Characterization and investigation of the role of QWs position in the optical degradation of GaN-based devices with different indium concentration QWs under high excitation conditions

The aim of this work is to optically characterize and investigate GaN-based devices with two InGaN QWs (Quantum Wells). In particular, we want to examine what happens when flipping the position of 10% and 20% indium concentration QWs and how it impacts on efficiency and reliability when exposed to high intensity – using a 405 nm laser diode – and high temperature conditions (from 35 °C to 75 °C). Different characterization techniques have been used: current-voltage characterization both under dark and light conditions, photocurrent spectroscopy, spectrum analysis. Three devices have been considered: the first one, with the QW on the side of the p-contact composed of 10% indium concentration and the QW with 20% concentration on the side of the n-contact; the second one is similar to the previous one, just flipping the composition of the two QWs, hence with 20% and 10% indium concentration respectively from the p-contact; the third one is composed of an inner thick GaN layer with no QWs. In this way it is possible to characterize both devices with and without QWs, allowing to develop a fair comparative necessary to fully understand the beneath mechanisms related to the swap of the two QWs as well as the impact of QWs presence. Different stresses have been executed under high excitation conditions. Step stresses, with constant time steps of 30 minutes each with exposure to optical intensities from 100 W/cm2 up to 2500 W/cm2. Step stresses with similar duration steps, but varying the current flowing through the devices, where the current has been chosen such that the power spectral density on the cell corresponded to the one due to optical excitation used in the previous stress. Finally, constant stresses, with a total duration of 20,000 minutes under 1000 W/cm2 high intensity 405 nm excitation. The aim of these measurements is to analyze degradation of devices both with and without QWs under abovementioned stress conditions, and in the difference exhibited by the QWs devices with swapped position. Characterizing the three devices by varying temperature and intensity under 405 nm excitation, the sample with 20% indium concentration QW on the side of the p-contact shows a lower degradation under high excitation conditions with respect to the one with opposite indium concentration QWs. In addition, the former shows better performance under 405 nm, as highlighted by the IV light measurements. Under 405 nm excitation, the devices with QWs exhibit similar degradation kinetics, higher than in sample without QWs. It can be concluded that degradation of such devices is related to QWs, which absorb the radiation at 405 nm. The results of this work can help to develop commercial devices with increased performance for concentration solar power systems and wireless power transfer systems.