Investigation of high performance ultra-violet and visible nanostructure light emitting diodes
Date Issued
2010
Date
2010
Author(s)
Ke, Min-Yung
Abstract
Due to some superior optical and electrical characteristics, ZnO related materials and GaN nanostructure LEDs attract great interests as the new short wavelength lighting source. But ZnO materials still have some limitations like the lack of stable p-ZnO and large amounts of intrinsic defects which cause the difficulty in realizing high purity UV/blue ZnO homojunction LEDs. In order to achieve high purity UV light emission ZnO LEDs, GZO/p-GaN, GZO/p-GaN with a SiO2 layer and GZO/p-GaN with a sandwich structure heterojunction LEDs were designed for optimizing light emission properties.
From EL spectra, a broadband emission from 400nm to 800nm, sharp peak emission at 394nm and excitonic emission at 377nm can be observed from LEDs with different structure, respectively. Broadband emissions of LEDs without a SiO2 layer due to transition in the GaN along with defects in the ZnO layers. When LEDs with a SiO2 layer, light emission attributed to recombination of accumulated carriers between n-ZnO/SiO2 and p-GaN/SiO2 junctions. We also characterized samples with the light emitting ZnO layer sandwiched between two SiO2 thin films. Carriers can effective injection and confinement in the sandwich structure, result in a strong excitonic emission at the wavelength 377nm.
. We further investigate light output behaviors of GaN nanorod LEDs. GaN nanorod LEDs were fabricated by nanosphere lithography. The electroluminescence peak wavelengths of the nanorod LEDs nearly remain as constant for an injection current level between 25mA and 100mA, which indicates that the quantum confined stark effect is suppressed in the nanorod devices. The radiation profiles of the nanorod LEDs show the high directionality of emission light and waveguiding effect. The result is associated with the vertical guiding effect along the nanorod cylinder and the Bragg scattering of photons extracted from the sidewall by the rest of the rods.
Finally, we integrated the experimental results of ZnO based and GaN based nanorod LEDs, high purity UV/blue light emission ZnO nanorod LED can be achieved.
We grown ZnO nanorod by aqueous solution method on p-GaN surface with a MgO electron blocking layer. A sharp excitonic emission peak at 390nm can be observed of LEDs with MgO layer. This result indicates electrons can be blocked at MgO/ZnO interface effectively and holes can easily pass through the p-GaN/MgO interface due to the different band offset between ZnO/MgO and p-GaN/MgO. ZnO LEDs with nanorods also can act as a waveguide. A narrower radiation profile with divergence angle 30o and light emission can be redirected into the surface normal direction. From the results, ZnO LEDs with nanorods have great potential to become new UV light sources with high light emission directionality.
Subjects
light emitting diodes
ZnO
heterojunction
carrier confinement structure
nanosphere
nanorod
light emission directionality
Type
thesis
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