Study of the reliability of GaN-based optoelectronic devices: UV-LEDs and InGaN-based laser diodes

This thesis reports the main results obtained from the Ph.D. research activity of the candidate. The activity was focused on the study of the physical mechanisms responsible for the degradation of ultraviolet light emitting diodes (UV-LEDs) and InGaN-based laser diodes (LDs), in particular, we tried to understand the role of defects in the device degradation by means of advanced techniques such Deep-Level Transient Spectroscopy (DLTS) and Photocurrent (PC) Spectroscopy. The first part of the thesis will be dedicated to the work carried out on UV LEDs, in particular on four groups of UV-A LEDs, each of them with a different emission wavelength, and another group of (In)AlGaN-based UV-B LEDs. From this study, it was possible to demonstrate that UV LEDs submitted to constant current stress show a gradual degradation and this degradation has a strong dependence on the emission wavelength. The degradation process is ascribed to the generation of point defects within the active region of the device, causing an increase in the non-radiative recombination. Moreover, stress current level and temperature have a certain impact on the degradation. Regarding the UV-B LEDs, the analysis of the degradation of the optical power highlighted that degradation is related to an increase in the Shockley-Read-Hall (SRH) recombination. DLTS analysis and PC spectroscopy measurements allowed to identify a band of defects centered around 2.5 eV below the conduction band, close to the mid-gap, that can explain the increased SRH recombination. Moreover, DLTS measurements allowed to identify the signature of Mg-related acceptor traps. These UV-B LEDs present three parasitic bands ascribed to different processes. This section on the study of the degradation of AlGaN-based UV-B LEDs will be concluded with the investigation of the involved degradation mechanisms when the devices are submitted to a stress current level higher than the nominal value. The existence of two degradation mechanisms was demonstrated, and these have a significant impact on the relative amplitude of the main peak, related to the QW emission, and of the parasitic peak at ~340 nm, related to the electron overflow towards the last quantum barrier. The second part of thesis will be focused on the work carried out on a group of InGaN-based LDs with a nominal emission wavelength around 418 nm and another group with emission wavelength between 422-426 nm. The analysis carried out on the first group of samples has the aim to study the relation between the degradation and the presence of defects in these devices when submitted to constant current stress. We were able to show that constant current stress induces an increase in the threshold current of the devices, instead, from the capacitance-temperature measurements, it was possible to identify two main defects, which physical origin, studied by means of DLTS, could be related to acceptor-like defects, associated with dislocation. To conclude the study, the results obtained from the analysis carried out on three group of InGaN-based LEDs with different dislocation density, in order to have a better comprehension of the impact of dislocation on the degradation and generation of defects, will be presented. DLTS analysis was carried out before and after the stress and allowed to identify the presence of a hole trap in each sample, whose concentration increases with the dislocation density, together with an electron trap generated after the stress, probably related to the dislocation density. Moreover, devices submitted to constant current stress showed a significant decrease in their optical power, not related exclusively to the dislocation density