Ultra-highly doped Si_1-xGe_x(001):B gas-source molecular-beam epitaxy: Boron surface segregation and its effect on film growth kinetics

Si_1-xGe_x(001) layers doped with B concentrations C_B between 2 × 10¹⁶ and 2 × 10²¹ cm^-3 were grown on Si(001)2×1 at T_s = 500-700°C by gas-so urce molecular-beam epitaxy (GS-MBE) from Si₂H₆, Ge₂H₆, and B₂H₆. Secondary-ion mass spectrometry measurements of modulation-doped structures demonstrate that B doping has no effect on the Ge incorporation probability. Steady-state B and Ge surface coverages (θ_B and θ_Ge) were determined as a function of C_B using in situ isotopically tagged temperature-programmed desorption. Results for Si_0.82Ge_0.18 layers grown at T_s = 500°C show that θ_Ge, remains constant at 0.63 ML while the bulk B concentration increases linearly up to 4.6×10²⁰cm^-3 corresponding to saturation coverage at θ_B,sal = 0.5ML, with the incident precursor flux ratio ξ=_B2H6/(J_Si2H6 + J_Ge2H6). B is incorporated into substitutional electrically active sites over this entire concentration range. At higher B concentrations, C_B increases faster than ξ and there is a large decrease in the activated fraction of incorporated B. The B segregation enthalpy during Si_0.82Ge_0.18(001) growth is -0.42 eV, compared to -0.53 and -0.64 eV during Si(001):B and Ge(001):B GS-MBE, respectively. Measured segregation ratios r_B = θ_B/X_B, where X_B is the bulk B fraction, range from 15 to 500 with a temperature dependence which is consistent with equilibrium segregation. Film deposition rates R SiGe(C_B) decrease by up to a factor of 2 with increasing C_B ≥ 5 × 10¹⁹ cm^-3, due primarily to a B-segregation-induced decrease in the dangling bond density. The above results were used to develop a robust model for predicting the steady-state H coverage θ_H, θ_B, θ_Ge, and R_SiGe as a function of ξ and T_s.