Study of the efficiency and reliability of GaN-based visible light emitting diode

This thesis reports the results of an extensive analysis on the study of efficiency and reliability of GaN-based visible light emitting diode. The devices under test are prototype, designed with the aim to analyze and understand the well-known weaknesses of this type of devices to provide the knowledge for future improvements. By means of specific experiments on test structure we have been able to analyze: • The low injection efficiency in complex heterostructure, featuring potential barrier, multi quantum well LEDs, and p-doping with Mg; • Role of indium in the optical performance and degradation process; • Impact of deep level in the layers of the active region on the electrical and optical performance of the device; • How the generation and relocation of defects plays a key role during the ageing process. In particular, we have demonstrated that: • The presence of potential barrier is responsible for the low injection efficiency of the devices because they prevent carrier to be injected inside the well limiting the recombination. T-CAD simulation allow to model the physics of the devices and visualize how carrier are distributed inside the structure. In particular, in multi quantum well LEDS, carriers are not uniformly distributed between the well, according to the distance of the well from the contact. Moreover, the low injection efficiency can also be ascribed to a non-optimized doping profile, since in the tested structure, the p-doping was realized with Mg that has a high ionization energy in GaN and AlGaN at room temperature. • The main degradation mechanism individuated in the test structure is the generation and relocation of defects in the active region of the devices. At this regard, we analyzed the impact that different trap location and concentration have on the optical performance concluding that, when trap are inside the QW, with an activation energy equal to midgap, they act as non-radiative recombination center (NRRC), increasing especially the Shockley Read Hall recombination rate. Detailed studies on SRH recombination in GaN-based LEDs highlight that, the recombination rate mainly depends on trap density and capture cross section, whereas the energy trap level is almost irrelevant. • Degradation has different trend according to the bias regime. In the low injection regime, it can mainly be ascribed to increment of defects that act as NRRCs. Regarding the electrical performance, it has been observed that the ageing process causes the generation of leakage conduction path, i.e decreasing shunt resistance, increment of trap assisted tunneling (TAT), that increase the current in the sub-turn on forward voltage regime and “steal” carrier to the radiative recombination. In the high current regime, different mechanisms can be pinpointed, such as the lowering of the injection efficiency, the increment of defects acting as NRRCs, increment of Auger recombination or the variation of the intrinsic properties of the material. Advanced characterization techniques, such as deep level optical spectroscopy (DLOS), have been implemented with the aim to identify the characteristics of the deep level inside the active region, in order to know their activation energy, and capture cross section. The results of the activity are described in the following chapters. This thesis reports the most relevant outcome obtained during the PhD program of the candidate. Useful information on the activity can also be found in the papers co-authored by the candidate