Resistive Switching Of ZrO₂-based Nanoparticle Thin Films

With the traditional memory device reaching the size limit, resistive random access memory (RRAM) has attracted more and more attention due to its good performance, such as excellent scalability, low power consumption, high-speed, high density and simple preparation. Among the various materials with resistive switching phenomenon, binary metal oxides are very popular owing their countable stability and simple component. ZrO2 has lots of merits including the excellent resistive switching characteristics, but also high dielectric constant, the faster storage and compatible with complementary metal oxide semiconductor process. However, with the increasing market demand for high-density memory, RRAM devices based on one-dimensional nanomaterials and their performance improvement calls for further study. In this paper, the ZrO2 nanoparticles, ZrO2-CuO and ZrO2-NiO composite nano heterojunction were prepared using hydrothermal method. The influence of hydrothermal condition, doping elements and interface microstructure on the resistive characteristics was investigated. The specific contents of the dissertation are as follows:First, the ZrO2 nanoparticles prepared by hydrothermal method correspond to monoclinic phase with low crystallinity and particle size of 7-10 nm. The nanoparticles obtained by hydrothermal growth at 180℃ for 3 h has the relatively highest crystallinity and ON/OFF ratio. After annealed at 600℃, the crystallinity of ZrO2 nanoparticles is largely improved but the ON/OFF ratio is decreased to about 10. This may be due to the improvement of HRS current by the increased carrier mobility. The switching mechanism can be ascribed to the formation/rupture of filamentary paths due to the field-induced migration of oxygen vacancies and oxygen ions. For Li doped ZrO2, the Li impurity is expected to occupy the substitutional positions, enhancing the oxygen vacancies filaments. Thus the devcice ON/OFF ratio and set voltage is decreased. When Li doping concentration increases to 15%, excess Li ions exist in interstitial positions and restrain the formation of oxygen vacancies defects. As for La doped ZrO2, the La impurity is expected to occupy the interstitial positions, which can weaken conductive filaments. Meanwhile, the Lai3+ can increase the carrier concentration and further improve HRS current, thus leading to lower ON/OFF ratio.Second, the microstructure of CuO nanomaterials prepared by hydrothermal method is much different with that of ZrO2 nanoparticles. The CuO obtained by hydrothermal growth at 180℃ for 1 h and pH=7 shows 75 nm-flower clumps with monoclinic phase. And the switching ON/OFF ratio is about two orders of magnitudes with the threshold voltage of about 1.2 V. When the pH is 10, the CuO nanostructures become flower-sheets with thickness of about 20 nm. The switching mechanism can be attributed to the height change of Schottky barrier at the interface. The ZrO2-CuO composite nanomaterials are a uniform mixture of ZrO2/CuO hybrids with lower ON/OFF ratio and set voltage than that of pure ZrO2. The first-principles calculation indicates that the density of states near Fermi level is composed of Cu 3d orbits, which leads to the decrease of band gap and increase of carrier transition.For the ZrO2-NiO composite nanomaterials, the finely dispersed NiO nanoparticles with particle size of about 27 nm are obtained by adjusting the experiment parameters. With the annealed temperature increasing from 400℃ to 900℃, the sample crystallinity is largely improved while the ON/OFF ratio decreases from about 1400 to 11 with the set voltage retaining about 1.3 V. This may be because the grain boundary barrier decreases owing to the improved crystallinity, resulting in the increase of carrier mobility and the decrease of ON/OFF ratio. The width change of depletion layer at interface is responsible for the switching behavior. Similar to the ZrO2-CuO, ZrO2-NiO composite nanomaterials also have lower ON/OFF ratio and set voltage than that of pure ZrO2.In summary, reproducible resistive switching is observed in the as-fabricated ZrO2 nanostructures. The data analysis shows that the low-valent metals doping into ZrO2 by replacing tetravalent Zr can form acceptors and help to control the formation of oxygen vacancies. As a result, the ON/OFF ratio is improved with lower set voltage. The p-type compound of CuO or NiO can significantly decrease the set voltage of ZrO2 nanoparticles. This research shows that the switching properties of ZrO2 can be improved by controlling doping defects, optimizing device structure and heterojunction interface etc. This paper can provide useful information for optimizing ZrO2 memory devices.