Postgraduate Thesis: Tailoring optical properties of light-emitting diodes by nanostructuring with nanospheres

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Title	Tailoring optical properties of light-emitting diodes by nanostructuring with nanospheres
Authors	Zhang, Qian 张倩
Issue Date	2012
Publisher	The University of Hong Kong (Pokfulam, Hong Kong)
Abstract	III-V nitride based light-emitting diodes (LEDs) have experienced rapid developments during past decade, proving their potential to substitute conventional incandescent bulbs and fluorescent lamps to fulfil energy-efficient and sustainable lighting needs. Tremendous endeavours have been made to improve the performance of LEDs, most of which focused on enhancing the internal and external quantum efficiencies. However, other optical properties of LEDs remain to be explored for a more flexible way of using LEDs in various applications. Therefore, this thesis proposes two nanostructuring strategies through the use of nanospheres to tailor the optical properties of LEDs. The nanostructured LEDs are demonstrated enable light emission with reduced divergence, or becomes polarized. The monolithic modifications are free of external optics and thus eliminate light loss, meanwhile providing manipulability of optical emission from LEDs. Firstly, close-packed indium-tin-oxide (ITO) micron-lenses with dimension of the order of wavelength have been integrated onto InGaN LEDs aiming at reducing the emission divergence. The sub-micron lens arrays are patterned by nanosphere lithography with silica nanosphere serving as an etch mask on ITO layer, leaving the semiconductor layer damage-free. An enhancement of up to 63.5% on optical output power from the lensed LED has been observed. The LED with 500 nm lenses exhibits a 26.8° reduction in emission divergence (full width at half maximum) compared with the bare LED. Three-dimensional finite-difference time-domain simulations performed for light extraction and emission characteristics is found to be consistent with the observed results. Secondly, polarization behavior of light emitted from InGaN LEDs propagating through a self-assembled polystyrene nanosphere opal film has been studied. Angular-resolved optical transmission of transverse electric (TE) and transverse magnetic (TM) polarized light has been measured. An integrated p/s ratio of 2.16 is observed at a detection angle of 70°, attributed to the suppression of TE mode at particular frequencies by the three-dimensional photonic crystal. Polarization is found to depend strongly on both the photonic bandgap of the opal and the angle of incidence. Theoretical calculations by transfer matrix method yield results consistent with the experimental data.
Advisors	Choi, HW
Degree	Master of Philosophy
Subject	Light emitting diodes. Nanostructured materials.
Dept/Program	Electrical and Electronic Engineering

