| Summary: | 碩士 === 國立清華大學 === 電子工程研究所 === 103 === Ultraviolet light (200-400 nm) LEDs are not only developing rapidly but also used in many varied applications in recent years because of its short wavelength used for imperceptible biosensor, high color rendering, and excellent stability. Moreover, near ultraviolet light does less damage to human beings by comparison. However, efficiency droop of UV LED is a restriction and important issue. The reason of efficiency droop relates to current crowding effect, Auger recombination, and trap-assisted tunneling leakage by structural defects. Most of all, the main reason is that effect of quantum wells is gradually inefficient due to low In-doping in InGaN below 400 nm wavelength.
In order to increase the efficiency, surface-emitting 8×8 LED arrays with pixel size of 200 μm are designed and conventional broad-area LEDs (400μm×400μm) will also be fabricated from the same wafers for comparison studies. The atomic layer deposition (ALD) Ga-doped ZnO films with a thickness of 100 nm have an electron concentration of ~8 × 1020 cm-3, low resistivity of ~4 × 10-4 Ω-cm, and high transmittance (~88%) at the near ultraviolet wavelengths. ALD-GZO thin films are deposited as the internal wires connecting pixels because of excellent step coverage.
The peak wavelength is 370 nm and has a red-shift as the injection current increases. The single pixel of LED arrays reveals a maximum light output power density of 7.25 W/cm2 at 90 mA as compared to the broad-area LED of 1.34 W/cm2 at 70 mA. The LED arrays also exhibit an enhancement in divergence angle from 128o to 137o. These results reveal that LED arrays have significant improvement for the light output power in the UV region.
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