Hsyi-En Cheng

Morphological and Photoelectrochemical Properties of ALD TiO2 Films

Titanium dioxide TiO2 has been extensively studied for its electric, magnetic, catalytic, and electrochemical properties. In recent years there has emerged significant interest in the synthesis of TiO2 thin films for environmental photocatalysis and water-splitting photocatalysis. 1-3 Several growth techniques have been used to synthesize TiO2 thin films, including sputtering, pulsed laser deposition, sprayed coating, hydrothermal synthesis, metallorganic chemical vapor deposition, plasma-enhanced chemical vapor deposition, and atomic layer deposition ALD, 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. TiO2 films with high specific surface area have been obtained by depositing a thin TiO2 layer on high-aspect-ratio holes, porous substrates, and nanoparticles. 7,8

RHEED studies of nucleation of Ge islands on Si(001) and optical properties of ultra-small Ge quantum dots

Abstract The initial stages of Ge growth on the Si(001)-(2×1) surface have been studied by using a RHEED pattern zero-streak profile analysis technique. Thicknesses for {105} and {113} facets formation, corresponding to the nucleation of coherent ‘hut’-clusters and dislocated ‘dome’ three-dimensional (3D) islands respectively, were determined in a growth temperature range of about 200–600°C. Multilayer structures containing ultra-small Ge quantum dots (QDs) with a plane size of about 10 nm and a height of 1.5 nm have been studied by photoluminescence (PL). PL bands assigned to QDs show an intensity comparable to data in the literature, but a band width five times smaller.

Visible Light Activity of Nitrogen-Doped TiO2 Thin Films Grown by Atomic Layer Deposition

This article demonstrates that a visible light active N-doped TiO 2 film can be fabricated by atomic layer deposition (ALD) with titanium chloride and ammonia water as the precursors. A 1.60 atom % N-doping anatase TiO 2 film has been successfully grown on an n + -silicon substrate. The ratio of substitutional N doping to the total N doping as high as 70% is achieved that extends the long-wavelength cutoff of TiO 2 to 550 nm. Results from the analysis of incident photon-to-current efficiency and potential-dependent photocurrent density suggest that the visible light activity of ALD N-doped TiO 2 films can be further improved by reducing the carrier recombination rate and increasing optical absorption efficiency.

Growth Characteristics and Properties of TiN Coating by Chemical Vapor Deposition

Titanium nitride films obtained by chemical vapor deposition with TiCl{sub 4}, N{sub 2}, and H{sub 2} as gas sources at ambient pressure have been examined in this study. The results reveal that the deposition rate increases with increasing H{sub 2} and N{sub 2} partial pressures but has a maximum value with TiCl{sub 4} partial pressure. The Arrhenius plots show that the transition temperature from surface limited to diffusion limited is greatly influenced by H{sub 2}, N{sub 2}, and TiCl{sub 4} partial pressures. The film microstructurre was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It is found that the change of topography and microstructure of TiN films responds to deposition temperature and gas concentration used. The diffusion rate of as-deposited atoms and the total surface energy are considered as the determinant factors for the change. The growth texture of TiN films was identified to be {l_brace}422{r_brace} at lower deposition temperature and {l_brace}200{r_brace} at high temperature. The mechanisms for texture formation are discussed.

Growth Mechanism of TiO2 Nanotube Arrays in Nanopores of Anodic Aluminum Oxide on Si Substrates by Atomic Layer Deposition

In this study, we combined atomic layer deposition and anodic aluminum oxide (AAO) on a silicon substrate and developed self-aligned Ti0 2 nanotube arrays. We studied the growth mechanism of TiO 2 nanotubes on the inner wall of AAO at 100 and 400°C. We found that at 100°C, TiO 2 grew in a layer-by-layer manner. Therefore, it can be grown into Ti0 2 nanotube arrays with very thin walls. However, at 400°C, Ti0 2 needs to first form a 2.5-nm amorphous layer, before becoming crystalline Ti0 2 via a phase transformation, and growing into crystalline TiO 2 nanotube arrays along the preferred plane {101} by means of a space-limited growth mechanism.

