Abstract
A metalorganic vapor phase epitaxy-grown InGaN/GaN multiple-quantum-well (MQW) with three graded-thickness wells (the first-grown well had the greatest width) near the n-GaN was used as the active layer of an LED. For LEDs with an asymmetric quantum well (AQW), high-resolution X-ray diffraction and transmission electron microscopic reveal that the modified MQWs with a reasonable crystalline quality were coherently strained on the underlying GaN epilayers without any relaxation. In addition, the slight increase of indium segregation in the LED with an AQW may be attributed to variations in indium contents experienced during epitaxial growth of the wide well-containing MQWs. By preventing the energetic electrons from accumulating at the topmost quantum well nearest the p-GaN, the presence of light intensity roll-off in the LED with an AQW is shifted to higher currents and the corresponding maximum light output power is increased with a ratio 7.9% higher than that of normal LEDs. Finally, similar emission wavelengths were observed in the electroluminescence spectra of both LEDs, suggesting that light emitted mostly from the top quantum wells (near the p-GaN) while the emissions from the AQW region were insignificant.
Highlights
Solid-state lighting (SSL) based upon the brightness, efficiency, and long-term reliability of light-emitting diodes (LEDs) is extremely suitable for use in a wide variety of monochrome applications such as displays, traffic signals, and biological sensors [1]
The high contrast between the InGaN and GaN layers observed from the high-resolution transmission electron microscopy (HRTEM) image provides objective evidence that the sharp interface is not destroyed in the LEDs fabricated with an asymmetric quantum well (AQW)-containing structure
Because the epitaxial growth of InGaN/GaN MQWs is fully coherent to GaN, the slight increase in the carrier localization degree of the LED with an AQW may result from variations of the indium distribution within the wide wells caused by the composition pulling effect
Summary
Solid-state lighting (SSL) based upon the brightness, efficiency, and long-term reliability of light-emitting diodes (LEDs) is extremely suitable for use in a wide variety of monochrome applications such as displays, traffic signals, and biological sensors [1]. The phenomenon of efficiency droop (nonthermal rollover), which is commonly observed in InGaN LEDs at a high current density, could be effectively addressed by the use of a wide well as light emitting medium [10,11] This is attributed to the decreased carrier density in the wide wells, which helps to suppress nonradiative Auger recombination and reduce carrier leakage during the LED operation. Ni et al [12] reported that coupled quantum wells consisting of a thin barrier can help increase the uniformity of hole distribution among the InGaN MQWs. As a result, the fabricated LEDs exhibit improved efficiency droop with a high crossover current density of up to 1100 A/cm under pulsed mode operations. The effects of the AQW on the structural and optical properties of InGaN LEDs are characterized experimentally
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