High-rate growth of single crystal diamond in microwave plasma in CH4/H2 and CH4/H2/Ar gas mixtures in presence of intensive soot formation

• Single crystal (SC) diamond films are grown in a microwave plasma using H2-CH4 and H2-CH4-Ar gas mixtures in a broad range of methane concentration (2–15%) at the growth rates up to 105 μm/h without nitrogen addition. Optical absorption, Raman and photoluminescence spectroscopy confirm the high quality of the produced material.
• Spatial distribution of gas temperature Ts in the plasma as well as of Hα and C2 line intensities are determined with optical emission spectroscopy (OES), the temperature Ts profiling being obtained from the shape of continuous component of the OE spectra.
• The thermal conductivity as high as k ≈ 2300 W/mK at room temperature is measured by a laser flash technique for low growth rate samples and ≈ 2000 W/mK for high growth rate (≈ 60 μm/h) material.
• The soot is formed at high methane contents and/or Ar gas addition in the reaction mixture with the rate roughly proportional to CH4 percentage. The mass yield for the soot can be an order of magnitude higher than that for diamond.

The growth of single crystal diamond plates in a microwave plasma using H2-CH4 and H2-CH4-Ar gas mixtures in a broad range of methane concentration (2–15%) is studied. The growth rates up to 60 μm/h in H2-CH4 mixtures and up to 105 μm/h in Ar-H2-CH4 mixtures are achieved at high CH4 content, without adding nitrogen, still obtaining transparent crystals. The thermal conductivity k of the SCs in the temperature range of 220–420 K is measured by a laser flash technique. High thermal conductivity k ≈ 2300 W/m ⋅ K at room temperature is found for the sample grown at low growth rate in H2-CH4, this value reducing to k ≈ 2000 W/mK for the material produced in high rate regime at 15% CH4. The spatial profiles of Hα and C2 line intensities in the plasma were determined with optical emission spectroscopy (OES). Soot formation at high CH4 contents is observed at the plasma border both for Ar-free and Ar-containing mixtures, the soot yield being roughly proportional to diamond growth rate. The soot temperature Ts, as measured with OES, is almost constant (3800 ± 300 K) in H2-CH4 mixtures over the all methane concentration range explored, while for Ar-containing plasma the Ts is even higher (4100–4200 K) at [CH4] < 10%, reducing, however, below 3800 K in CH4-reach mixtures. Raman spectra of collected soot correspond to crystalline graphite with high density of defects. The estimated carbon conversion efficiency from CH4 precursor to the soot can exceed 10% and should be taken into account in overall carbon balance in the CVD process.

Figure optionsDownload as PowerPoint slide