Investigation of the Efficiency Droop in Partially Strain Relaxed InGaN/GaN Light-Emitting Diodes
Date Issued
2010
Date
2010
Author(s)
Sun, Yu-Hsuan
Abstract
External light extraction has been a very popular topic in the past few years for researchers in the field of GaN based light emitting diodes(LEDs). Various surface roughening techniques have been proposed to enhance extraction efficiency, include the growth of an additional rough p-GaN layer, the texturing on transparent conducting contacts and on p-GaN layer. On the other hand, the root causes of the internal quantum efficiency of an LED have been considered separately from those of the external light extraction. A commonly employed method to find internal quantum efficiency of a LED is by assuming complete frozen of defects states at very low temperature (~0K). The internal quantum efficiency is therefore assumed to be 100% as the injected carriers are all contributed to radiative recombination. Despite the simplicity, the method underestimates other factors for the internal quantum efficiency. Also, as will be explained in this work, it becomes difficult to identify the absolute internal quantum efficiency between devices with various structures. In addition to nonradiative light emission, other factor that affects internal quantum efficiency in a GaN based LED is the strain induced quantum confined stark effect (QCSE). For a GaN based LED epi-structure, lattice mismatch between InGaN and GaN results in strain. It creates the internal electric field that leads to the separation of electrons and holes in the quantum well region and decreases the internal quantum efficiency. In the literature, the influence of QCSE is observed to be suppressed from nanorods by etching through the LED epilayers, or from the LED structure on a pattern sapphire substrate .
In this work, partially strain relaxation from the LEDs with surface textured p-GaN layer has been observed. The effect of surface roughening on external light extraction is thus correlated to the improvement of internal quantum efficiency due to relaxed strain. We first performed Raman measurement to study the strain in the InGaN/GaN layers with the top p-GaN layer textured. The optical properties were then explored by comparing the textured p-GaN device with the planar one. And finally, the effect of QCSE on both textured and planar p-GaN devices was analyzed by photoluminescent (PL) measurement at low temperature.
A major obstacle for solid-state lighting is so-called "efficiency droop", the way that the efficiency falls at high drive currents. Its origins were unclear. There are several mechanisms and theories proposed to explain the origin of droop effect, but up to the present, the physical origin of efficiency droop is still under hot debate.
In this thesis, we identify the polarization field due to the strain resulted from lattice mismatch have closely relationship with droop effect. On the purpose of verification, we fabricated the surface-textured InGaN/GaN LED in order to achieve partial strain relaxation. In experiment result, the Raman scattering measurement and the blue shift of peak wavelength prove us that the reduction of strain is a key factor of droop effect and show that less strain in MQWs will have better internal quantum efficiency and have better LEDs’ droop performance. It prove that surface roughness not only improve the extraction efficiency but also the increase internal quantum efficiency. In addition, we also make ITO surface-textured LED to verify whether the behavior of droop change or not and the result show that it indeed have the better efficiency droop performance. Furthermore, by junction and ambient temperature experiment, we show that the electrical stress induced junction temperature raise of LED also have close relationship with efficiency droop. Finding the origin of efficiency droop is a very critical and important issue in our society now. Fabricating droop-free LEDs could be the key breakthrough to unlocking the general lighting market and make significant contributions to our human society.
Subjects
strain relaxation
surface texturing
efficiency droop
quantum confined stark effect
Type
thesis
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