Abstract

III-nitride materials have been extensively employed in a wide variety of applications attributed to their compact sizes, lower operating voltage, higher energy efficiency and longer lifetime. Although tremendous progress has been reported for III-nitride light-emitting diodes (LEDs), further enhancement in the external quantum effciency (η_EQE), which depends upon internal quantum efficiency, injection efficiency and light extraction efficiency (η_extraction), is essential in realizing next generation high-efficiency ultraviolet (UV) and visible LEDs. Several challenges such as charge separation issue, large threading dislocation density, large refractive index contrast between GaN and air, and anisotropic emission at high Al-composition AlGaN quantum wells in the deep-UV regime have been identified to obstruct the realization of high-brightness LEDs. As a result, novel LED designs and growth methods are highly demanded to address those issues.

The objective of this dissertation is to investigate the enhancement of η_extraction

for various nanostructured III-nitride LEDs. In the first part, comprehensive studies on the polarization-dependent η_extraction for AlGaN-based flip-chip UV LEDs with microdome-shaped patterned sapphire substrates (PSS) and AlGaN-based nanowire UV LEDs are presented. Results show that the microdome-shaped PSS acts as an extractor for transverse-magnetic (TM)-polarized light where up to ~11.2-times and ~2.6-times improvement in TM-polarized η_extraction can be achieved for 230 nm and 280 nm flip-chip UV LEDs, while as a reflector that limits the extraction of transverse-electric (TE)-polarized light through the sapphire substrate. Analysis for 230 nm UV LEDs with nanowire structure shows up to ~48% TM-polarized η_extraction and ~41% TE-polarized η_extraction as compared to the conventional planar structure (~0.2% for TM-polarized η_extraction and ~2% for TE-polarized η_extraction). Plasmonic green LEDs with nanowire structure have also been investigated for enhancing the LED performance via surface plasmon polaritons. The analysis shows that both η_extraction and Purcell factor for the investigated plasmonic nanowire LEDs are independent of the Ag cladding layer thickness (H_Ag), where a Purcell factor of ~80 and η_extraction of ~65% can be achieved when H_Ag > 60 nm. Nanosphere lithography and KOH-based wet etching process have been developed for the top-down fabrication of III-nitride nanowire LEDs. The second part of this dissertation focuses on alternative approaches to fabricate white LEDs. The integration of three-dimensional (3D) printing technology with LED fabrication is proposed as a straightforward and highly reproducible method to improve η_extraction at the same time to achieve stable white color emission. The use of optically transparent acrylate-based photopolymer with a refractive index of ~1.5 as 3D printed lens on blue LED has exhibited 9% enhancement in the output power at current injection of 4 mA as compared to blue LED without 3D printed lens. Stable white color emission can be achieved with chromaticity coordinates around (0.27, 0.32) and correlated color temperature ~8900 K at current injection of 10 mA by mixing phosphor powder in the 3D printed lens. Novel LED structures employing ternary InGaN substrates are then discussed for realizing high-efficiency monolithic tunable white LEDs. Results show that large output power (~170 mW), high η_EQE (~50%), chromaticity coordinates around (0.30, 0.28), and correlated color temperature ~8200 K can be achieved by engineering the band structures of the InGaN/InGaN LEDs on ternary InGaN substrates.

Publication Date

5-2019

Document Type

Dissertation

Student Type

Graduate

Degree Name

Microsystems Engineering (Ph.D.)

Department, Program, or Center

Microsystems Engineering (KGCOE)

Advisor

Jing Zhang

Advisor/Committee Member

Santosh Kurinec

Advisor/Committee Member

Parsian Mohseni

Campus

RIT – Main Campus

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