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.
KEYWORDS: Light emitting diodes, Luminous efficacy, Coating, Light scattering, Blue light emitting diodes, Human vision and color perception, Silicon, Scanning electron microscopy, Photonics, Reliability
The emission-adaptive phosphor layer was fabricated by self-exposure method to realize the pc-white LED. The effect of phosphor layer forming parameters such as phosphor content, exposure intensity and exposure time on the phosphor layer shape and LED characteristics was investigated. Through the fabrication process of phosphor layer, two types of phosphor layer shape were fabricated by phosphor layer forming parameters. In the case of low exposure intensity and/or short exposure time with high phosphor content, the conformal coating shape phosphor layer which is imitative of the irradiance pattern of an LED chip was formed, whereas in the high exposure intensity and/or long exposure time with low phosphor content, the light distribution proportional shape phosphor layer which reflects the near-filed pattern of an LED chip was formed. Therefore the latter was superior to the former in the angular color homogeneity due to a strong similarity between the phosphor content distribution and the light intensity distribution of an LED chip in all directions. It was concluded that the object-oriented emission-adaptive phosphor layer closely similar to the light distribution proportional shape phosphor layer was formed in the condition of high exposure intensity and/or long exposure time with high phosphor content and resulted in high luminous efficacy, low correlated color temperature as well as high angular color homogeneity.
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.
Indium-tin-oxide (ITO)/Ni/AgCu/Pt reflectors for high-performance GaN-based vertical light-emitting diodes (VLEDs)
were investigated. The ITO layer was first annealed at 650 °C in air to make an Ohmic contact and then the Ni/AgCu/Pt
reflectors were deposited and subsequently annealed at 400 °C in air to improve their reflectance and mechanical
adhesion with the ITO layer. It was shown that the reflectance of the ITO/Ni/AgCu/Pt reflectors at 460nm was slightly
increased from 82 to 87% after second annealing. Based on the secondary ion mass spectrometry depth profiles, this
improvement was attributed to the formation of a transparent Ni-oxide and the existence of Cu atoms near ITO/AgCu/Pt
interface regions suppressing the inter and out-diffusion of Ag. The VLEDs fabricated with the ITO/Ni/AgCu/Pt
reflectors showed an approximately 4.4% higher output power and much better current-voltage characteristics than those
with the Ag-based reflectors.
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.
We investigated the surface plasmon coupling behavior in InGaN/GaN multiple quantum wells at 460 nm by
employing Ag nanostructures on the top of a roughened p-type GaN. After the growth of a blue light emitting diode
structure, the p-GaN layer was roughened by inductive coupled plasma etching and the Ag nanostructures were formed
on it. This structure showed a drastic enhancement in photoluminescence and electroluminescence intensity and the
degree of enhancement was found to depend on the morphology of Ag nanostructures. From the time-resolved
photoluminescence measurement a faster decay rate for the Ag-coated structure was observed. The calculated Purcell
enhancement factor indicated that the improved luminescence intensity was attributed to the energy transfer from
electron-hole pair recombination in the quantum well to electron vibrations of surface plasmon at the Ag-coated surface
of the roughened p-GaN.
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