Silicon carbon(001) gas-source molecular beam epitaxy from methyl silane and silicon hydride: The effects of carbon incorporation and surface segregation on growth kinetics

Si_1−yC_y alloys were grown on Si(001) by gas-source molecular-beam epitaxy (GS-MBE) from Si₂H₆/CH₃ SiH₃ mixtures as a function of C concentration y (0 to 2.6 at %) and deposition temperature T_s (500–600°C). High-resolution x-ray diffraction reciprocal lattice maps show that all layers are in tension and fully coherent with their substrates. Film growth rates R decrease with both y and T_s, and the rate of decrease in R as a function of y increases rapidly with T_s. In-situ isotopically-tagged D₂ temperature-programmed desorption (TPD) measurements reveal that C segregates to the second-layer during steady-state Si_1−y C_y(001) growth. This, in turn, results in charge-transfer from Si surface dangling bonds to second-layer C atoms, which have a higher electronegativity than Si. From the TPD results, we obtain the coverage &thgr; _Si*(y, T_s) of Si* surface sites with C backbonds as well as H₂ desorption energies E_d from both Si and Si* surface sites. This leads to an increase in the H₂ desorption rate, and hence should yield higher film deposition rates, with increasing y and/or T_s during Si_1−yC_y(001) growth. The effect, however, is more than offset by the decrease in Si₂H ₆ reactive sticking probabilities at Si* surface sites. Film growth rates R(T_s, $JSi2H6,JCH3SiH3$) calculated using a simple transition-state kinetic model, together with measured kinetic parameters, were found to be in good agreement with the experimental data.; At higher growth temperature (725 and 750°C), superlattice structures consisting of alternating Si-rich and C-rich sublayers form spontaneously during the gas-source molecular beam epitaxial growth of Si_1−y C_y layers from constant Si₂H₆ and CH ₃SiH₃ precursor fluxes. The formation of a self-organized superstructure is due to a complex interaction among competing surface reactions. During growth of the initial Si-rich sublayer, C strongly segregates to the second layer resulting in charge transfer from surface Si atom dangling bonds of to C backbonds. This, in turn, decreases the Si₂H₆ sticking probability and, hence, the sublayer deposition rate. This continues until a critical C coverage is reached allowing the nucleation and growth of a C-rich sublayer until the excess C is depleted. At this point, the self-organized bilayer process repeats itself.