Reversible Electrical Bistable Thin Film Devices, And "dynamic" Molecular Rectifier Research

Part 1 Research on Reversible bistable film devicesTo overcome the potentially limiting scaling difficulties present in the silicon-based semiconductor devices and to fulfill huge demands for electronic products, there is a strong desire to develop "electrode / functional layer / electrode" reversible electrical bistable memory devices. The devices could be divided into inorganic and organic ones, based on the functional materials employed. Inorganic memory devices are thought to be hopeful for application because of their good stability. However, the complexity and high cost in fabricating limit their popularization. Up to date, majority studies on inorganic devices have been mainly focused on the properties and mechanisms of developed devices, whereas few researches have been concerned with exploring inorganic functional layer that could be prepared with simple methods. Developing novel easy-fabricating inorganic devices with high stability is of great importance for simplifying manufactory process, lowering cost and commercializing the memory devices. On the other hand, organic devices provide simplified manufacturing process yielding low-cost, flexible, and light-weight devices, and may accomplish multilevel conductance storage. So exploring novel organic memories is a major interest of many scientists.Herein, we develop simple ways to fabricate inorganic functional films through interface reactions, and obtain two novel inorganic electrical bistable devices. Firstly, vacuum thermal evaporation method is employed to deposit metal and inorganic precursor layers on the substrate. Through the solid-solid interface reactions between the two components, an inorganic film is formed, which can be employed as the functional layer in electrical bistable memory devices. Then based on the aforementioned method, an even simpler aqueous solution dipping method is developed to prepare inorganic functional layer through liquid-solid interface reaction. Through the solid-liquid method, electrical memories with excellent performances are realized and successive thousands of write-read-erase-read cycles are observed in the devices, promising high potential for application. The inorganic functional films are characterized with many types of measurements and the reaction mechanism for the formation of inorganic functional films are proposed. The switching mechanism and electrical memory characteristics are investigated. Besides, MC molecule-basedorganic devices are fabricated and their electrical switching memory characteristicsare studied, including the influences of parameters like electrodes and organic layerthickness.The experiments and results are summarized as follows:1. An AgK₂(SCN)₃ composite layer is in situ growing on the silver electrode through solid-solid interface reactions between silver and potassium thiocyanate films prepared by vacuum thermal evaporation. This method accomplishes easy-fabricating of inorganic memory devices. Visible spectrometry, Raman spectra, X-Ray diffraction (XRD) patterns and X-ray photoelectron spectroscopy (XPS) are employed to analyze the film and the mechanism of interface reactions in the formation of AgK₂(SCN)₃ composite films are proposed: KSCN dydrolyses to form HSCN, and HSCN further reacts with Ag₂O at the interface to form AgK₂(SCN)₃ composite film.2. The AgK₂(SCN)₃ composite layer-based devices show electrical bipolar reversible bistability and can be operated stably and successively in "write-read-erase-read" mode with 10⁶ ON/OFF ratio. Conductive model fittings are analyzed and the electrical characteristics of devices using different electrodes are investigated, based on which it is concluded that the switching is due to formation-annihilation of conducting channels; and the rupture of conducting channels is suggested to be due to both Joule heating effect and ionization of conducting channels.3. A simple aqueous solution dipping method is developed for in situ growing CuSCN films on Cu electrodes through liquid-solid interface reactions between thiocyanate salt solutions and Cu films. This method provides a facile way to prepare inorganic memories with excellent performances. The as-prepared films are analyzed by Raman spectra and XRD patterns. The sorts of thiocyanate salts and their solution concentration are found to have effect on CuSCN film growing. The forming process of CuSCN film is monitored by scanning electron microscope (SEM), showing that the CuSCN grows in an islands-growth mode.4. The CuSCN-based device exhibits stable electrical reversible bistability. An enhancement of memory switching properties is specified after an "electrical aging" process, which changes the microstructures and components of the medium layer, forms localized Cu diffusion channels, and accordingly stabilizes the device properties. More than 3000 write-read-erase-read cycles are achieved in the unpackaged devices, with ON/OFF ratio higher than 100 and narrow dispersions of both ON and OFF resistances; besides, the devices show good nonvolatile properties and perform stably at high temperatures, promising potential for applications. A localized conducting channels model is proposed to explain the switching and good resistance uniformity in the device.5. MC-based M/O/M structure devices are fabricated through vacuum thermal evaporation, where MC is melamine cyanuric acid. The Ag / MC / Al device with Ag top electrode exhibits multilevel conductance memory effects. With the analysis based on the curves fittings, film characterizations, comparison of devices using different electrodes and different organic thickness, it is suggest the evaporation of Ag top electrode is crucial for the multilevel conductance and the switching mechanism is studied.Part 2 Research on dynamic-molecular-based rectifierAs fundamental one of the molecular electronic devices, molecular rectifier is now the major interest of many scientists. In spite of some progress made in molecular rectifier area, the rectification ratios of most devices based on D-σ-A model are too small for application.Following previous researches in our lab, we systematically study the achievement of molecular rectification and further verify the novel "dynamic" model of molecular electronics under the condition of Scanning Tunneling Microscope (STM). We demonstrate that such molecular devices exhibit pronounced rectification with high ratios, and elucidate the rectification mechanism as well.The specific experiments and results are listed as follows:1. The DT-1 molecules are chemisorbed on STM tip and the adsorbed molecules exhibit stable pronounced rectification with ratio about 10⁴. The excellent characteristics are attributed to the "dynamic" mechanism as follows: the swing of molecules with dipoles under external electric field leads to the change of tunneling distances and accordingly alter tunneling currents, thus resulting in high rectification ratio.2. The influences of adsorbed molecules tightness and external voltage sweeping rates are investigated. More densely packed monolayer or faster voltage sweeping suppresses the rectification effect. And the results further demonstrate the "dynamic" molecule device model. Besides, other molecules (DT-2, DT-3, DT-4, and DT-5) with the same frameworks are adopted in the construction of dynamic molecular devices. All the molecular devices exhibit stable rectification effect.