Theoretical exploration of efficiency droop mechanisms in III-nitride visible light-emitting diodes

Tsai, Yi-Chia

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https://hdl.handle.net/2142/115718 Copy

Description

Title

Theoretical exploration of efficiency droop mechanisms in III-nitride visible light-emitting diodes

Author(s)

Tsai, Yi-Chia

Issue Date

2022-04-19

Director of Research (if dissertation) or Advisor (if thesis)

• Bayram, Can
• Leburton, Jean-Pierre

Doctoral Committee Chair(s)

Bayram, Can

Committee Member(s)

• Dallesasse, John
• Dragic, Peter D.
• Li, Zhanming

Department of Study

Electrical & Computer Eng

Discipline

Electrical & Computer Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• efficiency droop
• LED
• cubic gallium nitride
• polarization
• first-principles calculation

Abstract

The InGaN light-emitting diode (LED)-based solid-state lighting market is $60B per year and expected to grow with a compound annual growth rate of 14.4% (2018-2023) with increasing utilization in consumer electronics, as well as increasing usage for lighting in commercial, industrial, and residential spaces. While LEDs promise a “brighter” future, the best-performing LEDs are only halfway to the ultimate performance, with tremendous potential for further energy savings and R&D progress. According to recent reports, 1.1 PWh of global energy turns to waste heat annually, equivalent to 131 Clinton power stations and emitting 0.6 billion tons of CO2. All of these can be saved by tackling efficiency droop. The efficiency droop effect reduces quantum efficiency at high injection, imposing a design trade-off between light output power, efficiency, and costs. In this doctoral dissertation, the electronic properties of wurtzite and zincblende III-nitrides are updated by first-principles calculations. First, the lattice constants and the bandgaps are studied for optimizing the quantum well. Red, green, and blue emitters made by bulk zincblende InGaN are found to have 5% less In content than the wurtzite counterparts, ensuring better crystal quality and smaller nonradiative recombination rates. Second, the band offsets are studied for optimizing the quantum barrier. The conduction band offset of the zincblende GaN/InN interface is found to be 0.25 eV greater than that of the wurtzite interface, forming deeper quantum wells that suppress carrier leakage. Third, the activation energies of Si-doped and Mg-doped zincblende GaN are studied for designing the PN junction. Si is found to be an excellent donor with low activation energy (<30 meV), while the activation energy of Mg-doped zincblende GaN is found to be 73 meV less than that of the wurtzite GaN, enabling more than four-fold improvement in hole density and alleviating carrier asymmetry. An Open Boundary Quantum LED Simulator (OBQ-LEDsim) based on variational principles is developed to assess the effects of nonradiative mechanisms on efficiency droop without enforcing any artificial boundaries, thereby describing the carrier densities inside and outside the quantum wells with high accuracy. First, it is found that the hhe Auger process is the primary recombination channel because holes are more localized than electrons in the quantum well. Varying the ratio of electron (Cn) to hole (Cp), Auger coefficient Cn/Cp, from 0 to ∞ suppresses the hhe Auger recombination, resulting in 25% higher electron and hole sheet charge densities in the LED active layer and reducing the efficiency droop by half. Second, the coexistence of strong internal polarization and large carrier effective mass accounts for ~51% of the efficiency droop under high current densities in wurtzite InGaN LEDs compared to zincblende InGaN LEDs. OBQ-LEDsim further reveals that the zincblende LED efficiency droop is highly immune to the indeterminacy of Auger electron-hole asymmetry (Cn/Cp) and to the increase of ambipolar Auger coefficient value (Ca), and is also more robust to the adverse effects of efficiency degradation mechanisms. Finally, it is found that carrier leakage causes internal quantum efficiency (IQE) degradation but is not responsible for the efficiency droop since the radiative recombination rate is limited by poor electron-hole wavefunction overlap caused by strong polarization and hence most carriers captured recombine via Auger process, instead of a radiative one. This correlation between Auger recombination and carrier leakage suggests polarization elimination is paramount for achieving high IQE and low efficiency droop. A zincblende InGaN LED with a low efficiency droop of 3% at 100 A/cm2 is proposed, paving a clear pathway toward the ultimate energy-saving solid-state lighting.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Yi-Chia Tsai

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