We report on the fabrication of 280-nm AlGaN-based deep-ultraviolet light-emitting diodes (UV LEDs) on sapphire substrates with an integrated microlens array. Microlenses with a diameter of 12μm were fabricated on the sapphire substrate by resist thermal reflow and plasma dry etching. LED devices were flip-chip bonded on high thermal conductive AlN ceramic submounts to improve the thermal dissipation, and the emitted UV light was extracted through the sapphire substrates. With the integrated microlens array, a 55% enhancement in the output power at 20-mA dc driving was achieved compared with the same LED without microlens. The light extraction enhancement is the result of the reduced internal reflections of the light caused by the microlens surface profile.
Electrical and structural characterization of Mg-doped p -type Al 0.69 Ga 0.31 N films on SiC substrate Mg-doped Al 0.7 Ga 0.3 N epilayers ͑ϳ1 m͒ were grown on an AlN/sapphire template by metalorganic chemical vapor deposition and the electrical and optical properties of these epilayers were studied. For optimized Mg-doped Al 0.7 Ga 0.3 N epilayers, we have obtained a resistivity around 10 5 ⍀ cm at room temperature and confirmed p-type conduction at elevated temperatures ͑Ͼ700 K͒ with a resistivity of about 40 ⍀ cm at 800 K. From the temperature dependent Hall effect measurement, the activation energy of Mg acceptor is found to be around 400 meV for Al 0.7 Ga 0.3 N alloy. The optimized Mg-doped Al 0.7 Ga 0.3 N epilayers have been incorporated into the deep-ultraviolet ͑UV͒ ͑Ͻ300 nm͒ light-emitting diode ͑LED͒ structures as an electron blocking layer. An enhancement in the performance of the UV LEDs was obtained. LEDs with peak emission wavelengths at 280 nm were fabricated with a circular geometry ͑300 µm disk diameter͒. Output power reached 0.35 mW at 20 mA and 1.1 mW at 150 mA dc current. The importance of Mg-doped Al 0.7 Ga 0.3 N alloys to suppress the long-wavelength emission components in deep-UV LEDs and the fundamental limit for achieving p-type Al-rich AlGaN alloys are also discussed.
AlGaN-based deep-ultraviolet light-emitting diode (LED) structures, which radiate light at 305 and 290nm, have been grown on sapphire substrates using an AlN epilayer template. The fabricated devices have a circular geometry to enhance current spreading and light extraction. Circular UV LEDs of different sizes have been characterized. It was found that smaller disk LEDs had higher saturation optical power densities but lower optical powers than the larger devices. This trade-off between power and power density is a result of a compromise between electrical and thermal resistance, as well as the current crowding effect (which is due to the low electrical conductivity of high aluminum composition n- and p-AlGaN layers). Disk UV LEDs should thus have a moderate size to best utilize both total optical power and power density. For 0.85mm×0.85mm interdigitated LEDs, a saturation optical power of 2.9mW (1.8mW) at 305nm (290nm) was also obtained under dc operation.
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