Effects of Sn Dopant on Structural and Optical Properties of ZnO Thin Film Prepared by Sol-Gel Route

Tin doped zinc oxide (Sn:ZnO) thin films were prepared on glass substrates via sol-gel dip-coating technique starting from zinc acetate dehydrate, (CH3CO2)2Zn⋅2H2O and tin chloride, SnCl2. The consequences of various Sn doping on the behavior of the film was investigated. The atomic percentages of dopant in ZnO-based solution were [Sn4+]/[Zn2+] which is between 0% and 4%. The thin films were characterized using Field Emission Scanning Electron Microscope (FESEM) and UV-Vis-NIR spectrophotometer.


Introduction
ZnO-based thin films have received spacious attention due to their particular structural and optical characteristics which are widely adapted in numerous type of device fabrication, such as UV sensors [1], light emitting diodes and thin film solar cells [2]. The characteristics and properties of intrinsic and doped ZnO thin films have been tremendously investigated. Most of these investigations focused on various dopant such by using appropriate donor of group III elements like Aluminium (Al), Gallium (Ga), and Indium (In). In order to achieve higher conductivity of ZnO-based thin films, various tetravalent metal dopants are added to ZnO films, such as silicon (Si), germanium (Ge), tin (Sn), titanium (Ti), etc. Among these tetravalent metal dopants, Sn is a suitable material because it possess a good optical transmittance. Besides, Sn will becomes Sn 4+ when it substitutes Zn 2+ site in the ZnO crystal lattice. This will produce two more free electrons to contribute to the electric conduction. In the present work, our interest was focused on the structural and optical properties of Sn-doped ZnO thin films. Several techniques were employed to produce intrinsic and doped ZnO films, including chemical vapor deposition (CVD) [3], sputtering, spray pyrolysis [4] and the sol-gel process. Among the preparation techniques of ZnO films, the sol-gel route represents an easy low cost and efficient route to coat large area surfaces for technological applications. In addition, this technique is one of the most promising in order to its simplicity, customizable micro/nano-structure, excellent control of stoichiometry of precursor solution, ease to compositional modifications and minimum variables to control the growth of film. Moreover, incorporation of dopants is easier in this technique.

Experimental
The ZnO based solution procedure has been reported before by the authors elsewhere [5,6]. In this experiment, tin (IV) chloride pentahydrate (SnCl 4 .5H 2 O) was used as a dopant source. The Dip-coating method was performed to obtain the thin films. Borosilicate glass was chosen as a substrate. All the substrates was being ultrasonically cleaned with acetone, methanol and distilled (DI) water before the coating process. The glass substrates were deposited into the precursor solution at constant speed of 40 mm/min. After the coating process, the thin films were immediately being heated to evaporate the solvent. The thin films were then undergo post-heating treatment at 500 ºC in air ambient. The films structure morphology were characterised by Field Emission Scanning Electron Microscopy, FESEM (model: JEOL JSM 6701F). On the other hand, the optical properties were measured by UV-Vis-NIR spectrometer (model: Varian Cary 5000) in the wavelength range from 350 to 800 nm.

Structural Properties
The surface morphologies of Sn doped ZnO thin films that were prepared at different dopant concentration between 0% and 4.0% are shown in Fig. 2 (50 k magnification; 5 kV applied voltage). The FESEM images show that the films deposited from 0 to 3.0 % are uniformly deposited with nanoscale ZnO particles. Furthermore, the FESEM images show different morphologies of each film which depend on the concentration of Sn. A spherical grain structure was observed for undoped ZnO while the films doped at 1.0% of Sn revealed a flakes structure on a spherical grain. On the other hand, a faceted granular grain structure can be observed for films doped at 2%. This faceted granular grain structure becomes into single nanorods alike. It was also observed that an agglomerated particles were formed which create a boundary between each other at higher Sn concentration which is 4.0%. This might be originated from an increase in lattice strain/stress which is due to the difference in ionic radius between Zn 2+ and Sn 4+ [7].

Optical Properties
The optical transmittance spectra of Sn doped ZnO thin films as a function of a doping concentration in the wavelength range (350-800 nm) are shown in Fig. 2. It is shown that all films are highly transparent where the average optical transmittance values are higher than 90 % in the visible region due to the homogeneous structure with uniform distributed particles in the film which reduces the optical scattering at the grain boundaries. The thin films had a sharp absorption onset at about 375 nm which is attributed to the intrinsic band gap of ZnO. Besides, it can be observed that the optical transmittance spectra of films represent a strong dependance on the Sn concentration. After doping, the transmittance decreased when the dopant concentration was being increased to 2%. On the other hand, we note that the transmittance increased again when the concentration of Sn=3.0% which might attributed to the presence of the interference fringes characteristics of uniform thickness and homogenous layers. However, the transmittance of the doped films always lower than those of undoped ones. The decrement of the transmittance after doping might be due to the increase of the thickness. Band gap energies, E g estimated from the absorption edges of Sn doped ZnO films are shown in Fig.  3. Similar to the structure of ZnO, the Sn doped ZnO film has a direct band gap. The absorption edge for direct interband transition is given by Eqs. (1) and (2) below [8]:

Applied Mechanics and Materials Vols. 773-774
where B is a constant for direct transition, and hν is the photon energy. The value of absorption coefficient, α is determined from the transmittance spectra. The optical energy gap, E g can then be obtained from the intercept of (αhv) 2 versus hv for direct transitions. Better linearity for (αhv) 2 versus hv is observed as shown in Fig. 3 and the energy gap was obtained by extrapolating the linear absorption edge part of the curve using equation (1) and (2) It is noteworthy that a significant blue-shift in optical energy gap, E g was examined while the Sn contents were raised to 4.0%. A slightly higher band gap of the films is due to the Burstein-Moss effect which described the blueshifting of the absorption edge of a degenerate semiconductor with an increasing carrier concentration [9,10]. According to the Moss-Burstein theory, the donor electrons occupy states at the bottom of the conduction band in heavily doped zinc oxide thin films. Since the Pauli principle prevents states from being doubly occupied and optical transitions are vertical, the valence electrons require extra energy to be excited to higher energy states in the conduction band. Therefore, the optical band gap of doped zinc oxide is broader than that of intrinsic zinc oxide films [10].

Conclusion
In this experiment, Sn doped zinc oxide were successfully prepared onto borosilicate glass substrates by sol-gel route. The introduction of Sn in the deposition process affected the growth of films because the thickness and the morphology of these thin layers varied when the amount of Sn was being varied. On the other hand, a blue-shift in the band gap energy is observed when the Sn dopant was being increased. Finally, we conclude that the structural and optical parameters of ZnO thin films were affected by Sn doping.

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