Observations concerning self-damage in GaAs injection lasers

Abstract

A study has been made of the gradual degradation in the output of GaAs lasers at 300°K. The process of catastrophic degradation, which involves mechanical facet damage, has been previously described. The lasers investigated were fabricated by Nelson using the solution regrowth method. Typically, the lasers were 10 mils long and 5 mils wide and were not provided with reflective coatings. We have found that the degradation process is 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 microscope 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 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 may be the result of the formation of recombination centers in the recombination region. These centers may be similar to those introduced by high-energy (1 MeV) electron irradiation. We find that an irradiation flux of1.5 \times 10^{16}electrons/cm²(resulting in the formation of an estimated density on the order of 10¹⁵defects/cm³) has a severe effect on laser performance, similar in many respects to that observed in gradual degradation: the emission pattern may become nonuniform, while the threshold current increases and the efficiency decreases. All this also occurs without any changes in the spectral energy distribution in the range investigated (1 to 1.5 eV) An evaluation of the factors affecting the gradual degradation rate indicates that it is primarily affected by the current density. The degradation rate is also influenced by the initial junction quality-lasers that initially exhibit highly nonuniform emission degrade faster than those that are relatively uniform. By limiting the device operation of a typical solution-grown laser to a current level between two and three times the threshold current, long-term (hundreds of hours) stable operation has been obtained at 300°K at repetition rates up to several kilohertz using a 100-ns pulse width.