Quality of diamond wafers grown by microwave plasma CVD: effects of gas flow rate

We found a strong impact of gas flow rate on diamond growth process in a 5 kW microwave plasma chemical vapour deposition reactor operated on CH4-H-2 gas mixtures. Diamond films of 0.1-1.2 mm thickness and 2.25 in. in diameter were produced at H-2 flow rates varied systematically from 60 seem to 1000 seem at 2.5% CH4. The highest growth rate, 5 mu m h(-1), was observed at intermediate F values (approximate to 300 sccm). Carbon conversion coefficient (the number of C atoms going from gas to diamond) increases monotonically up to 57% with flow rate decrease, however, this is accompanied with a degradation of diamond quality revealed from Raman spectra, thermal properties and surface morphology. High flow rates were necessary to produce uniform films with thermal conductivity >18 W cm(-1) K-1. Diamond disks with very low optical absorption (loss tangent tg delta <10(-5)) in millimetre wave range (170 GHz) have been grown at optimized deposition conditions for use as windows for high-power gyrotrons. (C) 1999 Elsevier Science S.A. All rights reserved.