Ultrahigh B doping (<= 10(22) cm(-)3) during Si(001) gas-source molecular-beam epitaxy: B incorporation, electrical activation, and hole transport

Si(001) layers doped with B concentrations C-B between 1 X 10(17) and 1.2 x 10(22) cm(-3) (24 at %) were grown on Si(001)2 X 1 at temperatures T-s = 5 00- 850 degrees C by gas-source molecular-beam epitaxy from Si2H6, and B2H6 ,. Cg increases Linearly with the incident precursor flux ratio J(B2H6)/J(S i2H6) and B is incorporated into substitutional electrically active sites a t concentrations up to C-B*(T-s) which, for T-s = 600 degrees C, is 2.5 X 1 0(20) cm(-3). At higher B concentrations, C-B increases faster than J(B2H6) /J(Si2H6) and there is a large and discontinuous decrease in the activated fraction of incorporated B. However, the total activated B concentration co ntinues to increase and reaches a value of N-B = 1.3 X 10(21) cm-3 with C-B = 1.2 X 10(22) cm(-3). High-resolution x-ray diffraction (HR-XRD) and reci procal space mapping measurements show that all films, irrespective of C-B and T-s, are fully strained. No B precipitates or misfit dislocations were detected by HR-XRD or transmission electron microscopy. The lattice constan t in the film growth direction a(perpendicular to), decreases linearly with increasing C-B up to the limit of full electrical activation and continues to decrease, but nonlinearly, with C-B > C-B*. Room-temperature resistivit y and conductivity mobility values are in good agreement with theoretical v alues for B concentrations up to C-B = 2.5 X 10(21) and 2 X 10(21) cm(-3), respectively. All results can be explained on the basis of a model which ac counts for strong B surface segregation to the second-layer with a saturati on coverage theta(B,sat) Of 0.5 ML (corresponding to C-B = C-B*) At higher C-B (i.e., theta(B) > theta(B,sat)) B accumulates in the upper layer as sho wn by thermally programmed desorption measurements, and a parallel incorpor ation channel becomes available in which B is incorporated into substitutio nal sites as B pairs that are electrically inactive but have a low charge-s cattering cross section.