Ageing in ZnSe:Mn light emitting diodes

We report the fabrication of ZnSe light emitting diodes with useful lifetimes of at least 10⁴ hr, whereas early ZnSe Schottky diodes suffered from degradation after a few tens of hours of operation. This ageing took the form of an increase in series resistance, logarithmic with time for the initial 10³ hr, and later increasing as the square root of time. The change is consistent with the current-assisted growth of an insulating layer beneath the Schottky contact. The initial growth rate is probably controlled by oxidation proceeding preferentially at stress-relieving cracks in the layer, and the later rate is characteristic of oxidation limited by transport through the layer. Both processes can be inhibited: by adding a known insulating layer before deposition of the Schottky contact, to seal any cracks which may develop, and by using effective epoxy encapsulation.