Reversible Degradation of GaN LEDs Related to Passivation

This paper analyzes the stability of gallium nitride blue light emitting diodes (LEDs) at high temperature levels. It is shown that high temperature storage can induce significant degradation of the electrical and optical characteristics of the samples. The most important failure modes detected after thermal stress are output power decrease, emission crowding and forward voltage increase. The degradation process is attributed to the interaction between hydrogen in the PECVD-deposited SiN passivation layer and the p-side of the diodes. This mechanism induces the deterioration of the characteristics of the metal/semiconductor interface and of the p-type semiconductor layer, as confirmed by the results obtained on transfer length method (TLM) structures on p-GaN submitted to thermal storage. The degradation process has been found to be reversible. In particular it is shown that passivation removal and subsequent annealing are sufficient for an almost complete recovery of the electrical and optical properties of the LEDs. Finally, it is shown that the use of a sputtered SiN passivation layer can be an effective alternative to usually adopted PECVD for the reduction of high temperature instabilities of GaN LEDs, due to its reduced hydrogen content.