device fabrication that results in large and poorly controlled variability in device performance. In this contribution we present a numerical investigation of the electroforming process. In our model, drift and Ficks and Soret diffusion processes are responsible for movement of vacancies in the oxide material. Simulations predict filament formation and qualitatively agreed with a reduction of the forming voltage in structures with a top electrode. The forming and switching results of the study are compared with numerical simulations and show a possible pathway toward more repeatable and controllable resistive memory devices.

We present a detailed study of memory performance of titanium oxide (TiO2-x)-based resistive switching memories by modifying critical parameters of the films involved in the memory stack grown by reactive sputtering at room temperature. The device includes a Ti nanolayer at the Au/TiO2-x interface and it is defined by the following material stack: Au/Ti/TiO2-x/Au/SiO2/Si. We investigate the memory performance optimization of the device in terms of the Ti nanolayer thickness using as a starting point for the TiO2-x growth conditions these identified by varying the ratio of oxygen concentration to argon concentration by our previous results. Due to the superb ability of Ti to absorb oxygen atoms from the dielectric matrix, a large amount of oxygen vacancies is created, which are crucial for the stable function of the memory devices. We observe the existence of an optimum Ti thickness that if further increased gradually degrades the resistive switching behavior. The induced interface oxide thickness is found also to affect the fluctuation of the ON/OFF processes. In terms of electrical performance self-rectifying characteristics were recorded for all samples in the both resistance states. We then demonstrate that at least five-level resistance states could be obtained by modifying the compliance current, exhibiting excellent resistance uniformity and retention capability. The results are supported by C-AFM measurements demonstrating the scaling potential of the large area device discussed above.

We use the combinatorial sputter technique to simultaneously sputter HfO2 and Al2O3 targets to obtain a film of HfxAl1-xOy of specific compositions. The effect of oxide thickness, oxide composition (i.e. Hf:Al ratio) and oxygen gettering layer thickness on DC sweep based resistive switching performance of RRAM is investigated. The oxide thickness primarily affects forming voltage and causes the memory window to increase for the thinnest oxide (6 nm, other thicknesses – 12 nm, 18 nm). The composition of oxide has a non-linear effect on the memory window (high to low resistance ratio) and variability of resistance states whereas the variability of set voltage improves significantly for ternary oxide compared to individual binary oxides. Finally, electrode interlayer for oxygen-gettering is also critical where a thin Ti layer of 1.5nm maintains the memory window but reduces variability in HRS, LRS, reduces Vset and Vreset and improves the variability in Vset.

We study W | Ta | CoFeB | MgO stacks for spin-orbit torque MRAM applications. A strong perpendicular magnetic anisotropy is obtained after annealing for CoFeB layer thickness of 0.9 nm or 1.2 nm and for specific W/Ta ratios, were the Ta layer thickness is between 0.3 nm and 1 nm. Furthermore, the desired high-spin orbit coupling β-phase of W is preserved even after annealing at 350°C. We argue that an efficient B getter, like Ta, is necessary for the coherent crystallization of the CoFeB | MgO interface that allows for the establishment of perpendicular magnetic anisotropy.

Ultra thin films of chromia (Cr2O3), less than 3 nm thick, grown epitaxial on α-Al2O3 (sapphire), and are thus compressively strained in-plane. The resulting films show evidence of some magnetic ordering above the Néel temperature of chromia (307 K). The observed higher temperature hysteresis effect observed are very likely a strain effect, and not associated with the typical antiferromagnetic ordering expected of chromia.

Integrated ferroelectric capacitors Pt/PZT/Pt/Ti/SiO2/Si with sol-gel deposited PZT films are studied. The (111) textured polycrystalline films are shown to have nonconductive PZT grain boundaries. The short-circuited photocurrents measured under illumination of the films by light with the quantum energy of 2.7 eV indicate the polarization inside the film directed from the top to the bottom electrode. Using the modified method of depolarization hysteresis loops, we found a non-switchable part of polarization which was measured to be -16 μC/cm2 and directed from the top to the bottom electrode. We consider this result to be a giant self-polarization and explain it in terms of flexoelectricity caused by lattice mismatch between the PZT and bottom Pt layers. The strain gradient across the PZT film thickness is estimated from the in-plane lattice constants measured in Pt and PZT films to be ∼103cm-1, which can produce the downward flexoelectric polarization of ∼14 μC/cm2, coinciding well with the measured one. Nonsymmetrical depolarization loops are found in the films when the polarization switching itself becomes more difficult under the negative or positive driving voltage. We show experimentally how depolarization with compensating bias or film illumination can affect the film polarization switching.

We fabricated ferroelectric (Pb,La)(Zr,Ti)O3 (PLZT) capacitors with Sn:In2O3 (ITO) top electrodes using chemical solution deposition. Then, the effects of a thin conductive ITO buffer layer between the Pt bottom electrode and PLZT thin film were investigated in combination with top electrode (ITO/PLZT/ITO/Pt). The H2 degradation resistance of ITO/PLZT/ITO/Pt capacitors with a 3- and 28-nm-thick buffer layer was improved to 78 and 85%, respectively, from 60% without a buffer layer. The time-of-flight secondary ion mass spectrometry profiles indicated the intensity of H ion increased after 45 min forming gas (3% H2/balance N2) annealing.

