Project Report

Submitted12/29/2011 The PI spent approximately three months in Suncheon, South Korea at Sunchon National University in order to design, develop, and test GaN multi-quantum-well (MQW) light-emitting diode (LED) structures with the implementation of core/shell Cd/ZnSe colloidal nano-crystal quantum dots (NCQD) with the goal to fabricate a more efficient white-light emitter based on solid-state technologies. Furthermore, the ability to measure non-radiant cavity-control on the direct energy transfer between the core/shell Cd/ZnSe NCQDs and GaN MQW LEds was to be investigated. The project was a collaborative effort between the Pennsylvania State University and Sunchon National University. The PI and colleagues who worked on this project successfully designed, developed, and tested MQW GaN blue-emitting LEDs while using several different microstructures in P-type GaN regions of the device in order to reduce operating current, leakage currents, and improve blue emission efficiency. The two different microstructures were patterned as micro-holes and nano-pillar top-emitting LEDs, while employing c-plane and a-plane crystal orientations. The micro-holes ranged from 3 to 20 microns in order to see differences in current-voltage characteristics and efficiencies. C-plane GaN-based devices proved to operate with less leakage currents and defects due to crystalline stability, whereas the operational a-plane GaN-based devices offered large leakage currents low efficiency due to defects in the crystalline structure during fabrication. These devices were saturated with NCQDs of red and green wavelength photoluminescence in order to mix blue-emitting LED and create a white-light emitter. Several different QD deposition methods were utilized from spin-coating, drop-casting, and immersion techniques. Drop-casting NCQDs from solution proved to yield the highest quantum yields and efficiency between the LED and QDs themselves. However, due to the size of the 550nm green-emissive QDs, the efficiencies were limited to <10%, which is typically reported in literature reviews. The tested LEDs coupled with NCQDs offered strong optical emission, but a green-shift in the color hue is needed to further produce a white-light emitter. Optically, the devices appeared a light pink/ soft white emission when tested. The non-radiative direct energy transfer between the GaN/NCQD structures are currently being measured at the Pennsylvania State University. In order to create a circularly-polarized GaN/NCQD white-light emitter, a thin-film of non-homogeneous dielectric material must be deposited via physical vapor deposition (PVD). A recipe and initial devices have been made with the employment of this film, and currently undergoing initial experiments with already existing devices. In conclusion, the PI and colleagues successfully designed, fabricated, and tested GaN-based LEDs with different microstructures in P-type GaN regions, and incorporated solution-based NCQDs in order to yield a nearly white-light emitter. The energy transfer between the GaN and NCQDs are currently being tested, and the implementation of a thin film to produce circularly-polarized light is in the initial stages of development at this time.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Carter Kimsey
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Swiontek Stephen E
State College
United States
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