ICMCTF1998 Session D2: Synthesis and Characterization of Diamond and Related Materials

To grow highly oriented diamond film, the d.c. bias pre-treatment is crucial, since the initial orientation of the diamond nuclei controls the subsequent growth of the diamond film. It is well-known that an amorphous interlayer with silicon carbide particles imbedded predominantly formed between the substrate Si and the diamond nuclei during the bias stage. Thus the control and elimination of these interfacial layers are essential to the achievement of direct epitaxy of diamond on Si.

Samples prepared by MPECVD with various bias voltages and CH₄/H₂ ratios were investigated by HRTEM and SEM as well as Raman spectroscopy.

Experimental results show that colored rings, which were formed during bias stage on the Si substrate, correspond to amorphous layers of different thickness. Diamonds nucleate subsequently on these amorphous layers. The nucleation density is found to be a function of bias voltage with fixed bias time. Also, the amounts of interfacial silicon carbide show similar results. For higer CH₄/H₂ ratios and more negative bias voltage, the spatial distributions of the nuclei are more even and uniform morphologically. All Raman spectra show similar trends, but with broadened diamond and graphite peaks.

Detailed analysis of HRTEM results and microstructural evolution, etc. will be discussed in this paper.

It is common to divide hard materials into two groups: ultrahard with diamond, boron nitride and hard materials listed as SiC, TiC, ect. Between these two groups a few ternary phases have hardness overlapping this of cBN. They can also be attractive because of their wide band gap characteristics as well as protective coatings.

One material, B-C-Si has atomic structure of boron carbide with silicon atoms located in the carbon chains linking the boron icosahedra. Synthesis has been conducted by Rf plasma CVD and by melting the elements. The Vickers hardness was measured as 6340 kg mm ^-2 which compares with 5000 kg mm ^-2 usually assigned to cBN.

The second material composed of Si-N-C is an analog of Si₃N₄ structure. In the crystalline form it was synthesized in the nitrogen microwave plasma by etching both graphite and silicon. An amorphous form of silicon carbonitride exhibits Vickers microhardness of 6500 kg mm ^-2 and oxydation resistance up to 1600C in air. SiNC crystals were also created on Si substrate instead of a hypothetical C₃N₄.

The third material composed of Si-O-C is an analog to SiO ₂ and is known in the amorphous form. It can be formed during nucleation of CVD diamond on SiO ₂. We have discovered the formation of a new crystalline phase during attempts at the hydrothermal synthesis of diamond by hydrolysis of SiC.

We will report on synthesis of ternary phases which has been conducted by a microwave plasma ignited in the mixture of gases N ₂, O ₂, CH ₄, B ₂H ₆. Chemical etching of substrates or solids immersed in the plasma contributes to the synthesis of ternaries.

The mechanisms of nitrogen incorporation in diamond are still an unsolved riddle. This is mainly due to the complexity of the processes involved as they not only depend on empirical parameters (e.g. vessel pressure, substrate temperature, type and concentration of the N-containing compound, ) but also on the plasma chemistry and the surface chemical reactions.

In this study, small quantities (ppm range) of two nitrogen containing compounds (nitrogen and vaporised nitromethane) are added to a conventional hydrogen-methane feed gas mixture to study nitrogen incorporation in diamond films prepared by Microwave Plasma Assisted Chemical Vapour Deposition. Optical Emission Spectroscopy (OES) is used to survey the plasma composition during depositions. The intensities of the CN and C₂ emitting radicals and the Balmer atomic hydrogen lines are correlated to the Raman film quality and to the nitrogen content in the film studied by SIMS.