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Characterization of Co:TiO2 Thin Film Grown by MOCVD Technique

The Co:TiO2 thin film was grown on n-type Si(100) by metal organic chemical vapor deposition (MOCVD) technique. The film’s growth parameters are as follow: substrate temperature of 450°C, bubbler temperature of 70°C, reactor chamber pressure of 2 militorr, and growth time of 2 hours. We characterized the structure of film by X-ray Difractometer (XRD), the morphology was characterized by Scanning Electron Microscope (SEM), and the fraction of Co atoms in TiO2 was characterized by Energy Dispersive x-ray Spectroscopy (EDS). The XRD result shows that the Co:TiO2 thin film is an anatase phase crystal dominated by A(213) orientation. Using Warren-Scherrer’s formula, the average grain size of Co:TiO2 is 169 nm. The SEM result shows that the Co:TiO2 film surface is quite coarse with relatively homogeneous grain shape. the average growth rate of Co:TiO2 film is 0.78 μm/h. In addition, the EDS result shows that Co atoms have been incorporated into the film replacing a portion of the Ti atoms by 0.085%.


Introduction
Titanium dioxide, TiO2, is one of semiconductor materials that studied for many purposes. Because of its properties, TiO2 can be applied in various fields [1]. TiO2 have been applied in optical components [2] and photocatalysis [3] and have a high potential application in as a nonlinear optic material, sensitive solar cell, and dynamic random access memory [4]. TiO2 also have an interesting properties for spintronic application. TiO2 doped by Co atoms shows the ferromagnetic properties at room temperature [5].
In this paper, we focused on a structural a morphology characterization of TiO2 doped by Co atoms (Co:TiO2).
The process of the film growth was done in the following way. The vaporized tube containing the 20 mL TTIP and 900 mg Co(TMHD)2 (in 20 mL THF) were heated to a constant temperature of 70 o C. TTIP and Co(TMHD)2 vapor pressurized 20 kPa was driven by argon gas at a rate of 70 sccm to the reactor chamber through a connecting pipe. In the reactor chamber, n-type Si(100) substrate has been pasted at molybdenum disk for the film growth process at temperature of 450° C. The total reactor pressure was 2 militorr. The growth time was 2 hours.
Characterization of Co:TiO2 thin films was done using XRD, SEM, and EDS. XRD was used to find out the structure and orientation of the Co:TiO2 film. In this case, XRD uses Cu Kα radi radiation at a wavelength of 1,54060 angstrom (Philips PW3710). SEM (JEOL type JSM-6510LA) was used to characterize the surface morphology and the thickness of Co:TiO2 film. Meanwhile, EDS was used to measure the fraction of Co atoms in TiO2. Figure 1 shows the XRD result of Co:TiO2 thin film. There are two diffraction peaks that appear at 2θ = 55.00 degree and 2θ = 61.70 degree. These peaks belongs to anatase phase, i.e., A(211) and A(213), respectively. Intensity of A(213) is more higher than A(211). Therefore, it can be revealed that the Co:TiO2 thin film is an anatase phase crystal dominated by A(213) orientation.

XRD
Referring to the standard diffraction peak for TiO2, the peak of A(213) is present at 2θ = 62.119 o and A(211) at 2θ = 55.060 o . This means that there is a diffraction peak shift for Co:TiO2. Such a shift may occur because the bit doping of Co atoms in TiO2 films change the film lattice constants. The size (diameter) of the crystals was obtained by using the following Warren-Scherer equations [11]:

≈ cos
where D is the diameter of the crystal, λ is X-ray wavelength, θ is the angle of Bragg diffraction, B is half of FWHM (full-width at half-maximum) of the selected peak, and K is the constant. Generally, the value of K chosen is 0.89. By substituting the value of K = 0.89, λ = 1.5046 angstrom, B = 0.000955 rad (for 2θ = 61.70 degree), and θ = 0.53804066 rad into the Warren-Scherrer equation, the obtained crystalline diameter is 169 nm. The size of this crystal diameter indicates that the MOCVD method has successfully grown film by grain size in the order of nanometers.

SEM
The surface morphology and cross section of the film was characterized by SEM. Figure 2(a) shows the SEM image of the Co:TiO2 film surcae. It can be seen that, in general, all surfaces of the film appear to be quite coarse with relatively homogeneous grain shape.
The SEM cross-sectional image of the film is shown in Figure 2(b). The film is composed of crystallike grains of crystal. In the substrate-film interface, the crystal grains appear irregular. This is because the growth of crystals is competitive so that the grains formed depend on the diffusion process of the surfaces of the precursor atoms. This competitive crystal growth is due to the diverse orientation of crystal grains that are bound to the surface of the substrate [12].
For 2 hours growth of the Co:TiO2 film, the average thickness of the film is 1.56 um. This shows that the average growth rate of the Co:TiO2 film is 0.78 m/h.

EDS
EDS characterization was performed to determine the composition of the atoms of the film. Figure 3 shows the EDS graph of the Co:TiO2 film. Based on the tabulation of the percentage of atoms, it is obtained that O: Ti: Co = 64.84: 35.13: 0.03. Referring to this comparison, the fraction of Co atoms to Ti is 0.03 : 35.13 × 100% = 0.085%. Thus, the Co atoms have been incorporated into the film replacing a portion of the Ti atoms by 0.085%.

Conclusion
The Co:TiO2 thin film have been successfully grown by MOCVD technique. XRD result show that the Co:TiO2 thin film is an anatase phase crystal dominated by A(213) orientation. From the SEM image, in general, all surfaces of the film appear to be quite coarse with relatively homogeneous grain shape, the film is composed of crystal-like grains of crystal, and the average growth rate of Co:TiO2 film is 0.78 m/h. EDS results show that Co atoms have been incorporated into TiO2 film.