Physical basis of noncatastrophic degradation in GaAs injection lasers

A study was made of the gradual degradation in the output of GaAs injection lasers in the course of operation at 300°K and below. The degradation process was found to be a bulk, rather than a surface, effect during which the near-field emission patterns decrease in uniformity. Except in rare cases, there is no external evidence of mechanical damage to the devices. This has been confirmed by scanning electron microscopy observations of the facets before and after gradual degradation. Softening of the I-V characteristics occurs in addition to increases in the threshold current density and decreases in both the stimulated and the spontaneous exterior differential efficiency. No evidence was found for significant changes in either the junction impurity profile or the optical emission spectra. From a detailed analysis of the change in the threshold currents and the efficiency, it is concluded that the internal quantum efficiency is gradually reduced during laser operation. This decrease is primarily attributed to the formation of nonradiative recombination centers in the recombination region. In addition, the optical loss is increased in some lasers. An evaluation of the factors affecting the degradation rate indicates that it is a superlinear function of the current density of operation. Furthermore, the degradation rate is strongly influenced by the initial junction quality--lasers which initially exhibit highly nonuniform emission patterns degrade faster than those which are relatively uniform.