Thermal degradation of InGaN/GaN LEDs ohmic contacts

This paper analyzes the instabilities of ohmic contacts on p-GaN during thermal stress, by means of the Transfer Length Method (TLM). Before stress, the I-V curves of the TLMs showed linear shape, indicating good ohmic behaviour of the contacts. High temperature treatment induced the increase of the sheet resistance of the p-GaN layers and the non-linearity of the electrical characteristics of the contacts. Instabilities of ohmic contacts have been attributed to the interaction between TLM surface and the PECVD-SiN passivation layer. Thermally-activated interaction between hydrogen in the passivation and the acceptor dopant at TLMs surface implies the decrease of the effective acceptor concentration, and the subsequent non-linearity of the contacts and sheet resistance increase. Degradation process was found to be reversible, after passivation removal and subsequent annealing. Finally, a technique for the deposition of passivation layers alternative to PECVD and stable at high temperature levels is proposed and demonstrated.