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On the mechanisms of InGaN electron cooler in InGaN/GaN light-emitting diodes

Zi-Hui Zhang, Wei Liu, Swee Tiam Tan, Zhengang Ju, Yun Ji, Zabu Kyaw, Xueliang Zhang, Namig Hasanov, Binbin Zhu, Shunpeng Lu, Yiping Zhang, Xiao Wei Sun, and Hilmi Volkan Demir

Open Access Open Access

Abstract

Electron overflow limits the quantum efficiency of InGaN/GaN light-emitting diodes. InGaN electron cooler (EC) can be inserted before growing InGaN/GaN multiple quantum wells (MQWs) to reduce electron overflow. However, detailed mechanisms of how the InGaN EC contributes to the efficiency improvement have remained unclear so far. In this work, we theoretically propose and experimentally demonstrate an electron mean-free-path model, which reveals the InGaN EC reduces the electron mean free path in MQWs, increases the electron capture rate and also reduces the valence band barrier heights of the MQWs, in turn promoting the hole transport into MQWs.

© 2014 Optical Society of America

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Figures (6)
Fig. 1
Fig. 1 EL spectra for (a) LED I and (b) LED II under various injection current levels of 10, 20, 30 and 40 A/cm2.

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Fig. 2
Fig. 2 (a) Experimentally measured and (b) numerically simulated optical output power and EQE for LEDs I and II, along with the optical output power and EQE in the semi-log scale in the insets.

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Fig. 3
Fig. 3 Schematic energy diagrams for (a) LED I and (b) LED II, along with which four electron transport/transition processes are depicted in the InGaN/GaN MQWs: ① electrons are captured into the quantum well, ② electrons recombine with holes and at defects, ③ electrons re-escape from the quantum well and ④ electrons directly fly over to a remote position without being captured by the quantum well.

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Fig. 4
Fig. 4 Calculated electric filed as a function of position within the EC layer at 20 A/cm2. The positive direction of the electric field is along the growth orientation, i.e., C + .

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Fig. 5
Fig. 5 Energy band diagrams for (a) LED I and (b) LED II.

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Fig. 6
Fig. 6 Simulated (a) electron concentration along with the inset depicting the electron leakage out of the active region, (b) normalized electron current, (c) hole concentration, and (d) radiative recombination rates at 20 A/cm2 across the InGaN/GaN MQW region for LEDs I and II, respectively.

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Tables (1)

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Table 1 Effective Valence Band Barrier Heights of InGaN/GaN MQWs for LEDs I and II

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Equations (7)

Equations on this page are rendered with MathJax. Learn more.

N t = N 0 ×[ 1−exp(− t QW / l MFP ) ]=( N 1 + N 2 )×[ 1−exp(− t QW / l MFP ) ]
M t = N 1 ×[ 1−exp(− t QW / l MFP ) ]+ N 2 ×[ 1−exp(− t QW / l MFP cooler ) ]
N 2 = N 0 ×[ 1−exp(− t cooler / l MFP ) ]
l M F P = v t h × τ S C
l M F P c o o l e r = v t h c o o l e r × τ S C
v t h = 2 × [ E + q V ] / m e
v t h c o o l e r = 2 × [ E + Δ E c − ℏ ω L O + q V − Δ E c ] / m e = 2 × [ E + q V − ℏ ω L O ] / m e
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