Effects of B doping on hydrogen desorption from Si(001) during gas-source molecular-beam epitaxy from Si₂H₆ and B₂H₆

Boron doping concentrations ≳6×10¹⁹ cm^-3 were found to increase Si(001) growth rates R_Si at low temperatures while decreasing R_Si at higher temperatures during gas-source molecular beam epitaxy (GS-MBE) from Si₂H₆ and B₂H₆. In order to probe the mechanisms governing these effects, Si(001) samples with B coverages θ_B ranging from <0.05 to ≃0.5 ML were prepared by exposing clean Si(001)2×1 wafers to B₂H₆ doses between 2×10¹⁷ and 4×10²⁰ cm^-20 at 200-400°C. The samples were then heated to 700°C to desorb the hydrogen, cooled to 200°C, and exposed to atomic deuterium until saturation coverage. D₂ temperature programmed desorption spectra exhibit β₂ and β₁ peaks due to dideuteride and monodeuteride desorption at 405 and 515°C as well as new B-induced peaks, β*₂ and β*₁, at 330 and 470°C. Increasing θ_B increases the area under β*₂ and β*₁ at the expense of β₂ and β₁. Moreover, the total D coverage continuously decreases from ≃1.23 ML in the absence of B to ≃0.74 ML al θ_B=0.5 ML. We propose that the observed B-induced decrease in the Si*-D bond strength, where Si* represents surface Si atoms bonded to second-layer B atoms, is due to charge transfer and increased Si* dimer strain. The Si* to B charge transfer also deactivates Si surface dangling bonds causing the decrease in θ_D. These results are used to explain the GS-MBE growth kinetics.