High Rate Deposition of High Quality ZnO:Al by Filtered Cathodic Arc
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High Rate Deposition of High Quality ZnO:Al by Filtered Cathodic Arc Rueben J. Mendelsberg1, 2, Sunnie H.N. Lim1, Delia J. Milliron2, and André Anders1 1 Plasma Applications Group, Lawrence Berkeley National Laboratory Berkeley, CA 94720, U.S.A. 2 Inorganic Nanostructures Facility of the Molecular Foundry, Lawrence Berkeley National Laboratory
ABSTRACT High quality ZnO:Al (AZO) thin films were prepared on glass substrates by direct current filtered cathodic arc deposition. Substrate temperature was varied from room temperature to 425°C, and samples were grown with and without the assistance of low power oxygen plasma (75W). For each growth condition, at least 3 samples were grown to give a statistical look at the effect of the growth environment on the film properties and to explore the reproducibility of the technique. Growth rate was in the 100-400 nm/min range but was apparently random and could not be easily traced to the growth conditions explored. For optimized growth conditions, 300-600 nm AZO films had resistivities of 3-6 x 10-4 ȍcm, carrier concentrations in the range of 2-4 x 1020 cm3, Hall mobility as high as 55 cm2/Vs, and optical transmittance greater than 90%. These films are also highly oriented with the c-axis perpendicular to the substrate and a surface roughness of 2-4 nm. INTRODUCTION Transparent conducting oxides (TCOs) are an important part of many emerging technologies, including energy saving windows with dynamic optical properties. Window applications as well as TCOs for solar cells or displays require high rate growth processes which are capable of depositing uniform coatings over several square meters. The current industry standard for oxide deposition is reactive magnetron sputtering, with several companies supplying sputtering equipment for in-line coating systems up to 3.5 m wide. However, standard reactive sputtering typically produces inferior AZO compared to other techniques, such as pulsed laser deposition (PLD) 1 2 or cathodic arc deposition 3 4 where the material condensing on the substrate is highly ionized plasma. Increasing the ionization by sputtering in a high power impulse mode 5 typically reduces the deposition rate and has the disadvantage of producing highly energetic negative oxygen ions 6.These ions are even a problem for standard sputtering in oxygen since they damage the growing film, leading to strong spatial variations in the electrical properties 7. In this work, high quality AZO films were deposited by direct current filtered cathodic arc deposition at growth rates over 10 times faster than magnetron sputtering. The resistivity of the films is nearly as low as that obtained by optimized PLD growth, which typically produces very high quality material but only over small areas and at low growth rates. Unlike PLD, cathodic arc doesn’t require an expensive high-power laser which means scaling up to large area coatings may be as simple as arranging multiple cathodes in a suitable geometry.
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EXPERIMENT For a statistical study of dc cathodic arc growth of AZO, a la
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