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.

Figure 1. Schematic cross-section diagram of the epitaxial structure for samples I, II, and III, respectively.

The d_active and d_well are fixed to 56.2 nm and 4 nm, respectively, while the Nw increases with decreasing the d_barrier.

Figure 2. Electrical characteristic for samples I, II, and III.

Semi-logarithmically plotted normalized light output power versus injection current at 300 K. The regions are divided two with variation of gradient. (red and white).

Figure 3. Analysis of carrier distribution by simulation for samples I, II, and III.

(a) single conduction band diagram, (b) conduction band diagram (solid line) and quasi-Fermi level (dash line), (c) valence band diagram (solid line) and quasi-Fermi level (dash line), (d) hole concentration, and (e) radiative recombination rate for samples I to III at 50 A/cm².

Figure 4. Time-resolved optical property for samples I, II, and III.

Recombination lifetime from sample I to III measured at low temperature (10 K).

Figure 5. The variation of FWHM for samples I, II, and III as a function of injection current.

The FWHM decreases at the low injection current region (yellow region), while it increases again with increasing injection current (white region). The former can be explained by the screening effect of internal electric field, while the latter by the band filling effect. We confirmed that the influence of the screening and band filling in QW was reduced with decreasing the barrier thickness.

Figure 6. Internal quantum efficiency for samples I, II, and III as a function of injection current.

(a) IQE and (b) normalized IQE versus the injection current for samples I to III. The IQE_max increases, and the droop decreases with decreasing the barrier thickness.

Figure 7. Correlation between the onset of droop and band filling, internal electric field, droop, and IQE_peak with varying barrier thickness.

The onset voltages of droop (◻) and band filling (◾), internal electric field in well (⚪), droop (▾), and IQE_peak (▴) are shown for samples I, II, and III. As the barrier thickness decreases and the QW number increases, the internal electric field within QWs and droop decreases and the IQE_peak increases.