In this study, an asymmetric multiple quantum well (AMQW) with quantum wells of different thicknesses was used as the active regions of InGaN light-emitting diodes (LEDs) to achieve improved performance. The active region of normal LEDs is composed of a six-pair InGaN (2.5 nm)/GaN (10 nm) MQW for 455 nm emission; nevertheless, in the proposed LEDs, three quantum wells adjacent to the n-GaN have gradually increased well thickness from the p-side to n-side layers while the remaining structure is identical to those of normal LEDs. High-resolution transmission electron microscopy (HRTEM) image and high-resolution X-ray diffraction (HRXRD) analysis show that the modified MQWs have a reasonable crystalline quality even though two kinds of quantum wells with different thicknesses are combined together to form the active region of the InGaN LEDs. In wafer-level testing, both LEDs have similar effective series resistance (~4 Ω) and forward voltage (~3.1 V @ 20 mA). This indicates that the incorporation of an AMQW into the LED structures has little impact on the electrical properties of the fabricated LEDs. In addition to the peak intensity shifted towards higher current levels, the maximum light output power of the LEDs with an AMQW is superior to that of their counterparts, i.e., 14.97 and 13.88 mW at 580 and 500 mA for the LEDs with and without an AMQW. For the temperature dependent photoluminescence (PL) measurements, the thermal activation energy (E_a) of the LEDs with and without an AMQW extracted from the high-temperature section (> 100 K) of the Arrhenius plots is evaluated as 36.8 and 26.7 meV, respectively. Such result suggests that the localization effect is relatively strong in LEDs with an AMQW, which will help injection carriers radiatively recombine at the indium-rich localized states [1]. Experimentally, variations in indium content and well width during epitaxial growth may be responsible for the different levels of indium segregation in the InGaN MQWs [2]. For the LEDs with an AMQW, the phenomenon of light intensity degraded at elevated current levels could be attributed to improved uniformity of carrier distribution among the quantum wells provided a graded-thickness MQW was used [3].

Reference

[1] T. Wang, J. Bai, S. Sakai, and J. K. Ho, “Investigation of the emission mechanism in InGaN/GaN-based light-emitting diodes,” Appl. Phys. Lett., vol. 78, pp. 2617–2619 (2001).

[2] J. H. Na, R. A. Taylor, K. H. Lee, T. Wang, A. Tahraoui, P. Parbrook, A. M. Fox, S. N. Yi, Y. S. Park, J. W. Choi, and J. S. Lee, “Dependence of carrier localization in InGaN/GaN multiple-quantum wells on well thickness,” Appl. Phys. Lett., vol. 89, pp. 253120-1~3 (2006).

[3] J. Y. Zhang, L. E. Cai, B. P. Zhang, X. L. Hu, F. Jiang, J. Z. Yu, and Q. M. Wang, “Efficient hole transport in asymmetric coupled InGaN multiple quantum wells,” Appl. Phys. Lett., vol. 95, pp. 161110-1~3 (2009).

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