Ultra-highly doped Si1−xGex(001):B gas-source molecular-beam epitaxy: Boron surface segregation and its effect on film growth kinetics

Abstract

Si 1−x Ge x (001) layers doped with B concentrations CB between 2×1016 and 2×1021 cm−3 were grown on Si(001)2×1 at Ts=500–700 °C by gas-source 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 (θB and θGe) were determined as a function of CB using in situ isotopically tagged temperature-programmed desorption. Results for Si0.82Ge0.18 layers grown at Ts=500 °C show that θGe remains constant at 0.63 ML while the bulk B concentration increases linearly up to 4.6×1020 cm−3, corresponding to saturation coverage at θB,sat=0.5 ML, with the incident precursor flux ratio ξ=JB2H6/(JSi2H6+JGe2H6). B is incorporated into substitutional electrically active sites over this entire concentration range. At higher B concentrations, CB increases faster than ξ and there is a large decrease in the activated fraction of incorporated B. The B segregation enthalpy during Si0.82Ge0.18(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 rB=θB/xB, where xB is the bulk B fraction, range from 15 to 500 with a temperature dependence which is consistent with equilibrium segregation. Film deposition rates RSiGe(CB) decrease by up to a factor of 2 with increasing CB⩾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 θH, θB, θGe, and RSiGe as a function of ξ and Ts.