Infrared optical properties of CVD diamond films
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Diamond films of 15-20 /xm thicknesses were prepared by microwave plasma enhanced chemical vapor deposition onto crystalline silicon substrates. The growth surfaces of the films were rough with polycrystalline crystallographic habits, while the substrate sides of these films were smooth and featureless as viewed by optical microscopy. A heated cast iron scaife was used to polish the rough growth surfaces, and free-standing films were removed from the silicon substrates by dissolving the silicon in an aqueous HF. Both infrared optical transmission and reflection spectra were measured over the range of 600-4000 cm"1. For polished films, near 70% transmittance was obtained over the whole range, while the transmittance for nonpolished films was much lower and varied strongly with the wave number. Absorptions due to carbon-hydrogen stretching bands as well as a silicon carbide phase were observed in the transmission spectra. The optical absorption coefficient and the refractive index were found to vary from as high as 150 to as low as 7 c m 1 and 2.41 to 2.49, respectively (depending on the film quality and the wave number). A weak signature of the two-phonon absorption band of diamond was observed. The relationship between deposition conditions and infrared optical properties of diamond films before as well as after polishing is discussed.
I. INTRODUCTION
During the past few years, significant progress has been made on experimental techniques for synthesizing diamond films by low pressure chemical vapor deposition (CVD).^4 Unique optical, electrical, thermal, and mechanical properties make diamond the material of choice for many applications. For instance, the optical transparency in the infrared-visible-ultraviolet (^225 nm) regions and the low atomic mass, combined with the excellent mechanical properties, make diamond a promising lens coating material for IR or x-ray windows. Unfortunately, the growth surfaces of almost all CVD diamond films are optically rough. Scattering from these rough surfaces results in a low transmittance which is defined as belonging to the direct incident beam, especially in the visible and infrared regions. Nevertheless, work on the infrared properties of rough CVD diamond films has been reported. Yarbrough et al.5 measured both transmission and reflection spectra of a 22.5 ixm thick diamond film over a middle-infrared range from 400-4000 cm"1. They reduced the raw data by using the method of Cody et al.6 and found two distinct absorption bands, one broad and weak at —1300 cnT1 and the other much stronger at —2800 cm"1. The absorption coefficient was found to be extremely high (20-212 cm"1) primarily due to losses caused by scattering at the rough surface. Wild et al.1 reported an absorption spectrum of a 400 ju.m thick free-standing diamond film by fitting a cubic spline curve to the measured spectrum and thus obtained the
J. Mater. Res., Vol. 5, No. 11, Nov 1990
http://journals.cambridge.org
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absorption spectrum by subtraction. The intrinsic twophonon absorpt
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