Single crystal diamond growth by MPCVD at subatmospheric pressures

Microwave plasma assisted chemical vapor deposition (MPCVD) is the established technique to produce high quality single crystal diamond (SCD). While typical pressures for SCD growth regimes in methane-hydrogen plasma are currently within 100–300 Torr, a transition to much high pressures promises enhanced growth rates. Here, we report on successful SCD synthesis by MPCVD in CH4-H2 gas mixtures at pressures up to 600 Tорр. A strong change of the plasma shape and volume (the latter shrinks by 10 times at fixed MW power) with pressure rise from 100 to 600 Torr was observed, still keeping the plasma stable. The record high absorbed MW power density of ≈1800 W/cm3 was achieved at 600 Torr. Optical emission spectroscopy (OES) was used for the plasma analysis via monitoring emission intensities of radicals Hα, C2 and CH. The gas temperature Tg determined from analysis of rotational fine structure of OES Swan transitions of dimer C2 (516 nm) turned out to be essentially constant ∼ 3100 ± 150 K over the pressure range explored. The diamond growth rate is found to increase by an order of magnitude with pressure to achieve 57 μm/h at 500 Torr at relatively low (4%) CH4 concentration, as measured in situ using low-coherence interferometry, but declined at further pressure increase. The produced films were characterized with SEM, XRD, Raman and photoluminescence spectroscopy, and a high/moderate quality of the obtained material was confirmed.