Boron-doped diamonds were deposited by microwave plasma chemical vapor deposition (MPCVD) method in order to investigate the influence of inlet boron concentration on the film properties. The substrate material of the specimens was pure titanium (99.9 ). Boron source was introduced into the vacuum chamber by bubbling of B_2O_3, acetone and methanol mixture. Samples were produced with different B_2O_3 concentrations in mixture (1000 ppm, 5000 ppm, and 10000 ppm). The surface morphology of the samples was observed by scanning electron microscope (SEM). X-ray diffraction was used to identify crystal structures of the films. Secondary ion mass spectroscopy was used to examine the qualitative boron contents in the films. For low B_2O_3 concentrations in liquid mixture (1000 ppm), the surface morphology of the film showed both micro crystalline diamond and nano crystalline diamond. For medium B_2O_3 concentrations in liquid mixture (5000 ppm), the surface morphology of the film was also consisted of micro crystalline diamond and nano crystalline diamond. However, the content of micro crystalline diamond decreased in comparison with low B_2O_3 concentration. For high B_2O_3 concentration in liquid mixture (10000 ppm), the surface morphology of the film was almost dominated by nano crystalline diamond. Therefore, the crystal size of boron doped diamond decreased with increasing boron concentration. From these results, it appears that boron will restrain the growth of diamond crystal during deposition.
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