Physical mechanisms for hot-electron degradation in GaN light-emitting diodes

Abstract

We report investigations on the degradation of GaN-based light-emitting diodes due to high dc current stress by examining two types of devices with the same fabrication procedures except for the growth conditions for the InGaN quantum wells (QWs). Higher trimethylindium and triethylgallium fluxes are used for type A devices resulting in a threefold increase in the InGaN QWs growth rate compared to type B devices. Detailed structural and optoelectronic properties of the devices are investigated by transmission electron microscopy, atomic force microscopy, thermal imaging, I-V measurements, and the low-frequency noise properties of the devices as a function of the stress time, t[sub S]. The experimental data show that the QWs in type B devices are dominated by spiral growth and they have substantially higher strain nonuniformity than type A devices. The highly strained GaN/InGaN interfaces in device B are also responsible for the faster increase in the defect density due to hot-electron injection. The defects enhance the trap-assisted tunneling in the multiple quantum wells (MQWs) resulting in the development of hot spots among type B devices after high current stressing of the MQWs. This in turn leads to an increase in the defect generation rate resulting in a thermal run-away condition that ultimately resulted in the failure of the device. The data show that an increase in the growth rate in the InGaN layer led to the domination by the step flow growth mode over the spiral growth mode in the MQWs. This is the main reason for the reduction in the dislocation density in type A devices and hence their increase in device reliability.