Effective suppression of efficiency droop in GaN-based light-emitting diodes: role of significant reduction of carrier density and built-in field

Abstract

A critical issue in GaN-based high power light-emitting diodes (LEDs) is how to suppress the efficiency droop problem occurred at high current injection while improving overall quantum efficiency, especially in conventional c-plane InGaN/GaN quantum well (QW), without using complicated bandgap engineering or unconventional materials and structures. Although increasing thickness of each QW may decrease carrier density in QWs, formation of additional strain and defects as well as increased built-in field effect due to enlarged QW thickness are unavoidable. Here, we propose a facile and effective method for not only reducing efficiency droop but also improving quantum efficiency by utilizing c-plane InGaN/GaN QWs having thinner barriers and increased QW number while keeping the same single well thickness and total active layer thickness. As the barrier thickness decreases and the QW number increases, both internal electric field and carrier density within QWs are simultaneously reduced without degradation of material quality. Furthermore, we found overall improved efficiency and reduced efficiency droop, which was attributed to the decrease of the built-in field and to less influence by non-radiative recombination processes at high carrier density. This simple and effective approach can be extended further for high power ultraviolet, green, and red LEDs.

High-Efficiency InGaN/GaN Quantum Well-Based Vertical Light-Emitting Diodes Fabricated on β-Ga2O3 Substrate

“…Figure 4b shows that the PL peak energy and the corresponding FWHM remained roughly constant at RT as the carrier density increases to 50×10 3 kW/cm 2 and 2×10 3 kW/cm 2 , respectively, indicating a negligible QCSE and confirming our EL results. When the excitation density increases further (> 10×10 3 kW/cm 2 ), the FWHM gradually increases due to the contribution of the band-filling effect of the high-energy localized centers, 27,29 resulting in a peak blue-shift above 50×10 3 kW/cm 2 . The IQE is calculated from the PDPL measurements using the ABC model (Equation S1, generated in terms of the optically generated carriers ‫ܩ(‬ ௧ ) in the Supporting Information); by using the fitting parameter (ܳ ଶ ) (Supporting Information Equation S7), the IQE can be expressed as 25…”

Realization of Highly Efficient InGaN Green LEDs with Sandwich-like Multiple Quantum Well Structure: Role of Enhanced Interwell Carrier Transport

“…11,12 In addition, the straininduced internal polarization field will tilt the green LED band diagrams of QWs more seriously than those of blue ones, leading to a so-called "efficiency droop" in InGaN-based green LEDs, which has been attributed to the inefficient hole transport and increased electron overflow out of the active region. 13,14 These two problems have seriously hampered the development of SSL, especially for high-power LEDs, whose operating current is usually greater than 350 mA and even up to 1 A, having dimensions of approximately 1 × 1 mm 2 . 15 Many efforts have been made to solve these two problems, such as designing novel MQW structures 14−16 or optimizing growth parameters.…”

“…Additionally, in order to incorporate In sufficiently, it is usually accomplished by sacrificing the growth temperature of InGaN quantum wells (QWs), resulting in a significant deterioration of the material quality in the QWs due to crystal defects. , On the other hand, the higher In composition in well layers induces a larger piezoelectric polarization electric field, resulting in a quantum confinement Stark effect (QCSE). The QCSE restricts the internal quantum efficiency (IQE) by reducing the overlap integral between the electron and hole wave functions, resulting in a lower recombination efficiency. , In addition, the strain-induced internal polarization field will tilt the green LED band diagrams of QWs more seriously than those of blue ones, leading to a so-called “efficiency droop” in InGaN-based green LEDs, which has been attributed to the inefficient hole transport and increased electron overflow out of the active region. , These two problems have seriously hampered the development of SSL, especially for high-power LEDs, whose operating current is usually greater than 350 mA and even up to 1 A, having dimensions of approximately 1 × 1 mm 2 …”

A comparative study of efficiency droop and internal electric field for InGaN blue lighting-emitting diodes on silicon and sapphire substrates

“…As the strength of the internal electric field increases, electron leakage from the MQWs to the p-GaN layer increases and hole injection into the MQW active region becomes increasingly inefficient, which aggravates the efficiency droop problem. In addition, the internal electric field was reported to increase the Auger recombination rate, which also leads to an increase in efficiency droop 13 14 15 . Therefore, a number of studies have been devoted to reducing the polarization-induced field in InGaN QWs.…”