DC Field	Value
dc.contributor.advisor	Choi, HW
dc.contributor.author	Zhang, Qian
dc.contributor.author	张倩
dc.date.hkucongregation	2012
dc.date.issued	2012
dc.description.abstract	III-V nitride based light-emitting diodes (LEDs) have experienced rapid developments during past decade, proving their potential to substitute conventional incandescent bulbs and fluorescent lamps to fulfil energy-efficient and sustainable lighting needs. Tremendous endeavours have been made to improve the performance of LEDs, most of which focused on enhancing the internal and external quantum efficiencies. However, other optical properties of LEDs remain to be explored for a more flexible way of using LEDs in various applications. Therefore, this thesis proposes two nanostructuring strategies through the use of nanospheres to tailor the optical properties of LEDs. The nanostructured LEDs are demonstrated enable light emission with reduced divergence, or becomes polarized. The monolithic modifications are free of external optics and thus eliminate light loss, meanwhile providing manipulability of optical emission from LEDs. Firstly, close-packed indium-tin-oxide (ITO) micron-lenses with dimension of the order of wavelength have been integrated onto InGaN LEDs aiming at reducing the emission divergence. The sub-micron lens arrays are patterned by nanosphere lithography with silica nanosphere serving as an etch mask on ITO layer, leaving the semiconductor layer damage-free. An enhancement of up to 63.5% on optical output power from the lensed LED has been observed. The LED with 500 nm lenses exhibits a 26.8° reduction in emission divergence (full width at half maximum) compared with the bare LED. Three-dimensional finite-difference time-domain simulations performed for light extraction and emission characteristics is found to be consistent with the observed results. Secondly, polarization behavior of light emitted from InGaN LEDs propagating through a self-assembled polystyrene nanosphere opal film has been studied. Angular-resolved optical transmission of transverse electric (TE) and transverse magnetic (TM) polarized light has been measured. An integrated p/s ratio of 2.16 is observed at a detection angle of 70°, attributed to the suppression of TE mode at particular frequencies by the three-dimensional photonic crystal. Polarization is found to depend strongly on both the photonic bandgap of the opal and the angle of incidence. Theoretical calculations by transfer matrix method yield results consistent with the experimental data.
dc.description.nature	published_or_final_version
dc.description.thesisdiscipline	Electrical and Electronic Engineering
dc.description.thesislevel	master's
dc.description.thesisname	Master of Philosophy
dc.identifier.hkul	b4807990
dc.language	eng
dc.publisher	The University of Hong Kong (Pokfulam, Hong Kong)
dc.relation.ispartof	HKU Theses Online (HKUTO)
dc.rights	The author retains all proprietary rights, (such as patent rights) and the right to use in future works.
dc.rights	Creative Commons: Attribution 3.0 Hong Kong License
dc.source.uri	http://hub.hku.hk/bib/B4807990X
dc.subject.lcsh	Light emitting diodes.
dc.subject.lcsh	Nanostructured materials.
dc.title	Tailoring optical properties of light-emitting diodes by nanostructuring with nanospheres
dc.type	PG_Thesis

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Firstly, close-packed indium-tin-oxide (ITO) micron-lenses with dimension of the order of wavelength have been integrated onto InGaN LEDs aiming at reducing the emission divergence. The sub-micron lens arrays are patterned by nanosphere lithography with silica nanosphere serving as an etch mask on ITO layer, leaving the semiconductor layer damage-free. An enhancement of up to 63.5% on optical output power from the lensed LED has been observed. The LED with 500 nm lenses exhibits a 26.8&#176; reduction in emission divergence (full width at half maximum) compared with the bare LED. Three-dimensional finite-difference time-domain simulations performed for light extraction and emission characteristics is found to be consistent with the observed results. 

Secondly, polarization behavior of light emitted from InGaN LEDs propagating through a self-assembled polystyrene nanosphere opal film has been studied. Angular-resolved optical transmission of transverse electric (TE) and transverse magnetic (TM) polarized light has been measured. An integrated p/s ratio of 2.16 is observed at a detection angle of 70&#176;, attributed to the suppression of TE mode at particular frequencies by the three-dimensional photonic crystal. Polarization is found to depend strongly on both the photonic bandgap of the opal and the angle of incidence. Theoretical calculations by transfer matrix method yield results consistent with the experimental data.</description.abstract><language>eng</language><publisher>The University of Hong Kong (Pokfulam, Hong Kong)</publisher><relation.ispartof>HKU Theses Online (HKUTO)</relation.ispartof><rights>The author retains all proprietary rights, (such as patent rights) and the right to use in future works.</rights><rights>Creative Commons: Attribution 3.0 Hong Kong License</rights><source.uri>http://hub.hku.hk/bib/B4807990X</source.uri><subject.lcsh>Light emitting diodes.</subject.lcsh><subject.lcsh>Nanostructured materials.</subject.lcsh><title>Tailoring optical properties of light-emitting diodes by nanostructuring with nanospheres</title><type>PG_Thesis</type><identifier.hkul>b4807990</identifier.hkul><description.thesisname>Master of Philosophy</description.thesisname><description.thesislevel>master&apos;s</description.thesislevel><description.thesisdiscipline>Electrical and Electronic Engineering</description.thesisdiscipline><description.nature>published_or_final_version</description.nature><date.hkucongregation>2012</date.hkucongregation><bitstream.url>http://hub.hku.hk/bitstream/10722/161585/1/FullText.pdf</bitstream.url></item>