The Formation Mechanism and Suppression Methods of Dark Features in Optical Polycrystalline Diamond

Suppressing the formation of dark features during diamond growth has been a key research focus in the chemical vapor deposition (CVD) of high-quality optical polycrystalline diamond. In this study, the formation mechanism and suppression effect of dark features in diamond under different silicon wafer processing and etching conditions were analyzed. Characterization results from X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy indicated that the formation of dark features was primarily associated with the quality of crystals, uniformity of grain size, and whether the crystals exhibiting columnar growth. Reducing the silicon substrate scratching duration from 10 min to 1 min, combined with oxygen etching during growth, significantly reduced the concentration of dark features. The shortened scratching duration promoted uniform columnar diamond growth, therefore suppressing the formation of dark features. Meanwhile, the etching of amorphous carbon and graphite by oxygen during the growth process further suppressed the formation of dark features. As a result, by reducing the substrate scratching duration to 1 min and employing growth conditions with gas flow rates of 500 sccm H2 + 30 sccm CH4 + 2.5 sccm O2 at a temperature of approximately 970 °C, the area fraction of dark features on the surface of the diamond optical window was reduced from 4.44% to 0.22%. Its optical transmittance reached 70.8% at a wavelength of 10.6 μm with an absorption coefficient of 0.081 cm−1, and 66.6% at 1.064 μm with an absorption coefficient of 0.246 cm−1.