A model for the thermal degradation of metal/(p-GaN) interface in GaN-based light emitting diodes

This paper analyzes the thermal degradation of GaN-based light emitting diodes with hydrogen-rich passivation layer by combined capacitance and current measurements. The decrease of optical power arising during thermal treatment at T=250°C is well related to a decrease in the high-frequency capacitance and to the generation of a further peak in the conductance/frequency curves. Deep level transient spectroscopy measurements excluded the role of further deep levels introduced/generated in the p-n junction region. Transmission line method analysis showed that stress induces nonlinearity of the characteristics of the contacts, related to the increase of Ohmic contact resistivity. All these modifications are explained using a small-signal model in which a parasitic impedance arises as a consequence of stress in the portion of metal/(p-GaN) interface exposed to diffusion of hydrogen from the passivation layer. Therefore, degradation is shown to be related to the worsening of the properties of the metal/semiconductor interface at the p side of the diode, due to the interaction between the hydrogen-rich passivation layer and the acceptor dopant. This interaction is well confined to the immediate proximity of the interface, leaving the activated acceptor concentration in the bulk almost unaffected.