Dopant induced modifications in the physical properties of sprayed ZnO:In films
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Shailaja Mahamuni and S. K. Kulkarni CAS in Materials Science and Solid State Physics, Department of Physics, University of Poona, Pune 411 007, India (Received 4 September 1992; accepted 29 December 1992)
Indium-doped zinc oxide (IZO) films were prepared by the spray pyrolysis technique. The effect of gradual incorporation of indium cations on the structural, electrical, and compositional properties of IZO films was studied in detail. It was observed that even a small addition of indium modifies the preferred growth of IZO film from the [002] direction to the [101] direction. Such a modification in growth pattern is a result of more nucleating centers created by indium doping. Indium dopant improves the electrical properties of the films. The carrier concentration depends mainly on the indium dopant level while the mobility is affected by the changes in crystal orientation that take place due to addition of dopants. X-ray photoelectron spectroscopy results show that indium doping does not lead to any stoichiometric changes in the IZO films and the dopant incorporation in the film is linearly proportional to that in the solution.
I. INTRODUCTION Transparent conducting films like zinc oxide (ZnO), tin oxide (SnO2), and indium oxide (In 2 O 3 ) are widely used in various opto-electronic devices1 and solar cell related fields.2'3 Of these, only ZnO films have been found to be stable during the development of hydrogenated, amorphous, and microcrystalline silicon solar cells.4 ZnO films are also advantageous because zinc compounds are nontoxic and easily available. These films can be prepared by a variety of methods like sputtering,5 chemical vapor deposition,6 and spray pyrolysis.7 Among these, spray pyrolysis is widely used due to its commercial viability and simplicity. Physical properties, mainly the electrical conductivity of sprayed ZnO films, can further be improved by doping with indium at cation sites.8 Considerable work on sprayed IZO films has been done by Major et al.,9 which shows that indium doping affects the structural and electrical properties of these films. Still, a systematic study of the effect of indium doping on various physical properties and co-relation between them is lacking. The present work is mainly concerned with the detailed investigation of indium-doping-induced changes in the growth mechanism of the film. Consequent changes in the structural, electrical, and compositional properties were studied using x-ray diffraction, Hall measurements at room temperature, and x-ray photoelectron spectroscopy (XPS) measurements, respectively. 1052
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J. Mater. Res., Vol. 8, No. 5, May 1993
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The undoped ZnO films show [002] preferred orientation which changes to [101] after adding a small amount of indium (1.2 at. %) in the precursor solution. On the other hand, heavy doping leads to a preferential growth along the [100] direction. These results have been explained on the basis of different nucleation phenomena taking place during deposition. The
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