Reverse bias capacity effects and defect energy levels in electron irradiated gaasp+-njunctions

Abstract

Results are presented of capacity as a function of reverse bias for GaAs p ⁺ n solar cells before and after irradiation by fast electrons. The effect on capacity measurements of the presence of additional donor or acceptor defects on the n-type side is disccssed theoretically. It is shown that a variety of characteristic effects can occur depending on the position of the defect energy level in the band gap and the associated time constant for electron emission and capture. It is deduced that the defects introduced by irradiation with a predominant influence on the results were a uniform concentration of deep acceptors with levels in the upper half of the gap. Other defects with deep levels in the top half of the gap were present before irradiation. The time constants were greater than 3 × 10⁻³ secs. It is inferred that the levels of other acceptors known to be introduced by electron irradiation are shallow and in the top half of the gap, and those of donors in the lower half. More tentatively it is concluded that few, if any, donor defects were introduced by irradiation with levels in the top half of the gap, or acceptor defects with levels in the lower half. The pre-irradiation impurity and defect concentration was essentially both uniform and unaffected by the irradiation. It is also deduced that the formation of an abrupt p ⁺-i-n structure does not appear to be a mechanism affecting the results. After irradiation some recovery in the capacity values occurred after a long period at room temperature.