Electronic charge transfer contribution in adsorption of silicon at the SiC(0001) surface—A density functional theory (DFT) study

Abstract

Properties of the SiC(0001) surface under various Si coverage were studied using density functional theory (DFT) calculations. It was shown that the clean SiC(0001) surface has the Fermi level pinned by the Si broken bond state, located 0.8eV below the conduction band minimum (CBM). The single Si atom is adsorbed in the H3 site, saturating broken bonds of the three neighboring Si atoms and that leads to the 23×23 reconstruction. The energy of Si atom adsorption at the clean SiC(0001) surface is equal to 7.1eV for single atom. It is reduced to approximately 6.7eV and 5.0eV, for the coverage below and above 0.25 monolayer (ML), respectively. The adsorption energy jump of about 1.7eV is due to electron transfer in which Si adatom states accommodate four electrons from Si broken bonds states. In addition adsorption of Si atoms at coverage exceeding 0.4ML leads to occupation of H3 sites that share Si surface top atoms with other Si adatoms which further reduces the adsorption energy to 4.2eV. The Si on-top position has the same energy which indicates on transition to this occupation for higher coverage. Accordingly, the vapor-surface equilibrium shows decrease of Si pressure by 3 orders of magnitude in the 0.25–0.40 Si ML coverage range. It is therefore expected that in typical SiC vapor growth, the Si coverage of the SiC(0001) surface is close to 0.3ML.