Bulk degradation of GaP red LEDs

The external quantum efficiency of GaP red light-emitting diodes (LEDs) degrades under forward-biased operation. The degradation is a complex phenomenon which in the long run is dominated by changes in the bulk semiconductor. This paper is restricted to the discussion of bulk degradation. The symptoms of the degradation are: 1) for a given forward current, the activation energy of the degradation is 0.5-0.8 eV and 2) for a fixed forward voltage, the light output is virtually unchanged while the forward current increases. The nonradiative excess current responsible for the degradation is probably a tunneling current. The introduction of metallic impurities such as copper into the semiconductor material does not appreciably change the initial quantum efficiency of the device. On the other hand, it shortens the lifetime of the device by several orders of magnitude under forward-biased operation. The symptoms of the degradation for the copper-contaminated devices are identical to those of the noncontaminated devices including the activation energy of the degradation. It is inferred therefore that fast moving impurities such as copper are the major cause of the bulk degradation. Liquid gallium is a known getter for metallic impurities such as copper. For junctions grown in the liquid phase epitaxy (LPE) process using high-purity gallium, the longer life can be attained by the careful elimination of metallic impurities during material growth and device processing.