Micro-Raman spectroscopy: self-heating effects in deep UV light emitting diodes

Ultraviolet light emitting diodes (LED) based on GaN and its ternary alloy AlGaN are key devices for applications such as solid stale white lighting and chemical sensing. Ultraviolet LEDs are prone to self-heating effects, i.e., temperature rises during operation, contributing significantly to the commonly observed saturation of light output power at relatively low input currents. Rather little, however, is known about the actual device temperature of an operating ultraviolet LED. Using micro-Raman spectroscopy temperature measurements were performed as a function of input current on 325nm-Al0.18Ga0.82N/Al0.12Ga0.88N multiple quantum wells LEDs grown on sapphire substrates, flip-chip mounted on SiC for heat-sinking. Temperature maps were recorded over the active device area. Temperature rises of about 65 degreesC were measured at input currents as low as 50mA (at 8V) for 200 mum x 200 mum size LEDs despite flip-chip mounting the devices. Temperature rises at the device edges were found to be higher than in the device center, due to combined heat sinking and current crowding effects. Finite difference heat dissipation simulations were performed and compared to the experimental results.