Transcript (3) EE235_Paper1_Darsen

Fabrication of photonic crystal structures on light
emitting diodes by nanoimprint lithography
Authors: Sang Hoon Kim, Ki-Dong Lee, Ja-Yeon Kim, Min-Ki
Kwon and Seong-Ju Park
Presented by Darsen Lu (3/19/2007)
EE235 Class Presentation on Nanoimprint Lithography (Spring 2007)
Outline
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Introduction
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Mold Fabrication
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How to fabricate the mold for nanoimprint lithography
PCLED Device Fabrication
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Some background Information
The fabrication of a photonic crystal light emitting diode
(PCLED) using nanoimprint
Results
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LED Performance Enhancement due to Photonic Crystal
Structure
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Introduction
Some background Information
EE235 Class Presentation on Nanoimprint Lithography (Spring 2007)
Light Emitting Diode (LED)
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LED
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Definition: a semiconductor device that emits incoherent narrowspectrum light when electrically biased in the forward direction
LED v.s. Incandescent (Edison’s lightbulb) and Flourescent Bulbs
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Much longer life span (105 - 106 hrs v.s. 103 / 104 hrs)
Suitable for applications that are subject to frequent on-off cycling
Efficiency: better than incandescent but currently worse than
flourescent bulbs
Source: US Department of Energy
http://www.netl.doe.gov/ssl/faqs.htm
Courtesy of Wikipedia
http://en.wikipedia.org/wiki/LED
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LED Efficiency
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Internal Quantum Efficiency (ηint)
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Definition: ratio of the number of electrons
flowing in the external circuit to the number of
photons produced within the device
Has been improved up to 80%
External Quantum Efficiency
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Definition: The percentage of photons that can
be extracted to the ambient.
Typically 1% ~ 10%
Limiting factor of LED efficiency
Improvement techniques: dome-shaped
package, textured surface, photonic crystal, …
Source: Lecture Note of “Optoelectronic Devices” (by Sheng-fu
Horng, Dept. of Electrical Engrg, NTHU, Hsinchu, Taiwan)
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Photonic Crystal
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Photonic Crystal
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Definition: Periodic optical nanostructures that are designed
to affect the propagation of EM waves.
The periodic structure creates a “photonic bandgap.” No
light with frequency within the gap can propagate.
Source: SPIE Photonics West by Steven G. Johnson http://ab-initio.mit.edu/photons/tutorial/;
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Photonic Crystal for enhancing the
external quantum efficiency of an LED
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Part of the extrinsic loss in LED
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Photonic Crystal + LED
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LED: a thin slab serves as a waveguide
At some frequencies, spontaneously emitted light
can be coupled in to the waveguide
 Efficiency Loss
The “optical bandgap” prevents spontaneously
emitted light from coupling into the waveguide,
therefore enhancing the efficiency of the device.
PC Structure Design
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Simulators are available: Ex: R-soft
Source: Shanhui Fan, Pierre R. Villeneuve, and J. D. Joannopoulos, “High Extraction Efficiency of
Spontaneous Emission from Slabs of PhotonicCrystals”
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Mold Fabrication
How to fabricate the mold for nanoimprint
lithography
EE235 Class Presentation on Nanoimprint Lithography (Spring 2007)
Mold Fabrication Process (1)
PR
1. Start with a silicon wafer.
Grow/Deposit layers: SiO2, Cr,
Photoresist (PR)
Cr
SiO2
2. Pattern the PR twice using a Laser
Interference Lithography (LIL)
3. Develop the PR.
Use a thermal treatment method to
alter the pillar shape and increase its
diameter
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Mold Fabrication Process (2)
PR
Cr
SiO2
4. Use RIE to etch the Cr.
Then use the Cr as a mask and etch
SiO2 with RIE
(This allows a thin PR layer)
5. Remove the Cr mask
Coat Anti-sticking layer by vacuum
evaporation
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Thermal Treatment Method
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Procedure:
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Heat the wafer with PR at 120C for 5 minutes
Purpose:
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Increase the pillar diameter (110nm to 150nm)
Control the shape of the photoresist pattern (circular)
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Thermal Treatment Method (2)
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120°C is found to be the optimum temperature
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Temperature too low  no significant tampering effects
Temperature too high  PR becomes too thin and
lithography quality becomes poor
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PCLED Device Fabrication
The fabrication of a photonic crystal layer
using nanoimprint
EE235 Class Presentation on Nanoimprint Lithography (Spring 2007)
LED Device Fabrication
1. Prepare an LED substrate
sample. Deposit Cr and PR.
3. Remove the residual area using
O2 plasma
2. Nanoimprint Process (50bar,
145C)
4. Etch the Cr and p-GaN region
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Results
LED Performance Enhancement due to
Photonic Crystal Structure
EE235 Class Presentation on Nanoimprint Lithography (Spring 2007)
Device Performance Enhancement
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LED intensity as a function of etch depth (With PC)
With Photonic
Crystal Structure:
9x Enhancement
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LED intensity as a function of etch depth (Without PC)
Control Group:
4x Enhancement
(Due to “penetrating”)
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SEM Pictures
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(a) FESEM Image of the LED sample after removal of the
residual layer and Cr Patterning
(b) Final PC strctures on a p-GaN layer
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The diameter of the hole increases after each patterning
FESEM: Field Emission Scanning Electron Microscope
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Summary
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The energy efficiency of an LED can be improved by enhancing
the external quantum efficiency.
Photonic Crystal is one of the possible mechanisms to enhance
the external quantum efficiency
A mold is fabricated using laser interference lithography (LIL)
and thermal treatment method
A Photonic Crystal LED is fabricated using nanoimprint
lithography
The light intensity of the LED is significantly improved due to
the photonic crystal structure
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The End
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Thank you
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Questions?
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