Heteroepitaxial Growth of Diamond by Chemical Vapor Deposition on Platinum Thin Films

1. Background

@Diamond semiconductor is an ultimate solid state device material for high frequency modulation and high power handling due to its high breakdown field (1 x 10⁷ V/cm), high saturation velocity (2 x 10⁷ cm/s), low dielectric constant (5.7), and high thermal conductivity (20 W/cm-K at room temperature). Growth of single crystal diamond films thus has been an important research subject since thin films of this material were successfully obtained by chemical vapor deposition (CVD).
@Kobe Steel, Ltd., in collaboration with Japan Fine Ceramics Center, is working on development of diamond growth aiming to establish technologies to produce large-area single crystal diamond, in Frontier Carbon Technology Project for which a central research laboratory is located at National Institute of Materials and Chemical Research.
@We chose platinum (Pt) as a suitable substrate material for heteroepitaxial growth of diamond. The heteroepitaxial diamond films on Pt(�V) are featured with spontaneously coalesced smooth (�V) surface and excellent in-plane azimuthal alignment of the crystals. Large-sized bulk single crystal Pt, however, is expensive and commercially unavailable. We have been accordingly seeking for suitable substrate to scale up this technology, and found that Pt can be epitaxially formed on sapphire. Because large-sized sapphire substrates are commercially available up to several inches in diameter, Pt films epitaxially deposited on sapphire can be an ultimate solution for large-area single crystal diamond films which could be compatible to advanced semiconductor device fabrication technology.

2. Outline of the results

@The substrate was prepared by depositing platinum by sputtering at 600�� on mirror-polished sapphire(0001) surface. It was found that the full-width at half-maximum (FWHM) values of the Pt(�V) X-ray diffraction rocking curve are approximately 0.16��. On the Pt (111)/sapphire(0001) substrates diamond films were grown at 875�� from a gas mixture of CH4 and H₂ by microwave plasma CVD. (The structure of the sample is illustrated in Figure 1.)
@Figure 2 displays typical surface morphology of the diamond films. Good portion of the film surface consists of (�V)-oriented diamond crystals which have intensively coalesced together with indiscernible grain boundaries between the facets. The FWHM value of the rocking curve is 2.0�� for diamond(�V).

3. Future prospects

@Future goals will be to eliminate the grain boundaries in the diamond films by improving the growth process. As large-sized CVD system and growth process conditions are being developed in the present project, it would be possible to produce single crystal diamond wafers in the future. That will open a path toward application of diamond for various electronic devices including ultraviolet light emitting diodes and high-power high-frequency transistors.

Chart 1 Industrial Use of Diamond Films

Description of the heteroepitaxially grown diamond film on platinum film vapordeposited on sapphire substrate

Figure l: cross section structure of the sample

Figure 2: surface photograph of heteroepitaxially grown diamond