Thesis
Geometrical strategies for enhancement of light extraction efficiency from gallium antimonide based-mid-infrared superlattice light emitting diodes
University of Iowa
Master of Science (MS), University of Iowa
Autumn 2020
DOI: 10.17077/etd.005725
Abstract
Mid-infrared (mid-IR) optoelectronic devices are highly sought after for applications in environmental, agricultural, medical, and security industries. This is because several key molecules exhibit strong ro-vibrational absorption signatures in the mid-IR wavelength range (3 µm to 30 µm), which is important for gas sensing. Additionally, mid-IR devices can be utilized to simulate thermal scenes, which is useful in military applications.
In this work we developed and characterized geometrical structures for mid-IR superlattice light emitting diodes (SLEDs) based on a gallium antimonide (GaSb) substrate with indium arsenide (InAs)/GaSb superlattice epitaxial layers designed to emit at 3.8 µm. At a wavelength of 3.8 µm, GaSb has low light extraction efficiency with a baseline of 1.75% for an infinite slab. This is due to the high refractive index of 3.8 at a wavelength of 3.8 µm which creates a low critical angle, 15.3º. We sought to improve the extraction efficiency of GaSb based SLEDs by manipulating the substrate via deep etched mesas, lenslets, and substrate removal.
Using microfabrication techniques, we created deep etched mesas. This was fabricated using a two-step dry etch process which used a reactive ion etching inductively coupled plasma (RIE-ICP) etcher. During the second step of the etch we protected the emitting layers by encapsulating the base mesa. We then fabricated lenslets to help the extraction efficiency in the f/2 region. These were created by thermally reflowing the photoresist in the soft mask and dry etching in an RIE-ICP etcher. We used multiple methods to remove the GaSb substrate including a hydrochloric (HCl) acid-based wet etch and a dry etching process. Removing most of the substrate helped to further increase light extraction.
Our samples were then characterized by photoluminescence (PL) measurements. We decided to use this measurement to simplify our experiments which avoided electrically pumping the samples. We used the area under the curve of the PL spectra and normalized the spectra to an on-sample or outside reference to characterize the light extraction improvement in our devices.
As our initial modeling did not match the light extraction improvements we expected, we conducted new simulations to represent our experimental results more accurately. This new modeling aligned more closely with our experimental results than the previous models and revealed that the sidewall losses could be hindering our samples' performance. We also presented the design of a new back-contact reflector, known as the metacontact, which upon preliminary measurement showed increases in reflectivity over the existing back-contact used in our SLED devices.
Details
- Title: Subtitle
- Geometrical strategies for enhancement of light extraction efficiency from gallium antimonide based-mid-infrared superlattice light emitting diodes
- Creators
- Victoria Elizabeth Eng
- Contributors
- Fatima Toor (Advisor)John Prineas (Committee Member)David Andersen (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Electrical and Computer Engineering
- Date degree season
- Autumn 2020
- DOI
- 10.17077/etd.005725
- Publisher
- University of Iowa
- Number of pages
- x, 58 pages
- Copyright
- Copyright 2020 Victoria Elizabeth Eng
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (page 58).
- Public Abstract (ETD)
Mid-infrared (mid-IR) optoelectronic devices are highly sought after for applications in environmental, agricultural, medical, and security industries. In this work we developed mid-IR superlattice light emitting diodes (SLEDs) based on a gallium antimonide (GaSb) substrate with an indium arsenide (InAs)/GaSb superlattice emitting layer. The light extraction efficiency has a low baseline of 1.72% in GaSb based SLEDs. We sought to improve the extraction efficiency by manipulating the GaSb substrate to create geometrical structures that guide the light out more effectively.
The main feature used to guide the light out of these devices effectively was a deep-etched mesa. This mesa allowed for light to be reflected out more efficiently and thus escape the GaSb substrate. At a 3.8 μm wavelength GaSb had a high refractive index, 3.8, and thus low critical angle, 15.3°. As the mesa depth increased, we hypothesized we would see increased light extraction.
We also used substrate removal and lenslets to improve light extraction. Substrate removal provided less material for light to travel through which reduced the absorption, thus increasing the light extraction. Lenslets were a piece of microtexture on top of a mesa that created a hemispherical structure. Lenslets are used to aid in directing light in the forward direction.
After fabricating these samples, we characterized them with photoluminescence (PL) measurements to see the effect our device design had on increasing the light extraction in a GaSb based device. We then modeled our samples to gain further insight into factors affecting light extraction in mid-IR GaSb based devices.
- Academic Unit
- Electrical and Computer Engineering
- Record Identifier
- 9984036087102771
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