Morphological and Photoelectrochemical Properties of ALD TiO[sub 2] Films

Abstract

TiO 2 films were grown by atomic layer deposition ͑ALD͒ at 200-500°C with TiCl 4 and H 2 O as gas sources. The relationships among deposition temperature, microstructure, and photoelectrochemical properties were investigated. The results showed that the crystalline form is anatase at deposition temperatures of 200-400°C and is a mixture of anatase and rutile at 500°C. The grain size changed with the deposition temperature and had a minimum at around 400°C. The potential sweep voltammograms demonstrated that the photoelectrochemical properties of these TiO 2 films were deteriorated by either the high defect density or the small grain size, or the existence of rutile phase. In order to modify the morphological properties of ALD TiO 2 films, a two-stage ALD with first stage for a nucleation layer and second stage for stress release and grain growth was proposed. Expectedly, the photoelectrochemical properties were significantly improved by this approach.Titanium dioxide ͑TiO 2 ͒ has been extensively studied for its electric, magnetic, catalytic, and electrochemical properties. In recent years there has emerged significant interest in the synthesis of TiO 2 thin films for environmental photocatalysis and water-splitting photocatalysis. 1-3 Several growth techniques have been used to synthesize TiO 2 thin films, including sputtering, pulsed laser deposition, sprayed coating, hydrothermal synthesis, metallorganic chemical vapor deposition, plasma-enhanced chemical vapor deposition, and atomic layer deposition ͑ALD͒, etc. Among these techniques, ALD has received great attention due to its excellent uniformity, accurate thickness control, low deposition temperature, and almost 100% step coverage. 4-8 ALD is based on sequential surface reaction steps, during which gaseous precursors are alternatively introduced into the chamber. Between the precursor pulses, the reactor is purged with an inert gas to remove the extra reactants, leaving only a saturated chemisorption layer. The film growth is therefore self-limited by the amount of reactants adsorbed on the surface. The self-limiting nature of the sequences makes the deposited film highly conformal and the film thickness digitally controllable. TiO 2 films with high specific surface area have been obtained by depositing a thin TiO 2 layer on high-aspect-ratio holes, porous substrates, and nanoparticles. 7,8 In view of an efficient photocatalyst, except for a high specific surface area, a suitable crystalline form, high crystallinity, and low defect density are also very important. TiO 2 occurs in three crystalline polymorphic forms of rutile, anatase, and brookite. The anatase was found more suitable for environmental photocatalysis due to its lower conduction bandedge potential ͑higher potential energy of photogenerated electrons͒. 1 Miao et al. 9 pointed out that only anatase films exhibit bactericidal abilities. Besides the suitable crystalline form for sufficient potential energy, the carriers must have long enough lifetime for a long-range electric drift. Macak et al....

Titania‐based photocatalytic coatings on stainless steel hospital fixtures

“…Figure 7 shows that the mean crystallite size has a minimum at 400 o C and increases with increasing deposition temperature, as expected from previous studies [26]. Increasing crystallite size with increasing deposition temperature has been observed for atomic layer deposited TiO 2 coatings on silicon substrates and has been attributed to the composite effect of nucleus density and grain coalescence with temperature [31]. According to Cheng and Chen [31], increasing the deposition temperature yields a higher subsequent crystallite nucleation and coalescence resulting in a coarser crystallite size.…”

“…Increasing crystallite size with increasing deposition temperature has been observed for atomic layer deposited TiO 2 coatings on silicon substrates and has been attributed to the composite effect of nucleus density and grain coalescence with temperature [31]. According to Cheng and Chen [31], increasing the deposition temperature yields a higher subsequent crystallite nucleation and coalescence resulting in a coarser crystallite size. Figure 8 gives the EDS measurements of the O:Ti ratio and the presence of iron in the films deposited over a range of temperatures but having the same thickness.…”

Characterization of low temperature deposited atomic layer deposition TiO2 for MEMS applications

“…Same tendency of the surface morphology change was reported by others. [15][16][17] The effect of nucleus density at the lower temperature is more pronounced than that of grain coalescence, thus causes the larger grain size. 17 On the other hand, much denser nuclei are formed at higher temperatures, which results in smaller grains.…”

“…[15][16][17] The effect of nucleus density at the lower temperature is more pronounced than that of grain coalescence, thus causes the larger grain size. 17 On the other hand, much denser nuclei are formed at higher temperatures, which results in smaller grains. However, according to our RMS data, once crystallization happened during deposition, the roughness issue would exist even for the films with smaller grains.…”

Atomic Layer Deposition TiO2 Films and TiO2/SiNx Stacks Applied for Silicon Solar Cells

“…Then, wafers were rinsed in deionized water and dried with N 2 gas. Right after the cleaning, Si substrate was placed in the reaction chamber and then the chamber temperature was raised to 200 • C for the ALD deposition [28].…”