Abstract: Ti films of the same thickness and near normal deposition angle and the same deposition rate were deposited on glass substrates, at room temperature, under UHV conditions. Due to getting properties of Titanium atoms, they specially react with oxygen during evaporation and TixOy layers produced. Different annealing temperatures as 130 and 330 Celsius degree with uniform 9 cm3 sec-1 oxygen flow, were used for producing titanium di oxide layers. Their nano structures were determined by AFM and XRD methods. Roughness of the films changed due to annealing process. The getting property of Ti and annealing temperature, can play an important role on the nano-structure of the films.
INTRODUCTION
Titanium dioxide (TiO2) is widely studied by researchers in the basic sciences as well as in engineering. It’s phase transformation has been widely studied for optical and electronic applications. This material shows properties that are of special interest in wide range technological of applications such as: photo catalysis, solar cells, gas sensors, hard coating, self-cleaning windows (Karuppasamy and Subrahmanyam, 2007; Dakka et al., 1999), optical wave guiding, optical coatings and microelectronics (Garapon et al., 1996; Durand et al., 1995; Kim et al., 1994), antireflective coatings (Bange et al., 1991), thin film capacitors (Prasad et al., 1997), etc. Besides, these films are extensively used in optical thin film devices, because of their good transmittance in the visible region, high refractive index and chemical stability (Bach and Krause, 1997). As known, TiO2 crystallizes in three different crystallo-graphic structures: brookite (orthorhombic), rutile (tetragonal) and anatase (tetragonal). Brookite is formed only in extreme preparation conditions, while rutile is the most common TiO2 crystal phase in nature. Anatase is metastable and thus it can be synthesized only in the restricted range of growth conditions. It is well known, however, that anatase exhibits the highest photocatalytic activity among the crystallographic phases of TiO2, being more appropriate for related applications (DeLoach and Aita, 1998; Stromme et al., 1996). Nano-crystalline TiO2 thin films have been prepared up to now by a large variety of growth techniques, such as Metal-Organic Chemical Vapour Deposition (MOCVD) (Prasad et al., 1997; Bach and Krause, 1997), reactive magnetron sputtering (DeLoach and Aita, 1998; Stromme et al., 1996) filtered cathodic vacuum are (Zhao et al., 2004) and Pulsed Laser Deposition (PLD) (Hsieh et al., 2002; Matsui et al., 2005; Nakamura et al., 2005; Gyorgy et al., 2005; Caiteanu et al., 2006; Sharma et al., 2003; Moret et al., 2000; Tsai et al., 2005; Yamamoto et al., 2002), In this research we want to study the influence of annealing temperature and oxygen flow on the nano-structure and roughness of produced layers and also crystallographic directions and Reflectivity of layers and their dependence to mentioned parameters.
MATERIALS AND METHODS
Titanium films of 69 nm thickness were deposited on glass substrates at room temperature. The residual gas was composed mainly of H2, H2O, CO and CO2 as detected by the quad ro pole mass spectrometer. The substrate normal was at 7 degree to the direction of the evaporated beam and the distance between the evaporation crucible and substrate was 43 cm.
Just before use all glass substrates were ultrasonically cleaned in heated acetone, then ethanol. Other deposition conditions were the same during coating. Vacuum pressure was about 4.5x10-5 tour and deposition rate was 0.9 A° sec-1. Thickness of the layers were determined by quartz crystal technique. We used annealing oven and different annealing temperatures (130 and 330 Celsius degree) and uniform oxygen flow to change nano structure of layers and produce titanium dioxide layers. The nanostructure of these films was obtained using a Philips XRD X’pert MPD Diffractometer (CuKα radiation) with a step size of 0.03 and count time of 1 sec per step, while the surface physical morphology and roughness was obtained by means of AFM (Dual ScopeTM DS 95-200/50) analysis.
RESULTS AND DISCUSSION
Figure 1a-c show the morphology of the produced layers in this study (AFM images). Figure 1a shows the AFM image of as deposited Ti film, at room temperature with 69 nm thickness. As it can be seen, the surface is full of small grains.
In presence of annealing temperature at 130°C and in presence of uniform oxygen flow (9 cm3 sec-1), oxygen will penetrate to the grain structure and brake them down to tinnier needle like grains (Fig. 1b). In Fig. 1c annealing temperature increases to 330°C and as it can be seen the grains are domed and clearly bigger. This is because of surface diffusion in this temperature so migration of grains will dominate to penetrating oxygen flow and grains are domed and bigger.
Figure 2 shows the diagram of roughness for layers produced in this work.
| Fig. 1(a-c): | AFM images of the as-deposited (a) Ti film and films annealed at (b) 130 K and (c) 330 K. The flow rate of oxygen during annealing was the same for all films, 9 cm3 sec-1 |
In presence of annealing temperature, at first, roughness decreases, that is because of oxygen penetration to grain structure and break them down, that tends to uniform surface.
| Fig. 2: | The roughness diagram of the as-deposited. |
But at 330°C annealing temperature and in presence of oxygen flow, because of surface diffusions and migration of grains, bigger domed grains appear, so roughness increases.
As we know Ti is a getter metal and in presence of oxygen and heat will be Converted to TixOy combinations.
Figure 3a-c, shows XRD images for the layers produced in this work.
As it can be seen from Fig. 3a and b, the layers are amorphous and there is no crystallographic direction. By increasing annealing temperature to 330°C and in presence of oxygen flow, Titanium Dioxide produced. Although, we use a thick 69 nm thickness Titanium layer, as it can be seen from Fig. 3c, two anatase A (004) and A (105) crystallographic directions, appear.
Figure 4 shows the Reflection of the layers produced in this work. As it can be seen, as deposited layer has more Reflection than two other layers.
| Fig. 3: | The XRD patterns of the as-deposited (a) Ti film and films annealed at (b) 130 K and (c) 330 K. The flow rate of oxygen during annealing was the same for all films, 9 cm3 sec-1 |
| Fig. 4: | The reflectance diagram of the films annealed at 130 K (a) and 330 K (b). The flow rate of oxygen during annealing was the same for all films, 9 cm3 sec-1 |
By increasing annealing temperature in presence of uniform oxygen flow, because of surface diffusion and coalescence of grains more voids appear on layers and the Reflection has a decreasing trend.
CONCLUSION
The influence of annealing temperature and uniform oxygen flow on titanium layers of the same thickness were obtained. This was accomplished by studying the relationship between AFM and XRD results. The morphology of the layers changes by increasing heat and in presence of oxygen. By increasing annealing temperature and in presence of oxygen flow, at first, oxygen penetrates to grains structures and brake them down to needle like grains. By increasing heat, because of surface diffusion, grains get domed and bigger. Roughness decreases at first step and increases for the layer with 330°C annealing temperature, that is in agreement with AFM results. XRD patterns showed anatase structure in A (004) and A (105) crystallographic directions for the layer of 69 nm thickness in presence of 330°C annealing temperature. XRD patterns were amorphous for as deposited titanium layer in presence of 130°C annealing temperature. As it can be seen from XRD patterns, x is one and y is two and we have TiO2 for this research at 330°C temperature and the other layers are amorphous. By increasing annealing temperature reflection has a decreasing trend because of migration of grains and formation of more voids on the layers.