The effect of hydrogen plasma on the properties of indium-tin oxide films
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I. INTRODUCTION The recent interest in hydrogenated amorphous silicon-based optoelectronic devices has led to a renewed interest in the properties of transparent conducting oxide (TCO) films, used as electrodes in these devices. Since amorphous silicon is deposited by glow discharge decomposition of pure or hydrogen-diluted silane, the chemical stability of TCO films subjected to a reducing plasma needs to be assured. We have shown1 recently that the surfaces of indium-tin oxide (ITO) and fluorine-doped tin oxide (FTO) films get reduced to yield metallic indium and tin, respectively, when subjected to hydrogen plasma. We have now undertaken a detailed study of the nature and extent of changes in the surface composition, surface microstructure, and electrical and optical properties of ITO films when exposed to hydrogen plasma. We have also studied the reoxidation of the reduced surface of the plasma-exposed films, by annealing in oxygen ambient. II. EXPERIMENTAL DETAILS Transparent conducting indium-tin oxide films were deposited by spraying an alcoholic solution of stannic chloride and indium chloride onto glass substrates held at a temperature of 650 K. The Sn/In ratio in the solution was 0.02. Films of thickness ~0.75 /um, sheet resistance ~10-12 fl/sq, and average visible transmittance ~ 80% were obtained. The films were exposed to hydrogen plasma in an rf (13.56 MHz) glow discharge chamber. The conditions of hydrogen plasma exposure were pressure slOO mTorr, rf power ^0.25 W/cm 2 , and gas flow rate ~ 10 seem at a temperature of 500 K. The plasma conditions were kept constant and the duraPresent address: Physics Department, Indian Institute of Technology, Pawai, Bombay-400 076, India. b) Present address: Director, Indian Institute of Technology, Kharagpur-721 302, India. a>
J. Mater. Res. 3 (4),Jul/Aug 1988
http://journals.cambridge.org
tion of plasma exposure was varied to change the dosage of plasma exposure. The plasma-exposed films were annealed at 700 K in oxygen at atmospheric pressure for 1 h. The composition of the films was studied by x-ray photoelectron spectroscopy (XPS) with a SAM-ESCA(PH159O-A1O) system using Mg Ka radiation. Films were etched in situ with a 2 keV A r + ion gun for studying composition changes with film thickness. The surface topography of the films was studied with a JEOL JSM-35CF scanning electron microscope. The optical reflectance and transmittance in the visible and near infrared were measured using a Hitachi 330 UV, visible, near infrared (IR) double-beam spectrophotometer. The specular reflectance was measured at an angle of incidence of 5 deg. The total hemispherical reflectance and transmittance were measured using an integrating sphere attachment. The resistivity and carrier concentration were measured using van der Pauw geometry. Film thickness was measured by a Talystep instrument. III. RESULTS AND DISCUSSION A. Composition As already reported,1 the surface of ITO films exposed to hydrogen plasma is reduced. Figure 1 shows the In(3J 3 / 2 ) and In(3c?5/2) core levels
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