Investigation in the role of hydrogen on the properties of diamond films grown using Ar/H2/CH4 microwave plasma

The transition of diamond grain sizes from micron- to nano- and then to ultranano-size could be observed when hydrogen concentration is being decreased in the Ar/CH4 plasma. When grown in H2-rich plasma (H2 = 99% or 50%), well faceted microcrystalline diamond (MCD) surface with grain sizes of less than 0.1 μm are observed. The surface structure of the diamond film changes to a cauliflower-like geometry with a grain size of around 20 nm for the films grown in 25% H2-plasma. In the Ar/CH4 plasma, ultrananocrystalline diamond (UNCD) films are produced with equi-axed geometry with a grain size of 5–10 nm. The H2-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature. The induction of the grain growth process, either by using H2-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries.

Research Highlights
► Transition of diamond grain sizes and surface structures from micron (well faceted)-to nano (cauliflower-like geometry) and then to ultranano (equi-axed geometry) - size is observed when hydrogen concentration is being decreased in Ar/CH4 plasma.
► H2-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature.
► Either by using H2-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries.