Ultrahigh B doping during Si(001) gas-source molecular-beam epitaxy: B incorporation, electrical activation, and hole transport

Si(001) layers doped with B concentrations (Formula presented) between (Formula presented) and (Formula presented) (24 at %) were grown on (Formula presented) at temperatures (Formula presented) by gas-source molecular-beam epitaxy from (Formula presented) and (Formula presented) increases linearly with the incident precursor flux ratio (Formula presented) and B is incorporated into substitutional electrically active sites at concentrations up to (Formula presented) which, for (Formula presented) is (Formula presented) At higher B concentrations, (Formula presented) increases faster than (Formula presented) and there is a large and discontinuous decrease in the activated fraction of incorporated B. However, the total activated B concentration continues to increase and reaches a value of (Formula presented) with (Formula presented) High-resolution x-ray diffraction (HR-XRD) and reciprocal space mapping measurements show that all films, irrespective of (Formula presented) and (Formula presented) are fully strained. No B precipitates or misfit dislocations were detected by HR-XRD or transmission electron microscopy. The lattice constant in the film growth direction (Formula presented) decreases linearly with increasing (Formula presented) up to the limit of full electrical activation and continues to decrease, but nonlinearly, with (Formula presented) Room-temperature resistivity and conductivity mobility values are in good agreement with theoretical values for B concentrations up to (Formula presented) and (Formula presented) respectively. All results can be explained on the basis of a model which accounts for strong B surface segregation to the second-layer with a saturation coverage (Formula presented) of 0.5 ML (corresponding to (Formula presented) At higher (Formula presented) (i.e., (Formula presented) B accumulates in the upper layer as shown by thermally programmed desorption measurements, and a parallel incorporation channel becomes available in which B is incorporated into substitutional sites as B pairs that are electrically inactive but have a low charge-scattering cross section.