Rotating Disk Reactor-Low Pressure Metal Organic Chemical Vapor Deposition (Mocvd) of Optical Films
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ABSTRACT Over the past 30 years, the need for transparent conducting oxide coatings has been met almost exclusively by tin doped indium-oxide. As the display market advances in complexity, the demand for alternative transparent materials exhibiting high conductivity and stability has become greater. In this paper, we discuss briefly the merits of using doped ZnO as a superior transparent conducting oxide. We report here our results in scaling our ZnO MOCVD reactor technology from 5" to 12" diameter susceptors. Using Rotating Disk Reactor-Low Pressure Metal Organic Chemical Vapor Deposition, we have been able to obtain large area uniformity on multiple (14 cm x 9 cm) glass sheets per deposition run. Promising film characteristics suggest significant application in the field of flat panel displays and other optical systems may be possible. INTRODUCTION Although tin doped indium-oxide (ITO) has satisfied the need for transparent conducting oxide (TCO) coatings for more than quarter of a century [1-4], inherent problems with the material has limited progress in display systems development in general. Instability in hydrogen plasmas, a lack of deposition uniformity, and degradation at high deposition or operating (heater)
temperatures are all issues that have hindered advancement. These problems combined with the toxic nature of indium clearly identify the need for an alternative TCO. Intrinsic ZnO is highly resistive but can be made highly conductive by doping. Sheet resistance of ZnO:Ga should be comparable with ITO, 10-1 f,,/J. We have previously demonstrated MOCVD deposited transparent ZnO films on 5" diameter platters with sheet resistance of ca. 13 QE [5]. Other superior properties of ZnO include 90 % visible optical transparency (as well as through the far IR [6]) and good thermal and chemical stability. The doped material has the potential of achieving highly crystalline films on a variety of substrates at low temperature (ca. 400 'C) [5]. We consider Rotating Disk Reactor-Low Pressure Metal Organic Chemical Vapor Deposition to be the most versatile growth technique to achieve these properties, offering the ability to control the film stoichiometry and impurity level. MOCVD favorably yields two-dimensionally nucleated grains and is very forgiving with respect to "particle induced pin holes". The two latter statements are not applicable to sputtering for example. Alternatively, compositionally grading film alloys may allow for more wear resistant TCO's in optical applications. Superior chemical and stability properties could be obtained from oxides using non-stoichiometry to generate electron degeneracy but these are unstable. More favorable is impurity-doped films, which tend to be more stable. In this paper, we report the initial, operational results from our new scaled MOCVD reactor and showerhead which allows deposition of uniform, optical quality films of ZnO over 171 Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
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