Study of Structural and Optical Properties of Undoped and Cd Doped SnO 2 Thin Films Prepared by Sol-Gel Dip Coating Technique

In this study, we report the study of the surface morphology, the structural and optical proprieties of transparent cadmium doped tin dioxide (Cd:SnO2) thin films deposited on glass and Si(100) substrates by Sol Gel Dip Coating (SGDC) technique. The analysis was carried out using Grazing Incidence X-Ray Diffraction (GIXRD), Atomic Force Microscopy (AFM), UV-Vis spectrophotometry and Spectroscopic Ellipsometry (SE). The X-ray diffraction reveals that all films deposited on Si(100) substrate have tetragonal crystalline structure with preferential orientation along (310) plane, but an amorphous structure is obtained for all the films prepared on glass substrate. The surface roughness, observed by means of AFM varies from 19 to 4 nm. The optical measurements show that the deposited Cd:SnO2 films have a high transparency (~86%) in the visible spectrum and a band gap energy decreasing from 3.60 eV to 3.31 eV with increasing Cd concentration. The obtained values of the refractive index of the films are ranging between 1.559 and 1.613.

The aim of this study is to study the structural and optical properties of cadmium doped SnO 2 films deposited by SGDC technique onto silicon and glass substrates with different Cd doping concentrations.The effect of Cd doping on the physical properties of the fabricated SnO 2 thin films is elucidate.

MATERIALS AND METHODS
Undoped and Cd doped SnO 2 thin films were prepared by sol-gel dip-coating process using SnCl 2 -2H 2 O and CdCl 2 -5H 2 O as precursors.The starting sols of undoped and Cd doped SnO 2 with two different molar concentrations (6 and 10 at.% Cd) were prepared by a simple procedure.A mixture of CdCl 2 -5H 2 O and Fig. 1: Grazing incidence X-ray diffraction patterns of thin Cd: SnO 2 films deposited on glass (A) and Si (100) (B) substrates SnCl 2 -2H 2 O chemicals was dissolved in 20 ml of ethanol.After refluxing at 60°C during 2 hours under magnetic stirring, homogeneous mixtures of precursors were formed.Then, aging for two days at room temperature, the transparent sols were obtained.Subsequently, the sols were dip-coated on the glass and Si(100) cleaned substrates and then dried at 100°C for 15 min for the formation of the initial thin film.Different layers were obtained after repeating the operation several times.Finally, the samples were annealed in air at 550°C for 2 h.
The structural properties of the deposited films were studied by means of Grazing Incidence X-Ray Diffraction (GIXRD) using CuKα radiation (λ = 1.54056Å) from Bruker-AXS.D8 diffractometer.The surface morphology was observed using a Pacific Nanotechnology atomic force microscope.The Spectroscopic Ellipsometry (SE) measurements were performed on a Horiba-Jobin-Yvon Ellipsometer UVISEL operating in the wavelength range 260-800 nm.The optical transmittance was recorded on a Shimadzu 3101 PC UV-visible spectrophotometer.

