Infrared spectroscopy and X-ray diffraction studies on the crystallographic evolution of La2O3 films upon annealing

Rare earth oxides (REOs) have lately received extensive attention in relation to the continuous scaling down of non-volatile memories (NVMs). In particular, La₂O₃ films are promising for integration into future NVMs because they are expected to crystallize above 400 °C in the hexagonal phase (h-La₂O₃) which has a higher κ value than the cubic phase (c-La₂O₃) in which most of REOs crystallize. In this work, La₂O₃ films are grown on Si by atomic layer deposition using La(C₅H₅)₃ and H₂O. Within the framework of the h-La₂O₃ formation, we systematically study the crystallographic evolution of La₂O₃ films versus annealing temperature (200-600 °C) by Fourier transform infrared spectroscopy (FTIR) and grazing incidence X-ray diffraction (GIXRD). As-grown films are chemically unstable in air since a rapid transformation into monoclinic LaO(OH) and hexagonal La(OH)₃ occurs. Vacuum annealing of sufficiently thick (>100 nm) La(OH)₃ layers induces clear changes in FTIR and GIXRD spectra: c-La₂O₃ gradually forms in the 300-500 °C range while annealing at 600 °C generates h-La₂O₃ which exhibits, as inferred from our electrical data, a desirable κ ∼ 27. A quick transformation from h-La₂O₃ into La(OH)₃ occurs due to H₂O absorption, indicating that the annealed films are chemically unstable. This study extends our recent work on the h-La₂O₃ formation.     

Thermal oxidation of chemical vapour deposited tungsten layers on silicon substrates for embedded non-volatile memory application

This research is targeted to enhance the functionality of bipolar complementary metal-oxide-semiconductor by innovative concepts of embedded resistive random access memory (RRAM) cells integration in the back-end-of-line (BEOL) region. The material of our interest is tungsten oxide as an insulator in RRAM cells and we focussed on the growth and characterisation of closed tungsten oxide layers. In this materials science study, we investigated the tungsten oxidation process under BEOL constraints (< 450 °C). Thin films of tungsten oxide (6-50 nm) were prepared by oxidising, under an atmosphere of one bar oxygen, the chemical vapour deposited tungsten layers on TiN covered silicon wafers. The X-ray photoelectron spectroscopy investigations indicate that the stoichiometric WO₃ grows after oxidation at 300 °C for an hour. The tungsten oxide layers prepared above 300 °C for longer than 15 min were non-stoichiometric. The X-ray diffraction investigations reveal the crystallisation of the WO₃ layers in monoclinic phase above 350 °C when oxidised for longer than 30 min; above 400 °C the (001) growth texture becomes dominant.     

The effects of active layer thickness on Programmable Metallization Cell based on Ag–Ge–S

Programmable Metallization Cell (PMC) is a newly developed non-volatile memory device based on chalcogenide solid electrolytes. In this paper, we studied the PMC’s electrical properties as a function of its active layer thickness. The PMC devices are fabricated based on Ag-Ge-S materials using thermal evaporation method. The fabricated devices have their active layer thickness ranges from 8 nm to 30 nm. The I-V characteristics of the fabricated devices are studied as a function of their active layer thickness. It is observed that the ‘ON’ resistance of PMC displays a decreasing trend as we increase the active layer thickness. This is because less deposition nuclei can be formed on the cathode/electrolyte interface in thinner devices, which limits the number of conduction links that can be formed. The SET voltage also decreases slightly as increase of the active layer thickness. This is probably because thinner Ag-Ge-S layer localizes Ag ions’ movements due to the large size of Ag+ ion. On the other hand, the RESET voltage increases as we increase the active layer thickness, which is because there are more conduction links to be ionized in thicker devices.     

Role of tunneling layer in graphene-oxide based organic nonvolatile memory transistors

This paper demonstrates non-volatile memory transistor using solution processable graphene oxide (GO) as charge storage nodes in the configuration, p⁺⁺Si/SiO₂/GO/Tunneling layer/Pentacene/Au. The tunneling layers are polymethylmethacrylate (PMMA) and polyvinylphenol (PVP). GO film could be deposited as single layered flakes with a uniform distribution using spin coating technique. The devices with PMMA as charge tunneling layer exhibited higher mobility and on/off ratio than PVP based devices. The devices show a large positive threshold voltage shift (∼24 V for PMMA and ∼15 V for PVP) from initial value during programming at gate voltage of +80 V kept for 10 s. The transfer curves can be restored approximately to its initial condition by applying an erasing voltage of −30 V for 10 s for both the devices. Since such a large shift is not observed without GO layer, we consider that memory effect was due to electron trapping in GO. Further, retention of the initial memory window was measured to be 63% and 37% after 3000 s for PMMA and PVP based devices, respectively.     

Growth of amorphous,anatase and rutile phase TiO2 thin films on Pt/TiO2/SiO2/Si (SSTOP) substrate for resistive random access memory (ReRAM) device application

Memory structures play a basic role in providing integrated circuits of powerful processing capabilities. Even most powerful processors have nothing to offer without an accompanying memory and importantly, the development of mobile devices is dependent on the continual improvement of memory technology. Herein, we report the synthesis of TiO₂ thin films on SSTOP (Pt/TiO₂/SiO₂/Si) substrate via physical vapour deposition process for the first time. The layers consisted of Si, SiO₂, TiO₂ and Pt, hence the SSTOP shorthand is used throughout the text. Three different phases of TiO₂ thin films were obtained, i.e. amorphous, anatase and rutile phases, by controlling the reaction parameters which were examined by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM) and Raman-scattering spectroscopy in order to understand the crystallographic, morphological, compositional and scattering properties. The detailed studies confirmed the formation of various crystal phases of titania. The grown thin films on SSTOP substrates were further utilized to fabricate resistive random access memory (ReRAM) devices and the initial electrical screening was performed on capacitor-like structures which were prepared using platinum top electrodes (diameter = 250 μm) on a 14 × 14 array metal contact mask. Current-Voltage (I–V) measurements were implemented employing a range of current compliances (IC). The detailed electrical characterizations revealed that the forming field for a switchable unipolar device was found to be greatest on rutile titania and lowest on the amorphous titania phase. Similarity, the resistive contrast was greatest on the rutile titania phase and lowest on the anatase titania phase.     

