Growth Mechanism and Kinetics of Diamond in Liquid Gallium from Quantum Mechanics Molecular Dynamics Simulations

Abstract

Ruoff and co-workers recently demonstrated low-temperature(1193K) homoepitaxial diamond growth from liquid gallium solvent. To developan atomistic mechanism for diamond growth underlying this remarkabledemonstration, we carried out density functional theory-based moleculardynamics (DFT-MD) simulations to examine the mechanism of single-crystaldiamond growth on various low-index crystallographic diamond surfaces(100), (110), and (111) in liquid Ga with CH4. We findthat carbon linear chains form in liquid Ga and then react with thegrowing diamond surface, leading first to the formation of carbonrings on the surface and then initiation of diamond growth. Our simulationsfind faster growth on the (110) surface than on the (100) or (111)surfaces, suggesting the (110) surface as a plausible growth surfacein liquid Ga. For (110) surface growth, we predict the optimum growthtemperature to be & SIM;1300 K, arising from a balance between thekinetics of forming carbon chains dissolved in Ga and the stabilityof carbon rings on the growing surface. We find that the rate-determiningstep for diamond growth is dehydrogenation of the growing hydrogenated(110) surface of diamond. Inspired by the recent experimental studiesby Ruoff and co-workers demonstrating that Si accelerates diamondgrowth in Ga, we show that addition of Si into liquid Ga significantlyincreases the rate of dehydrogenating the growing surface. Extrapolatingfrom the DFT-MD predicted rates at 2800 to 3500 K, we predict thegrowth rate at the experimental growth temperature of 1193 K, leadingto rates in reasonable agreement with the experiment. These fundamentalmechanisms should provide guidance in optimizing low-temperature diamondgrowth.