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The intention of this work was to identify the physical reasons for the different behavior of green laser diodes compared to blue devices. Therefore the electro-optical properties of blue and green InGaN-based laser diodes were analyzed, based on a comparison of pulsed L-I-characteristics. In doing so, a deviation of the behavior of a perfect semiconductor laser diode was observed for the green laser. Although no explicit temperature dependence of the internal losses and the injection efficiency (ηinj) was observed in the range of 10-90°C, a variation, concerning the current dependence of the injection efficiency, was revealed. The injection efficiency of blue laser diodes can be taken as constant whereas ηinj decreases as a function of current for the green device. This reduction was correlated with an insufficient capture of carriers into the quantum wells.
In the following the question whether the decreasing injection efficiency was caused by overflow of electrons or holes was examined by the use of test structures.
By varying the aluminum content of the electron blocking layer (EBL) the insufficient capture of electrons by the green quantum wells was proven. Although the quality of the EBL has an enormous influence on the absolute values of the injection efficiency, there was no impact on the relative decrease of ηinj as a function of current. Instead of that a current dependent overshoot of holes was observed by the use of test structures with InGaN-detection layers on the n-side.
To analyze the influence of the current dependent injection efficiency on laser threshold, especially with regard to temperature dependent increase, the key parameters were quantified in the course of this work. These values were combined with an empirical model for laser threshold. The optical gain was analyzed by Hakki-Paoli-measurements for varying case temperatures and operation currents.
However, for the derivation of laser threshold condition the optical gain is required as a function of carrier density. The recombination coefficients that are necessary for the translation of current to carrier density were determined for case temperatures from 25 to 80°C. The experimental data of the Hakki-Paoli-analysis was used to determine the physical parameters of a linear gain model especially with regard to temperature dependence of transparency carrier density and differential gain. Based on this set of parameters, the determining factors for the temperature dependent increase of laser threshold was evaluated using a linear gain model. This way the carrier lifetime which is dominated by Auger-recombination processes in the corresponding operation regime was identified as the root cause for the temperature dependent increase of laser threshold.
In order to investigate the long-term stability of the injection efficiency, the acceleration factors of the degradation of green laser diodes were determined primarily. It was shown that the device degradation is electro-thermally activated and therefore comparable to the degradation behavior of Blu-Ray laser diodes. Current was identified to be the root cause for degradation of green laser diodes. For the corresponding analysis, a green laser diode was alternating operated in two different operation conditions that were chosen to keep the temperature of the active region constant throughout the experiment. In electrical operation, the threshold current of green laser diodes show a square-root like increase which is known from blue laser diodes. However, the slope efficiency and therefore also the injection efficiency above threshold show no decrease in degradation.
Optical gain was analyzed before and after degradation by Hakki-Paoli-measurements, but no change could be detected. However, it was shown that the carrier density within the quantum wells decreases during operation. Based on the recombination coefficients for green laser diodes that were determined within the framework of this thesis, it was estimated that the rate of defect-assisted recombination processes would have to increase by a factor of three in order to explain the experimentally observed 20 per cent increase of laser threshold. This unreasonable hypothesis was excluded by comparing the EL-behavior of a green laser diode below threshold before and after degradation. As an outcome of the investigations done on carrier transport, the existence of non-negligible carrier densities outside the green quantum wells is known. Therefore the degradation of green laser diodes is not necessarily limited to the quantum wells, but can also take place within the wave-guiding layers.