S. Bourgeois

SEM and XPS studies of titanium dioxide thin films grown by MOCVD

Abstract The metal organic chemical vapour deposition (MOCVD) method was used to prepare titanium dioxide thin films. Thin films of TiO2, about 100 nm thick, were deposited on (100)Si and (1102)Al2O3 sapphire substrates using titanium isopropoxide (Ti(OC3H7)4) as metal organic precursor. The morphology of the films and the presence of impurities on the thin films surfaces were studied using respectively, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The influence of the growth parameters such as the deposition temperature, the carrier gas (nitrogen) flow rate and the presence of an additional oxygen flow on the characteristics of the titanium dioxide films has been studied. The morphology of the deposits has been shown to be greatly influenced by the growth parameters (temperature, substrate and carrier gas flow rate). The main impurity of the layer, carbon, has been found by XPS to be a function of both the growth temperature and the nature of the substrate. Moreover, it has been evidenced that an additional oxygen flow is not essential to obtain stoichiometric titanium dioxide films.

Growth and characterization of AP-MOCVD iron doped titanium dioxide thin films

Abstract Atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) was used to prepare iron doped titanium dioxide thin films. Thin films, between 40 and 150 nm thick, were deposited on Si, SiO 2 and Al 2 O 3 substrates using titanium tetra isopropoxide and ferrocene as metal organic precursors. TiO 2 iron doping was achieved in the range of 1–4 at.%. The film morphology and thickness, polycrystalline texture and doping content were studied using respectively scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The influence of growth temperature, deposition time, substrate type and dopant partial pressure were studied. Electrical characterizations of the films were also performed.

Study by XPS of ultra-thin nickel deposits on TiO2(100) supports with different stoichiometries

Abstract X-ray photoemission spectroscopy was used to study thin nickel deposits on monocrystalline titanium dioxide surfaces. These surfaces were submitted before deposition to different kinds of treatment in order to obtain samples with various fixed bulk as well as surface stoichiometries. The influence of these stoichiometries on the electronic structure of deposited nickel was studied. Reducing thermal treatments were also performed in situ and their consequences studied by XPS.

Use of isotopic labelling in a SIMS study of the hydroxylation of TiO2(100) surfaces

The interaction of TiO2(100) surfaces with hydrogen and water vapour has been studied using mainly secondary ion mass spectrometry (SIMS). The influence of the stoichiometry of the surface on its reactivity has been checked. The existence of two kinds of hydroxyl groups is pointed out. The behaviour of these hydroxyl populations with the temperature was studied. Moreover the ability of SIMS to distinguish different isotopes of the same element was used: exposure of TiO2(100) surfaces to isotope labelled molecules, D2 and H218O, was performed in order to specify the nature of these two hydroxyl groups.

Effect of the surface stoichiometry on the interaction of Mo with TiO2 (110)

Abstract Molydenum has been deposited at room temperature on (110) TiO2 surfaces with different stoichiometries, roughnesses and crystallinities. Whatever the substrate preparation is, in-situ Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) studies as well as ex-situ atomic force microscopy (AFM) and reflexion high-energy electron diffraction (RHEED) studies reveal a Stranski–Krastanov growth mode: the completion of three monolayers followed by islands growth is observed in every case. The three monolayers are always composed of amorphous molybdenum oxide with an oxidation state of molybdenum less than IV. The oxidation of the molybdenum layers generates Ti3+ and Ti2+ in the substrate, and induces a reconstruction of the surface: during the layer formation, the roughness of the material strongly decreases. Moreover, if the substrate is prereduced prior to the deposition, the reduction induced by molybdenum oxidation can easily migrate in the TiO2 bulk. In the case of an initial stoichiometric surface, the reduction is more located in the interfacial layers. After the growth of three layers, metallic BC islands without preferential orientation appear.

