Smooth diamond films grown by hot filament chemical vapor deposition on positively biased silicon substrates
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C.H. Chao Electrical Engineering Department, University of Missouri, Columbia, Missouri 65211
M. A. Prelas Nuclear Engineering Department, University of Missouri, Columbia, Missouri 65211
E. J. Charlson and J. M. Meese Electrical Engineering Department, University of Missouri, Columbia, Missouri 65211 (Received 10 December 1993; accepted 27 April 1995)
Diamond films have been grown by hot filament chemical vapor deposition (CVD) on mirror-polished positively biased Si substrates. Very smooth films a few micrometers thick were obtained in only 30 min. SEM, x-ray diffraction patterns, and Raman were used to characterize the films. Not only diamond but other carbon phases, were also detected. The initial structure showed a high density of defects and large stresses. Structural changes in time were found to occur with films apparently undergoing a phase transformation.
I. INTRODUCTION Diamond films produced by low pressure chemical vapor deposition (CVD), are hard, chemically inert, and transparent. Such films have already been used as protective coatings for infrared optics. One major limitation in optical coatings is the surface roughness due to crystalline grains. The light scattering on the rough surface, rather than the absorption, is responsible for optical losses in diamond films.1 Therefore, smooth diamond films should be of interest for optical applications. It is well known that diamond has a large surface energy and thus grows three-dimensionally.2 A growing film becomes continuous at thickness, exceeding the average distance between nuclei. Correspondingly, the roughness of diamond films grown at low nucleation densities is high. The nucleation density of diamond on polished surfaces is low, of the order of 104 nuclei/cm.2'3 At such nucleation densities films become continuous when their thickness exceeds 100 /am.3 To enhance nucleation, scratching the substrate by diamond particles is routinely used. However, such a procedure is not acceptable for optical and some electronic applications. Another way to enhance nucleation is to bias, positively or negatively, the substrate surface during CVD.4^6 Nucleation densities over 1010 nuclei/cm 2 have been obtained in this way on mirror-polished substrates.5 At such nucleation densities, diamond films become continuous at a thickness of a few tenths of a micrometer. High nucleation densities lead to early coalescence of films J. Mater. Res., Vol. 10, No. 8, Aug 1995
and therefore to much smoother surfaces. Typical for the biased nucleation is a strong texture. The orientation of the films grown on negatively biased substrates is, to some extent, epitaxial.7'8 Usually after a short time (~15 min) nucleation on negatively biased substrates, the films are further grown unbiased.7-8 This paper reports the synthesis and properties of microscopically smooth diamond films grown by hot filament CVD on Si(100) positively biased substrates by nucleation and growth under dc bias.
II. EXPERIMENTAL The diamond films were grown by hot filament CVD with a dc electric bias a
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