Cadmium tin oxide Cd2SnO4 thin films with a thickness of 228.5 nm were prepared by RF magnetron sputtering technique on glass substrates at room temperature. AFM has been utilized to study the morphology of these films as a function of annealing temperature at the nanoscale. The optical properties of these films, such as the transmittance, T(λ), and reflectance, R(λ), have been studied as a function of annealing temperature. The optical constants, such as optical energy gap, width of the band tails of the localized states, refractive index, oscillatory energy, dispersion energy, real and imaginary parts of both dielectric constant and optical conductivity have been found to be affected by changing the annealing temperature of the films.
Tin oxide SnO2, Indium oxide InO3, and Zink oxide ZnO are good examples of degenerate semiconductors which are highly transparent and conducting in the family of binary transparent conducting oxides (TCOs). However, ternary oxide TCOs such as Zn2SnO4 [
The present work shows the possibilities of successfully prepare Cadmium Stannate (Cd2SnO4) thin films using RF sputtering technique. The obtained thin films have been studied under various conditions especially as a function of annealing temperature that to our knowledge has been paid a little attention.
Thin films of Cd2SnO4 with thickness of 228.5 nm are prepared using a target constitute of (Cd:SnO2 ratio of 2:1), the targets was purchased from Cathay Advanced Materials Limited (China). The films were deposited onto ultrasonically cleaned glass substrates using UNIVE 350 sputtering unit with RF power model Turbo drive TD20 classic (Leybold), RF power model CESAR, RF power generator and rate thickness monitor model INFICON SQM-160. The deposition conditions were: base pressure of about 2 × 10−3 Pa, substrate to target distance of 10 Cm, RF power of 150 W, self bias 400 V, deposition pressure of 2 Pa, argon flow rate of 10 SCCM. The substrate temperature during deposition was kept at room temperature. The prepared Cd2SnO4 films were annealed in air at different temperatures in the range from 150˚C to 550˚C. Atomic force microscope model (Veeco-di Innova Model-2009-AFM-USA) was used to obtain the topographical images of the film surface and to describe the changes of the grain size with changing the annealing temperature. The optical transmittance, T(λ), and reflectance, R(λ), of the films under investigation were measured by means of a computer programmable Jasco V-570 (Japan) double beam spectrophotometer in the wavelength range from 200 to 2000 nm at normal incidence. In case of reflectivity measurement, an additional attachment model ISN-470 was used. The absorption coefficient α of the films was determined directly from the spectrophotometer measurements using the formula [
where d is the film thickness, T is the transmittance and R is the reflectance of the films.
The optical energy band gap Eg was estimated from the optical measurements by analyzing the optical data with the expression for the optical absorption coefficient
where h is the Planck’s constant and A is a constant. Eg was obtained by extrapolating the linear portion of the plots of
where
Figures 1(a)-(e) show the morphology of annealed Cd2SnO2 thin films with a thickness of 228.5 nm at 150˚C, 250˚C, 380˚C, 450˚C and 550˚C obtained by AFM using tapping mode. AFM images in
The transmittance, T(λ), and reflectance, R(λ), spectra for the annealed Cd2SnO4 thin films of the same thickness 228.5 nm at different annealing temperatures (150˚C, 250˚C, 380˚C, 450˚C and 550˚C) are shown in
The optical energy gap is estimated from the optical measurements by analyzing the optical data with the expression for the optical absorbance, and the photon energy,
To calculate the width of the band tails Ee of the localized states, the following equation [
where
The variation of the refractive index with the wavelength is shown in
be attributed to the decrease in the packing density with increasing annealing temperature [
In the transparent region, the obtained data of refractive index, n, can be analyzed to obtain the lattice dielectric constant, εL, via a procedure describes the contribution of the free carriers and the lattice vibration modes of the dispersion. The relationship between the real dielectric constant, ε1, and the wavelength, λ, in normal dispersion region is given by [
where e is the elementary charge, εo is the permittivity of free space and, N/m* is the ratio of free carrier concentration to the effective mass of electrons.
Annealing temperature (˚C) | Average particle size (nm) | Root mean square roughness (RMS) (nm) | Average height (nm) | Maximum height (nm) |
---|---|---|---|---|
150 | 168 | 2.84 | 17.22 | 42.01 |
250 | 171 | 4.89 | 17.22 | 43.15 |
380 | 195 | 5.89 | 24.23 | 63.33 |
450 | 211 | 11.17 | 40.27 | 88.48 |
550 | 230 | 33.19 | 80.32 | 173.65 |
ratios N/m* are obtained. The disagreement between εL and ε∞ may be due to free carriers’ contribution [
The classic dispersion theory provides the description of the variation of n(λ) in the region of very small values of the extinction coefficient k(λ), under negligible damping. Wemple and DiDomenico [
where hυ represents the photon energy, Eo is the energy of the oscillator and Ed is the dispersion energy which describes the strength of the electronic transitions. The calculated values of the dispersion parameters (Eo and Ed) are obtained by plotting of (n2 − 1)−1 against (hν)2 for the annealed Cd2SnO4 thin films as shown in
The calculated dispersion energies increase with increasing annealing temperature and have the values of 1.845, 1.961, 1.951, 1.848, and 2.428 eV for the annealed Cd2SnO4 films at 150˚C, 250˚C, 380˚C, 450˚C and 550˚C, respectively. In addition, the calculated values of the single oscillator energies are 4.973, 4.593, 4.345, 4.788, and 5.967 eV for the annealed films at 150˚C, 250˚C, 380˚C, 450˚C and 550˚C, respectively, revealing tendency of increasing with elevating the annealing temperatures. The complex dielectric constant can be given by:
where
The real
where
Cadmium tin oxide Cd2SnO4 thin films of the same thickness (228.5 nm) were prepared by RF magnetron sputtering technique on glass substrates at room temperature. AFM images revealed the change of morphology at the nanoscale with increasing annealing temperature. The optical energy gap was found to increase with increasing the annealing temperature. The width of the band tails of the localized states was found to decrease with increasing annealing temperature. The refractive index decreased with increasing the incident wavelength and with increasing annealing temperature as well. The dispersion energy was found to have the tendency of decreasing with elevating the annealing temperatures, whereas the single oscillator energy was found to increase by increasing the annealing temperatures.
The author is thankful to Professor Mostafa M. Abdelraheem for his support throughout this work. The financial support is appreciated from the vice precedency for graduate studies and scientific research at Taif University.
Ateyyah M.Al-Baradi, (2015) Effect of Heat Treatment on the Nanoscale Structure and Optical Properties of Cd2SnO4 Thin Films Deposited by RF Magnetron Sputtering. Journal of Modern Physics,06,1803-1813. doi: 10.4236/jmp.2015.613184