Dissertation Defense, Yik-khoon Ee

Dissertation Defense, Yik-khoon Ee

Title:                      Reduced Dislocation Engineering and Enhanced
                             Light Extraction Efficiency of III-Nitride Light
                             Emitting Diodes

Date:                     Tuesday, November 3, 2009

Time:                     12:30pm to 2pm

Location:              Rauch Business Center Room 85


The rising energy cost has pushed for technological advances for high energy-efficiency technology. The United States spends more than $37 billion annually on energy for lighting alone, and lighting accounts for 21% of the electricity usage in the United States. One of the long term strategies adopted to reduce the energy consumption is solid state lighting through the use of light emitting diodes (LEDs). The efficiency of existing III-Nitride quantum wells LEDs are limited by (1) detrimental impact of electrostatic fields in InGaN quantum well reducing the transition matrix element, (2) high defect density in the GaN material which increases non-radiative recombination rate, and reduces the LEDs lifetime, (3) low light extraction efficiency due to the high refractive index of GaN as compared to air which in turns lead to significant light trapping in the LEDs by total internal reflection, and (4) the challenges in achieving high efficiency at high operation current density.

In this dissertation, methods to achieve high efficiency III-Nitride LEDs emitting in the visible regime, by addressing the second and third challenges as mentioned above have been proposed and investigated. The novel approaches used to overcome the second and third challenges include: (1) reducing the dislocation density of GaN and increasing the internal quantum efficiency of LEDs through the use of a novel abbreviated GaN epitaxy growth mode on nano-patterned AGOG sapphire substrate, and (2) enhancing the light extraction efficiency of LEDs through the use of novel techniques such as SiO2 / polystyrene microlens arrays, polydimethylsiloxane (PDMS) concave microstructure arrays, and TiO2 microspheres arrays.

The research works encompass various aspects: modeling and device design, epitaxial growth by metalorganic chemical vapor deposition, materials characterization and analysis, device fabrications, and device characterizations. The use of colloidal-based microlens arrays has led to significant enhancement of light extraction efficiency for both LEDs devices employing the concave and convex-based microlens arrays. The innovation in abbreviated growth mode on patterned substrate has led to the ability of realizing high quality GaN with reduced dislocation density and higher LED output power by employing a lower cost growth technique.

Ph.D. Committee:

Prof. Nelson Tansu (Chair, and Ph.D. Advisor)
Prof. Yujie Ding
Prof. Helen M. Chan
Prof. Richard P. Vinci
Prof. James F. Gilchrist