Infrared Absorption by Free Carriers in Si and Influence on Oxygen Determination by FTIR-Spectroscopy
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INFRARED ABSORPTION BY FREE CARRIERS IN SI AND INFLUENCE ON OXYGEN DETERMINATION BY FTIR-SPECTROSCOPY L.Kdster and F.Bittersberger Wacker-Chemitronic GmbH, 8263 Burghausen,
Germany
ABSTRACT Using FTIR-spectroscopy for the determination of the oxygen content in CZ Si, an additional absorption of IR light occurs for doping densities above 101 5 /cm 3 , which is caused by the free carriers. It increases nearly linearly with the carrier density and reduces multiple reflections within the sample. Excessively low oxygen values are obtained if this effect is neglected. The deviation from the true values can amount up to 15 % and depends on carrier density, type of doping, oxygen content and sample thickness. Measurements of the absorption coefficient induced by the free carriers were performed for pand n-type FZ material in dependence of the IR wavelength and of the doping concentration. The obtained values at the wavelength of the oxygen absorption band (1107 cm-I) were used to calculate correction factors for the oxygen content in the doping range from 1 * 101 5 /cm 3 to 1 * 1017/cm3 . An example shows that the correction reaches 5 % for carrier densities of 1 * 10 1 7 /cm 3 , n-type and 2 * 10 1 6 /cm 3 , p-type, respectively, for an assumed sample thickness of 0.7 mm.
The development of internal gettering techniques has led to ever increasing demands on accurate knowledge of both interstitially bound and precipitated oxygen in Czochralski-grown (CZ) silicon crystals. The content of interstitial oxygen is usually determined by the height of the IR absorption band at 1 1107 cm- . The accuracy is influenced by several physical factors such as temperature, sample thickness, multiple internal reflections within the sample, diffuse radiation scattering at rough surfaces and absorption by free carriers. The absorption by free carriers reduces the total transmitted intensity and the multiple internal reflections within the sample. Therefore, the oxygen content of highly doped silicon with resistivities in the mncm range cannot be measured by IR absorption spectroscopy. Procedures have been established for the determination of the oxygen content in n-type wafers with resistivities down to 0.02 ncm [I],[2]. Standard procedures for the determination of the oxygen content are given by ASTM and DIN [3]. Both of them include multiple reflections of the IR light, but do not explicitely take into account the influence of the free carrier absorption. Here we concentrate upon the resistivity range below 5 n cm, where the absorption by free carriers slowly begins with increasing carrier density. Excessively low oxygen values are obtained if this effect is neglected. Physical Basis The evaluation of IR spectra is equation for multiple reflection (4]
generally
based on the
Mat. Res. Soc. Symp. Proc. Vol. 262. @1992 Materials Research Society
272
(I
- R)
T =
2
1 - R
2
e-ad (1)
e-2ad
with T = ratio of transmitted by incident intensity (transmittance), R = reflectivity of silicon = 0.300, d = sample thickness, a = total abso
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