Material properties, coupled with typical device structures of GaN-based
light-emitting diode (LED) wafer give rise to Lambertian emission patterns with
large beam divergence. However, this pattern may not be useful or beneficial to
many applications. In some specific applications, such as spot lighting or light
sources for fiber coupling, emission with narrow beam divergence is required,
whereas in general lighting such as the street lamps and indoor lighting, a diffused
light source rather than a point source is needed. By manipulating the optical emission of LEDs at the chip level, some performance metrics of LEDs can be
enhanced and their applications can be extended into new fields rather than
merely for lighting. Additionally, the need for external optics can be eliminated,
thereby increasing the flexibility of design. In this thesis, five implementations
are reported to achieve emission control, namely chip design, optics design,
package design and system design, which are ordered according to the LED
fabrication process flow. Manipulation of optical emission can be observed by
comparing the proposed devices with the conventional devices, or the successful
demonstration of a new application.
By chip shaping via laser micromachining, a three-dimensional
truncated-conic LED (TC-LED) is proposed to cut off efficiently lateral emissions
from the LED sidewall, thus enhancing color uniformity from its top quantum-dot
coated surface. The optical properties of TC-LED are investigated: the beam
divergence is reduced by 32o and the power in the normal direction is enhanced by
21.7%. After applying quantum dots to achieve white-light emission, the top
emission color uniformity is improved by 37%.
By including optics on the chip level, beam divergence can be narrowed
down. The hemispherical lens LED (HL-LED) with directional beam is
proposed, achieving a 53.8% enhancement of fiber coupling efficiency. On top
of a flip-chip-packaged TC-LED, a hemispherical BK-7 lens is capillary-bonded
onto the sapphire surface. Compared with TC-LED, the divergence of HL-LED
is significantly reduced by 50o.
Vertically-mounted LED (vmLED) is proposed to broaden the emission
pattern at the packaging level. By mounting the LED die upright to expose two
large illumination surfaces instead of the traditional way of bonding the die flat
down, the optical emission pattern is converted from Lambertian to a two-lobed
pattern. Both the optical properties and thermal properties are investigated and it
is found that there is a trade-off between the heat dissipation and light output. A
sapphire-prism-mounted vmLED is further proposed to improve the heat sinking.
In the last two chapters, micro-LED arrays with smaller illuminated active
regions are introduced and the combination with external optics, including optical
fibers and projection lens sets are used to demonstrate novel LED applications.
By coupling a bi-linear micro-LED array into a fiber bundle, a portable
microdisplay system is demonstrated and this comprehensive system can be used
for image projection. Another application involved a linear UV-micro-LED
array coupled with a projection lens set; this optical system has been demonstrated
as a direct-write lithographic tool for the fabrication of polymer microlens arrays
on InGaN LEDs. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/146150 |
| Date | January 2011 |
| Creators | Zhu, Ling, 朱玲 |
| Contributors | Choi, HW, Lai, PT |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Source | http://hub.hku.hk/bib/B47297463 |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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