• bimodal distribution function;
  • focused microwave CVD;
  • linear antenna microwave CVD;
  • polycrystalline diamond films;
  • self-nucleation

In this work, the diamond nucleation is studied using two different deposition systems: (i) focused microwave plasma (labeled as “hot plasma”) and (ii) pulsed-linear antenna microwave plasma (“cold plasma”). For the hot plasma, the influence of CH4 concentration in the hydrogen rich gas mixture on the self nucleation is investigated. It is found that higher CH4 concentration leads to increased diamond nucleation density, but also the amount of sp2 carbon bonds increases within the deposits. In the cold plasma, we investigate the influence of CO2 addition into CH4/H2 and total process pressure. Experimental results reveal a strong dependence of CO2 and pressure on spontaneous nucleation. The nucleation density (Nd) typical for non-treated substrates (∼105 cm−2) is improved by four orders to 109 cm−2 in cold plasma thanks to increased CH4 and CO2 concentrations. The highest nucleation density in the hot plasma system is still one order lower (∼4 × 107 cm−2). Moreover, in cold plasma a bimodal distribution function of diamond clusters is observed for low ratios of CO2/H2. Observed differences in Nd values are attributed to plasma properties and surface chemistry. Based on the experimental results, possible mechanisms contributing to the spontaneous nucleation process are discussed.