We propose some chemical processing procedures for fabricating thin films in Hf-Zr-O system by a unique film deposition technique using supercritical carbon dioxide fluid (scCO2), i.e., supercritical fluid deposition (SCFD), which would be an prospective approach for fabricating metal-oxide films for integrated circuits because of its unique characteristics; e.g., extraction ability, transportation capability, and reaction equilibrium etc., are quite favorable for the film deposition from metal-complex precursors.

The SCFD was accomplished in a closed batch-type reaction apparatus, consisting of two steps; (a) material deposition and (b) subsequent post-treatment under scCO2 atmosphere. Thin films of amorphous Hf-Zr-O were deposited on platinized silicon [(111)Pt/TiO2/(100)Si] substrates by SCFD using metal-complex precursors M[OCH(CH3)]2(C9H11O2)2 (M = Hf or Zr) at reaction temperature of 100 – 300 °C, significantly lower than those for MOCVD. These films possessed dielectric permittivity’s of approximately 20 – 25, comparable to those from conventional processes, although they still included residue of organic species that prompt the dielectric degradation under lower-frequency bias application.

We report the evidence of ferroelectricity from LN-type ZnSnO3 nanostructure arrayed thick films (10 - 20 µm) on Si with remanent polarization value as high as ≈ 30 µC/cm2 in nanowire arrays. A combined pulsed-laser deposition (PLD) technique and a solvothermal synthesis scheme was adopted to effectively synthesize the nanostructured samples assisted by conducting ZnO template-layers. The similar crystal symmetry and comparable lattice parameter between ZnO and LN-type ZnSnO3 facilitated the dense growth of high-quality ZnSnO3 nanostructure arrays in the form of one-dimensional vertical nanowires, nanorods and two-dimensional nanoflakes. The strategic synthesis method allowed controlled tunability of the morphology, crystallinity, and packing density of ZnSnO3 nanostructures, which in turn facilitated the measurement of ferroelectric (FE) properties using a simple sandwich-device geometry. Analyses of the FE properties in relation to the structures are presented and their potential for designing future Pb-free FE devices for non-volatile memory applications is discussed.

We report the measurements of ferroelectricity in LiNbO3 (LN)-type ZnSnO3 /ZnO nanocomposite thick films deposited on Pt-Si substrates using a novel combined chemical/physical technique. Phase-pure LN-type ZnSnO3 nanorods (NRs) were first synthesized using a low temperature solvothermal process and characterized in detail using X-ray diffraction, electron microscopy and Raman spectroscopy. The prototype device for polarization measurements was fabricated by depositing the as-prepared LN-type ZnSnO3 NRs onto conducting Pt-Si substrates (also served as bottom electrodes). A dielectric filler-layer of polycrystalline ZnO was deposited on top using pulsed laser deposition to fabricate LN-type ZnSnO3 /ZnO nanocomposite films. Polarization measurements of the Pt/ZnSnO3+ZnO/Pt nanocomposite capacitors at 300K showed indication of polarization switching in the hysteresis loops with a remanent polarization (Pr) of 13 μC/cm2 at a low applied voltage of 8 V. The work provides information on the coherent design of future FE memory devices based on the emerging non-toxic Pb-free material LN-ZnSnO3.

We present the memory performance of devices with bistable electrical behavior based on polymer materials. We demonstrate that adding photosensitive particles to admixture allows us to control switching voltages and to observe photo-induced switching in addition to electrical one. From the properties of electrically-induced resistive switching and from the presence of light-induced switching we propose the necessity to consider crossover between to different switching mechanisms – filament formation and charge storage.

We present a nano-patterning process for semiconducting polymeric composites that could potentially be utilized for the development of polymer-based data storage devices. Nano-patterning (writing) operates on the basis of the mechanical interaction between the electrically unbiased tip of an atomic force microscope and the surface of polymeric composite films. Via friction forces, the tip/sample interaction produces a local increase of molecular disorder in the polymer matrix, inducing a localized lowering in the conductivity of the organic semiconductor. Herein we suggest a figure of merit for quantifying the efficiency of pattern formation and we address the dependence of the writing process on the thermal annealing temperature of the composite film. Control experiments on composite films deposited on substrates with different roughness suggest that the writing effect is invariant to the roughness of the substrate. The potential storage density of the writing process depends on the tip curvature.

Here we report our study of the electronic properties of [100]-textured gadolinium nitride (GdN) thin films synthesized using a chemical vapor deposition (CVD) method. The electronic properties of the films were investigated using photoemission and inverse photoemission spectroscopy coupled with computational modeling. Our density functional theory (DFT) calculations suggest that the theoretically predicted half-metallic electronic structure of GdN is likely due to its low density of states (DOS) at the Fermi level. These calculations are supported by our photoemission and inverse photoemission spectroscopic measurements which show a band gap for the prepared films of a few milli-electron volts, seemingly consistent with the predicted electronic structure. Additionally, the use of a CVD gallium nitride capping layer was found to decelerate the surface oxidation of our GdN samples.