High temperature instabilities of ohmic contacts on p-GaN

A few papers on high temperature degradation of GaN LEDs have been recently published. Degradation has been attributed to the interaction between PECVD passivation and LEDs surface, but a quantitative description of the degradation of contacts on p-GaN has not been given. The aim of this paper is to describe the degradation of ohmic contacts on p-GaN during thermal treatment. Analysis was carried out on TLM structures with PECVD passivation during storage at 250 °C. Before stress, the current–voltage curves of the TLM pads showed linear shape, indicating good ohmic behaviour. Stress tests indicated that high temperatures can induce an increase of the sheet resistance of the p-GaN layers and strong non-linearity of the electrical characteristics for low current values. Degradation process was found to be reversible: passivation removal and further storage at 250 °C were found to be sufficient for an almost complete recovery of the electrical properties of the contacts. The degradation process has been attributed to the thermally activated interaction between the passivation layer and the TLMs surface. In particular, hydrogen present in the passivation layer due to the precursors used for PECVD (SiH4 and NH3) can interact with the acceptor dopant at TLM surface, thus worsening the properties of the metal/semiconductor contact and of the p-GaN layer. These results are consistent with recent findings on temperature degradation of GaN LEDs, and provide better understanding of the role of PECVD passivation on high temperature stability of p-GaN contact layers.