Influence of the Annealing Temperature on the Thickness and Roughness of La 2 Ti 2 O 7 Thin Films

In this work, the impact of the substrate annealing temperature on the thickness and roughness of La 2 Ti 2 O 7 thin films was verified. A group of LTO Thin films was grown on Si (100) substrates successfully via pulsed laser deposition technique (PLD) at various annealing temperatures with a constant numbers of pulses and energy per pulse. Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) were used to investigate the thickness and roughness of the deposited La 2 Ti 2 O 7 thin films. The average thickness of the thin films was decreased due to the increasing in the annealing temperature linearly; the maximum thickness was found (231 nm) when LTO thin film deposited at 500˚C. The root mean square roughness was increased linearly with increasing the substrate Temperatures. The minimum roughness was found (0.254 nm) when LTO deposited at (500˚C). From the obtained results, its clear evidence that the annealing temperature has an influence on the thickness and roughness of the LTO thin films.


Introduction
During the last few years, many experimental and theoretical studies were blessed to the study of perovskite materials because of their fascinating properties in- cluding, ferroelectricity, superconductivity [1] [2] [3], pyroelectric and optical properties [4] [5]. Depending on these unique properties perovskite ceramics have distinct remarkable applications such as tunable microwave devices and piezoelectric devices [6] [7], sensors and wireless communications [8] [9], and some environmental applications [10]. Perovskites have the basic formula ABX 3 , where A and B are cations with A larger than that of B and X is often times oxygen but also other large ions are possible such as halides, sulfides and nitrides [11]. The perovskite structures exist in Layered perovskite, double perovskite, and Triple perovskite [12]. Lanthanum titanium oxide is a member of the layered perovskite family [13], which has high Curie temperature and excellent piezoelectric and electro-optic properties. This makes La 2 Ti 2 O 7 thin films a powerful candidate for a variety of applications in electrical and optical devices [14] [15].
The physicochemical properties of these materials are dependent on the crystal structure, lattice defect, exposed lattice plane, surface morphology, particle size, and specific surface area as well as the pore structure [16].

Experimental Methodology
Lanthanum titanium oxide powder was prepared using traditional solid-state  ). In order to improve the synthesis efficiency, the calcined product was pelleted using uni-axial pressing (Kimaya Engineers).
The formed pellet was then heat-treated in air at 1100˚C for 8 hours with immediately grinding. For the deposition process, a 45.8% density target was obtained by uni-axial pressing of the LTO powder under 20 MPa followed by a sintering at (1350˚C) for 10 hours in air. Then the sintered pellet was characterized by a (Rigaku) X-ray diffractometer and ultimately the LTO pellet was placed as a target in the film deposition process. Before the deposition, Si substrates were cleaned with RCA to eliminate all irrelevant objects from the surface and finally the substrate diced into (4 cm 2 ) pieces [24].
LTO thin film deposited on (100) Si substrates using PLD system as shown in Figure 1

Sample Characterization
The XRD pattern of the LTO target at room temperature was analyzed and determined using X'Pert High Score Plus software which was classified into a monoc-     Table 1 displays the average thickness of the thin films measured from SEM images (see Figure 3) at different annealing temperatures, As it's shown in Figure 4 the thickness of the film decreased with the increasing of the annealing temperature for all the three samples. because the deposited molecules become more active due to energy gained from the substrate holder as resulting in increasing their motion with increasing the temperature as well as reducing their potential force to the substrate which is appearing in fewer micro-porosity defects, higher compactness and thinner thickness of these films [23].

Thickness and Roughness Measurement
The surface seems very smooth with a small presence of monotonic grains (see Figures 5-7) that leads to a slightly increased in the roughness of the thin films with the increasing of the annealing temperature as shown in Figure 8.
The value of the root means square (RMS) of LTO films that is the average of height deviations taken from the mean image data plane calculated by the Nano scope Analysis software using the Equation (1) [27], and presented in Table 2.

Conclusions
In this work, a monoclinic La 2 Ti 2 O 7 target synthesized by a solid-state method successfully and confirmed by XRD using X'Pert High Score Plus software.
Pulsed laser deposition system was capable of synthesizing LTO thin films on Si (100) substrate at different annealing temperatures. During the deposition, the thermal annealing of LTO films in vacuum had a noticeable effect on thickness and the roughness of the films, the average thickness was found to be decreased while roughness (RMS) were linearly increased with the increasing of annealing Advances in Materials Physics and Chemistry temperatures. Based on SEM and AFM results, it can be concluded that the annealing temperature influences the thickness and the roughness of the LTO thin films.