Hydrogen incorporation in CVD diamond films

Concentration of bonded hydrogen (C-H) in polycrystalline diamond films grown in microwave plasma, DC plasma and hot filament CVD reactors using CH4/H-2 gas mixtures has been evaluated using IR spectroscopy. H content was found to be in the range of 30 ppm (optical quality thick films) to 20000 ppm (thin nanocrystalline films). As a consequence of decoration of defects and grain boundaries by hydrogen two effects were observed: (i) a decreases of thermal conductivity with concentration of incorporated hydrogen, (ii) a correlation of hydrogen and nitrogen impurities concentration in the diamond. High temperature annealing in vacuum results in breaking the C-H bonds followed by re-arrangement of defected regions and grain boundaries to graphitic (sp(2) - bonded) structures, that strongly increases the optical absorption of the material. It is observed that a H+ ion implantation followed by annealing causes a formation of buried graphitized islands and even blisters rather than uniform continuous layer. Porous diamond layers prepared by oxidation of nanocrystalline films are shown to accumulate up to 20 at% hydrogen upon storage in ambient atmosphere.