This paper investigates the factors that influence the efficiency and reliability of InGaN based visible light emitting diodes with non-optimized carrier injection. The devices under test are LEDs with a single quantum well with a nominal emission wavelength of 495 nm at I = 100 mA. We stressed the devices with a constant current density of J = 80 A/cm2, at room temperature, for 25000 minutes. We monitored the optical performance of the devices before and during stress. From the preliminary characterization we observed an increment in the optical power followed by a blue shift as a function of current. Simulation results highlight an asymmetric carrier injection, in particular a lack of electrons in the low bias regime. The low injection efficiency is also confirmed by temperature-dependent measurements, where we observed an increment in the OP with increasing temperature. During an ageing experiment, we observed an increment in OP for high injection level, accompanied with a blue shift in the peak emission wavelength. This result suggests an improvement in the injection efficiency and or a better carrier confinement. In this regard, we performed photoluminescence measurements during stress, which confirm the hypothesis of a better carrier confinement. In particular, PL signal show an increasing trend during the ageing process, which can be ascribed to the generation of negatively charged defects in the quantum barrier, with consequent impact on carrier confinement.
This paper investigates the effect of the properties and position of defects on the performance and reliability of InGaN/GaN Multi Quantum Well (MQW) LED. To this aim, we analyzed color-coded structures featuring two quantum wells, emitting respectively at 405 nm and 495 nm. In order to evaluate the mechanisms that limit the reliability of the devices, a constant current stress at I=80 A/cm2 and T=350K was carried out. From the degradation results, the electrical characteristics show an increment in the leakage current in the sub-turn on forward voltage regime compatible with a diffusion process. To interpret the measurement results, we implemented a simulation deck through Apsys Crosslight software, in order to model the effect that different trap concentrations and spatial locations have on the optical characteristics. The energy of the involved traps was derived by deep level optical spectroscopy analysis (DLOS). By comparing the simulation results with the experimental data, we observed that optical degradation is compatible with an increment of defect density in specific layers of the active region. Moreover, we found that the position of the traps (i.e. their proximity to the quantum wells or the EBL), modifies the band bending and influences the carrier density inside the wells, thus affecting the recombination rate and the peak emission wavelength. Therefore, we suggest that the optical degradation in MQW LED can be ascribed to a combined effect of defects with different spatial location.
III-N light-emitting-diodes (LEDs) are subject of intense investigations, thanks to their high efficiency and great reliability. The quality of the semiconductor material has a significant impact on the electro-optical performance of LEDs: for this reason, a detailed characterization of defect properties and the modeling of the impact of defects on device performance are of fundamental importance. This presentation addresses this issue, by discussing a set of recent case studies on the topic; specifically, we focus on the experimental characterization of defects, and on the modeling of their impact on the electro-optical characteristics of the devices.
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