Growth of titanium dioxide thin films by atomic layer epitaxy
Abstract
Titanium dioxide thin films were deposited by atomic layer epitaxy using TiCl4 and water as reactants. The film growth was performed over the temperature range 150–600 °C in order to study the effects of temperature on the growth rate. The effect of the substrate material on the growth rate and crystal structure was also investigated. Spectrophotometry, X-ray diffraction, Rutherford backscattering spectroscopy and nuclear reaction analysis were used to determine the chemical and physical characteristics of the films. The growth mechanism is discussed on the basis of literature and results obtained.
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Cited by (301)
TiO2 thin films prepared by atomic layer deposition (ALD) have attracted great attention due to the widespread application of the oxide as a promising charge storage material for lithium or proton batteries. In this work, we study TiO2 film grown on Si substrates by atomic layer deposition with tetrakis (dimethylamino) titanium as metal precursor (TDMAT) and water vapour as an oxidant. The chemical composition and impurity content of the film as a function of growth temperature is studied by Ion Beam Analysis (IBA). D2O (99.8%) was used as oxidant to study the film growth to distinguish between hydrogen atoms originating from the water oxidant or the metal precursor. Combining ellipsometry with Rutherford Backscattering Spectrometry (RBS), Nuclear Reaction Analysis (NRA) and Elastic Recoil Detection Analysis (ERDA) reveals film density as a function of growth temperature. Atomic force microscopy (AFM) was used to characterize the film surface structure. By investigating the structural and compositional range of ALD TiO2 films will open the new opportunities of application.
The study investigated hybrid coatings CrN + Cr2O3/TiO2 type obtained using the hybrid PVD/ALD technology on a substrate made of Cr-Ni-Mo (316L) stainless steel. Coating of CrN + Cr2O3 was obtained by the Arc-PVD method. TiO2 using atomic layers deposition was investigated. It were deposited at a variable number of cycles from 200 to 1000 cycles at a constant temperature of 200 °C. Structural studies using scanning and transmission electron microscopy and atomic force microscopy were performed. The chemical composition analysis in the micro-regions was performed using EDS. The study of electrochemical properties was performed using potentiodynamic method and electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution. As a result of the study, it was found that the best electrochemical properties were shown by hybrid coatings for which the TiO2 layer was obtained in 500 ALD cycles. Based on microscopic studies with diffraction and spectroscopic analysis, the structure of the tested coatings was related to electrochemical properties, and the influence of deposition conditions on corrosion resistance was explained. In addition, the tests were supplemented with tests of tribological properties and adhesion of coatings to the substrate.
Mechanical and optical properties of as-grown and thermally annealed titanium dioxide from titanium tetrachloride and water by atomic layer deposition
2021, Thin Solid FilmsThe use of thin-films made by atomic layer deposition (ALD) is increasing in the field of optical sensing. ALD TiO2 has been widely characterized for its physical and optical properties, but systematic information about the influence of thermal history to optical and mechanical properties of the film is lacking. Optical applications require planar surface and tunability of the refractive index and residual stress. In addition, mechanical properties such as elastic modulus and film hardness influence the performance of the layer, especially, when optics is integrated with microelectromechanical systems. In this work, optical properties, density, elemental analysis, residual stress, elastic modulus and hardness of as-grown ALD TiO2 thin films on silicon were studied at temperature range from 80 to 350 °C and influence of post-ALD thermal annealing was studied on films annealed up to 900 °C. ALD TiO2 films were under tensile stress in the scale of hundreds of MPa. The stress depended both on the ALD temperature and film thickness in a complex way, and onset of crystallization increased the residual stress. Films grown at 110 and 300 °C were able to withstand post-ALD annealing at 420 °C without major change in residual stress, refractive index or extinction coefficient. Elastic modulus and hardness increased upon crystallization with increasing ALD temperature. The results presented here help to improve the design of the optical devices by choosing films with desired optical properties, and further help to design the post-ALD thermal budget so that films maintain their desired features.
