Growth behavior of titanium dioxide thin films at different precursor temperatures
© Nam et al; licensee Springer. 2012
Received: 9 September 2011
Accepted: 26 January 2012
Published: 26 January 2012
The hydrophilic TiO2 films were successfully deposited on slide glass substrates using titanium tetraisopropoxide as a single precursor without carriers or bubbling gases by a metal-organic chemical vapor deposition method. The TiO2 films were employed by scanning electron microscopy, Fourier transform infrared spectrometry, UV-Visible [UV-Vis] spectroscopy, X-ray diffraction, contact angle measurement, and atomic force microscopy. The temperature of the substrate was 500°C, and the temperatures of the precursor were kept at 75°C (sample A) and 60°C (sample B) during the TiO2 film growth. The TiO2 films were characterized by contact angle measurement and UV-Vis spectroscopy. Sample B has a very low contact angle of almost zero due to a superhydrophilic TiO2 surface, and transmittance is 76.85% at the range of 400 to 700 nm, so this condition is very optimal for hydrophilic TiO2 film deposition. However, when the temperature of the precursor is lower than 50°C or higher than 75°C, TiO2 could not be deposited on the substrate and a cloudy TiO2 film was formed due to the increase of surface roughness, respectively.
Since a TiO2 film showing a high refractive index is transparent in the visible light range, it can be used as an antireflection coating on a SiO2 thin film . It can also act as a photocatalyst because of its chemical stability, high quantum yield, and nontoxic property . For all of these optical applications, it is necessary to control polymorphs of TiO2, which have different structural and optical properties. It is well known that TiO2 exists in three different polymorphs: rutile, anatase, and brookite . To our knowledge, brookite is an orthorhombic structure and has not been observed in thin films. Both rutile and anatase phase are tetragonal structures. The anatase phase is a low-temperature polymorph with a less dense structure (3.894 g/cm3) , an optical bandgap of 3.25 eV [3–5], and a refractive index of 2.5 . TiO2 has been attracting much interest due to a wide range of applications such as in dye-sensitized solar cells [7, 8], photocatalysts [9–12], optical coatings , and capacitors for large-scale integrated devices . TiO2 photocatalysts have been applied in various fields, in which the anti-fogging, self-cleaning, or automobile windows should be quite attractive. The photocatalytic activities of TiO2 materials strongly depend on surface morphology, crystal structure, and crystallization of the concerned TiO2 photocatalyst. Various deposition techniques have been developed for depositing TiO2 thin films, including evaporation , sputtering , thermal oxidation of titanium , and the chemical vapor deposition [CVD] method . Among them, the CVD technique using a metal-organic compound as a precursor [MOCVD] has many advantages, such as a good conformal coverage, the possibility of epitaxial growth and selective deposition, and the application to large-area deposition. Also, this method is of low cost, and it is easy to control the deposition growth parameters. Thus, the MOCVD method is well known as one of the most powerful techniques and is suitable for stoichiometric and microstructural thin film deposition .
In this experiment, therefore, we deposited TiO2 thin films on glass substrates with a single molecular precursor as titanium tetraisopropoxide at different precursor temperatures such as 75°C (sample A) and 60°C (sample B) by the MOCVD method. Also, we discuss the influence of the precursor temperature on the surface energy of TiO2 thin films.
TiO2 thin films were deposited on a glass substrate using a MOCVD reactor, whose system was fabricated using a quartz tube and stainless steel bodies connected through O-ring joints. The MOCVD apparatus was evacuated using a rotary pump. The glass substrate was pretreated with acetone, ethanol, and deionized water in an ultrasonic cleaner and mounted onto the graphite holder that was laid in the center of the MOCVD chamber. To fix the glass substrate onto the graphite holder tightly, we grooved the graphite holder and tilted it at an angle to get a thin film with a uniform surface. The graphite holder was heated using a DC power through a super-Kanthal wire (Sandvik Korea Ltd., Seoul, South Korea) inserted in it with a substrate temperature at 400°C. The general deposition conditions are a temperature of 400°C, a working pressure of 8.2 × 10-2 Torr, and a working time of 30 min. Titanium tetraisopropoxide (Ti[OCH(CH3)2]4) [TTIP] was used as a precursor with heating at 60°C and 75°C and without a bubbler gas. The as-grown films were characterized with X-ray diffraction [XRD], scanning electron microscopy [SEM], atomic force microscopy [AFM], Fourier transform infrared spectrometry [FT-IR], contact angle measurement, and ultraviolet-visible [UV-Vis] spectroscopy.
Results and discussion
The hydrophilic TiO2 films were successfully deposited on slide glass substrates using TTIP as a single precursor without carriers or bubbling gases by the MOCVD method. The amorphous phase of the TiO2 thin films on the glass substrate at 400°C exists up to the precursor temperature of 60°C. The anatase phase appears at a precursor temperature of 75°C. The columnar structure can be evidenced by SEM images taken on cut edges perpendicular to the growth direction. Sample A has a denser surface than sample B. This phenomenon can be explained by precursor nucleation, above the substrate, the clusters being then adsorbed on the surface. XRD data show that the phase was diverted from amorphous to anatase, and FT-IR analysis was performed for the verification of changed O-H surface functional group. At the precursor temperature of 60°C, the thin film was grown in the amorphous phase and has an increasing coordination number.
This research was supported by grants NRF-20100029417 (SRC Program of Center for Plasma Bioscience Research) and NRF-20100029699 (Priority Research Centers Program).
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