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

Abstract

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 60μm/h in H2-CH4 mixtures and up to 105μm/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 220–420K is measured by a laser flash technique. High thermal conductivity k≈2300W/m⋅K at room temperature is found for the sample grown at low growth rate in H2-CH4, this value reducing to k≈2000W/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±300K) in H2-CH4 mixtures over the all methane concentration range explored, while for Ar-containing plasma the Ts is even higher (4100–4200K) at [CH4]<10%, reducing, however, below 3800K 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.