Highly conducting transparent thin films based on zinc oxide

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Highly conducting transparent thin films based on zinc oxide Ruiping Wang,a) Laura L. H. King,b) and Arthur W. Sleight Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003 (Received 24 July 1995; accepted 6 February 1996)

Doped zinc oxide thin films were prepared by rf magnetron sputtering using the dopants Al, Ga, In, and Ge. The best results were obtained with Al and Ga doping where room temperature conductivities were as high as 1600 and 1800 ohm21 cm21 , respectively. Hall measurements were performed at 77 K and 298 K. The Hall mobility as in the range of 9 to 22 cm2yVs, and there was generally very little temperature dependence of the mobility or conductivity. Cation doping levels were as high as 10 at. %, but the conductivities did not increase beyond 3 at. % doping level. For films with high conductivity, electron carrier concentrations from Hall measurements were significantly lower than the concentrations of dopants. Optical measurements on the films showed that the average transmittance though the visible range is higher than 85%. The measurements also indicated a blueshift of the absorption edge with doping.

I. INTRODUCTION

Materials possessing high electrical conductivity coupled with high transmittance of visible light are termed transparent conductors. These materials generally have strong optical absorption in both the UV and IR regions of the spectrum and can, thus, also be used for shielding in those regions. Such materials find utility either as films or powders. The films can be used as transparent electrodes for optoelectronic devices, as transparent heat mirrors for solar energy utilization, or as a coating to heat the surface of glass windows. Appropriate materials are insulating oxides that have been suitably doped. Four hosts have shown promise for producing transparent conductors. These are Cd2 SnO4 , In2 O3 , SnO2 , and ZnO. Conductivity is induced in all four cases by doping such that carriers are injected into an otherwise empty s band. Because of cost and toxicity issues, there is considerable impetus to develop films based on ZnO that have properties comparable to or better than those based on the In2 O3 , SnO2 , or Cd2 SnO4 . We report here on our effort to produce high quality transparent conducting films based on ZnO. We have also characterized our films in such a way that we might better understand a material that also has important applications in catalysis, luminescence, surface acoustic wave devices, and varistors. Zinc oxide based thin films have been prepared by sputtering,1–11 MOCVD,12–14 simple vapor transport,15 and the Pyrosol process.16 The dopants that have been used in these studies to produce n-type conducting thin a)

Current address: Applied Materials, 3100 Bowers Avenue, Mail Stop 0225, Santa Clara, California 95054. b) Current address: Conductus, 969 West Maude Avenue, Sunnyvale, California 94086. J. Mater. Res., Vol. 11, No. 7, Jul 1996

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