Kinetics of formation of the midgap donor EL2 in neutron irradiated GaAs materials

Fast neutron irradiation of n-GaAs mainly induces two deep electron traps in the band gap. The first of these is referred to as EL6 and has an energy level at Ec −0.35 eV, where Ec is the conduction band minimum; the second one has a wide energy distribution around Ec −0.5 V and is referred to as the U band. The annealing kinetics of these two levels is studied, and it is found that EL6 vanishes by a pair-defect (short-range) type recombination while the U band anneals by a long range migration process. Both annealing processes can be observed between 400 and 500 °C. In this annealing temperature range, the concentration of deep donor level EL2 (Ec −0.75 eV) increases with temperature. It is suggested that the defect giving rise to the EL2 level is created during irradiation but that medium range (tens to hundreds of Å) interactions with other neighboring defects strongly influence the electric properties of the overall defect, which is then detected as the U band. Due to long range migration, annealing reduces medium range interaction and the U band disappears in favor of the well-defined EL2 level (Ec −0.75 eV). This allows us to reconcile the apparent contradiction between DLTS (deep level transient spectroscopy) and EPR (electron paramagnetic resonance) and to identify the EL2 level with the EPR signal attributed to the AsGa defect.