Optical characterization of sol–gel ZnO:Al thin films
Introduction
Nanostructured metal oxide films have been extensively studied due to their numerous applications namely in optoelectronic applications. Among these materials, ZnO gains a great scientific research in respect to their interesting properties such as wide band gap (3.37 eV at room temperature) with large exciton binding energy (60 meV), high chemical stability, low dielectric constant, large electrochemical coupling coefficient, high thermal conductivity, binding, antibacterial and UV protection properties [1]. ZnO thin films posses high transmittance in the infrared and visible light regions and high refractive index, which extend the application range of ZnO films [2]. It can be used as chemical and biological gas sensors [3], ultraviolet light emitters, thin film transistors, solar cell windows [4].
Recently, there has been a huge research of ZnO films as transparent conducting oxides (TCOs) as it is known that ZnO can exhibit n-type conductivity due to its non-stoichiometry. This is attributed to native defects such as oxygen vacancies (Vo) and zinc interstitials (Zni) which can create shallow donor states within the forbidden gap. To increase its conductivity further without sacrificing its optical transmission as, Group III metal dopants such as Al, B and In can be added [5]. Doping with Group III elements (B, In, Al and Ga) is usually necessary to improve the electrical and optical properties of these films [6]. Aluminium doped zinc oxide coatings manifest high transparency, low resistivity. The doping concentration influences on the optical, structural properties [7], [8].
ZnO and doped ZnO thin films can be deposited by several techniques as magnetron sputtering, plasma enhanced chemical vapour deposition, pulsed laser deposition, reactive evaporation, spray pyrolysis and molecular beam epitaxy, etc [9], [10].
Among these deposition methods, thee sol–gel technology is a facile technological approach offering possibility for production of high quality homogeneous thin films on large area, excellent control of the stoichiometry, composition modification (mixing on a molecular level), cost effective process, inexpensive equipments, etc [11], [12].
The aim of this study is to obtain tailoring of the optical properties of Al doped ZnO films. This work presents a sol–gel method for depositing thin films of ZnO, ZnO:Al and Al2O3 thin films. Optical and structural behaviour is investigated as a function of the doping concentrations of Al. The influence of different annealing temperatures is also studied. The measurement techniques used are XRD, FTIR and UV–VIS spectrophotometry.
Section snippets
Experimental
Two sol solutions have been prepared for deposition of ZnO and Al2O3 films. They were synthesized as follows:
Solution (1) the zinc acetate has been dissolved in an absolute ethanol (0, 4 M concentration). The complexing agent and stabilizer used is monoethanolamine (MEA). The molar ratio MEA/Zn is fixed to 1. The solution has been mixed and stirred on a magnetic stirrer at 50 °C/2 h. The clear and transparent solution is obtained and ultrasonically treated at 40 °C. More information for Zn sol
Results and discussions
X-ray diffraction has been used for crystal phase identification, determination of lattice constants and crystallite sizes. It must be noted that XRD study of Al2O3 films revealed that the films are fully amorphous, presenting typical spectra for amorphous material without any specific diffraction lines. The XRD spectra were recorded for Al2O3 films after 700 and 800 °C annealing. On the other hand, ZnO films began to crystallize at lower annealing temperatures.
ZnO and ZnO:Al thin films have
Conclusions
Sol–gel technology has been successfully applied for deposition of transparent ZnO and ZnO:Al films by spin coating method. XRD analysis reveals predominantly wurtzite ZnO crystal phase with significantly deterioration of the film crystallization for Al doped ZnO films. The crystallite sizes of ZnO and ZnO:Al films increased with annealing as a sign of improved crystallization. The Al doped ZnO films have smaller crystallite sizes, significantly influenced by Al contents. XRD analysis reveals a
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