Iron deposition on TiO2(110): effect of the surface stoichiometry and roughness

Abstract Characterizations of ultra-thin iron films deposited on TiO 2 (110) surfaces with different stoichiometries, roughnesses and crystallinities have been carried out by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). For a high initial roughness of the substrate, a 2D growth mode is observed up to three monolayers. But, if the initial roughness is low, clusters grow on the TiO 2 surface. Whatever the initial surface stoichiometry, electronic exchanges occur between titanium and iron leading to a reduction of titanium and an oxidation of iron. This interaction between iron and titanium dioxide surface takes place only at the interface between the metal and the oxide surface: during the completion of the first layer for deposition on a non-stoichiometric and rough surface (2D growth) and at the base and periphery of the islands for 3D growth (in the case of deposition on a flat and stoichiometric surface). Moreover, the amount of electrons exchanged between titanium and iron is lower when the substrate oxide surface is prereduced. The interaction between the deposited metal and the oxide surface is related to the amount of available oxygen.

Molybdenum deposition on TiO2 (110) surfaces with different stoichiometries

Abstract The deposition of ultra thin molybdenum films has been carried out on three different TiO 2 surfaces: a stoichiometric and flat one obtained after annealing, a non stoichiometric and rough surface made by Ar + bombardment and a stoichiometric and rough surface obtained by oxygen bombardment. Whatever the substrate preparation, in situ AES and XPS studies and ex situ AFM and RHEED characterizations have revealed a Stranski–Krastanov growth mode: the completion of three monolayers followed by island growth is observed in any case. The three monolayers are composed of amorphous molybdenum oxide with a molybdenum oxidation state between III and IV. The oxidation of the molybdenum layers generates a reduction of the substrate with the formation of Ti 3+ and Ti 2+ and induces a reconstruction of the surface: during the formation of the molybdenum oxide layers the roughness of the surface strongly decreases. After the growth of the three layers, the surface is flat whatever the initial roughness. Then, the molybdenum atoms can diffuse on the surface and generate clusters. The resulting islands are metallic (BC structure) but without preferential orientation.

In-situ small-angle x-ray scattering study of nanoparticles in the plasma plume induced by pulsed laser irradiation of metallic targets

Small angle x-ray scattering was used to probe in-situ the formation of nanoparticles in the plasma plume generated by pulsed laser irradiation of a titanium metal surface under atmospheric conditions. The size and morphology of the nanoparticles were characterized as function of laser irradiance. Two families of nanoparticles were identified with sizes on the order of 10 and 70 nm, respectively. These results were confirmed by ex-situ transmission electron microscopy experiments.

Thermal effects on the growth by metal organic chemical vapour deposition of TiO2 thin films on (100) GaAs substrates

Abstract TiO 2 thin films were deposited on (100) GaAs substrates by LP-MOCVD with deposition temperatures ( T d ) ranking from 450 to 750 °C. The structure of these layers was studied by X-ray diffraction (XRD) and Raman spectroscopy. The growth of the TiO 2 anatase phase was observed for T d T d >600 °C. Finally, X-ray photoelectron spectrometry (XPS) and secondary ion mass spectroscopy (SIMS) experiments showed the presence of small quantities of Ga and As through the whole film thickness, slightly increasing at the surface of the layers. This result was related to the SEM observations and explained by considering the growth conditions.

A surface EXAFS study of thin nickel deposits on (110) TiO2 surfaces

Abstract The first stages of nickel deposition on a clean (110) TiO 2 surface were studied using surface extended X-ray absorption fine structure (EXAFS). Experiments were performed on two kinds of nickel deposits, one of 0.8 equivalent monolayers and another of 1.6 equivalent monolayers. Depositions were performed on well characterized TiO 2 (110)-p(1 × 1) surfaces. The correlation of Auger experiments with EXAFS results showed that no islands were present on the surface; only two-dimensional growth of nickel was observed. Moreover, it was shown, in the case of the thinner deposit, that a model with nickel atoms forming chains in the channels determined by oxygen atoms on the surface is consistent with the EXAFS data.