Resistive Switching Of Several NiO-based Nanoparticle Composite Thin Films

With the rapid development of science and technology, the traditional memory device can not meet the high demand of the people. The resistance random access memory (RRAM) is attracting more and more attention due to its good performance, such as low power consumption, high-speed, high density and simple preparation. Among the materials, the NiO has lots of merits except the good resistance switching characteristics, such as the faster storage and simple composition and compatible with complementary metal oxide semiconductor process. The studies found that the optimization of process parameters and the doped elements made its threshold voltage drop to 1 V, but the on/off had not significantly increased. The NiO-TiO2 and NiO-SnO2 composite nano-particles were prepared using sol-hydrothermal, and then the composite films were doped with different rare earth elements to improve the characteristics. The specific contents of the dissertation are as follows:First, TiO2 nanoparticles and NiO-TiO2 composite nanoparticles were synthetized by sol-hydrothermal, and then the influences of the mole ratio of La and Ce on the structure, and resistive switching of the composite nanoparticels thin film are investigated. The results show that TiO2 thin films annealed at low temperatures has not resistive switching, while the films composed with the nanoparticles synthetized in the solution of PH=1 and PH=3 present the resistive switching. When the PH is about 6 or 9, the nanoparticles are anatase and the conductivity of the films is high than that of the rutile TiO2 films. The conductive mechanism of all films is dominated by the space charge limited conduction (SCLC) and thermal emission (RS); when the ratio of Ni/Ti reaches 7:1, the resistive switching of the NiO-TiO2 is the best. And then the threshold voltage of the composite thin films with the doping of 7% La-7% Ce decreases to 1.0 V, but the switching on/off ratio is about two orders of magnitudes, which show little dependence on the dopants.Second, the NiO-SnO2 composite nanoparticle thin films were prepared using sol-hydrothermal spin-coating method, and studied the structure, morphology and resistive switching properties.The results show the grain size of NiO-SnO2 prepared in the solution of PH=10 are about 70 nm and the nanoparticle films exhibit reproducible resistive switching with threshold voltage of about 1 V and on/off ratio of 500. The current-voltage measurements indicated that the conduction is governed by the Ohmic law for the low resistive state, while the high resistive state is dominated by the space charge limited conduction. The switching mechanism can be ascribed to the charge trapping/detrapping process induced by the applied threshold electrical field. The DOS near fermi surface of NiO is determined by the electron orbit of Ni 3p, Ni 3d and O 2s, while the influence from the electron orbit of O 2s on the DOS of NiO-SnO2 and NiO-TiO2. Because of the Generalized Gradient Approximation, the bandgap of the whole materials is lower than the experiment value. The light absorption peak from theoretical calculation is consistent with the experiment value.In a word, reproducible resistive switching is observed in the as-fabricated composite thin films. The data analysis shows that the charge transport of the high resistive state film is dominated by the SCLC and RS mechanism, and the DOS near fermi surface of NiO determined by the electron orbit has changed with the compound of the TiO2 or SnO2. The switching mechanism of the thin films using the sol-gel method is shown to be resulted from the filament formation and rupture of oxygen vacancies under the threshold electrical field and/or Joule heating, while the mechanism of the nanoparticles thin films can be ascribed to the charge trapping/detrapping process induced by the applied threshold electrical field. The doping of Eu, Ru and La, Ce increases the optical transmittance, but has little effects on the resistive switching.