We have developed a new light emitting device named ZOGAN LED. It is composed with the p-layer comprising p-ZnO and p-GaN. ZOGAN LED and GaN LED are dramatically different in device performance. For example, it is well known for GaN LED that the EQE of GaN LED becomes worse, as either the device size is getting smaller, or the injection current is larger. However, unlike GaN LED, ZOGAN LED shows no EQE droop with current increase or size decrease, while keeping its EQE level exceptionally high. No EQE droop and no size-dependent EQE decrease are unique features of ZOGAN LED, which is never ever reported before. The non-radiative leakage current is almost negligible and not increased as the chip size decreases even below 10um for ZOGAN microLED. For these unique properties, ZOGAN microLED is critical to accomplish the high-efficient microLED displays with >5,000PPI resolution.
In this presentation, we will report the features of ZOGAN microLED, based on experimental results of electrical/optical measurements. After brief discussion and review on physical properties of ZOGAN LEDs, ZOGAN microLEDs in different chip sizes (20, 10, and 5 microns in diameter) will be demonstrated.
The influence of phosphor sedimentation on the white light-emitting diode with different structure chip was
investigated. The phosphor sedimentation phenomenon occurred seriously as encapsulant viscosity lowers. The influence
of phosphor sedimentation on the white light-emitting diode with the vertical structure chip whose one side only emits is
larger than that of lateral structure chip whose all sides emit. Hence, Difference in luminous efficacy by the phosphor
sedimentation reached about 20 % in the case with the vertical structure chip due to optical loss stemmed from the
phosphor sediment layer.
Various types of nano-to-micron scale patterned structure have been employed into nitride light-emitting diodes
(LEDs) in order to investigate the optical performances of device. The patterned structure was formed on top of the LED
epitaxial structure or was embedded between epitaxial layers and sapphire substrate. The patterned structure affected to
the LED performances in terms of light distribution and anisotropic increase of light extraction as well as increase of
external quantum efficiency. The controllability of light extraction by forming a patterned structure with different index
material is applicable to flip chip devices or chips on board in which light is supposed to be extracted toward a certain
direction with the straight forward directionality. The index matched nano-patterned AlN template played such a role of
anisotropic directionality of light extraction in the device. Periodic (photonic band gap) and non-periodic (random hole)
patterned structure also showed different extraction efficiency and characteristics of light distribution. The experimental
result was well matched with the simulated estimation.
White light-emitting diodes (LEDs) have dramatically developed and gradually taken over from the conventional light
source as the solid-state lighting during the last decade. It is now sufficient for illumination application in performance,
while it is still insufficient in color quality. Especially, most of phosphor-converted white LEDs have the poor angular
color homogeneity. In this study, we adopted a distinctive phosphor conformal coating technique in the packaging
process to reduce the variance of correlated color temperature (CCT) among the packages and spatial CCT in the
package. Also, to reduce the spatial CCT variance without considerable shrinkage of luminous efficacy, we applied submicrometer
scale TiO2 powder as diffuser in the phosphor layer or in the encapsulation layer of white LED with a
phosphor conformal coating layer and investigated the effects of titania diffuser on angular color homogeneity and
optical performance. Regardless of the diffuser content, spatial CCT variance and luminous efficacy were decreased with
the increase of the diffuser content. Nevertheless, among the conditions for achievement of the equivalent in color
uniformity, the luminous efficacy in the case of 0.1 wt% diffuser mingled in the encapsulation layer was 20 % higher
than in the case of 5 wt% diffuser mingled in the phosphor layer. These phenomena result from differences of light
scattering loss caused by 10 times more volume of diffuser mixed in the phosphor layer than in the encapsulation layer.
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