Compositional variation of microstructure in ion-implanted AlxGa1-xAs

The ion damage produced in alloys of AlxGa1-xAs (x = 0.6, 0.7, 0.8, and 0.8 5) by implantation at 77 K with Kr ions (500, 700, and 1500 keV) was studie d by using Rutherford backscattering channeling and transmission electron m icroscopy, In addition, the accumulation of ion damage at 50 K was studied by performing the ion implantations in situ in the transmission electron mi croscope. In Al0.8Ga0.2As, damage accumulation at 77 K was independent of d ose rate, indicating that dynamic annealing is not occurring at 77 K, The i n situ studies demonstrated that planar defects are produced on warm-up fro m 50 K to room temperature, indicating that they are not the nucleation sit e for amorphization. The lower energy implantations revealed that amorphiza tion initiated within the AlxGa1-xAs layer, showing that heterointerfaces a re not required for amorphization. These results, along with the similarity of the room-temperature microstructures in the different alloys, imply tha t the amorphization mechanism is independent of Al content. It is proposed that the observed dependence of the amorphization dose on Al content is rel ated to an increase in the number of cascade overlaps needed to initiate an d to produce a continuous amorphous layer. A mechanism explaining the micro structural changes with composition, based on the thermal and physical prop erties of the alloy and on the distribution of energetic cascade events, is presented.