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

Si1-xGex(001) layers doped with B concentrations C-B between 2 x 10(16) and 2 x 10(21) cm(-3) were grown on Si(001)2 x 1 at T-s = 500-700 degreesC by gas-source molecular-beam epitaxy (GS-MBE) from Si2H6, Ge2H6, and B2H6. Sec ondary-ion mass spectrometry measurements of modulation-doped structures de monstrate that B doping has no effect on the Ge incorporation probability. Steady-state B and Ge surface coverages (theta (B) and theta (Ge)) were det ermined as a function of C-B using in situ isotopically tagged temperature- programmed desorption. Results for Si0.82Ge0.18 layers grown at T-s = 500 d egreesC show that theta (Ge) remains constant at 0.63 ML while the bulk B c oncentration increases linearly up to 4.6 x 10(20) cm(-3), corresponding to saturation coverage at theta (B,sat) = 0.5 ML, with the incident precursor flux ratio xi = J(B(2))H((6))/(J(Si(2))H((6)) + J(Ge(2))H((6))). B is inco rporated into substitutional electrically active sites over this entire con centration range. At higher B concentrations, C-B increases faster than xi 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, com pared to -0.53 and -0.64 eV during Si(001):B and Ge(001):B GS-MBE, respecti vely. Measured segregation ratios r(B) = theta (B)/x(B), where x(B) is the bulk B fraction, range from 15 to 500 with a temperature dependence which i s consistent with equilibrium segregation. Film deposition rates R-SiGe(C-B ) decrease by up to a factor of 2 with increasing C(B)greater than or equal to5 x 10(19) 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 theta (H), theta (B), thet a (Ge), and R-SiGe as a function of xi and T-s. (C) 2001 American Institute of Physics.