Operation of InGaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with quaternary AlInGaN barriers at room and elevated temperatures is reported. The devices outperform conventional GaN/InGaN MQW LEDs, especially at high pump currents. From the measurements of quantum efficiency and total emitted power under dc and pulsed pumping, we show the emission mechanism for quaternary barrier MQWs to be predominantly linked to band-to-band transitions. This is in contrast to localized state emission observed for conventional InGaN/InGaN and GaN/InGaN LEDs. The band-to-band recombination with an increased quantum-well depth improves the high-current performance of the quaternary barrier MQW LEDs, making them attractive for high-power solid-state lighting applications.
Study of the spectral noise density and its dependence on current density in as fabricated and degraded blue light emitting diodes (LEDs) based on InGaN/GaN quantum well structures are reported. It is shown that defects are generated nonuniformly in the course of degradation, being concentrated along extended defects penetrating into the active region of LEDs. It is demonstrated that the decrease in the exter nal quantum efficiency in the course of aging is due to the enhancement of charge transport uniformity, which leads to the formation of shunts and local overheating regions. Typically, in blue LEDs, these effects are responsible for the ambiguous development of the degradation process, which hinders prognostication of LED service life. The effect of noise suppression is observed in a narrow current density range (10 -2 to 10 -1 A cm -2 ) corresponding to the onset of radiative recombination.
We report on the study of heat 2D-distribution in InGaN LEDs with the stress made on local device overheating and temperature gradients inside the structure. The MQW InGaN/GaN/sapphire blue LEDs are designed as bottom emitting devices where light escapes the structure through the transparent GaN current spreading layer and sapphire substrate, whereas the LED structure with high-reflectivity Ni/Ag p-contact is bonded to the thermally conductive Si submount by a flip-chip method. The measurements are performed with an IR microscope operating in a time-resolved mode (3-5 um spectral range, <20 µm spatial and 10 µs temporal resolution), while scanning a heat emission map through a transparent sapphire substrate. We show how current crowding (which is difficult to avoid) causes a local hot region near the ncontact pads and affects the performance of the device at a high injection level.
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