This paper reports the optical properties of prepared samarium oxide Sm 2 O 3 thin films nanoparticles using RF sputtering technique. X-ray diffraction is used to examine and characterize the prepared films. Optical measurements are carried out by employing U-V-Visible spectroscopy to study optoelectronic properties of Sm 2 O 3 thin films. These films are highly transparent in the visible range. The average value of the optical gap belonging to the thin films deposited under different pressure of the gas is 4.33 eV. The refractive index (n) behaves as normal dispersion and decreases with increasing the pressure of the gas. The dispersion energy and single oscillator energy increase with increasing the pressure of the gas whereas the optical conductivity decreases with increasing the power on Sb target.
High-dielectric constant (k) materials have excessively been investigated to replace the conventional SiO2-based
gate dielectric [
As far as the authors know little attention has been paid to study the optical properties of sputtered Sm2O3 thin films under different conditions of preparation, therefore this paper focused in this point.
Samarium oxide thin films in this study were deposited on pre-cleaned glass substrates using UNIVEX 350 SPUTTERING UNIT with RF power MODEL Turbo drive TD20classic (Leybold) and thickness monitor model INFICON AQM 160. The ceramic Sm2O3 target, from Cathey Advanced Materials Limited Company was used as a sputtering source. The sputtering chamber was evacuated to a base pressure of 2 × 10−6 torr and then back filled with mixture of 98% pure argon +2% pure O2 and up to the sputtering pressure of 2 × 10−2 mbar and the sputtering pressure was maintained constant throughout the coating. Prior to deposition the substrates were cleaned using a 10% by volume solution of hydrofluoric acid followed by a rinse in deionizer water. The target substrate distance was 10 cm with an angle 65˚. The standard cubic centimeter per minute (sccm) maintained at value of 20 cm3/min with substrate rotation 2 rpm and desired power depending on the condition of preparation. The rate of deposition was 2 nm/min. Film thicknesses were determined accurately after deposition by using multiple-beam Fizeau fringes in reflection, Tolansky method [
Transmittance, T(λ), and reflectance, R(λ), of the as-deposited films were measured at normal incidence in the wavelength range 250 - 1200 nm by means of a double beam spectrophotometer (JASCO model V-670 UV-Vis- NIR) attached with constant angle specular reflection attachment (5˚), to determine the spectral behavior of the optical constants and to deduce some optical parameters of Sm2O3 thin film. The spectral data obtained directly from the spectrophotometer were transformed to absolute values by making a correction to eliminate the absorbance and reflectance of the substrate. The absolute values of T and Rare given by M. M. El-Nahass [
where Ift and Ig are the intensities of the light passing through the film-glass system and that passing through the reference glass, respectively and Rg is the reflectance of the glass substrate, and
where Im is the intensity of light reflected from the reference mirror, Ifr is the intensity of light reflected from the film and Rm is the mirror reflectance.
In order to estimate the optical energy gap in the absorption region of the spectra, it is better to calculate the absorption coefficient α, and the absorption index, k, of the films at different wavelengths, therefore, the following equations are used [
where α is the absorption coefficient and d is the film thickness. The experimental error in measuring the film thickness was taken as ±2%, in T and R as ±1% respectively.
X-ray diffraction patterns of as-prepared thin films show SmO2 peak at 2θ = 40.6˚ representing the plane (403) for the three films deposited under Ar pressure 20, 40, and 60 psi. Second peak of SmO is detected at 37.5˚. The second peak at 37.5˚ and preferred (111) plane is reported to by Jun-Gill Kang et al. [
where λ is the X-ray wavelength of CuKα (0.15418 nm), β is the width of the peak at half maximum intensity, and θ is the corresponding Bragg angle. The constant K is of the order of 0.95.
It is found that the crystallite size decreases from 36 nm to 23 nm with increasing the pressure of the gas from 20 to 60 psi for the peak at 2θ = 40.6˚, besides it decreases from 24 nm to 17 nm with increasing the pressure from 20 to 60 psi for the second peak.
To get information about direct or indirect inter-band transitions, the optical band gap was determined from the analysis of the spectral dependence of the absorption near the fundamental absorption edges within the framework of one electron theory [
In the above equation, Eg represents the optical band gap energy and A is the characteristic parameter, independent on photon energy, for direct transition. The
Some tail states lie in the gap region which arise due to the presence of defects in the material. Therefore, the absorption of photons occurs even at energies lower than Eg. The absorption tail can be determined mainly due to structural disorder existing at the grain boundaries [
where αo is a constant characterizes the materials and Ed is the Urbach energy refers to the width of the tail states.
but changes slowly over the wavelength range λ > 360 nm (normal dispersion). At longer wavelength, the calculated value of the refractive index decreases with increasing the pressure of the gas. Also, there are slight variations in the intensity of the refractive index peak as a result of increasing the gas pressure. It is found that the refractive index has the same peak at 360 nm.
The variation of the refractive index n in the region of very small values of the extinction coefficient k, under negligible damping are provided by the classic dispersion theory. Wemple and DiDomenico described the wavelength dependence of the refractive index, n, in the transparent region for various different solids by using the single-oscillator model of the form [
where hυ represents the photon energy, Eo is the energy of the oscillator and Edis is the dispersion energy which describes the strength of the electronic transitions. The calculated values of the dispersion parameters are obtained by plotting of
Pressure | ε∞ | εL | N/m* (1045 kg−1∙m−3) | Eo (eV) | Edis (eV) |
---|---|---|---|---|---|
20 psi | 3.46 | 3.71 | 48.4 | 9.83 | 24.15 |
40 psi | 3.43 | 3.66 | 46.9 | 10.17 | 24.69 |
60 psi | 3.44 | 3.65 | 46.4 | 10.58 | 25.79 |
The dispersion theory provides the description of the variation of n2 versus λ2 to obtain the lattice dielectric constant, εL, free carrier concentration to the free carrier effective mass N/m* and permittivity of free space εo.
It is noticed from
The spectral distribution of the complex dielectric constant ε describes the propagation, reflection and loss of light in multilayer structures, besides, it can be expressed by the following equation:
where
The real σ1 and imaginary σ2 components of optical conductivity are described as [
where ω is the angular frequency, εo is the permittivity of free space. The spectra of real and imaginary parts of the optical conductivity are shown in
The volume and surface loss functions (VELF and SELF) are proportional to the characteristics energy loss of fast electrons traveling the bulk and surface of the material, respectively. They related to the real and imaginary parts of the dielectric constant and can be calculated by using the relations [
RF magnetron sputtering is used for preparing Sm2O3 thin films. Two peaks characterizing Sm2O3 are detected by X-ray diffraction patterns. The optical gap decreases with increasing argon pressure. Refractive index and optical conductivity decrease wit increasing the pressure of the gas. Dispersion and single oscillator energies increase with increasing the pressure of the gas. VELF and SELF found to decrease with the increase of the pressure of the gas.
The project was financially supported by King Saud University, Vice Deanship of research chairs.
M. M. Abd El-Raheem,M. H. El Bogami,Ateyyah M. Al-Baradi,S. A. Amin,A. M. El-Naggar,A. A. Albassam, (2015) Optical Properties of RF Sputtered Samarium Oxide Thin Films. Open Access Library Journal,02,1-12. doi: 10.4236/oalib.1102065