| B-doped Si1-xGex(001) layers, with concentrations CB to 1.7 x 1022 cm-3, were grown by gas-source molecular beam epitaxy (GS-MBE) from Si2H 6, Ge2H6 and B2H6 at temperatures Ts = 500--800°C. D2 temperature-programmed desorption (TPD) spectra were used to determine B coverages theta B as a function of CB and Ts. B surface segregation to the second-layer was observed with surface-to-bulk B concentration ratios to 1200. Saturation B coverage thetaB,sat was 0.5 ML.; TPD results from Si(001) layers were used to determine the B segregation enthalpy, --0.53 eV. Film deposition rates R increase by ≥50% with increasing CB >∼ 1 x 1019 cm-3 at Ts ≤ 550°C, due to increased H desorption rates from B-backbonded Si adatoms, and decrease by corresponding amounts at Ts ≥ 600°C due to decreased adsorption site densities.; B segregation enthalpy during Si0.82Ge0.18(001) growth decreases from --0.53 and --0.64 eV for Si(001) and Ge(001) GS-MBE, respectively, to --0.42 eV. The Ge concentration x and theta Ge are independent of B2H6 flux. Film growth rates RSiGe(CB), decrease by up to a factor of two with increasing CB. The above results were used to develop a robust model, to predict the steady-state H coverage thetaH, thetaB, theta Ge, RSi, and RSiGe as functions of impingement flux ratios and Ts.; CB in Si1-xGex(001) increases with precursor flux ratio and B is incorporated into active sites at concentrations to C*B (Ts) = 2.5 x 1020 and 4.6 x 1020 cm-3 for x = 0 and 0.18 at Ts = 600 and 500°C, respectively. At higher B concentrations, there is a large decrease in the activated fraction of incorporated B. The total acceptor concentration continues to increase. No B precipitates or misfit dislocations were detected by HR-XRD or TEM. The out-of-plane lattice constant a⊥ decreases linearly with increasing C*B and non-linearly, for CB > C*B . Electrical properties are in good agreement with theoretical values to C*B . When thetaB > thetaB,sat, B accumulates in the upper layer, and a parallel incorporation channel becomes available in which B is incorporated as B-pairs which are electrically inactive. |
