A combined µ-Cathodoluminescence and µ-Photoluminescence Investigation of the Degradation of InGaN/GaN Laser Diodes

Despite the excellent potential of InGaN laser diodes (LDs), the reliability of these devices is still limited by a number of factors. The parametric degradation (i.e. the gradual increase in threshold current, Ith, induced by electrical stress) has been ascribed to an increase in non-radiative recoInbination within the active region [ToIniya2010, Meneghini2010], or to a decrease in the injection efficiency [Marona2008]; however, from the experiments presented so far in the literature (mostly based on EL measurements), it is difficult to distinguish between these two hypothesis, since both mechanisms produce siInilar effects on the EL characteristics of LDs. The aiIn of this paper is to contribute to the understanding of the degradation of InGaN LDs, by presenting the first study based on combined micro-CL and micro-PL measurements, carried out on untreated and aged devices. Results strongly suggest that degradation originates froIn an increase in non-radiative recoInbination within the QWs, and provide information on the geometry and si7:e of the degraded regions. The results of the stress tests indicate that: (i) when submitted to constant current stress, the LDs show a gradual increase in Ith; Ith increase has a power law dependence on time, which is consistent with previous literature studies, and suggests that degradation proceeds through a diffusion process; (ii) the Ith increase is well correlated to the decrease in sub-threshold emission, indicating that that degradation is related to an increase in the losses within the active region. To understand the physical origin of degradation, we removed the ridge Inetallization fronl a set of untreated and aged lasers, and we characteri7:ed the devices by means of f.LCL measurements (Ebeam=20 keY). Monochromatic CL maps (measured at 430 nm, i.e. at the QW peak) indicate that stress induces a significant decrease in the radiative efficiency of the LDs; this occurs only in the region close to the ridge, i.e. in the area where the current density (and heat dissipation) is maximum (>4 kA/cm2) during stress. From CL measurements it is difficult to understand if degradation occurs only in the barriers (where carriers are mostly generated with electron beaIn excitation), or also in the QWs. To give an answer to this question, we carried out resonant micro-PL mapping(380 nm excitation): also in this case results demonstrated the existence of a significant darkening of the region close to the ridge, thus confirming that stress induced a worsening of the radiative efficiency of the QWs (and not only in the barriers). In sUInInary, results strongly suggest that the degradation of the LDs is related to an increase in non-radiative recombination, rather than to a variation in injection efficiency, and occurs also in the QWs, and not only in the barriers: in fact, by It-PL analysis, we measured a significant darkening of the ridge region even with sub-bandgap excitation, i.e. by directly generating carriers only in the QWs.