Atmospheric pressure MOCVD of TiO2 thin films using various reactive gas mixtures
Introduction
TiO2 is a well known and very attractive material owing to its chemical stability, biocompatibility and remarkable electrical and optical properties. Indeed, it exhibits a high dielectric constant and resistivity, a high refractive index and a good optical transparency over a wide spectral range, as well as a relatively high hardness. As a result, TiO2 thin films have several applications like electric insulators, antireflective layers for optical devices [1], [2] and protective coatings [3], [4]. TiO2 films have also been considered as future candidates as thin dielectrics in dynamic random access memory (DRAM) storage capacitors [5]. Furthermore, anatase nanoparticles are well known photocatalysts which exhibit a band gap (Eg=3.2 eV) compatible with the redox potential of the H2O/OH couple (−2.8 eV). It is also self-regenerating and recyclable. Owing to these properties TiO2 is a major candidate in new devices, including gas sensors [6] and supported photocatalysts used for the decontamination and purification of environmental pollutants [7], [8].
TiO2 films have been deposited by different techniques such as anodization [9], electrodeposition [10], sol–gel [11], activated reactive evaporation [12], reactive dc magnetron sputtering [13], chemical vapor deposition (CVD) [14], [15], [16], plasma enhanced chemical vapor deposition [17], electrostatic sol-spray deposition [18], spray pyrolysis [19] and pyrosol [20]. Among these processes, atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) has a promising industrial potential because no vacuum system is required and it is known for its good capability for large-scale production and uniform coverage. Additionally, MOCVD permits a good control of the deposition parameters and, subsequently, of the stoichiometry and microstructure of the films.
This paper deals with the deposition of TiO2 thin films by atmospheric pressure MOCVD using titanium(IV) tetraisopropoxide (TTIP) in oxidizing atmosphere on Si(100) and stainless steel substrates. Deposition on steel substrates should lead to new applications. The deposition of TiO2 on various metallic substrates was previously reported but a low pressure CVD process was used [4]. Furthermore, we are aware of several papers on the growth mechanisms of TiO2 using TTIP as starting material, without oxidizing ambient [21] and with addition of O2 [22] or H2O [4], [23] but, again, these processes operated under low pressure and it is not the goal of this work to address mechanistic issues. Here, we report a study on the structural characteristics of the films and their growth kinetics in relation with the deposition conditions.
Section snippets
Experimental
A cold-wall vertical CVD quartz reactor, 5 cm in diameter, was used for the deposition of the layers. Si(100) wafers and 304 L stainless steel coupons were used as substrates and placed on a stainless steel sample holder (3.2 cm in diameter) heated by HF induction. The substrate temperature (350–700 °C) was measured using a thermocouple inserted in the sample holder. The gas streams (O2 and N2) were monitored using mass flowmeters. N2 was used as carrier gas and the total flow rate was varied
Morphology and structure of the films
The deposition conditions of a series of CVD runs are listed in Table 1. The TiO2 films showed a good uniformity with different bright colors depending on their thickness (interferential colors). The films grown at low temperature (400–450 °C) exhibit a columnar structure and a smooth surface morphology (Fig. 1a). By increasing the temperature in the range 500–650 °C, the columnar structure is confirmed but many elongated and pointed crystallites grow faster than the others leading to
Conclusions
TiO2 coatings were deposited by MOCVD under atmospheric pressure on various substrates including stainless steel. Their morphology, structure and purity can be controlled by changing the process parameters, specially the growth temperature and the TTIP mole fraction. Addition of O2 as oxidizing reagent has no significant influence on the morphology, the microstructure and the deposition rate of the layers. By contrast, in the presence of H2O vapor, the growth rate is significantly increased and
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