Optimal design of novel precursor materials for the atomic layer deposition using computer-aided molecular design
2021, Chemical Engineering ScienceNanomaterials and nanostructures with multi-functional properties found widespread applications such as electronics, optics, and coatings that can be fabricated using Atomic Layer Deposition (ALD). ALD is a vapor phase deposition technique to generate thin films of metals and metal oxides on a substrate. In this process, a precursor, which often comprises of organic functional groups that surround the depositing metal, chemisorbs on the substrate or reacts with the surface sites and with each other. Precursor chemisorption on the substrate leads ALD to be a self-limiting process. Thus, the precursor(s) should be chosen in a way to enhance deposition based on the ALD conditions. For a given application, it is practically impossible to carry out a large number of experiments using numerous precursors with varied deposition conditions to find the one that maximizes the growth rate in ALD. In addition, only existing precursors can be tested experimentally. The overall objective of this work is to develop a computational tool for the in-silico design of precursor materials using adsorbate solid solution theory (ASST). In the first part of this paper, we apply the ASST to derive properties of the functional groups present in the precursor using a new Group Contribution Method (GCM). The GCM is successfully derived and compared with the experimental data from ALD. The method shows good agreement and is useful for the design of novel materials. In the second part of this paper, using the thermodynamic properties as obtained from GCM, we develop a computer-aided molecular design (CAMD) framework for the optimal design of novel precursor materials with enhanced deposition properties for the ALD of metal oxides and metals. The metaheuristic efficient ant colony optimization algorithm (EACO) developed in-house is used for both parts. CAMD is a combinatorial optimization methodology, where molecules with optimal desired properties are generated from functional groups. The precursor selection optimization problem is posed as a mixed integer nonlinear programming problem, which is solved using EACO. The ALD growth kinetics is used as an objective of optimization and a solution is validated with thermodynamic constraints. Along with the number of groups, the temperature is also included in the optimization framework as decision variables. Forty-one novel titanium precursor molecular structures for ALD are generated with growth rates ranging from 1.23 Å/cycle to as high as 1.65 Å/cycle. Thus, these precursors have shown growth rates higher than the known titanium precursors. ALD growth rate is found to be a function of the combination of the precursor functional groups as well as temperature with a complex correlation among them.
TiO Atomic Layer Deposition (ALD) is used in microelectronics due to its ability to produce conformal thin films whose thickness is controlled at the sub-nanometer scale. Tetrakis(dimethylamido)titanium(IV) and water are frequently used precursors for TiO ALD, although there are still some differences in growth behavior in the literature. In this parametric study, the growth of TiO was controlled in situ by Multi-Wavelength Ellipsometry in combination with ex situ thickness measurements by X-ray Reflectometry. The injection and purge times were optimized to reach self-saturation on the surface. We put in evidence two regions of the Growth Per Cycle (GPC) as a function of substrate temperature: GPC decreases from 0.8 Å cy to 0.5 Å cy as the temperature rises from 50 °C to 200 °C, and then GPC increases from 0.6 Å cy to 0.9 Å cy as the temperature rises from 250 °C to 350 °C. There is no evidence of an ALD window - the temperature region where the growth rate is the same. The stoichiometry of the layer grown at 200 °C, determined by X-ray Photoelectron Spectroscopy, is TiO (1 Ti atom per 2 O atoms). The 200 °C as-grown sample becomes crystalline (Anatase crystals) after annealing 3 h in air at 400 °C, or at 600 °C. The initial growth stages of TiO were studied by Multi-Wavelength Ellipsometry and Atomic Force Microscopy. Regarding TiO ALD on (100)Si substrate (with a native SiO), we observed a substrate enhanced growth which turned into steady-state ALD beyond 20–30 cycles. Additionally, TiO ALD on (001)InGaAs substrate demonstrates substrate inhibited growth of type 2 (islands formation) which turned into TiO steady-state ALD beyond 20–30 cycles. The results of TiO ALD are compared with those of the existing literature.
Influence of oxygen precursors on atomic layer deposition of HfO<inf>2</inf> and hafnium-titanium oxide films: Comparison of O<inf>3</inf>- and H<inf>2</inf>O-based processes
2020, Applied Surface ScienceAtomic layer deposition (ALD) of HfO2 and hafnium-titanium oxide (HTO) in O3- and H2O-based processes by using a flow-type reactor was studied. Growth per cycle (GPC) recorded for the HfCl4-O3 process at substrate temperatures of 225–600 °C was 0.05–0.13 nm. At temperatures exceeding 300 °C, the O3-based process yielded films with lower GPC and marked thickness gradients, but with lower chlorine contamination levels than the HfCl4-H2O process did. In the HTO films grown from HfCl4, TiCl4 and O3, the thickness gradients decreased with increasing TiO2 content to values that were smaller than those of the films deposited from HfCl4, TiCl4 and H2O. The O3-based ALD of HTO resulted in lower chlorine concentration and higher GPC in the films with Hf/(Hf + Ti) atomic ratios of 0–0.8 and 0.3–0.8, respectively. Independently of the oxygen precursor used, the as-grown HTO films contained anatase at Hf/(Hf + Ti) values of 0–0.16, monoclinic phase with inclusions of cubic, tetragonal or orthorhombic phase at Hf/(Hf + Ti) values of 0.71–1.00, and predominantly amorphous phase at intermediate Hf/(Hf + Ti) values. Differently form the O3-based process, the H2O-based one allowed growth of monoclinic phase with well-developed preferential orientation in the films with Hf/(Hf + Ti) atomic ratios of 0.88–1.00.