Characterization of Nanocones Grown During DC Magnetron Sputtering of an ITO Target
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Characterization of Nanocones Grown During DC Magnetron Sputtering of an ITO Target J.F. Conley, Jr.,1 D. McClain,2 J. Jiao,2 W. Gao,1 D. Evans,1and Y. Ono1 1
IC Process Technology Laboratory, Sharp Labs of America, 5700 NW Pacific Rim Blvd., Camas, WA, 98607 2 Portland State University, P.O. Box 751, Portland, OR, 97207 ABSTRACT A low temperature method for uniform growth of In2O3 nanostructures on Si wafers that does not require separate catalyst materials or template-assistance is investigated. Nanostructures are uniformly deposited on either bare or SiO2 thin film coated Si substrates via DC magnetron sputtering at 200-400°C using a 90% In2O3 / 10% SnO2 (ITO) target. The nanostructures are approximately 500 nm long, sit atop an accompanying underlying 100 nm conductive film, and are conically shaped, with a diameter of about 80 nm at the base, tapering to a point that is capped with a spherical "ball". X-ray diffraction (XRD) indicates a cubic In2O3 phase. Field emission from the tips is observed at a base pressure of 10-8 Torr with turn-on fields in a range between 45-75 V/cm and threshold fields from 64-105 V/cm. Nanocone growth is investigated with respect to O2 and Ar flow rates, temperature, power, pressure, wafer rotation, and time. INTRODUCTION Nanostructured materials such as nanowires, nanorods, nanofibers, whiskers, etc., exhibit interesting optical and electronic properties and have been demonstrated in many applications such as chemical and bio sensors, LEDs, transistors, lasers, field emitters, etc..1,2 In2O3, a transparent wide bandgap semiconductor material with high gas sensitivity, is an excellent candidate for nanostructure device applications such as gas sensors or electrodes.3,4 One of the primary methods for nanowire formation is the vapor-liquid solid (VLS) transport method in which a catalyst can be used to grow a nanowire from the gas phase.5 (Other methods, such as laser ablation, arc discharge, template assistance, etc. have also been used.3,4,6,7) The VLS process typically requires a catalyst and deposition temperatures >700ºC. For example, a very thin film (~3nm) or clusters of a catalyst such as Au, is often used. Due to fast diffusion and levels in the middle of the Si forbidden gap, Au is not a welcome material in a microelectronics fabrication facility. Therefore, a low temperature method to prepare In2O3 nanowires that does not involve a catalyst would be desirable. In this paper, we demonstrate a simple method for formation In2O3 nanowires that does not require a separate catalyst and can be performed at relatively low temperatures (200-400ºC). Nanostructure growth is investigated with respect to O2 and Ar flow rates, temperature, power, pressure, wafer rotation, and time. X-ray diffraction (XRD) and field emission (FE) results are also reported. EXPERIMENTAL DETAILS DC magnetron sputtering was performed using a 99.99% pure In2O3/SnO2 target (10% SnO2) at a pressure range of 2-10 mTorr, a temperature range of 100-400ºC, O2 levels of 1-5% O2, gas flow rates in the range
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