Crystalline-to-amorphous phase transformation in ion-irradiated GaAs

The crystalline-to-amorphous phase transformation in GaAs irradiated with B, C, O, Si, Ar, or Ge ions has been investigated by transmission electron microscopy and Rutherford backscattering with ion channeling. The critical relationships between ion flux and substrate temperature which define the threshold conditions for amorphization in GaAs by irradiation with various ions have been measured. The amorphous phase forms over a wide range of irradiation conditions by a collapselike nucleation process when a critical free energy is exceeded. The threshold conditions for nucleation are shown to be thermally activated, with a single activation energy of (formula presented) independent of ion species, energy, or fluence. However, despite the independence of the measured activation energy on ion species, the critical temperature for amorphization does not scale linearly with the rate of energy deposition or displacement density, indicating that details of the collision cascade which influence defect quenching, trapping, clustering and annihilation processes determine the pathway to amorphization. We speculate that this results from the difficulty of nucleating the amorphous phase at temperatures where defects are highly mobile, and that nucleation occurs locally at stable defect complexes which are formed more readily in the dense collision cascades created by heavier ions.