Dependence of degradation on InGaN quantum well position: a study based on color coded structures

Abstract

To study the optical degradation of InGaN-based LEDs, we designed an experiment based on color-coded devices, having two quantum-wells with different positions and emission wavelengths. We analyzed a structure (A) with 20% AlGaN EBL, a QW emitting at 495 nm close to the n-side, a QW emitting at 405 nm close to the p-side; a second structure (B) with reversed QW position (495 nm closer to the p-side). The 495 nm QW is the reference QW, whose degradation is investigated during stress time, aiming at analyzing the impact of QW position on degradation rate. We submitted devices to 80 A/cm2 constant current stress, monitoring optical power and voltage by I-V and L-I characterization at each step. All the structures showed an increase in reverse leakage and low forward bias current, possibly due to trap-assisted tunneling ascribed to an increase in trap concentration. Reverse current was found to increase with the square root of stress time, indicating the presence of a diffusion process. The intensity of both QWs decreased during stress time; remarkably, degradation rate of reference QW (495 nm) was found to be much stronger for device B, where the 495 nm QW is closer to the p-side. The defects responsible for degradation were characterized by Steady-State Photocapacitance measurements, indicating the presence of a ~2 eV level, whose signal changes during stress time. Shallower defects were detected by C-DLTS, that identified a level with 0.284 eV activation energy, possibly related to VN, whose concentration decreases during stress, due to defect annealing.