Preparation And Properties Of Monometal Oxides Grown By Atomic Layer Deposition

In recent years, atomic layer deposition (ALD) has become one of the solutions for the ultra thin films. ALD employs self-limiting surface reactions that result in the growth of one atomic layer per ALD cycle, and high quality thin films are achieved layer by layer with precisely controlled film thickness to single atomic layer, and excellent conformity in very high aspect ratio geometries and porous structures. Moreover, ALD deposition can perform at much lower temperature than some other techniques.Recently, multi-component metal oxides ultra thin films preparing via ALD method becomes a research hotspot. Bi-based ferroelectric thin films such as SrBi2Ta2O9,(Bi,La)4Ti3O12, BiAlO3, BiFeO3have attracted considerable attention. However, there are little reports about deposition of Bi-based complex ferroelectrics films by ALD. As an important component for SrBi2Ta2O9, Bi4Ti3O12, BiFeO3, high quality Bi2O3films had not been deposited successfully via ALD method.In this work, some previous works for ALD Bi-based complex ferroelectrics films were done. The main results are as follow:(1) α-Bi2O3ultra thin films were successfully synthesized on silicon and quartz substrates by means of ALD using Bi(thd)3and H2O as precursors. The marked steric hindrance originate from the bulky ligands of Bi(thd)3was found.The structure of Bi(thd)3(thd:2,2,6,6-tetramethyl-3,5-heptanedionato) molecule, volatility, thermal stability, the reaction mechanism for Bi(thd)3and H2O was investigated. The optimum ALD window was about270-300℃,and an ALD-type growth mechanism via surface saturation reaction was identified, the growth rate was about0.1A/cycle. The growth rate fits well to a Langmuir adsorption model. The X-ray Diffraction (XRD), Raman scattering and High-resolution transmission electron microscopy investigations revealed that Bi2O3films crystallized into a predominant alpha phase above250℃. The leakage mechanism of the films is Poole-Frenkel effect. Fourier transform infrared spectroscopy investigation implies the reaction is complete, and no organic residue remained in the films. It was found that the band gap increased with the decreasing temperature, and the relationship of Eg～T fits well to the Bose-Einsteinmodel. In addition, the α-Bi2O3films (synthesized by ALD) transformed into meta-stable y-Bi2O3. with preferred orientation (222) after annealing above512℃, and y-phase could persist at room temperature.(2) AI2O3thin films were deposited by atomic layer deposition (ALD) method with trimethylaluminum (TMA) and ozone. The deposition was accomplished at room temperature without any assistance. The breakdown field of the films is as high as7MV/cmThe GPC (about1.25A/cycle) is almost independent of TMA purge time range from2to12s. This indicates that the minimum of the purge time2s is long enough to eliminate multilayer adsorption and prevent subsequent CVD-type growth. The ALD cycle time can be reduced two orders of magnitude with the using of O3instead of H2O. The Al2O3films surface are atomically smooth. The growth possibly follows2D layer-by-layer growth mode. The capacitance-voltage loop implies that there are much less defects density (order of magnitude of1010cm-2) in the03-based Al2O3film than H2O-based one. Using of O3for the Al2O3film can suppress the presence of-OH radicals compares to H2O-based process, much lower defects density in the03-based films facilitates the improvement of the insulating behavior and C-V characteristics. The excellent insulating behavior of03-based films corresponds to resistivity higher than1017Ω·-cm. The breakdown field is about7MV/cm, and it is roughly twice that of H2O-based film (deposited at33℃)..(3) The anatase TiO2films and nanotubes have been synthesized successfully by atomic layer deposition method using titanium isopropoxide and water.The XRD studies show the anatase phase (101) preferred orientation. The characteristic of leakage is consistent with space-charge-limited conduction mechanism. The morphology, composition of the prepared TiO2nanotubes was investigated by scanning electron microscope and energy dispersive X-ray spectroscopy (EDS), respectively. The TiO2nanotubes are composed of Ti and O, and the ratio of Ti to O approximate to1:2. The was confirmed and certified by XRD and Raman scattering studies. The lattice constants of the TiO2nanotubes is slightly larger than that of powder anatase TiO2crystal, which may due to the the strain resulting from the crimp of the TiO2crystal lattice.