Transparent conducting ZnO:Al, In and Sn thin films deposited by the sol–gel method
Introduction
Metal oxide semiconductor films have been widely studied and have received considerable attention in recent years due to their optical and electrical properties. Some of them are good candidates for transparent conductive oxide films. Among them, ZnO is one of the metal oxide semiconductors suitable for use in optoelectric devices. It is an alternative material to tin oxide and indium tin oxide, which have been most used to date [1], [2].
ZnO is an n-type wide band gap semiconductor (Eg=3.2 eV), and its electrical conductivity is due to intrinsic and extrinsic defects. The conductivity of pure ZnO was produced by the former defects such as zinc excess at the interstitial position and the lack of oxidation. As pure ZnO thin films are sensitive to oxidation, absorption of O2 in the films is inclined to decrease the electrical conductivity. In cases of doped ZnO with different dopants, the electrical properties are enhanced by extrinsic defects and these trials have been attempted [3], [4], [5], [6], [7]. Additionally, electrical properties of ZnO could be modified by thermal treatment in a reducing atmosphere [8]. The optical properties of ZnO were mainly affected by a surface morphology [9] and the change of the optical energy band gap followed heavy doping [10]. The morphology was also modified by thermal treatment in a reducing atmosphere [9] and by an appropriate doping process [11].
Ohyama reported that the use of 2-methoxyethanol and monoethanolamine (MEA), solvents with high boiling point, resulted in transparent ZnO films with strongly preferred orientation [12] and that better electrical and optical properties had been obtained in 0.5 at.% aluminum doped ZnO thin films heated in reducing atmosphere [3]. Nunes found that when the doping concentrations of Al, In and Ga were 1, 1 and 2 at.%, respectively, electrical and optical properties of doped ZnO were superior [13].
ZnO thin films have been prepared by a variety of thin film deposition techniques, such as pulsed-laser deposition [14], RF magnetron sputtering [15], chemical vapor deposition [16], spray pyrolysis [17] and the sol–gel process [18]. While physical deposition such as pulsed-laser deposition and RF magnetron sputtering produce films with good electrical and optical properties at lower deposition temperature, it has disadvantages of a relative low deposition rate and a high cost for equipment. However, the sol–gel technique offers the greatest possibility of preparing a small as well as large-area coating of ZnO thin films at low cost for technological applications.
In this work, the effects of dopants and their doping concentration on the microstructure and electrical properties of doped ZnO thin films, as well as the interrelationship between the degree of crystal orientation and the properties of films were investigated.
Section snippets
Experimental details
ZnO thin films were prepared by the sol–gel method. As a starting material, zinc acetate dihydrate (Zn(CH3COO)2·2H2O) was used. 2-methoxyethanol and MEA were used as a solvent and stabilizer, respectively. The dopant sources of aluminum, indium and tin were aluminum chloride (AlCl3), indium chloride (InCl3) and tin chloride (SnCl4), respectively. Zinc acetate dihydrate and a dopant were first dissolved in a mixture of 2-methoxyethanol and MEA solution at room temperature. The molar ratio of MEA
Results and discussion
Fig. 1 shows the X-ray diffraction patterns of ZnO thin films doped with aluminum, indium and tin. The doping concentration for each dopant changed from 0 to 3 at.%. All films, undoped and doped films regardless of dopants and their doping concentrations, had only a (0 0 2) diffraction peak, indicating the preferred grain growth along the (0 0 2) plane. The 1 at.% aluminum and indium doped ZnO thin film had the highest (0 0 2) diffraction peak intensity. Moreover, the peak intensities of those films
Conclusions
Aluminum, indium and tin doped ZnO thin films were prepared by the sol–gel method. All films were oriented preferentially along the (0 0 2) direction. Films doped with 1 at.% aluminum concentration, 1 at.% indium and 2 at.% tin for each dopant had a stronger c-axis orientation perpendicular to the substrate, larger grain, more smooth surface morphology and higher conductivity and transmittance than the others. The aluminum doped film with 1 at.% had a thickness of 200 nm and a columnar structure.
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