Ab initiostudy of the initial growth mechanism of silicon nitride onSi(100)−(2×1)usingNH3

Abstract

Density functional theory (DFT) is used to examine the reaction mechanisms of nitridation of the $\mathrm{Si}\mathrm{}\left(100\right)\mathrm{}-(2\mathrm{}\times 1)$ surface by ${\mathrm{NH}}_3.$ The surface is modeled using the cluster approximation. A detailed reaction mechanism that includes ammonia adsorption and decomposition, insertion of nitrogen into Si-Si bonds, hydrogen diffusion on the surface and ${\mathrm{H}}_2$ desorption is investigated. We find that nitrogen-containing species prefers to be on the surface, bonded to the two surface Si atoms. The energy barriers are also calculated. We find that the activation barrier of the nitridation rate-limiting step is 0.43 eV higher than the activation barrier for ${\mathrm{NH}}_3$ desorption. This confirms the experimental observation that a large fraction of ${\mathrm{NH}}_3$ that dissociates upon adsorption recombines and desorbs. We also find the hydrogen desorption barrier from the nitrided silicon surface to be 3.34 eV. As the surface hydrogen concentration decreases, we find that the activation barriers of the nitridation reactions become smaller. Our calculations show that the dissociation barrier decreases from 2.65 to 1.95 eV when two surface hydrogen atoms are removed from the reactive silicon dimer. This result agrees with the experimental findings that further nitride growth does not occur until hydrogen desorption takes place. Therefore, we conclude that the rate-limiting step for the nitridation reaction is the hydrogen desorption. Further dissociation reactions leading to stable species such as bridge-bonded $\mathrm{NH}\mathrm{}\left(b\right)\mathrm{}$ and $\mathrm{N}\mathrm{}\left(b\right)\mathrm{}$ species are also studied. Vibrational frequencies of the surface species are calculated and found to agree with experiments. This confirms our prediction of the nature of the surface species and the reaction mechanisms leading to the initial growth of silicon nitride films on $\mathrm{Si}\mathrm{}\left(100\right)\mathrm{}-(2\mathrm{}\times 1).$