Evidence of Gas Phase Nucleation of Nano Crystalline Diamond Through the Analysis of Activation Energy

The mechanism of ballas like nano crystalline diamond (NCD) formation still remains elusive, and this work attempts to analyze it in the framework of gaseous chemistry. NCD thin films were grown from H₂/CH₄ plasma in a 2.45 GHz chemical vapor deposition system. While the activation energy values (E_a) ∼10 kcal/mol calculated from the Arrhenius equation using substrate temperature (∼1000−1300K) match with the E_a for NCD formation throughout the literature, they are lower compared with 13.6−22.5 kcal/mol for standard single crystal diamond (SCD) formation, concluding thus far, that the energetics and processes involved in the formation of NCD and SCD are different.

To investigate this, we modified the substrate preparation and sample collection method while keeping the growth parameters constant: un-seeded Si substrates were physically separated from the plasma by a metal stub with a pinhole through which sample was collected. Electron microscopy and Raman spectroscopy of the collected sample found that NCD self nucleates in the plasma and flows to the substrate which acts as a mere collection plate meaning that the location of formation of NCD is in the gas phase. This warrants a revisit to the calculation of E_a where we propose to substitute substrate temperature with gas temperature (∼2000K) that we obtained by simulating a basic model of H₂/CH₄ plasma for our reactor geometry. The new calculations give values in the range of SCD formation suggesting that the formation processes for NCD and SCD could be the same.