Y.W. Lin. Outline Introduction Experiments Results and Discussion Conclusion References.

Y.W. Lin

Outline Introduction Experiments Results and Discussion Conclusion References

IntroductionInGaN/GaN light-emitting diodes (LEDs) grown on c-plane sapphire substrate have recently become a favorable choice forapplications in energy saving solid-state lighting.One key feature that still needs to be solved is the so-called“efficiency droop.”

Among these factors, poor transport of holes might be the mostimportant one. Because it could lead to the accumulation of thesecarriers at the quantum wells near p-GaN,which increase the possibilities for the overflow of electrons and Auger recombinationat high carrier concentration.

As a result, improving hole transport in the active region hasbeen proposed as a mean to reduce droop behavior.

In this paper, we report a new design of the barrier layers in MQWs by grading the composition of barriers from In x Ga 1-x N to GaN along [0001] direction, to form a graded-composition multiple quantum barriers (GQB) and show the improvement intransport of holes in active region and substantial reduction inefficiency droop behavior.

For the GQB LED, the composition of indium was gradedfrom 5% to 0% along [0001] direction.

Experiments

Ni/Au

Ni/Au

GaN nucleation layertion layer

n- contact

p- contact

20 nm p-Al0.15Ga0.85N EBL

200 nm p-GaN layer(p-doping ＝ 1 ×1018cm-3)

4µm n-GaN layer(n-doping ＝ 2 ×1018cm-3)

Sapphire

In0.15Ga0.85 N/GaN MQWssix pairs ： 2.5 nm wells /10 nm barriers

300 × 300 µm 2

ITO

InGaN/GaN SPS layer

The composition of indium was graded from 5% to 0% along [0001] direction.

percentage of screening effect ： 50%Shockley-Read-Hall recombination lifetime ： 1 nsAuger recombination coefficient ： 10-

31cm6/sEmission wavelengths ： 450 nm at 22 A/cm2

Results and Discussion

FIG. 1. (Color online) Calculated band diagrams of the conventional andGQB LEDs.

FIG. 2. (Color online) Calculated carrier concentrations at current densityof 20 A/cm2 for (a) conventional and (b) GQB LEDs.

last well fifth well8.5 ×1028 cm-3 1.3 ×1028 cm-3

Hole concentrations：

last well fifth well3.5 ×1028 cm-3 5.8 ×1028 cm-3

Conventional LED

GQB LED

FIG. 3. (Color online) Calculated carrier concentrations at current densityof 200 A/cm2 for (a) conventional and (b) GQB LEDs.

FIG. 4. (Color online) Experiment and simulation normalized efficiency forconventional and GQB LEDs. The inset figure shows I-V characteristics ofconventional and GQB LEDs.

Con. LED GQB LED8.2 Ω 6.5 Ω

series resistance ：

Con. LED GQB LED3.4 V 3.27 V

Vf at 22 A/cm2 ：

Con. LED GQB LED34% 6%

efficiency droop：

FIG. 5. (Color online) Calculated radiative recombinations of conventionaland GQB LEDs at current density of (a) 20 A/cm2 and (b) 200 A/cm2.

Conclusion As a result,the hole transport in MQWs was significantlyenhanced at either low or high current density, which is beneficial for droop reduction. The GQB LED was realized by MOCVD,and the I-V curveshowed that GQB LED has lower series resistance than theconventional one and the efficiency droop was reduced from 34% in conventional LED to only 6% in GQB LED, which is inagreement with our simulation results.These results indicate that although the improvement in holetransport facilitates the reduction of efficiency droop, spatialdistribution between electrons and holes should be taken intoconsideration.

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