Organic small-molecule heterointerface for use in transistor-type non-volatile memory

A chargeable layer is an essential element for charge transfer and trapping in a transistor-based non-volatile memory device. Here we demonstrate that a heterointerface layer comprising of two different small molecules can show electrical memory characteristics. The organic heterointerface layer was fabricated with a pentacene and tris(8-hydroxyquinoline) aluminum (Alq3) layers by sequential vapor deposition without breaking the vacuum state. Pentacene was adopted as the active layer on the top, and Alq3 was used as the bottom layer for charge trapping. The bottom-gate top-contact transistor with an organic heterointerface layer showed distinct non-volatile memory behaviors and showed high air stability and reliability. We investigated the energy structure of the pentacene/Alq3 heterointerface layer to reveal the operation mechanism of the non-volatile memory and suggested that the writing/erasing gate bias-dependent energy barrier originating from the difference between the energy levels of the pentacene and Alq3 layers controls the charge transfer at the heterointerface layer. Our approach suggests a simple way to fabricate heterointerface layers for organic non-volatile memory applications with high air stability and reliability.     

Insights on the chemical basis of the astringency of Spanish red wines

The main goal of the present study is to provide an insight on the role played by non-volatile molecules on the different in-mouth attributes, particularly astringency. For achieving such goal, the main in-mouth sensory attributes of 34 oaked Spanish red wines were measured by a trained panel. The wine content in 30 sensory-active molecules was analysed by different HPLC based methodologies together with classical enological parameters and two proanthocyanidin indexes. Fourteen compounds (aconitic acids, polymeric proanthocyanidins, caftaric, caffeic and coutaric acids and seven quercetins) were found to be at concentrations above reported taste thresholds and to have a reasonably high range of occurrence. Two highly statistically significant models for astringency were built with those compounds. Even if the models could not be fully validated by sensory addition experiments, the research has demonstrated that wine astringency is driven by polymeric proanthocyanidins and by certain phenolic acids, the rate trans/cis-aconitic acid and flavonol profiles. The research has highlighted the existence of extremely complex interactions between non-volatile compounds on the in-mouth sensory perception. Particularly remarkable is the lack of additivity and potential antagonism found between the pairs cis/trans-aconitic acids, between aconitic and caffeic acids and between quercetin-3-O-galactoside and quercetin-3-O-glucoside. Also remarkable was the sweetness × astringent interaction and the matrix-dependence of the sensory responses elicited by flavonols. These results suggest the need for new paradigms and experimental procedures for fully decoding the real sensory relevance of individual non-volatile compounds in the overall wine flavour.     

Electrostatic force microscopy measurements of charge trapping behavior of Au nanoparticles embedded in metal–insulator–semiconductor structure

Charge trapping properties of electrons and holes in Au nanoparticles embedded in metal–insulator–semiconductor (MIS) on p-type Si (1 0 0) substrates were investigated by electrostatic force microscopy (EFM). The Au nanoparticles were prepared with a unique laser irradiation method and charged by applying a bias voltage between EFM tip and sample. The EFM system was used to image charged areas and to determine quantitatively the amount of stored charge in the Au nanoparticle-inserted MIS structure. In addition, charge trapping characteristics of the samples were carried out with electrical measurements, such as capacitance–voltage and current–voltage measurement for memory characteristics. Finally, the comparison of EFM results with the electrically measured data was done to determine the amount of stored charge in the Au nanoparticle-inserted MIS structure, confirming the usefulness of EFM system for the characterization of nanoparticle-based non-volatile devices.     

Phase Change Memory lifetime enhancement via online data swapping

As DRAM technology is facing scalability limitations due to its excessive leakage power in nano-scale technologies, various non-volatile memory technologies have been emerged to replace it in memory hierarchy. Among these technologies, Phase Change Memory (PCM) is a promising technology for main memory due to its near-zero leakage power, higher density, non-volatility and soft error immunity. However, its major drawbacks, including high write energy and limited write endurance, have prevented its usage as a drop-in replacement of DRAM technology. In this paper, we propose a technique to swap data between memory lines with goal of reducing bit flips. The proposed swapping technique finds the best place to write a chunk of data among a limited set of lines to minimize number of bit flips. The proposed swapping operation works online i.e, does not require any data profiling. Moreover, it does not require major modifications of existing solutions and works only by the addition of a proposed circuitry. It is remarkable that, this technique is additive to various other architectures aiming at PCM lifetime enhancement. Experimental results carried out on a quad core CMP system show that the proposed technique prolongs PCM main memory lifetime by 48% which is achieved at the price of 1% and 2% overhead in read and write latencies respectively.     

Rare earth-based high-k materials for non-volatile memory applications

A study of a La-based high-k oxide to be employed as active dielectric in future scaled memory devices is presented. The focus will be held on La_xZr_1−xO_2−δ (x = 0.25) compound. In order to allow the integration of this material, its chemical interaction with an Al₂O₃ cap layer has been studied. Moreover, the electrical characteristics of these materials have been evaluated integrating them in capacitor structures. The rare earth-based ternary oxide is demonstrated to be a promising candidate for future non-volatile memory devices based on charge trapping structure.