Cs incorporation in semiconductors during low-energy bombardment: A dynamic computer simulation study

The implantation and retention of Cs atoms during low-energy irradiation of different materials (Si, Ge, and InP) was investigated by dynamic computer simulations using the Monte-Carlo code T-DYN that takes into account the gradual change of the target composition due to the Cs irradiation. The Cs incorporation was studied for four impact energies (0.2, 0.5, 1, and 3 keV) and for incidence angles ranging from 0° to 85°. For selected irradiation conditions, different values of the Cs surface binding energy UCs (0.4, 0.8, and 2.4 eV) were used. The total implantation fluences were chosen to lie well above the values required to reach a stationary state. The steady-state Cs surface concentration, cCs, exhibits a distinct dependence on impact energy and angle, and on UCs; it decreases with increasing incidence energy and angle. Furthermore, increasing UCs results in a pronounced increase of cCs. The computed values of cCs appear to be higher than corresponding data from experiment. Under equilibrium, the partial sputtering yields of the target, YX, depend only weakly on UCs, but vary with the Cs energy and angle. YX exhibits the usual dependence on incidence angle, first increasing up to a maximum value (at ∼65°-75°) and declining sharply for larger angles. For all bombardment conditions a clear preferential sputtering of Cs atoms as compared to the target atoms is found; typically, preferential sputtering of Cs increases with decreasing irradiation energy and incidence angle, and with decreasing UCs.