Evidence for avalanche generation in reverse-biased InGaN LEDs

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.