The heterojunction effects of TiO2 nanotubes fabricated by atomic layer deposition on photocarrier transportation direction

The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated. For ITO/TiO2/Si diodes, the photocurrent is controlled either by the TiO2/Si heterojunction (p-n junction) or the ITO-TiO2 heterojunction (Schottky contact). In the short circuit (approximately 0 V) condition, the TiO2-Si heterojunction dominates the photocarrier transportation direction due to its larger space-charge region and potential gradient. The detailed transition process of the photocarrier direction was investigated with a time-dependent photoresponse study. The results showed that the diode transitioned from TiO2-Si heterojunction-controlled to ITO-TiO2 heterojunction-controlled as we applied biases from approximately 0 to -1 V on the ITO electrode.

Substrate Materials and Deposition Temperature Dependent Growth Characteristics and Photocatalytic Properties of ALD TiO2 Films

Titanium dioxide (TiO 2 ) films were grown on Ni-, Ta-, and Ti-coated glass substrates at temperatures between 200 and 500°C by atomic layer deposition (ALD) from TiCl 4 and H 2 O precursors. The as-deposited films were anatase crystalline at the deposition temperature of 200°C and became rutile crystalline with the increase in deposition temperature. The temperature for the transition of anatase to rutile varied with the substrate material; the Ni-coated substrate has the highest anatase-to-rutile transition temperature, followed by the Ta-coated substrate, and then the Ti-coated substrate. The grain size of as-deposited films had a minimum with deposition temperature for each substrate due to the composite effects of nucleus density and grain coalescence with temperature. The photocatalytic activity of ALD TiO 2 films for the decomposition of methylene blue was highly influenced by the growth characteristics and the underlying material. The 200 and 300°C deposited films on Ni-coated substrates possessed the highest photocatalytic activity among all experimental samples due to their pure anatase structure and the Schottky contact of Ni/TiO 2 .

Effect of Geometric Nanostructures on the Absorption Edges of 1-D and 2-D TiO2 Fabricated by Atomic Layer Deposition

2-Dimensional (2-D) TiO2 thin films and 1-dimensional (1-D) TiO2 nanotube arrays were fabricated on Si and quartz substrates using atomic layer deposition (ALD) with an anodic aluminum oxide (AAO) template at 400 °C. The film thickness and the tube wall thickness can be precisely controlled using the ALD approach. The intensities of the absorption spectra were enhanced by an increase in the thickness of the TiO2 thin film and tube walls. A blue-shift was observed for a decrease in the 1-D and 2-D TiO2 nanostructure thicknesses, indicating a change in the energy band gap with the change in the size of the TiO2 nanostructures. Indirect and direct interband transitions were used to investigate the change in the energy band gap. The results indicate that both quantum confinement and interband transitions should be considered when the sizes of 1-D and 2-D TiO2 nanostructures are less than 10 nm.

Ultraviolet Photoresponse of TiO2 Nanotube Arrays Fabricated by Atomic Layer Deposition

We used atomic layer deposition with anodic aluminum oxide, to fabricate self-aligned TiO2 nanotube arrays, on a Si substrate, at 400 C. Numerous p-n nanojunctions can be fabricated using this approach. In the absence of the external application of a bias voltage, these crystalline nanotubes make very sensitive ultraviolet sensors. The sensitivity is a result of the built-in voltage, generated by the nanojunction formed between the TiO2 nanotubes and the Si substrate, which separates electron-hole pairs and produces photocurrents. In addition, the one-dimensional structure of the TiO2 nanotubes provides a well-defined path for the transportation of the photo-generated carriers. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3576076] All rights reserved.

A novel structure in Ge/Si epilayers grown at low temperature

Abstract We report the growth of Ge/Si strained layer at low temperature. A new growth mode is observed where ‘groove islands’ are formed beneath the Ge wetting layer. The new structure exhibits an Ge concentration dependent profile along the growth direction. This effect is tentatively attributed to the stress-driven intermixing of Ge/Si during the growth.