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(a) Schematic diagram of the flip-chip bonded deep-UV LED structure with an integrated microlens array on the sapphire substrate. (b) AFM image of photoresist after reflow. (c) Optical microscopy image and (d) SEM image of a fabricated sapphire microlens array.
The EL spectrum of a flip-chip bonded deep-UV LED with an integrated microlens array. The inset shows the optical microscopy image of the same circular LED taken from sapphire side and its characteristics. Under a dc driving, the output power of our disk UV LEDs with a diameter of fabricated from different wafers varies between 0.2 and .
Comparison of the deep-UV LED output power with and without the microlens array. A 55% increase in the optical output power by the microlens array formation was measured at dc driving.
Schematic diagrams to illustrate the light extraction enhancement by the microlens array formation. In (a), the planar interface limits the light extraction in a cone with most of the light entrapped in the wave-guided sapphire slab and nitride layers, while the curvature interface formed by the microlens array in (b) enhances the light extraction by reducing the incident angle and multiple reflection.
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