Electrical and optical transport in undoped and indium-doped zinc oxide films
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I. INTRODUCTION Zinc oxide is a technologically important material, well known for its electronic, opto-electronic, and acousto-electronic applications. The properties of bulk ZnO have been studied extensively and a very detailed review has recently been published.1 In the thin film form, the focus has been essentially on sputtered films for SAW device applications. However, recent work on thin films of ZnO, deposited by various techniques has shown that the opto-electronic properties of ZnO films can be conveniently tailored to obtain high-quality transparent conductors.2 The thermal stability of undoped ZnO films is poor3 but indium-4 and aluminum-5 doped films have shown extremely stable transparent conducting properties. In the present work a detailed study of the electrical and optical transport phenomena in transparent conducting films of undoped and indium-doped ZnO deposited by spray pyrolysis has been undertaken. Owing to their heavily doped as well as polycrystalline nature, the conduction in these films is controlled by their microstructure, composition, and the level of doping. The presence of species at the grain boundaries, such as chemisorbed oxygen and segregated dopant atoms also plays a very important role in determining the nature of grain boundary scattering mechanism. The conduction mechanism in these films has been analyzed considering both ionized impurity scattering within the grains and grain boundary scattering. Their dependence on the microstructure and doping level has also been studied. The optical properties of the films in the UV, visible, and IR 300
J. Mater. Res. 1 (2), Mar/Apr 1986
http://journals.cambridge.org
regions (0.3 /iva-50 /nm) have been studied and correlated with the corresponding electrical parameters. The optical studies have also been used to obtain information about the scattering mechanism within the grains.
II. EXPERIMENTAL DETAILS Undoped and indium-doped ZnO films were deposited by the spray pyrolysis technique. An alcoholic solution of zinc acetate mixed with indium chloride (in case of doped films) was sprayed onto glass substrates held at about 675 K. Details of the deposition process and the role of various deposition parameters have been discussed in an earlier paper.6 The films were annealed at about 650 K for 30 min in two different ambients, namely, oxygen at atmospheric pressure and vacuum ~ 10 ~ 3 Pa. The annealing was done on a molybdenum sheet which was resistively heated. The thickness of the films was measured with a Talystep instrument and unless otherwise specified, the specimens for the present study have thicknesses in the range of 0.8-1 .Ofim. The composition of the films was studied by Auger electron spectroscopy using a PHI 590 A Scanning Auger Microprobe (SAM) system. Here van der Pauw's geometry was used to measure the electrical resistivity (p) and Hall coefficient of the films to determine the carrier concentration (N) and mobility (fi) as well as their temperature dependence. The specular reflectance (R) and transmittance (71) in the
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