| Abstract Scope |
Even though ultraviolet light emitting diodes (UV LEDs) are now commercially available with wavelengths as short as 240nm, there are a number of formidable material issues that act as roadblocks to the production of efficient, long lifetime UV LEDs. In this work, we expand the use of thermally-induced voltage alteration (TIVA) and light-induced voltage alteration (LIVA) to localize potential precursors that may lead to premature failure in deep UV LEDs. UV LEDS were imaged using lasers with wavelengths of 532, 1064 and 1340nm. In LIVA and TIVA, a laser is rastered across a device under constant current bias. Dark or bright spots are generated in an applied voltage map, dictated by increases or decreases in the voltage of the LED and localized by the laser position. When the UV LEDs were scanned, dark and distinct defect spots were localized. Some defect spots which only appeared only under 532nm stimulation are likely areas of electron-hole pair generation since the spots appeared dark. Other defect spots were present under all wavelengths. Current-versus-voltage (IV) curves were also measured under laser interaction. When the devices were subjected to uniform heating, the current over the entire measured voltage range (-3 to 8V) increased with increasing temperature. In contrast, an increase in current occurred only on a specific part of the measured voltage range (-3 to ~4V) when the devices were subjected to localized heating with the 532nm laser on the defect position. This suggests that stimulation with the 532nm laser produces an effect that is not entirely thermal. Also, this increase in current only occurred with any significance when the defect was spotted with the 532nm laser. This again suggests that the effect is more of an e-h pair transition, rather than a thermal effect, involving energies of 2.33eV or higher. Plotting the voltage map intensity of the imaged TIVA/LIVA defect spots against the IV curve showed that a secondary slope in the IV curve between ~3 and 4V matched the onset and termination voltage of the electrically active defect spots identified in the applied voltage map. Almost all of the devices with no defects in the voltage maps do not have the secondary slope. When the 532nm laser was spotted on the defect, the slope did not change but the current increased. When the laser was spotted off of the defect, the slope decreased or disappeared, mimicking a device IV curve with no electrically active defects. This suggests that the secondary slope within the IV curve directly relates to the presence of the observed defect within the material. This work highlights how the TIVA/LIVA techniques can be used in characterizing electrically active defects in UV LEDs. |