Effects of surface steps on oxygen and boron deposited on Si(1 0 0) studied quantum mechanically

Experimental evidence, gained from studies of high temperature deposition, indicates that both oxygen and boron interact strongly with steps on the Si(1 0 0) vicinal surface. This study describes these interactions using calculations with quantum mechanical detail. A large cluster, containing some hundreds atoms, represents the steps on Si(1 0 0). The impurity is deposited on this cluster in an adatom (oxygen), substitutional or interstitial (boron) location. A map of the total energy of the system, Et, is constructed by displacing the atom on the terraces and down the step. Et is evaluated quantum mechanically at semiempirical level and the focus of the calculations is on preferential binding sites and on their dependence on the type of impurity and on the step structure. In the case of oxygen, due to the attraction of the underlying silicon dimers, a barrier in the range 0.1–0.15eV is observed along paths on top of the dimer rows. This barrier prevents both intralayer coupling between the upper and lower terrace and diffusive motions on terraces. Boron impurities of both substitutional and interstitial type are stably attached to the steps by the formation of bonds considerably shorter than the normal interatomic distance in crystalline silicon. Due to the tensile strain so generated, the step reconstructs strongly. In spite of the strength of silicon–boron bonds, clusters containing up to five boron atoms do not show any tendency toward dissolution. These results are discussed in the light of available theories and experimental observations.