A myriad of engineering applications involve contact between two surfaces, which induces localized plastic deformation near the surface asperities. As a generic problem in studying nanometer scale plastic deformation of solid surfaces, a unit process model of dislocation formation near a surface step under contact loading of a flat rigid surface is considered. The driving force on the dislocation is calculated using conservation integrals. The effect of surface adhesion, step size and lattice resistance on the dislocation driving force are analyzed in a continuum dislocation model, while the nucleation process is simulated atomistically. The driving force formula is used for a dislocation nucleation criterion and to get the equilibrium distance traveled by the dislocation away from the surface step. Results of the unit process model show that under a normal contact load dislocations nucleated in certain slip planes can only stay in a thin layer near the surface, while dislocations nucleated along other slip planes easily move away from the surface into the bulk material. The former dislocation is named anti-load dislocation and the latter dislocation is called pro-load dislocation. Embedded atom method (EAM) is utilized to perform the atomistic simulation of the unit-process model. As predicted by the continuum dislocation model, the atomistic simulations also indicate that surface adhesion plays significant role in dislocation nucleation process. Varying the surface adhesion leads to three different regimes of load-deflection instabilities, namely, just dislocation nucleation instability for no adhesive interaction, two distinct surface adhesion and dislocation nucleation instabilities for weak adhesive interaction and a simultaneous surface adhesion and dislocation nucleation instability for strong adhesive interaction. The atomistic simulations provide additional information on dislocation nucleation and growth near the surface steps. The results of dislocation segregation predict existence of a thin tensile-stress layer near the deformed surface and the results on the adhesion effect provides a cold-welding criterion.