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This thesis describes low temperature growth of wide band gap metal oxide thin films deposited by thermal (T-) and plasma-enhanced (PE-) atomic layer deposition (ALD) techniques in which high quality materials are grown with atomic level precision.
Metal oxides are extensively investigated due to their exceptional physical and chemical properties, including relatively wide band gap, high dielectric constant and high thermal stability. This variety of properties results in a wide range of different applications. Thin films of indium oxide (InOx), gallium oxide (GaOx), zinc oxide (ZnOx), and quaternary InOx/GaOx/ZnOx (IGZO), in addition to the well-known aluminum oxide (AlOx), and the catalyst cerium oxide (CeOx), have proven to be superior candidates for many applications; from microelectronics and optoelectronics to gas sensor devices. The demanding requirements of low-temperature deposition processes for thermal sensitive substrates, which include high layer homogeneity and conformality over large areas, makes ALD a pioneer deposition technique. Although many oxides have been grown by TALD and PEALD, the deposition of wide band gap oxides at low temperatures are rarely reported and/or being investigated.
In this work, the deposition method of the individual binary oxide films and combining the respective binary processes into the developed super-cycle growth of quaternary compound have been investigated at relatively low-temperatures by TALD and PEALD. Besides, the growth characteristics and chemical properties of the deposited films were evaluated by in-situ and ex-situ characterization techniques such as spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS), where the influence of ALD process parameters on the growth mechanism and films composition are discussed in detail for any potential applications.
Role of substrates morphology and chemistry in ALD HfO₂ on Si(111)-H terminated surfaces as model
(2017)
This work presents an approach to investigate fundamental aspects concerning the early stage of the atomic layer deposition (ALD) growth process on stepped surfaces. The first interaction between precursors and surface is strongly important for the ALD growth that it is still far away from the status to be completely understood.
For this purpose, a few ALD-cycles withtetrakis(dimethylamido)hafnium (TDMAH) and trimethylaluminum (TMA) as metallic precursors and water (H₂O) as oxidant has been performed in order to study the initial metal oxide film growth on stepped surfaces such as silicon Si(111)-H terminated, highly oriented pyrolytic graphite (HOPG) and silver deposited HOPG (Ag-HOPG). These investigations have been carried out at various substrate temperatures, where scanning tunneling microscopy (STM) has been used systematically to probe the ALD features. This technique is delivering unique knowledge about the locality and the density of nucleation’s sites on the different substrates. The data collected are then subjected to a mathematical model to understand the growth and to determine the effect of the surface morphology and chemistry on the behavior of the nucleation.
The in-situ cycle-by-cycle STM investigation of 4 initial ALD cycles of TDMAH and H₂O on Si(111)-H terminated at room temperature (RT) and at 280°C displays two regimes of growth: In Regime I (1st - 2nd cycle) an increase in roughness in the first cycle to 0.2nm and 0.34nm respectively for RT and 280°C with a partial surface coverage of 71% and 54% is observed. In the 2nd cycle, the coverage increased to ~98% and 94% maintaining the same film height of the 1st cycle. A complete layer is formed in this regime. The results are discussed in reference to the Puurunen model. Following this model, the determination of the reaction mechanism in relation to the number of Hf atoms/nm² attached to the surface reveals that two ligands exchanges occur at RT and one ligand exchange at 280°C in the first regime. In addition, the origin of the reaction saturation was determined to be caused by the steric hindrance effect. In this first regime, the growth model is governed by random deposition followed by Mullins diffusion as determined from the universal values found for the roughness dynamic exponents (α, β, 1/z) of the film.
This thesis focuses on the deposition of thin TiO₂ films on p-type Si using atomic layer deposition (ALD) technique, on the study of the electronic proprieties of the grown films and on the electrochemical characterization of TiO₂/Si photoelectrodes. The deposition parameters, electronic properties and electrochemical performance and stability of the TiO₂/Si samples are correlated.
The ALD technique is used to deposit TiO₂ with two different precursors namely Titanium isopropoxide and Titanium methoxide onto Si substrates. Laboratory as well as synchrotron based X-ray spectroscopy techniques are used to characterize these films. The growth quality of the TiO₂ ALD films is determined by analyzing X-Ray photoelectron spectroscopy (XPS) data in terms of stoichiometry, defect states and Ti³⁺:Ti⁴⁺ ratios. The ALD technique was modified with different heating arrangements to obtain various polymorphs of TiO₂.
The ALD and anatase TiO₂ films are characterized using synchrotron radiation to study their electronic properties and these films are compared with single crystal rutile TiO₂. X-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (res-PES) measurements are performed with synchrotron radiation. XAS measurements are used to determine the polymorphs as well as the electronic structure of TiO₂ Res-PES measurements are conducted at the O1s and Ti2p edges to study multiple hole Auger decay processes and polaronic and charge transfer states as well as to determine the electronic band gap of the TiO₂ layers. One of the main findings of this thesis is the determination of the partial density of states (pDOS) of O and Ti in the conduction and valence band. The combination of the pDOS and the band edge positions obtained from res-PES measurements are used to calculate the charge neutrality level of the TiO₂ polymorphs.
The photoelectrochemical measurements are conducted on bare-Si and TiO₂/Si photoelectrodes. The electrochemical performance of these photoelectrodes is studied in electrolytes having pH values ranging from 1 to 13. The deposition of TiO₂ on Si enhances the photoelectrochemical performance of the Si photoelectrode. The TiO₂ increases the stability of the photoelectrode in all electrochemical media over 12 hours of experimental condition. Moreover, it is also observed that the TiO₂/Si photoelectrode is less responsive to the pH value of the electrolyte. The electrochemical findings are explained on the basis of the electronic properties of the TiO₂ layer. The electronic band gap obtained from spectroscopic measurement and the photoelectrochemical measurements are used to explain the performance and stability of the TiO₂/Si photoelectrodes. The thesis also addresses the stability of Si microstructured photoelectrodes (SiMPs) prepared by an electrochemical method. The stability of the SiMPs deteriorates more rapidly than that one of the planar Si photoelectrode. However, using a protective ALD TiO₂ layer on these SiMPs the overall performance is even more enhanced than on the TiO₂/planar Si system.
We have grown HfO2 on Si(001) by atomic layer deposition (ALD) using HfCl4, TEMAHf, TDMAHf and H2O as precursors. The early stages of the ALD were investigated with high-resolution photoelectron spectroscopy and x-ray absorption spectroscopy. We observed the changes occurring in the Si2p, O1s, Hf4f, Hf4d, and Cl2p (for HfCl4 experiment) core level lines after each ALD cycle up to the complete formation of two layers of HfO2. The investigation was carried out in-situ giving the possibility to determine the properties of the grown film after every ALD cycle or even after a half cycle. This work focused on the advantages of the in-situ approach in comparison to ex-situ experiments. The study provides to follow the evolution of the important properties of HfO2: contamination level, density and stoichiometry, and influence of the experimental parameters to the interface layer formation during ALD. Our investigation shows that the in-situ XPS approach for ALD gives much more information than ex-situ experiments.