RESULTS AND DISCUSSION
The X-ray diffraction patterns of the undoped and Cd doped SnO 2 thin films deposited on glass and Si (100) substrates are shown in the Fig. 1A and 1B respectively.
As can be seen from the Fig. 1A, the X-ray diffraction spectra recorded in thin SnO 2 films deposited on a glass substrate do not contain any peaks, indicating that the films are amorphous.However, the X-ray analysis shows that SnO 2 films deposited on a monocrystalline silicon are polycrystalline with tetragonal structure (Fig. 1B).The most intense peak In Fig. 2, also the (310) peak position undergoes a shift which is due to mechanical strains created by the incorporation of a high Cd concentration in the SnO 2 lattice.
The optical transmittance spectra of pure and Cd doped SnO 2 thin films are recorded in the range 300-800 nm and plotted in the Fig. 3.
Figure 3 shows that the optical transmittance of all the studied samples is more than 80% in the spectral UV-Vis region and their absorption band is entirely in   (Mariappan et al., 2013).A slight shift of the absorption edge toward higher wavelength region is found with increasing Cd concentration.The higherenergy electronic transitions from valence to conduction bands confirm the direct type semiconducting nature of the material (Pejova and Grozdanov, 2007).The E g band gap of the deposited films is calculated by using the following Tauc formula (Tauc, 1974): where, ν : The frequency A : A constant m : Assumes values 1/2, 2, 3/2 and 3 depending on the mode of interband transition, i.e., direct, allowed indirect, direct forbidden and indirect forbidden transition, respectively.
Typical plots of (αhν) 2 versus hν for pure and Cd doped SnO 2 films are shown in Fig. 4. The extrapolation of the linear portion of the (αhν) 2 curves to the hν energy axis gives the value of the energy gap for pure and Cd doped SnO 2 films.As can be seen, the optical band gap decreases from 3.60 eV for the undoped SnO 2 films to 3.31 eV for 10 at.% Cd doped SnO 2 films.The deduced 3.60 eV value is in agreement with the band gap energy of pure SnO 2 (Bhat et al., 2007).The reduction in the band gap energy may be due to the decrease of holes concentration with the increase of Cd doping (Mariappan et al., 2013).
Figure 5 shows AFM images of the undoped and Cd-doped SnO 2 thin films for different atomic Cd contents.By analyzing the AFM images, the roughness of the films is estimated through the root mean square (RMS) parameter.The Cd doping leads to a significant decrease in RMS values from 19.2 nm for SnO 2 to 4.3 nm for 6 at.% Cd:SnO 2 .For the 10 at.% Cd:SnO 2 sample, the RMS is intermediate.
AFM images indicate the formation of polycrystalline thin films with uniform and smooth surface.Moreover, it is evident that lower surface roughness with more compact surface is achieved by addition of 6 Cd at.% doping.However, more compact surface significantly decreases the RMS factor of the films, which leads to TCO with higher transparency.It seems that the latter factor plays a key role in improving of properties of TCO films.As a result, the combination of more compact and homogenous surface with less RMS roughness provides new TCO with better properties compared to TCO films for devices applications.
The knowledge of optical constants of the thin films is very significant since they can determine the exact application of the films.Figure 6A shows the dispersion of the refractive index for pure and Cd doped SnO 2 thin films, derived from SE measurements.
It is clear from the Fig. 6A that the spectral behavior of refractive index for the pure and Cd doped films is almost similar and it is decreasing with wavelength.The refractive index is 1.559 for undoped SnO 2 films and this value increases to 1.600 and 1.613 for 10 at.% and 6at.% Cd doping respectively (Fig. 6B).

CONCLUSION
The cadmium doped tin oxide (Cd:SnO 2 ) thin films are deposited on glass and Si(100) substrates with different Cd concentrations.The XRD show an amorphous structure for the thin films deposited on glass substrate.However, the tetragonal crystalline structure with preferential orientation along (310) plane is revealed for the films elaborated on Si(100) substrate.The all obtained thin films are transparent with optical transmittance ~86%.From the optical studies it is observed that the band gap energy decreases from 3.60 eV to 3.31 eV with increase of Cd concentration.AFM images show that the surface roughness is reduced to lower values with addition of Cd.The measured refractive index reveals that the Cd:SnO 2 films grown by the sol-gel dip coating technique are suitable for interest applications such as optoelectronic devices.

Fig. 2 :
Fig. 2: X-ray diffraction (310) line of thin Cd:SnO 2 films deposited on Si observed corresponds to the (310) line and weak additional (101), (301), (041), (212) and (223) lines are registered.It is easy to notice that the most important (310) line becomes more sharp and more intense with the increasing of Cd concentration from 6 to 10 at.%.It means that the high Cd doping leads to the formation of well-crystallized SnO 2 films.The peaks obtained experimentally are identified and compared with standard values of Joint Committee on the Powder Diffraction Spectra data (JCPDS No. 88-0287).In Fig.2, also the (310) peak position undergoes a shift which is due to mechanical strains created by the incorporation of a high Cd concentration in the SnO 2 lattice.The optical transmittance spectra of pure and Cd doped SnO 2 thin films are recorded in the range 300-800 nm and plotted in the Fig.3.Figure3shows that the optical transmittance of all the studied samples is more than 80% in the spectral UV-Vis region and their absorption band is entirely in

Fig. 3 :
Fig. 3: Transmission spectra of undoped and Cd doped SnO 2 thin films deposited on glass

Fig. 6 :
Fig. 6: (A) Dispersion of the refractive index for pure and Cd doped SnO 2 films, derived from SE measurements and (B) refractive index vs Cd concentration the UV light region of 300-350 nm.The observed transmission values are compared with earlier reported values(Mariappan et al., 2013).A slight shift of the absorption edge toward higher wavelength region is found with increasing Cd concentration.The higherenergy electronic transitions from valence to conduction bands confirm the direct type semiconducting nature of the material(Pejova and Grozdanov, 2007).The E g band gap of the deposited films is calculated by using the following Tauc formula(Tauc, 1974):