|Ph.D Student||Heiman Alexey|
|Subject||Mechanism of Formation and Properties of Nanocrystalline|
|Department||Department of Chemistry||Supervisor||Professor Alon Hoffman|
This research was focused on the study of the properties and formation mechanism of thin carbon layers deposited by direct current glow discharge (DC GD) CVD from a methane/hydrogen mixture under applied voltage of 500 Volts.
Such deposition leads to formation of a nanocrystalline diamond or graphitic phase, depending on the substrate temperature, deposition time, and gas composition. Nanocrystalline diamond attracts much attention in the scientific world due to its promising tribological, field emitting, and electrochemical properties.
Nanocrystalline diamond is deposited from a graphitic precursor at a temperature of 880 0C after deposition of 30 minutes with [CH4]=9%. The diamond content of the nano-diamond films reaches ~80% in the near-surface area. The offered mechanism of nano-diamond formation is stress-induced subsurface nano-graphite - nano-diamond phase transition. The main stress source is the subplantation process of the energetic ions under the reactive surface. While the stress is relaxed in the films deposited at higher and lower temperatures, leading to deposition of graphitic phase, no efficient stress relaxation mechanism exists in the 880 0C films and the intrinsic stresses are evaluated to reach 5 GPa. This enables graphite-diamond phase transformation.
Hydrogen was shown to play an indispensable role in the nano-diamond deposition. Its concentration increases to 19 at.% upon the nano-diamond formation. Hydrogen diffusion through the surface layers is critical for stabilization of the formed nano-diamond crystallites. Hydrogen saturation of the grain boundaries leads to thermodynamic stabilization of the diamond nano-clusters. Significant stresses in the nano-diamond films and stabilizing effect of the underlying diamond particles result in enhanced secondary nucleation of new nano-diamond particles.
The most interesting feature of the deposited nano-graphite is preferential vertical orientation of the basal planes in the films deposited at 800 0C and horizontal in the 950 0C films close to the substrate interface. This alignment is caused by hydrogen content of the films and temperature-induced mobility of the surface atoms. The following film evolution leads to preferential vertical alignment of the basal planes, indicating presence of large compressive stresses in the films.