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With this paper we describe an extensive analysis of the reliability of InGaN-based laser diodes, emitting at 405 nm. These devices have excellent characteristics for application in the next-generation optical data storage systems. The analysis aims at describing the degradation process, as well as at investigating the role of current in determining the degradation rate. The results obtained within this paper suggest that the degradation of the laser diodes is correlated to the increase in the non-radiative recombination rate, with subsequent worsening of the optical properties of the devices. Furthermore, our findings support the hypothesis that current is the main driving force for degradation, while temperature and optical power play only a limited role in determining the degradation kinetics.

The gradual increase in threshold current in InGaN-based blue laser diodes (LDs) submitted to stress tests is generally attributed to diffusion-related phenomena. In fact, diffusion into active layers can increase the concentration of non-radiative recombination centers (NRCs). However, this hypothesis has not been verified yet for InGaN LDs. In this paper, we analyze the variation of the non-radiative recombination lifetime τ nr in LDs submitted to stress tests: the analysis of τ nr provides information on the density evolution of NRCs. Furthermore, we propose a novel model for the investigation of the degradation kinetics. This model assumes that the depth distribution of NRCs obeys to Fick's diffusion equation. The obtained results suggest that the increase in 1/τ nr can be correlated to a diffusion process. For the first time, we have estimated the value of the diffusion coefficient that is related to degradation, which is equal to 1.9×10−19 cm2/sec. This value is very large if compared with the one obtained in bulk material. Therefore, we suggest that the diffusion process is driven by the presence of dislocations. The presented method for degradation analysis constitutes a powerful tool for the study of the degradation in InGaN-based LDs.

This paper presents an analysis of the role of current in the long-term degradation of InGaN-based laser diodes (LDs). The analysis has been carried out by means of a wide set of stress tests under different driving conditions. During stress, the optical and electrical characteristics of the samples have been continuously monitored, with the aim of determining the physical mechanisms responsible for degradation and the dependence of the degradation rate on the driving conditions. The results reported in this paper demonstrate that: (i) constant current stress determines the increase of the threshold current of the devices, that is well related to the decrease of the sub-threshold emission; (ii) stress induces the increase of the reverse current of the devices, but does not affect their forward-bias characteristics; (iii) degradation rate has an almost linear dependence on stress current level; (iv) degradation takes place also below lasing threshold and for very low stress current levels, i.e. with very limited optical field levels. The results described in this paper indicate that the degradation kinetics are mostly determined by the stress current level used during operation: optical field and temperature are thought to play only a minor role in determining the degradation rate. Furthermore, the strong correlation between the threshold current increase and the sub-threshold emission decrease suggests that degradation can be ascribed to the increase of the non-radiative recombination rate in the active region of the devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This letter describes an analysis of the degradation of InGaN-based laser diodes. The influence of current, temperature, and optical power level on the degradation kinetics has been analyzed by means of a wide set of stress tests carried out under different operating conditions. We demonstrate the following: 1.) the degradation rate is strongly related to the operating current level; 2.) high-temperature stress does not determine significant degradation of lasers characteristics; and 3.) the intensity of the optical field does not significantly influence the degradation rate. Degradation process is found to be electrothermally activated and is ascribed to the increase of the nonradiative recombination rate in the active layer, with subsequent decrease of the efficiency of the devices.