Electronic transfer contribution in adsorption of silicon at SiC(0001) surface – density functional theory (DFT) study
Abstract
Physical properties of SiC(0001) surface under various coverage by Si adatoms was studied using density functional theory (DFT) calculations. The clean SiC(0001) surface has the Fermi level pinned by Si broken bond state located 0.8 eV below CBM. The most stable position of single Si atom adsorbed at SiC(0001) surface is H3 site bonded to three neighboring Si atoms, saturating their broken bonds. The adsorption energy is about 7.1 eV that leads to creation of stable surface reconstruction. For more atoms up to 0.25 ML coverage the energy gain is smaller by 0.4 eV. The stability of the adatoms at H3 position is related to electronic properties of the system so that Si accepts four electrons from Si broken bonds states. That creates energy effect of about 1.5 eV, thus the adsorption energy in this range is about 6.7 eV. According to EECR further increase exhausts Si broken bond electron source at about 0.25 ML coverage that lowers the adsorption energy down to about 5 eV. Further adsorption of Si atoms occupied H3 sites that have common Si top atoms with other Si adatoms that lower the adsorption energy gain to 4.2 eV at the coverage exceeding 0.4 ML in which H3 and on top configurations have roughly the same energy so the transition to SiC lattice position by subsequent Si adatoms is possible. The reduction of the adsorption energy leads to drastic decrease of the equilibrium pressure of silicon by 3 orders of magnitude in the region between 0.25 ML and 0.40 ML coverage. Generally it is expected that for typical conditions for SiC growth from the vapor, the coverage of SiC(0001) surface is close to 0.3 ML.