Dose-rate effects in GaAs investigated by discrete pulsed implantation using a focused ion beam

The dependence of the retained lattice damage upon dose rate was investigated by focused ion beam (FIB) implantation of 210 keV Si++ into GaAs at room temperature. The as-implanted and postannealed states were characterized by ion channeling and Hall-effect measurements, respectively. Dose-rate effects arise from stabilizing interactions between populations of defects produced by different ions, and these experiments were designed to probe the time constants of those interactions. In the context of dose-rate experiments, direct-write FIB represents a much more controllable means of implantation over conventional broad beams since the exact timing of dose delivery may be precisely defined and varied. In this work, the final implanted dose was achieved by the successive application of individual flux pulses of constant intensity but of varying duration t(d) and repetition period t(r). A consistent trend toward a greater concentration of displaced atoms directly after implantation and a higher sheet resistance after annealing was observed for longer t(d) and for shorter t(r). This effect did not manifest itself simply in terms of the average current density, J(avg)proportional to t(d)/t(r). Furthermore, it was observed on all time scales accessible in this experiment, suggesting that the important self-annealing mechanisms have a wide range of time constants, from less than 1 mu s to more than 1 s. A heterogeneous model of damage nucleation is discussed whereby the defect track of an individual ion event self-anneals until it is overlapped by a following event. (C) 1996 American Institute of Physics.