Effects of substrate pretreatment and methane fraction on the optical transparency of nanocrystalline diamond thin films
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Effects of substrate pretreatment and methane fraction on the optical transparency of nanocrystalline diamond thin films D. M. Bhusari Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
J. R. Yang Department of Mechanical Engineering, National Taiwan Institute of Technology, Taipei, Taiwan
T. Y. Wang Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
K. H. Chen Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
S. T. Lin Department of Mechanical Engineering, National Taiwan Institute of Technology, Taipei, Taiwan
L. C. Chen Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan (Received 29 August 1997; accepted 19 February 1998)
Optical transmittance of the nanocrystalline diamond films has been studied as a function of grain size of the diamond powder used for substrate pretreatment and the methane fraction in the source gas. It has been observed that for CH4 fractions below 13%, the films grown on substrates pretreated with finer diamond powder are more transparent, while this trend reverses for CH4 fractions above 13%. These variations in the transparency of the films correlate very well with their corresponding surface roughness. Nanocrystalline/amorphous diamond films with transmittance of greater than 80% beyond ˚ have been obtained for CH4 700 nm and with average surface roughness as low as 61 A fractions as high as 42% in the source gas. Interestingly, these films do not show an obvious presence of any graphitic carbon, and the structural ordering of the amorphous sp 3 -bonded phase also seems to be insensitive to the CH4 content of the source gas.
I. INTRODUCTION
Highly transparent diamond films with smooth surfaces are extremely desirable for applications as optical coatings and wear-resistant coatings. Although the present best quality diamond films deposited under optimized growth conditions possess high electronic and structural quality,1,2 these films lack the desired optical transparency and also exhibit high surface roughness, due to their polycrystalline nature with large grain sizes. Such a rough surface causes excessive light scattering and also produces unacceptably high friction. On the other hand, attempts to compromise the grain size, in order to achieve a smooth surface, inadvertently result in a much higher content of sp 2 -bonded carbon in the films, and the ensuing opacity and softness precludes their aforementioned applications. Nevertheless, the recent development of nanocrystalline diamond films3–8 offers a very promising alternative to polycrystalline diamond films, since these films exhibit high surface smoothness as well as high transmittance (Tr) in the near-ir region. Growth of nanocrystalline diamond films with Tr greater than 80% beyond 700 nm and an av˚ has erage surface roughness (Ra ) as low as 100 A J. Mater. Res., Vol. 13, No. 7, Jul 1998
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