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.
We investigate the electroluminescence of blue LEDs in low bias (500 pA – 9 μA) at different temperature (15°C – 75°C). From 500 pA to 100 nA, the OP increases with bias current up to 10nA, and is stronger at higher temperature, as expected by radiative recombination through deep levels. The stronger contribution is at λ > 800 nm, i.e. at energies lower than the QW and GaN barrier midgap (720 nm). Above 100 nA the OP increases with current, and is compatible with QW emission. Its intensity decreases at higher temperature, as expected for non-radiative recombination. The experimental findings indicate that radiative recombination through deep levels can significantly influence the low current characteristics of the devices, even when those states are not at midgap.
Within this paper, we summarize some of the degradation mechanism that still affect GaN-based optoelectronic devices. The most common source of the degradation is the creation of lattice defects, which lower the optical efficiency due to their role as non-radiative recombination centers, as proven in the case of UV-B LEDs. The local generation of defects is not the only possibility, with diffusion of impurities (possibly hydrogen from the p-side) being shown to be the limiting factor in the case of green laser diodes. Under extreme bias conditions, such as the EOS events, the robustness of the current carriers and spreading structures is critical, as shown by failure of bonding wires, metal lines and vias in white LEDs. In every optoelectronic device photons themselves possess an energy at least equal to the bandgap, and can be an additional source of degradation that cannot be eliminated.
Avalanche generation is a physical mechanism responsible for the breakdown at extremely high field, such as in the reverse bias conditions typical of ESD discharges. In this work, for the first time we provide experimental evidence that avalanche generation can take place in state-of-the-art InGaN-based blue LEDs. We measured the current-voltage and electroluminescence curves of the devices while pulsing them with increasing reverse voltages. We investigated a wide span of temperatures (from cryogenic to room temperature) in order to verify that the increase in leakage current detected below -80 V is related to avalanche generation (positive temperature-coefficient). Numerical simulations show that in this bias condition the band-to-band tunneling barrier thickness is low, leading to the possible injection of highly-energetic electrons from the p-side to the n-side that can start the avalanche process. The spectral shape shows a broad emission, covering the spectral range between 1.25 and 3.5 eV; the low energy side slowly decreases below 2.2 eV, and two sharp edges are seen at the high-energy side. Since an avalanche generation process is present, we can interpret the spectrum as follows: (i) hole and electron pairs generated by the avalanche process recombine, emitting photons; (ii) high-energy side: reabsorption of the emitted photons in the In-containing layers and nGaN side, confirmed by the red-shift at higher temperature; (iii) low-energy side: internal photoluminescence of the defects in the n-GaN layer, confirmed by PL measurements with external excitation. A theoretical computation based on this model is able to reproduce the experimental data.
This paper demonstrates that when InGaN LEDs are submitted to a constant reverse bias, they can show a time-dependent breakdown, that leads to the catastrophic failure of the devices. By submitting green and blue LEDs to constant voltage stress in the range between -40 V and -60 V we demonstrate that: (i) under reverse bias conditions, current is focused on localized paths, whose positions can be identified by electroluminescence measurements, and that originate from the presence of extended defects; (ii) during a constant voltage stress, the reverse current of the LEDs gradually increases; (iii) for longer stress times, all devices show a time-dependent breakdown; (iv) time-to-failure has an exponential dependence on stress voltage, and is Weibull-distributed.
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