Reliability analysis of Gan-Based LEDs for solid state illumination

High-brightness, high-efficiency GaN-based LEDs have already found many applications, and are extremely promising devices for solid-state lighting, including general illumination [1]. While reliability of LEDs is significantly better in most aspects compared to traditional light sources, lumen maintenance remains one of the critical issues concerning this new technology. Early reliability studies [2] observed a rapid loss of light output, severe degradation of the transparent encapsulating material under blue and UV illumination and elevated temperatures; non-optimized packaging also induced other reliability problems due to difficulties in thermal management. As the packaging technology improves, the interest is focussing on failure modes and mechanisms related with the semiconductor material and the die technology, which have been less investigated in the past. This work presents the results of extensive high-current accelerated aging tests carried out on blue InGaN/GaN LEDs packaged with the usual epoxy encapsulation (lamps) as well as mounted without encapsulation (chips), in order to isolate the chip-related failure modes. As a result, 5 distinct failure modes were identified: (i) in encapsulated devices, testing at high current levels and, consequently, at high junction temperatures, induces degradation of the epoxy material in contact with the heated device surface, leading to the formation of an opaque layer on the device surface; (ii) high current and temperature affect the semi-transparent ohmic contact on top of the device, leading to increase in series resistance, with consequent current crowding effects that reduce the optical power; (iii) an increase in doping of the p-type layer takes place, possibly due to decomposition of Mg complexes [3,4], and Mg reactivation; (iv) Deep Level Transient Spectroscopy (DLTS) detects changes in the distribution of deep levels already present in untreated devices, and also a generation of shallow traps; (v) both DLTS and photocurrent spectra indicate the creation of extended defects in devices treated at high current density. Identification of the specific failure mechanisms allows an accurate extrapolation of LED lifetime, and suggests specific actions for improving device reliability.