Characteristics of Lanthanum Titanium Oxide Thin Films Deposited by Pulsed Laser Deposition under Different Annealing Temperatures

Abstract

In this work, the influence of annealing temperature and the variation of the thermal stability during the deposition were investigated. Pulsed laser deposition system was capable of manufacturing two groups of La2Ti2O7 thin films on Si (100) substrate at different annealing temperatures based on the pulse rate frequency 3 and 4 Hz for 85 and 64 minutes, respectively. Both groups of samples were deposited with a KrF excimer laser with fixed parameters at different annealing temperatures 25, 400, 450, 500, 600, 650, and 700 0C, separately. In order to study the effect of temperature on the microstructure, optical properties of La2Ti2O7 films, the fabricated films were investigated using x-ray diffractometer (XRD), scanning electron microscope (SEM) associated with energy dispersive x-ray spectrometer (EDS), atomic force microscopy (AFM), and the ellipsometry. The XRD patterns of the LTO target and films were analyzed into monoclinic structural and an amorphous nature, respectively. While the EDS spectra confirmed that, the film's chemical composition was of lanthanum, oxygen, titanium with the disappearance of any impurities. The SEM images illustrated that the average thicknesses of the LTO thin films were found to be decreased linearly with the increasing of annealing temperatures for the thin films that were deposited in 3 and 4 HZ together. The AFM reported that all the LTO film were smooth, dense, and uniform. At room temperature, the highest roughness was recorded for the two groups of thin films then become smoother when the temperature increased up to 500 0C, and they were slightly rougher with the increase of temperature up to 700 0C for both groups of films were deposited with 3 and 4 Hz. The obtained results from the ellipsometry pointed out that the refractive indices of the first group of LTO films in the wavelength range of (400– V 1000) nm is within (2.42 – 2.12) changing with the annealing temperatures with the variation of 0.09, particularly at 1000 nm. While it was found within (2.38 to 2.11) changing with the annealing temperatures as well with a small variation of 0.12 specifically at the 1000 nm for the second group of LTO films. As such as the refractive indices of both groups reduced with the increasing of wavelengths following a normal dispersion. On the other hand, the extinction coefficient of the two groups was invariable rose as the temperature rises up to 700 °C together. Further, the real part of the dielectric constant of LTO films in the wavelength range 400–1000 nm is within (4.43 – 5.90) and (4.47 – 5.67) adjusting with the different annealing temperatures for the first and the second group, respectively. While the imaginary part of the dielectric constant of the two groups gradually increases (with peaks shift) during the temperature rises from 25 to 700 °C. Furthermore, the maximum depolarization at 450 nm was found (65%) and (28%) when the thin films deposited with 600 0C and 450 0C for the first and the second group, respectively. While the minimums depolarization were (3%) for the both groups at the same wavelength. Based on the achieved results from the XRD, SEM, AFM, EDX, and ellipsometry, it can be concluded that the annealing temperature controls the characteristics of the LTO thin films.