Electrical and optical characterization of Sb : SnO 2
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M. F. Arendt Department of Chemistry and Biochemistry, University of Texas, Austin, Texas 78712
John S. Swinnea Department of Materials Science and Engineering, University of Texas, Austin, Texas 78712 (Received 14 May 1993; accepted 17 August 1993)
Films of Sb: SnO 2 have been formed by vacuum e-beam evaporation. The structural, electrical, and optical properties of these films have been investigated with respect to annealing time and temperature. After heat treatment in an oxygen atmosphere, thin films with a peak transmittance of 98% and 4 - 9 X 10~3 Ocm resistivity have been obtained. The barrier heights and energy band diagrams of Sb: SnO 2 /Si n-n and p-n heterojunctions have been determined by C-V measurements.
I. INTRODUCTION Transparent conducting oxides have been utilized for numerous optoelectronic devices. One application where conducting oxides such as indium tin oxide (ITO) have been projected to have significant impact is low-cost photovoltaic cells. This approach to solar cell fabrication has been pursued as a means of reducing contact shadowing. ITO would appear to be very useful for photovoltaics because it forms a Schottky-barrier on most semiconductors,1"3 its band gap is wide enough to be transparent to visible radiation, and it has relatively good conducting properties. On the other hand, the diffusioninduced degradation observed in ITO/Si heterojunctions has led to poor reliability. It has previously been reported that Sb: SnO 2 does not exhibit this degradation mechanism and further that it has higher conductivity than SnO2-4 In this paper, we describe the fabrication of Sb: SnO 2 thin films by e-beam evaporation. We have investigated the structural, electrical, and optical properties for a wide range of annealing temperatures in an oxygen ambient. In addition, the energy diagrams for Sb: SnO 2 on /?-type and n-type Si are inferred from C-V measurements. II. EXPERIMENTAL It has been shown that Sb:SnO 2 films with 110 wt. % Sb exhibit low resistance and high transmission.4"6 To achieve a composition in this region, a mixture of 92% SnO 2 :8% Sb 3 O 4 was used as the source evaporation material. The thin films were deposited by vacuum e-beam evaporation onto quartz and silicon substrates. During the deposition, the substrate temperature was maintained at 150 "C. For comparison, undoped SnO 2 was also evaporated. The Sb: SnO 2 films J. Mater. Res., Vol. 8, No. 12, Dec 1993 http://journals.cambridge.org
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formed on quartz were subsequently annealed in oxygen at temperatures in the range from 500 to 900 °C for various times. Their resistivity was determined using the four-point probe method, and the optical transmission in the wavelength range from 300 to 1000 nm was measured using a spectrophotometer. X-ray diffraction measurements were performed using CuK a radiation. The films on the silicon wafers were annealed in oxygen at 500-600 °C for 1 h. Al was evaporated onto the back surface of the silicon wafers in order to make an ohmic contact. The junction capacitances o
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