Investigation of Efficiency Droop Behaviors of InGaN/GaN Multiple-Quantum-Well LEDs With Various Well Thicknesses Yun-Li Li, Member, Yi-Ru Huang, and Yu-Hung Lai IEEE

Outline Introduction Experiment Results and Discussions Conclusions References

Introduction InGaN/GaN MQW LEDs, a well-known fundamental problem needs to be overcome, namely the efficiency “droop”.

sapphire substratrs Experiment P N n-GaN(4 μ m) P-GaN(120 nm) p-type AlGaN/GaN EBL(10 nm/2nm) Mg-doped p-AlGaInN (10nm) N 10-period InGaN/GaN MQW n-GaN(4 μ m) LT-GaN (20nm) sapphire substratrs LED size 350 ×350 μ m2

Results and Discussions Fig. 1. Normalized EQE measurements of the MQW LEDs with varied well thicknesses. A reduced efficiency droop behavior is demonstrated for the samples with thicker quantum wells. The inset shows absolute values of the external quantum efficiencies for the samples.

Fig. 3. Simulation results of IQE at different current densities.

Fig. 2. Schematic figure of total current density Jtotal = Jrad + Jnrad +Joverflow .

RAuger RAuger Rdis Rdis Fig. 4. Ratios of dislocation recombination rates to total carrier recombination rates, Rdis/Rdis + Rrad + RAuger , and Auger recombination rates to total carrier recombination rates, RAuger/Rdis + Rrad + RAuger, versus total current density. The ratios of dislocation recombination rates are higher than that of Auger recombination rates at low current densities. As current densities increase, ratios of dislocation recombination rates decrease and ratios of Auger recombination rates increase.

Fig. 5. Ratios of nonradiative current densities to total current densities Jnrad /Jtotal and carrier overflow current densities to total current densities Joverflow /Jtotal at different current densities. Jnrad /Jtotal saturates at current higher than 100 A/cm2 . Joverflow /Jtotal increases as total current densities increase.

TABLE I VARIATIONS OF IQE, RATIO OF NONRADIATIVE CURRENT DENSITIES TO TOTAL CURRENT DENSITIES (JNRAD /JTOTAL ) AND RATIO OF CARRIER OVERFLOW CURRENT DENSITIES TO TOTAL CURRENT DENSITIES (JOVERFLOW /JTOTAL ) IN DIFFERENT CURRENT DENSITY RANGES

Conclusions Simulation results strongly suggest that at current densities from 10 to 100 A/cm2 , Auger recombination is the dominant mechanism for efficiency droop behaviors, while at current densities from 100 to 200 A/cm2 , carrier overflow becomes the dominant mechanism for efficiency droop behaviors.

On the importance of radiative and Auger losses in GaN-based quantum wells J. Hader, J. V. Moloney, B. Pasenow, S. W. Koch, M. Sabathil, N. Linder, and S. Lutgen APPLIED PHYSICS LETTERS

Outline Introduction Experiment Results and Discussions Conclusions

FIG. 1. Confinement potentials thick and wavefunctions thin lines for a 2 nm wide In0.37Ga0.63N/GaN well. Black: nominal well. Gray: well with Gaussian alloy composition. Dashed lines: energies of the states and zeros of the wavefunctions. Left: N=0. Middle: N=1014 cm−2. Right: N=0 and internal fields turned off.

325 K FIG. 2. Carrier lifetime due to radiative recombination for an In0.37Ga0.63N/GaN well at 300 K. Black/gray: nominal/Gaussian well. Solid/dashed/dotted: for full/half/zero strength of the internal fields. Thin lines: fields of full strength and 325 K.

Rdis RAuger FIG. 3. Loss current for the Gaussian well. Solid: Jrad+JAug; dashed: Jrad; dotted: JAug. Black/dark gray/light gray: for full/half/zero strength of the internal fields. Thin black: fits according to Jrad=BN2, B=3.5 10×12 cm2 / s and JAug=CN3, C=3.510×34 cm6 / s.

FIG. 4. Experimental and theoretical threshold current densities for various InGaN-based laser diodes. Triangles/squares/stars: from Refs.

References Yun-Li Li, Member, Yi-Ru Huang, and Yu-Hung Lai,”Investigation of Efficiency Droop Behaviors of InGaN/GaN Multiple-Quantum-Well LEDs With Various Well Thicknesses”. J. Hader, J. V. Moloney, B. Pasenow, S. W. Koch, M. Sabathil, N. Linder, and S. Lutgen,” On the importance of radiative and Auger losses in GaN-based quantum wells”.