Selective compositional mixing in GaAs/AlGaAs superlattice induced by low dose Si focused ion beam implantation

Abstract

The Al-Ga interdiffusion induced by Si focused ion beam implantation and subsequent rapid thermal annealing (RTA) was investigated in an Al0.3Ga0.7As/GaAs superlattice structure with equal 3.5 nm barrier and well widths. Si++ was accelerated to either 50 or 100 kV and implanted parallel to sample normal at doses ranging from 1013 to 1015/cm2. The effect of rapid thermal anneal of 10 s at 950 °C was characterized by the secondary ion mass spectrometry technique. In the implanted region, the interdiffusion causing compositional mixing was significantly enhanced by the Si implantation. An ion dose as low as 1×1014/cm2 results in a two-order of magnitude increase in the interdiffusion coefficient, to a value of 4.5×10−14 cm2/s, producing a mixing effectiveness of ∼90%. In contrast, the RTA-only case produces an interdiffusion coefficient of 1.3×10−16 cm2/s and very little mixing. A strong depth dependence of the mixing process was observed at 100 keV implantation energy, with a ‘‘pinch-off’’ (more heavily mixed) region being formed at a certain depth. It is noticed that the depth where this enhancement occurred is not associated with either the maximum concentration of Si ions or of vacancies. Instead, it coincides with the positive maximum of the second derivative of the vacancy profile, which in turn represents a maximum in the vacancy injection generated by the presence of a transient vacancy concentration gradient. Based on these findings, a theoretical model was developed using vacancy injection as responsible for mixing.