Manipulating optical emission from light-emitting diodes and their applications

Abstract

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.