Analysis of degradation mechanisms induced by electrical over-stress on high efficiency gallium nitride LEDs

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Metadata	Details
Publication Date	2019-12-02
Journal	Padua Research Archive (University of Padua)
Authors	Nicola Renso

Abstract

This thesis investigates the reliability of state-of-the-art InGaN LEDs for lighting applications and the impact of the diffusion-related mechanisms on optoelectronic GaN-based wafers, with the aim to identify the physical mechanisms responsible for the premature degradation of those devices. By means of custom experimental setups, developed during the triennial research activity, it is possible to identify the dominant failure modes and degradation mechanisms of GaN LEDs subjected to electrical over-stress (EOS), both in forward and reverse bias, and to correlate specific failures with the epitaxy related weaknesses of state-of-the-art LEDs. In particular, to understand the role of defects in the device degradation, advanced techniques such as Deep-Level Transient Spectroscopy (DLTS) and Deep-Level Optical Spectroscopy (DLOS) are employed. An extensive analysis aimed to correlate the epitaxial growth parameters (indicated with letters from A to E due to confidentiality agreements) and LEDs electro-static discharge (ESD) robustness in reverse bias is presented. The analysis investigates separately the roles of the epitaxial features on the n- and p-side by means of DC and pulsed characterizations. The results suggest: i) on the n-side, the value of the parameter A is critical in the robustness to ESD events; ii) on the p-side, C is the critical parameter; iii) leakage paths in the structure can act as radiative recombination centers in reverse bias conditions and can be responsible for the failure via junction shorting; iv) nitrogen vacancies may be the physical origin of those defects. The information about the physical mechanisms responsible for degradation are used as a feedback for devices manufacturers, for the improvement of the technological processes. The analysis of the failure modes trigged by EOS events in forward bias describes the power dissipation as the main cause of damage: according to the amount of power delivered to the chip, due to the strong self-heating, the failure can interest the chip (leading to a short-like failure) or the whole package (inducing an open-like failure). Further experiments allow to identify four different regions before the failure of the device: i) radiative recombination is the dominant recombination process and non-uniform band-filling can be detected, ii) by increasing the current density, strong self-heating can be noticed, and then iii) the saturation of the quantum wells (QWs) induces a strong overflow and may lead to additional power dissipation. Finally, (iv) the extreme high current density induces the current crowding effect, leading to a progressive decay of the optical properties of the device and to the device failure. Avalanche generation in state-of-the-art high power InGaN LEDs is detected: the extremely high electric field generated by strong reverse biases triggers band-to-band tunneling, leading to impact ionization. Current-voltage characterizations at cryogenic temperatures detect the shift of IV curves with increasing temperatures, confirming the role of avalanche generation. Further analysis of the electro-luminescence spectral distribution, in this extreme bias conditions, shows that (i) hole and electron pairs generated by the avalanche process recombine radiatively, generating photons, (ii) which are partially re-absorbed in the In-containing layers and n-GaN side and then (iii) re-emitted as internal photoluminescence of the yellow-emitting defects in the n-GaN layer. Experiments on color coded structures with different QW order and electron blocking layer (EBL) Al content show that i) leakage current increase in reverse and low forward bias conditions is related to diffusion, ii) the optical degradation is not dominated by diffusion but is related to the defects propagation triggered by the energy released by SRH recombination, iii) the optical degradation starts from the p-side and iv) the physical origin of those defects are impurities on the AlGaN/GaN interface or nitrogen vacancies.

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