Grain size dependent physical and chemical properties of thick CVD diamond films for high energy density physics experiments

• Thick diamond films were grown by microwave-plasma assisted CVD.
• Grain size control was achieved by adjusting the CH4 content of the CH4/H2 feed gas.
• The density of the CVD diamond films decreases with decreasing grain size.
• Hydrogen and graphitic carbon impurities increase with decreasing grain size.
• Nanocrystalline films have a pronounced [110] oriented fiber texture.

We report on the grain size dependent morphological, physical and chemical properties of thick microwave-plasma assisted chemical vapor deposited (MPCVD) diamond films that are used as target materials for high energy density physics experiments at the Lawrence Livermore National Laboratory. Control over the grain size, ranging from several μm to a few nm, was achieved by adjusting the CH4 content of the CH4/H2 feed gas. The effect of grain size on surface roughness, morphology, texture, density, hydrogen and graphitic carbon content was systematically studied by a variety of techniques. For depositions performed at 35 to 45 mbar and 3000 W microwave power (power density ~ 10 W cm− 3), an abrupt transition from micro-crystalline diamond to nanocrystalline diamond was observed at 3% CH4. This transition is accompanied by a dramatic decrease in surface roughness, a six percent drop in density and an increasing content in hydrogen and graphitic carbon impurities. Guided by these results, layered nano-microhybrid diamond samples were prepared by periodically changing the growth conditions from nano- to microcrystalline.