Growth, Characterization And Properties Of Several Kinds Of Zn-Based Oxide Thin Films By Atomic Layer Deposition

Materials science has gained substantial progress in recent decades. With the new information age and interdisciplinary need, the demand for new materials with excellent performances is more and more urgent. Microelectronics has been growing aggressively according to Moore’s law during the past half century, so the fabrication technology of thin films has also been developed rapidly, which plays an important role in high-tech fields. Atomic layer deposition (ALD) has emerged as an important technique to deposit thin films for scaling down semiconductor industry. Due to its precise thickness control down to atomic level, large area uniformity, and excellent3D conformality, this technique is being applied in more and more fields.ZnO is a kind of semiconductor materials with direct band gap, which has been widely used in piezoelectricity, gas sensor, thin film transistor, white light illumination, photocatalysis, chemical industry, and medicine. As a star material, ZnO shows great commercial value with advantages of abundant resource in the earth, low price, non-toxicity, safety and friendly environment. The research on ZnO system and its doping/modification has been a hotspot in a number of scientific frontiers.In this thesis, we focused on the atomic layer deposition (ALD) technique and several kinds of valuable Zn-based oxides materials. ZnO, Al doped ZnO, and Zn-Ti-O ternary thin films have been prepared by ALD. The effect of the processing parameters on their growth behavior, composition, phase evolution, and morphology has been investigated systematically. The related electrical, optical and photocatalytic properties have been characterized deeply. Main achievements are summarized as follows:1. The growth behavior of ALD ZnO, Al2O3and TiO2films was investigated, including ALD window, deposition rate, transmittance, and optical band gap. Al2O3films were deposited on Si by ALD using water as oxygen source, and trimethylaluminum (TMA), Al isopropoxide (AIP), and AICl3as Al source, respectively. During the deposition temperature range from200to250℃, the growth of Al2O3films with three kinds of Al precursors shows typical ALD self-limiting and self-saturation features. The source temperature of AIP, and AICl3were optimized to130and120℃, respectively, to generate enough source vapor pressure for ALD saturated chemisorption process. In typical ALD growth, growth per cycle (GPC) of A12O3were1.0,1.6, and1.0A/cycle with TMA, AIP and AICl3as Al precursor, respectively. The ALD window of ZnO deposited with diethylzinc (DEZ) and water as precursors was130～220℃, and the corresponding GPC was about1.5A/cycle. With the substrate temperature of200～250℃, the GPC of ALD TiO2with isopropyl titanate (TTIP) and water as precursors was found to constant as low as0.3A/cycle. ALD ZnO and TiO2films on-OH terminated Si surface show better surface smoothness with roughness less than0.8nm in RMS. While deposited on-H terminated Si surface, the roughness of ZnO increases to7.9nm, ascribed to the island growth. The optical band gap of ALD ZnO and TiO2films is determined to be3.3eV.2. The growth, composition, structures and electrical properties of ALD Al doped ZnO (AZO) films were investigated in depth. The impact of Al precursor type and3cycle ratios on doping concentration and resistivity of AZO films were compared. The processing in ALD AZO with AIP as A1precursor was optimized. While using TMA and AIP as A1precursor, at optimal Zn/Al cycle ratio of19:1, the AZO films show the lowest resistivity of3～4×10-3Ω cm with the Al/(A1+Zn) atomic ratio of2～2.3%, which is explained by an effective field model. While using AICl3as Al precursor, the resistivity of AZO films increases by two orders of magnitude, attributed to the residual C1element in the AZO films. For the first time, by changing the AIP source temperature, the doped Al atoms in one doping layer is adjusted so as to optimize the film resistivity. At optimal Zn/Al cycle ratio of19:1, by changing the AIP temperature from115℃to135℃, the Al dopant atomic concentration (Al/(A1+Zn)) in AZO films varies from0.25%to2.32%. The60nm-thick AZO films deposited at AIP temperature of120℃and substrate temperature of200℃shows the lowest resistivity of9.4×10-4Ω cm with the Al/(A1+Zn) atomic ratio of0.61%, relatively low roughness (RMS=0.91nm) and better optical transparency (>90%). In addition, after annealed at400℃in containing oxygen atmosphere, both ZnO and AZO films show enhanced resistivity, which can be attributed to lower carrier concentration of Zni and Vo caused by incorporation of oxygen into the films.3. Zn-Ti-O (ZTO) ternary films with various Zn/Ti cycle ratios were deposited on Si substrates using DEZ and TTIP as Zn and Ti sources by ALD. The effect of Zn/Ti cycle ratio and post-annealing temperature on the growth rate, composition, phase structure, and morphology of ZTO films were investigated deeply. It is found that for ALD ZTO films, the growth per cycle (GPC) of TiO2deposited on ZnO-terminated surface is faster than that of pure TiO2, while the GPC of ZnO deposited on TiO2-terminated surface becomes slower than that of pure ZnO. This makes the Zn/Ti cycle ratio effect on the film composition become weak. The post-annealing temperature and ALD sequence play important roles in facilitating the ZTO phase evolution. Pure spinel Zn2TiO4phase can be obtained in the (1:2)-ZTO films with40.3mol.%Ti content post-annealed at900℃. For (2:5)-ZTO samples with48.8mol.%Ti content, pure hexagonal ZnTiO3phase can be formed at700℃with better thermal stability. At700℃and above, the rutile TiO2phase appears for (1:3)-ZTO samples with52.1mol.%Ti content. The SEM images confirm with increasing the post-annealing temperature, the grain size becomes large with the inhomogenous morphologic change due to the h-ZnTiO3phase decomposition into Zn2TiO4and rutile TiO2phases. Dielectric and photocatalytical properties of the (2:5)-ZTO films with h-ZnTiO3phase were characterized.(2:5)-ZTO annealed at700℃show the best dielectric properties of dielectric constant and dielectric loss of29.6and0.012at1MHz, respectively. The800℃annealed (2:5)-ZTO film with optical band gap of3.75eV show better photocatalytical activity in degradation of methyl violet under UV-light. This study confirms that Zn-Ti-O films with controllable composition and phase can be achieved by optimizing the ALD sequences and cycle ratios. Further more, this work provides an example for ALD application in fabrication of ternary compounds.In summary, ALD technique was explored to prepare ZnO, Al doped ZnO and Zn-Ti-O ternary films. The growth behavior and related properties were investigated deeply and some meaningful progress has been made. The improved electrical property of ALD Al doped ZnO films at low temperature deposition on complex three-dimensional structures is of importance for some applications such as solar cell and organic light-emitting diodes.