Effect of Nano-structure on Internal Quantum Efficiency in InGaN/GaN Nanorod Light Emitting Diodes
Date Issued
2011
Date
2011
Author(s)
Chang, Chun-Hsiang
Abstract
With the tremendous growth of compound semiconductors, gallium nitride (GaN) has attracted considerable attentions in the field of solid state lighting due to the superior optical and electrical characteristics. As one of its applications, GaN-based nanorod light emitting diodes (LEDs) have been regarded to have the potential for improving optical properties. Nevertheless, there are some issues, such as the lack of efficient nano-fabrication, large leakage current under reversed bias and low optical output efficiency, that limit the performance and simplicity of manufacturing GaN nanorod LEDs.
In this thesis, we demonstrated a novel and practical approach to fabricate InGaN/GaN nanorod LED arrays with p-i-n structure using nanosphere lithography for nanorod formation, PECVD (plasma-enhanced chemical vapor deposition) grown SiO2 layer for sidewall passivation, and chemical mechanical polishing (CMP) process for parallel metal contact. With such a nano-device, we achieve a reverse leakage current of 4.77nA at -5V, an ideality factor of 7.35, and an optical output intensity 6807mW/cm2 at the injection current density of 32A/cm2. Based on the high performance, the temperature and current dependent electroluminesence (EL) of blue planar and nanorod LEDs was compared over a wide temperature range. With the nearly constant photon energy at room temperature, it reveals that the nanorod structures can effectively mitigate the strain induced quantum confined Stark effect (QCSE) and improve the internal quantum efficiency (IQE) of GaN LEDs.
From the result of low temperature EL, the coexistence of strain relaxation and etching-induced defect states was observed in our nano-structured devices. Thus we further explore the correlation between these effects by two samples with different length of nanorods. Since the dominant recombination mechanism is dependent on the injection current, the variation of external quantum efficiencies (EQEs) between two devices can be clarified and explained by the defect-state-induced nonradiative recombination and the mitigation of strain-induced QCSE. Longer nanorods may cause a stronger strain relaxation but more defect state distribution, resulting in a mildly increasing EQE with less droop. As we excluded the effect of defect states at low temperature, the IQE characteristics were further verified. While the influence of defect-state-induced nonradiative recombination still dominates the overall performance, it is fact that the longer nanorod will contribute to the increase of IQE. With an optimized nanorod etching mechanism, high performance LEDs with long nanorods can thus be realized soon.
Subjects
GaN
nanorod
LED
internal quantum efficiency
strain relaxation
quantum confined Stark effect
chemical mechanical polishing
Type
thesis
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